The Southern Ocean south of 30° S represents only one-third of the total ocean area, yet absorbs half of the total ocean anthropogenic carbon and over two-thirds of ocean anthropogenic heat. In the past, the Southern Ocean has also been one of the most sparsely measured regions of the global ocean. Here we use pre-2005 ocean shipboard measurements alongside novel observations from autonomous floats with biogeochemical sensors to calculate changes in Southern Ocean temperature, salinity, pH and concentrations of nitrate, dissolved inorganic carbon and oxygen over two decades. We find local warming of over 3 °C, salinification of over 0.2 psu near the Antarctic coast, and isopycnals are found to deepen between 65° and 40° S. We find deoxygenation along the Antarctic coast, but reduced deoxygenation and nitrate concentrations where isopycnals deepen farther north. The forced response of the Earth system model ESM2M does not reproduce the observed patterns. Accounting for meltwater and poleward-intensifying winds in ESM2M improves reproduction of the observed large-scale changes, demonstrating the importance of recent changes in wind and meltwater. Future Southern Ocean biogeochemical changes are likely to be influenced by the relative strength of meltwater input and poleward-intensifying winds. The combined effect could lead to increased Southern Ocean deoxygenation and nutrient accumulation, starving the global ocean of nutrients sooner than otherwise expected.
Druffel, E R., S Griffin, Nina Wang, N G Garcia, A P McNichol, Robert M Key, and B D Walker, May 2019: Dissolved Organic Radiocarbon in the Central Pacific Ocean. Geophysical Research Letters, 46(10), DOI:10.1029/2019GL083149. Abstract
We report marine dissolved organic carbon (DOC) concentrations, and DOC ∆14C and δ13C values in seawater collected from the central Pacific. Surface ∆14C values are low in equatorial and polar regions where upwelling occurs, and high in subtropical regions dominated by downwelling. A core feature of these data is that 14C aging of DOC (682±86 14C yr) and dissolved inorganic carbon (DIC) (643±40 14C yr) in Antarctic Bottom Water between 54.0°S and 53.5°N are similar. These estimates of aging are minimum values due to mixing with deep waters. We also observe minimum ∆14C values (–550 to –570‰) between the depths of 2000–3500m in the North Pacific, though the source of the low values cannot been determined at this time.
Gruber, Nicolas, Dominic Clement, B R Carter, Richard A Feely, S van Heuven, M Hoppema, Masao Ishii, and Robert M Key, et al., March 2019: The oceanic sink for anthropogenic CO2 from 1994 to 2007. Science, 363(6432), DOI:10.1126/science.aau5153. Abstract
We quantify the oceanic sink for anthropogenic carbon dioxide (CO2) over the period 1994 to 2007 by using observations from the global repeat hydrography program and contrasting them to observations from the 1990s. Using a linear regression–based method, we find a global increase in the anthropogenic CO2 inventory of 34 ± 4 petagrams of carbon (Pg C) between 1994 and 2007. This is equivalent to an average uptake rate of 2.6 ± 0.3 Pg C year−1 and represents 31 ± 4% of the global anthropogenic CO2 emissions over this period. Although this global ocean sink estimate is consistent with the expectation of the ocean uptake having increased in proportion to the rise in atmospheric CO2, substantial regional differences in storage rate are found, likely owing to climate variability–driven changes in ocean circulation.
The Global Ocean Data Analysis Project (GLODAP) is a synthesis effort providing regular compilations of surface to bottom ocean biogeochemical data, with an emphasis on seawater inorganic carbon chemistry and related variables determined through chemical analysis of water samples. This update of GLODAPv2, v2.2019, adds data from 116 cruises to the previous version, extending its coverage in time from 2013 to 2017, while also adding some data from prior years. GLODAPv2.2019 includes measurements from more than 1.1 million water samples from the global oceans collected on 840 cruises. The data for the 12 GLODAP core variables (salinity, oxygen, nitrate, silicate, phosphate, dissolved inorganic carbon, total alkalinity, pH, CFC-11, CFC-12, CFC-113, and CCl4) have undergone extensive quality control, especially systematic evaluation of bias. The data are available in two formats: (i) as submitted by the data originator but updated to WOCE exchange format and (ii) as a merged data product with adjustments applied to minimize bias. These adjustments were derived by comparing the data from the 116 new cruises with the data from the 724 quality-controlled cruises of the GLODAPv2 data product. They correct for errors related to measurement, calibration, and data handling practices, taking into account any known or likely time trends or variations. The compiled and adjusted data product is believed to be consistent to better than 0.005 in salinity, 1 % in oxygen, 2 % in nitrate, 2 % in silicate, 2 % in phosphate, 4 µmol kg−1 in dissolved inorganic carbon, 4 µmol kg−1 in total alkalinity, 0.01–0.02 in pH, and 5 % in the halogenated transient tracers. The compilation also includes data for several other variables, such as isotopic tracers. These were not subjected to bias comparison or adjustments.
The original data, their documentation and DOI codes are available in the Ocean Carbon Data System of NOAA NCEI (https://www.nodc.noaa.gov/ocads/oceans/GLODAPv2_2019/, last access: 17 September 2019). This site also provides access to the merged data product, which is provided as a single global file and as four regional ones – the Arctic, Atlantic, Indian, and Pacific oceans – under https://doi.org/10.25921/xnme-wr20 (Olsen et al., 2019). The product files also include significant ancillary and approximated data. These were obtained by interpolation of, or calculation from, measured data. This paper documents the GLODAPv2.2019 methods and provides a broad overview of the secondary quality control procedures and results.
Talley, L D., I Rosso, I V Kamenkovich, Matthew R Mazloff, J Wang, E S Boss, Alison R Gray, K S Johnson, Robert M Key, S C Riser, N L Williams, and Jorge L Sarmiento, January 2019: Southern Ocean biogeochemical float deployment strategy, with example from the Greenwich Meridian line (GO‐SHIP A12). Journal of Geophysical Research: Oceans, 124(1), DOI:10.1029/2018JC014059. Abstract
Biogeochemical Argo floats, profiling to 2000 m depth, are being deployed throughout the Southern Ocean by the Southern Ocean Carbon and Climate Observations and Modeling program (SOCCOM). The goal is 200 floats by 2020, to provide the first full set of annual cycles of carbon, oxygen, nitrate and optical properties across multiple oceanographic regimes. Building from no prior coverage to a sparse array, deployments are based on prior knowledge of water mass properties, mean frontal locations, mean circulation and eddy variability, winds, air‐sea heat/freshwater/carbon exchange, prior Argo trajectories, and float simulations in the Southern Ocean State Estimate (SOSE) and Hybrid Coordinate Ocean Model (HYCOM). Twelve floats deployed from the 2014‐2015 Polarstern cruise from South Africa to Antarctica are used as a test case to evaluate the deployment strategy adopted for SOCCOM's 20 deployment cruises and 126 floats to date. After several years, these floats continue to represent the deployment zones targeted in advance: (1) Weddell Gyre sea ice zone, including the Antarctic Slope Front, Maud Rise, and the open gyre; (2) Antarctic Circumpolar Current (ACC) including the topographically‐steered Southern zone ‘chimney' where upwelling carbon/nutrient‐rich deep waters produce surprisingly large carbon dioxide outgassing; (3) Subantarctic and Subtropical zones between the ACC and Africa; and (4) Cape Basin. Argo floats and eddy‐resolving HYCOM simulations were the best predictors of individual SOCCOM float pathways, with uncertainty after 2 years on the order of 1000 km in the sea ice zone and more than double that in and north of the ACC.
Toggweiler, J R., E R M Druffel, Robert M Key, and Eric D Galbraith, April 2019: Upwelling in the Ocean Basins north of the ACC Part 1: On the Upwelling Exposed by the Surface Distribution of Δ14C. Journal of Geophysical Research: Oceans, 124(4), DOI:10.1029/2018JC014794. Abstract
The upwelling associated with the ocean's overturning circulation is hard to observe directly. Here, a large data set of surface Δ14C measurements is compiled in order to show where deep water is brought back up to the surface in the ocean basins north of the Antarctic Circum‐polar Current (ACC). Maps constructed from the data set show that low‐Δ14C deep water from the ACC is drawn up to the surface in or near the upwelling zones off Northwest Africa and Namibia in the Atlantic, off Costa Rica and Peru in the Pacific, and in the northern Arabian Sea in the Indian Ocean. Deep water also seems to be reaching the surface in the subarctic Pacific gyre near the Kamchatka Peninsula. The low‐Δ14C water drawn up to the surface in the upwelling zones is also shown to spread across the ocean basins. It is easily seen, for example, in the western Atlantic off Florida and in the western Pacific off New Guinea and Palau. The spreading allows one to estimate the volumes of upwelling, which, it turns out, are similar to the volumes of large‐scale upwelling derived from inverse box models. This means that very large volumes of cool subsurface water are reaching the surface in and near the upwelling zones − much larger volumes than would be expected from the local winds.
Toggweiler, J R., E R M Druffel, Robert M Key, and Eric D Galbraith, April 2019: Upwelling in the Ocean Basins north of the ACC Part 2: How Cool Subantarctic Water Reaches the Surface in the Tropics. Journal of Geophysical Research: Oceans, 124(4), DOI:10.1029/2018JC014795. Abstract
Large volumes of cool water are drawn up to the surface in the tropical oceans. A com‐panion paper shows that the cool water reaches the surface in or near the upwelling zones off northern and southern Africa and Peru. The cool water has a subantarctic origin and spreads extensively across the Atlantic and Pacific basins after it reaches the surface. Here, we look at the spreading in two low‐resolution ocean general circulation models and find that the spreading in the models is much less extensive than observed. The problem seems to be the way the upwelling and the spreading are connected (or not connected) to the ocean's large‐scale over‐turning. As proposed here, the cool upwelling develops when warm buoyant water in the western tropics is drawn away to become deep water in the North Atlantic. The “drawing away” shoals the tropical thermocline in a way that allows cool subantarctic water to be drawn up to the surface along the eastern margins. The amounts of upwelling produced this way exceed the amounts generated by the winds in the upwelling zones by as much as four times. Flow restrictions make it difficult for the warm buoyant water in our models to be drawn away.
Sulpis, O, B P Boudreau, A Mucci, C Jenkins, D S Trossman, Brian K Arbic, and Robert M Key, November 2018: Current CaCO3 dissolution at the seafloor caused by anthropogenic CO2. Proceedings of the National Academy of Sciences, 115(46), DOI:10.1073/pnas.1804250115. Abstract
Oceanic uptake of anthropogenic CO2 leads to decreased pH, carbonate ion concentration, and saturation state with respect to CaCO3 minerals, causing increased dissolution of these minerals at the deep seafloor. This additional dissolution will figure prominently in the neutralization of man-made CO2. However, there has been no concerted assessment of the current extent of anthropogenic CaCO3 dissolution at the deep seafloor. Here, recent databases of bottom-water chemistry, benthic currents, and CaCO3 content of deep-sea sediments are combined with a rate model to derive the global distribution of benthic calcite dissolution rates and obtain primary confirmation of an anthropogenic component. By comparing preindustrial with present-day rates, we determine that significant anthropogenic dissolution now occurs in the western North Atlantic, amounting to 40–100% of the total seafloor dissolution at its most intense locations. At these locations, the calcite compensation depth has risen ∼300 m. Increased benthic dissolution was also revealed at various hot spots in the southern extent of the Atlantic, Indian, and Pacific Oceans. Our findings place constraints on future predictions of ocean acidification, are consequential to the fate of benthic calcifiers, and indicate that a by-product of human activities is currently altering the geological record of the deep sea.
We introduce a composite tracer, Alk*, that has a global distribution primarily determined by CaCO3 precipitation and dissolution. Alk* also highlights riverine alkalinity plumes that are due to dissolved calcium carbonate from land. We estimate the Arctic receives approximately twice the riverine alkalinity per unit area as the Atlantic, and 8 times that of the other oceans. Riverine inputs broadly elevate Alk* in the Arctic surface and particularly near river mouths. Strong net carbonate precipitation lowers basin mean Indian and Atlantic Alk*, while upwelling of dissolved CaCO3 rich deep waters elevates Northern Pacific and Southern Ocean Alk*. We use the Alk* distribution to estimate the carbonate saturation variability resulting from CaCO3 cycling and other processes. We show regional variations in surface carbonate saturation are due to temperature changes driving CO2 fluxes and, to a lesser extent, freshwater cycling. Calcium carbonate cycling plays a tertiary role. Monitoring the Alk* distribution would allow us to isolate the impact of acidification on biological calcification and remineralization.
Manizza, Manfredi, M J Follows, Stephanie Dutkiewicz, C Hill, and Robert M Key, December 2013: Changes in the Arctic Ocean CO2 sink (1996–2007): A regional model analysis. Global Biogeochemical Cycles, 27(4), DOI:10.1002/2012GB004491. Abstract
The rapid recent decline of Arctic Ocean sea ice area increases the flux of solar radiation available for primary production and the area of open water for air-sea gas exchange. We use a regional physical-biogeochemical model of the Arctic Ocean, forced by the National Centers for Environmental Prediction/National Center for Atmospheric Research atmospheric reanalysis, to evaluate the mean present-day CO2 sink and its temporal evolution. During the 1996–2007 period, the model suggests that the Arctic average sea surface temperature warmed by 0.04°C a−1, that sea ice area decreased by ∼0.1 × 106 km2 a−1, and that the biological drawdown of dissolved inorganic carbon increased. The simulated 1996–2007 time-mean Arctic Ocean CO2 sink is 58 ± 6 Tg C a−1. The increase in ice-free ocean area and consequent carbon drawdown during this period enhances the CO2 sink by ∼1.4 Tg C a−1, consistent with estimates based on extrapolations of sparse data. A regional analysis suggests that during the 1996–2007 period, the shelf regions of the Laptev, East Siberian, Chukchi, and Beaufort Seas experienced an increase in the efficiency of their biological pump due to decreased sea ice area, especially during the 2004–2007 period, consistent with independently published estimates of primary production. In contrast, the CO2 sink in the Barents Sea is reduced during the 2004–2007 period due to a dominant control by warming and decreasing solubility. Thus, the effect of decreasing sea ice area and increasing sea surface temperature partially cancel, though the former is dominant.
Plancherel, Y, Keith B Rodgers, Robert M Key, A R Jacobson, and Jorge L Sarmiento, July 2013: Role of regression model selection and station distribution on the estimation of oceanic anthropogenic carbon change by eMLR. Biogeosciences, 10(7), DOI:10.5194/bg-10-4801-2013. Abstract
Differencing predictions of linear regression models generated from hydrographic data collected at different times (the eMLR method) was proposed as a means of quantifying the dominant patterns of change in oceanic anthropogenic carbon in the context of sparse data sets subject to natural variability. The ability of eMLR to recover the anthropogenic carbon signal in the North Atlantic was tested using a global circulation and biogeochemistry model. Basin-scale applications of eMLR on horizontal layers can estimate the change in anthropogenic carbon inventory with an accuracy typically better than 10%. Regression model selection influences the distribution of the recovered anthropogenic carbon change signal. The systematic use of statistically optimum regression formulae does not produce the best estimates of anthropogenic carbon change if the distribution of the station locations emphasizes hydrographic features differently in time. Additional factors, such as a balanced station distribution and vertical continuity of the regression formulae should be considered to guide model selection. Accurate results are obtained when multiple formulae are used throughout the water column. Different formulae can yield results of similar quality. The fact that good results are obtained in the hydrographically complex North Atlantic suggests that eMLR can produce accurate estimates in other basins.
Abalos, Marta, Nicolas Gruber, Robert M Key, S Khatiwala, and X Giraud, October 2012: Changing controls on oceanic radiocarbon: New insights on shallow-to-deep ocean exchange and anthropogenic CO2 uptake. Journal of Geophysical Research: Oceans, 117(C10), DOI:10.1029/2012JC008074. Abstract
The injection of radiocarbon (14C) into the atmosphere by nuclear weapons testing in the 1950s and 1960s has provided a powerful tracer to investigate ocean physical and chemical processes. While the oceanic uptake of bomb-derived 14C was primarily controlled by air-sea exchange in the early decades after the bomb spike, we demonstrate that changes in oceanic 14C are now primarily controlled by shallow-to-deep ocean exchange, i.e., the same mechanism that governs anthropogenic CO2 uptake. This is a result of accumulated bomb 14C uptake that has rapidly decreased the air-sea gradient of 14C/C (Δ14C) and shifted the main reservoir of bomb 14C from the atmosphere to the upper ocean. The air-sea Δ14C gradient, reduced further by fossil fuel dilution, is now weaker than before weapons testing in most regions. Oceanic 14C, and particularly its temporal change, can now be used to study the oceanic uptake of anthropogenic CO2. We examine observed changes in oceanic Δ14C between the WOCE/SAVE (1988-1995) and the CLIVAR (2001-2007) eras and simulations with two ocean general circulation models, the Community Climate System Model (CCSM) and the Estimating the Circulation and Climate of the Ocean Model (ECCO). Observed oceanic Δ14C and its changes between the 1980s-90s and 2000s indicate that shallow-to-deep exchange is too efficient in ECCO and too sluggish in CCSM. These findings suggest that mean global oceanic uptake of anthropogenic CO2 between 1990 and 2007 is bounded by the ECCO-based estimate of 2.3 Pg C yr-1 and the CCSM-based estimate of 1.7 Pg C yr-1.
Downes, S M., and Robert M Key, et al., December 2012: Tracing Southwest Pacific Bottom Water using potential vorticity and Helium-3. Journal of Physical Oceanography, 42(12), DOI:10.1175/JPO-D-12-019.1. Abstract
This study uses potential vorticity and other tracers to identify the pathways of the densest form of Circumpolar Deep Water in the South Pacific, termed "Southwest Pacific Bottom Water" (SPBW), along the 28.2 kg m−3 surface. This study focuses on on the potential vorticity signals associated with three major dynamical processes occurring in the vicinity of the Pacific-Antarctic Ridge: (1) the strong flow of the Antarctic Circumpolar Current (ACC); (2) lateral eddy stirring, and (3) heat and stratification changes in bottom waters induced by hydrothermal vents. These processes result in southward and downstream advection of low potential vorticity along rising isopycnal surfaces. Using δ3He released from the hydrothermal vents, the influence of volcanic activity on the SPBW may be traced across the South Pacific along the path of the ACC to Drake Passage. SPBW also flows within the southern limb of the Ross Gyre, reaching the Antarctic Slope in places, and contributes via entrainment to the formation of Antarctic Bottom Water. Finally, it is shown that the magnitude and location of the potential vorticity signals associated with SPBW have endured over at least the last two decades, and that they are unique to the South Pacific sector.
Manizza, Manfredi, M J Follows, Stephanie Dutkiewicz, D Menemenlis, J W McClelland, C Hill, B J Peterson, and Robert M Key, December 2011: A model of the Arctic Ocean carbon cycle. Journal of Geophysical Research: Oceans, 116, C12020, DOI:10.1029/2011JC006998. Abstract
A three dimensional model of Arctic Ocean circulation and mixing, with a horizontal
resolution of 18 km, is overlain by a biogeochemical model resolving the physical, chemical
and biological transport and transformations of phosphorus, alkalinity, oxygen and carbon,
including the air-sea exchange of dissolved gases and the riverine delivery of dissolved
organic carbon. The model qualitatively captures the observed regional and seasonal trends
in surface ocean PO4, dissolved inorganic carbon, total alkalinity, and pCO2. Integrated
annually, over the basin, the model suggests a net annual uptake of 59 Tg C a-1, within the
range of published estimates based on the extrapolation of local observations (20–199 Tg C a-1). This flux is attributable to the cooling (increasing solubility) of waters moving into
the basin, mainly from the subpolar North Atlantic. The air-sea flux is regulated seasonally
and regionally by sea-ice cover, which modulates both air-sea gas transfer and the
photosynthetic production of organic matter, and by the delivery of riverine dissolved
organic carbon (RDOC), which drive the regional contrasts in pCO2 between Eurasian and
North American coastal waters. Integrated over the basin, the delivery and remineralization
of RDOC reduces the net oceanic CO2 uptake by ~10%.
Bianchi, Daniele, Jorge L Sarmiento, Anand Gnanadesikan, Robert M Key, P Schlosser, and R Newton, August 2010: Low helium flux from the mantle inferred from simulations of oceanic helium isotope data. Earth and Planetary Science Letters, 297(3-4), DOI:10.1016/j.epsl.2010.06.037. Abstract
The high 3He/4He isotopic ratio of oceanic helium relative to the atmosphere has long been recognized as the signature of mantle 3He outgassing from the Earth's interior. The outgassing flux of helium is frequently used to normalize estimates of chemical fluxes of elements from the solid Earth, and provides a strong constraint to models of mantle degassing. Here we use a suite of ocean general circulation models and helium isotope data obtained by the World Ocean Circulation Experiment to constrain the flux of helium from the mantle to the oceans. Our results suggest that the currently accepted flux is overestimated by a factor of 2. We show that a flux of 527 ± 102 mol year− 1 is required for ocean general circulation models that produce distributions of ocean ventilation tracers such as radiocarbon and chlorofluorocarbons that match observations. This new estimate calls for a reevaluation of the degassing fluxes of elements that are currently tied to the helium fluxes, including noble gases and carbon dioxide.
Tanhua, T, Robert M Key, M Hoppema, A Olsen, Masao Ishii, and C L Sabine, November 2010: Expanding carbon data collection from the ocean's interior. EOS, 91(48), 457-458.
Rodgers, Keith B., Robert M Key, Anand Gnanadesikan, Jorge L Sarmiento, John P Dunne, and A R Jacobson, et al., September 2009: Using altimetry to help explain patchy changes in hydrographic carbon measurements. Journal of Geophysical Research, C09013, DOI:10.1029/2008JC005183. Abstract
Here we use observations and ocean models to identify mechanisms driving large seasonal to interannual variations in dissolved inorganic carbon (DIC) and dissolved oxygen (O2) in the upper ocean. We begin with observations linking variations in upper ocean DIC and O2 inventories with changes in the physical state of the ocean. Models are subsequently used to address the extent to which the relationships derived from short-timescale (6 months to 2 years) repeat measurements are representative of variations over larger spatial and temporal scales. The main new result is that convergence and divergence (column stretching) attributed to baroclinic Rossby waves can make a first-order contribution to DIC and O2 variability in the upper ocean. This results in a close correspondence between natural variations in DIC and O2 column inventory variations and sea surface height (SSH) variations over much of the ocean. Oceanic Rossby wave activity is an intrinsic part of the natural variability in the climate system and is elevated even in the absence of significant interannual variability in climate mode indices. The close correspondence between SSH and both DIC and O2 column inventories for many regions suggests that SSH changes (inferred from satellite altimetry) may prove useful in reducing uncertainty in separating natural and anthropogenic DIC signals (using measurements from Climate Variability and Predictability's CO2/Repeat Hydrography program).
Correction: 10.1029/2009JC005835
Submarine groundwater discharge is defined as any flow of water at continental margins from the seabed to the coastal ocean, regardless of fluid composition or driving force1. The flux of submarine groundwater discharge has been hypothesized to be a pathway for enriching coastal waters in nutrients, carbon and metals. Here, we estimate the submarine groundwater flux from the inventory of 228Ra in the upper Atlantic Ocean, obtained by interpolating measurements at over 150 stations. Only 46% of the loss in 228Ra from radioactive decay is replenished by input from dust, rivers and coastal sediments. We infer that the remainder must come from submarine groundwater discharge. Using estimates of 228Ra concentrations in submarine groundwater discharge, we arrive at a total flux from submarine groundwater discharge of 2–4x1013 m3 yr-1, between 80 and 160% of the amount of freshwater entering the Atlantic Ocean from rivers. Submarine groundwater discharge is not a freshwater flux, but a flux of terrestrial and sea water that has penetrated permeable coastal sediments. Our assessment of the volume of submarine groundwater discharge confirms that this flux represents an important vehicle for the delivery of nutrients, carbon and metal to the ocean.
McNeil, B I., N Metzl, Robert M Key, R Matear, and A Corbiere, 2007: An empirical estimate of the Southern Ocean air-sea CO2 flux. Global Biogeochemical Cycles, 21, GB3011, DOI:10.1029/2007GB002991. Abstract
Despite improvements in our understanding of the Southern Ocean air-sea flux
of CO2, discrepancies still exist between a variety of differing
ocean/atmosphere methodologies. Here we employ an independent method to
estimate the Southern Ocean air-sea flux of CO2 that exploits all
available surface ocean measurements for dissolved inorganic carbon (DIC)
and total alkalinity (ALK) beyond 1986. The DIC concentrations were
normalized to the year 1995 using coinciding CFC measurements in order to
account for the anthropogenic CO2 signal. We show that
independent of season, surface-normalized DIC and ALK can be empirically
predicted to within ~8 µmol/kg using standard hydrographic
properties. The predictive equations were used in conjunction with World
Ocean Atlas (2001) climatologies to give a first estimate of the annual
cycle of DIC and ALK in the surface Southern Ocean. These seasonal
distributions will be very useful in both validating biogeochemistry in
general circulation models and for use in situ biological studies within the
Southern Ocean. Using optimal CO2 dissociation constants, we
then estimate an annual cycle of pCO2 and associated net air-sea
CO2 flux. Including the effects of sea ice, we estimate a
Southern Ocean (>50°S) CO2 sink of 0.4 ± 0.25 Pg C/yr. Our
analysis also indicates a substantial CO2 sink of 1.1 ± 0.6 Pg
C/yr within the sub-Antarctic zone (40°S–50°S), associated with strong
cooling and high winds. Our results imply the Southern Ocean CO2
flux south of 50°S to be very similar to those found by Takahashi et al.
(2002), but on the higher end of a range of atmospheric/oceanic CO2
inversion methodologies. This paper estimates for the first time basic
seasonal carbon cycle parameters within the circumpolar Southern Ocean,
which have up to now been extremely difficult to measure and sparse. The
application of such an empirical technique using more widely available
hydrographic parameters in the Southern Ocean provides an important
independent estimate to not only CO2 uptake, but also for other
future biogeochemical studies. Refining and testing these empirical methods
with new carbon measurements will be important to further reduce
uncertainties and extend our understanding of Southern Ocean CO2
dynamics.
Sarmiento, Jorge L., J Simeon, Anand Gnanadesikan, Nicolas Gruber, Robert M Key, and R Schlitzer, March 2007: Deep ocean biogeochemistry of silicic acid and nitrate. Global Biogeochemical Cycles, 21, GB1S90, DOI:10.1029/2006GB002720. Abstract
Observations of silicic acid and nitrate along the lower branch of the global conveyor belt circulation show that silicic acid accumulation by diatom opal dissolution occurs at 6.4 times the rate of nitrate addition by organic matter remineralization. The export of opal and organic matter from the surface ocean occurs at a Si:N mole ratio that is much smaller than this almost everywhere (cf. Sarmiento et al., 2004). The preferential increase of silicic acid over nitrate as the deep circulation progresses from the North Atlantic to the North Pacific is generally interpreted as requiring deep dissolution of opal together with shallow remineralization of organic matter (Broecker, 1991). However, Sarmiento et al. (2004) showed that the primary reason for the low silicic acid concentration of the upper ocean is that the waters feeding the main thermocline from the surface Southern Ocean are depleted in silicic acid relative to nitrate. By implication, the same Southern Ocean processes that deplete the silicic acid in the surface Southern Ocean must also be responsible for the enhanced silicic acid concentration of the deep ocean. We use observations and results from an updated version of the adjoint model of Schlitzer (2000) to confirm that this is indeed the case.
Sweeney, C, M Gloor, A R Jacobson, Robert M Key, Galen McKinley, Jorge L Sarmiento, and R Wanninkhof, 2007: Constraining global air-sea gas exchange for CO2 with recent bomb 14C measurements. Global Biogeochemical Cycles, 21, GB2015, DOI:10.1029/2006GB002784. Abstract
The 14CO2 released
into the stratosphere during bomb testing in the early 1960s provides a
global constraint on air-sea gas exchange of soluble atmospheric gases like
CO2. Using the most complete database of dissolved inorganic
radiocarbon, DI14C, available to date and a suite of ocean
general circulation models in an inverse mode we recalculate the ocean
inventory of bomb-produced DI14C in the global ocean and confirm
that there is a 25% decrease from previous estimates using older DI14C
data sets. Additionally, we find a 33% lower globally averaged gas transfer
velocity for CO2 compared to previous estimates (Wanninkhof,
1992) using the NCEP/NCAR Reanalysis 1 1954–2000 where the global mean winds
are 6.9 m s−1. Unlike some earlier ocean radiocarbon studies, the
implied gas transfer velocity finally closes the gap between small-scale
deliberate tracer studies and global-scale estimates. Additionally, the
total inventory of bomb-produced radiocarbon in the ocean is now in
agreement with global budgets based on radiocarbon measurements made in the
stratosphere and troposphere. Using the implied relationship between wind
speed and gas transfer velocity ks= 0.27
u102
(Sc/660)−0.5
and standard partial pressure difference climatology of CO2 we
obtain an net air-sea flux estimate of 1.3 ± 0.5 PgCyr−1 for
1995. After accounting for the carbon transferred from rivers to the deep
ocean, our estimate of oceanic uptake (1.8 ± 0.5 PgCyr−1)
compares well with estimates based on ocean inventories, ocean transport
inversions using ocean concentration data, and model simulations.
Orr, James C., V J Fabry, Olivier Aumont, Laurent Bopp, Scott C Doney, Richard A Feely, Anand Gnanadesikan, Nicolas Gruber, A Ishida, Fortunat Joos, Robert M Key, Keith Lindsay, E Maier-Reimer, R Matear, Patrick Monfray, A Mouchet, R G Najjar, G-K Plattner, Keith B Rodgers, C L Sabine, Jorge L Sarmiento, R Schlitzer, Richard D Slater, I J Totterdell, M-F Weirig, Y Yamanaka, and Andrew Yool, September 2005: Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms. Nature, 437(7059), DOI:10.1038/nature04095. Abstract
Today's surface ocean is saturated with respect to calcium carbonate, but increasing atmospheric carbon dioxide concentrations are reducing ocean pH and carbonate ion concentrations, and thus the level of calcium carbonate saturation. Experimental evidence suggests that if these trends continue, key marine organisms such as corals and some plankton will have difficulty maintaining their external calcium carbonate skeletons. Here we use 13 models of the ocean-carbon cycle to assess calcium carbonate saturation under the IS92a 'business-as-usual' scenario for future emissions of anthropogenic carbon dioxide. In our projections, Southern Ocean surface waters will begin to become undersaturated with respect to aragonite, a metastable form of calcium carbonate, by the year 2050. By 2100, this undersaturation could extend throughout the entire Southern Ocean and into the subarctic Pacific Ocean. When live pteropods were exposed to our predicted level of undersaturation during a two-day shipboard experiment, their aragonite shells showed notable dissolution. Our findings indicate that conditions detrimental to high-latitude ecosystems could develop within decades, not centuries as suggested previously.
Gnanadesikan, Anand, John P Dunne, Robert M Key, K Matsumoto, Jorge L Sarmiento, Richard D Slater, and P S Swathi, December 2004: Oceanic ventilation and biogeochemical cycling: Understanding the physical mechanisms that produce realistic distributions of tracers and productivity. Global Biogeochemical Cycles, 18(4), GB4010, DOI:10.1029/2003GB002097. Abstract
Differing models of the ocean circulation support different rates of ventilation, which in turn produce different distributions of radiocarbon, oxygen, and export production. We examine these fields within a suite of general circulation models run to examine the sensitivity of the circulation to the parameterization of subgridscale mixing and surface forcing. We find that different models can explain relatively high fractions of the spatial variance in some fields such as radiocarbon, and that newer estimates of the rate of biological cycling are in better agreement with the models than previously published estimates. We consider how different models achieve such agreement and show that they can accomplish this in different ways. For example, models with high vertical diffusion move young surface waters into the Southern Ocean, while models with high winds move more young North Atlantic water into this region. The dependence on parameter values is not simple. Changes in the vertical diffusion coefficient, for example, can produce major changes in advective fluxes. In the coarse-resolution models studied here, lateral diffusion plays a major role in the tracer budget of the deep ocean, a somewhat worrisome fact as it is poorly constrained both observationally and theoretically.
Key, Robert M., A Kozyr, C L Sabine, K Lee, R Wanninkhof, J L Bullister, Richard A Feely, F J Millero, C Mordy, and T-H Peng, 2004: A global ocean carbon climatology: Results from Global Data Analysis Project (GLODAP). Global Biogeochemical Cycles, 18(4), DOI:10.1029/2004GB002247. Abstract
During the 1990s, ocean sampling expeditions were carried out as part of the World Ocean Circulation Experiment (WOCE), the Joint Global Ocean Flux Study (JGOFS),
and the Ocean Atmosphere Carbon Exchange Study (OACES). Subsequently, a group of U.S. scientists synthesized the data into easily usable and readily available products. This collaboration is known as the Global Ocean Data Analysis Project (GLODAP). Results were merged into a common format data set, segregated by ocean. For comparison purposes, each ocean data set includes a small number of high-quality historical cruises. The data were subjected to rigorous quality control procedures to eliminate systematic data measurement biases. The calibrated 1990s data were used to estimate anthropogenic CO2, potential alkalinity, CFC watermass ages, CFC partial pressure, bomb-produced radiocarbon, and natural radiocarbon. These quantities were merged into the measured data files. The data were used to produce objectively gridded property maps at a 1 resolution on 33 depth surfaces chosen to match existing climatologies for temperature, salinity, oxygen, and nutrients. The mapped fields are interpreted as an annual mean distribution in spite of the inaccuracy in that assumption. Both the calibrated data and the gridded products are available from the Carbon Dioxide Information Analysis Center. Here we describe the important details of the data treatment and the mapping procedure, and present summary quantities and integrals for the various parameters.
Matsumoto, K, Jorge L Sarmiento, Robert M Key, Olivier Aumont, J L Bullister, K Caldeira, J-M Campin, Scott C Doney, H Drange, J-C Dutay, M J Follows, Y Gao, Anand Gnanadesikan, Nicolas Gruber, A Ishida, Fortunat Joos, Keith Lindsay, E Maier-Reimer, J Marshall, R Matear, Patrick Monfray, A Mouchet, R G Najjar, G-K Plattner, R Schlitzer, Richard D Slater, P S Swathi, I J Totterdell, M-F Weirig, Y Yamanaka, Andrew Yool, and James C Orr, April 2004: Evaluation of ocean carbon cycle models with data-based metrics. Geophysical Research Letters, 31, L07303, DOI:10.1029/2003GL018970. Abstract
New radiocarbon and chlorofluorocarbon-11 data from the World Ocean Circulation Experiment are used to assess a suite of 19 ocean carbon cycle models. We use the distributions and inventories of these tracers as quantitative metrics of model skill and find that only about a quarter of the suite is consistent with the new data-based metrics. This should serve as a warning bell to the larger community that not all is well with current generation of ocean carbon cycle models. At the same time, this highlights the danger in simply using the available models to represent the state-of-the-art modeling without considering the credibility of each model.
Roussenov, V, R G Williams, M J Follows, and Robert M Key, 2004: Role of bottom water transport and diapycnic mixing in determining the radiocarbon distribution in the Pacific. Journal of Geophysical Research, 109(C6), C06015, DOI:10.1029/2003JC002188. Abstract
The mechanisms controlling the distribution of radiocarbon over the deep Pacific are examined using, firstly, a simplified one-and-a-half-layer model and, secondly, an isopycnic circulation model with parameterized radiocarbon sources and sinks. Two mechanisms control the radiocarbon at depth: relatively fast, lateral transport of bottom waters determining the horizontal distribution and a slower balance between advection-diffusion and radio decay in the vertical. In the isopycnic model, there is a strong topographic control of the bottom water spreading, which is more complex than in the idealized model. Sensitivity studies reveal that altering the bottom intensified diapycnic mixing leads to significant changes in the radiocarbon distribution through both the direct diapycnic transfer and, indirectly, by modifying the bottom water transport and northward penetration of young radiocarbon waters.
Sabine, C L., Richard A Feely, Nicolas Gruber, Robert M Key, K Lee, J L Bullister, R Wanninkhof, C S Wong, D W R Wallace, B Tilbrook, F J Millero, T-H Peng, A Kozyr, T Ono, and A F Rios, 2004: The oceanic sink for anthropogenic CO2. Science, 305(5682), 367-371. Abstract PDF
Using inorganic carbon measurements from an international survey effort in the 1990s and a tracer-based separation technique, we estimate a global oceanic anthropogenic carbon dioxide (CO2) sink for the period from 1800 to 1994 of 118 ± 19 petagrams of carbon. The oceanic sink accounts for ~ 48% of the total fossil-fuel and cement-manufacturing emissions, implying that the terrestrial biosphere was a net source of CO2 to the atmosphere of about 39" 28 petagrams of carbon for this period. The current fraction of total anthropogenic CO2 emissions stored in the ocean appears to be about one-third of the long-term potential.
Chung, S-N, K Lee, Richard A Feely, C L Sabine, F J Millero, R Wanninkhof, J L Bullister, Robert M Key, and T-H Peng, 2003: Calcium carbonate budget in the Atlantic Ocean based on water column inorganic carbon chemistry. Global Biogeochemical Cycles, 17(4), 1093, DOI:10.1029/2002GB002001. Abstract
Recent independent lines of evidence suggest that the dissolution of calcium carbonate (CaCO3) particles is substantial in the upper ocean above the calcite 100% saturation horizon. This shallow-water dissolution of carbonate particles is in contrast with the current paradigm of the conservative nature of pelagic CaCO3 at shallow water depths. Here we use more than 20,000 sets of carbon measurements in conjunction with CFC and 14C data from the WOCE/JGOFS/OACES global CO2 survey to estimate in situ dissolution rates of CaCO3 in the Atlantic Ocean. A dissolution rate is estimated from changes in alkalinity as a parcel of water ages along an isopycnal surface. The in situ CaCO3 dissolution increases rapidly at the aragonite 100% saturation horizon. Estimated dissolution rates north of 40ºN are generally higher than the rates to the south, which is partly attributable to the production of exported CaCO3 being higher in the North Atlantic than in the South Atlantic. As more CaCO3 particles move down the water column, more particles are available for in situ dissolution. The total water column CaCO3 dissolution rate in the Atlantic Ocean is determined on an annual basis by integrating estimated dissolution rates throughout the entire water column and correcting for alkalinity input of approximately 5.6 × 1012 mol C yr-1 from CaCO3-rich sediments. The resulting water column dissolution rate of CaCO3 for the Atlantic Ocean is approximately 11.1 × 1012 mol C yr-1. This corresponds to about 31% of a recent estimate (35.8 × 1012 mol C yr-1) of net CaCO3 production by Lee [2001] for the same area. Our calculation using a large amount of high-quality water column alkalinity data provides the first basin-scale estimate of the CaCO3 budget for the Atlantic Ocean.
Lee, K, S-D Choi, G-H Park, R Wanninkhof, T-H Peng, Robert M Key, C L Sabine, Richard A Feely, J L Bullister, F J Millero, and A Kozyr, December 2003: An updated anthropogenic CO2 inventory in the Atlantic Ocean. Global Biogeochemical Cycles, 17(4), 1116, DOI:10.1029/2003GB002067. Abstract
This paper presents a comprehensive analysis of the basin-wide inventory of anthropogenic CO2 in the Atlantic Ocean based on high-quality inorganic carbon, alkalinity, chlorofluorocarbon, and nutrient data collected during the World Ocean Circulation Experiment (WOCE) Hydrographic Program, the Joint Global Ocean Flux Study (JGOFS), and the Ocean-Atmosphere Carbon Exchange Study (OACES) surveys of the Atlantic Ocean between 1990 and 1998. Anthropogenic CO2 was separated from the large pool of dissolved inorganic carbon using an extended version of the ΔC* method originally developed by Gruber et al. [1996]. The extension of the method includes the use of an optimum multiparameter analysis to determine the relative contributions from various source water types to the sample on an isopycnal surface. Total inventories of anthropogenic CO2 in the Atlantic Ocean are highest in the subtropical regions at 20°–40°, whereas anthropogenic CO2 penetrates the deepest in high-latitude regions (>40°N). The deeper penetration at high northern latitudes is largely due to the formation of deep water that feeds the Deep Western Boundary Current, which transports anthropogenic CO2 into the interior. In contrast, waters south of 50°S in the Southern Ocean contain little anthropogenic CO2 . Analysis of the data collected during the 1990–1998 period yielded a total anthropogenic CO2 inventory of 28.4 ± 4.7 Pg C in the North Atlantic (equator-70°N) and of 18.5 ± 3.9 Pg C in the South Atlantic (equator-70°S). These estimated basin-wide inventories of anthropogenic CO2 are in good agreement with previous estimates obtained by Gruber [1998] , after accounting for the difference in observational periods. Our calculation of the anthropogenic CO2 inventory in the Atlantic Ocean, in conjunction with the inventories calculated previously for the Indian Ocean [ Sabine et al., 1999 ] and for the Pacific Ocean [ Sabine et al., 2002 ], yields a global anthropogenic CO2 inventory of 112 ± 17 Pg C that has accumulated in the world oceans during the industrial era. This global oceanic uptake accounts for approximately 29% of the total CO2 emissions from the burning of fossil fuels, land-use changes, and cement production during the past 250 years.
Matsumoto, K, Anand Gnanadesikan, Nicolas Gruber, Robert M Key, and Jorge L Sarmiento, 2003: Inconsistent model uptake of anthropogenic tracers in the Southern Ocean. Geochimica et Cosmochimica Acta, 67(18), Suppl 1, A278. PDF
McNeil, B I., R Matear, Robert M Key, J L Bullister, and Jorge L Sarmiento, 2003: Anthropogenic CO2 uptake by the ocean based on the Global Chlorofluorocarbon Data Set. Science, 299(5604), 235-239. Abstract PDF
We estimated the oceanic inventory of anthropogenic carbon dioxide (CO2) from 1980 to 1999 using a technique based on the global chorofluorocarbon data set. Our analysis suggests that the ocean stored 14.8 petagrams of anthropogenic carbon from mid-1980 to mid-1989 and 17.9 petagrams of carbon from mid-1990 to mid-1999, indicating an oceanwide net uptake of 1.6 and 2.0 ± 0.4 petagrams of carbon per year, respectively. Our results provide an upper limit on the solubility-driven anthropogenic CO2 flux into the ocean, and they suggest that most ocean general circulation models are overestimating oceanic anthropogenic CO2 uptake over the past two decades.
Toggweiler, J R., and Robert M Key, 2003: Ocean Circulation / Thermohaline Circulation In Encyclopedia of Atmospheric Sciences, Vol. 4, San Diego, CA, Academic Press, 1549-1555.
Watson, A J., James C Orr, Anand Gnanadesikan, Robert M Key, Jorge L Sarmiento, and Richard D Slater, 2003: Carbon dioxide fluxes in the global ocean In Ocean Biogeochemistry: A Synthesis of the Joint Global Ocean Flux Study (JGOFS), Berlin, Germany, Springer-Verlag, 123-143.
Feely, Richard A., C L Sabine, K Lee, and Robert M Key, et al., 2002: In situ calcium carbonate dissolution in the Pacific Ocean. Global Biogeochemical Cycles, 16(4), 1144, DOI:10.1029/2002GB001866. Abstract PDF
Over the past several years researchers have been working to synthesize the WOCE/JGOFS global CO2 survey data to better understand carbon cycling processes in the oceans. The Pacific Ocean data set has over 35,000 sample locations with at least two carbon parameters, oxygen, nutrients, CFC tracers, and hydrographic parameters. In this paper we estimate the in situ CaCO3 dissolution rates in the Pacific Ocean water column. Calcium carbonate dissolution rates ranging from 0.01–1.1 µmol kg-1 yr-1 are observed in intermediate and deepwater beginning near the aragonite saturation horizon. In the North Pacific Intermediate Water between 400 and 800 m, CaCO3 dissolution rates are more than 7 times faster than observed in middle and deep water depths (average = 0.051 µmol kg-1 yr-1). The total amount of CaCO3 that is dissolved within the Pacific is determined by integrating excess alkalinity throughout the water column. The total inventory of CaCO3 added by particle dissolution in the Pacific Ocean, north of 40°S, is 157 Pg C. This amounts to an average dissolution rate of approximately 0.31 Pg C yr-1. This estimate is approximately 74% of the export production of CaCO3 estimated for the Pacific Ocean. These estimates should be considered to be upper limits for in situ carbonate dissolution in the Pacific Ocean, since a portion of the alkalinity increase results from inputs from sediments.
Keller, K, Richard D Slater, M Bender, and Robert M Key, 2002: Possible biological or physical explanations for decadal scale trends in North Pacific nutrient concentrations and oxygen utilization. Deep-Sea Research, Part II, 49(1-3), 345-362. Abstract PDF
We analyze North Pacific GEOSECS (1970s) and WOCE (1990s) observations to examine potential decadal trends of the marine biological carbon pump. Nitrate concentrations ([NO3]) and apparent oxygen utilization (AOU) decreased significantly in intermediate waters (by -0.6 and -2.9 μmol kg-1, respectively, at = 27.4 kg m-3, corresponding to 1050 m). In shallow waters (above roughly 750 m) [NO3] and AOU increased, though the changes were not statistically significant. A sensitivity study with an ocean general circulation model indicates that reasonable perturbations of the biological carbon pump due to changes in export production or remineralization efficiency are insufficient to account for the intermediate water tracer trends. However, changes in water ventilation rates could explain the intermediate water tracer trends and would be consistent with trends of water age derived from radiocarbon. Trends in AOU and [NO3] provide relatively poor constraints on decadal scale trends in the marine biological carbon pump for two reasons. First, most of the expected changes due to decadal scale perturbations of the marine biota occur in shallow waters, where the available data are typically too sparse to account for the strong spatial and temporal variability. Second, alternative explanations for the observed tracer trends (e.g., changes in the water ventilation rates) cannot be firmly rejected. Our data analysis does not disprove the null-hypothesis of an unchanged biological carbon pump in the North Pacific.
Key, Robert M., P D Quay, P Schlosser, A P McNichol, K F von Reden, R J Schneider, K L Elder, M Stuiver, and H Göte Ostlund, 2002: WOCE Radiocarbon IV: Pacific Ocean results. Radiocarbon, 44(1), 239-392. Abstract PDF
The World Ocean Circulation Experiment, carried out between 1990 and 1997, provided the most comprehensive oceanic survey of radiocarbon to date. Approximately 10,000 samples were collected in the Pacific Ocean by U.S. investigators for both conventional large volume ß counting and small volume accelerator mass spectrometry analysis techniques. Results from six cruises are presented. The data quality is as good or better than previous large-scale surveys. The 14C distribution for the entire WOCE Pacific data set is graphically described using mean vertical profiles and sections, and property-property plots.
Lamb, M F., C L Sabine, Richard A Feely, R Wanninkhof, and Robert M Key, et al., 2002: Consistency and synthesis of Pacific Ocean CO2 survey data. Deep-Sea Research, Part II, 49(1-3), 21-58. Abstract PDF
Between 1991 and 1999, carbon measurements were made on twenty-five WOCE/JGOFS/OACES cruises in the Pacific Ocean. Investigators from 15 different laboratories and four countries analyzed at least two of the four measurable ocean carbon parameters (DIC, TAlk, fCO2, and pH) on almost all cruises. The goal of this work is to assess the quality of the Pacific carbon survey data and to make recommendations for generating a unified data set that is consistent between cruises. Several different lines of evidence were used to examine the consistency, including comparison of calibration techniques, results from certified reference material analyses, precision of at-sea replicate analyses, agreement between shipboard analyses and replicate shore based analyses, comparison of deep water values at locations where two or more cruises overlapped or crossed, consistency with other hydrographic parameters, and internal consistency with multiple carbon parameter measurements. With the adjustments proposed here, the data can be combined to generate a Pacific Ocean data set, with over 36,000 unique sample locations analyzed for at least two carbon parameters in most cases. The best data coverage was for DIC, which has an estimated overall accuracy of ~3 :mol kg-1. TAlk, the second most common carbon parameter analyzed, had an estimated overall accuracy of ~5 :mol kg-1. To obtain additional details on this study, including detailed crossover plots and information on the availability of the compiled, adjusted data set, visit the Global Data Analysis Project web site at: http://cdiac.esd.ornl.gov/oceans/glodap.
Millero, F J., D Pierrot, K Lee, R Wanninkhof, Richard A Feely, C L Sabine, Robert M Key, and T Takahashi, 2002: Dissociation constants for carbonic acid determined from field measurements. Deep-Sea Research, Part I, 49(10), 1705-1723. Abstract PDF
A number of workers have recently shown that the thermodynamic constants for the dissociation of carbonic acid in seawater of Mehrbach et al. are more reliable than measurements made on artificial seawater. These studies have largely been confined to looking at the internal consistency of measurements of total alkalinity (TA), total inorganic carbon dioxide (TCO2) and the fugacity of carbon dioxide (fCO2). In this paper, we have examined the field measurements of pH, fCO2, TCO2 and TA on surface and deep waters from the Atlantic, Indian, Southern and Pacific oceans to determine the pK1, pK2 and pK2-pK1. These calculations are possible due to the high precision and accuracy of the field measurements. The values of pK2 and pK2-pK1 over a wide range of temperatures (-1.6–38°C) are in good agreement (within ±0.005) with the results of Mehrbach et al. The measured values of pK1 at 4°C and 20°C are in reasonable agreement (within ±0.01) with all the constants determined in laboratory studies. These results indicate, as suggested by internal consistency tests, that the directly measured values of pK1+pK2 of Mehrbach et al. on real seawater are more reliable than the values determined for artificial seawater. It also indicates that the large differences of pK2-pK1 (0.05 at 20°C) in real and artificial seawater determined by different investigators are mainly due to differences in pK2. These differences may be related to the interactions of boric acid with the carbonate ion.
The values of pK2-pK1 determined from the laboratory measurements of Lee et al. and Lueker et al. at low fCO2 agree with the field-derived data to ±0.016 from 5°C to 25°C. The values of pK2-pK1 decrease as the fCO2 or TCO2 increases. This effect is largely related to changes in the pK2 as a function of fCO2 or TCO2. The values of fCO2 calculated from an input of TA and TCO 2, which require reliable values of pK2-pK1, also vary with fCO2. The field data at 20°C has been used to determine the effect of changes of TCO2 on pK2 giving an empirical relationship:
pK2TCO2=pK2-1.6×10-4(TCO2-2050) which is valid at TCO2>2050 :mol kg-1. This assumes that the other dissociation constants such as KB for boric acid are not affected by changes in TCO2. The slope is in reasonable agreement with the laboratory studies of Lee et al. and Lueker et al. (–1.2x10-4 to –1.9x10-4). This equation eliminates the dependence of the calculated ƒCO2 on the level of ƒCO2 or TCO2 in ocean waters (` =29.7 :atm in ƒCO2). An input of pH and TCO2 yields values of ƒCO2 and TA that are in good agreement with the measured values ("22.3 F atm in ƒCO2 and "4.3 :mol kg-1 in TA). The cause of the decrease in pK2 at high ƒCO2 is presently unknown. The observed inconsistencies between the measured and computed ƒCO2 values may be accounted for by adding the effect of organic acid (~8 :mol kg-1) to the interpretation of the TA. Further studies are needed to elucidate the chemical reactions responsible for this effect.
Rubin, S I., and Robert M Key, 2002: Separating natural and bomb-produced radiocarbon in the ocean: the potential alkalinity method. Global Biogeochemical Cycles, 16(4), DOI:10.1029/2001GB001432. Abstract PDF
the use of radiocarbon as a tracer for oceanic processes generally requires differentiation of naturally occurring radiocarbon from the bomb component produced by atmospheric nuclear weapons testing. We present a new separation method based on the strong linear correlation between radiocarbon and potential alkalinity. Unlike previous techniques the new algorithm is applicable at all latitudes. Additionally, the potential alkalinity method provides an estimate of surface ocean prebomb radiocarbon concentrations. Predictions with the technique appear to be unbiased and have uncertainties which are less than previous techniques.
Sabine, C L., Richard A Feely, Robert M Key, J L Bullister, F J Millero, K Lee, T-H Peng, B Tilbrook, T Ono, and C S Wong, 2002: Distribution of anthropogenic CO2 in the Pacific Ocean. Global Biogeochemical Cycles, 16(4), 1083, DOI:10.1029/2001GB001639. Abstract
This work presents an estimate of anthropogenic CO2 in the Pacific Ocean based on measurements from the WOCE/JGOFS/OACES global CO2 survey. These estimates used a modified version of the C* technique. Modifications include a revised preformed alkalinity term, a correction for denitrification, and an evaluation of the disequilibrium terms using an optimum multiparameter analysis. The total anthropogenic CO2 inventory over an area from 120°E to 70°W and 70°S to 65°N (excluding the South China Sea, the Yellow Sea, the Japan/East Sea, and the Sea of Okhotsk) was 44.5 ± 5 Pg C in 1994. Approximately 28 Pg C was located in the Southern Hemisphere and 16.5 Pg C was located north of the equator. The deepest penetration of anthropogenic CO2 is found at about 50°S. The shallowest penetration is found just north of the equator. Very shallow anthropogenic CO2 penetration is also generally observed in the high-latitude Southern Ocean. One exception to this is found in the far southwestern Pacific where there is evidence of anthropogenic CO2 in the northward moving bottom waters. In the North Pacific a strong zonal gradient is observed in the anthropogenic CO2 penetration depth with the deepest penetration in the western Pacific. The Pacific has the largest total inventory in all of the southern latitudes despite the fact that it generally has the lowest average inventory when normalized to a unit area. The lack of deep and bottom water formation in the North Pacific means that the North Pacific inventories are smaller than the North Atlantic.
Sabine, C L., Robert M Key, Richard A Feely, and D Greeley, 2002: Inorganic carbon in the Indian Ocean: distribution and dissolution processes. Global Biogeochemical Cycles, 16(4), 1067, DOI:10.1029/2002GB001869. Abstract
This study uses nearly 25,000 carbon measurements from the WOCE/JGOFS global CO2 survey to examine the distribution of dissolved inorganic carbon (DIC) and total alkalinity (TA) in the Indian Ocean. Shallow and intermediate distributions of inorganic carbon do not strictly follow temperature and salinity because of differing surface gradients and vertical biological processes that work to modify the circulation derived features. Anthropogenic CO2 has increased the shallow DIC by as much as 3%, decreasing the vertical DIC gradient. Deep ocean DIC and TA increase toward the north because of the decomposition and dissolution of organic and inorganic particles. Calcite saturation depths range from 2900–3900 m with the deepest saturation depth in the central Indian Ocean. Variations of aragonite saturation depth (200–1400 m) are similar to calcite, but the deepest saturations are in the southwestern Indian Ocean. The shallowest aragonite saturation depths are found in the Bay of Bengal. In the northern Arabian Sea and Bay of Bengal, the current aragonite saturations are 100 and 200 m shallower, respectively, than in preindustrial times. Estimates of carbonate dissolution rates on isopycnal surfaces range from 0.017 to 0.083 µmol kg-1 yr-1 in deep waters. Upper water column dissolution rates range from 0 to 0.73 µmol kg-1 yr-1, with a local maximum occurring in intermediate waters just below the aragonite saturation horizon. Dissolution is also generally higher north of the Chemical Front at 10–20°S. There is some evidence for significant sedimentary sources in the northern Indian Ocean.
We use an ocean biogeochemical-transport box model of the top 100 m of the water column to estimate the CaCO3 to organic carbon export ratio from observations of the vertical gradients of potential alkalinity and nitrate. We find a global average molar export ratio of 0.06 ± 0.03. This is substantially smaller than earlier estimates of 0.25 on which a majority of ocean biogeochemical models had based their parameterization of CaCO3 production. Contrary to the pattern of coccolithophore blooms determined from satellite observations, which show high latitude predominance, we find maximum export ratios in the equatorial region and generally smaller ratios in the subtropical and subpolar gyres. Our results suggest a dominant contribution to global calcification by low-latitude nonbloom forming coccolithophores or other organisms such as foraminifera and pteropods.
Deutsch, Curtis A., Nicolas Gruber, Robert M Key, Jorge L Sarmiento, and A Ganachaud, 2001: Denitrification and N2 fixation in the Pacific Ocean. Global Biogeochemical Cycles, 15(2), 483-506. Abstract PDF
We establish the fixed nitrogen budget of the Pacific Ocean based on nutrient fields from the recently completed World Ocean Circulation Experiment (WOCE). The budget includes denitrification in the water column and sediments, nitrogen fixation, atmospheric and riverine inputs, and nitrogen divergence due to the large-scale circulation. A water column denitrification rate of 48 ± 5 Tg N yr-1 is calculated for the Eastern Tropical Pacific using N* [Gruber and Sarmiento, 1997] and water mass age tracers. On the basis of rates in the literature, we estimate sedimentary denitrification to remove an additional 15 ± 3 Tg N yr-1 . We then calculate the total nitrogen divergence due to the large scale circulation through the basin, composed of flows through a zonal transect at 32°S, and through the Indonesian and Bering straits. Adding atmospheric deposition and riverine fluxes results in a net divergence of nitrogen from the basin of -4 ± 12 Tg N yr-1 . Pacific nitrogen fixation can be extracted as a residual component of the total budget, assuming steady state. We find that nitrogen fixation would have to contribute 59 ± 14 Tg N yr-1 in order to balance the Pacific nitrogen budget. This result is consistent with the tentative global extrapolations of Gruber and Sarmiento [1997], based on nitrogen fixation rates estimated for the North Atlantic. Our estimated mean areal fixation rate is within the range of direct and geochemical rate estimates from a single location near Hawaii [Karl et al., 1997]. Pacific nitrogen fixation occurs primarily in the western part of the subtropical gyres where elevated N* signals are found. These regions are also supplied with significant amounts of iron via atmospheric dust deposition, lending qualitative support to the hypothesis that nitrogen fixation is regulated in part by iron supply.
Key, Robert M., 2001: Radiocarbon In Encyclopedia of Ocean Sciences, Academic Press, 2338-2353. PDF
Orr, James C., E Maier-Reimer, U Mikolajewicz, Patrick Monfray, Jorge L Sarmiento, J R Toggweiler, N K Taylor, J Palmer, Nicolas Gruber, C L Sabine, C Le Quéré, Robert M Key, and J Boutin, 2001: Estimates of anthropogenic carbon uptake from four three-dimensional global ocean models. Global Biogeochemical Cycles, 15(1), 43-60. Abstract PDF
We have compared simulations of anthropogenic CO2 in the four three-dimensional ocean models that participated in the first phase of the Ocean Carbon-Cycle Model Intercomparison Project (OCMIP), as a means to identify their major differences. Simulated global uptake agrees to within ± 19%, giving a range of 1.85±0.35 Pg C yr -1 for the 1980-1989 average. Regionally, the Southern Ocean dominates the present-day air-sea flux of anthropogenic CO2 in all models, with one third to one half of the global uptake occurring south of 30°S. The highest simulated total uptake in the Southern Ocean was 70% larger than the lowest. Comparison with recent data-based estimates of anthropogenic CO2 suggest that most of the models substantially overestimate storage in the Southern Ocean; elsewhere they generally underestimate storage by less than 20%. Globally, the OCMIP models appear to bracket the real ocean's present uptake, based on comparison of regional data-based estimates of anthropogenic CO2 and bomb 14C. Column inventories of bomb 14C have become more similar to those for anthropogenic CO2 with the time that has elapsed between the Geochemical Ocean Sections Study (1970s) and World Ocean Circulation Experiment (1990s) global sampling campaigns. Our ability to evaluate simulated anthropogenic CO2 would improve if systematic errors associated with the date-based estimates could be provided regionally.
Gruber, Nicolas, K Keller, and Robert M Key, 2000: What story is told by oceanic tracer concentrations?Science, 290(5491), 455-456.
Ortiz, J D., Alan C Mix, P A Wheeler, and Robert M Key, 2000: Anthropogenic CO2 invasion into the northeast Pacific based on concurrent 13CDIC and nutrient profiles from the California Current. Global Biogeochemical Cycles, 14(3), 917-929. Abstract PDF
The stable isotopic signature of dissolved inorganic carbon ( 13CDIC ) in the northeast Pacific Ocean is lower in near-surface waters by 1.1‰ relative to values predicted from global oceanic trends of 13CDIC versus nutrients. A combination of anthropogenic carbon uptake from the atmosphere and thermodynamic, air-sea gas exchange processes in different water mass source areas account for the isotopic depletion. Here we evaluate the efficacy of using a concurrent nutrient- 13C strategy to separate these two effects, with the goal of improving estimates of anthropogenic carbon uptake over the course of the Industrial Revolution. In depth profiles from the sea surface to 2500 m at four stations across the California Current (42°N), nitrate, rather than phosphate, is best correlated to 13CDIC providing the best choice for this experiment. On the basis of an assumption of no anthropogenic carbon in North Pacific Deep Waters between 1000-2500m depth (potential densities, ~ 27.3-27.7), the "anthropogenic - preanthropogenic" carbon isotope shift 13Ca-p ) in near-surface waters of the northeast Pacific is inferred to be -0.62 ± 0.17‰, while the thermodynamic air-sea gas exchange signature is estimated at -0.48 ± 0.17‰. Values of 13 Ca-p (similar to the regional patterns of 14C and Tritium penetration) approach zero for >26.8, indicating little penetration of anthropogenic carbon into the North Pacific Intermediate Water or the upper North Pacific Deep Water. Our results suggest an upper North Pacific sink of anthropogenic carbon over the past ~200 years that is ~40% greater than that estimated for the interval between ~1970 and ~1990 by Quay et al., [1992]. Our estimate of the North Pacific inventory of anthropogenic carbon, added to published estimates from the North Atlantic and Indian Ocean, is smaller than model predictions of the total carbon sink, suggesting that a significant portion of anthropogenic carbon enters the deep sea via the Southern Ocean.
Sabine, C L., Robert M Key, K M Johnson, F J Millero, A Poisson, Jorge L Sarmiento, D W R Wallace, and C D Winn, 1999: Anthropogenic CO2 inventory of the Indian Ocean. Global Biogeochemical Cycles, 13(1), 179-198. Abstract PDF
This study presents basin-wide anthropogenic CO2 inventory estimates for the Indian Ocean based on measurements from the World Ocean Circulation Experiment/Joint Global Ocean Flux Study global survey. These estimates employed slightly modified d C* and time series techniques originally proposed by Gruber et al. [1996] and Wallace [1995], respectively. Together, the two methods yield the total oceanic anthropogenic CO2 and the carbon increase over the past 2 decades. The highest concentrations and the deepest penetrations of anthropogenic carbon are associated with the Subtropical Convergence at around 30° to 40°S. With both techniques, the lowest anthropogenic CO2 column inventories are observed south of 50°S. The total anthropogenic CO2 inventory north of 35°S was 13.6 ± 2 Pg C in 1995. The inventory increase since GEOSECS (Geochemical Ocean Sections Program) was 4.1 ± 1 Pg C for the same area. Approximately 6.7 ± 1 Pg C are stored in the Indian sector of the Southern Ocean, giving a total Indian Ocean inventory of 20.3 ± 3 Pg C for 1995. These estimates are compared to anthropogenic CO2 inventories estimated by the Princeton ocean biogeochemistry model. The model predicts an Indian Ocean sink north of 35°S that is only 0.61-0.68 times the results presented here; while the Southern Ocean sink is nearly 2.6 times higher than the measurement-based estimate. These results clearly identify areas in the models that need further examination and provide a good baseline for future studies of the anthropogenic inventory.
Broecker, W, S L Peacock, S Walker, R Weiss, M Schroeder, U Mikolajewicz, C Heinze, Robert M Key, T-H Peng, S I Rubin, and E Fahrbach, 1998: How much deep water is formed in the Southern Ocean?Journal of Geophysical Research, 103(C8), 15,833-15,843. Abstract PDF
Three tracers are used to place constraints on the production rate of ventilated deep water in the Southern Ocean. The distribution of the water mass tracer PO4* ("phosphate star") in the deep sea suggests that the amount of ventilated deep water produced in the Southern Ocean is equal to or greater than the outflow of North Atlantic Deep Water from the Atlantic. Radiocarbon distributions yield an export flux of water from the North Atlantic which has averaged about 15 Sv over the last several hundred years. CFC inventories are used as a direct indicator of the current production rate of ventilated deep water in the Southern Ocean. Although coverage is as yet sparse, it appears that the CFC inventory is not inconsistent with the deep water production rate required by the distributions of PO4* and radiocarbon. It has been widely accepted that the major part of the deep water production in the Southern Ocean takes place in the Weddell Sea. However, our estimate of the Southern Ocean ventilated deep water flux is in conflict with previous estimates of the flux of ventilated deep water from the Weddell Sea, which lie in the range 1-5 Sv. Possible reasons for this difference are discussed.
Cochran, J K., and Robert M Key, et al., 1995: Natural and anthropogenic radionuclide distributions in the Nansen Basin, Arctic Ocean: Scavenging rates and circulation timescales. Deep-Sea Research, Part II, 42(6), 1495-1517. Abstract PDF
Determination of the naturally occurring radionuclides 232Th, 230Th, 228Th and 210Pb, and the anthropogenic radionuclides 241Am, 239,240Pu, 134Cs and 137Cs in water samples collected across the Nansen Basin from the Barents Sea slope to the Gakkel Ridge provides tracers with which to characterize both scavenging rates and circulation timescales in this portion of the Arctic Ocean. Large volume water samples ( ~ 1500 l) were filtered in situ to separate particulate (> 0.5 µm) and dissolved Th isotopes and 241Am. Thorium-230 displays increases in both particulate and dissolved activities with depth, with dissolved 230Th greater and particulate 230Th lower in the deep central Nansen Basin than at the Barents Sea slope. Dissolved 228Th activities also are greater relative to 228Ra, in the central basin. Residence times for Th relative to removal from solution onto particles are ~ 1 year in surface water, ~ 10 years in deep water adjacent to the Barents Sea slope, and ~ 20 years in the Eurasian Basin Deep Water. Lead-210 in the central basin deep water also has a residence time of ~ 20 years with respect to its removal from the water column. This texture of scavenging is reflected in distributions of the particle-reactive anthropogenic radionuclide 241Am, which shows higher activities relative to Pu in the central Nansen Basin than at the Barents Sea slope.
Distributions of 137Cs show more rapid mixing at the basin margins (Barents Sea slope in the south, Gakkel Ridge in the north) than in the basin interior. Cesium-137 is mixed throughout the water column adjacent to the Barents Sea slope and is present in low but detectable activities in the Eurasian Basin Deep Water in the central basin. At the time of sampling (1987) the surface water at all stations had been labeled with 134Cs released in the 1986 accident at the Chernobyl nuclear power station. In the ~ 1 year since the introduction of Chernobyl 134Cs to the Nansen Basin, it had been mixed to depths of ~ 800 m at the Barents Sea Slope and to ~ 300 m in the central basin. "Pre-Chernobyl" inventories of 137Cs (as well as 239, 240Pu) are 10 times those expected from global atmospheric fallout from nuclear weapons testing and are derived principally from releases from the Sellafield, U.K., nuclear fuel reprocessing facility on the Irish Sea. Based on the sources of 137Cs to the Nansen Basin, mixing time scales are 9 - 18 years for the upper water column (to 1500 m) and ~ 40 years for the deep water. These mixing time scales, combined with more rapid scavenging at the basin margin relative to the central basin, produce residence times of particle-reactive radionuclides in the Nansen Basin comparable to other open ocean areas (e.g., north-west Atlantic) despite the presence of permanent ice cover and long periods of low-light levels that limit productivity in the Arctic.
Sarmiento, Jorge L., G Thiele, Robert M Key, and W S Moore, 1990: Oxygen and nitrate new production and remineralizaion in the North Atlantic subtropical gyre. Journal of Geophysical Research, 95(C10), 18,303-18,315. Abstract PDF
New estimates are obtained of oxygen utilization rates on isopycnal surfaces in the North Atlantic subtropical gyre thermocline based on tritium inventories (2.4-3.5 mol m-2yr-1) and 228Ra measurements (8.5 ± 0.8 mol m-2yr-1). Arguments are given for why the tritium inventory oxygen utilization rate estimate may be too low. The 228Ra results are combined with recent estimates of oxygen utlization within the thermocline (Jenkins, 1987) as well as estimates of oxygen production in the mixed layer (Spitzer and Jenkins, 1989; Musgrave et al., 1988), to suggest a tentative overall oxygen balance for the whole water column. The new production of oxygen in the surface ocean (~4.6 ± 1.6 mol m-2yr-1) appears to be lower than the estimated utilization within the thermocline (~8.5 ± 0.8 mol m-2yr-1), suggesting that there may be a net lateral import of organic matter into the thermocline equivalent to a new production of ~3.9 ± 1.8 mol m-2yr-1. The nitrogen balance is consistent with these results. An estimate for the total nitrogen remineralization rate in the thermocline is obtained from the oxygen utilization rate by using an -O2:N Redfield ratio of 9.1 ± 0.4 for remineralization (Minster and Boulahdid, 1987), giving a nitrogen remineralization rate of ~0.93 ± 0.10 mol m-2yr-1. Subtracting off the estimated lateral export of nitrate of ~0.51 ± 0.21 mol m-2yr-1, which is presumed to be balanced by a lateral import of dissolved organic nitrogen (Rintoul and Wunsch, 1990), gives a nitrate flux into the surface of ~0.42 ± 0.23 mol m-2yr-1, which is comparable to the estimate of 0.6 ± 0.2 mol m-2yr-1 obtained by Jenkins (1988) near Bermuda as well as the 100-m particulate nitrogen flux of 0.33 mol m-2yr-1 obtained Altabet (1989) near Bermuda.
Moore, W S., Jorge L Sarmiento, and Robert M Key, 1986: Tracing the Amazon component of surface Atlantic water using 228Ra, salinity and silica. Journal of Geophysical Research, 91(C2), 2574-2580. Abstract PDF
High 228Ra/226Ra activity ratios characteristic of waters in the Amazon estuary provide a sensitive indicator of the presence of these waters in the Atlantic Ocean. A conservative mixing model utilizing the 228Ra/226activity ratio (AR) tied to absolute measurements in the estuary allows us to estimate that 20-34% of the surface water east of the Antilles during June, 5-9% from the same area during December, and 15-20% of the eastern Caribbean surface water during December are derived from the Amazon estuary. Differences in 228Ra input occur in response to variable stratification of water near the river mouth. During high discharge, intense vertical mixing enriches the water in the estuary in 228Ra. A large fraction of this water moves to the north and east of Antilles, where its relatively high 228Ra/226 AR distinguishes it over 1500 km from its source. During low discharge (northern hemisphere fall) a significant fraction of river water passes northwest of the zone of intense mixing into a vertically stratified region where 228Ra gain is lower. This water is transported by the Guiana Current along the coast of South America and into the Caribbean.
Key, Robert M., R F Stallard, W S Moore, and Jorge L Sarmiento, 1985: Distribution and flux of 226Ra and 228Ra in the Amazon River estuary. Journal of Geophysical Research, 90(C4), 6995-7004. Abstract PDF
Measurements of 226Ra and 228Ra in the Amazon River estuary show that desorption from river-borne suspended particulate matter in the estuary increases the riverine flux of both isotopes to the ocean by a factor of approximately 5 over the flux attributable to radium dissolved in the river water alone. The total Amazon flux supplies approximately 0.20% of the 226Ra and approximately 2.6% of the 228Ra standing crops in the near-surface Atlantic (0-200 m). Diffusive flux from estuarine and shelf sediments and desorption from resuspended sediments in the region of the estuary approximately double the estuarine 226Ra concentration and quadruple the estuarine 228Ra concentration above that caused by the dissolved and desorbed river components alone.
Moore, W S., Robert M Key, and Jorge L Sarmiento, 1985: Techniques for precise mapping of 226Ra and 228Ra in the Ocean. Journal of Geophysical Research, 90(C4), 6983-6994. Abstract
Improvements in the analyses of 226Ra and 228Ra in seawater made possible by better extraction and processing techniques reduce significantly the errors associated with these measurements. These improvements and the extensive sampling for Ra isotopes conducted on the TTO North Atlantic Study should enable us to use the distribution of 228Ra to study mixing processes on a 3-15 year time scale in both the upper and deep North Atlantic. The 228Ra profiles already analyzed show a closer resemblance to GEOSECS tritium data than to TTO tritium data in the upper ocean. This is because the transient tracer tritium was responding on a 10-year time scale during GEOSECS and a 20-year time scale during TTO. The steady state tracer 228Ra should always respond on a time scale of 8 years. Thus the 228Ra data obtained on TTO should provide a means to extend the features of the GEOSECS tritium field to the regions of the TTO study. The 226Ra data are of high enough quality to identify features associated with different water masses. Changes in the positions of the deep-water masses since the GEOSECS cruise are revealed by the 226Ra data
