The deep ocean releases large amounts of old, pre-industrial carbon dioxide (CO2) to the atmosphere through upwelling in the Southern Ocean, which counters the marine carbon uptake occurring elsewhere. This Southern Ocean CO2 release is relevant to the global climate because its changes could alter atmospheric CO2 levels on long time scales, and also affects the present-day potential of the Southern Ocean to take up anthropogenic CO2. Here, year-round profiling float measurements show that this CO2 release arises from a zonal band of upwelling waters between the Subantarctic Front and wintertime sea-ice edge. This band of high CO2 subsurface water coincides with the outcropping of the 27.8 kg m−3 isoneutral density surface that characterizes Indo-Pacific Deep Water (IPDW). It has a potential partial pressure of CO2 exceeding current atmospheric CO2 levels (∆PCO2) by 175 ± 32 μatm. Ship-based measurements reveal that IPDW exhibits a distinct ∆PCO2 maximum in the ocean, which is set by remineralization of organic carbon and originates from the northern Pacific and Indian Ocean basins. Below this IPDW layer, the carbon content increases downwards, whereas ∆PCO2 decreases. Most of this vertical ∆PCO2 decline results from decreasing temperatures and increasing alkalinity due to an increased fraction of calcium carbonate dissolution. These two factors limit the CO2 outgassing from the high-carbon content deep waters on more southerly surface outcrops. Our results imply that the response of Southern Ocean CO2 fluxes to possible future changes in upwelling are sensitive to the subsurface carbon chemistry set by the vertical remineralization and dissolution profiles.
Chen, Haidi, Adele K Morrison, Carolina O Dufour, and Jorge L Sarmiento, March 2019: Deciphering patterns and drivers of heat and carbon storage in the Southern Ocean. Geophysical Research Letters, 46(6), DOI:10.1029/2018GL080961. Abstract
The storage of anomalous heat and carbon in the Southern Ocean in response to increasing greenhouse gases greatly mitigates atmospheric warming and exerts a large impact on the marine ecosystem. However, the mechanisms driving the ocean storage patterns are uncertain. Here using recent hydrographic observations, we compare for the first time the spatial patterns of heat and carbon storage, which show substantial differences in the Southern Ocean, in contrast with the conventional view of simple passive subduction into the thermocline. Using an eddy-rich global climate model, we demonstrate that redistribution of the preindustrial temperature field is the dominant control on the heat storage pattern, whereas carbon storage largely results from passive transport of anthropogenic carbon uptake at the surface. Lastly, this study highlights the importance of realistic representation of wind and surface buoyancy flux in climate models to improve future projection of circulation change and thus heat and carbon storage.
Chen, Haidi, and Galen McKinley, June 2019: Isopycnal processes allow for summertime heterotrophy despite net oxygen accumulation in the lower‐euphotic zone of the western North Atlantic subtropical gyre. Global Biogeochemical Cycles, 33(6), DOI:10.1029/2018GB006094. Abstract
In the oligotrophic subtropical gyre of the North Atlantic, the processes that allow for an imbalance between annual biological productivity and organic carbon export have been sought for decades. We use biogeochemical data from profiling floats and 26‐years bottle samples off Bermuda to provide the first evidence for a mechanism that allows for heterotrophy in the presence of oxygen accumulation in the lower‐euphotic zone (50‐100m) during the stratified season. After the spring bloom, surface waters that are enriched in oxygen and organic matter, but low in nitrate, are subducted and transported along the seasonal isopycnals that progressively displace downward. Due exclusively to this downward displacement, a positive 50‐100m depth‐integrated O2 anomaly appears (1688±545 mmol O2 m‐2) from mid‐May to mid‐October. Neglecting this effect of isopycnal displacement would suggest an excess of biological productivity over remineralization at 50‐100m (344±330 mmol O2 m‐2). Yet, when these changes are differenced, significant along‐isopycnal oxygen consumption (‐1344±537 mmol O2 m‐2) is identified. After accounting for mixing, net biological‐driven oxygen consumption is still found (‐827±509 mmol O2 m‐2), which indicates heterotrophy. Remineralization of sinking and suspended organic matters at 50‐100m could support 90±67% of the heterotrophic demand. Our analysis also shows that the spread in the biological‐driven oxygen sink is linked to the strength of isopycnal displacement that modulates the supply of nutrients and organic matters. This along‐isopycnal transport and heterotrophy in the lower‐euphotic zone reduces carbon export at 100m, and helps to resolve previously noted imbalances between surface biological productivity and total organic carbon export.
