Priscu, J C., B C Christner, J E Dore, Marian B Westley, B N Popp, K L Casciotti, and W B Lyons, November 2008: Supersaturated N2O in a perennially ice-covered Antarctic lake: Molecular and stable isotopic evidence for a biogeochemical relict. Limnology and Oceanography, 53(6), 2439–2450. Abstract
The east lobe of Lake
Bonney, a permanently ice-covered lake in the McMurdo Dry Valleys,
Antarctica, has a mid-depth maximum N2O concentration of 43.3
µmol N L-1 (>700,000% saturation with respect to air),
representing one of the highest concentrations reported for a natural
aquatic system. 15N
and 18O measurements indicate that this
is the most isotopically depleted N2O yet observed in a natural
environment (minimum 15N-N2O
of -79.6‰ vs. air-N2; minimum 18O-N2O
of -4.7‰ vs. Vienna standard mean ocean water), providing new end points for
these parameters in natural systems. The extremely depleted nitrogen and
oxygen isotopes, together with nitrogen isotopic isomer data for N2O,
imply that most of the N2O was produced via incomplete
nitrification and has undergone virtually no subsequent consumption.
However, molecular evidence provides little support for metabolically active
nitrifying populations at depths where the maximal N2O
concentrations occur and contemporary biogeochemical reactions cannot
explain the extreme excesses of N2O in Lake Bonney. The gas
appears to be a legacy of past biogeochemical conditions within the lake,
and in the absence of a significant sink and the presence of a highly stable
water column, gradients in N2O produced by past microbial
activity could persist in the cold saline waters of Lake Bonney for >104
years.
Yamagishi, H, Marian B Westley, B N Popp, S Toyoda, N Yoshida, S Watanabe, K Koba, and Y Yamanaka, 2007: Role of nitrification and denitrification on the nitrous oxide cycle in the eastern tropical North Pacific and Gulf of California. Journal of Geophysical Research, 112, G02015, DOI:10.1029/2006JG000227. Abstract
Nitrous oxide (N2O) is an important atmospheric greenhouse gas
and is involved in stratospheric ozone depletion. Analysis of the isotopomer
ratios of N2O (i.e., the intramolecular distribution of 15N
within the linear NNO molecule and the conventional N and O isotope ratios)
can elucidate the mechanisms of N2O production and destruction.
We analyzed the isotopomer ratios of dissolved N2O at a site in
the eastern tropical North Pacific (ETNP) and a site in the Gulf of
California (GOC). At these sites, the flux of N2O to the
atmosphere is extremely high but denitrification activity in the oxygen
minimum zone (OMZ) also reduces N2O to N2. We
estimated the isotopomeric enrichment factors for N2O reduction
by denitrification. The factor was -11.6 ± 1.0‰ for the bulk (average) N,
-19.8 ± 2.3‰ for the center N (a-site nitrogen), -3.4 ± 0.3‰ for the
end N (ß-site nitrogen), and -30.5 ± 3.2‰ for the 18O of N2O.
Isotopomer analysis of N2O suggests that nitrifiers should
contribute to N2O production more than denitrifiers at the
oxycline above the OMZs in the ETNP (50–80 m) and in the GOC (80–300 m). In
contrast, denitrifiers should largely contribute to the N2 O
production and consumption in the OMZs both in the ETNP (120–130 m) and in
the GOC (600–800 m). The N2O isotopomer analysis will be a useful
tool for resolving the distribution of water masses that carry a signal of N
loss by denitrification.
Westley, Marian B., H Yamagishi, B N Popp, and N Yoshida, 2006: Nitrous oxide cycling in the Black Sea inferred from stable isotope and isotopomer distributions. Deep-Sea Research, Part II, 53(17-19), DOI:10.1016/j.dsr2.2006.03.012. Abstract
The low-oxygen regions of the world's oceans have been shown to be major sources of nitrous oxide, a trace gas in the atmosphere that contributes to both greenhouse warming and the destruction of stratospheric ozone. Nitrous oxide can be produced as a by-product of nitrification or an intermediate of denitrification; low oxygen conditions enhance the yield of nitrous oxide from both pathways. We measured the concentration and isotopic composition of dissolved nitrous oxide at several stations in the Black Sea, an anoxic basin with a well-defined suboxic layer that separates the ventilated surface waters from the sulfidic deep waters. Our data show that in contrast to other low-oxygen marine regions, nitrous oxide does not accumulate in the Black Sea at significant levels. Moreover, whereas the reduction of nitrous oxide by denitrification usually yields residual gas that is enriched in both stable isotopes, in the Black Sea declining nitrous oxide concentrations are accompanied by enrichment in 18O-N2O but depletion in 15N-N2O. We measured a minimum δ15N-N2O value of −10.8±0.8‰ vs. air N2, by far the lowest measured to date for seawater. Measurements of the distribution of 15N within the linear nitrous oxide molecule reveal that this unusual isotopic signal is most pronounced in the end-position nitrogen, and that site preference, or the tendency for 15N to be found in the center-position nitrogen, co-varies positively with 18O-N2O. We surmise that the highly unusual isotopic composition of Black Sea nitrous oxide is the result of two processes: production of 15N-depleted nitrous oxide by ammonium oxidation followed by its reduction by denitrification, which causes enrichment in 18O and enhancement of 15N-site preference. Bottle incubation experiments with 15N-ammonium and 15N-nitrite reveal that both oxidation and reduction pathways to nitrous oxide are active in the Black Sea suboxic zone.
