科研之友
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摘要
O-2 deficient zones (ODZs) of the world's oceans are important locations for microbial dissimilatory nitrate (NO3-) reduction and subsequent loss of combined nitrogen (N) to biogenic N-2 gas. ODZs are generally coupled to regions of high productivity leading to high rates of N-loss as found in the coastal upwelling region off Peru. Stable N and O isotope ratios can be used as natural tracers of ODZ N-cycling because of distinct kinetic isotope effects associated with microbially mediated N-cycle transformations. Here we present NO3- and nitrite (NO2-) stable isotope data from the nearshore upwelling region off Callao, Peru. Subsurface oxygen was generally depleted below about 30m depth with concentrations less than 10 mu M, while NO2- concentrations were high, ranging from 6 to 10 mu M, and NO3- was in places strongly depleted to near 0 mu M. We observed for the first time a positive linear relationship between NO2- delta N-15 and delta O-18 at our coastal stations, analogous to that of NO3- N and O isotopes during NO3- uptake and dissimilatory reduction. This relationship is likely the result of rapid NO2- turnover due to higher organic matter flux in these coastal upwelling waters. No such relationship was observed at offshore stations where slower turnover of NO2- facilitates dominance of isotope exchange with water. We also evaluate the overall isotope fractionation effect for N-loss in this system using several approaches that vary in their underlying assumptions. While there are differences in apparent fractionation factor (epsilon) for N-loss as calculated from the delta N-15 of NO3-, dissolved inorganic N, or biogenic N-2, values for epsilon are generally much lower than previously reported, reaching as low as 6.5 %. A possible explanation is the influence of sedimentary N-loss at our inshore stations which incurs highly suppressed isotope fractionation.
- 出版日期2016
