Isotopic constraints on the pre-industrial oceanic nitrogen budget

[摘要] The size of the bioavailable (i.e., "fixed") nitrogen inventory in the oceaninfluences global marine productivity and the biological carbon pump.Despite its importance, the pre-industrial rates for the major source andsink terms of the oceanic fixed nitrogen budget, N₂ fixation anddenitrification, respectively, are not well known. These processes leavedistinguishable imprints on the ratio of stable nitrogen isotopes, δ¹⁵N, which can therefore help to infer their patterns and rates. Herewe use δ¹⁵N observations from the water column and a newdatabase of seafloor measurements to constrain rates of N₂ fixation anddenitrification predicted by a global three-dimensional Model of OceanBiogeochemistry and Isotopes (MOBI). Sensitivity experiments were performedto quantify uncertainties associated with the isotope effect ofdenitrification in the water column and sediments. They show that the levelof nitrate utilization in suboxic zones, that is the balance between nitrateconsumption by denitrification and nitrate replenishment by circulation andmixing (dilution effect), significantly affects the isotope effect of watercolumn denitrification and thus global mean δ¹⁵NO₃^–.Experiments with lower levels of nitrate utilization within the suboxic zone(i.e., higher residual water column nitrate concentrations, ranging from20 to 32 μM) require higher ratios of benthic to water columndenitrification, BD : WCD = 0.75–1.4, to satisfy the global meanNO₃^– and δ¹⁵NO₃^– constraints in the modernocean. This suggests that nitrate utilization in suboxic zones plays animportant role in global nitrogen isotope cycling. Increasing the netfractionation factor ϵ_BD for benthic denitrification(ϵ_BD= 0–4‰) requires even higherratios, BD : WCD = 1.4–3.5. The model experiments that bestreproduce observed seafloor δ¹⁵N support the middle to high-endestimates for the net fractionation factor of benthic denitrification(ϵ_BD = 2–4‰). Assuming a balancedfixed nitrogen budget, we estimate that pre-industrial rates of N₂fixation, water column denitrification, and benthic denitrification werebetween 195–350 (225), 65–80 (76), and 130–270 (149) Tg N yr⁻¹,respectively, with our best model estimate (ϵ_BD = 2‰) in parentheses. Although uncertainties still exist,these results suggest that marine N₂ fixation is occurring at muchgreater rates than previously estimated and the residence time for oceanicfixed nitrogen is between ~ 1500 and 3000 yr.

[学科分类] 地球化学与岩石