Tracing the role of Arctic shelf processes in Si and N cycling and export through the Fram Strait: insights from combined silicon and nitrate isotopes

[摘要] Nutrient cycles in the Arctic Ocean are being altered bychanging hydrography, increasing riverine inputs, glacial melt and sea-iceloss due to climate change. In this study, combined isotopic measurements ofdissolved nitrate ( δ 15 N-NO 3 and δ 18 O-NO 3 ) and silicic acid ( δ 30 Si(OH) 4 ) are usedto understand the pathways that major nutrients follow through the ArcticOcean. Atlantic waters were found to be isotopically lighter ( δ 30 Si(OH) 4 = +  1.74 ‰) than their polarcounterpart ( δ 30 Si(OH) 4 = +  1.85 ‰)owing to partial biological utilisation of dissolved Si (DSi) within theArctic Ocean. Coupled partial benthic denitrification and nitrification onEurasian Arctic shelves lead to the enrichment of δ 15 N-NO 3 and lighter δ 18 O-NO 3 in the polarsurface waters ( δ 15 N-NO 3 =  5.44 ‰, δ 18 O-NO 3 =  1.22 ‰) relative toAtlantic waters ( δ 15 N-NO 3 =  5.18 ‰, δ 18 O-NO 3 =  2.33 ‰). Using apan-Arctic DSi isotope dataset, we find that the input of isotopically light δ 30 Si(OH) 4 by Arctic rivers and the subsequent partialbiological uptake and biogenic Si burial on Eurasian shelves are the keyprocesses that generate the enriched isotopic signatures of DSi exportedthrough Fram Strait. A similar analysis of δ 15 N-NO 3 highlights the role of N-limitation due to denitrification losses on Arcticshelves in generating the excess dissolved silicon exported through FramStrait. We estimate that around 40 % of DSi exported in polar surfacewaters through Fram Strait is of riverine origin. As the Arctic Ocean isbroadly N-limited and riverine sources of DSi are increasing faster thannitrogen inputs, a larger silicic acid export through the Fram Strait isexpected in the future. Arctic riverine inputs therefore have the potentialto modify the North Atlantic DSi budget and are expected to become moreimportant than variable Pacific and glacial DSi sources over the comingdecades.

[学科分类] 大气科学