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

Abstract

Nutrient cycles in the Arctic Ocean are being altered by changing hydrography, increasing riverine inputs, glacial melt and sea-ice loss due to climate change. In this study, combined isotopic measurements of dissolved nitrate (d15N-NO3 and d18O-NO3) and silicic acid (d30Si(OH)4) are used to understand the pathways that major nutrients follow through the Arctic Ocean. Atlantic waters were found to be isotopically lighter (d30Si(OH)4Combining double low line+ 1.74 ‰) than their polar counterpart (d30Si(OH)4Combining double low line+ 1.85 ‰) owing to partial biological utilisation of dissolved Si (DSi) within the Arctic Ocean. Coupled partial benthic denitrification and nitrification on Eurasian Arctic shelves lead to the enrichment of d15N-NO3 and lighter d18O-NO3 in the polar surface waters (d15N-NO3Combining double low line 5.44 ‰, d18O-NO3Combining double low line 1.22 ‰) relative to Atlantic waters (d15N-NO3Combining double low line 5.18 ‰, d18O-NO3Combining double low line 2.33 ‰). Using a pan-Arctic DSi isotope dataset, we find that the input of isotopically light d30Si(OH)4 by Arctic rivers and the subsequent partial biological uptake and biogenic Si burial on Eurasian shelves are the key processes that generate the enriched isotopic signatures of DSi exported through Fram Strait. A similar analysis of d15N-NO3 highlights the role of N-limitation due to denitrification losses on Arctic shelves in generating the excess dissolved silicon exported through Fram Strait. We estimate that around 40 % of DSi exported in polar surface waters through Fram Strait is of riverine origin. As the Arctic Ocean is broadly N-limited and riverine sources of DSi are increasing faster than nitrogen inputs, a larger silicic acid export through the Fram Strait is expected in the future. Arctic riverine inputs therefore have the potential to modify the North Atlantic DSi budget and are expected to become more important than variable Pacific and glacial DSi sources over the coming decades.