Seasonal cycles of biogeochemical fluxes in the Scotia Sea, Southern Ocean: A stable isotope approach

Abstract. The biological carbon pump is responsible for much of the decadal variability in the ocean carbon dioxide (CO₂) sink, driving the transfer of carbon from the atmosphere to the deep ocean. A mechanistic understanding of the ecological drivers of particulate organic carbon (POC) flux is key to both the assessment of the magnitude of the ocean CO₂ sink, as well as for accurate predictions as to how this will change with changing climate. This is particularly important in the Southern Ocean, a key region for the uptake of CO₂ and the supply of nutrients to the global thermocline. In this study we examine sediment trap derived particle fluxes and stable isotope signatures of carbon (C), nitrogen (N) and biogenic silica (BSi) at a study site in the biologically productive waters of the northern Scotia Sea in the Southern Ocean. Both deep (2000 m) and shallow (400 m) sediment traps exhibited two main peaks in POC, particulate nitrogen and BSi flux, one in austral spring and one in summer, reflecting periods of high surface productivity. Particulate fluxes and isotopic compositions were similar in both deep and shallow sediment traps, highlighting that most remineralisation occurred in the upper 400 m of the water column. Differences in the seasonal cycles of isotopic compositions of C, N and Si provide insights into the degree of coupling of these key nutrients. We measured increasing isotopic enrichment of POC and BSi in spring, consistent with fractionation during biological uptake. Since we observed isotopically light particulate material in the traps in summer, we suggest physically-mediated replenishment of lighter isotopes of key nutrients, enabling full expression of the isotopic fractionation associated with biological uptake. The change in the nutrient and remineralisation regimes, indicated by the different isotopic baselines of the spring and summer productive periods suggests to a change in the source region of material reaching the traps, and associated shifts in phytoplankton community structure. This, combined with the occurrence of advective inputs at certain times of the year, highlights the need to make synchronous measurements of physical processes to be able to better track changes in the source regions of sinking particulate material. We also highlight the need to conduct particle specific (e.g. faecal pellet, phytoplankton detritus, zooplankton moults) isotopic analysis to improve the use of this tool in assessing particle composition of sinking particulate material and develop our understanding of the drivers of biogeochemical fluxes.