Unprecedented sea-ice minima enhances algal production deposited at the Arctic seafloor

Abstract

Abstract Sea-ice in the Arctic is declining, with 2018 a particularly low year for ice extent, driven by anomalously warm atmospheric circulation in winter 2017/18. This is consistent with a multi-decadal trend to an earlier ice-free Barents Sea as climate change rapidly warms the Arctic. Here we investigate a N–S transect in the Barents Sea, crossing the Polar Front from Atlantic waters in the south to Arctic waters in the north, focusing on the organic geochemical signature (pigments and lipids) in surface sediments sampled in summer, between the years of 2017–19. Early ice-out in summer 2018 was confirmed by satellite imagery, tracking the evolution of Arctic sea-ice extent between years. Consistent with less extensive sea-ice cover in 2018 we found increases in multiple chlorophyll and carotenoid pigments as well as fatty acids (reflecting recent phytoplankton delivery) in the northern part of our transect at the seafloor. We attribute this to nutrient and organic matter release from earlier 2018 ice-out leading to stratification, post-melt phytoplankton blooms and the deposition of organic matter to the seafloor, evidenced by pigments and lipids. Organic matter delivered to the seafloor in 2018 was reactive and highly labile, confirming its deposition in the most recent season, pointing to rapid deposition. Correlations were found during ice-free periods between satellite-derived chlorophyll a and multiple indicators of water column productivity deposited at the seafloor. We also found convincing evidence of multi-year biogeochemical change across the Polar Front, where sedimentary change is marked by chlorophyll degradation products providing evidence of grazing, indicative of a tightly coupled ecosystem close to the marginal ice zone. Overall, our results show the tight coupling of Arctic productivity with the delivery and quality of organic matter to the seafloor and how this varies across the Barents Sea. More frequent early summer sea-ice loss driven by climate warming in the Barents Sea will have consequences for the delivery of organic matter to the seafloor with impacts for benthic organisms, microbiology and the sequestration of carbon.