The rise in atmospheric CO 2 during Heinrich Stadial 1 (HS1; 14.5-17.5 kyr B.P.) may have been driven by the release of carbon from the abyssal ocean. Model simulations suggest that wind-driven upwelling in the Southern Ocean can liberate 13 C-depleted carbon from the abyss, causing atmospheric CO 2 to increase and the δ 13 C of CO 2 to decrease. terminations. New data from 2700 to 3000 m show that the deep SW Atlantic was isotopically distinct from the abyss during HS1. As a result, we find that mid-depth δ 13 C minima were most likely driven by an abrupt drop in δ 13 C of northern component water. Low δ 13 C at the Brazil Margin also coincided with an~80‰ decrease in Δ 14 C. Our results are consistent with a weakening of the Atlantic meridional overturning circulation and point toward a northern hemisphere trigger for the initial rise in atmospheric CO 2 during HS1.

The initial rise in atmospheric CO2 during the last deglaciation was likely driven by input of carbon from a 13C‐depleted reservoir. Here we show that high resolution benthic foraminiferal records from the mid‐depth Brazil Margin display an abrupt drop in δ13C during Heinrich Stadial 1 (HS1) that is similar to but larger than in the atmosphere. Comparing the Brazil Margin results to published records from the North Atlantic, we are unable to account for the South Atlantic δ13C data with conservative mixing between northern and southern component water masses. Rapid input of abyssal water from the Southeast Atlantic could account for deglacial δ13C anomalies at the Brazil Margin but it would require a reversal in deep water flow direction compared to today. A new mid‐depth water mass may explain similar HS1 δ13C values in both the North and South Atlantic, but contrasting oxygen isotopic values between the two basins do not support such a scenario. Instead, it appears that δ13C behaved non‐conservatively during the deglaciation, possibly reflecting the input of carbon from an isotopically depleted source.

The Barents Sea is experiencing long-term climate-driven changes, e.g. modification in oceanographic conditions and extensive sea ice loss, which can lead to large, yet unquantified disruptions to ecosystem functioning. This key region hosts a large fraction of Arctic primary productivity. However, processes governing benthic and pelagic coupling are not mechanistically understood, limiting our ability to predict the impacts of future perturbations. We combine field observations with a reaction-transport model approach to quantify organic matter (OM) processing and disentangle its drivers. Sedimentary OM reactivity patterns show no gradients relative to sea ice extent, being mostly driven by seafloor spatial heterogeneity. Burial of high reactivity, marine-derived OM is evident at sites influenced by Atlantic Water (AW), whereas low reactivity material is linked to terrestrial inputs on the central shelf. Degradation rates are mainly driven by aerobic respiration (40–75%), being greater at sites where highly reactive material is buried. Similarly, ammonium and phosphate fluxes are greater at those sites. The present-day AW-dominated shelf might represent the future scenario for the entire Barents Sea. Our results represent a baseline systematic understanding of seafloor geochemistry, allowing us to anticipate changes that could be imposed on the pan-Arctic in the future if climate-driven perturbations persist. This article is part of the theme issue ‘The changing Arctic Ocean: consequences for biological communities, biogeochemical processes and ecosystem functioning’.

Burial of organic material in marine sediments represents a dominant natural mechanism of long-term carbon sequestration globally, but critical aspects of this carbon sink remain unresolved. Investigation of surface sediments led to the proposition that on average 10-20% of sedimentary organic carbon is stabilised and physically protected against microbial degradation through binding to reactive metal (e.g. iron and manganese) oxides. Here we examine the long-term efficiency of this rusty carbon sink by analysing the chemical composition of sediments and pore waters from four locations in the Barents Sea. Our findings show that the carbon-iron coupling persists below the uppermost, oxygenated sediment layer over thousands of years. We further propose that authigenic coprecipitation is not the dominant factor of the carbon-iron bounding in these Arctic shelf sediments and that a substantial fraction of the organic carbon is already bound to reactive iron prior deposition on the seafloor.