Abstract. There is generally a lack of knowledge on how marine organic carbon accumulation is linked to vertical export and primary productivity patterns in the Arctic Ocean. Despite the fact that annual primary production in the Arctic has increased as a consequence of shrinking sea ice, its effect on flux, preservation, and accumulation of organic carbon is still not well understood. In this study, a multi-proxy geochemical and organic-sedimentological approach is coupled with organic facies modelling, focusing on regional calculations of carbon cycling and carbon burial on the western Barents Shelf between northern Scandinavia and Svalbard. OF-Mod 3-D, an organic facies modelling software tool, is used to reconstruct and quantify the marine and terrestrial organic carbon fractions and to make inferences about marine primary productivity changes across the marginal ice zone (MIZ). By calibrating the model against an extensive set of sediment surface samples, we improve the Holocene organic carbon budget for ice-free and seasonally ice-covered areas in the western Barents Sea. The results show that higher organic carbon accumulation rates in the MIZ are best explained by enhanced surface water productivity compared to ice-free regions, implying that shrinking sea ice may reveal a significant effect on the overall organic carbon storage capacity of the western Barents Sea shelf.

The production of high-salinity brines during 9 sea-ice freezing in circum-arctic coastal polynyas is 10 thought to be part of northern deep water formation as it 11 supplies additional dense waters to the Atlantic meridional 12 overturning circulation system. To better predict the effect 13 of possible future summer ice-free conditions in the Arctic 14 Ocean on global climate, it is important to improve our 15 understanding of how climate change has affected sea-ice 16 and brine formation, and thus finally dense water formation 17 during the past. Here, we show temporal coherence 18 between sea-ice conditions in a key Arctic polynya (Stor-19 fjorden, Svalbard) and patterns of deep water convection in 20 the neighbouring Nordic Seas over the last 6500 years. A 21 period of frequent sea-ice melting and freezing between 6.5 22 and 2.8 ka BP coincided with enhanced deep water 23 renewal, while near-permanent sea-ice cover and low brine 24 rejection after 2.8 ka BP likely reduced the overflow of 25 high-salinity shelf waters, concomitant with a gradual slow 26 down of deep water convection in the Nordic Seas, which 27 occurred along with a regional expansion in sea-ice and 28 surface water freshening. The Storfjorden polynya sea-ice 29 factory restarted at *0.5 ka BP, coincident with renewed 30 deep water penetration to the Arctic and climate amelioration over Svalbard. The identified synergy between Arctic polynya sea-ice conditions and deep water convection during the present interglacial is an indication of the potential consequences for ocean ventilation during states with permanent sea-ice cover or future Arctic icefree conditions.

In this study we focus on late Holocene primary productivity (PP) variability in the western Barents Sea and its response to variable sea ice coverage by combining PP reconstructed from several sediment cores with regional PP trends simulated with a well-constrained organic facies model, OF-Mod 3D. We find that modern production rates reconstructed from buried marine organic matter (''bottomup'') resemble simulated export production at 50 m water depth inferred from numerical simulations of surface water PP in a 3D ocean model, SINMOD (''top-down''). Paleoproductivity rates in the northern Barents Sea are more variable and generally higher (30-150 gC m -2 year -1 ) than in the SW Barents Sea region (\75 gC m -2 year -1 ) throughout the last 6000 years BP. In the SW Barents Sea, PP rates and terrestrial organic matter (TOM) supply remain constantly low indicating present-day-like oceanographic conditions with only marginal influence of sea ice related processes during the last 6000 years BP. PP rates in the northern Barents Sea indicate a shift from stable modern-like conditions prior to 2800 BP to denser, more permanent sea ice coverage along the marginal ice zone (MIZ) between 2800 and 1000 years BP and low PP rates. PP rates increase around 1000 years BP indicating a northward shift of the MIZ and accelerated export towards the seabed. During the last 500 years a pronounced decline in PP rates towards the present day indicates reduced annual duration of the MIZ in the area due to global warming. Our results suggest that a combination of first-year ice and higher PP in a warming pan-Arctic may point to a potential Arctic carbon sink while sea ice is still present.