Understanding past variability in Antarctic sea ice is of critical importance to determine how it regulates global climate processes, biogeochemistry, and Southern Ocean marine ecosystems. Records of changes in Holocene sea-ice conditions in the Weddell Sea are limited to a few marine sediment cores and inferences from continental ice cores. Here we present a novel record of sea ice and climate from ∼ 6390 to 1830 cal. yr BP based on accumulation rates and multi-proxy geochemical analyses of a snow petrel stomach-oil deposit from the Heimefrontfjella Range, Dronning Maud Land, East Antarctica. Three different sea-ice configurations are interpreted from the record. In the first interval, from 6390 to 5960 cal. yr BP, we see evidence of high productivity and inputs of krill, which suggests foraging both at the continental shelf edge in the marginal ice zone (MIZ) and extending offshore over pelagic waters. We infer that the MIZ lay within the foraging range of Heimefrontfjella. In the second interval, from 5960 to 4320 cal. yr BP, productivity remained high, but there was a reduced influence of krill and likely more fish in the diet. This is consistent with foraging both over the continental shelf edge and offshore, supportive of the summer sea ice retreat reaching the shelf edge more frequently. Finally, in the final interval, between 4320 to 1830 cal. yr BP we infer very low productivity where increased sea-ice conditions restricted access to foraging grounds and open waters, with a less accessible MIZ resulting in a more dense sea-ice pack. Our results highlight how specific Holocene sea-ice configurations can be interpreted from the geochemical composition of snow petrel stomach-oil deposits. We also show, for the first time, the utility of phytol and cholesterol analysis for tracking past avian diet.

Proxy-based reconstructions of past changes in atmospheric carbon dioxide concentrations (pCO(2)) are essential for understanding climate dynamics. A common method for reconstructing past pCO(2) is based on the carbon isotopic fractionation during photosynthesis by Rubisco (epsilon(p)). This proxy method is based upon the difference (epsilon(p)) between the stable carbon isotopic composition (delta C-13) of dissolved CO2 and the delta C-13 of marine photoautotroph biomass, which depends on the concentration of dissolved CO2, related to pCO(2) through Henry's Law. This method has been applied to the general phytoplankton biomarker chlorophyll (preserved as isoprenoids like phytol, phytane, and pristane in the sedimentary record) to reconstruct photoautotroph biomass delta C-13. The long-term stability of these chlorophyll-derived biomarkers in the sedimentary record has currently allowed the reconstruction of pCO(2) across the Phanerozoic (similar to 450 million years). However, the chlorophyll-derived biomarker proxy currently lacks a robust validation within modern settings. Here we investigate the relationship between the delta C-13 of chlorophyll (as phytol) and the concentration of dissolved CO2 in the modern ocean using a globally distributed set of 30 marine core top sediments and 75 suspended particulate matter samples. Our results demonstrate a positive relationship between the extent of fractionation (higher phytol epsilon(p)) and dissolved CO2 concentration. This marks the first empirical calibration between phytol epsilon(p) and the concentration of dissolved CO2 in natural settings. We find that terrestrial input negatively affects this observed relationship, and the exclusion of coastal samples from our dataset improves the correlation. When applied to previously published Pleistocene proxy data, our new calibration provides an improved pCO(2) reconstruction with estimates that are statistically like direct pCO(2) measurements from the Antarctic ice cores. When applied to published data from the entire Phanerozoic, our calibration provides estimates in line with those of other proxy methods, emphasizing the potential of chlorophyll for reconstructions of pCO(2) across geological time.

Sea ice in Antarctica is closely coupled to the climate system, influencing water mass upwelling, albedo and the exchange of heat and gas between the ocean and atmosphere. Sea ice also supports a diverse ecosystem which is sensitive to changes in climate and biogeochemistry. The Heimefrontfjella mountain range in East Antarctica features the nesting sites of the snow petrel (Pagodroma nivea) where finely laminated stomach-oil deposits (regurgitated dietary contents) are deposited. Such deposits can provide valuable information on Holocene dietary changes of the snow petrel that may relate to palaeoclimatic variations. Snow petrel feeding grounds in the Weddell Sea range from neritic (coastal) zones rich in fish, to the productive open ocean where Antarctic krill (Euphausia superba) become increasingly important. Distinct dietary signatures are recorded in the biomarkers of these deposits, providing new evidence of changing sea-ice and climate in the Weddell Sea.Here we focus on stomach-oil deposits from Heimefrontfjella. A highly resolved radiocarbon-dated (14C-AMS) sequence spanning ~6,500 to 2,000 cal. yr BP has been investigated for organic biomarkers (fatty acids, sterols), stable isotopes of carbon and nitrogen (δ13C, δ15N) and inorganic composition by X-ray fluorescence (XRF). From ~6,500 to 6,000 cal. yr BP fatty acid markers were generally high in concentration, with particularly high levels of C14:0 mirrored by high δ15N suggesting food sources rich in Antarctic krill and periods of enhanced feeding in the open ocean. Subsequently between 6,000 and 4,500 cal. yr BP there was a marked reduction in C14:0, C18:0 and δ15N, although phytol concentration remained high. This trophic shift suggests a transitional Weddell Sea still rich in productivity with snow petrels feeding in both the open ocean and close to the shore on a mixture of fish, krill and squid. This is consistent with regional mid-Holocene warmth, and also suggests dynamic variable meteorological and oceanographic conditions during this period. Subsequently, between ~4,500 and 2,000 cal. yr BP organic marker concentrations were markedly lower, suggesting a relatively low productivity period, which we anticipate required more coastal feeding by snow petrels. This change is consistent with evidence from regional reconstructions suggesting movement into neoglacial conditions.Together these findings highlight that the Weddell Sea experienced relatively short-term decadal and centennial-scale changes in sea ice and climate during the Holocene. Our results support existing regional proxies (e.g. offshore sediment records, lake records, ice-core records and palaeo-glacial thinning history) and highlight the importance of snow petrel deposits in recording palaeo-dietary and ecosystem changes in Antarctic marine systems.

Rates of peptide bond hydrolysis and other diagenetic reactions are not favourable for Mesozoic protein survival. Proteins hydrolyse into peptide fragments and free amino acids that, in open systems such as bone, can leach from the specimen and be further degraded. However, closed systems are more likely to retain degradation products derived from endogenous proteins. Amino acid racemisation data in experimental and subfossil material suggests that mollusc shell and avian eggshell calcite crystals can demonstrate closed system behaviour, retaining endogenous amino acids. Here, high-performance liquid chromatography reveals that the intra-crystalline fraction of Late Cretaceous (estimated ~80 Ma) titanosaur sauropod eggshell is enriched in some of the most stable amino acids (Glx, Gly, Ala, and possibly Val) and those that racemise are fully racemic, despite being some of the slowest racemising amino acids. These results are consistent with degradation trends deduced from modern, thermally matured, sub-fossil, and ~3.8 Ma avian eggshell, as well as ~30 Ma calcitic mollusc opercula. Selective preservation of certain fully racemic amino acids, which do not racemise in-chain, along with similar concentrations of free versus total hydrolysable amino acids, likely suggests complete hydrolysis of original peptides. Liquid chromatography-tandem mass spectrometry supports this hypothesis by failing to detect any non-contamination peptide sequences from the Mesozoic eggshell. Pyrolysis-gas chromatography-mass spectrometry reveals pyrolysates consistent with amino acids as well as aliphatic hydrocarbon homologues that are not present in modern eggshell, suggestive of kerogen formation deriving from eggshell lipids. Raman spectroscopy yields bands consistent with various organic molecules, possibly including N-bearing molecules or geopolymers. These closed-system amino acids are possibly the most thoroughly supported non-avian dinosaur endogenous protein-derived constituents, at least those that have not undergone oxidative condensation with other classes of biomolecules. Biocrystal matrices can help preserve mobile organic molecules by trapping them (perhaps with the assistance of resistant organic polymers), but trapped organics are nevertheless prone to diagenetic degradation even if such reactions might be slowed in exceptional circumstances. The evidence for complete hydrolysis and degradation of most amino acids in the eggshell raises concern about the validity of reported polypeptide sequences from open-system non-avian dinosaur bone and other Mesozoic fossils.

ABSTRACT To reconstruct past environmental changes, a range of indirect or proxy approaches can be applied to Quaternary archives. Here, we review the complementary and novel insights that have been provided by the analysis of chemical fossils (biomarkers). Biomarkers have a biological source that can be highly specific (e.g. produced by a small group of organisms) or more general. We show that biomarkers are able to quantify key climate variables (particularly water and air temperature) and can provide qualitative evidence for changes in hydrology, vegetation, human–environment interactions and biogeochemical cycling. In many settings, biomarker proxies provide the opportunity to simultaneously reconstruct multiple climate or environmental variables, alongside complementary and long‐established approaches to palaeoenvironmental reconstruction. Multi‐proxy studies have provided rich sets of data to explore both the drivers and impacts of palaeoenvironmental change. As new biomarker proxies continue to be developed and refined, there is further potential to answer emerging questions for Quaternary science and environmental change.

The protection of organic carbon through association with iron minerals (Fe R ) is an important factor in its stabilisation, long-term storage, and burial efficiency in marine sediments. However, large uncertainties still exist concerning the sources, lability, age, and composition of the organic matter associated with Fe R in natural sediments. Therefore, the timing and environmental setting of the carbon-iron bonding process remain elusive. Here we use radiocarbon (Δ 14 C) and stable isotopes (δ 13 C) of downcore bulk sedimentary organic matter, benthic foraminifera and the organic carbon fraction bound to Fe R to interrogate the source and age of the organic carbon pool associated with Fe R in Arctic marine sediments. In the Barents Sea, we find that the organic carbon associated with Fe R is younger overall than the bulk organic matter and is probably marine derived. The comparison to other investigations of OC-Fe R origins reveals that in large parts of Arctic shelf regions Fe R associated organic carbon is radiocarbon enriched and has a higher δ 13 C org value compared to the bulk sediment, irrespective of sediment depth/age. Our findings suggest a rapid and preferential binding of fresh and marine organic matter with Fe R . Hence, labile organic matter prone to decomposition is protected and stabilised, underlining the potential of the organic carbon–iron association as an efficient carbon burial mechanism.

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.

Unprecedented and dramatic transformations are occurring in the Arctic in response to climate change, but academic, public, and political discourse has disproportionately focussed on the most visible and direct aspects of change, including sea ice melt, permafrost thaw, the fate of charismatic megafauna, and the expansion of fisheries. Such narratives disregard the importance of less visible and indirect processes and, in particular, miss the substantive contribution of the shelf seafloor in regulating nutrients and sequestering carbon. Here, we summarise the biogeochemical functioning of the Arctic shelf seafloor before considering how climate change and regional adjustments to human activities may alter its biogeochemical and ecological dynamics, including ecosystem function, carbon burial, or nutrient recycling. We highlight the importance of the Arctic benthic system in mitigating climatic and anthropogenic change and, with a focus on the Barents Sea, offer some observations and our perspectives on future management and policy.

Eutrophication substantially alters biotic communities and has become a major threat to biodiversity conservation in lake ecosystems. However, little is known about how long-term diversity dynamics respond to nutrient enrichment among lakes in different ecological states. Based on a dataset of diatom records in 210Pb-dated cores collected from eight shallow lakes in the middle reaches of the Yangtze River (central China), this study evaluates temporal changes in species richness and ss-diversity over recent decades, highlighting three distinct trends in diversity dynamics. In heavily-polluted lakes (e.g., Shahu and Sanliqi), severe eutrophication caused the replacement of many resident species by few pollution-tolerant species. However, in transitional lakes (e.g., East Dongting and Luhu), slightly increased nutrients promoted a net species gain at an accelerating rate. While in macrophyte-dominated lakes (e.g., Futou), nutrient enrichment caused species gain to slow down. Slightly increased nutrients probably promoted the spread of some cosmopolitan species, but severe eutrophication caused the extinction of many resident species. Given that the expansion of cosmopolitan species would cause biotic homogenization at a regional scale, species gain in individual lakes cannot be assumed to be beneficial to ecosystem functioning. For conserving local diatom diversity, individual lakes characterized by species gain at an accelerating rate are clear management targets. For conserving regional diatom diversity, controls on both catchment external nutrient inputs and in-lake internal loads are required to promote heterogenous habitats and maintain diatom diversity. Exploring past diversity dynamics is an essential solution to inform and direct the sustainable management of ecologically diverse floodplain lakes.

Modelling, monitoring, and experimental data have shown that global climate change can impact aquatic phytoplankton communities directly, through the effects of warming on primary producers, as well as indirectly through cascading effects from higher trophic levels. Although both concepts are common in modern limnological studies, it remains unclear whether the 'top-down' effects from higher trophic levels on phytoplankton exert strong effects in natural systems over long (centennial) timescales. Here, we use multiproxy data including pigments, zooplankton remains, nutrient concentrations, and paleoclimate indicators from a sediment core in Dagze Co, Central Tibet (a two-trophic level lake) to reconstruct algal production, zooplankton community, nutrient and salinity changes. Our results show that top-down effects of higher trophic levels offset effects from warming and nutrient addition on algal growth. Warming enhanced glacial meltwater inflow to the lake, and intensive human activities increased nutrient inputs. Changes in lake salinity and N:P ratios coincided with zooplankton community shifts during the past 600 years, and Daphnia tibetana replaced the brine shrimp, Artemia tibetiana, after the relocation of a town to upstream of the lake in the 1980s led to overharvesting of the brine shrimp. These shifts contributed strongly to changes in algal communities, with changes in zooplankton leading to strong top-down effects that decreased algal production through increasing grazing pressure despite increasing nutrient concentrations. Our results suggest that the typical external drivers (climate and nutrients) of lake ecosystems may be suppressed by internal shifts in plankton communities in lakes.

Climate variability can induce rapid changes in peatland ecosystems, affecting both carbon cycling and vegetation succession. Diatoms are an important group of ubiquitous and diverse algae in peatlands. Until now, the responses of diatom communities to climate variability have rarely been explored in peatlands, especially in subtropical regions. In this study, monitoring and paleolimnological datasets were combined to decipher environmental changes of a subtropical montane peatland (central China) over the last two centuries. Seasonal monitoring data revealed that diatom communities were closely correlated with precipitation, depth to the water table, conductivity, nitrate and temperature. Sedimentary records revealed that temporal changes in diatom assemblages and geochemical elements displayed similar trends in two peat cores after the 1950s. The first gradient in diatom composition represented a shift from Pinnularia species to taxa preferring less-acidic habitats, which was closely linked to climate warming and the enrichment of inorganic elements (e.g. sodium and calcium) since the early 20th century. Meanwhile, changes in diatom communities were further related to precipitation variability, atmospheric deposition and local hydrogeomophic setting. Taken together, the succession of diatom communities was closely linked to climate-regulated availability of nutrients and moisture in this subtropical peatland over the last two centuries. In order to achieve sustainable management of these scarce peatlands, further biological monitoring and paleoecological studies are needed to improve our knowledge of peatland ecosystem evolution in response to future climate change.

The Arctic is rapidly changing, disrupting biogeochemical cycles and the processing, delivery and sedimentation of carbon (C), in linked terrestrial–aquatic systems. In this investigation, we coupled a hydrogeomorphic assessment of catchment soils, sediments and plants with a recent lake sediment sequence to understand the source and quality of organic carbon present in three Arctic upland lake catchments on Disko Island, located just south of the low–high Arctic transition zone. This varied permafrost landscape has exposed soils with less vegetation cover at higher altitudes, and lakes received varying amounts of glacial meltwater inputs. We provide improved isotope and biomarker source identifications for palaeolimnological studies in high-latitude regions, where terrestrial vegetation is at or close to its northerly and altitudinal range limit. The poorly developed catchment soils lead to lake waters with low dissolved organic carbon (DOC) concentrations (≤1.5 mg L−1). Sedimentary carbon/nitrogen (C/N) ratios, the C isotope composition of organic matter (δ13Corg) and biomarker ratios (n-alkanes, n-alkanols, n-alkanoic acids and sterols) showed that sedimentary organic matter (OM) in these lakes is mostly derived from aquatic sources (algae and macrophytes). We used a 210Pb-dated sediment core to determine how carbon cycling in a lake–catchment system (Disko 2) had changed over recent centuries. Recent warming since the end of the Little Ice Age (LIA∼1860 CE), which accelerated after ca. 1950, led to melt of glacier ice and permafrost, releasing nutrients and DOC to the lake and stimulating pronounced aquatic algal production, as shown by a >10-fold increase in β-carotene, indicative of a major regime shift. We also demonstrate that recent increases in catchment terrestrial vegetation cover contributed to the autochthonous response. Our findings highlight that in Arctic lakes with sparsely developed catchment vegetation and soils, recent Anthropocene warming results in pronounced changes to in-lake C processing and the deposition of more reactive, predominately autochthonous C, when compared with extensively vegetated low-Arctic systems.

Summary Increases in Arctic sea ice extent have been observed over the past decades with warming at the high latitudes especially pronounced, including in the northern Barents Sea. To track variability between years in the timing and extent of ice-out we compared surface sediment pigment, fatty acid and sterol compositions between ice abundant (2017 and 2019) and ice-free (2018) summer sampling seasons across a south to north 30° E gradient. We found total chlorophyll-derived pigments varied predictably across the transect, and were usually markedly higher at station closest to the Polar Front transition. In 2018 which had an unusually low ice-extent and an early ice-out in the northern Barents Sea confirmed by satellite imagery, highest concentrations were present in the northerly station, indicative of increased production from phytoplankton. This trend was mirrored by total fatty acid methyl esters (FAMEs) which in 2018 showed progressive increases from southern to northern stations, with the pattern driven predominantly by short-chain FAMEs which are abundant in phytoplankton. Our findings have consequences for the functioning of the Arctic carbon cycle – if retreat of Arctic sea ice persists and ‘atlantification’ continues, then delivery of phytoplankton bloom-derived carbon to the seafloor could intensify.

Abstract. The Arctic is rapidly changing, disrupting biogeochemical cycles and the processing, delivery and sedimentation of carbon (C), in linked terrestrial-aquatic systems. In this investigation, we coupled a hydrogeomorphic assessment of catchment soils, sediments and plants with a recent lake sediment sequence to understand the source and quality of organic carbon present in three Arctic upland lake catchments on Disko Island, located just south of the Low-High Arctic transition zone. This varied permafrost landscape has exposed soils with less vegetation cover at higher altitudes, and all lakes received varying extent of glacial meltwater inputs. We provide improved isotope and biomarker source identifications for palaeolimnological studies in high latitude regions, where terrestrial vegetation is at or close to its northerly and altitudinal range limit. The poorly developed catchment soils lead to lake waters with low dissolved organic carbon (DOC) concentrations (≤ 1.5 mg L−1). Sedimentary Carbon / Nitrogen (C / N) ratios, the C isotope composition of organic matter (δ13Corg) and biomarker ratios (n-alkanes, n-alkanols, n-alkanoic acids and sterols) showed that sedimentary organic matter (OM) in these lakes is mostly derived from aquatic sources (algae and macrophytes). We used a 210 Pb dated sediment core to determine how carbon cycling in a lake-catchment system (Disko 2) had changed over recent centuries. Recent warming since the end of the Little Ice Age (LIA ~1860 AD), which accelerated after ca. 1950, led to melt of glacier ice and permafrost releasing nutrients and DOC to the lake, stimulating pronounced aquatic algal production, as shown by a > 10 fold increase in β-carotene, indicative of a major regime shift. Our findings highlight that in Arctic lakes with sparsely developed catchment vegetation and soils, recent Anthropocene warming results in pronounced changes to in-lake C processing and the deposition of more reactive, predominately autochthonous C, compared with extensively vegetated low Arctic systems.

Transformation of organic matter in Arctic ocean sediments across the redox interface M.A. STEVENSON1, J.C. FAUST2, L.L. ANDRADE1, F.S. FREITAS3, N.D. GRAY1, K.TAIT4, K.R. HENDRY3, R.G. HILTON5, S.F. HENLEY6, P.LEARY1, S.PAPADAKI3, C.MÄRZ2, G.D. ABBOTT1 1Newcastle University, Newcastle upon Tyne, NE1 7RU, UK (mark.stevenson@newcastle.ac.uk) 2 University of Leeds, Leeds, LS2 9JT, UK 3 University of Bristol, Bristol, BS8 1RJ, UK 4 Plymouth Marine Laboratory, Plymouth, PL1 3DH, UK 5 Durham University, Durham, DH1 3LE, UK 6 University of Edinburgh, Edinburgh EH9 3FE, UK

Process-based, mechanistic investigations of organic matter transformation and diagenesis directly beneath the sediment–water interface (SWI) in Arctic continental shelves are vital as these regions are at greatest risk of future change. This is in part due to disruptions in benthic–pelagic coupling associated with ocean current change and sea ice retreat. Here, we focus on a high-resolution, multi-disciplinary set of measurements that illustrate how microbial processes involved in the degradation of organic matter are directly coupled with inorganic and organic geochemical sediment properties (measured and modelled) as well as the extent/depth of bioturbation. We find direct links between aerobic processes, reactive organic carbon and highest abundances of bacteria and archaea in the uppermost layer (0–4.5 cm depth) followed by dominance of microbes involved in nitrate/nitrite and iron/manganese reduction across the oxic-anoxic redox boundary (approx. 4.5–10.5 cm depth). Sulfate reducers dominate in the deeper (approx. 10.5–33 cm) anoxic sediments which is consistent with the modelled reactive transport framework. Importantly, organic matter reactivity as tracked by organic geochemical parameters ( n -alkanes, n -alkanoic acids, n -alkanols and sterols) changes most dramatically at and directly below the SWI together with sedimentology and biological activity but remained relatively unchanged across deeper changes in sedimentology. This article is part of the theme issue ‘The changing Arctic Ocean: consequences for biological communities, biogeochemical processes and ecosystem functioning’.

Datasets utilised within this study

Urbanization is altering water quality and biotic communities of lake ecosystems worldwide, especially in developing regions. In this study, the diatom community structure and water quality of the epilimnion were analyzed in 38 lakes (40 sampling sites) in Wuhan City (central China) to assess the impacts of urbanization on lake ecosystems. Diatom communities displayed clear variation along the urban-to-rural gradient, with blooms of eutrophic species found in urban lakes in contrast to higher abundances of mesotrophic or oligotrophic taxa in rural lakes. Redundancy analyses revealed that the diatom community recognition to species, genus, and ecological guild level were all significantly correlated with Chl a, Si, and secchi depth, indicating that the changes in diatom communities were mainly explained by differences in nutrient concentrations and light condition. Considering the rapid urbanization and population expansion, it is important to carry out effective measures for the protection of these lake ecosystems, especially by reducing sewage discharge. The high consistency found between diatom community classification and water quality status highlights the applicability of diatoms for bioindication of shallow urban lakes, especially in rapidly developing cities. Even the taxonomic determination at the level of genera or ecological guilds can enable rapid water-quality assessment.

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’.