Description: A basin-wide transect of nitrate isotopes (δ15NNO3, δ18ONO3), across the UK GEOTRACES 40°S transect in the South Atlantic is presented. This data set is used to investigate Atlantic nutrient cycling and the communication pathways of nitrogen cycling processes in the global ocean. Intermediate waters formed in the subantarctic are enriched in δ15NNO3 and δ18ONO3 from partial utilization of nitrate by phytoplankton and distant denitrification processes, transporting heavy isotope signatures to the subtropical Atlantic. Water mass modification through the Atlantic is investigated by comparing data from 40°S (South Atlantic) and 30°N (North Atlantic). This reveals that nitrate in the upper intermediate waters is regenerated as it transits through the subtropical Atlantic, as evidenced by decreases in δ18ONO3. We document diazotrophy-producing high N:P particle ratios (18–21:1) for remineralization, which is further confirmed by a decrease in δ15NNO3 through the subtropical Atlantic. Thesemodifications influence the isotopic signatures of the North Atlantic Deep Water (NADW) which is subsequently exported from the Atlantic to the Southern Ocean. This study reveals the dominance of recycling processes and diazotrophy on nitrate cycling in the Atlantic. These processes provide a source of low δ15NNO3 to the Southern Ocean via the NADW, to counteract enrichment in δ15NNO3 from water column denitrification in the Indo/Pacific basins. We hence identify the Southern Ocean as a key hub through which denitrification and N2 fixation communicate in the ocean through deepwater masses. Therefore, the balancing of the oceanic N budget and isotopic signatures require time scales of oceanic mixing.

Description: Climate change will enhance the release of organic and inorganic compounds such as carbon (C) and nitrogen (N) via increased permafrost thawing and river runoff in the Arctic. However, the amount of C and N as well as its composition during runoff is largely unknown. Our study aims to characterize and define the source and fate of riverine C and N and its effect on primary productivity in Arctic coastal waters. In order to assess the alteration of the erosion signal from land to sea, we analyzed a fastly degrading permafrost cliff in the Lena River delta (Sobo Sise Island). Furthermore, we collected Lena river water samples from 13 locations (CACOON on Ice Expedition, spring 2019) between the Sobo Sise Cliff and the coast. 28 permafrost samples were analyzed for terrestrial parameters (grain size, total organic carbon, stable isotopes and lipid biomarkers. In order to analyze the thaw impact on Lena river water, we used a hydrochemical approach to determine DON, TDN and respective isotope analysis of the water samples. Describing organic matter fluxes and characteristics will lead to a better understanding of the nutrient load associated with permafrost thaw, and thus warming climate in Arctic environments.

Description: Organic carbon (OC) stored in Arctic permafrost represents one of Earth’s largest and most vulnerable terrestrial carbon pools. Amplified climate warming across the Arctic results in widespread permafrost thaw. Permafrost deposits exposed at river cliffs and coasts are particularly susceptible to thawing processes. Accelerating erosion of terrestrial permafrost along shorelines leads to increased transfer of organic matter (OM) to nearshore waters. However, the amount of terrestrial permafrost carbon and nitrogen as well as the OM quality in these deposits is still poorly quantified. We define the OM quality as the intrinsic potential for further transformation, decomposition and mineralisation. Here, we characterise the sources and the quality of OM supplied to the Lena River at a rapidly eroding permafrost river shoreline cliff in the eastern part of the delta (Sobo-Sise Island). Our multi-proxy approach captures bulk elemental, molecular geochemical and carbon isotopic analyses of Late Pleistocene Yedoma permafrost and Holocene cover deposits, discontinuously spanning the last ~52 kyr. We showed that the ancient permafrost exposed in the Sobo-Sise cliff has a high organic carbon content (mean of about 5 wt %). The oldest sediments stem from Marine Isotope Stage (MIS) 3 interstadial deposits (dated to 52 to 28 cal ka BP) and are overlaid by last glacial MIS 2 (dated to 28 to 15 cal ka BP) and Holocene MIS 1 (dated to 7–0 cal ka BP) deposits. The relatively high average chain length (ACL) index of n-alkanes along the cliff profile indicates a predominant contribution of vascular plants to the OM composition. The elevated ratio of iso and anteiso-branched fatty acids (FAs) relative to mid- and long-chain (C�20) n-FAs in the interstadial MIS 3 and the interglacial MIS 1 deposits suggests stronger microbial activity and consequently higher input of bacterial biomass during these climatically warmer periods. The overall high carbon preference index (CPI) and higher plant fatty acid (HPFA) values as well as high C=N ratios point to a good quality of the preserved OM and thus to a high potential of the OM for decomposition upon thaw. A decrease in HPFA values downwards along the profile probably indicates stronger OM decomposition in the oldest (MIS 3) deposits of the cliff. The characterisation of OM from eroding permafrost leads to a better assessment of the greenhouse gas potential of the OC released into river and nearshore waters in the future.

Description: Anthropogenic global warming is changing the global nitrogen cycle. The Arctic ecosystems, a oligotrophic region, are limited in nitrogen, , which is warming approximately twice as rapidly as the global average and therefore sensitive to changes in nitrogen cycling. Arctic warming intensifies thawing of permafrost soils, which releases their large organic nitrogen reservoir. Released organic nitrogen reaches hydrological systems and is transported by large rivers to the Arctic Ocean. We estimate the load of nitrogen supplied from terrestrial sources into the Arctic Ocean by sampling Lena River water along one of the major deltaic channels in winter and summer in 2019 and at a stationary location in the central delta over a one-year cycle. Additionally, we investigate the potential release of reactive nitrogen including nitrous oxide from soils. We found that the Lena Delta region added approximately 25% of total nitrogen to the river and transported mainly organic nitrogen to the ocean. This has the potential to increase the primary production locally in the river and downstream in the coastal ocean. The higher availability of inorganic nitrogen is the source to enhance N2O emissions from terrestrial and aquatic sources to the atmosphere.