Particulate and dissolved organic carbon in the Lena Delta – the Arctic Ocean interface

Rapid Arctic warming accelerates permafrost thaw releasing aged organic matter (OM) to inland aquatic ecosystems and ultimately, after transport via estuaries or deltas, to the Arctic Ocean nearshore. Despite the importance of Arctic deltas, their functioning is still poorly studied. Here, we examined seasonal fluctuations and spatial differences in the quantity and composition of OM in the Lena Delta, measuring dissolved and particulate organic carbon (DOC and POC) concentrations, carbon isotopes (δ13C and Δ14C), and total suspended matter (TSM). We compared deltaic POC to the POC in the Lena River main stem over a ~1600 km transect, from Yakutsk to the Lena Delta. We further examined and compared dynamics of DOC and POC in summer and winter across a ~140 km transect in the Lena Delta. TSM and POC concentrations decreased by 75 % during transit from Yakutsk to the Lena Delta. 18 % of deltaic and 5 % of river main stem POC originated from Yedoma deposits. Thus, despite lower concentrations of POC in the delta, amount of POC from Yedoma deposits in deltaic waters were almost twice as large as in the main stem (0.07 ±0.02 and 0.04 ±0.02 mg L-1, respectively). Deltaic POC was strongly depleted in 13C due to significant phytoplankton contributions (~-68 ±6 %). Strong differences between winter and summer samples in DOC and POC concentrations and their properties in the Lena Delta were also found. Combined analyses of DOC and POC revealed that Pleistocene-aged Yedoma deposits were still actively degrading in winter influencing the quantity and composition of OM of the Lena Delta and exported OC loads. Deltaic processes control the type and amount of OM exported to the Arctic Ocean and require deeper investigations as crucial processes for the riverine and oceans pathways in a warming Arctic

Particulate organic matter in the Lena River and its Delta: From the permafrost catchment to the Arctic Ocean

Rapid Arctic warming accelerates permafrost thaw, causing an additional release of terrestrial organic matter (OM) into rivers, and ultimately, after transport via deltas and estuaries, to the Arctic Ocean nearshore. The majority of our understanding of nearshore OM dynamics and fate has been developed from freshwater rivers, despite the likely impact of highly dynamic estuarine and deltaic environments on transformation, storage, and age of OM delivered to coastal waters. Here, we studied OM dynamics within the Lena River main stem and Lena Delta along an approximately ∼1600 km long transect from Yakutsk, downstream to the delta disembogue into the Laptev Sea. We measured particulate organic carbon (POC), total suspended matter (TSM), and carbon isotopes (δ13C and ∆14C) in POC to compare riverine and deltaic OM composition and changes in OM source and fate during transport offshore. We found that TSM and POC concentrations decreased by 55 and 70 %, respectively, during transit from the main stem to the delta and Arctic Ocean. We found deltaic POC to be strongly depleted in 13C relative to fluvial POC, indicating a significant phytoplankton contribution to deltaic POC (∼68 ±6 %). Dual-carbon (∆14C and δ13C) isotope mixing model analyses suggested an additional input of permafrost-derived OM into deltaic waters (∼18 ±4 % of deltaic POC originates from Pleistocene deposits vs ∼ 5 ±4 % in the river main stem). Despite the lower concentration of POC in the delta than in the main stem (0.41 ±0.10 vs. 0.79 ±0.30 mg L-1, respectively ), the amount of POC derived from Pleistocene deposits in deltaic waters was almost twice as large as POC of Yedoma origin in the main stem (0.07 ±0.02 and 0.04 ±0.02 mg L-1, respectively). We assert that estuarine and deltaic processes require consideration in order to correctly understand OM dynamics throughout Arctic nearshore coastal zones and how these processes may evolve under future climate-driven change

Circumpolar dataset of Soil Organic Carbon north of treeline derived from ENVISAT ASAR GM, link to GeoTIFF

A new approach for the estimation of soil organic carbon (SOC) pools north of the tree line has been developed based on synthetic aperture radar (SAR; ENVISAT Advanced SAR Global Monitoring mode) data. SOC values are directly determined from backscatter values instead of upscaling using land cover or soil classes. The multi-mode capability of SAR allows application across scales. It can be shown that measurements in C band under frozen conditions represent vegetation and surface structure properties which relate to soil properties, specifically SOC. It is estimated that at least 29 Pg C is stored in the upper 30 cm of soils north of the tree line. This is approximately 25 % less than stocks derived from the soil-map-based Northern Circumpolar Soil Carbon Database (NCSCD). The total stored carbon is underestimated since the established empirical relationship is not valid for peatlands or strongly cryoturbated soils. The approach does, however, provide the first spatially consistent account of soil organic carbon across the Arctic. Furthermore, it could be shown that values obtained from 1 km resolution SAR correspond to accounts based on a high spatial resolution (2 m) land cover map over a study area of about 7 × 7 km in NE Siberia. The approach can be also potentially transferred to medium-resolution C-band SAR data such as ENVISAT ASAR Wide Swath with ~120 m resolution but it is in general limited to regions without woody vegetation. Global Monitoring-mode-derived SOC increases with unfrozen period length. This indicates the importance of this parameter for modelling of the spatial distribution of soil organic carbon storage

Enhanced river runoff and permafrost thaw affect Arctic shelf processes

Enhanced river runoff and coastal erosion are causing greater amounts of terrestrial material supply to Arctic shelf waters. Increasing freshwater export of carbon and nutrient loads from land (terr-OM) together with compositional shifts - due to changing hydrologic flow paths and permafrost thaw, can modify shelf water chemistry and biogeochemical processes. Here, we examine how shifts in land-ocean terr-OM supply may alter shelf primary productivity, respiration and ultimately net regional CO2 air–sea fluxes. Unique insights into terr-OM dynamics and composition during transit through riverine, deltaic and shelf waters were collected through multiple field campaigns on the Lena River and Laptev Sea shelf region. Harnessing these field data, we examine the effects of contemporary and future terr-OM supply to shelf waters using newly developed 1-D and 3-D regional biogeochemical models specifically capable of parameterising terr-OM, composition and degradation. In agreement with prior studies, we find that land-derived nutrients could strengthen coastal production sustaining up to ~50% of primary productivity under current terr-OM conditions. However, we also found that additional terr-OM supply caused increased light limitation in coastal waters, offsetting nutrient fertilization effects and stimulating zooplankton grazing. Model experiments indicate that future increases in terr-OM of between 25-50% and/ or shifts to more biologically reactive coastal OM -such as to be expected with permafrost thaw, will reduce net CO2 uptake and lead to positive CO2 feedback from Arctic shelf waters. Our results question the capacity of the coastal Arctic Ocean to serve as a net sink for atmospheric CO2 with future increasing land-ocean connectivity and terr-OM supply

Storage and transformation of organic matter fractions in cryoturbated permafrost soils across the Siberian Arctic

In permafrost soils, the temperature regime and the resulting cryogenic processes are important determinants of the storage of organic carbon (OC) and its small-scale spatial variability. For cryoturbated soils, there is a lack of research assessing pedon-scale heterogeneity in OC stocks and the transformation of functionally different organic matter (OM) fractions, such as particulate and mineral-associated OM. Therefore, pedons of 28 Turbels were sampled in 5 m wide soil trenches across the Siberian Arctic to calculate OC and total nitrogen (TN) stocks based on digital profile mapping. Density fractionation of soil samples was performed to distinguish between particulate OM (light fraction, LF, 1.6 g cm−3), and a mobilizable dissolved pool (mobilizable fraction, MoF). Across all investigated soil profiles, the total OC storage was 20.2 ± 8.0 kg m−2 (mean ± SD) to 100 cm soil depth. Fifty-four percent of this OC was located in the horizons of the active layer (annual summer thawing layer), showing evidence of cryoturbation, and another 35 % was present in the upper permafrost. The HF-OC dominated the overall OC stocks (55 %), followed by LF-OC (19 % in mineral and 13 % in organic horizons). During fractionation, approximately 13 % of the OC was released as MoF, which likely represents a readily bioavailable OM pool. Cryogenic activity in combination with cold and wet conditions was the principle mechanism through which large OC stocks were sequestered in the subsoil (16.4 ± 8.1 kg m−2; all mineral B, C, and permafrost horizons). Approximately 22 % of the subsoil OC stock can be attributed to LF material subducted by cryoturbation, whereas migration of soluble OM along freezing gradients appeared to be the principle source of the dominant HF (63 %) in the subsoil. Despite the unfavourable abiotic conditions, low C / N ratios and high δ13C values indicated substantial microbial OM transformation in the subsoil, but this was not reflected in altered LF and HF pool sizes. Partial least-squares regression analyses suggest that OC accumulates in the HF fraction due to co-precipitation with multivalent cations (Al, Fe) and association with poorly crystalline iron oxides and clay minerals. Our data show that, across all permafrost pedons, the mineral-associated OM represents the dominant OM fraction, suggesting that the HF-OC is the OM pool in permafrost soils on which changing soil conditions will have the largest impact.Russian Ministry of Education and Science/14.B25.31.0031German Federal Ministry of Education and Research/03F0616AEvangelisches Studienwerk VilligstDF

Organic matter characteristics of a rapidly eroding permafrost cliff in NE Siberia (Lena Delta, Laptev Sea region)

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

High-resolution bathymetry models for the Lena Delta and Kolyma Gulf coastal zones

Arctic river deltas and deltaic near-shore zones represent important land-ocean transition zones influencing sediment dynamics and nutrient fluxes from permafrost-affected terrestrial ecosystems into the coastal Arctic Ocean. To accurately model fluvial carbon and freshwater export from rapidly changing river catchments, as well assessing impacts of future change on the Arctic shelf and coastal ecosystems, we need to understand the sea floor characteristics and topographic variety of the coastal zones. To date, digital bathymetrical data from the poorly accessible, shallow and large areas of the eastern Siberian Arctic shelves are sparse. We have digitized bathymetrical information for nearly 75,000 locations from large-scale (1:25,000 – 1:500,000) current and historical nautical maps of the Lena Delta and the Kolyma Gulf region in Northeast Siberia. We present the first detailed and seamless digital models of coastal zone bathymetry for both delta/gulf regions in 50 m and 200 m spatial resolution. We validated the resulting bathymetry layers using a combination of our own water depth measurements and a collection of available depth measurements, which showed a strong correlation (r > 0.9). Our bathymetrical models will serve as an input for a high-resolution coupled hydrodynamic-ecosystem model to better quantify fluvial and coastal carbon fluxes to the Arctic Ocean but may be useful for a range of other studies related to Arctic delta and near-shore dynamics such as modelling of submarine permafrost, near-shore sea ice, or shelf sediment transport

Seasonal nitrogen fluxes of the Lena River Delta

The Arctic is nutrient limited, particularly by nitrogen, and is impacted by anthropogenic global warming which occurs approximately twice as fast compared to the global average. Arctic warming intensifies thawing of permafrost-affected soils releasing their large organic nitrogen reservoir. This organic nitrogen reaches hydrological systems, is remineralized to reactive inorganic nitrogen, and is transported to the Arctic Ocean via large rivers. We estimate the load of nitrogen supplied from terrestrial sources into the Arctic Ocean by sampling in the Lena River and its Delta. We took water samples along one of the major deltaic channels in winter and summer in 2019 and sampling station in the central delta over a one-year cycle. Additionally, we investigate the potential release of reactive nitrogen, including nitrous oxide from soils in the Delta. We found that the Lena transported nitrogen as dissolved organic nitrogen to the coastal Arctic Ocean and that eroded soils are sources of reactive inorganic nitrogen such as ammonium and nitrate. The Lena and the Deltaic region apparently are considerable sources of nitrogen to nearshore coastal zone. The potential higher availability of inorganic nitrogen might be a source to enhance nitrous oxide emissions from terrestrial and aquatic sources to the atmosphere

High-resolution bathymetry models for the Lena Delta and Kolyma Gulf coastal zones

Arctic river deltas and deltaic near-shore zones represent important land–ocean transition zones influencing sediment dynamics and nutrient fluxes from permafrost-affected terrestrial ecosystems into the coastal Arctic Ocean. To accurately model fluvial carbon and freshwater export from rapidly changing river catchments as well as assess impacts of future change on the Arctic shelf and coastal ecosystems, we need to understand the sea floor characteristics and topographic variety of the coastal zones. To date, digital bathymetrical data from the poorly accessible, shallow, and large areas of the eastern Siberian Arctic shelves are sparse. We have digitized bathymetrical information for nearly 75 000 locations from large-scale (1:25 000–1:500 000) current and historical nautical maps of the Lena Delta and the Kolyma Gulf region in northeastern Siberia. We present the first detailed and seamless digital models of coastal zone bathymetry for both delta and gulf regions in 50 and 200 m spatial resolution. We validated the resulting bathymetry layers using a combination of our own water depth measurements and a collection of available depth measurements, which showed a strong correlation (r>0.9). Our bathymetrical models will serve as an input for a high-resolution coupled hydrodynamic–ecosystem model to better quantify fluvial and coastal carbon fluxes to the Arctic Ocean, but they may be useful for a range of other studies related to Arctic delta and near-shore dynamics such as modeling of submarine permafrost, near-shore sea ice, or shelf sediment transport. The new digital high-resolution bathymetry products are available on the PANGAEA data set repository for the Lena Delta (https://doi.org/10.1594/PANGAEA.934045; Fuchs et al., 2021a) and Kolyma Gulf region (https://doi.org/10.1594/PANGAEA.934049; Fuchs et al., 2021b), respectively. Likewise, the depth validation data are available on PANGAEA as well (https://doi.org/10.1594/PANGAEA.933187; Fuchs et al., 2021c)