ALBERT

Description: Rapid climate warming in the Arctic region intensifies permafrost thaw, increases active layer depth in summer and enhances riverbank and coastal erosion. This causes the release of organic matter (OM) into streams and rivers and result in discharge of terrestrial OM into the Arctic Ocean. The focus of our study, particulate OM (POM) consists of a complex mixture of compounds from different sources with different chemical/physical resistance towards decomposition and mineralization. Particulate organic carbon (POC) transported by the Lena River represents a quantitatively important carbon pool exported to the Laptev Sea, but its quantity, quality, transport and transformation features are still not fully understood. With this study, we determined the sources (e.g., permafrost, soil, peat, phytoplankton, vegetation, etc.), quality and age of organic carbon transported by Arctic rivers to understand the effects of climate change on the river watersheds as well as on the Arctic coastal nearshore zone. Our study is embedded into the project ‘Changing Arctic Carbon cycle in the cOastal Ocean Nearshore (CACOON)’, which aims to investigate composition, seasonal effect, and changes during the transport and further fate of organic matter discharged by the Lena River to the Laptev sea (see Strauss et al 2021, this abstract book). To assess these data, we have had an intensive fieldwork in the Russian Arctic in summer 2019. Samples were collected across a ~1500 km transect from the Yakutsk through the centre of the Lena Delta to the Nearshore zone, covering the fresh-salt water transition. We analysed water samples from one to three different water depths to capture stratification in the water column. In a next step, the water was filtered at Samoylov Research Station through precombusted GF/F filters (25 mm diameter). Filters with POM were stored frozen in precombusted glass petri dishes. Later, the filters were analysed at AWI Bremerhaven for total suspended matter, total POC concentration, stable (δ13C) and radiocarbon (Δ14C) isotopes. We found significant qualitative and quantitative differences between the OM composition in the Lena River main channel and its delta. Further, we found suspended matter and POC concentrations decreased during the transit from the river to the Arctic Ocean. We demonstrate that deltaic POC is depleted in 13C relative to fluvial POC, and that its 14C signature suggests a modern composition indicating phytoplankton origin. This observation likely reflects the difference in hydrological conditions between the delta and the river main channel, caused by lower flow velocity and average water depth.

Description: Massive Arctic rivers are feeding ≈11% of the global river discharge into the Arctic Ocean, while the ocean stores only ≈1% of the global ocean volume. The ongoing rapid climate warming has led to pronounced changes in precipitation, active layer thickening, increased air and soil temperatures, increased riverbank and coastal erosion rates, extensive permafrost thaw and increasing freshwater discharge to the Arctic Ocean. Since most studies have focused on rivers or oceans itself and mainly during the late summer, near-shore coastal regions are understudied and crucial in determining the amount of carbon transported and/or released into the Arctic Ocean. Here, we investigated river-derived carbon dioxide (CO2) and methane (CH4) emissions from seven repeated transects of the Kolyma River and nearshore (120 km between Cherskiy and Ambarchik) over the entire open water season between June and September 2019. We estimated the cumulative gross delivery of river-derived CH4 and CO2 to the coastal ocean to be around 0.0008 Tg CH4 (800 000 kg) and 0.2 Tg CO2 (200 000 000 kg). Measurements reveal that more than 50% of the cumulative gross delivery is happening during the fresh period, making the season dynamics extremely important.

Description: No other region has warmed as rapidly in the past decades as the Arctic. Funded by the British Natural Environment Research Council (NERC) and the German Federal Ministry of Education and Research (BMBF), the Changing Arctic Carbon cycle in the Coastal Ocean Near-shore (CACOON) project investigates how this warming influences Arctic coastal-marine ecosystems. Arctic rivers annually carry around 13% of the globally transported dissolved organic carbon (despite the Arctic Ocean making up only approx. 1% of the Earth's ocean volume). Arctic shelf waters are therefore dominated by terrestrial organic carbon pools, so that shelf ecosystems are intimately linked to freshwater supplies. Arctic ecosystems also contain permafrost organic carbon that may be released with warming. Climate change already thaws permafrost, reduces sea ice and increases riverine discharge, triggering important feedbacks. The importance of the near-shore region, consisting of several tightly connected ecosystems that include rivers, deltas, and the shelf, is however often overlooked. Year-round studies are scarce but needed to predict the impact of shifting seasonality, fresher water, changing nutrient supply and greater proportions of permafrost-derived organic carbon on coastal waters. The aims of the CACOON project are to quantify the effect of changing freshwater export and permafrost thaw on the type and fate of river-borne organic matter (OM) delivered to the Arctic shore and resulting changes on ecosystem functioning in the coastal Arctic Ocean. We are achieving this through a combined observational, experimental, and modelling approach. We conduct laboratory experiments to parameterise the susceptibility of terrigenous organic carbon to abiotic and biotic transformation and losses, then use the results from these to deliver a marine ecosystem model capable of representing major biogeochemical cycles. We apply this model to assess how future changes to freshwater runoff and carbon fluxes alter the ecosystems. To reach these aims, we conducted 4 field campaigns in 2019 in the Lena (see https://epic.awi.de/id/eprint/53575/) and Kolyma Delta region. In the Lena Delta, during spring we were using a mobile camp on sledges to collect water samples, ice cores, surface sediments, gas samples as well as CTD profiles. A permafrost cliff (Sobo-Sise) was sampled to analyse terrestrial endmembers of organic matter entering the deltaic and eventually marine system following erosion and transport. During the summer campaign we retrieved samples along a 200 km transect from the centre of the Delta to the Laptev Sea covering the fresh-salt water transition. The aim of Kolyma field sampling was to capture the open water season from the ice breakup to re-freezing and sample the Kolyma River and the near shore area. The lab work on these samples is currently ongoing with first papers lead by CACOON or with project contributions being published already (available here: https://www.researchgate.net/project/CACOON-Changing-Arctic-Carbon-cycle-in-the-coastal-ocean-near-shore).

Description: The Laptev sea in the Russian Arctic receives organic rich sediments from both erosion of coastal cliffs and from the Lena river, the worlds 13th largest by discharge, which drains a large permafrost rich area of the Asian continent. These sediments differ both in their physical characteristics and in the bioavailability of their carbon. The comparative distribution and residence time of these two sediment types will therefore have an impact on the shelf biogeochemistry. We use the unstructured grid hydrodynamic model FVCOM to model the Laptev sea around the Lena outflow. Output from this model drives a simple model of sediment tracers, with seperate classes for coastal and riverine input. Existing sediment observations are used to calibrate the model setup. This setup is used to study the spatial distribution and residence times of the two sediment sources both under current conditions and under a future scenario reflecting the expected change in freshwater input and ice cover.

Description: The land-ocean transition in the Arctic is a highly sensitive environment facing severe changes due to increasing temperatures. To assess these changes, the CACOON project conducted intensive fieldwork in the Russian Arctic with four field campaigns in 2019 in the Lena and Kolyma delta region. Lena Delta: In the Lena Delta region CACOON lead a 19 day spring campaign (CACOON Ice). Using a mobile camp on sledges, we were able to collect water samples, ice cores, surface sediments, gas samples as well as CTD profiles. For getting the terrestrial endmember, a Pleistocene permafrost cliff was sampled. During the summer campaign (CACOON Sea) more than 1300 samples were retrieved along a 200 km long transect from the centre of the delta into the Laptev Sea covering the freshsalt water transition. This expedition included sampling for the CAO-EISPAC project and revisited several sampling locations from the spring expedition to investigate seasonality. Kolyma Delta: The aim of field sampling was to capture the open water season (spring, summer and autumn) from the ice breakup in early June to re-freezing in late September. During the two long campaigns with 71 field days, we were able to sample the Kolyma River and the near shore area in seven independent transects. During that period, we collected more than 1200 samples, filtered and preserved for further analysis during the coming months. The collected sample material will arrive in Potsdam in December 2019. For requests, please contact Jens.Strauss@awi.de and paul.mann@northumbria.ac.uk Modelling the comparative influence of riverine input and coastal erosion on sediments in the Laptev Sea.

Description: The Lena River is one of the biggest Arctic rivers. Continuous permafrost covers about 72-80% of the drainage area, which will thaw with ongoing climate warming. Thus, the Lena potentially releases large quantities of permafrost-derived organic matter (OM) to the world’s largest continental shelf: the East Siberian Arctic Shelf. This OM consists of a complex mixture of compounds from different aquatic and terrigenous sources with different chemical/physical resistance towards decomposition and mineralization. Determining the sources (e.g., permafrost, soil, peat, phytoplankton, vegetation, etc.), quality and age of organic carbon (OC) transported by Arctic rivers is important to understand the effects of climate change on the river watersheds as well as on the Arctic coastal zone. Here we show results on composition and age of OM in Lena River. We collected the samples during the summer seasons of 2016, 2017 at the southern limit of the Lena Delta (Stolb Island). Water was sampled from three depths (surface, middle of stream and above the riverbed), two times per season. Stable carbon isotopes (δ13C) and radiocarbon values (Δ14C) were determined for particulate and dissolved organic carbon (POC and DOC, respectively). Our data show significant variations of POC isotopic composition between the studied years and within every season as well, while DOC isotopic values are more stable. POM appears to originate from older sources than DOM, as DO14C values are higher and indicative of more modern sources like recent terrestrial vegetation and aquatic primary production. Thus, isotopic survey illustrates spatial and temporal variability of POM composition and age in Lena Delta.

Description: Arctic rivers deliver ≈11% of global river discharge into the Arctic Ocean, while this ocean represents only ≈1% of the global ocean volume. Ongoing climate warming across the Arctic, and specifically Siberia, has led to regional-scale changes in precipitation patterns, greater rates of permafrost thaw and active layer deepening, as well as enhanced riverbank and coastal erosion. Combined, these climatic and cryospheric perturbations have already resulted in increased freshwater discharge and changes to constituent loads (e.g. dissolved organic carbon - OC) supplied from land to the Arctic Ocean. To date, the majority of studies examining terrestrial organic matter (OM) delivery to the Arctic Ocean have focused almost entirely on freshwater (riverine) or fully-marine environments and been conducted during late summer seasons – often due to logistical constraints. Despite this, an improved understanding of how OC is transformed, mineralised and released during transit through the highly reactive nearshore estuarine environment is critical for examining the fate and influence of terrestrial OM on the Arctic Ocean. Capturing seasonality over the open water period is also necessary to identify current OM fluxes to the ocean vs the atmosphere, and aid in constraining how future changes may modify them. Here we focus upon carbon dioxide (CO2) and methane (CH4) measurements collected during six repeated transects of the Kolyma River and nearshore zone (covering ~120 km) from 2019. Transects spanned almost the entirety of the riverine open water season (June to September). We use these results, in parallel with gas concentrations derived from prior studies, to develop and validate a simple box-model of gas emissions from the nearshore zone. Observations and model‐derived output data reveal that more than 50% of the cumulative gross delivery of CH4 and CO2 to the coastal ocean occurred during the freshet period with dissolved CH4 concentrations in surface water reaching 660 Nanomole per liter [nmol/l]. These results demonstrate the relevance of seasonal dynamics and its spatial variability which are needed in order to estimate greenhouse gas fluxes on an annual basis. More accurate understanding of land-ocean carbon fluxes in the Arctic is therefore crucial to mitigate the effects of climate change and to support the decisions of policy makers.

Description: 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.