Degrading permafrost river catchments and their impact on Arctic Ocean nearshore processes

Terrestrial inputs of nutrients and dissolved organic carbon to the Arctic Ocean and their influence on primary production

The Arctic region, warming at approximately four times the global average, is experiencing rapid climatic shifts that could result in a summer ice-free Arctic Ocean by mid-century. This review compiles recent studies on Arctic biogeochemistry, highlighting the significant role of continental runoff-including rivers, permafrost, and glaciers-in nutrient cycling, carbon dynamics, and pelagic primary production. Particularly in the East Siberian Shelf, terrestrial inputs substantially contribute to the export of dissolved organic carbon (DOC) and nutrients, thus impacting regional ecosystems and primary productivity. Subsea permafrost emerges as a key DOC exporter, with estimated fluxes reaching 700 to 1000 Tg yr-1 under extreme scenarios. In the Arctic's low-light environment, photodegradation plays a vital role in transforming terrestrial dissolved organic matter (DOM) into nutrients. Notably, phytoplankton levels in the Arctic Ocean have surged by about 30 % since the 1990s. Projections indicate that by this century's end, the Arctic Net Primary Productivity (NPP) could approach 700 Tg C yr-1, with a more significant increase in the Eurasian Arctic than in the American and Barents Sea regions. This trend is mainly due to terrestrial inputs and permafrost thawing effects. Research in the Arctic, particularly on biogeochemistry and phytoplankton dynamics in response to climate change, faces challenges from extreme weather, data scarcity, and complex environmental processes. Therefore, continuous monitoring and targeted research, especially in the East Siberian Shelf and subsea permafrost regions, are crucial for overcoming these challenges and improving our understanding of the changing Arctic Ocean ecosystem.

Modelling terrigenous DOC across the north west European Shelf: Fate of riverine input and impact on air-sea CO2 fluxes

Terrigenous carbon in aquatic systems is increasingly recognised as an important part of the global carbon cycle. Despite this, the fate and distribution of terrigenous dissolved organic carbon (tDOC) in coastal and oceanic systems is poorly understood. We have implemented a theoretical framework for the degradation of tDOC across the land to ocean continuum in a 3D hydrodynamical-biogeochemical model on the North West European Shelf. A key feature of this model is that both photochemical and bacterial tDOC degradation rates are age dependant constituting an advance in our ability to describe carbon cycling in the marine environment. Over the time period 1986-2015, 182±17 Gmol yr-1 of riverine tDOC is input to the shelf. Results indicate that bacterial degradation is by far the most important process in removing tDOC on the shelf, contributing to 73±6 % (132±11 Gmol yr-1) of the total removal flux, while 21±3 % (39±6 Gmol yr-1) of riverine tDOC was advected away from the shelf and photochemical degradation removing 5±0.5 % of the riverine flux. Explicitly including tDOC in the model decreased the air-sea carbon dioxide (CO2) flux by 112±8 Gmol yr-1 (4±0.4 %), an amount approximately equivalent to the CO2 released by the UK chemical industry in 2020. The reduction is equivalent to 62 % of the riverine tDOC input to the shelf while approximately 17 % of riverine input is incorporated into the foodweb. This work can improve the assumptions of the fate of tDOC by Earth System Models and demonstrates that the inclusion of tDOC in models can impact ecosystem dynamics and change predicted global carbon budgets for the ocean.

Spatiotemporal variability of organic carbon in streams and rivers of the Northern Hemisphere cryosphere

The earth's cryosphere is the outpost of climatic warming, which leads to rapid changes of organic carbon (OC) transport from terrestrial to aquatic ecosystems. OC in the cryosphere rivers plays vital roles in the carbon cycle and river ecosystem health. Yet, we still lack a comprehensive assessment of the spatiotemporal patterns of riverine OC across the Northern Hemisphere cryosphere. Here, we compiled OC concentration, radiocarbon (14C), and the specific ultraviolet absorbance (SUVA) of dissolved OC (DOC) at 254 nm data from 1007 unique sites, extracted from 138 published literature between 1972 and 2022. Overall, the average DOC and particulate OC (POC) concentrations are 6.34 and 2.61 mg C L-1, respectively, with the average age of DOC and POC being ~1100 and ~4300 years BP, respectively, indicating the release of aged carbon pools. Seasonal variations in DOC and POC concentrations, Δ14C-DOC and SUVA254 were observed, with distinct spatial variations closely linked to specific watershed characteristics. We found permafrost-impacted watersheds displayed significantly higher DOC concentrations, younger OC ages but lower POC concentrations compared to glacier-impacted watersheds. Meanwhile, in boreal forest watersheds, DOC is the most concentrated and youngest in varied ecoregions. Additionally, in permafrost regions characterized by higher permafrost extent, ground ice content, or lowlands with thick overburden cover, riverine DOC is more concentrated and aromatic. We estimated that specific OC fluxes in glacier rivers are higher than that in permafrost rivers (4.77 and 1.86 g C m-2 yr-1, respectively). Our results highlight the complex and variable spatiotemporal patterns of riverine OC in the northern cryosphere, which are essential for assessing the impact of OC on the global carbon cycle and climate warming.

Arctic soil CO2 release during freeze-thaw cycles modulated by silicon and calcium

Arctic soils are the largest pool of soil organic carbon worldwide. Temperatures in the Arctic have risen faster than the global average during the last decades, decreasing annual freezing days and increasing the number of freeze-thaw cycles (temperature oscillations passing through zero degrees) per year as the temperature is expected to fluctuate more around 0 °C. At the same time, proceeding deepening of seasonal thaw may increase silicon (Si) and calcium (Ca) concentrations in the active layer of Arctic soils as the concentrations in the thawing permafrost layer might be higher depending on location. We analyzed the importance of freeze-thaw cycles for Arctic soil CO₂ fluxes. Furthermore, we tested how Si (mobilizing organic C) and Ca (immobilizing organic C) interfere with the soil CO₂ fluxes in the context of freeze-thaw cycles. Our results show that with each freeze-thaw cycle the CO₂ fluxes from the Arctic soils decreased. Our data revealed a considerable CO₂ emission below 0 °C. We also show that pronounced differences emerge in Arctic soil CO₂ fluxes with Si increasing and Ca decreasing CO₂ fluxes. Furthermore, we show that both Si and Ca concentrations in Arctic soils are central controls on Arctic soil CO₂ release, with Si increasing Arctic soil CO₂ release especially when temperatures are just below 0 °C. Our findings could provide an important constraint on soil CO₂ emissions upon soil thaw, as well as on the greenhouse gas budget of high latitudes. Thus we call for work improving understanding of freeze-thaw cycles as well as the effect of Ca and Si on carbon fluxes, as well as for increased consideration of those factors in wide-scale assessments of carbon fluxes in the high latitudes.