Dissolved organic matter properties in arctic coastal waters are strongly influenced by fluxes from permafrost coasts and by local meteorology.

Under future climate change scenarios, Arctic coastal waters are believed to receive higher terrestrial organic matter (OM) fluxes. Permafrost carbon is increasingly mobilized upon thaw from rivers draining permafrost terrain and from eroding permafrost coasts. Once received, the coastal waters are the transformation zone for terrestrial OM, although quantities, especially those of dissolved organic matter (DOM) released by coastal erosion, are largely unknown. This nearshore zone plays a crucial role in Arctic biogeochemical cycling, as here the released material is destined to be (1) mineralized into greenhouse gases, (2) incorporated into marine primary production, (3) buried in nearshore sediments or (4) transported offshore. In this presentation, we show data on DOM quantities in surface water in the nearshore zone of the southern Beaufort Sea from two consecutive summer seasons under different meteorological conditions. Colored dissolved organic matter (cDOM) properties help to differentiate the terrestrial from the marine DOM component. Figure 1 shows DOC concentrations and salinities for 23 and 24 days in the summer seasons of 2013 and 2014, respectively. DOC concentrations in the nearshore zone of the southern Beaufort Sea vary between about 1.5 and 5 mg C L-1. In the Lena River Delta, bay water, river water, and permafrost meltwater creeks yielded similar values between 5.8 and 5.9 mg C L-1 (Dubinenkov et al., 2015). Similarly, Fritz et al. (2015) found DOC concentrations in ice wedges between 1.6 and 28.6 mg C L-1. In 2013, the first half of July was characterized by low salinity between 8 and 15 psu and high DOC concentrations of 3.5 to 5 mg C L-1. Then, a sudden change in water properties occurred after a major storm which lasted for at least 2 days. This storm led to strongly decreased DOC (1.5 to 2.5 mg C L-1) concentration and increasing salinity (14 to 28 psu) in surface water, probably due to upwelling In 2014, a more stable situation in both salinity and DOC prevailed, with relatively high salinity (23 to 29 psu) and low DOC concentration (1.5 to 2.5 mg C L-1). This pattern was due to rather windy and wavy conditions throughout the whole season. The water column in 2014 was likely well-mixed and DOC-poor because saline waters have probably been transported from the offshore to the nearshore. We recognized a significant negative correlation between DOC and salinity, independent from varying meteorological conditions. In general, this suggests a conservative mixing between DOC derived from terrestrial/permafrost runoff and marine DOC. The low salinity in July 2013 was probably due to prolonged sea-ice presence in the sampled area. This leads to the assumption that DOC also originates from melting sea ice. Quantitatively more important will be terrestrial runoff which is relatively rich in DOC. A stable stratification in the nearshore zone and calm weather conditions will increase the influence of terrestrial-derived DOM. The strength of the terrestrial influence can be estimated by salinity measures as they directly correlate with DOC concentrations; the lower the salinity the stronger the terrestrial influence. We conclude that the terrestrial imprint of coastal erosion on DOM concentrations in the nearshore zone is significant. We see that DOC concentrations are significantly elevated also compared to riverine input in front of river mouths and deltas. Meteorological conditions play a major role for the strength of the terrestrial DOM signal, which can vary on short timescales. Our approach is different from ship-based oceanography because we study DOM that is directly derived from thawing permafrost coasts, explicitly excluding rivers. When qualifying DOM origin from permafrost landscapes apart from rivers we have to take into consideration the different DOM mobilization pathways. 1) Surface runoff and near-surface groundwater flow mainly drain and flush the active layer. 2) Melting ground ice releases DOM. 3) Ground ice meltwater leaches DOM from sedimentary OM upon permafrost thaw on land. 4) DOM is leached from sedimentary OM upon contact with sea water. The latter three will mobilize old OM which is believed to be highly bioavailable (see Vonk et al., 2013a, b). References: Dubinenkov, I., Flerus, R., Schmitt-Kopplin, P., Kattner, G., Koch, B.P., 2015. Origin-specific molecular signatures of dissolved organic matter in the Lena Delta. Biogeochemistry 123, 1-14. Fritz, M., Opel, T., Tanski, G., Herzschuh, U., Meyer, H., Eulenburg, A., Lantuit, H., 2015. Dissolved organic carbon (DOC) in Arctic ground ice. The Cryosphere 9, 737-752. Vonk, J.E., Mann, P.J., Davydov, S., Davydova, A., Spencer, R.G.M., Schade, J., Sobczak, W.V., Zimov, N., Zimov, S., Bulygina, E., Eglinton, T.I., Holmes, R.M., 2013a. High biolability of ancient permafrost carbon upon thaw. Geophysical Research Letters 40, 2689-2693. Vonk, J.E., Mann, P.J., Dowdy, K.L., Davydova, A., Davydov, S.P., Zimov, N., Spencer, R.G.M., Bulygina, E.B., Eglinton, T.I., Holmes, R.M., 2013b. Dissolved organic carbon loss from Yedoma permafrost amplified by ice wedge thaw. Environmental Research Letters 8, 035023

Impetuous CO2 release from eroding permafrost coasts

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Impacts of stronger winds and less sea ice on Canadian Beaufort Sea shelf ecosystems since the late 1990s

Continuous and multi-decadal records of faunal abundance and diversity helping to identify the impacts of ongoing global warming on aquatic ecosystems are rare in the coastal Arctic. Here, we used a 50-year-long microfaunal record from a sediment core collected in the Herschel Basin (YC18-HB-GC01; 18 m water depth) to document some aspects of the environmental responses of the southern Canadian coastal Beaufort Sea to climate change. The microfaunal indicators include benthic foraminiferal assemblages, ostracods and tintinnids. The carbonate shells of two foraminiferal species were also analyzed for their stable isotope signatures (δ13C and δ18O). We compiled environmental parameters from 1970 to 2019 for the coastal region, including sea ice data (break-up date, freeze-up date, open season length and mean summer concentration), the wind regime (mean speed, direction of strong winds and the number of storms), hydrological data (freshet date, freshet discharge and mean summer discharge of the Firth and the Mackenzie rivers), and air temperature. Large-scale atmospheric patterns were also taken into consideration. Time-constrained hierarchal clustering analysis of foraminiferal assemblages and environmental parameters revealed a near-synchronous shift around the late 1990s. The microfaunal shift corresponds to an increased abundance of taxa tolerant to variable salinity, turbulent bottom water conditions, and turbid waters towards the present. The same time interval is marked by stronger easterly winds, more frequent storms, reduced sea-ice cover, and a pervasive anticyclonic circulation in the Arctic Ocean (positive Arctic Ocean Oscillation; AOO+). Deeper vertical mixing in the water column in response to intensified winds was fostered by increased open surface waters in summer leading to turbulence, increased particle loading and less saline bottom waters at the study site. Stronger easterly winds probably also resulted in enhanced resuspension events and coastal erosion in addition to a westward spreading of the Mackenzie River plume, altogether contributing to high particulate-matter transport. Increase food availability since ∼2000 was probably linked to enhanced degradation of terrestrial organic carbon, which also implies higher oxygen consumption. The sensitivity of microfaunal communities to environmental variations allowed capturing consequences of climate change on a marine Arctic shelf ecosystem over the last 50 years

Landscape-related ground ice variability on the Yukon coastal plain inferred from computed tomography and remote sensing

Warming in the Arctic causes strong environmental changes with degradation of permafrost (permanently frozen ground). Active layer deepening (gradual thaw) and permafrost erosion (abrupt thaw) results in the mobilization and lateral transport of organic carbon, altering current carbon cycling in the Arctic. Ground ice content is a crucial factor limiting our understanding and ability to determine the rates and dynamics of permafrost thaw and its impact on potential thaw subsidence rates, changes in lateral hydrological pathways and its driving mechanisms on a landscape scale. In this study we investigate ground ice content and its characteristics across the most dominant landscape units of the Yukon coastal plain (Canadian Arctic), using two spatially and technically contrasting approaches. In our bottom-up approach, twelve permafrost cores were collected from moraine, lacustrine, fluvial and glaciofluvial deposits using a SIPRE corer (mean drilling depth of 2 m) in spring of 2019. Ground ice and sediment contents within polygon centers were analyzed and classified using computed tomography and image recognition software (k-means). Our top-down approach quantified ice-wedge volumes from remote sensing imagery tracing the circumference of polygon troughs over the same area. Preliminary results - extrapolated to the entire coastal plain - show that the ground-ice content in polygon centers vary significantly from massive ice in the polygon troughs (wedge-ice). Total ice volume was estimated around 80.2 vol.-%, of which 68.2 ± 18.1 vol.-% was attributed to ground ice in polygon centers, and 12 ± 3.1 vol.-% of the landscape is massive ice in wedge-ice along polygon troughs. Additionally, differences among and between landscape units are also substantial, with highest ice volume contents in moraines landscapes, where polygon centers contain 58.8 vol.-% ground ice and wedge-ice volume is 16.2 vol.-%), while the lowest ice contents are found in glacio-fluvial deposits (22.1 vol.-% resp. 9.1 vol.-%). Our results reveal a higher average and a larger variability in ground ice contents than previously found, suggesting a need of both ground-based measurements and remote sensing imagery to further our understanding of the future landscape subsidence, but also to avoid a likely under- or overestimation associated with the chosen approach. We conclude that due to the high ground ice contents on the Yukon coastal plain, substantial changes of the permafrost landscape will occur under current warming trends. These will include subsidence, abrupt erosion, changes in hydrology and organic carbon mobilization, degradation and export processes, which will differ between landscape units

Rapid CO2 release from eroding permafrost in seawater

Permafrost is thawing extensively due to climate warming. When permafrost thaws, previously frozen organic carbon (OC) is converted into carbon dioxide (CO2) or methane, leading to further warming. This process is included in models as gradual deepening of the seasonal non‐frozen layer. Yet, models neglect abrupt OC mobilization along rapidly eroding Arctic coastlines. We mimicked erosion in an experiment by incubating permafrost with seawater for an average Arctic open‐water season. We found that CO2 production from permafrost OC is as efficient in seawater as without. For each gram (dry weight) of eroding permafrost, up to 4.3 ± 1.0 mg CO2 will be released and 6.2 ± 1.2% of initial OC mineralized at 4 °C. Our results indicate that potentially large amounts of CO2 are produced along eroding permafrost coastlines, onshore and within nearshore waters. We conclude that coastal erosion could play an important role in carbon cycling and the climate system

Impacts of collapsing permafrost coasts: the fate of carbon, nutrients and sediments in the Arctic nearshore zone

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Transformation of terrestrial organic matter along thermokarst-affected permafrost coasts in the Arctic

The changing climate in the Arctic has a profound impact on permafrost coasts, which are subject to intensified thermokarst formation and erosion. Consequently, terrestrial organic matter (OM) is mobilized and transported into the nearshore zone. Yet, little is known about the fate of mobilized OM before and after entering the ocean. In this study we investigated a retrogressive thaw slump (RTS) on Qikiqtaruk - Herschel Island (Yukon coast, Canada). The RTS was classified into an undisturbed, a disturbed (thermokarst-affected) and a nearshore zone and sampled systematically along transects. Samples were analyzed for total and dissolved organic carbon and nitrogen (TOC, DOC, TN, DN), stable carbon isotopes (δ13C-TOC, δ13C-DOC), and dissolved inorganic nitrogen (DIN), which were compared between the zones. C/N-ratios, δ13C signatures, and ammonium (NH4-N) concentrations were used as indicators for OM degradation along with biomarkers (n-alkanes, n-fatty acids, n-alcohols). Our results show that OM significantly decreases after disturbance with a TOC and DOC loss of 77 and 55% and a TN and DN loss of 53 and 48%, respectively. C/N-ratios decrease significantly, whereas NH4-N concentrations slightly increase in freshly thawed material. In the nearshore zone, OM contents are comparable to the disturbed zone. We suggest that the strong decrease in OM is caused by initial dilution with melted massive ice and immediate offshore transport via the thaw stream. In the mudpool and thaw stream, OM is subject to degradation, whereas in the slump floor the nitrogen decrease is caused by recolonizing vegetation. Within the nearshore zone of the ocean, heavier portions of OM are directly buried in marine sediments close to shore. We conclude that RTS have profound impacts on coastal environments in the Arctic. They mobilize nutrients from permafrost, substantially decrease OM contents and provide fresh water and nutrients at a point source