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West Siberia contains some of the largest soil carbon stores on Earth owing to vast areas of peatlands and permafrost, with the region warming far faster than the global average. Organic matter transported in fluvial systems is likely to undergo distinct compositional changes as peatlands and permafrost warm. However, the influence of peatlands and permafrost on future dissolved organic matter (DOM) composition is not well characterized. To better understand how these environmental drivers may impact DOM composition in warming Arctic rivers, we used ultrahigh resolution Fourier‐transform ion cyclotron resonance mass spectrometry to analyze riverine DOM composition across a latitudinal gradient of West Siberia spanning both permafrost‐influenced and permafrost‐free watersheds and varying proportions of peatland cover. We find that peatland cover explains much of the variance in DOM composition in permafrost‐free watersheds in West Siberia, but this effect is suppressed in permafrost‐influenced watersheds. DOM from warm permafrost‐free watersheds was more heterogenous, higher molecular weight, and relatively nitrogen enriched in comparison to DOM from cold permafrost‐influenced watersheds, which were relatively enriched in energy‐rich peptide‐like and aliphatic compounds. Therefore, we predict that as these watersheds warm, West Siberian rivers will export more heterogeneous DOM with higher average molecular weight than at present. Such compositional shifts have been linked to different fates of DOM in downstream ecosystems. For example, a shift toward higher molecular weight, less energy‐rich DOM may lead to a change in the fate of this material, making it more susceptible to photochemical degradation processes, particularly in the receiving Arctic Ocean.

 

Abstract. The Pleistocene sand sea on the Arctic Coastal Plain (ACP) ofnorthern Alaska is underlain by an ancient sand dune field, a geologicalfeature that affects regional lake characteristics. Many of these lakes,which cover approximately 20 % of the Pleistocene sand sea, are relativelydeep (up to 25 m). In addition to the natural importance of ACP sand sealakes for water storage, energy balance, and ecological habitat, the needfor winter water for industrial development and exploration activities makeslakes in this region a valuable resource. However, ACP sand sea lakes havereceived little prior study. Here, we collect in situ bathymetric data totest 12 model variants for predicting sand sea lake depth based on analysisof Landsat-8 Operational Land Imager (OLI) images. Lake depth gradients weremeasured at 17 lakes in midsummer 2017 using a Humminbird 798ci HD SI Comboautomatic sonar system. The field-measured data points were compared tored–green–blue (RGB) bands of a Landsat-8 OLI image acquired on 8 August2016 to select and calibrate the most accurate spectral-depth model for eachstudy lake and map bathymetry. Exponential functions using a simple bandratio (with bands selected based on lake turbidity and bed substrate)yielded the most successful model variants. For each lake, the most accuratemodel explained 81.8 % of the variation in depth, on average. Modeled lakebathymetries were integrated with remotely sensed lake surface area toquantify lake water storage volumes, which ranged from 1.056×10-3 to 57.416×10-3 km3. Due to variations in depthmaxima, substrate, and turbidity between lakes, a regional model iscurrently infeasible, rendering necessary the acquisition of additional insitu data with which to develop a regional model solution. Estimating lakewater volumes using remote sensing will facilitate better management ofexpanding development activities and serve as a baseline by which toevaluate future responses to ongoing and rapid climate change in the Arctic.All sonar depth data and modeled lake bathymetry rasters can be freelyaccessed at https://doi.org/10.18739/A2SN01440 (Simpson and Arp, 2018) andhttps://doi.org/10.18739/A2HT2GC6G (Simpson, 2019), respectively. 

Abstract. Light transmission into bare glacial ice affects surfaceenergy balance, biophotochemistry, and light detection and ranging (lidar)laser elevation measurements but has not previously been reported for theGreenland Ice Sheet. We present measurements of spectral transmittance at350–900 nm in bare glacial ice collected at a field site in the westernGreenland ablation zone (67.15∘ N, 50.02∘ W). Empirical irradianceattenuation coefficients at 350–750 nm are ∼ 0.9–8.0 m−1 for ice at 12–124 cm depth. The absorption minimum is at∼ 390–397 nm, in agreement with snow transmissionmeasurements in Antarctica and optical mapping of deep ice at the SouthPole. From 350–530 nm, our empirical attenuation coefficients are nearly1 order of magnitude larger than theoretical values for optically pureice. The estimated absorption coefficient at 400 nm suggests the ice volumecontained a light-absorbing particle concentration equivalent to∼ 1–2 parts per billion (ppb) of black carbon, which is similar topre-industrial values found in remote polar snow. The equivalent mineraldust concentration is ∼ 300–600 ppb, which is similar to values forNorthern Hemisphere warm periods with low aeolian activity inferred from icecores. For a layer of quasi-granular white ice (weathering crust)extending from the surface to ∼ 10 cm depth, attenuationcoefficients are 1.5 to 4 times larger than for deeper bubbly ice. Owing tohigher attenuation in this layer of near-surface granular ice, opticalpenetration depth at 532 nm is 14 cm (20 %) lower than asymptoticattenuation lengths for optically pure bubbly ice. In addition to thetraditional concept of light scattering on air bubbles, our results implythat the granular near-surface ice microstructure of weathering crust isan important control on radiative transfer in bare ice on the Greenland IceSheet ablation zone, and we provide new values of flux attenuation,absorption, and scattering coefficients to support model development andvalidation.