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Abstract Rapid warming in the Arctic threatens to destabilize mercury (Hg) deposits contained within soils in permafrost regions. Yet current estimates of the amount of Hg in permafrost vary by ∼4 times. Moreover, how Hg will be released to the environment as permafrost thaws remains poorly known, despite threats to water quality, human health, and the environment. Here we present new measurements of total mercury (THg) contents in discontinuous permafrost in the Yukon River Basin in Alaska. We collected riverbank and floodplain sediments from exposed banks and bars near the villages of Huslia and Beaver. Median THg contents were 49+13/−21ng THg g sediment−1and 39+16/−18ng THg g sediment−1for Huslia and Beaver, respectively (uncertainties as 15th and 85th percentiles). Corresponding THg:organic carbon ratios were 5.4+2.0/−2.4Gg THg Pg C−1and 4.2+2.4/−2.9Gg THg Pg C−1. To constrain floodplain THg stocks, we combined measured THg contents with floodplain stratigraphy. Trends of THg increasing with smaller sediment size and calculated stocks in the upper 1 m and 3 m are similar to those suggested for this region by prior pan-Arctic studies. We combined THg stocks and river migration rates derived from remote sensing to estimate particulate THg erosional and depositional fluxes as river channels migrate across the floodplain. Results show similar fluxes within uncertainty into the river from erosion at both sites (95+12/−47kg THg yr−1and 26+154/−13kg THg yr−1at Huslia and Beaver, respectively), but different fluxes out of the river via deposition in aggrading bars (60+40/−29kg THg yr−1and 10+5.3/−1.7kg THg yr−1). Thus, a significant amount of THg is liberated from permafrost during bank erosion, while a variable but generally lesser portion is subsequently redeposited by migrating rivers.
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Arctic Permafrost Thawing Enhances Sulfide Oxidation
Abstract Permafrost degradation is altering biogeochemical processes throughout the Arctic. Thaw‐induced changes in organic matter transformations and mineral weathering reactions are impacting fluxes of inorganic carbon (IC) and alkalinity (ALK) in Arctic rivers. However, the net impact of these changing fluxes on the concentration of carbon dioxide in the atmosphere (pCO2) is relatively unconstrained. Resolving this uncertainty is important as thaw‐driven changes in the fluxes of IC and ALK could produce feedbacks in the global carbon cycle. Enhanced production of sulfuric acid through sulfide oxidation is particularly poorly quantified despite its potential to remove ALK from the ocean‐atmosphere system and increasepCO2, producing a positive feedback leading to more warming and permafrost degradation. In this work, we quantified weathering in the Koyukuk River, a major tributary of the Yukon River draining discontinuous permafrost in central Alaska, based on water and sediment samples collected near the village of Huslia in summer 2018. Using measurements of major ion abundances and sulfate () sulfur (34S/32S) and oxygen (18O/16O) isotope ratios, we employed the MEANDIR inversion model to quantify the relative importance of a suite of weathering processes and their net impact onpCO2. Calculations found that approximately 80% of in mainstem samples derived from sulfide oxidation with the remainder from evaporite dissolution. Moreover,34S/32S ratios,13C/12C ratios of dissolved IC, and sulfur X‐ray absorption spectra of mainstem, secondary channel, and floodplain pore fluid and sediment samples revealed modest degrees of microbial sulfate reduction within the floodplain. Weathering fluxes of ALK and IC result in lower values ofpCO2over timescales shorter than carbonate compensation (∼104 yr) and, for mainstem samples, higher values ofpCO2over timescales longer than carbonate compensation but shorter than the residence time of marine (∼107 yr). Furthermore, the absolute concentrations of and Mg2+in the Koyukuk River, as well as the ratios of and Mg2+to other dissolved weathering products, have increased over the past 50 years. Through analogy to similar trends in the Yukon River, we interpret these changes as reflecting enhanced sulfide oxidation due to ongoing exposure of previously frozen sediment and changes in the contributions of shallow and deep flow paths to the active channel. Overall, these findings confirm that sulfide oxidation is a substantial outcome of permafrost degradation and that the sulfur cycle responds to permafrost thaw with a timescale‐dependent feedback on warming.
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- PAR ID:
- 10476302
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Global Biogeochemical Cycles
- Volume:
- 37
- Issue:
- 11
- ISSN:
- 0886-6236
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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