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 increase pCO2, 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 (SO42‐) 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 on pCO2. Calculations found that approximately 80% of SO42‐ in mainstem samples derived from sulfide oxidation with the remainder from evaporite dissolution. Moreover, 34S/32S ratios, 13C/12C ratios of dissolved inorganic carbon, 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 of pCO2 over timescales shorter than carbonate compensation (∼104 yr) and, for mainstem samples, higher values of pCO2 over timescales longer than carbonate compensation but shorter than the residence time of marine SO42‐ (∼107 yr). Furthermore, the absolute concentrations of SO42‐ and Mg2+ in the Koyukuk River, as well as the ratios of SO42‐ 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.This article is protected by copyright. All rights reserved.

Storage and Distribution of Organic Carbon and Nutrients in Acidic Soils Developed on Sulfidic Sediments: The Roles of Reactive Iron and Macropores

“…There is growing evidence that intensified thaw/degradation of permafrost across many terrains in the (sub)arctic regions is frequently accompanied by the liberation of massive acidic sulfate-rich drainages due to the oxidation of sulfidic shales and glacial tills that were previously frozen underground or covered by glaciers. 29 , 31 − 33 , 111 , 112 The results of our study suggest that, during the oxidative weathering of these (sub)arctic sulfidic materials, most of the OM may remain intact at least in the short term. The production and outflow of the acidic drainages also result in massive formation of Fe(III) hydroxides and oxyhydroxysulfates (identified/modeled as schwertmannite and jarosite) in the associated active layers and along perennial/intermittent “rusting” streams/rivers.…”

“…Subsoil, despite being poor in OC compared to topsoil, has a greater thickness and higher degree of compactness/deformation and thus holds a larger pool of reactive Fe minerals . These physical and compositional properties enable the subsoil to act as a crucial, yet frequently overlooked, reservoir of terrestrial OC and nutrients. , Subsoil also contains a considerable volume of interconnected cracks, fissures, and tubular pores (hereafter collectively referred to as “macropores” or “macropore system”) that vary greatly in three-dimensional geometry, mainly depending on soil structure, fauna activities, and preferential water flow patterns. , These macropores serve as hotspots for (bio)­geochemical reactions/processes, microbial activities, and the exchange and advection/movement of liquid and gaseous phases, − which in turn regulates OC/nutrient distribution, transformation, and storage in subsoil systems. ,, During the last two decades, our understanding of the factors and mechanisms controlling OC/nutrient sequestration, transformation, and stabilization in subsoil has greatly expanded. − However, it is still unclear how macropores and associated physical/(bio)­geochemical reactions/processes could reshape and contribute to the distribution and storage of OC and nutrients in acidic sulfate-rich subsoil systems that are widespread on many coastal plains − and certainly increasing in extent in thawing (sub)­arctic regions. − …”

“… 21 − 25 However, it is still unclear how macropores and associated physical/(bio)geochemical reactions/processes could reshape and contribute to the distribution and storage of OC and nutrients in acidic sulfate-rich subsoil systems that are widespread on many coastal plains 26 − 28 and certainly increasing in extent in thawing (sub)arctic regions. 29 − 33 …”

Metal mobilization from thawing permafrost to aquatic ecosystems is driving rusting of Arctic streams

“…While considerable research has focused on the fate of carbon and nutrients transported from permafrost to aquatic ecosystems 12 , few studies have focused on the causes of iron (Fe) and trace metal mobilization from thawing soils to streams 13,14 and less is known about the consequences for water quality and aquatic food webs. Abrupt transitions in water chemistry on the timescale of weeks to months may represent an unforeseen risk of permafrost thaw for food security 15 , as subsistence fisheries and drinking water supplies may become degraded in some Arctic river networks.Recent observations from the Arctic indicate that waters draining permafrost landscapes may be vulnerable to the mobilization of Fe and other metals following thaw through changing redox conditions and microbial activity 13,14,16 and enhanced chemical weathering of minerals 9,17 . One striking indication of altered Fe-cycling processes is the abrupt change in the color of streams that reflects a dramatic shift in water quality 18 .…”

“…Recent observations from the Arctic indicate that waters draining permafrost landscapes may be vulnerable to the mobilization of Fe and other metals following thaw through changing redox conditions and microbial activity 13,14,16 and enhanced chemical weathering of minerals 9,17 . One striking indication of altered Fe-cycling processes is the abrupt change in the color of streams that reflects a dramatic shift in water quality 18 .…”