Glacial weathering, sulfide oxidation, and global carbon cycle feedbacks

Abstract

Connections between glaciation, chemical weathering, and the global carbon cycle could steer the evolution of global climate over geologic time, but even the directionality of feedbacks in this system remain to be resolved. Here, we assemble a compilation of hydrochemical data from glacierized catchments, use this data to evaluate the dominant chemical reactions associated with glacial weathering, and explore the implications for long-term geochemical cycles. Weathering yields from catchments in our compilation are higher than the global average, which results, in part, from higher runoff in glaciated catchments. Our analysis supports the theory that glacial weathering is characterized predominantly by weathering of trace sulfide and carbonate minerals. To evaluate the effects of glacial weathering on atmospheric pCO 2 , we use a solute mixing model to predict the ratio of alkalinity to dissolved inorganic carbon (DIC) generated by weathering reactions. Compared with nonglacial weathering, glacial weathering is more likely to yield alkalinity/DIC ratios less than 1, suggesting that enhanced sulfide oxidation as a result of glaciation may act as a source of CO 2 to the atmosphere. Back-of-the-envelope calculations indicate that oxidative fluxes could change ocean-atmosphere CO 2 equilibrium by 25 ppm or more over 10 ky. Over longer timescales, CO 2 release could act as a negative feedback, limiting progress of glaciation, dependent on lithology and the concentration of atmospheric O 2 . Future work on glaciation-weathering-carbon cycle feedbacks should consider weathering of trace sulfide minerals in addition to silicate minerals.weathering | carbon cycle | glaciation | oxidation | sulfide E pisodes of glaciation represent some of the most dramatic changes in the history of Earth's climate. Basic questions remain unanswered about why the planet has occasionally but not always supported glaciers, about why some glaciations have been more intense than others, and about how glaciers have shaped Earth's landscapes, chemical cycles, and climate. Before the Phanerozoic (i.e., before ∼540 Mya), several glaciations are thought to have approached complete or near-complete planetary ice cover ("Snowball Earth" events; ref. 1). In contrast, more recent glaciations, including the Quaternary glaciations of the last million years, have been characterized by oscillating climate with ice extent limited to high latitudes even during glacial maxima (2). The formation of ice cover has long been known to fundamentally alter Earth's environment, changing albedo (3), influencing atmospheric and ocean circulation (4), and reshaping landscapes (5). These diverse effects may contribute to planetary-scale feedbacks that influence the transition to and the evolution of glaciation.Chemical weathering influences the composition of the atmosphere by releasing ionic species from minerals that alter the alkalinity and redox condition of the ocean-atmosphere system. Weathering of silicate minerals produces alkalinity, removing carbon from the at...

Pyrite Oxidation Drives Exceptionally High Weathering Rates and Geologic CO2 Release in Mountaintop‐Mined Landscapes

“…Scaled to the 5,900 km 2 of mountaintop mined land in central Appalachia, we estimate that mine lands export 3.24 * 10 12 g/year of pyrite derived sulfate, accounting for 5–7% of the current global budget. Estimates of worldwide pyrite oxidation rates are likely significant underestimates given more recent work showing similarly high pyrite oxidation rates in glaciated landscapes (Calmels et al, ; Torres et al, ), heavy rainfall catchments (Das et al, ), and deep mines (Raymond & Oh, ). Even if the global estimate increases substantially, the rates of pyrite oxidized sulfate production from mined Appalachian catchments will still be significant and highly disproportionate to their area.…”

“…Under alkaline conditions, much of this CO 2 is converted to HCO 3 − via carbonate equilibrium reactions (Stumm & Morgan, ), which yields the following net reaction (5): Bicarbonate ions released by this weathering reaction (equation ) will be exported downstream with other weathering products but may also be evaded as CO 2 when waters become less alkaline downstream (Marcé et al, ). Mining‐derived HCO 3 − that enters the ocean can act as a CO 2 source over geologic timescales that occur over 10 6 years (Torres et al, , ) via Despite ample research on the high concentrations of rock‐derived ions in streams draining mountaintop mines, there has been insufficient research on chemical weathering fluxes and almost no work looking at elevated pyrite oxidation and carbonate weathering effects on the geologic C cycle. Here we use a paired‐catchment approach to estimate the impact of mountaintop mining on weathering rates and geologic C cycling.…”

Seasonal variations of biogeochemical matter export along the Langtang-Narayani river system in central Himalaya

“…In general, the oxidation of sulfides can be relevant for weathering rates of affected basins (e.g., Calmels et al, 2007). Sulfide oxidation can also be a cause for substantial CO 2 -degassing from the water (Torres et al, 2016(Torres et al, , 2017. Note that clearly elevated pCO 2 values can be observed below station LNS-7 (Fig.…”

“…Sulfide oxidation, often coupled with carbonate dissolution, influences the flux of dissolved inorganic carbon (DIC) in subglacial drainage systems within the Himalayan region and elsewhere (Tranter and Raiswell, 1991;Tranter et al, 1993;Galy and France-Lanord, 1999;Hasnain and Thayyen, 1999;Bhatt et al, 2000;Millot et al, 2003;Bhatt et al, 2009;Wolff-Boenisch et al, 2009;Bhatt et al, 2016;Torres et al, 2017). Sulphur oxidation may also be a relevant long-term source of CO 2 to the atmosphereocean system over long geological time scales (Torres et al, 2014).…”

Ice cap erosion patterns from bedrock 10Be and 26Al, southeastern Tibetan Plateau

“…Erosion by glaciers and ice sheets over multiple glacial cycles is responsible for producing diverse landscapes, generating and reducing relief, perturbing geochemical processes, and thereby contributing to climate oscillations during the Quaternary Period (Montgomery, 2002;Brook et al, 2006;Koppes and Montgomery, 2009;Brocklehurst, 2010;Jaeger and Koppes, 2016;Egholm et al, 2017;Torres et al, 2017). Erosion plays a dominant role in landscape evolution, so placing constraints on glacial erosion depths and patterns will enable a better understanding of the relative importance of the different drivers of these erosional processes (Hallet et al, 1996;Koppes et al, 2015;Herman et al, 2018).…”