Abstract
Abstract. Growing evidence points to the dynamic role that kerogen is playing on Earth's surface in controlling atmospheric chemistry over geologic time. Although quantitative constraints on the weathering of kerogen remain loose, its changing weathering behavior modulated by the activity of glaciers suggests that this largest pool of reduced carbon on Earth may have played a key part in atmospheric CO2 variability across recent glacial–interglacial cycles and beyond. This work enunciates the possibility of kerogen oxidation as a major driver of atmospheric CO2 increase in the wake of glacial episodes. This hypothesis of centennial- and millennial-timescale relevance for this chemical weathering pathway is substantiated by several lines of independent evidence synthesized in this contribution, including the timing of atmospheric CO2 increase, atmospheric CO2 isotope composition (13C and 14C), kerogen oxidation kinetics, observations of kerogen reburial, and modeling results. The author hypothesizes that the deglaciation of kerogen-rich lithologies in western Canada contributed to the characteristic deglacial increase in atmospheric CO2, which reached an inflection point ≤ 300 years after the Laurentide Ice Sheet retreated into the kerogen-poor Canadian Shield. To reconcile the release of isotopically light carbon via kerogen oxidation with Earth surface carbon pool constraints, major oceanic degassing and biospheric regrowth must have acted in concert across glacial–interglacial transitions. Additionally, a process such as a strong shift in the ratio of C3 to C4-derived organic matter must be invoked to maintain isotope mass balance, a point key for reconciling the hypothesis with the carbon isotope record of marine dissolved inorganic carbon. In order to test this hypothesis, quantitative constraints on the contribution of kerogen oxidation to CO2 rise at glacial terminations are needed through systematic studies on (1) CO2 fluxes emanating from the weathering of different lithologies, (2) oxidation kinetics of kerogen along glacial chronosequences, and (3) high-resolution temporal changes in the aerial extent of glacially exposed lithological units and glacial flour.
Highlights
Over geologic timescales, atmospheric CO2 is controlled by the combined effects of chemical weathering of silicates and carbonates and the organic carbon cycle (Berner et al, 1983; Berner, 1990; Torres et al, 2014)
The author hypothesizes that the deglaciation of kerogen-rich lithologies in western Canada contributed to the characteristic deglacial increase in atmospheric CO2, which reached an inflection point ≤ 300 years after the Laurentide Ice Sheet retreated into the kerogen-poor Canadian Shield
Atmospheric CO2 is controlled by the combined effects of chemical weathering of silicates and carbonates and the organic carbon cycle (Berner et al, 1983; Berner, 1990; Torres et al, 2014)
Summary
Atmospheric CO2 is controlled by the combined effects of chemical weathering of silicates and carbonates and the organic carbon cycle (Berner et al, 1983; Berner, 1990; Torres et al, 2014). The decay of kerogen on Earth’s surface is incomplete (e.g., Hemingway et al, 2018; Leythaeuser, 1973), with physical erosion followed by riverine transport (Galy et al, 2015) and reburial in lacustrine and marine settings (e.g., Blattmann et al, 2018a, 2019b; Cui et al, 2016; Sackett et al, 1974; Sparkes et al, 2020) The operation of this “simple cycle besides the more complicated common circulation of carbon”, enunciated by Sauramo (1938), begs the following questions: (i) what is the reburial efficiency of kerogen, (ii) what is the weathering efficiency of kerogen, (iii) what are their controlling factors, and (iv) how do reburial and weathering efficiency vary over geologic time and space? The operation of this “simple cycle besides the more complicated common circulation of carbon”, enunciated by Sauramo (1938), begs the following questions: (i) what is the reburial efficiency of kerogen, (ii) what is the weathering efficiency of kerogen, (iii) what are their controlling factors, and (iv) how do reburial and weathering efficiency vary over geologic time and space? The answers presented will suggest that the exogenous kerogen cycle behaves dynamically, with this contribution hypothesizing a connection between kerogen oxidation and millennial/centennial-scale atmospheric CO2 increases during glacial terminations
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