Abstract
The Paleoproterozoic Lomagundi–Jatuli Event (LJE) is generally considered the largest, in both amplitude and duration, positive carbonate C-isotope ( δ 13 C carb ) excursion in Earth history. Conventional thinking is that it represents a global perturbation of the carbon cycle between 2.3–2.1 Ga linked directly with, and in part causing, the postulated rise in atmospheric oxygen during the Great Oxidation Event. In addition to new high-resolution δ 13 C carb measurements from LJE-bearing successions of NW Russia, we compiled 14 943 δ 13 C carb values obtained from marine carbonate rocks 3.0–1.0 Ga in age and from selected Phanerozoic time intervals as a comparator of the LJE. Those data integrated with sedimentology show that, contra to consensus, the δ 13 C carb trend of the LJE is facies (i.e. palaeoenvironment) dependent. Throughout the LJE interval, the C-isotope composition of open and deeper marine settings maintained a mean δ 13 C carb value of +1.5 ± 2.4‰, comparable to those settings for most of Earth history. In contrast, the 13 C-rich values that are the hallmark of the LJE are limited largely to nearshore-marine and coastal-evaporitic settings with mean δ 13 C carb values of +6.2 ± 2.0‰ and +8.1 ± 3.8‰, respectively. Our findings confirm that changes in δ 13 C carb are linked directly to facies changes and archive contemporaneous dissolved inorganic carbon pools having variable C-isotopic compositions in laterally adjacent depositional settings. The implications are that the LJE cannot be construed a priori as representative of the global carbon cycle or a planetary-scale disturbance to that cycle, nor as direct evidence for oxygenation of the ocean–atmosphere system. This requires rethinking models relying on those concepts and framing new ideas in the search for understanding the genesis of the grandest of all positive C-isotope excursions, its timing and its hypothesized linkage to oxygenation of the atmosphere. Supplementary material : C–O isotope data, figure S1, tables S1–S5 and the dataset for δ 13 C carb values are available at https://doi.org/10.6084/m9.figshare.c.5471815
Highlights
The connection between key fluxes of carbon as carbonate precipitated out of the marine dissolved inorganic carbon (DIC) reservoir and the amount buried as organic matter, including the respective isotopic compositions of each flux, has been commonly evaluated within a framework of mass balance and used in evaluating the pattern and tempo of the global carbon cycle (e.g. Schidlowski 1988; Kump 1991; Hayes and Waldbauer 2006; Mason et al 2017)
We propose that other postulated open- and deeper-marine units with very positive δ13Ccarb values (e.g. Silverton Formation) may reflect redeposition of 13C-rich carbonate transported from adjacent shallower environments
Our findings show that the C-isotope pattern of the Lomagundi–Jatuli Event (LJE) is a record of contemporaneous lateral gradients in the isotopic composition of DIC pools, ranging from normal open-marine values of c. 0‰ δ13Ccarb to extremely high values of +16‰ and even higher, in nearshore-marine–coastal settings
Summary
The connection between key fluxes of carbon as carbonate precipitated out of the marine DIC reservoir and the amount buried as organic matter, including the respective isotopic compositions of each flux, has been commonly evaluated within a framework of mass balance and used in evaluating the pattern and tempo of the global carbon cycle (e.g. Schidlowski 1988; Kump 1991; Hayes and Waldbauer 2006; Mason et al 2017). Other models invoke tectonics driving exhumation and erosion of older sedimentary rocks that, in turn, drive fluctuations in the isotopic composition of the carbon flux entering the marine realm to generate C-isotope excursions in lieu of changes to primary productivity Our work assesses the key premise that underpins the LJE paradigm, namely, that the magnitude and duration of its δ13Ccarb values are an accurate archive of the C-isotopic composition of the global ocean between 2.3 and 2.1 Ga. All concepts and models that use the LJE as evidence of the redox evolution of Earth’s surface environments and initial rise and proliferation of free di-oxygen rely on that premise to be true
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