Making Sense of Massive Carbon Isotope Excursions With an Inverse Carbon Cycle Model

Abstract

The beginning and end of the Proterozoic Eon are marked by extreme variations in carbonate carbon isotope values that have been interpreted to record massive perturbations to the global carbon cycle. The lower Proterozoic contains an extended interval of strata characterized by positive carbonate δ13C values. Conversely, uppermost Proterozoic carbonate strata contain thick intervals with extremely negative δ13C values and multiple large swings in carbonate δ13C. Previous attempts to model these pronounced carbon isotope excursions as shifts in the global marine dissolved inorganic carbon (DIC) reservoir have proved to be problematic, as the direction and magnitude of these positive and negative carbon isotope excursions require unrealistic amounts of either organic carbon burial or organic carbon oxidation, respectively. Here we present a modified global carbon cycle model—coupled with oxygen and sulfur cycle mass balances—that includes a parameterization of the recycling of sedimentary isotope anomalies and allows the extent of organic carbon oxidation to vary as a function of atmospheric oxygen levels. Our model is designed to match carbon isotope records while maintaining redox and mass balance with a given set of initial conditions and carbon cycle parameterizations. Using this approach, we demonstrate that there is a range of plausible biogeochemical perturbations that could induce substantial δ13C excursions in the global marine DIC reservoir. However, we also find that there are multiple, nonunique Earth system states for any observed marine δ13C value.

A sluggish mid‐Proterozoic biosphere and its effect on Earth's redox balance

“…In contrast, our model predicts that oxidative weathering of organic matter would have been suppressed under mid-Proterozoic conditions, consistent with a number of recent models (Daines et al, 2017;Miyazaki, Planavsky, Bolton, & Reinhard, 2018). This results in δ 13 C values for the integrated carbon flux to the ocean-atmosphere system that are significantly higher than the canonical mantle value (−0.3‰ and −1.1‰ for our "Low O 2 " and "High O 2 " scenarios, respectively; Supporting information Table S8), compared to a canonical value of ~15% to 20% (e.g., Krissansen-Totton et al, 2015).…”

“…This results in δ 13 C values for the integrated carbon flux to the ocean-atmosphere system that are significantly higher than the canonical mantle value (−0.3‰ and −1.1‰ for our "Low O 2 " and "High O 2 " scenarios, respectively; Supporting information Table S8), compared to a canonical value of ~15% to 20% (e.g., Krissansen-Totton et al, 2015). These results add to a growing body of work indicating that the quantitative scaling between the δ 13 C values of marine sedimentary carbonate rocks and organic carbon burial fluxes from the exogenic system is somewhat obscure, particularly at low atmospheric pO 2 (Bjerrum & Canfield, 2004;Daines et al, 2017;Miyazaki et al, 2018;Schrag et al, 2013).…”

On the co‐evolution of surface oxygen levels and animals

“…With respect to geochemical data, there is no sound empirical evidence for a large DOC reservoir. The Shuram carbon isotope excursion—if it is recording marine DIC values—can be easily explained via modest changes to atmospheric O 2 and net pyrite burial in the sedimentary rock cycle without any need for a large DOC reservoir (Miyazaki, Planavsky, Bolton, & Reinhard, ). Decoupling of carbonate and organic carbon isotope values is also not sound evidence for a large DOC reservoir (cf.…”

“…The geochemical data in these three papers are considered robust, but additional sampling and new intellectual frameworks for interpreting isotopic data have made the picture murkier. Regarding carbon and sulfur isotope data from carbonates, it is now recognized that (a) local oceanographic and diagenetic controls play a major role on the resulting signals and (b) isotopic data cannot be simply read in a mass‐balance framework relating burial of oxidized and reduced species (Ahm et al, ; Fike, Bradley, & Rose, ; Miyazaki et al, ). The Avalon Peninsula succession may have become more oxygenated in the mid‐Ediacaran, but new iron speciation data from other successions, as well as recent reports of banded iron formations in the Cambrian (Li, Cheng, et al, ; Li, Zhang, et al, ), demonstrate this was likely a regional phenomenon and that widespread deep‐water anoxia persisted into the Paleozoic (Sperling et al, ).…”

A productivity collapse to end Earth’s Great Oxidation

“…Importantly, this rise in O 2 has been suggested to exceed not only Archean, but also background Proterozoic (6,7) and possibly Phanerozoic levels in a so-called "oxygen overshoot" (8,9). However, this high-O 2 interpretation can be tempered under different assumptions regarding changes in the isotopic value of carbon inputs to the global dissolved inorganic carbon (DIC) reservoir as well as the fractionation associated with carbon fixation by primary producers (10). Moreover, some researchers question whether these ca.…”