A DOM regulation model for dolomite versus calcite precipitation in the Ediacaran ocean: Implications for the “dolomite problem”

Abstract

The Ediacaran Period (∼635–539 Ma) witnessed the largest negative carbonate carbon isotope (δ13Ccarb) excursion in Earth history (to −12‰ VPDB), known as the Shuram Excursion (SE), which has been attributed to oxidation of a massive dissolved organic matter (DOM) reservoir, yielding highly 13C-depleted oceanic dissolved inorganic carbon (DIC). Limestone and dolostone are intimately interbedded in the Ediacaran Doushantuo Formation (South China) as well as in coeval successions globally, suggesting frequent shifts between calcite and dolomite precipitation in the Ediacaran ocean. In this study, we compiled paired Mg/(Mg + Ca) and δ13Ccarb data for 991 well-preserved (i.e., minimally altered) carbonate-rich samples from 17 SE-associated sections with a wide global distribution. δ13Ccarb compositions vary among the dolomite-dominated (−10‰ to +7‰), calcite-dominated (mainly in −10‰ to −5‰), and mineralogically mixed samples (with a bimodal distribution of −10‰ to −5‰ or +4‰ to +8‰). Inspired by the recent DOM catalyzation hypothesis and the highly-stratified redox model as well as the large DOM reservoir hypothesis for the Ediacaran ocean, we propose a “DOM regulation model” to account for controls on the precipitation of dolomite versus calcite in the Ediacaran ocean and, thus, for observed Mg/(Mg + Ca)-δ13Ccarb patterns. In this model, calcite was precipitated under low-DOM conditions, which existed in oxic surface waters and the nearshore part of a mid-depth DOM-anaerobic oxidation zone. The former had δ13CDIC > +4‰, buffered by atmospheric CO2, whereas the latter had δ13CDIC decreasing from +4‰ to −10‰ due to increasing contributions of DOM oxidation. On the other hand, dolomite precipitation was catalyzed by high-DOM conditions, which existed in distal anoxic deep waters and the offshore part of the DOM-anaerobic oxidation zone. The former had δ13CDIC > +4‰, ultimately buffered by atmospheric CO2, whereas the latter had δ13CDIC increasing from −10‰ to +4‰ due to decreasing contributions of DOM oxidation. Our model is well supported by coexisting redox data. Our study highlights a possible link between massive dolomite formation and the existence of a large DOM reservoir in the Ediacaran ocean, providing an alternative perspective on the origin and widespread distribution of dolomite in deep-time marine systems (i.e., the “dolomite problem”).