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The abundance of dolomitic strata in the geological record contrasts with the general rarity of locations where dolomite forms today, a discrepancy that has long posed a problem for their interpretation. Recent culture experiments show that dolomite can precipitate at room temperature, raising the possibility that many ancient dolomites may be of syngenetic origin. We compiled a large geodata set of secular variations in dolomite abundance in the Phanerozoic, coupled with compilations of genus richness of marine benthic invertebrates and sulfur-isotope variations in marine carbonates. These data show that dolomite abundance is negatively correlated to genus diversity, with four dolomite peaks occurring during mass extinctions. Dolomite peaks also correspond to the rapid increase in sulfur-isotope composition (δ34S), an indicator of enhanced sulfate reduction, in anoxic oceans. These results confirm that variations in dolomite abundance during the Phanerozoic were closely linked with changes in marine benthic diversity, with both in turn related to oceanic redox conditions.

The Permian-Triassic boundary (PTB) crisis caused major shortterm perturbations in ocean chemistry, as recorded by the precipitation of anachronistic carbonates. Here, we document for the first time a global dolomitization event during the Permian-Triassic transition based on Mg/(Mg + Ca) data from 22 sections with a global distribution representing shallow- to deep-marine environments. Ten of these sections show high Mg/(Mg + Ca) ratios bracketing the PTB, recording a short-term spike in dolomite formation. The dolomite consists mainly of micron-scale anhedral to subhedral crystals that are associated with abundant fossilized bacterial bodies and extracellular polymeric substances, suggesting that dolomite precipitation was induced by microbial metabolic activity. Sections showing a dolomite spike at the PTB are widely distributed geographically, but mostly encountered in mid-shelf to upper-slope settings. Because the dolomitization event coincided with a rapid expansion of oceanic anoxia and high rates of sulfate reduction, we hypothesize that it was triggered by enhanced microbial sulfate reduction within the oceanic chemocline.