A high-resolution record of early Paleozoic climate

Abstract

The spatial coverage and temporal resolution of the Early Paleozoic paleoclimate record are limited, primarily due to the paucity of well-preserved skeletal material commonly used for oxygen-isotope paleothermometry. Bulk-rock ${\delta }^{18}\mathrm{O}$ datasets can provide broader coverage and higher resolution, but are prone to burial alteration. We assess the diagenetic character of two thick Cambro–Ordovician carbonate platforms with minimal to moderate burial by pairing clumped and bulk isotope analyses of micritic carbonates. Despite resetting of the clumped-isotope thermometer at both sites, our samples indicate relatively little change to their bulk ${\delta }^{18}\mathrm{O}$ due to low fluid exchange. Consequently, both sequences preserve temporal trends in ${\delta }^{18}\mathrm{O}$ . Motivated by this result, we compile a global suite of bulk rock ${\delta }^{18}\mathrm{O}$ data, stacking overlapping regional records to minimize diagenetic influences on overall trends. We find good agreement of bulk rock ${\delta }^{18}\mathrm{O}$ with brachiopod and conodont ${\delta }^{18}\mathrm{O}$ trends through time. Given evidence that the ${\delta }^{18}\mathrm{O}$ value of seawater has not evolved substantially through the Phanerozoic, we interpret this record as primarily reflecting changes in tropical, nearshore seawater temperatures and only moderately modified by diagenesis. Focusing on the samples with the most enriched, and thus likely least-altered, ${\delta }^{18}\mathrm{O}$ values, we reconstruct Late Cambrian warming, Early Ordovician extreme warmth, and cooling around the Early–Middle Ordovician boundary. Our record is consistent with models linking the Great Ordovician Biodiversification Event to cooling of previously very warm tropical oceans. In addition, our high-temporal-resolution record suggests previously unresolved transient warming and climate instability potentially associated with Late Ordovician tectonic events.