Carbon and Sulfur Cycling in Early Paleozoic Oceans

Here, I evaluate biospheric evolution during the Ordovician using high-resolution inorganic carbon and sulfur (carbonate-associated sulfate and pyrite) isotope profiles for Early Ordovician to early Late Ordovician strata from geographically distant sections in Western Newfoundland and the Argentine Precordillera. Additionally, I present new, high-resolution U-Pb ages for volcanic ash beds within strata of the Argentine Precordillera. Carbon isotope data record subdued variation that is typical of Early- to Middle Ordovician strata worldwide. By contrast, sulfur-isotopic compositions of carbonate-associated sulfate reveal a complex signal of short-term, rhythmic variation superimposed over a longer-term signal. This short-term, rhythmic variation occurs in all sections and appears to be unrelated to lithology or depositional environment, suggesting preservation of an oceanographic signal. I interpret this signal to reflect a combination of a marine sulfate reservoir that was likely much smaller than the modern, the persistence of a substantial deep-ocean hydrogen sulfide reservoir, and the episodic oxidation of a portion of the deep-ocean euxinic reservoir. Persistent euxinia likely resulted from decreased solubility of oxygen in warmer water and/or sluggish oceanic circulation during greenhouse conditions that reduced vertical ventilation. A dramatic change in the behavior of carbonate-associated sulfate and pyrite in the Middle Ordovician is interpreted to reflect a major oceanographic event that records the initial transition from Ordovician greenhouse to icehouse states. I suggest that the initiation of downwelling of increasingly cool, oxygen-rich surface water resulted in widespread oxidation of much of the deep ocean hydrogen sulfide reservoir and concomitant limitation of marine pyrite formation. It is unknown, however, why sea surface temperatures declined through the Early to Middle Ordovician. Explosive volcanism does not appear to be a primary climate driver, based on the timing of Argentinian K-bentonite formations relative to marine records of sea surface temperature, carbon and strontium isotopic composition. Rather, long-term positive feedback between organic carbon burial rates and productivity may have increased carbon dioxide drawdown, ultimately driving a gradual decrease in sea surface temperatures in the Early to Middle Ordovician.