CALCIUM ISOTOPES, SEAWATER DIAGENESIS, AND END ORDOVICIAN EARTH HISTORY (Invited Presentation)

Stratigraphic variability in stable isotope ratios of biogeochemically important elements such as carbon and sulfur provide critical data for interpreting paleoenvironmental change in deep time. However many pre-Jurassic isotope records derive from shallow water carbonate strata, which are particularly susceptible to diagenetic alteration of their geochemical archives. Recognizing and quantifying the diagenetic overprint of stable isotope records in ancient carbonate rocks is an ongoing challenge. Here we use the calcium isotope system (δ^44/40Ca) in conjunction with field observations and petrography to elucidate the diagenetic history of carbonate strata along an offshore-onshore transect of an Upper Ordovician carbonate shelf exposed in the Great Basin. Deep water limestones of the Hanson Creek Formation record a 0.6‰ negative δ^44/40Ca excursion coincident with increasing Sr content, a 7‰ positive δ¹³C excursion, and stratigraphic evidence for sea level fall associated with the Hirnantian glacial maximum (Holmden et al., 2012). In contrast, new δ^44/40Ca data from coeval shallow water dolostones of the Ely Springs Formation have little stratigraphic variability. Instead, we observe a geographic gradient in the mean δ^44/40Ca value across the shallow platform transect, with higher δ^44/40Ca values in the landward direction. We interpret the Hanson Creek data to reflect a change from calcite to aragonite sedimentation in the deep basin. The enrichment in δ^44/40Ca values across the shallow platform indicates increasing influence of open system seawater diagenesis toward the continent, likely driven by multiple episodes of glacioeustatic sea level fluctuation. That the magnitude of the δ¹³C excursion recorded in the shallow water sections is only 3-4‰ suggests seawater diagenesis altered the magnitude of the primary carbon isotope excursion by buffering if toward post-excursion seawater δ¹³C values. Seawater diagenesis may explain isotopic gradients in ancient shallow water platforms. This process could also “erase” carbonate-associated sulfate sulfur isotope excursions, with the magnitude of alteration dependent on the ratio of dissolved inorganic carbon to sulfate in the seawater.