N. J. Pester

Effect of paleoseawater composition on hydrothermal exchange in midocean ridges

Significance Circulation of seawater through midocean ridges results in large-scale chemical transfer between the solid Earth and oceans. Driven by magmatic heat, seawater undergoes hydrothermal reactions that affect the concentrations of Mg, Ca, SO4, and Sr in the oceans over millions of years. Changes in the composition of seawater during the past 500 million years may have previously unrecognized effects on the composition of hydrothermal fluids flowing back into the oceans through time. This work presents a model suggesting that Ca, and Sr exchange in particular, may have been much different in the geologic past, which has important implications for the interpretation of Sr isotope records in paleoseawater and ancient oceanic crust. Variations in the Mg, Ca, Sr, and SO4 concentrations of paleoseawater can affect the chemical exchange between seawater and oceanic basalt in hydrothermal systems at midocean ridges (MOR). We present a model for evaluating the nature and magnitude of these previously unappreciated effects, using available estimates of paleoseawater composition over Phanerozoic time as inputs and 87Sr/86Sr of ophiolite epidosites and epidote-quartz veins as constraints. The results suggest that modern hydrothermal fluids are not typical due to low Ca and Sr relative to Mg and SO4 in modern seawater. At other times during the last 500 million years, particularly during the Cretaceous and Ordovician, hydrothermal fluids had more seawater-derived Sr and Ca, a prediction that is supported by Sr isotope data. The predicted 87Sr/86Sr of vent fluids varies cyclically in concert with ocean chemistry, with some values much higher than the modern value of ∼0.7037. The seawater chemistry effects can be expressed in terms of the transfer efficiency of basaltic Ca and Sr to seawater in hydrothermal systems, which varies by a factor of ∼1.6 over the Phanerozoic, with minima when seawater Mg and SO4 are low. This effect provides a modest negative feedback on seawater composition and 87Sr/86Sr changes. For the mid-Cretaceous, the low 87Sr/86Sr of seawater requires either exceptionally large amounts of low-temperature exchange with oceanic crust or that the weathering flux of continentally derived Sr was especially small. The model also has implications for MOR hydrothermal systems in the Precambrian, when low-seawater SO4 could help explain low seawater 87Sr/86Sr.