The Nd and Sr isotopic evolution of Proterozoic seawater

Abstract

Nd and Sr isotopic measurements from a variety of marine chemical precipitates provide insight into processes controlling the chemistry of seawater during the Proterozoic. Nd isotope measurements on banded iron formations and phosphorites, and Sr isotope measurements on carbonates indicate that during both the Early and Late Proterozoic, hydrothermal input to the oceans was a significant fraction of the total input to ocean chemistry. Isotopic data from Early Proterozoic clastic sediments show systematic differences from coeval chemical sediments. These differences become less marked toward the end of the Proterozoic. This implies a higher hydrothermal water to river water flux ratio during the Early Proterozoic. The significant changes in seawater isotopic composition during the Proterozoic reflect the transition from mantle dominated Archean oceans to a typically modern system. Isotopic and mass balance constraints from banded iron formations indicate that oxidation of hydrothermal Fe2+ in seawater could have been an important control on the redox state of Proterozoic ocean‐atmosphere system. The present isotopic balances of carbon and sulfur could have been decoupled as a result.