A normalised seawater strontium isotope curve: possible implications for Neoproterozoic-Cambrian weathering rates and the further oxygenation of the Earth

Abstract

The strontium isotope composition of seawater has varied over geological time in re-sponse to changes in the rates of continental weathering relative to ocean crust alter-ation. However, the potential of the seawater 87 Sr/ 86 Sr curve to trace globally inte-grated chemical weathering rates has not been fully realised because ocean 87 Sr/ 86 Sr 5 is also strongly influenced by the isotopic evolution of Sr sources to the ocean. A first at-tempt is made here to normalise the seawater 87 Sr/ 86 Sr curve to plausible trends in the 87 Sr/ 86 Sr ratios of the three major Sr sources: carbonate dissolution, silicate weath-ering and submarine hydrothermal exchange. The normalised curve highlights the Neoproterozoic-Cambrian interval as a period of exceptionally high chemical weath-10 ering rates, which can be linked to increased nutrient availability, bioproductivity and oxygenation of Earth's surface environment. Use of normalised seawater 87 Sr/ 86 Sr curves will, it is hoped, help to improve future geochemical models of Earth System dynamics.