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

The strontium isotope composition of seawater is strongly influenced on geological time scales by changes in the rates of continental weathering relative to ocean crust alteration. However, the potential of the seawater ⁸⁷Sr/⁸⁶Sr curve to trace globally integrated chemical weathering rates has not been fully realised because ocean ⁸⁷Sr/⁸⁶Sr is also influenced by the isotopic evolution of Sr sources to the ocean. A preliminary attempt is made here to normalise the seawater ⁸⁷Sr/⁸⁶Sr curve to plausible trends in the ⁸⁷Sr/⁸⁶Sr ratios of the three major Sr sources: carbonate dissolution, silicate weathering and submarine hydrothermal exchange. The normalised curve highlights the Neoproterozoic-Phanerozoic transition as a period of exceptionally high continental influence, indicating that this interval was characterised by a transient increase in global weathering rates and/or by the weathering of unusually radiogenic crustal rocks. Close correlation between the normalised ⁸⁷Sr/⁸⁶Sr curve, a published seawater ?³⁴S curve and atmospheric pCO₂ models is used here to argue that elevated chemical weathering rates were a major contributing factor to the steep rise in seawater ⁸⁷Sr/⁸⁶Sr from 650 Ma to 500 Ma. Elevated weathering rates during the Neoproterozoic-Cambrian interval led to increased nutrient availability, organic burial and to the further oxygenation of Earth's surface environment. Use of normalised seawater ⁸⁷Sr/⁸⁶Sr curves will, it is hoped, help to improve future geochemical models of Earth System dynamics.