The Relationship Between 87Sr/86Sr and Seawater Chemistry

When compared with sea level curves, which are thought to reflect changes in the volume of mid-ocean ridge spreading (through increased rates or increased length), there is a notable offset with ⁸⁷Sr/⁸⁶Sr. By understanding this offset we should be able to better understand how plate tectonics has controlled ocean chemistry through time. To first order, marine ⁸⁷Sr/⁸⁶Sr is high when Mg concentrations are high and low when Mg concentrations are low. This is consistent with radiogenic ⁸⁷Sr/⁸⁶Sr's and Mg concentrations being derived from continental weathering, whereas hydrothermal alteration produces unradiogenic ⁸⁷Sr/⁸⁶Sr and is a significant sink for Mg. Moreover, Sr and Ca concentrations are high when ⁸⁷Sr/⁸⁶Sr is high and low when ⁸⁷Sr/⁸⁶Sr is low. Ca is derived from continental weathering and has a significant contribution from hydrothermal alteration of ocean crust; Sr appears to be derived solely from the continents (based on the observation that [Sr] in hydrothermal fluids are unchanged from seawater). We use the differences in sources and sinks of Mg, Ca, and Sr, as well as constraints imposed by the ⁸⁷Sr/⁸⁶Sr ratios, to investigate the relative roles of continental run-off versus hydrothermal alteration of ocean basalts in affecting seawater chemistry through time.