Reconstructing seawater Sr/Ca during the last 70 My using fossil fish tooth enamel

The chemical composition of past oceans records the combined effects of the evolution of life, climate changes and solid Earth dynamics. Variations in the rate of hydrothermal alteration of the oceanic crust, continental weathering input and burial of marine carbonates are thought to drive the seawater Sr/Ca ratio. Several methods exist for reconstructing past seawater Sr/Ca ratio which are based on the partitioning of Sr between biogenic and inorganic carbonates, and seawater. A compilation of reconstructed seawater Sr/Ca ratios shows that results at variance with each other are obtained for the last 100 My, leaving the question of the Sr/Ca composition of seawater over this period unresolved. Here, a new method for reconstructing the Sr/Ca ratio of past seawater based on the partitioning of Sr between tooth apatite of fossil fish enamel and seawater is proposed. Previously reported values of Sr/Ca and δ18OPO4 for a collection of fish teeth and new data allow the reconstruction of the seawater Sr/Ca ratio evolution for the last 70 My using a new thermometer based on the Sr/Ca ratio in fish teeth. Calculated Sr/Ca ratios decrease from ~ 14 mmol·mol− 1 at 70 Ma to ~ 8 mmol·mol− 1 at 50 Ma and further increase to present day values during the Pliocene. The results are in agreement with values calculated from Cenozoic benthic foraminifera obtained from ODP and DSDP sites. Once the biological offsets of the partitioning of Sr between apatite and water are determined, fossil fish tooth bioapatite represents a material of choice for reconstructing the past seawater Sr/Ca considering its better resistance to diagenetic alteration than for calcite and aragonite.

Research highlights
► Hydrothermal alteration, continental weathering and carbonate burial drive the seawater strontium/calcium (Sr/Ca) ratio.
► The Sr/Ca ratio in fish teeth records both the temperature and the ambient water Sr/Ca ratio.
► The seawater Sr/Ca ratio for the last 70 millions years is reconstructed using Sr/Ca and δ18O measured in fossil fish teeth.
► The evolution of the seawater Sr/Ca ratio is in agreement with data from foraminifera but not from corals and calcite veins.