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

摘要

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 100My, 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 δ~(18)O_(PO4) for a collection of fish teeth and new data allow the reconstruction of the seawater Sr/Ca ratio evolution for the last 70My using a new thermometer based on the Sr/Ca ratio in fish teeth. Calculated Sr/Ca ratios decrease from ~14mmol·mol~(-1) at 70Ma to ~8mmol·mol~(-1) at 50Ma 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.