Variability in strontium and lithium composition of ancient seawater from fluid inclusions in haliteimplications for reconstructing drivers of seawater secular variability

Secular variations in the major ion chemistry and isotopic composition of seawater on multimillion-year time scales over the Phanerozoic are well documented, but the causes of these changes are debated. 7Li and 87Sr/86Sr are widely utilized to interpret the driving mechanisms of secular changes in seawater chemistry, the tectonic history of the Earth and the link between paleo-ocean chemistry and the carbon cycle. These interpretations and models, however, are based on (1) few quantitative data on strontium concentration SrSW in seawater calculated from the Sr/Ca ratios of marine carbonates and (2) the assumption that the Li concentration Lisw of seawater has been similar to modern Lisw. But those assumptions, if inaccurate, could undermine the validity of modeling results. The marine strontium and lithium cycles through time could be better reconstructed using coupled marine records of SrSW, 87Sr/86Sr, Lisw and 7Li. SrSW and Lisw in ancient seawater would be particularly useful for examining which global processes, continental weathering or global volcanicity at seafloor hydrothermal systems and subduction zones, exerted the dominant control on the changes in seawater chemistry. Recent analytical advances using combined cryo-SEM-EDS and laser ablation ICP-MS now allow quantitative measurement of SrFI and LiFI in fluid inclusions in halite. SrSW and Lisw, reconstructed from chemical analyses of >1,000 fluid inclusions in more than 100 halite samples with marine 87Sr/86Sr values, varied seven-ten-fold and oscillated twice between high- and low-Sr and Li concentrations over the past 550 million years, in rhythm with Ca-rich and SO4-poor paleoseawater intervals, calcite-aragonite seas, supercontinent breakup, dispersal, and assembly cycles, greenhouseicehouse climates, and modeled atmospheric pCO2. These data enable us to better constrain the Sr and Li cycle, and offer new insights into geochemical modeling of Phanerozoic seawater chemistry using multiple isotope systems and seawater concentrations.