Variability in strontium and lithium composition of ancient seawater from fluid inclusions in halite—implications 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. δ⁷Li 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 [Sr]_SW in seawater calculated from the Sr/Ca ratios of marine carbonates and (2) the assumption that the Li concentration [Li]sw of seawater has been similar to modern [Li]sw. 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 [Sr]_SW, ⁸⁷Sr/⁸⁶Sr, [Li]sw and δ⁷Li. [Sr]_SW and [Li]sw 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 [Sr]_FI and [Li]_FI in fluid inclusions in halite. [Sr]_SW and [Li]sw, 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, greenhouse–icehouse 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.