The Jurassic Period (ca. 201–145 Ma) is marked by fundamental reorganizations of paleogeography, paleoceanography, ecosystems, and the progressive shift from aragonite to calcite as the favored marine biogenic carbonate polymorph. Sr/Ca ratios of well-preserved Jurassic oysters and belemnites from sections in the UK and Poland demonstrate that the Sr/Ca ratio of seawater varied systematically throughout the Early and Middle Jurassic in parallel with already documented seawater 87Sr/86Sr. The Sr flux from increased mid-ocean ridge activity in the Early Jurassic outbalanced the input of riverine Sr, leading to gradually lower seawater 87Sr/86Sr associated with the parallel and strong decrease in seawater Sr/Ca ratios. This downward trend was reversed by enhanced continental input in the Toarcian and Aalenian, but resumed in the Bajocian and continued to the Callovian. Parallel changes of seawater 87Sr/86Sr and Sr/Ca suggest a common cause for these long-term variations and are best explained by changes in the balance of continental weathering and hydrothermal fluxes. These findings underline the strong control of global tectonic processes on the evolution of biomineralization and downplay the role of biomineral evolution in influencing strontium chemistry of seawater in the mid-Mesozoic.

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