Strontium isotope variations in Middle Jurassic (Late Bajocian–Callovian) seawater: Implications for Earthʹs tectonic activity and marine environments

Strontium isotope ratios of well-preserved belemnite rostra and ammonite shells are presented from stratigraphically well-dated Upper Bajocian–Callovian sediments of the Polish Jura Chain in central Poland. Nekto-benthic belemnites show a narrow trend of changes of 87Sr/86Sr ratio that are interpreted to record secular variations in the strontium isotope composition of seawater. A new fit of the seawater strontium isotope curve for the Late Bajocian–Callovian is presented based on current and literature data. The data show a gradual decrease of seawater 87Sr/86Sr ratio throughout the Late Bajocian–earliest Middle Callovian, lowest 87Sr/86Sr ratios (0.706850) and the flat minimum of the curve were reached in the latest Middle and the Late Callovian. The Middle Jurassic decrease in seawater 87Sr/86Sr ratio is interpreted to have been a result of the increased volcanic activity of the seafloor. Studied nektonic ammonites are considered to have lived in surface or shallow near-coastal waters of the Polish Jura Chain basin and been affected by the freshwater influx. A slight enrichment of ammonite shells in 87Sr isotope is linked to a change in strontium isotope composition of the surface or near-coastal waters due to the riverine strontium input.