Revised Middle-Upper Jurassic strontium isotope stratigraphy

A study is conducted to supplement the uppermost Lower Jurassic-lowermost Cretaceous marine strontium isotope dataset and to present new statistical fits of the Middle-Late Jurassic seawater strontium isotope curve based on a numerical time scale and a detailed biostratigraphical zonal scheme. The use of the stratigraphical scheme allows reduction of dating errors related to uncertainty of numerical age determinations. The presented correlation charts enable direct calibration between strontium isotope stratigraphy and regional biostratigraphical frameworks. New strontium isotope data have been obtained from well-preserved Lower Bajocian, uppermost Callovian, Oxfordian, Kimmeridgian, and Upper Volgian belemnite rostra. The presented seawater Sr-87/Sr-86 curve is characterized by reliable 95% confidence limits (mean width of +/- 0.000007), which take into account precision of dating of particular data points. A decrease of seawater Sr-87/Sr-86 ratio, from ca. 0.70730 to ca. 0.70683, is observed throughout the Middle Aalenian-Early Oxfordian (172.1-160.8 Ma ago). The steepest Phanerozoic fall of the ratio, with a rate of change of up to 0.00015 per 1 Ma, is recorded in the Bajocian segment of the strontium isotope curve. The Phanerozoic minimum of the marine Sr-87/Sr-86 ratio occurred at the Early-Middle Oxfordian transition. The Sr-87/Sr-86 ratio increased starting from the Middle Oxfordian till the end of the Jurassic (160.8-145.7 Ma) reaching a value of ca. 0.70720 at the Jurassic-Cretaceous transition. The Middle-earliest Late Jurassic decrease in seawater Sr-87/Sr-86 ratio is interpreted as a result of the increased hydrothermal activity of the seafloor during the breakup of Gondwana, and the formation of new Atlantic-Tethyan oceanic basins. The successive rise of the ratio is probably related to the decelerated hydrothermal venting of the oceanic crust although a partial increase in radiogenic strontium input from continental weathering cannot be excluded.