STRONTIUM ISOTOPES IN EARLY JURASSIC SEAWATER

The analysis of well-preserved, well-dated belemnites and oysters from the Jurassic of Great Britain has resulted in a well-constrained, detailed seawater strontium isotope curve for the Early Jurassic. The preservation of fossil low-Mg calcite was monitored using Mn, Fe, deltaC-13, and deltaO-18. Iron was the most useful indicator, with about 75% of the samples containing more than 150 ppm Fe showing Sr-87/Sr-86 ratios elevated relative to adjacent points on the curve. High Mn concentrations less often correlated with elevated Sr-87/Sr-86 ratios; however, low Mn concentrations (<25 ppm Mn) were consistently associated with apparently well-preserved Sr-87/Sr-86. deltaC-13 and deltaO-18 proved to be insensitive to diagenesis as it affects Sr-87/Sr-86. The principal features of the strontium isotope curve include a rise to about 0.70772 in the latest Triassic and earliest Jurassic (Hettangian). From the Hettangian, the curve begins a roughly linear descent through the Sinemurian and Pliensbachian. Following a small levelling off and increase in the late Pliensbachian, the curve falls rapidly to its Early Jurassic minimum of 0.70706. It then gently increases through the Toarcian until the falciferum zone, where it shows an apparently abrupt increase to 0.70719 before continuing its slow increase to 0.70728 in the Aalenian-Bajocian (Middle Jurassic). This reversal of the downwards trend established in the Sinemurian and Pliensbachian had not been previously identified. The Sinemurian and Pliensbachian section of the curve potentially allows correlation and dating to within 1 or 2 ammonite subzones (+/-0.5 to 1 Ma).