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, det ailed seawater strontium isotope curve for the Early Jurassic. The pre servation 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 rat ios elevated relative to adjacent points on the curve. High Mn concent rations less often correlated with elevated Sr-87/Sr-86 ratios; howeve r, low Mn concentrations (<25 ppm Mn) were consistently associated wit h apparently well-preserved Sr-87/Sr-86. deltaC-13 and deltaO-18 prove d to be insensitive to diagenesis as it affects Sr-87/Sr-86. The princ ipal 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 inc rease in the late Pliensbachian, the curve falls rapidly to its Early Jurassic minimum of 0.70706. It then gently increases through the Toar cian until the falciferum zone, where it shows an apparently abrupt in crease to 0.70719 before continuing its slow increase to 0.70728 in th e Aalenian-Bajocian (Middle Jurassic). This reversal of the downwards trend established in the Sinemurian and Pliensbachian had not been pre viously identified. The Sinemurian and Pliensbachian section of the cu rve potentially allows correlation and dating to within 1 or 2 ammonit e subzones (+/-0.5 to 1 Ma).