Strontium isotope profile of the early Toarcian (Jurassic) oceanic anoxic event, the duration of ammonite biozones, and belemnite palaeotemperatures

Abstract

We profile 87Sr/ 86Sr, δ 13C, δ 18O, Sr/Ca, Mg/Ca, and Na/Ca in belemnites through Pliensbachian and Toarcian strata on the Yorkshire coast, UK, which include the early Jurassic oceanic anoxic event. The 87Sr/ 86Sr profile shows that the relative duration of ammonite subzones differ by a factor of up to 30: the Lower Jurassic exaratum subzone is 30 times longer than the clevelandicum subzone because the exaratum subzone in Yorkshire, which contains the anoxic event, is condensed by a factor of between 6.5 and 12.2 times, relative to adjacent strata. Using our 87Sr/ 86Sr profile, the resolution in correlation and dating attainable in the interval is between ±1.5 m and ±15 m of section, and better than 0.25 Myr. In parts of the sequence, this stratigraphic resolution equals that attainable with ammonites. A new age model is provided for late Pliensbachian and early Toarcian time that is based on the 87Sr/ 86Sr profile. Through the sequence, the Sr/Ca, Mg/Ca, Na/Ca and δ 18O of belemnite carbonate covary, suggesting that elemental ratios may be useful for palaeotemperature measurement. Our δ 13C belemnite data splits into three the previously reported positive isotope excursion (to +6.5‰) in the early Toarcian. We speculate that the excursion(s) resulted from addition to surface waters of isotopically heavy CO 2 via ebullition of methanogenic CO 2 from the sediment during early burial of organic rich (>10% TOC) sediments