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

We profile 87Sr/86Sr, δ13C, δ18O, 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 δ18O of belemnite carbonate covary, suggesting that elemental ratios may be useful for palaeotemperature measurement. Our δ13Cbelemnite 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 CO2 via ebullition of methanogenic CO2 from the sediment during early burial of organic rich (>10% TOC) sediments