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

We profile Sr-87/Sr-86,delta(13)C, delta(18)O, 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 Sr-87/Sr-86 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 Sr-87/Sr-86 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 lime that is based on the Sr-87/Sr-86 profile. Through the sequence, the Sr/Ca, Mg/Ca, Na/Ca and delta(18)O of belemnite carbonate covary, suggesting that elemental ratios may be useful for palaeotemperature measurement. Our delta(13)C(belemnite) data splits into three the previously reported positive isotope excursion (to +6.5%(0)) 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 (C) 2000 Elsevier Science B.V. All rights reserved.