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 b elemnites 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, w hich 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, th e resolution in correlation and dating attainable in the interval is betwee n +/- 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 amm onites. A new age model is provided for late Pliensbachian and early Toarci an 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, suggesti ng that elemental ratios may be useful for palaeotemperature measurement. O ur delta(13)C(belemnite) data splits into three the previously reported pos itive isotope excursion (to +6.5%(0)) in the early Toarcian. We speculate t hat the excursion(s) resulted from addition to surface waters of isotopical ly heavy CO2 via ebullition of methanogenic CO2 from the sediment during ea rly burial of organic rich (>10% TOC) sediments (C) 2000 Elsevier Science B .V. All rights reserved.