ISOTOPIC (C, O, ⁸⁷SR/⁸⁶SR) AND TRACE ELEMENT (NA, MG, SR) TRENDS THROUGH TIME IN LATE CRETACEOUS BELEMNITES: PROXIES FOR TEMPERATURE AND ICE-VOLUME?

MCARTHUR, John M., Earth Sciences, UCL, Gower Street, London, WC1E6BT, JANSSEN, Nico N., Geertekerkhof 14bis, Utrecht, 3511 XC, THIRLWALL, Matthew F., Dept Geology, RHUL, Egham Hill, Egham, TW20 0EX and LENG, Melanie J., NERC Isotope Geoscience Laboratory, British Geological Survey, Keyworth, Nottingham, NG12 5GG, j.mcarthur@ucl.ac.uk

New ⁸⁷Sr/⁸⁶Sr data from Lower Cretaceous belemnites of south-western France (Vocontian Basin) provide a global standard for Sr-isotopic dating in the interval. Values of ⁸⁷Sr/⁸⁶Sr increase from 0.707208 ± 0.000 006, at the Tithonian/Berriasian boundary, through 0.707295 ± 0.000 002 (2 s.e.) at the Berriasian/Valanginian boundary, 0.707389 ± 0.000 005 (2 s.e.) at the Valanginian/Hauterivian boundary, and thereafter rise through the Hauterivian. A plateau in ⁸⁷Sr/⁸⁶Sr through the upper Verrucosum and Peregrinus ammonite zones (midle-to upper Valanginian) prevents precise dating or correlation using ⁸⁷Sr/⁸⁶Sr in this interval.

Values of δ¹³C in belemnites scatter greatly but still define the well-known positive C isotope excursion in Valanginian times of around 2.5 . In Duvalia binerva, the Vocontian excursion in δ¹³C is accompanied by an excursion of around 0.4 in δ¹⁸O to heavier values without a change in Mg/Ca. Assuming that both Mg/Ca and δ¹⁸O reflect temperature, whilst δ¹⁸O additionally reflects ice-volume, these proxies are tentatively interpreted as showing that the excursion in δ¹⁸O represents the formation and decay of ice-caps in late Valanginian times of a mass about one-third of those found today.

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Earth System Processes 2 (8–11 August 2005)

ISOTOPIC (C, O, 87SR/86SR) AND TRACE ELEMENT (NA, MG, SR) TRENDS THROUGH TIME IN LATE CRETACEOUS BELEMNITES: PROXIES FOR TEMPERATURE AND ICE-VOLUME?

ISOTOPIC (C, O, ⁸⁷SR/⁸⁶SR) AND TRACE ELEMENT (NA, MG, SR) TRENDS THROUGH TIME IN LATE CRETACEOUS BELEMNITES: PROXIES FOR TEMPERATURE AND ICE-VOLUME?