17 GeoSystems and CHRONOS: Probing Earth’s Deep-Time Climate and Linked Systems

Tuesday, 9 August 2005
1:30 PM - 4:30 PM

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

Paper number: 17-7
Program: THEME SESSIONS
Time: 3:30 PM – 3:50 PM

Authors

Abstract

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.