Palaeotemperatures, polar ice-volume, and isotope stratigraphy (Mg/Ca, δ18O, δ13C, 87Sr/86Sr):: The Early Cretaceous (Berriasian, Valanginian, Hauterivian)

Temporal trends through Early Cretaceous time of delta C-13, delta O-18, Mg/Ca, and Sr-87/Sr-86 in calcite, and delta O-18 in seawater, are discussed using belemnites from SE France and SE Spain. Both positive and negative excursions in delta C-13(c) are seen in the Berriasian-Hauterivian interval, but none appear to be connected to Parana-Etendeka volcanism, and none can be tied convincingly to changes in sea level. Negative excursions to -2%o in delta C-13(c) occur in the Upper Berriasian and in the Lower Valanginian. Small positive excursions in delta C-13(c) occur in the uppermost Valanginian (upper C. furcillata Zone) and uppermost Hauterivian (B. balearis/P. ohmi Zones). A major positive excursion in delta C-13(c) in the Valanginian rises to + 1.5%o through the upper K. biassalense Subzone (upper B. campylotoxus ammonite Zone of the Lower Valanginian), which correlates to Chron M11 An. 1n., and continues through the S. verrucosum Zone (Upper Valanginian). Extrapolation from carbon-isotope correlations of the onset of this excursion shows that the base of the Hauterivian (F.A. of Acanthodiscus ammonite genus) coincides with the base of Chron M10n and has a numerical age of 133.9 Ma. In Berriasian, Lower Valanginian and Upper Hauterivian belemnites, 6180, is mostly negative (around -0.3%0, three-point mean) but becomes positive (up to +0.4%o, three-point mean) in the Upper Valanginian and Lower Hauterivian before returning to negative values in the Upper Hauterivian. The transition from negative to positive values, through the S. verrucosum Zone, is accompanied by a 30% decrease in Mg/Ca in belemnite calcite, confirming that the trend in delta O-18(c) represents mostly cooling. The trend of delta O-18(SW), computed from Mg/Ca and delta O-18(c), lags trends in Ca/Mg and delta O-18(c) and becomes around 0.8%o more positive through the Upper Valanginian and Lower Hauterivian in response, we postulate, to the formation of substantial amounts of polar ice after a period of global cooling. By Late Hauterivian times, temperature proxies (6180, and Mg/Ca) show substantial warming had occurred and delta O-18(SW) had returned to less positive values, presumably as a result of waning ice-volume. Sea level lowstands of up to 90 m, reported to occur in the Late Berriasian and Early Valanginian, are not recorded in our delta O-18(c) or Mg/Ca data, so they were either not real or were tectonic in origin. Values of Sr-87/Sr-86 in seawater rose monotonically by 0.000294 through Berriasian, Valanginian and Hauterivian time, except in Late Valanginian time, when a plateau in Sr-87/Sr-86 occurred. Through extrapolation, the value of Sr-87/Sr-86 is estimated to be 0.707180 +/- 0.000010 at the base of the Berriasian and 0.707474 +/- 0.000010 at the base of the Barremian; it is fixed by regression analysis to be 0.707294 +/- 0.000005 at the base of the Valanginian and 0.707383 +/- 0.000005 at the base of the Hauterivian. (C) 2007 Published by Elsevier B.V.