| Abstract: | New and previously published oxygen-isotope data, derived from calcareous deep-sea sedimentary components recovered by the Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP), are presented in the context of long-term mid- to Late Cretaceous (Aptian to Maastrichtian) temperature change. Oxygen-isotope records include data from the Southern Hemisphere (ODP Holes 762C, 763B and 766A cored on Exmouth Plateau, off western Australia), equatorial Pacific Ocean (DSDP Site 463, Mid-Pacific Mountains) and Northern Hemisphere (Italian Scaglia and English Chalk outcrop sections). Interpolation of the majority of these isotopic data onto a numerical timescale, using independent magnetostratigraphic and biostratigraphic datums, facilitates direct correlation between different localities. Furthermore, comparison to other palaeoclimate data sets indicates that globally synchronous climate variability occurred on both hemispheres of the Cretaceous globe during Aptian to Maastrichtian time.
A marked decrease in oxygen-isotope ratios occurred across the Aptian-Albian transition, a change that is conventionally interpreted as a marked warming event. However, a preliminary application of Mg/Ca palaeothermometry suggests that the Aptian-Albian oxygen-isotope record could in fact reflect massive input of oxygen-16 into the global oceans, most likely in relation to a deglaciation episode. Intriguingly, this latter interpretation may help to reconcile the glendonite and dropstone evidence for a cool, or possibly even sub-freezing, late Aptian climate with the purported mid-Cretaceous 'greenhouse' world. Subsequent to the Aptian-Albian isotopic transition, steady long-term warming occurred until Albian-Cenomanian boundary time, with cooling post-dating the earliest Cenomanian warm event, and a climate optimum attained sometime during Cenomanian-Turonian boundary time. The most rapid climate change, a marked cooling of climate, occurred between the mid- and late Turonian, and is consistent only broadly with an hypothesis that sedimentary organic-carbon burial during the Cenomanian-Turonian boundary Ocean Anoxic Event sequestered atmospheric carbon dioxide and initiated a reverse greenhouse effect. Subsequently, temperatures increased, or at the very least remained constant, during Coniacian and Santonian time, before continually decreasing through the Campanian and Maastrichtian; the latter long-term deterioration of climate apparently slowed during the middle Campanian.
Initial comparison of oxygen-isotope data to available geological atmospheric carbon-dioxide proxies (sea-floor spreading and oceanic plateaux production rates, strontium isotopes, and continental land area and a sea level curve) indicates an inconsistent relationship between a greenhouse-forcing mechanism and mid- to Late Cretaceous climate evolution. For example, the late Aptian is associated with the highest crustal-production rates, and hence greatest inferred volcanic out-gassing of carbon dioxide, during the mid- to Late Cretaceous, but paradoxically is associated with oxygen-isotope and sedimentological (glendonite and dropstone occurrence) evidence for cool, and possibly even sub-freezing, palaeotemperatures. Consequently, additional research is required in order to understand more fully those climate-forcing mechanisms that resulted in the long-term evolution of climate during the enigmatic mid- to Late Cretaceous geological interval. |