| Abstract: | The mid-Cretaceous (~120 to 80 Ma) witnessed some of the
warmest polar temperatures yet experienced by multi-cellular
life on Earth, repeated reef drowning in the tropics and a
series of "oceanic anoxic events" (OAEs) that promoted
widespread deposition of organic carbon (C-org)-rich marine
sediments and biotic turnover. The underlying cause of
mid-Cretaceous warmth is widely attributed to tectonically
driven increases in atmospheric levels of greenhouse gases
(e.g. carbon dioxide) while a wide range of competing
hypotheses seek to explain the climatic causes and effects
of OAEs.
Here, I present new stable isotope records from individual
species of remarkably well-preserved clay-hosted planktonic
and benthic foraminifera in western tropical Atlantic sites
(ODP 1052, Blake Nose and DSDP 144, Demerara Rise).
Stable isotope records from benthic foraminifera from ODP
Site 1052 provide new constraints on the problem of the
underlying cause of oceanic anoxia during the latest Albian.
Planktonic foraminifera of Turonian age from DSDP Site 144
yield the warmest equivalent oxygen isotope sea-surface
temperatures (SSTs) yet reported for the entire
Cretaceous-Cenozoic. These data (i) lend support to the
hypothesis of a "Cretaceous greenhouse" and (ii) strengthen
the case for a Turonian age for the Cretaceous thermal
maximum (KTM). At the same time however, these data
highlight a 20-40 m.y. mismatch between peak
Cretaceous-Cenozoic global warmth and peak inferred tectonic
CO2 production.
This mismatch is either an artifact of a "hidden" Turonian
pulse in global ocean-crust cycling or real evidence of the
influence of some other factor on atmospheric CO2 and/or
SSTs. A hidden pulse in crust cycling would explain the
timing of peak Cretaceous-Cenozoic sea level (also Turonian)
but other factors are needed to explain high-frequency
(~10-100 ka) instability in middle Cretaceous SSTs reported
previously (Wilson and Norris, 2001, Nature 412, p. 425-429). |