Timing and magnitude of early Aptian extreme warming:: Unraveling primary δ18O variation in indurated pelagic carbonates at Deep Sea Drilling Project Site 463, central Pacific Ocean

In order to elucidate early Aptian marine paleotemperature evolution across the period of enhanced organic carbon (C-org)-burial [Oceanic Anoxic Event (OAE) 1a], stable isotope analyses were performed on pelagic limestones at Deep Sea Drilling Project Site 463, central Pacific Ocean. The delta O-18 data exhibit a distinct anomaly by similar to-2 parts per thousand spanning the OAE la interval (i.e., a similar to 6 m-thick, phytoplanktonic C-org-rich unit constrained by magneto-, bio- and delta C-13 stratigraphy). Elucidation of paleotemperature significance of the delta O-18 shift is made by taking account of recent Sr/Ca evidence at the same section, which revealed that geochemical signals in carbonate-poor lithologies are relatively unaltered against burial diagenesis. By discriminating delta O-18 values from carbonate-poor samples (CaCO3 contents=5-30 wt.%), it appears that an abrupt rise in sea-surface temperatures (SSTs) by 8 degrees C (=-1.7 parts per thousand shift in delta O-18) occurred immediately before OAE la, whereas a cooling mode likely prevailed during the peak C-org-burial. In terms of its stratigraphic relationship as to the C-org-rich interval and to a pronounced negative delta C-13 excursion, as well as its timescale, the observed SST rise resembles those associated with the Paleocene-Eocene thermal maximum and, more strikingly, Jurassic Toarcian OAE. This observation is consistent with the hypothesis that these paleoenvironmental events were driven by a common causal mechanism, which was likely initiated by the greenhouse effect via massive release of CH4 or CO2 from the isotopically-light carbon reservoir and terminated by a negative productivity feedback.(C) 2008 Elsevier B.V. All rights reserved.