Greenhouse World and the Mesozoic Ocean

Determination of Mesozoic Ocean Temperature HistoryAn important achievement of DSDP and ODP was the reconstruction of the history of Mesozoic ocean temperature changes based on geochemical methods such as oxygen isotopes, TEX 86 and alkenone analyses.Oxygen isotope data have provided the greatest source of paleotemperature reconstructions from ancient oceans.However, the increasing prevalence of diagenetic alteration in older or more deeply buried rocks limits or prevents reliable isotopic data from biogenic calcite preserved in terrestrial outcrops.Compared to many land-based sections, calcareous microfossils of Cretaceous age recovered from samples drilled at DSDP and ODP sites are often better preserved, and usually have not been as seriously affected by complex tectonic and/or weathering processes.Exquisitely preserved foraminifera from the low-latitude Demerara Rise (ODP Site 1258-1261; 4-15˚N), mid-latitude Blake Nose (ODP Sites 1049, 1050, 1052; 30˚N) and high-latitude Falkland Plateau (DSDP Site 511; 60˚S) have been especially useful for reconstructing vertical and latitudinal temperature gradients of the midthrough Late Cretaceous ocean (Figs. 2 & 3).The TEX 86 method is especially useful for organic carbon-rich sediments, and has provided excellent paleo-temperature determinations (e.g., Schouten et al., 2003;Jenkyns et al., 2004).According to isotopic records of surface dwelling planktic foraminifera, sea surface temperatures reached a maximum of 42˚C at Demerara Rise (Bice et al., 2006), 33˚C at Blake Nose (Huber et al., 2002), and 31˚C at Falkland Plateau (Huber et al., 2002;Bice et al., 2003) during the Turonian (Figs. 2 & 3).At comparable latitudes in the modern ocean, August surface water temperatures are 25-28˚C at 0-20˚N, 20-28˚C at 20-40˚N, and 0-5˚C at 60˚S (Thurman and Trujillo, 1999).These data consequently suggest that Cretaceous warming was most prominent at high latitudes where the difference of temperature between the mid-Cretaceous and the present oceans is nearly 30˚C (Fig. 2).Bice and Norris (2002) estimate that at least 4500 ppm CO 2 is required to match the above-mentioned maximum temperatures, which is >11 times the modern atmospheric concentration.Using a more recent climate model Bice et al. (2006) conclude that 3500