| Title: | Mid and Late Cretaceous SST and CO2 from Exceptionally Well-Preserved Planktic Foraminifera |
| Author: | Richard D Norris |
| Date Submitted: | 05/02/2002 |
| Address: | MS-23
Woods Hole
MA
USA
02543-1541
|
| Phone: | 508-389-2839 |
| Email: | RNorris@whoi.edu |
| Co-Authors: | Bice, Karen, Woods Hole Oceanographic Institution, kbice@whoi.edu; Wilson, Paul, Southampton Oceanography Centre, paw1@mail.soc.soton.ac.uk; Huber, Brian, Smithsonian Institution of Natural History, huber.brian@nmnh.si.edu |
| Affiliation: | Woods Hole Oceanographic Institution |
| | |
| Abstract URL: | http://cis.whoi.edu/science/GG/ccod/viewAbstracts.cfm?RefNumber=19725624 |
| Keywords: | Sea surface temperature, Carbon dioxide, Cenomanian, Turonian, Planktic foraminifera, Stable isotopes, tropics |
| Abstract: | Paleotemperature proxies such as the biogeography of plants and vertebrates as well as stable isotope records from marine carbonates demonstrate that the Cenomanian-Turonian interval records the warmest high latitude climates of the past ~200 million years. Curiously, most published tropical records of sea surface temperature (SST) are no warmer than today or are even cooler. Yet Late Cretaceous climate models that take into account the generally higher concentration of greenhouse gases and modern-like heat transport suggest that the tropics should be warmer and the high latitudes should be cooler than is indicated by most proxy temperature estimates. We have addressed this apparent paradox by analyzing exceptionally well-preserved foraminifera with glassy skeletons and primary shell textures. Our data sets now include analyses of four different sites spanning 30°N to 60°S. In all cases our data show oxygen isotope ratios that are more negative than analyses from chalk-hosted foraminifera at similar latitudes.
We used foraminifera preserved in claystone that have glassy shells similar to foraminifera caught in plankton tows. In some cases, the excellent foraminifer preservation is supported by the presence in the same strata of ammonites with aragonitic shells. Isotopic analysis of different species from closely sampled sequences has allowed us to recognize species with near-surface and thermocline habitats as well as to assess the variability in inferred temperatures. Species believed to be surface-dwellers based on their isotopic signature yield  of ~-3.5 to -4  in the Albian and early Cenomanian of ODP Sites 1050 and 1052; -3.9 to -4 in the late Cenomanian of DSDP 144, -4.2 to -4.5 in the late Turonian of DSDP 144, and -3.9 to -4.6 in the late Turonian of DSDP 511.
SST determinations depend upon a variety of assumptions about the isotopic composition of Cretaceous seawater. We must consider the effects of runoff-enriched slope water. The preservation of foraminifera in clay is consistent with significant runoff as is the tendency for many of the foraminifera to be unusually small. However, isotopic analysis of modern foraminifera across a depth transect (30 m to >1000 m) on the east Texas shelf and slope reveals no onshore-offshore trends in despite large changes in average shell size and species diversity. Small errors in paleolatitude determination can also be magnified in tropical sites where the for seawater can differ substantially inside and outside the Intertroipcal Convergence Zone. Recent estimates of paleo-pH have suggested that tropical temperatures may be underestimated by a few degrees owning to the high pH associated with high pCO2 in the mid Cretaceous. Conversely, calculations of the amount of low temperature diagenesis in Cretaceous basalts suggest that Cretaceous seawater may have had less 18O than today, causing paleotemperature estimates to be too high by about two degrees.
Conservative estimates of SST from our data, assuming an ice-free planet and the global mean of seawater (-1 ), suggest temperatures ranging from ~28-30 ± 2°C. More realistic estimates based on allowance for variation in evaporation and precipitation with latitude suggest temperatures between ~30-33 ± 2°C. Tropical temperatures might have been even higher (up to ~36°C) if we allow for higher surface ocean pH associated with generally higher pCO2 in the Cretaceous. The data also reinforce published evidence for a remarkably low thermal gradient between the Turonian tropics and the high southern latitudes of perhaps only 6°C, compared to the modern gradient of ~24-28°C. These SST estimates also imply that atmospheric CO2 must have been very high (~6500-7500 ppm) during parts of the early Late Cretaceous to sustain both high tropical SST and warm southern latitudes. |
|