A wealth of fossil data (terrestrial and marine) from northern and southern high latitudes provide dramatic evidence that the Cretaceous was warm. However, these fossils provide incomplete snapshots of Cretaceous climate as their occurrences are discontinuous and their distributions do not provide a quantitative measure of Earth's temperature. A more continuous history of Earth's temperature change during the Cretaceous can be inferred from delta18-O ratios preserved in the Calcium Carbonate (CaCO3) shells of foraminifera. Unfortunately, interpreting paleotemperatures from delta18-O measurements is complicated by diagenesis and uncertainties regarding the delta18-O composition of contemporary seawater. Discovery of new sites that yield Cretaceous foraminifera with truly exceptional preservation has helped address diagenetic concerns, and parallel analyses of magnesium-calcium (Mg/Ca) ratios can help separate seawater and temperature effects on measured delta18-O values. This proposal requests funds to collect, isolate, and analyze exceptionally well preserved foraminifera from up to 1150 m of Late Cretaceous cores from four key stratigraphic intervals at four localities in coastal Tanzania. Results generated will greatly improve temporal and spatial control of the Cretaceous tropical temperature record. Such improvement is necessary to advance our understanding of the linkages between climate forcing mechanisms, ocean circulation, and patterns of biotic evolution and extinction in the Cretaceous greenhouse world. In combination with climate model experiments, this grant will test hypotheses regarding greenhouse climate dynamics including tropical temperature history, purported greenhouse glacial episodes, and the degree to which tropical and 'global' temperatures co-vary. This project focuses on 1) delta18-O analyses of exceptionally well-preserved foraminifera across Late Cretaceous intervals of extreme warmth and relative cold including glacial intervals, 2) parallel measurements of Mg/Ca in selected samples to help constrain changes in seawater composition, and 3) climate model experiments and data-model comparisons that will allow better evaluation of model sensitivity to Carbon Dioxide (CO2).
Broader Impacts: The project involves partners from the United States (Smithsonian Museum of Natural History, University of Missouri-Columbia, and Woods Hole Oceanographic Institution), the United Kingdom (Cardiff University, Bristol University, Southampton University, and University College London), the Republic of Ireland (Trinity College Dublin), and the Tanzania Petroleum Development Corporation. The project has held regular science meetings and has enhanced the involvement of Tanzanian scientists. Graduate and undergraduate students are involved in all phases of this research, and the analyses are done at an NSF-funded stable isotope facility. Local Tanzanians are hired to support many aspects of field work. An additional benefit is improvement in the knowledge of the structural geology and stratigraphy of the southeast coastal region of Tanzania, which is useful for Tanzanian environmental policy decisions including possible hydrocarbon potential and drilling of shallow water wells.
Over the course of three drilling seasons the Tanzania Drilling Project recovered over 1.7 km of core spanning most of the middle and Late Cretaceous (~120-65 Ma) as well as a brief interval in the early Paleocene (~63-62 Ma). Exceptional preservation is not present in all samples, but we have found this style of preservation across the entire Turonian (the time of peak Cretaceous warmth), as well as portions of the Cenomanian, the Coniacian through the Campanian (times of warming and cooling before and after the Turonian, respectively) and the early Paleocene. Our drilling also obtained stratigraphic intervals spanning the late Aptian-early Albian (~115-111 Ma) and the late Campanian-Maastrichtian (~74-66 Ma). Collaborating scientists come from eight countries and three continents. Much of our work to date has focused on sample collection and primary documentation and description, but efforts now have shifted more to hypothesis testing. For example, the Turonian record is well represented in part due to our concerted efforts to recover a complete sequence of the Cenomanian/Turonian (C/T) boundary interval (~93.5 Ma), which includes Oceanic Anoxic Event 2, one of the largest perturbations to the carbon cycle in the last 250 million years. While a number of aspects of the C/T interval in Tanzania are puzzling, the rest of the Turonian record seem to be relatively straightforward. Globally, the Turonian is the warmest portion of the Cretaceous greenhouse climate, but it includes proposed intervals of greenhouse glaciations. Our isotopic analyses of glassy foraminifera suggest that, in the western subtropical Indian Ocean, conditions were remarkably stable and warm over the entire age from ~ 93.5 Ma to 89 Ma. The results suggest both that (1) on geological timescales climate sensitivity to forcing from greenhouse gasses is at the high end of the range of estimates in the literature and that (2) the Tanzanian record provides no support for a Turonian glacial age. Excellent preservation has also allowed novel paleobiological and taxonomic studies as well as novel examination of geochemical signals. Evidence of an operculum and a wall-crystal architecture based on lath-shaped micro crystals suggests all studied Pithonelloideae, which are an unusual group of spherical to oval shaped calcareous microfossils, are dinoflagellate cysts. The measured oxygen isotopic values on the Pithonelloideae indicate a surface water habitat, whereas quite high carbon isotopic values suggest strong vital effect. Work is continuing to try to understand the source of the apparent fractionation. Similarly, because of the unusually complete nature of the assemblages recovered, we have been able to extend the range of an unusual benthic foraminifer Tubulogenerina ~40 million years and refine the definition of and systematic relationships among species of an aragonitic benthic foraminifer assigned to Colomia. Finally among other ongoing projects, an Italian Ph.D. student Francesca Falzoni has spent ~ 4 months of the past year in the States working on Campanian foraminiferal systematics at the Smithsonian and temporal and taxonomic isotopic trends at the University of Missouri whereas MU master’s student Shannon Haynes worked last summer with UK collaborator Dr. Richard Pancost to attempt to determine the source of unexpected C/T carbon isotopic results in bulk analyses using compound specific techniques. In short, we hope and think we have accomplished our goals of collecting a remarkably good sample set for the study of the Late Cretaceous greenhouse climate. We have documented our results well and are beginning to disseminate some of more provocative implications of analyses of these samples. We further hope that we will be successful in demonstrating the importance of continued work on the samples in the requests for ongoing support.
