Albian paleoceanography and paleotemperatures of the tropical western Atlantic (ODP Leg 207, Demerara Rise) - the onset of the mid-Cretaceous super-greenhouse climate.
Zusammenfassung der Projektergebnisse
Research on organic-rich sediments at Demerara Rise (ODP Leg 207, tropical Atlantic Ocean) mainly focussed on the Cenomanian to Santonian interval. Hardly any research has been done, however, on the organic-rich Albian succession at Demerara Rise; and only few datasets exist for the Albian that reconstruct SST and bottom-water paleotemperatures by using stable oxygen isotopes of well-preserved benthic and planktic foraminifers. Therefore, this proposal aimed to investigate the continental slope sequence of Albian organic-rich clays recovered during Leg 207 (with a special focus on OAE 1d) by using stable isotopes of pristinely preserved foraminifera as well as assemblage counts. Unfortunately, the chemostratigraphic signature of OAE 1d wasn’t found in Site 1258 samples, identifying a sedimentation gap. Therefore the project was shifted towards the establishment of a long-term Albian SST and bottom-water record. While measuring the bulk of samples for foraminiferal stable isotopes, however, the mass spectrometer had a software problem leading to a total loss of most samples. Therefore, only sparse foraminiferal isotope data are available that were measured in a BSc thesis and as test samples. Due to this loss of data, the entire project was again shifted towards planktic foraminiferal assemblage counts to gain information about cyclic changes in surface-water hydrography. Stable isotope data show bottom-water temperatures that are in the range of 20 to 26 °C and therefore comparable to the high paleotemperatures that are shown for the Cenomanian. This suggests that Albian bottom-water formation was triggered by the same mechanisms as proposed for the younger intervals of the Cenomanian, namely the formation of warm saline bottom waters. Planktic foraminiferal oxygen isotope data suggest Albian SST of 25 to 32 °C that are slightly lower than the few TEX86 data (33 and 34 °C) that exist so far. Based on the planktic foraminiferal assemblages, the Albian record at Demerara Rise can be subdivided into three intervals. Intervals 1 and 3 are characterized by a “typical” mid- Cretaceous planktic foraminiferal assemblage reflecting a deep and/or weak OMZ at Demerara Rise. Interval 2, in contrast, is characterized by high abundances of clavate forms and H. pulchra, suggesting a strong and/or shallow OMZ that reaches within the depth habitat of these planktic foraminiferal species. The cyclic pattern observed in the foraminiferal assemblages within Interval 2 is interpreted to reflect changes in the strength and/or depth of the OMZ triggered by Milancovitch cyclicity. This interpretation is based on similar observations from younger sediments at Demerara Rise. For the Albian record presented here, it can be assumed that the observed cyclicity during Interval 2 is triggered by the same effects, suggesting the same mechanisms for the development and strength of the OMZ in the tropical Atlantic.
