Stable isotope analyzes on benthic foraminifera from the Late Cretaceous (Turonian - Santonian) of Demerara Rise (ODP Leg 207) and comparison with the Pacific Ocean
Final Report Abstract
Benthic foraminiferal stable isotopes and benthic as well as planktic foraminiferal assemblages of the Turonian to earliest Campanian have been investigated to reconstruct paleoclimatic and paleoceanographic changes in the tropical Atlantic Ocean (ODP Leg 207, Demerara Rise). Moreover, the aim of this study was to establish a continuous benthic foraminiferal stable isotope record of the mid- to Late Cretaceous. Therefore, stable isotopes were also analyzed from tropical Pacific DSDP Sites 305 and 463. In general the obtained results show that especially the Late Cretaceous bottom-water environment in the tropical Atlantic was much more variable than previously thought. Most significant results of the performed study are: (1) Establishment of a benthic foraminiferal stable isotope compilation for the last 50 Ma of the Cretaceous (Albian-Late Maastrichtian, 115-65 Ma). This compilation shows that Cretaceous deep-water temperatures were significantly warmer than today (>20C) and can be separated into four intervals. The first two intervals are characterized by the Cretaceous thermal maximum, massive storage of organic carbon in sediments, and significant formation of warm and saline deep-water formation in low-latitudinal epicontinental seas. The following interval (Late Turonian-mid Campanian) shows decreasing oxygen isotope values in the South and North Atlantic basins, interpreted to be the result of a beginning opening of the Equatorial Atlantic Gateway. A full connection of all ocean basins is proposed for the last 13 Ma of the Cretaceous based on similar carbon and oxygen isotope values on a global scale. (2) Stable isotope records of well-preserved planktic foraminifera and the TEX86 proxy from the tropical Atlantic show that the Turonian (93.5-89.3 Million years ago) presents one of the warmest periods of the Phanerozoic eon with tropical sea-surface temperatures >35°C (and therefore much warmer than today). A combination of these data with benthic foraminiferal oxygen isotopes show synchronous shifts in the chemistry of both the surface ocean and deep sea, proposed to reflect a ~200,000 year period of glaciation ~91.2 Million years ago during the middle Turonian. Performed calculations of the possible size of this ice sheet implies that this short lived period of glaciation built glaciers of at least half the size of the modern Antarctic ice cap. This study shows that even the extreme warmth of the Cretaceous super-greenhouse climate was not a barrier to the formation of large ice sheets, calling into question the common assumption that the poles were always ice free during past periods of intense global warming. (3) High resolution investigations of planktic and benthic foraminiferal assemblages of the Turonian to Coniacian show that there is no relation between the observed deflection in the stable isotope data and foraminiferal abundance. Instead, planktic foraminiferal faunas are characterized by an uninterrupted 400 kyr cyclicity, interpreted to reflect fluctuations in the mean latitudinal position of the Late Cretaceous Intertropical Convergence Zone and associated fluctuations in precipitation and trade wind strength.
Publications
- (2006): Cenomanian/Turonian benthic foraminiferal faunas of the Demerara Rise depth transect (ODP Leg 207). FORAMS 2006, Natal, Brasil
Friedrich, O., Erbacher, J. & Mutterlose, J.
- (2006): Stable isotope composition of Cretaceous benthic foraminifera: Biological and environmental effects. FORAMS 2006, Natal, Brasil
Friedrich, O., Schmiedl, G. & Erlenkeuser, H.
