CHARACTERIZATION OF GEOCHEMICAL AND FOSSIL VARIABILITIES IN CENOMANIAN-TURONIAN SEDIMENTS OF DSDP LEG 43, SITE 386, BERMUDA RISE

Geochemical studies of Cenomanian strata at Site 386 reveal that TOC values range from 0.0% to 31.6%, with an average of 2.0%, and highest values occur at the stratigraphic level attributable to OAE 2 (Core 43). Levels rich in TOC contain predominantly marine planktonic organisms (radiolarians, nannoplankton) and tend to be isotopically lighter compared to most Tertiary samples. The d¹³C_org values for bulk organic matter range between -27.8 and -19.7‰ with an average of -25.3‰, showing a positive excursion of about 2‰ during OAE 2, preceded by a slight negative excursion.

XRD analyses of clay content imply both oceanic and continental provenances. The oceanic component of Cenomanian sediments is discrete smectite, whereas the associated illite-smectite indicates a continental origin. The highly expandable illite-smectite and the kaolinite of Turonian sediments indicate a continental origin from subtropical soils, while the clinoptilolite is of marine origin. The OAE 2 interval yields abundant microscopic quartz interpreted to be diagenetically generated from remobilized biogenic silica, which reprecipitated in pore waters with lower pH due to high content of organic matter.

Younger levels show systematic increase in the proportion of calcareous nannoplankton (50-75%) represented mostly by Watznaueria barnesae, a solution resistant taxon suggestive of dissolution effect. However, in the Early Cenomanian this species shows a negative correlation with high productivity species and a positive correlation with carbonate content, which indicates a fluctuating CCD. The elevated organic matter content is related to the overall decreased oxygen content in the Cretaceous greenhouse oceans, particularly during that time interval.