Ocean Drilling Program, United States

A principal objective of Leg 207 was to recover relatively expanded, shallowly buried Cretaceous and Paleogene sediments from Demerara Rise off Suriname, South America, that could be used for paleoceanographic study of the tropical Atlantic. This period of the Earth's history involved episodes of ocean anoxia, rapid climate change, mass extinction, and opening of the Equatorial Atlantic Gateway. Five sites were drilled in a depth transect from 3200 to 1900 meters below sea level (mbsl) (modern water depth), which resulted in recovery of multiple sequences of Cenomanian-Turonian black shales, Campanian-Maastrichtian chalk, and Paleocene, lower Eocene, and middle Eocene chalk. In aggregate, the recovered sections form a continuous record of tropical sedimentation from the late early Cenomanian ( approximately 98 Ma) to the Santonian ( approximately 83.5 Ma) and from the late Campanian ( approximately 76 Ma) to the late middle Eocene ( approximately 38 Ma). The oldest sedimentary rocks recovered during Leg 207 are lower and middle Albian claystones (Site 1258) and the youngest are Pliocene-Pleistocene clay-rich nannofossil oozes (Site 1261). In total, approximately 650 m of black shales were recovered. Critical intervals recovered include multiple copies of oceanic anoxic events (OAEs) 2 and 3. OAEs result from major shifts in ocean circulation patterns and represent significant perturbations in the global carbon cycle, with massive deposition of organic carbon in marine environments. They are hypothesized to have played a major role in the evolution of Earth's climatic and biotic history. The entire sequence of black shale sediments has a cyclical overprint of organic matter-rich black shale alternating with laminated foraminiferal packstone and occasional glauconitic bioturbated intervals. These alternations reflect varying levels of bottom water dysoxia and surface water productivity and may show Milankovitch periodicities. Interstitial water chemical analyses show that approximately 90 m.y. after deposition of the black shales these sediments continue to act as a bioreactor that dominates organic matter degradation via sulfate reduction and methanogenesis. The other prominent feature seen in the Leg 207 pore water is the presence of a brine at three sites characterized by chloride concentrations >60% higher than standard seawater. Data suggest that the shales act as an aquifer for the brines at these sites. Six copies of the Cretaceous/Tertiary (K/T) boundary were recovered from three sites. Each of these cores contains a 1.7- to 1.9-cm-thick graded spherule ejecta layer, presumably resulting from fallout of the meteorite impact and representing the first discrete ejecta layer found on the South American craton. The interval is accompanied by the disappearance of many species of microfossils and a bloom in new species following the event. Strong sediment physical property contrasts around the K/T boundary make this event a prominent reflection horizon in seismic profile that is correlated from site to site and throughout the study area. The Paleocene/Eocene (P/E) boundary was recovered at all five sites, with 10 cores spanning the boundary interval. The P/E boundary was a period of significant and rapid global warming (5 degrees -7 degrees at the poles), mass extinction in oceanic microorganisms, and widespread shoaling of the carbonate compensation depth. The entire episode of global warming is estimated to have lasted approximately 84 k.y., whereas noticeably light 13C in marine carbonates persisted for approximately 220 k.y. A dark green clay-rich bed that is in sharp contact with underlying chalk represents the P/E boundary at all the sites. Site 1260 has distinct lamination from just below to approximately 10 cm above the boundary. Magnetic susceptibility measurements suggest that the clay-rich part of the boundary sequence ranges from 1 to 2 m thick. All sites display pronounced cyclicity in physical property measurements and sediment color. The pervasive cyclicity in physical property records offers the possibility not only of refining the chronology around critical intervals but also crosschecking results between sites and understanding past climate forcing mechanisms.