Ocean Drilling Program, United States

Rapid and extreme perturbations of climate and the global carbon cycle occurred during the Cretaceous and Paleogene periods. Demerara Rise, a submarine plateau in the western equatorial Atlantic Ocean, is ideally located and hosts the sedimentary sequences to sample and investigate these events. Ocean Drilling Program (ODP) Leg 207 was designed to core a depth transect of these sediments on the outer flank of Demerara Rise, spanning modern water depths of 1900-3200 m. During the leg, near-continuous records of Late Cretaceous and Paleogene sedimentary successions from this locale were successfully recovered, including the mid-Cenomanian ocean anoxic event (MCE), Late Cretaceous ocean anoxic event (OAE 2), Cretaceous/Paleogene (K/P) boundary event, and Paleocene/Eocene Thermal Maximum (PETM). The sampled stratigraphy is dominated by two lithologic components: terrigenous detritus and biogenic carbonate and silica. This stratigraphy is broadly categorized into three main styles of deposition: (1) synrift/syntransform clastics, (2) restricted marine "black shales," and (3) open marine chalks and calcareous claystones. Strong physical property contrasts in each of these litho-types allows for excellent core to seismic reflection profile correlation. High-quality seismic reflection data through each drill site permits excellent linkage between drill sites. Oldest sediments recovered during Leg 207 are late Albian in age, and marine conditions are suggested by their composition. A major regional unconformity tops these sediments, leading into organic-rich, laminated claystones (black shales) indicative of oxygen depletion within a restricted basin. The onset of black shale deposition was transgressive from the latest Albian to latest Cenomanian and coincided with transform margin separation of South America and Africa. Carbon and oxygen isotope studies indicate bottom water temperatures were warm through the Cenomanian, and a pronounced delta (super 13) C excursion in the mid-Cenomanian reflects the MCE. During the late Cenomanian, delta (super 18) O isotope values and TEX86 biomarker data indicate warm surface waters and decreased salinity suggesting restricted ocean circulation conditions at Demerara Rise. Positive carbon isotope values, high total organic carbon contents, elevated Fe/Al and Co/Al ratios, and lack of benthic foraminifers indicate severe anoxia and an increase in surface water productivity during OAE 2, a global ocean oxygen depletion event. The Late Cretaceous saw a return of salinity conditions and gradual sea-surface water temperature cooling. Reoxygenation gradually occurred during the Santonian, and black shale deposition ceased on Demerara Rise in the early Campanian. Chalk and calcareous claystone deposition followed through the late Campanian-Maastrichtian interval. The organic matter of the Cretaceous black shale sequence is dominantly marine in origin. Although results of microbial observations through this interval are inconclusive, geochemical data (e.g., linear gradients of ammonium and sulfate from the top of the black shales to the seafloor and trace metal anomalies) indicate that these sediments are active bioreactors dominating the pore water chemistry. At three of the Leg 207 drill sites (1258, 1259, and 1260) excellent examples of the Cretaceous/Paleogene boundary were recovered, marked by a 2-cm-thick graded spherule layer and a spike in iridium concentration. This layer resulted from atmospheric fallout from the proposed Chicxulub meteorite impact. Faunal turnover at this boundary is dramatic. The Paleocene/Eocene boundary event was recovered at all five sites, marked again by faunal turnover and a significant drop in carbon isotope values as well as a change in sediment texture and color. The Eocene sedimentary succession is well represented in Leg 207 cores. delta (super 13) C and delta (super 18) O data from benthic foraminifers reveal a general middle Eocene cooling with short-lived warming events that were previously unrecognized. A regional mid- or late Miocene erosional event, possibly brought on by marked changes in global ocean circulation, sporadically removes much of the upper Eocene and lower Neogene succession on the outer Demerara Rise. The locale is largely sediment-starved at this time, with only a thin Quaternary sediment cover.