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Overview of Scientific Objectives

The best examples in the geologic record of rapid (1 k.y. to 1 m.y.) wholesale extinctions linked to massive perturbations of the global carbon cycle and extreme changes in Earth's climate come from the Cretaceous and Paleogene Periods (e.g., oceanic anoxic events [OAEs] and the Paleocene/Eocene Thermal Maximum [PETM]). Little is known about the underlying causes and effects of these critical events in Earth's history, however. To a significant extent, these gaps in our understanding arise because of a lack of modern high-resolution paleoceanographic records from ocean drill sites, particularly from the tropics, that are so important in driving global ocean-atmospheric circulation. Drilling at Demerara Rise was targeted to access expanded sections of Cretaceous- and Paleogene-age deep-sea sediments, fulfilling priorities of the Ocean Drilling Program (ODP) Extreme Climates Program Planning Group and Long Range Plan. Demerara Rise represents an ideal drilling target for this purpose because the target sediments (1) are shallowly buried and, in places, crop out on the seafloor, (2) exist with good stratigraphic control in expanded sections, (3) contain spectacularly well preserved microfossils, and (4) were deposited within the core of the tropics in a proximal location to the equatorial Atlantic gateway.

During Leg 207, five sites were cored on the northern margin of Demerara Rise (Figs. F1, F2). The sites are located in a depth transect (present water depths are 1900–3192 m) along a grid of high-resolution multichannel seismic reflection lines supplemented by existing industry lines (Fig. F2). The transect of Cretaceous and Paleogene cores will be used to evaluate the following:

  1. The history of Cretaceous anoxia in an equatorial setting and thereby test competing hypotheses for the causes and climatological effects of OAEs (particularly in relation to rapid emission and drawdown of greenhouse gases);
  2. The detailed response of oceanic biotic communities across a range of paleowater depths to extreme perturbations in the geochemical carbon cycle and global climate;
  3. Short- and long-term changes in greenhouse forcing and tropical sea-surface temperature (SST) response;
  4. Key Cretaceous/Paleogene events of biotic turnover and/or inferred climate extremes, particularly across the Cretaceous/Paleogene (K/T) and the Paleocene/Eocene (P/E) boundaries; and
  5. The role of equatorial Atlantic gateway opening in controlling paleoceanographic circulation patterns, OAEs, and cross-equatorial ocean heat transport into the North Atlantic.
Geologic History of Demerara Rise

Demerara Rise is a prominent submarine plateau located at ~5°N off the coasts of Surinam and French Guyana (Figs. F1, F2). The rise stretches ~380 km along the coast and is ~220 km wide from the shelf break to the northeastern escarpment, where water depths increase quickly from 1000 to >4500 m. Most of the plateau lies in shallow water (~700 m), but the northwest margin is a gentle ramp that reaches depths of 3000–4000 m. Much of the plateau is covered by 2–3 km of sediment. The sedimentary cover thins near the northeastern escarpment and exposes the lower parts of the sediment column and underlying basement at depths of 3000 to >4500 m. In contrast, the gentle ramp on the northwest margin is covered by a nearly uniform drape of pelagic sediment down to water depths >4000 m.

Demerara Rise is built on rifted continental crust of Precambrian and early Mesozoic age. Tectonic reconstruction of the equatorial Atlantic places Demerara Rise south of Dakar, Senegal, prior to rifting of Africa from South America. The South American margin in the vicinity of Demerara Rise was one of the last areas in contact with West Africa during opening of the equatorial Atlantic. Rifting processes and related transform faulting separated the Guinea and Demerara Plateaus along an east-west-striking fault system during the earliest Cretaceous (Fig. F3). Barremian basaltic volcanics have been recovered in industry wells from the eastern Demerara Rise, suggesting that rifting began in the Early Cretaceous. Early Cretaceous en echelon faulting along the northwestern edge of Demerara Rise was caused by extensional movements and created a gently dipping ramp that reaches from present water depths between 1500 and 4000 m (Gouyet et al., 1994; Benkhelil et al., 1995).

Late Jurassic sandstones have been dredged in a water depth of 4400 m at the foot of the northern slope (Fox et al., 1972). The first known marine sediments on Demerara Rise are Neocomian in age (Fig. F4), and prior to Leg 207, the northern edge of the plateau is thought to have subsided rapidly and reached water depths of nearly 2 km by late Cenomanian time (Arthur and Natland, 1979). A striking angular unconformity is present across Demerara Rise, separating pre-Albian synrift sequences from Albian to present-age sediments. Upper Albian sediments are mostly green clayey carbonate siltstones. The Cenomanian–Santonian sequence consists almost exclusively of laminated black shale, with occasional stringers of limestone and chert. The black shale is a principal source rock for oil production in coastal French Guyana and Surinam and has total organic carbon (TOC) contents of up to 6–8 wt% in industry wells near the middle of the plateau. Laterally equivalent shales are important source rocks in basins west of Demerara Rise, and they are known as the Canje Formation (Guyana), Naparima Hill Formation (Trinidad), and La Luna Formation (Venezuela and Colombia). Campanian–Paleogene sediments are calcareous to siliceous oozes and chalks. A prominent submarine channel system and erosional surface developed in the late Oligocene–early Miocene. This surface can be traced across the entire northwestern plateau. The channels carried sediment east to west over the flank of the plateau and into feeder channels for a submarine fan that formed northwest of Demerara Rise. The channel system was short lived, and most of the Neogene sediments (hemipelagic and pelagic deposits) are thin or absent from the distal portions of the plateau.

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