Demerara Rise: Equatorial Cretaceous and Paleogene Paleoceanographic Transect, Western Atlantic

Abstract

The best examples in the geologic record of rapid (1-k.y. to 1-m.y. timescale) wholesale extinctions linked to massive perturbations of the global carbon cycle and extreme changes in the Earth's climate come from the Cretaceous and Paleogene periods (e.g., oceanic anoxic events [OAEs] and the Late Paleocene Thermal Maximum [LPTM]). Little is known about the underlying causes and effects of these critical events in Earth history, in large part because of the lack of modern high-resolution paleoceanographic records from this time interval, particularly from tropical regions that are so important in driving global ocean-atmospheric circulation. Ocean Drilling Program Leg 207 drilling on Demerara Rise is designed to fill these gaps in our understanding by recovering continuous samples from expanded sections at shallow burial depths of Cretaceous-and Paleogene-age deep-sea sediments that contain records of these extreme events. During Leg 207, we plan to drill a series of sites representing a mid-Cretaceous and Paleogene paleoceanographic transect on Demerara Rise (Surinam margin, western Atlantic). The Demerara Rise is ideal for this purpose because the target sediments (1) crop out on the seafloor, (2) exist with good stratigraphic control in expanded unlithified 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. The four primary drill sites are located in a depth transect that falls in present water depths of 1895-3215 m. The sites have been imaged with a grid of high-resolution multichannel seismic reflection lines supplemented by existing industry lines with stratigraphic control from Deep Sea Drilling Project (DSDP) Site 144, industry well Demerara A2-1, and gravity cores of Paleogene sediments from Meteor cruise 49/4. DSDP Site 144 was spot cored toward the escarpment in a highly condensed section and recovered sequences of dark clays and shales correlative to at least three Cretaceous OAEs plus a further 150-m-thick sequence of upper Paleocene to lower Oligocene carbonate ooze. These sections thicken inboard, and records of at least five OAEs (OAE-1b,-1c,-1d,-2, and-3) can probably be penetrated by transect drilling on the Demerara Rise with good potential for recover of the LPTM and Eocene/Oligocene boundary. The drilling strategy for Leg 207 will be to recover continuous sections by multiple coring at each site and logging to maximize the potential for high-resolution investigations. The proposed transect of Cretaceous and Paleogene cores will be used to evaluate, at high resolution, the following: 1. The history of multiple Cretaeous OAEs in an equatorial setting and thereby test competing hypotheses for their causes and climatological effects (particularly in relation to rapid emission and draw-down 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 clim...

Nd isotopic excursion across Cretaceous ocean anoxic event 2 (Cenomanian-Turonian) in the tropical North Atlantic

“…If Cretaceous eolian input from the Guyana shield was analogous to volcanics in the modem Pacific, low e^^ values should be recorded for surface waters with increasing values at depth. However, we observed no separation among values in Campanian-Maastrichtian samples where results are available for three Demerara sites separated by >1 km of depth (Erbacher et al, 2004). Furthermore, contrary to the modem Pacific, the area affected by airborne material could not have been very large, because Blake Nose e^^^ values are distinctly higher than those at Demerara Rise (Fig.…”

“…Single data points from Angola (Grandjean et al, 1987) and Sweden (Puceat et al, 2005) plot with the background values observed at Demerara, but these samples were both recovered from relatively shallow, isolated basins that likely received runoff with low s^^ values derived from nearby Precambrian cratons, similar to the situation in modem Baffin Bay (Stordal and Wasserburg, 1986). During the Late Cretaceous, Site 1258 would have been hundreds of kilometers from the South American coastline and is estimated to have been at depths >1000 m, with no barriers between it and the rest of the North Atlantic (Arthur and Natland, 1979;Erbacher et al, 2004;Jimenez Berrocoso et al, 2008). Isolation is therefore an unlikely explanation for low values on Demerara Rise.…”

“…1). In the Demerara sites, the Cenomanian-early Santonian is represented by 50-90 m of laminated black shale separated from overlying late Campanian-Paleocene clayey chalk by a condensed interval and/or hiatus (Erbacher et al, 2004). The Demerara sequence is unusual in that deposition of laminated black shales began millions of years before and continued millions of years after OAE2.…”

Nitrogen cycling during the Cretaceous, Cenomanian‐Turonian Oceanic Anoxic Event II

“…Lithologies in the sequences range from TOC-rich laminated claystones to TOC-rich laminated marls and chalks. Carbon compositions range from 2-30% TOC and 0-95% CaCO 3 [Erbacher et al, 2004[Erbacher et al, , 2005.The carbonate is primarily calcareous nannofossil and planktic foraminiferal tests, with varying amounts of carbonate cement in some indurated beds. Fish skeletal material and phosphatic fecal pellets are also observed.…”

“…Lithologies below the ''black shale'' sequence were sedimentary in origin and where dateable were Albian in age. [Erbacher et al, 2004].…”