Demerara Rise is a promontory of the South American continental crust that is conjugate to the Guinea Plateau of west Africa (Fig. F2). The northern margin of Demerara Rise is bounded by a transform fracture system that extends fully eastward to the coast of West Africa, north of which is the Guinea Plateau (Fig. F2). In the Late Jurassic, Demerara Rise and the Guinea Plateau formed the southern margin of the North Atlantic Ocean. At that time, Demerara Rise was likely a marginal sea in which evaporites and shallow-water carbonates formed (Gouyet et al., 1994; Benkhelil et al., 1995). By the Early Cretaceous, mid-Atlantic Ridge extension, rifting, and dextral transform shear motion separated the two plateaus. Following initial rifting and transform motion, subsidence was presumably rapid throughout the Early and mid-Cretaceous as a result of thermal cooling (Flicoteaux et al., 1988). Additionally, global sea level continued to rise rapidly through this period (Haq et al., 1988). This subsidence and sea level rise created accommodation space for thick sediment accumulation on this portion of the continental margin. These sediments have been imaged with industry multichannel seismic (MCS) data but were not sampled during Leg 207 (Fig. F3). The overlying postrift upper Mesozoic and Cenozoic sediments were best imaged with site survey high-resolution MCS reflection data (e.g., Fig. F4), however, and were sampled at the five Leg 207 drill sites (Fig. F1).
The most distinctive seismic reflection event apparent on all seismic profiles imaged from outer Demerara Rise is a regionally correlative horizon, designated "C" (Fig. F3). Reflections overlying the C horizon appear largely conformable. Underlying reflections truncate upward against the C surface, forming an angular unconformity. This underlying section consists of pre-Albian synrift and syntransform sediments. High-amplitude reflectors are more or less continuous and parallel, although tilted and locally divergent. Near-vertical strike-slip faults and a near-vertical outer (eastern) margin of Demerara Rise are evidence of transform tectonics. Apparent normal listric faults are probably representative of margin subsidence. The C horizon is referred to as a "break-up" unconformity, likely representing the final separation of Demerara Rise and the Guinea Plateau concurrent with opening of the Atlantic Gateway.
The principal stratigraphic section of interest for Leg 207 was the interval above the C horizon, which is largely Late Cretaceous and Paleogene in age. Four regionally correlated reflection events within this section provided the seismic stratigraphic framework for the Leg 207 drilling program (Fig. F4). These events were correlated with drilling results through generation of synthetic seismograms (Fig. F5). Above the C surface is a ~70- to 100-ms-thick sequence of medium-amplitude, parallel, largely flat lying coherent reflections. This sequence corresponds to Cenomanian–Turonian organic-rich claystone observed at the drill sites (lithologic Unit IV of Erbacher, Mosher, Malone, et al., 2004). At the top of this seismic sequence is a reflection event (B´) that corresponds to the top of the black shale sequence. A ~50-ms-thick sequence above B´ and below Reflector B typically shows lower amplitude but conformable reflections corresponding to the Campanian–Maastrictian section of nannofossil chalk to calcareous silt and claystone (lithologic Subunit IIIB) observed during Leg 207. O'Regan and Moran (this volume) found significant physical property differences including a marked reduction in permeability in the transition between the lithologies of Units IV and III, explaining the distinctive seismic signatures each produces and the good correlations of seismic to core data. The high-amplitude B reflector at the top of lithologic Subunit IIIB ties to the K/P boundary, shown particularly well with core-seismic correlations at three of the Leg 207 sites (1259, 1260, and 1261).
Between the B reflector and the overlying A horizon is a variably thick unit largely composed of parallel, coherent, flat-lying reflections that correlates with a Paleogene sequence of foraminifer nannofossil chalk to clayey nannofossil chalk (lithologic Subunits IIIA, IIC, and IIB) (Fig. F5). The A horizon is an erosional surface of probable Miocene age (it was not specifically sampled during drilling) that apparently resulted in significant removal of section on the outer flank, leaving mostly Paleocene and lower to middle Eocene sediments. This latter section thins toward the north and toward the flanks of the rise. The Neogene section above the A surface is nearly absent on the outermost portions of Demerara Rise but thickens inboard. It consists of nannofossil clay (lithologic Unit I) and is >300 m thick at Site 1261.
Hetzel et al. (this volume) produced quantitative geochemical characterization of the five lithologic units using bulk total inorganic carbon, total organic carbon, sulfide, a suite of major and minor elements, and rare earth elements. They divided the Leg 207 lithostratigraphy into three depositional modes: synrift clastics (Unit V), laminated black shales (Unit IV), and open-marine chalk and calcareous claystones (Units III, II, and I). Their results indicate that major element compositions are dominated by a mixture of two components: terrigenous detritus and biogenic carbonate and silica. The terrigenous component in most of the sedimentary section has the composition of average shale. In contrast, Unit I is composed of a more clay dominated weathered terrigenous component. The Cretaceous black shales of Unit IV are enriched in redox-sensitive and stable sulfide elements (Mo, V, Zn, and As). Combined with high phosphate contents and a pronounced Ce anomaly, these data confirm high paleoproductivity and oxygen depletion in the water column during deposition of Unit IV.
Oldest sediments recovered during Leg 207 are lower upper Albian claystones and silty claystones. These claystones are rich in organic matter and phosphatic concretions at Site 1258 and contain less organic matter at Sites 1257 and 1260. A dominance of marine organic matter as well as the presence of fossils indicating normal open-marine conditions (e.g., ammonites, calcareous nannofossils, and foraminifers) demonstrate that the distal parts of Demerara Rise were occupied by an open-marine and epicontinental basin (Meyers et al., 2006; Meyers and Bernasconi, this volume; Owen and Mutterlose, 2006; Kulhanek and Wise, 2006). Lower upper Albian claystones are unconformably overlain by organic-rich, laminated black shales. At the deepest site (Site 1258), these black shales are latest Albian in age (Kulhanek and Wise, 2006), potentially representing OAE 1d.
The hiatus between lower upper Albian claystones and uppermost Albian black shales, recognized as the C horizon on seismic data, dates an important stage in the evolution of the continental margin from the rift to drift stage and from sedimentation in an epicontinental basin to open oceanic conditions. O'Regan (this volume) used log and core physical property data from multiple holes at each drill site to generate composite stratigraphic depth scales (in units of equivalent log depth [eld]) for the black shale interval. The onset of black shale deposition is transgressive, starting in the latest Albian at Site 1258, the early Cenomanian at Sites 1260 and 1257, the late Cenomanian at Site 1261, and the latest Cenomanian at Site 1259 (Erbacher et al., 2005). These results indicate establishment of an apparently stable and long-lasting oxygen minimum zone at Demerara Rise.
Several chemical indicators suggest severe dysoxia to even anoxic conditions during the Cenomanian–Santonian at Demerara Rise. Böttcher et al. (this volume) compared conditions on Demerara Rise with those from the present-day Black Sea, as the abundance of reactive iron indicates anoxic conditions in the water column and within the sediment. Their observation is supported by high C/N ratios and low nitrogen isotope values, which might be the result of depressed organic matter degradation and microbial nitrogen fixation in an anoxic water column (Meyers et al., 2006). Benthic foraminifers, however, were observed throughout the black shale interval, arguing for frequent reoxygenation of the seafloor (Friedrich et al., 2006).
Low-resolution stable oxygen isotope data from Norris et al. (2002), Bornemann and Norris (in press), and Bice et al. (2006), as well as high-resolution data from Friedrich et al. (2006) and Moriya et al. (in press), demonstrated that sea-surface temperatures (SSTs) and bottom water temperatures at Demerara Rise were warm throughout the Cenomanian. Stable carbon isotopes from foraminiferal calcite and bulk organic matter record a pronounced positive excursion in the mid-Cenomanian that represents the mid-Cenomanian Event (MCE). At Demerara Rise, the MCE marks a major turnover in calcareous nannofloras (Hardas and Mutterlose., 2006) and benthic foraminifers (Friedrich et al., 2006). Above the MCE, 
18O values of benthic foraminifers increase, suggesting either a pronounced cooling of bottom waters or a significant influence of saline waters at the seafloor (Friedrich et al., 2006). Cyclic sediment composition variations through this interval are inferred to represent eccentricity and precession cycles (Nederbragt et al., this volume).
Although benthic foraminiferal assemblages document oxygen-poor conditions at the seafloor throughout the Cenomanian, above the MCE oxygen seems to have been even more deficient (Friedrich et al., 2006). Friedrich et al. (2006) favor a model where warm arid conditions during the late Cenomanian led to increased evaporation in tropical epicontinental basins such as the La Luna Sea west and southwest of Demerara Rise that served as sources of saline deep or intermediate waters that bathe Demerara Rise. 18O values of benthic foraminifers decrease in the uppermost Cenomanian at the base of OAE 2, which is interpreted as a decrease of bottom water salinity associated with a shutdown of deepwater production in tropical epicontinental basins, probably due to an increase of precipitation associated with OAE 2.
Arguably, the most prominent and widespread of the mid-Cretaceous oceanic anoxic events is OAE 2 near the Cenomanian/Turonian boundary. The isotope excursion as well as the distribution of organic-rich sediments is truly global, and both have been described from numerous outcrops and deep-sea cores around the world (for example, see
Nederbragt et al., this volume). At Demerara Rise, a pronounced positive carbon isotope excursion of as much as 6.5
, during OAE 2, was recorded by Erbacher et al. (2005) (Fig. F6). Rather complete OAE 2 successions are preserved at Sites 1258, 1260, and 1261. High-resolution carbon isotope and calcareous nannoplankton stratigraphies across the OAE enabled a detailed correlation of the Leg 207 sites with sections elsewhere in the world (Erbacher et al., 2005; Hardas and Mutterlose, 2006;
Nederbragt et al., this volume).
Late Cenomanian surface water temperatures at Demerara Rise were warmer than today (27°–29°C) (Norris et al., 2002; Forster et al., 2007). Oxygen isotope values of benthic foraminifers indicate a severe warming or a decrease in salinity of bottom waters at Demerara Rise at the base of OAE 2 (Friedrich et al., 2006). Rapid warming of SSTs was also suggested by Forster et al. (2007), who, on the basis of oxygen isotope values of planktonic foraminifers and TEX86 biomarker data, observed a rise of 6°C (to 35°–36°C) at the base of the OAE, paralleling the initial increase of carbon isotopes of OAE 2. SSTs drop shortly and for <150 k.y. after the initial warming to values lower than the pre-OAE conditions, reflecting the drawdown of atmospheric carbon dioxide, probably related to the intensive carbon burial during OAE 2 (Forster et al., 2007). Above this cooling interval, temperatures rise again, suggesting that circumstances leading to OAE 2 were stronger than the short-term storage capability of CO2. Following OAE 2, SSTs remain in the thirties (Wilson et al., 2002; Forster et al., 2007). Positive carbon isotope values, high total organic carbon (TOC) contents, elevated Fe/Al and Co/Al ratios, and a lack of benthic foraminifers indicate severe anoxia, sometimes even euxinia, and an increase of surface water productivity during OAE 2 (Erbacher et al., 2005; Hetzel et al., 2006; Friedrich et al., 2006).
SSTs remain warm following OAE 2 and throughout the Turonian (Forster et al., 2007; Bornemann and Norris, in press), whereas benthic oxygen isotope values indicate relatively fast cooling or an increase of bottom water salinity (Friedrich et al., 2006). A gradual SST decrease occurred in the latest Turonian and Coniacian with some short-term cooling events, which might be related to the buildup of ice shields at high latitudes (Bornemann and Norris, in press).
Black shale sedimentation at Demerara Rise ceased in the early Campanian. Based on benthic foraminiferal faunas, Friedrich and Erbacher (2006) suggested bottom water reoxygenated gradually during the late Santonian at the shallower parts of the Demerara Rise depth transect, whereas the deep end of the transect remained anoxic until the early Campanian. These authors proposed that this gradual ventilation from shallow to deep environments was related to the ongoing opening of the equatorial Atlantic Gateway.
Upper Campanian–Maastrichtian strata are represented by >200 m of chalk and calcareous claystones. Based on calcareous nannofossil assemblages, Thibault and Gardin (2006) described two cooling events during the Maastrichtian. A severe warming event in the latest Maastrichtian resulted in a decrease of surface water productivity prior to the K/P boundary. Stable carbon isotope studies of this latest Maastrichtian interval likely were compromised by a diagenetic overprint (MacLeod, this volume, and discussion below).
Forster et al. (2004), Meyers et al. (2006), and
Meyers and Bernasconi (this volume) showed that Cenomanian–Santonian black shale sequences of Unit IV contain between 2 and 15 wt% organic carbon. High Rock-Eval hydrogen index (HI) values, low Tmax values, and sterane and hopane biomarkers indicate that the bulk of organic matter is marine in origin and thermally immature. These authors determined that this organic matter is derived from mostly marine algal and microbial sources. Although 13Corg values and TOC/TN ratios mimic those of land-plant organic matter, contributions from land plants are considered minor. Elevated C/N ratios suggest depressed organic matter degradation, most likely associated with low-oxygen conditions in the water column that favored preservation of nitrogen-poor forms of organic matter over nitrogen-rich components. 15Ntotal values of the black shales fall between –4 and +1, which is much lower than modern ocean values of approximately +5. The low 15N values indicate that greatly amplified microbial nitrogen fixation accompanied black shale deposition and imply existence of strong near-surface stratification of the ocean.
During Leg 207, a high-resolution interstitial water sampling program was conducted to understand the impact of relatively deep seated organic matter–rich black shale sequences on diagenetic processes. Initial shipboard results suggested that the Cretaceous black shales are still active biogeochemically despite deposition >100 m.y. ago (Erbacher, Mosher, Malone, et al., 2004; Erbacher et al., 2004) (Fig. F7). Arndt et al. (2006) used an interstitial water transport-reaction model to characterize dominant biogeochemical processes operating in Demerara Rise sediments. Their results clearly indicate a single stratigraphic biogeochemical source and sink, the Cretaceous black shales, coupled with simple diffusion to and from the sediment/water interface, drives the entire biogeochemical system. Two processes dominate the system: organic matter degradation by methanogenesis within the shales and anaerobic methane oxidation (AMO) in the overlying sediments. Consumption of upward-diffusing methane and downward-diffusing sulfate during AMO drives an enhanced diffusional sulfate flux, producing linear profiles (Fig. F7). Deviations of linear sulfate gradients are the result of local porosity variability. Consumption of sulfate promotes remobilization of biogenic barium and precipitation of barite in the zone of oversaturation in the overlying sediments where sulfate content increases. Temporal shifts in the barite precipitation zone inhibited formation of barite fronts (Brumsack, 2006). Calculated methanogenic and AMO rates are much lower than reaction rates from shallow near-shore marine sediments but compare with the lower range of rate studies from other deep-marine sedimentary sections.
Meyers et al. (2004) suggested that it is likely that methane generation within the black shale units stemmed from in situ microbial activity because of the low thermal maturity of the organic carbon and the weak correspondence of higher TOC concentration and greater methane concentration. Attempts to quantify the microbial population were challenging despite these conclusions and the fact that interstitial water geochemistry clearly indicates an active biosphere. Schippers and Neretin (this volume) utilized direct microscopic counts using acridine orange stain (AODC) and ribosomal ribonucleic acid (rRNA) techniques, specifically, catalyzed reporter deposition–fluorescence in situ hybridization (CARD-FISH), to investigate the microbial population within the black shales. Prokaryotes were detected in 2 of 13 black shale samples using both techniques, with total numbers comparable to sediments from previous studies of ODP cores. A few living cells of archaea were detected, but their numbers were too low to quantify using CARD-FISH. The absence of prokaryotes in most samples suggests that the black shales do not support a thriving microbial community. However, the presence of prokaryotes in high numbers in two samples indicates at least a patchy distribution of archaea, perhaps enough to catalyze the lower rates of reduction suggested by interstitial water studies. Fredricks and Hinrichs (this volume) analyzed black shales and overlying organic-lean sediments for the presence of intact polar lipids (IPL), which constitute the cell membranes of living microorganisms. Results document the presence of IPL in the organic-lean sediments, which likely are related to methane oxidation. IPL were difficult to detect in black shales, which was interpreted to be the result of the poor signal-to-noise ratio (low cell content in a high organic matter matrix). Initial studies of Demerara Rise sedimentary microbial populations suggest low microbial activity; however, cell distribution may be patchy in the black shales. Adequate substrate exists in the black shales to maintain an active biogeochemical system to dominate the interstitial water geochemical profiles.
Clayton et al. (this volume) measured iron isotopes in 10 black shale samples from Hole 1260B to gain a better understanding of oceanic redox during the mid-Cretaceous. Three extraction procedures separated Fe components in the sediments, which is a little-used technique for marine sediments. However, technical problems limited the scope of the planned work. 56Fe values of Fe oxide-sulfide-carbonate extracts range between 0.02 and –0.77 and negatively correlate to C/N ratios. In contrast, 56Fe values of two Fe oxide extracts are more positive (0.74 and 0.63), suggesting measurable fractionation within different Fe components in the cores. These results demonstrate the potential for selective extraction procedures to study fractionation of Fe isotopes and redox recycling is marine sediments.
At three sites along the Demerara Rise depth transect (1258, 1259, and 1260) excellent examples of the K/P boundary were recovered, marked by a 2-cm-thick graded spherule layer at each site (Fig. F8). At a paleodistance of ~4500 km from the proposed Chicxulub meteorite impact crater, the K/P boundary succession at Demerara Rise is remarkably complete, and fine details of the sedimentological and paleontological expression of the event are recorded. These details include a record of seafloor sediment resuspension within minutes of the impact followed by deposition of a primary air fall over subsequent hours to weeks and a well-resolved record of the recovery and radiation of foraminifers over the first few million years (MacLeod et al., 2007) (Fig. F8). Faunal turnover at this boundary is dramatic. Sections near this interval include the uppermost Maastrichtian, a relatively extended P0 planktonic foraminiferal zone, and a complete succession of the lowermost Paleogene planktonic foraminiferal zones (MacLeod et al., 2007). Iridium concentrations reach a maximum of ~1.5 ppb at the top of the spherule bed; Ir concentrations are otherwise below detection limits elsewhere in the core (MacLeod et al., 2007) (Fig. F8).
The PETM is an interval of extreme and abrupt global warming. All five of the Leg 207 sites recovered the Paleocene/Eocene boundary along a transect of 1300 m present water depth. With a thickness of ~2.5 m, the most expanded PETM interval at Demerara Rise was recovered at Site 1258 (Nuñes and Norris, 2006; Sexton et al., 2006) (Fig. F9). Low carbon isotope values (Fig. F9), however, argue for secondary geochemical overprinting of foraminifers. Calcareous nannofossils reacted with the appearance of classic "excursion taxa" during the PETM, which, in the case of Demerara Rise, parallel a sudden occurrence of malformed and eutrophic species. The latter are interpreted to be related to increased runoff with subsequent fertilization of the ocean off South America (Jiang and Wise, 2006). Ongoing high-resolution studies focusing on the inorganic geochemistry and nannofossil ecology of the PETM will contribute additional information about temperature excursions and their consequences during this event.
Eocene chalks, particularly through the lower–middle Eocene, are nearly continuous and expanded (~200 m thick) at Sites 1258 and 1260. Detailed stratigraphic studies based on calcareous nannofossils (Lupi and Wise, 2006) and paleomagnetism (Suganuma and Ogg, this volume) will serve as the basis for further high-resolution paleoceanographic and cyclostratigraphic studies. Sexton et al. (2006) present the first high-resolution benthic foraminiferal 13C and 18O records derived at one locality and not representing a composite from numerous sites (Fig. F10). In revealing a temporary warming period punctuating general middle Eocene cooling, data from Sexton et al. (2006) support the validity of the Eocene composite from Zachos et al. (2001). Sexton et al.'s (2006) 13C record suggests minimal interocean 13C gradients during the warm early and middle Eocene due either to a lack of deepwater sources derived from high latitudes or a reduction in the efficiency of the biological pump during these warm periods. Several intervals in Sexton et al.'s (2006) stable isotope record seem to indicate previously unknown short-lived warming events (e.g., at 52.6, 50.5, and 40.3 Ma).
Broadly, the entire Cenozoic succession thickens inboard toward the shelf, where it is nearly 400 m thick at Site 1259, for example. Within this succession Neogene stratigraphy is distinctly separated from the Paleogene by a regional lower upper Miocene (Zone NN11a) unconformity that is clearly identifiable in seismic profile (Reflector A in Fig. F11) and in recovered cores (Ingram and Wise, 2006). O'Regan and Moran (this volume) estimate in excess of 220 m of sediment removal during this erosional event. In addition, evidence of ongoing faulting through the section and widespread sediment failure immediately above this unconformity resulted in a slump deposit observed at Site 1261 (Zone NN11b) (Ingram and Wise, 2006).
It is possible that strong currents swept across the rise, eroding Miocene and older sediments. Expansion of Antarctic ice sheets during the middle to late Miocene influenced bottom water formation and upwelling along Atlantic continental margins. Concurrently, the Central American Seaway (CAS) shoaled, preventing deepwater exchange between the Pacific and Atlantic Oceans (Nisancioglu et al., 2003). These broad-scale changes in ocean circulation may have been responsible for generating currents that swept across outer Demerara Rise, causing the late Miocene unconformity. Alternatively, the close proximity of the slump identified at Site 1261 with faulting shown in seismic profiles indicates a relationship between possible late-stage tectonic activity and this mass failure deposit. Ingram and Wise (2006) believe that a single mass failure event may have been responsible for late Miocene erosion and creation of the A reflection horizon as shown in Figure F11.
Critical to further paleoceanographic investigations is the establishment of an accurate chronostratigraphic framework and recognition of hiatuses for the stratigraphic sections investigated.
MacLeod (this volume), for example, points out the difficulty in correlating site to site results, given the present understanding of age control. Although not yet developed, robust chronostratigraphy will come about through contributions pertaining to integration of site results (e.g., modified composite depth scales) and establishment of time marker horizons and cyclostratigraphic tuning. For example,
Suganuma and Ogg (this volume) identified paleomagnetic polarity Chrons C18n–C27r of middle Eocene to late Paleocene age and Chrons C29r through potentially C34n of Maastrichtian–Campanian age. Erbacher et al. (2005) established the position of the Cenomanian/Turonian boundary interval and position of OAE 2 from 13C measurements, with this being a critical time horizon that provides a base for subsequent paleoceanographic studies on upper Cenomanian to lower Turonian sediments.
O'Regan (this volume) established an equivalent log depth scale of the Cretaceous section by tying core depths to logging depths using curve matching of physical property data. This improved depth scale will permit cyclostratigraphic analysis and orbital tuning to further refine age control, such as the work of
Nederbragt et al. (this volume). Ingram and Wise (2006) studied nannofossils of Cenozoic sections to establish the age stratigraphy and recognize a number of hiatuses and used 18O and 13C to establish the Oligocene/Eocene boundary.
