Bottom water anoxia, inoceramid colonization, and benthopelagic coupling during black shale deposition on Demerara Rise (Late Cretaceous western tropical North Atlantic)

Abstract

[1] The bulk rock geochemistry and inoceramid isotopic composition from Cenomanian to Santonian, finely laminated, organic-rich black shales, recovered during Ocean Drilling Program Leg 207 on Demerara Rise (western tropical North Atlantic), suggest persistent anoxic (free H 2 S) conditions within the sediments and shortterm variations within a narrow range of anoxic to episodically dysoxic bottom waters over a $15 Ma time interval. In addition to being organic-rich, the 50-90 m thick sections examined exhibit substantial bulk rock enrichments of Si, P, Ba, Cu, Mo, Ni, and Zn relative to World Average Shale. These observations point to high organic burial fluxes, likely driven by high primary production rates, which led to the establishment of intensely sulfidic pore waters and possibly bottom waters, as well as to the enrichments of Cr, Mo, U, and V in the sediments. At the same time, the irregular presence of benthic inoceramids and foraminifera in this facies demonstrates that the benthic environment could not have been continuously anoxic. The d 13 C and d 15

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

“…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.…”

Nutrient trap for Late Cretaceous organic-rich black shales in the tropical North Atlantic

“…1B), in the absence of lithological changes, suggest water-mass control on the ε Nd . At both sites, the Cenomanian sequence is composed of fi nely laminated black shales deposited under dysoxic to euxinic bottom waters (Jiménez Berrocoso et al, 2008), and there is no evidence for a difference in terrigenous sources through time or between sites. Evidence against diagenetic control on the ε Nd(t) values of Demerara Rise is that no isotopic shift is observed between the black shales and the overlying Campanian-Paleocene chalks, despite the dramatic ~8 unit positive excursion within the black shale unit during OAE2 (Fig.…”

“…Oxygen-depleted bottom waters in the Demerara region would be a consequence of high rates of organic carbon export to the seafl oor due to high surface productivity (Jiménez Berrocoso et al, 2008), partly fueled by relatively high nutrient inputs from proximal continental regions under the infl uence of intense tropical greenhouse weathering and by the proposed circulation-controlled nutrient trapping. Nutrients released during organic-matter degradation, especially phosphate, which is effi ciently recycled under severe oxygen depletion, would be present in relatively high concentrations in bottom waters.…”

“…1A). To explain how suffi cient nutrients levels needed for high productivity (Jiménez Berrocoso et al, 2008) might persist over Demerara Rise for ~15 m.y. given this geometry, we invoke the circulation pattern outlined above as a means of creating a dynamic, circulation-controlled nutrient trap and associated positive feedbacks among oxygen depletion in nutrient-rich bottom waters, effi cient phosphate recycling, and high productivity.…”

Diagenetic barium cycling in Black Sea sediments – A case study for anoxic marine environments

“…The present study, however, shows that the size of barite crystals is not necessarily indicative of their origin, especially in anoxic ocean basins. Similar interpretations of Ba-records that cover OAE2 successions include the Sopeira section in Spain (Drzewiecki and Simo, 1997), the proto-North Atlantic (Kuypers et al, 2002), the Spława section of the Skole Nappe, Poland (Bąk, 2007), and the Demerara Rise (Jiménez Berrocoso et al, 2008). Scopelliti et al (2004) attempted to use Ba xs as a productivity proxy for OAE deposits in Sicily and anticipated that Ba xs enrichments in the Bonarelli Level equivalent would reflect high productivities.…”