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

Abstract

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 H2S) conditions within the sediments and short‐term 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 δ13C and δ15N values of the inoceramid shell organics provide no evidence of chemosymbiosis and are consistent with pelagic rain as being a significant food source. Demerara Rise inoceramids also exhibit well‐defined, regularly spaced growth lines that are tracked by δ13C and δ18O variations in shell carbonate that cannot be simply explained by diagenesis. Instead, productivity variations in surface waters may have paced the growth of the shells during brief oxygenation events suitable for benthic inoceramid settlement. These inferences imply tight benthopelagic coupling and more dynamic benthic conditions than generally portrayed during black shale deposition. By invoking different temporal scales for geochemical and paleontological data, this study resolves recent contradictory conclusions (e.g., sulfidic sedimentary conditions versus dysoxic to suboxic benthic waters) drawn from studies of either sediment geochemistry or fossil distributions alone on Demerara Rise. This variability may be relevant for discussions of black shales in general.