Nitrogen Fixation Enhanced Organic Matter Production in Demerara Rise Black Shales

The high concentrations of organic carbon that are common to Cretaceous black shales imply levels of sustained primary production that are unknown in the modern ocean. How organic matter productivity could be maintained for many millennia remains an open question. Cenomanian to Santonian black shale sequences in the five sites drilled by Leg 207 on the Demerara Rise range in thickness from 56m to 93m. The finely laminated sequences contain between 1 and 28 percent organic carbon, and their organic geochemical properties reveal aspects of the exceptional conditions of organic matter production and preservation involved in their formation. The results of Rock-Eval pyrolysis show that the bulk of the organic matter originates from marine primary production. Improved preservation of the organic matter is implied by C/N ratios that increase to 40 as organic carbon concentrations increase; land-plant organic matter appears to be important only in the lower Cenomanian black shales from Site 1260 in which C/N ratios sometimes reach 60. Nitrogen isotope compositions that become lighter as organic carbon concentrations increase indicate that organic matter production was enhanced by a consortium of primary producers that included nitrogen fixing bacteria. Expansion of an intensified oxygen minimum zone into the photic zone probably permitted coexistence of algae and the photosynthetic microbes that function best under dysaerobic and anaerobic conditions and that are not limited by nitrate availability.