2-Methylhopanoids as indicators of paleoproductivity in paleooceans during times of black shale deposition

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2-Methylhopanoids are biological markers of a significant number of cyanobacteria species. This project investigates if these fossil organic compounds are indicators of high primary productivity in the paleoocean involving nitrate limitation and subsequent nitrogen fixation. Cretaceous black shales from Shatsky Rise in the central Pacific Ocean drilled during ODP Leg 198 are the test case for this study. 2-Methylhopanoid profiles across these black shales are contrasted to those in over- and underlying sedimentary rocks. In addition, the distribution patterns across the black shale are studied for steady or oscillating changes of paleoproductivity. A novel approach is the combined analysis of free 2-methylhopanoids in the extractable bitumen fraction and bound analogs in the insoluble macromolecular organic matter. For comparison, sapropels from the eastern Mediterranean Sea (ODP Leg 160) representing a significantly younger age and a different paleoceanographic setting are investigated in the same way. Cretaceous black shales from the North and South Atlantic Ocean are possible further targets for comparison. Finally, cultured cyanobacteria are investigated for their 2-methylhopanoid content to extend the basis for interpretation in fossil organic matter.