CHEMICAL COMPOSITION OF LAMINATED MARINE SEDIMENTS DURING OCEANIC ANOXIC EVENT 2, DEMERARA RISE, EQUATORIAL ATLANTIC OCEAN

The Cenomanian/Turonian Oceanic Anoxic Event 2 (OAE2) was a global episode of elevated carbon burial in the oceans accompanied by a worldwide positive carbon isotope excursion, widespread anoxia and super-greenhouse conditions. It is unclear what triggered the event, what sustained it for so long (approximately 400 kyr), and why it ended. At this time, Demerara Rise was part of a broad marine shelf in the equatorial Atlantic Ocean onto which a thick sequence of organic-carbon rich laminated sediments accumulated. These sediments were cored during the ODP Leg 207 cruise; Site 1261 was found to have the most complete OAE2 record. The sediment laminations suggest a high accumulation rate, potentially at the annual scale. To capture the geochemical characteristics and thicknesses of the laminations, high-resolution X-ray fluorescence (XRF) core scanner data were collected with a 200 micron slit size at 100-micron step size intervals from Site 1261, Hole B, 13 R, Sections 1,2 and 3. Preliminary analysis indicates that the majority of the laminations are mm-scale, but a substantial number are much thinner, approaching the scanning Nyquist resolution of 200 microns. In terms of count area, the prevalence of measured elements is, from highest to lowest: Ca, S, Si, Fe, K, Ti, Al, and P. Ca is interpreted to be associated with carbonate, Si, K, Al, Fe, and Ti are linked with siliciclastics, S is related to autogenic pyrite, and P is connected with nutrient availability. The repeated alternation of these elements indicates a cycling of two main processes: in-situ biological productivity (carbonate laminae), which is enhanced by upwelling of high nutrient waters and terrigenous input (siliciclastic laminae) controlled by climate and river fluxes. Oscillations of autogenic elements such as S indicate varying euxinic conditions of bottom ocean waters. The XRF data also exhibit Milankovitch-scale variations. These chemical composition observations will be used to develop a dynamical model of the climatic and oceanographic conditions that affected the equatorial Atlantic during OAE2.