MARINE BLACK SHALE AT DEMERARA RISE, EQUATORIAL ATLANTIC OCEAN, AT THE INCEPTION OF CRETACEOUS OCEANIC ANOXIC EVENT 2: A RECORD OF DISTURBANCE IN TERRESTRIAL SEDIMENT INFLUX, MARINE PRODUCTIVITY, NUTRIENT CYCLING, OCEANIC ANOXIA AND CIRCULATION

The organic-carbon rich laminated marine black shale of early Late Cretaceous Oceanic Anoxic Event 2 (OAE2) at Demerara Rise, western equatorial Atlantic Ocean provide a detailed record of greenhouse climate, continental runoff, marine productivity, carbon burial, oceanic anoxia and euxinia, and diagenesis. Here we present a multivariate analysis of sub-mm scale X-Ray Fluorescence (XRF) elemental composition (100 micron spacing) and photo scans (70 micron spacing) of 4.5 m of black shale core sections from ODP Leg 207, Site 1261B. The timescale for the section was estimated by application of the Average Spectral Misfit method to an extended grayscale scan of Site 1261 cores that simultaneously tested for the presence of astronomical frequencies. The results confirm the presence of astronomical frequencies in the shale, and indicate a 1.071 cm/kyr sedimentation rate for the OAE2 interval, which is 9.97 m thick at site 1261, i.e., a duration of 931 kyr. The 4.5 m scanned section includes: (a) 120 kyr of pre-OAE2, (b) onset-OAE2 marked by a thin (1 cm) volcanic ash layer, followed by the start of the well-known global carbon isotope excursion (CIE) for 150 kyr, and (c) full-OAE2 for 150 kyr during CIE maximum conditions. Principal component analysis of Ca, S, Si, Fe, K, Ti, Al, and P show progressive and dramatic changes in element associations from pre-OAE2, onset-OAE2, to full-OAE2. Notably, Si, Fe, K, Ti, Al and S act together during pre-OAE2, partially disengage during onset-OAE2, and then further disengage during full-OAE2. At the start of OAE2, P collapses, Ca destabilizes, and S increases. The photo scans also indicate three orders of persistent laminations, 10 to 50 mm, 1 mm, and 0.2 mm, which represent decadal to millennial scale cycles. A short interval of anomalously high-amplitude mm-scale cycling occurs in all elements except Ca during full-OAE2 conditions. These observations will be interpreted together with other published data from Demerara Rise to develop a dynamical model of the equatorial Atlantic Ocean and its surroundings at the inception of OAE2.