High-resolution Records of Major Element Ratios for the Plio-/Pleistocene at the Agulhas Plateau: Implications for Orbital Variations in Sediment Provenance, the Formation of Seismic Reflectors and Deep Water Flow Changes

Contourite deposits found in the Indian-Atlantic ocean gateway hold detailed information on past changes in the bottom water flow history over long time intervals of the Cenozoic. Until IODP Exp. 361 only late Pleistocene paleoceanographic studies for the region were carried out using sediment samples obtained from piston cores. We present preliminary results from Site U1475 (Agulhas Plateau), a location proximal to the entrance of North Atlantic Deep Water (NADW) to the Southern Ocean and South Indian Ocean. The site is located over a sediment drift in 2669 m water depth and comprises a complete stratigraphic section of the last ∼7 Ma. The whole spliced sediment record (292 meters) of Site U1475 was measured using an X-ray fluorescence (XRF) core scanner to derive multi-centennial resolution records of major element intensities. Based on these measurements it is possible to derive of biogenic (e.g. %CaCO₃) and siliciclastic (e.g. TiO₂, K₂O) mineral phases. Because the Ca counts almost exclusively stem from biogenic carbonate formed by microfossil shells the ratio of biogenic components vs. terrigenous is reflected in e.g. the Ca/Ti or Ca/Fe records. On the other hand, changes of elemental ratios such as K/Fe or Al/Ti show the variability within the terrigenous sediment fraction. While long-term changes in elemental ratios can be linked to the seismic reflection pattern associated with deep water circulation changes, short-term cyclicities in sediment provenance reflect orbital scale Plio-Pleistocene climate variations. E. g. power spectra performed on the ln(Ca/Ti) and ln(K/Fe) records for a peculiar Pliocene ( 5.7 - 4.1 Ma) high sedimentation rate interval reveal significant spectral density peaks at periods close to the Milankovitch precession band (19 - 23 kyr). Such a high variability in the precession band is also evident in a number of other element ratios while changes in physical properties (e.g. density, seismic impedance) for the same interval seem to be dominated by eccentricity. We present evolutionary spectral analyses revealing how the orbital response of the different parameters have changed over time and derive an improved age model based on cyclostratigraphy.