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Sedimentation rates off SW Africa since the late Miocene deciphered from spectral analyses of borehole and GRA bulk density profiles: ODP Sites 1081–1084 |
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| Authors: | Thomas J Gorgas Roy H Wilkens |
| Institution: | a Department of Geology and Geophysics, SOEST, University of Hawai’i, 1680 East-West Road, Honolulu, HI 96822, USA b Office of Naval Research, Code 322 GG, 800 North Quincy Street, Arlington, VA 22217-5660, USA |
| Abstract: | Sedimentation rates (SRs) off SW Africa were calculated by performing spectral analyses in the depth domain on borehole and gamma-ray attenuation (GRA) bulk density data from ODP Sites 1081–1084. Inversion and integration of SRs versus depth from spectral analysis yielded detailed SR profiles in the time domain. Our technique allowed the detection of excursions in calculated SRs that not only often differed from those established through coarse-scaled biostratigraphic data, but also revealed a greater regional variability in the sediment accumulation over time. High-resolution bulk density data exhibited distinct periodicity in the waveband of Milankovitch cycles (precession at 19–23 kyr; obliquity at 41 kyr; eccentricity at 100 kyr). The pronounced Milankovitch cyclicity suggests that climate variability and trends in SRs along the Benguela Current System (BCS) were responding to insolation patterns during the past 6 Myr. We find relatively low SRs when evolutive amplitude spectra are dominated by obliquity and eccentricity periods. In contrast, significant SR peaks at all sites often occur when strong precessional amplitudes coexist with obliquity and eccentricity cycles. Episodes of high SRs often coincide with peaks in organic carbon mass accumulation rates (MAR Corg) and reduced sea surface temperature (SST) in the southern Walvis Basin, which have been associated with increased regional upwelling. This suggests that the high SRs reflect high productivity (high MAR Corg) attributed to strong wind and upwelling intensity during cool climate periods (low SSTs) in accordance with orbital forcing patterns observed in our spectra. |
| Keywords: | South Atlantic Milankovitch Benguela Current SW Africa bulk density marine sediments spectral analysis Fourier sedimentation rates SST |
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