Sedimentation rates (SR) off the southwest African coast were calculated by performing spectral analyses on borehole and gamma-ray attenuation (GRA) wet bulk density records. Our SR profiles, calculated for selected Sites 1081, 1082, and 1084, Ocean Drilling Program Leg 175, were consistent with those obtained from biostratigraphic analyses in both depth and time domain. However, our calculations revealed greater details in SR variations than those obtained from biostratigraphy because of a much higher sampling rate of GRA bulk density and well-log measurements. We generated high-resolution SR profiles by applying a fast Fourier transform on the autocorrelation function, which was moved over the wet bulk density records. Our method relies on the correct identification of spectral peaks that coincide with Milankovitch cycles. The detection of main orbital periods, namely precession (19 to 23 k.y.), obliquity (41 k.y.), and eccentricity (100 k.y.), allows the conversion of cycles per meter into SR values (meters per million years). Inversion and integration of these SR values yielded high-resolution SR profiles in the time domain.
We calculated a maximal age of ~9.0 Ma for sediments on the Walvis Ridge (Site 1081), and 5.6 and 4.6 Ma in the Walvis Basin (Sites 1082 and 1084, respectively). Sedimentation cycles in the Milankovitch waveband characterized our power spectra at all sites. Waxing and waning of the cycle amplitudes were interpreted as variations in orbital forcing that has influenced heterogeneously local changes in bioproductivity and upwelling vigor across the study area. Time segments of high deposition before 1.2 Ma correlated strongly with a contemporaneous presence of all three Milankovitch cycles, whereas low SR values were noticed when not all of the main cycles coexisted. However, after 1.2 Ma, the correspondence between spectral and deposition patterns was less consistent than before.
We tested our methodology on synthetic density compaction curves that were perturbed with known frequencies. This enabled us to investigate the influence of variable Fourier and autocorrelation windows, smooth factors, noise contamination, and aliasing effects on our results. These models were aimed to optimize our technique in resolving the correct depositional rates at our selected sites by quantifying Milankovitch cyclicity in wet bulk density records.
1Gorgas, T.J., Kronen, J.D., and Wilkens, R.H., 2001. Data report: Sedimentation rates from Milankovitch periodicity in log and GRA bulk density records off southwest Africa, Sites 1081, 1082, and 1084. In Wefer, G., Berger, W.H., and Richter, C. (Eds.), Proc. ODP, Sci. Results, 175 [Online]. Available from World Wide Web: <http://www-odp.tamu.edu/publications/175_SR/chap_09/chap_09.htm> [Cited YYYY-MM-DD]
2Department of Geology and Geophysics, SOEST, University of Hawaii, 1680 East-West Road, Honolulu HI 96822, USA. Correspondence author: tgorgas@soest.hawaii.edu
3Office of Naval Research Code 322-GG, 800 North Quincy Street, Arlington VA 22217-5660, USA.
Initial
receipt: 2 February 2000
Acceptance: 14 September 2000
Web publication: 10 April 2001
Ms 175SR-208