Approximately 500 closely spaced pelagic sediment samples, representing the latest Miocene, middle Miocene, Oligocene/Miocene boundary, and mid-Oligocene, were obtained from two sites on the Ceara Rise. Inductively coupled plasma emission spectroscopy was used to measure the concentrations of Al, Ca, P, K, Si, Ti, Ba, V, and Cu. Normalization by Al was used to allow for variable carbonate dilution of the siliciclastic components. For comparison, Holocene hemipelagic silty clay samples from the Amazon Fan were also analyzed. The elemental ratios were used to infer varying sediment compositions and surface productivity.
On average, K/Al and Ti/Al values are similar in the latest Miocene to Holocene values, but are lower in all the older samples. This reflects the switch from kaolinite to illite-dominated clay minerals in the late Miocene, during the growth of the Amazon Fan. Average Ba/Al, P/Al, and Si/Al values from the mid-Oligocene and Oligocene/Miocene boundary samples are higher than those of the middle Miocene to Holocene samples. The higher ratios suggest phases of relatively high surface productivity, which at least partly accounts for the occurrence of biosiliceous plankton microfossils in these intervals.
In the mid-Oligocene and Oligocene/Miocene boundary samples, variations in Ba/Al, P/Al, and Si/Al are correlated, suggesting variable productivity over short time periods. Maximum productivity occurred during times of maximum burial/preservation of calcium carbonate. Spectral analysis reveals statistically significant regular cyclicity in all of these parameters from these intervals. The cyclicity is associated with the 40,000-yr obliquity orbital-climatic cycles. The dominance of this cyclicity, and lack of evidence for 20,000-yr cycles, suggests that short-term variations in productivity were driven by high-latitude climatic processes perhaps acting through bottom-water production.
1Shackleton, N.J., Curry, W.B., Richter, C., and Bralower, T.J. (Eds.), 1997. Proc. ODP, Sci. Results, 154: College Station, TX (Ocean Drilling Program).
2Department of Geology, University of Luton, Park Square, Luton, Bedfordshire LU1 3JU, United Kingdom. graham.weedon@luton.ac.uk
3Godwin Laboratory, Department of Earth Sciences, University of Cambridge, Free School Lane, Cambridge, Cambridgeshire, United Kingdom.
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