A geochemical study of sediments from Ocean Drilling Program Site 983 was conducted to examine low-frequency variations in carbonate content as expressed by blue-band reflectance (450-500 nm) over the last 1.2 Ma. Sedimentary percent organic carbon, percent carbonate, and excess barium (Ba[ex]) were used as the primary tools to evaluate the factors responsible for these long-term changes. We observe positive correlation between the mass-accumulation rate of various biogenic components and the mass-accumulation rate of Ba(ex), especially in sediments younger than ~600 ka. Deeper in the section (~600-1200 ka), the correlation between Ba(ex) and the other biogenic tracers is weak. The lack of correlation between Ba(ex) and biogenic carbonate likely results either from a higher supply of terrigenous material at that time (which confounds Ba[ex] estimation), or remobilization of Ba resulting from low pore-water sulfate ion concentrations, or both. Nonbiogenic sediments at Site 983, represented by Th, K2O, and the molar Ti/Al ratio, exhibit cyclic variations that represent mixing between continental and oceanic (i.e., basaltic) terrigenous sources. The timing of these cycles matches that of the major glacial-interglacial cycles, which suggests that they result from the supply of continental material as ice-rafted debris during glacial periods and fine-grained basaltic material by bottom currents during interglacial periods. Given these observations, the most likely causes for the low-frequency carbonate variations observed in the Site 983 sediments are shifts in surface productivity and, to a lesser extent, dilution by the input of terrigenous material.
1Raymo, M.E., Jansen, E., Blum, P., and Herbert, T.D.
(Eds.), 1999. Proc. ODP, Sci. Results, 162: College Station, TX (Ocean Drilling
Program).
2Ocean
Research Institute, University of Tokyo, 1-15-1 Minamidai, Nakano-ku, Tokyo 164, Japan.
3Present
address: Korea Ocean Research and Development Institute (KORDI), Ansan, P.O. Box 29, Seoul
425-600, Korea. smhyun@sari.kordi.re.kr
4Lamont-Doherty
Earth Observatory of Columbia University, Route 9W, Palisades, NY 10964, U.S.A.
5Department
of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology
E34-254, Cambridge, MA 02139, U.S.A.
Date of initial
receipt: 14 August 1997
Date of acceptance: 11 June 1998
Ms 162SR-019