This paper presents a rock magnetic study of cyclic deep-ocean sediment from the Ceara Rise in the equatorial Atlantic Ocean off the coast of Brazil. Whole-core magnetic susceptibility data have been used as a proxy for climate change in these sediments. This study is aimed at testing this assumption and at determining the source of the magnetic susceptibility variations. We analyzed hysteresis properties, high-field and low-field susceptibilities, anhysteretic remanent magnetization, and thermomagnetic behavior of 114 sediment samples. Hysteresis measurements show that the magnetic carrier is of pseudosingle domain size. Low-temperature demagnetization data indicate the Verwey transition (diagnostic of magnetite) at 118 K. The magnetite concentration was estimated from the saturation magnetization values and varies between 43 ppm in light, carbonate-rich layers and 94 ppm in dark, carbonate-poor layers. The average low-field mass susceptibility is 12.56 X 108 m3/kg, and the average high-field susceptibility is 4.8 X 108 m3/kg. The contribution of magnetite to the low-field susceptibility is 63% in carbonate-poor layers and 53% in carbonate-rich layers. Concentration-independent rock magnetic ratios revealed small-scale and large-scale variations in the magnetic grain size. The variations in magnetic properties mirror changes in clay content, which provides excellent evidence that the susceptibility variations reflect fluctuations in sedimentary input from the Amazon River. The dominant control on Amazon-derived terrigenous sediment is sea level. Therefore, it appears that magnetic susceptibility is a valid climate proxy for the Ceara Rise sediments.
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).
2Ocean Drilling Program, Texas A&M University, College Station, TX 77845, U.S.A. Richter@tamu.edu
3Institut de Physique du Globe, 4 Place Jussieu, 75252 Paris Cedex 05, France.
4Institute for Rock Magnetism, University of Minnesota, Minneapolis, MN 55455, U.S.A.