Variations in calcium carbonate sedimentation on Ceara Rise have been dominated by dilution of noncarbonate material derived from the Amazon. There has been a continuous increase in the input of this material during the last 1 m.y., which may be related to a long-term increase in erosion in the Amazon drainage basin. In addition, large glacial-interglacial differences in carbonate concentration are caused mainly by increased dilution by noncarbonate material during low sea levels. In the shallowest sites, long-term average carbonate accumulation has been relatively constant, suggesting that average carbonate productivity has been constant. But there are indications that accumulation has varied on shorter time scales: a prominent maximum at about 410 ka, for instance, exhibits accumulation rates that are twice the average value. Bathymetric differences in carbonate accumulation imply that dissolution was more severe in the deepest site prior to about 433 ka. Throughout the last 1 m.y., carbonate accumulation rates and concentrations have been converging toward similar values at all water depths, implying that the bathymetric gradient in [CO32], which marks the transition from North Atlantic Deep Water to Circumpolar Deep Water, has been reducing. Therefore, the calcite saturation levels of the two water masses have become more similar over the last 1 m.y.
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 and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, U.S.A. email@example.com
3Department of Geological Sciences, Salem State College, Salem, MA 01970, U.S.A.