The accumulation and composition of sediment preserved on the deep-ocean floor reflects the tectonic evolution, volcanic history, climate changes, and weathering rates of the nearby continental regions. The terrigenous and volcanic components of the sediment result from physical and chemical weathering that act on the exposed crustal rocks at any particular time period in Earth's history. Studying the chemical composition of terrigenous matter in deep-sea sediment can help to answer questions of how sediment is weathered and transported, address the composition and geographic location of the sediment source(s), and contribute to our understanding of regional and global chemical mass balances.
The Caribbean region is an opportune place to study these and other topics for several reasons. First, the Caribbean Sea region is unlike other Atlantic-type ocean basins. The Caribbean plate is allochthonous with respect to its neighbors, making it difficult to reconstruct its paleogeology. For example, there are many models of the plate tectonic evolution of the Gulf of Mexico and the Caribbean, all of which are inconclusive in some regard (Burke et al., 1978; Duncan and Hargraves, 1984; Burke, 1988; Donnelly, 1989; Pindell and Barrett, 1990; Morris et al., 1990). In this context, the past volcanic activity in the region is not thoroughly known. Secondly, the region (Fig. 1) holds a central tectonic and geographic position because it lies between the North and South American plates and the Pacific and Atlantic Oceans. Accordingly, it played a vital role in global ocean circulation until the closing of the Isthmus of Panama, which began at ~13 Ma and was completed at ~1.9 Ma (Farrell et al., 1995, and references therein). Return flow from the South Atlantic that feeds the Gulf Stream moves through the Caribbean, and paleoceanographic studies of carbonate preservation (e.g., Droxler and Burke, 1995) illustrate the Caribbean's important geographic location today.
For ~20 yr before Leg 165, there was no recovery of deeply buried sediment in the circum-Caribbean region. Five sites were drilled during Leg 165, and, of these, we discuss chemical records from Sites 998, 999, and 1001 here. These sites cover nearly 80 m.y. of Earth's history (Sigurdsson, Leckie, Acton, et al., 1997). The two sites visited during Leg 165 and not discussed here (Sites 1000 and 1002) have unique characteristics that make them inapplicable to this study: Site 1000 is a drowned carbonate platform (Droxler et al., 1998) and Site 1002 is a large anoxic marine basin (Cariaco Basin) with only a 600,000 yr record (Peterson et al., 1998).
Our results indicate that throughout the region the sediment is dominated by the calcium carbonate (CaCO3) fraction. The noncarbonate fraction is composed of detrital sediment (hereafter referred to as "terrigenous") and dispersed ash. Dispersed ash is different from discrete ash layers (Carey and Sigurdsson, 1998; Carey and Sigurdsson, Chap. 5, this volume) and refers to ash that is physically and chemically mixed with the carbonate and other noncarbonate sediments. In this paper, we use the bulk sediment chemistry to quantitatively determine the absolute percentages of CaCO3, terrigenous matter, and dispersed ash in the sediment sequences at the three sites. We will discuss the temporal and paleogeographic changes in terrigenous supply, the timing and magnitude of increases and decreases in the accumulation of dispersed ash, and interpret the paleoceanographic patterns in the context of the changing tectonics of the Cenozoic Caribbean Sea.