Ocean Drilling Program, United States

Records of long-term sediment deposition in the Caribbean Sea were recovered during Ocean Drilling Program Leg 165. Samples from the Cayman Rise (Site 998), the Colombian Basin (Site 999), and the Hess Escarpment (Site 1001) were analyzed for calcium carbonate (CaCO (sub 3) ) by coulometry and for selected major and trace elements by X-ray fluorescence and inductively coupled plasma-emission spectrometry. These data were used to quantify in the bulk sediment the absolute concentrations of CaCO (sub 3) , terrigenous matter, and dispersed ash (as opposed to discrete ash layers). The weight percent of terrigenous matter was computed using a Cr-based normative calculation, and dispersed ash content was calculated by difference; the assumption of a three component system (CaCO (sub 3) , ash, and terrigenous matter) is justified by and consistent with petrographic analysis. Sites 998 and 999 broadly exhibit the same pattern of terrigenous accumulation. Both show a general decrease in terrigenous accumulation rates during the Oligocene and early Miocene, except for a sharp increase at Site 998 during the early Oligocene (30-40 Ma) and significant increases in the late Miocene and late Pliocene/early Pleistocene. The same pattern in terrigenous accumulation is recorded at Sites 925 and 929 in the Ceara Rise (Atlantic Ocean), which receives input from an Amazon River source, demonstrating that Sites 925, 929, 998, and 999 collectively provide a circum-Andean record of tectonic uplift, with the Leg 165 sites responding to inputs from the Magdelena River system. Both Sites 998 and 999 appear to be responding to South and Central American inputs, particularly after the middle Miocene; however, the variation in the terrigenous, carbonate, and dispersed ash at Site 998 point to an erosional event during the Oligocene that is apparently unique to this site's location. Dispersed ash commonly accounts for 15-20 wt% of the bulk sediment, and in some cases up to 45 wt%. The accumulation of dispersed ash typically leads the accumulation of discrete layers by 2-10 m.y. These changes in sediment composition could signify (1) the distance from the source of volcanism, (2) periods of small volume volcanic activity preceding the large eruptions, or (3) the transportation to the deep sea of terrestrially deposited ash preceding the large eruptions.