BARIUM IN EQUATORIAL PACIFIC CARBONATE SEDIMENT - TERRIGENOUS, OXIDE,AND BIOGENIC ASSOCIATIONS

We have analyzed carbonate sediment from Ocean Drilling Program Site 8 50 (equatorial Pacific Ocean) in order to assess the changing effects of sedimentation style on Ba accumulation through time. Formed along t he East Pacific Rise similar to 12 m.y. ago at 1 degrees-2 degrees S a nd now located at 1.3 degrees N, Site 850 has experienced many changes through its northwesterly migration, including a crossing of the equa tor at similar to 4 Ma. We divide sedimentation into three stages (pha ses I, II, and III; with phase III being youngest) according to geogra phic position, lithostratigraphy, and bull; accumulation rate. Of grea test importance are contrasts between phase II (7.5-4.0 m.y. ago) and phase III (4.0-0 m.y. ago). Phase II includes the previously described ''biogenic bloom'' as well as the depositional record of elevated pro ductivity near the equator. In phase II the accumulation of Ba shows t he strongest correlation with the accumulation of CaCO3 (r(2) = 0.69), opal (r(2) = 0.44), and C-org (r(2) = 0.41) compared with elsewhere t hrough the sequence, although the correlation with terrigenous accumul ation is also very strong (r(2) = 0.63). In phase III, which records d eposition in the northern hemisphere and is thus closer to terrigenous input at the Intertropical Convergence Zone (ITCZ), there are no stat istically significant correlations between the accumulations rates of Ba and CaCO3, Ba and opal, and Ba and C-org. Most significantly, throu gh phase III the Ba accumulation rate is extremely strongly tied to te rrigenous accumulation (r(2) = 0.89), which is the strongest correlati on of any in our database, and to the accumulation of Fe-excess (r(2) = 0.74), which we use to track the Fe-oxide component. Cross-equator s urface sediment transects have previously indicated that beneath the I TCZ there is a pronounced local maximum in Ba accumulation, and the st rong tie between Ba and terrigenous accumulation and associated Fe-oxi des at Site 850 also corresponds with its tectonic migration toward th e ITCZ. We conclude that the putative link between Ba accumulation and export production may be obscured by changes in particle composition even within a dominantly biogenic sedimentary regime, as well as by ea rly diagenetic transfer between phases, and that the non-barite elemen tal Ba inventory may complicate the use of elemental Ba as a quantitat ive proxy for barite in the bulk sediment.