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We have analyzed carbonate sediment from Ocean Drilling Program Site 850 (equatorial Pacific Ocean) in order to assess the changing effects of sedimentation style on Ba accumulation through time. Formed along the East Pacific Rise ∼12 m.y. ago at 1°–2°S and now located at 1.3°N, Site 850 has experienced many changes through its northwesterly migration, including a crossing of the equator at ∼4 Ma. We divide sedimentation into three stages (phases I, II, and III; with phase III being youngest) according to geographic position, lithostratigraphy, and bulk accumulation rate. Of greatest 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 productivity near the equator. In phase II the accumulation of Ba shows the strongest correlation with the accumulation of CaCO3 (r² = 0.69), opal (r² = 0.44), and Corg (r² = 0.41) compared with elsewhere through the sequence, although the correlation with terrigenous accumulation is also very strong (r² = 0.63). In phase III, which records deposition in the northern hemisphere and is thus closer to terrigenous input at the Intertropical Convergence Zone (ITCZ), there are no statistically significant correlations between the accumulations rates of Ba and CaCO3, Ba and opal, and Ba and Corg. Most significantly, through phase III the Ba accumulation rate is extremely strongly tied to terrigenous accumulation (r² = 0.89), which is the strongest correlation of any in our database, and to the accumulation of Feexcess (r² = 0.74), which we use to track the Fe-oxide component. Cross-equator surface sediment transects have previously indicated that beneath the ITCZ there is a pronounced local maximum in Ba accumulation, and the strong tie between Ba and terrigenous accumulation and associated Fe-oxides at Site 850 also corresponds with its tectonic migration toward the 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 early diagenetic transfer between phases, and that the non-barite elemental Ba inventory may complicate the use of elemental Ba as a quantitative proxy for barite in the bulk sediment. |
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