Phosphorus components, trace metals and Ba excess of ODP Site 202-1237 sediments

We determined the sedimentary concentrations of phosphorus (P), barium (Ba), manganese (Mn), titanium (Ti), aluminum (Al), and uranium (U) for sediment samples from the southeast Pacific Nazca Ridge, Ocean Drilling Program Site 1237. This unique record extends to 31 Ma over 360 meters composite depth (mcd), recording depositional history as the site progressed eastward over its paleohistory. We sampled with a temporal resolution of ~0.2 m.y. throughout the sequence, equivalent to an average spacing of 1.63 m/sample. Concentrations of sequentially extracted components of P (oxide-associated, authigenic, organic, and detrital) increase toward the modern. Al/Ti ratios indicate that the background detrital source material is consistent with upper continental crust. U enrichment factors (U EFs) generally exceed crustal values and indicate slightly reducing environments. However, authigenic U precipitation can also be influenced by the organic carbon rain rate and may not be solely an indicator of redox conditions. Dramatic changes in Mn EFs at ~162 mcd, from values between 12 and 93 to values <12 after this depth, and a sharp color contact boundary lead us to believe that a paleoredox boundary from an oxygenated to a more reducing depositional environment occurred near this depth. Estimates of biogenic barite concentrations from a total sediment digestion technique (Ba excess) are greater than those from a barite extraction (Ba barite) for selected samples across the entire depth range. Applying a range of Ba/Ti ratios from different source materials to correct for detrital inputs does not change the lack of agreement with Ba barite concentrations. Reactive P (P reactive) concentrations (the sum of oxide-associated, authigenic, and organic P concentrations) increase toward the modern with values typically <12 µmol P/g from the base of our record through ~100 mcd, with a gradual increase to concentrations >15 µmol P/g. Ba excess follows the same general trends as Preactive, with concentrations <14 µmol Ba/g in the lower portion of the record to values >15 µmol Ba/g. Accumulation rate records of these proxies will be needed to infer paleoproductivity. P reactive/Ba excess ratios, an indicator of the relative burial of the nutrient P to organic carbon export, exhibit higher values, similar to modern, from the base of our record through ~180 mcd. The remainder of the record exhibits values lower than modern, indicating that organic carbon export to the sediments was higher relative to nutrient burial.