The Fate of Barite in Sediments Under a Coastal Upwelling System.

Marine barite has been associated with decaying organic matter that carries particulate Ba to the seafloor and consequently it has been directly related to marine organic carbon flux. Ba normalized to Al otherwise known as bariumexcess is often used to demonstrate past changes in oceanic productivity. However, it is well known that barite is extremely susceptible to dissolution in suboxic conditions. The solubility of barite in deep ocean waters remains low, until depletion in SO42- begins to occur. At which point dissolution of barite occurs leading to increased dissolved barium in the sediment porewaters. ODP Hole 1017E (34°32'N, 121°6'W) on the Southern California Margin provides an ideal location to examine the relative contribution of productivity and redox history to the production and preservation of barite in sediments. This site lies beneath an upwelling cell and has yielded high-resolution records of paleoproductivity. The location of the site within the lower oxygen minimum zone (OMZ) along with high organic carbon rain results in changes in pore water oxygen concentration. Presently the OMZ is a significant feature along the California Margin. However, during the last glacial, large changes in porewater oxygen have been documented suggesting the waxing and waning of the OMZ in close concert with rapid climate change in the North Atlantic. Multiproxy results from ODP Hole 1017E have allowed researchers to determine both the history of past productivity changes as well as understand the redox chemistry of the sediments. Here the record of bariumexcess is presented demonstrating a history of marine barite delivery to the sediments as well as changes in the shallow sulphate reduction depth. Significant barite enrichment occurs as a diagenetic front at the modern sulphate reduction boundary. This boundary also intersects a sediment facies containing fine grained-organic rich sediments deposited during Interstadial Event 7 (35 Ka) of the last glacial. Changes in diffusion rates associated with sediment facies change produce a strong but misleading correlation between barite enrichment and rapid climate change. Thus within the bariumexcess record at ODP Hole 1017E is a history of the shallow (upper tens of cm) redox conditions and an indication of modern deep redox chemistry.