Authigenic Uranium Formation and Preservation in Two Contrasting Settings: Santa Barbara Basin and Middle Atlantic Bight, and a Comparison with Molybdenum

Abstract

U and Mo are among a group of redox sensitive elements which are highly soluble in oxygenated ocean water, but become much less soluble in reducing sediments. In the case of Mo, this reduction in solubility causes enrichment of Mo in sediments underlying sulphidic anoxic waters. Another mechanism causing Mo enrichment in sediments under more oxygenated waters is co-precipitation with Mn-oxides. In the case of U, authigenic precipitation is caused by chemical or microbial reduction at a lower redox potential than required formation of authigenic Mo under sulphidic conditions. The difference in redox potential at which U and Mo are reduced can therefore be exploited to reconstruct the redox state of ocean sediments in the past. Bottom water oxygen (BWO) and organic carbon flux (OCF) are the main determinants of the redox state of sediment. We present results from the Santa Barbara basin (SBB) and the Middle Atlantic Bight (MAB) to compare the behaviour of U and Mo (SBB only) under different BWO ( 1 gM sulphide in the water column. We also measured a Mo flux of 87 gg/cm 2 ka for sinking particles collected in a sediment trap. This source of particulate Mo can account for most of total sediment authigenic Mo in sediments at BWO> 10 p.M, but only 10 to 20% of total sediment authigenic Mo MAR at BWO <5 laM. The SBB data demonstrate that authigenic Mo formation is not observed when BWO is above 10 laM even under very high OCF.