What do we learn from peaks of uranium and of manganese in deep sea sediments?

Abstract

Uranium and manganese are two redox-sensitive trace elements with complementary behavior. Under anoxic conditions, U is immobile, while reduced Mn is dissolved in pore water of sediments. Uranium diffuses from sea water into reducing sediments and is enriched there. The depth of the reducing layers and the sediment accumulation rate determine the enrichment of authigenic uranium. On the contrary, Mn is dissolved in the reducing sediment sections, diffuses in pore water and forms peaks of Mn-oxide as soon as oxygen is available. Thus, the profiles of both Mn and authigenic U record the depth of the redoxcline during periods of oxygen limitation in the segments. Estimates of the depth of incorporation of uranium during the last glacial were derived for several cores at localities in the Atlantic. Numerical modeling of the redoxcline as a function of the oxygen content of the water column and of the flux of organic matter to the sediments, constrains these values during periods of extreme shallow redoxcline depths. Burn-down of organic matter and deepening of the redoxcline during the Holocene may have significantly altered the organic matter and uranium signals of glacial sediments. At present-day conditions, sediments accumulating at rates smaller than 2 cm ky-1 may have lost the U signals. Peaks of manganese are then the only reliable record for a less ventilated sediment column in the past. © 2001 Elsevier Science B.V.