Molybdenum and uranium geochemistry in continental margin sediments: Paleoproxy potential

We measured solid-phase Mo and U concentrations in marine sediments from the California, Mexico, Peru, and Chile margins to ascertain the behavior of Mo and U during early diagenesis in continental margin settings. At sites along the California, Mexico, and Peru margins where there are estimates of mass accumulation rates, authigenic U accumulation rates range from similar to 0-50 nmol m(-2) day(-1). At the California and Mexico margin sites Mo accumulation rates range from 0 to 134 nmol m(-2) day(-1) whereas at the Peru margin site rates may be as high as 550 nmol m(-2) day(-1). We observe relationships between metal accumulation rates and the delivery and burial rates of organic carbon (C-org). In the case of Mo there appears to be at least two relationships between metal accumulation rate and organic carbon burial. For most of the data presented in this manuscript, continental margin sediments have a MO:C-org accumulation rate ratio of similar to 20 mu mol mol(-1). This value is significantly lower, however, than those reported for anoxic basins similar to 100 mu mol mol(-1), but is consistent with reported Mo:C-org rain ratios from Mexico margin sediment traps. In contrast to Mo, there appears to be a single U:C-org burial ratio of similar to 5 mu mol mol(-1), which includes a range of environments from anoxic basins and open ocean sites. We interpret the relationships between metal accumulation and organic carbon to indicate that the reactions that govern authigenic metal accumulation are primarily sensitive to the delivery and burial of organic carbon in these particular settings. However, we note that any relationship between metal accumulation and organic carbon could be indirect. In the particular case of Mo, based on what is known about Mo geochemistry from the literature, it is possible that Mo authigenesis is coupled to sulfur cycling in many of the environments covered by this study, and that the observed association between Mo and organic carbon burial is a consequence of the coupling between carbon and sulfur burial. Using the observed relationships between Mo and C-org burial as well as constraints from the Mo isotope budget we estimate that continental margin sediments are an important sink for Mo. The magnitude of this sink (similar to 0.4 x 10(8) mol y(-1), or larger) may be as much as one quarter of the oceanic removal term, and is likely to be larger than the modern anoxic basin sink. (c) 2006 Elsevier Inc. All rights reserved.