Exploration on variations in chromium/cobalt and vanadium/cobalt ratios in pelagic sediments

In this paper we explored variation in Cr/Co and V/Co ratios with time in 21 pelagic clay samples from various locations in Pacific Ocean. We also included data from previous researchers to illustrate the trend we observed. Reasons for the observed change are explained and pros and cons for each one are examined. What motivated us to start this research is that redox evolution of oceans through time is one of the most desired but elusive parameters in paleo-oceanography. Most attempts have focused on tracking variations in redox-sensitive elements in anoxic margin sediments. However, redox conditions in marginal basins vary widely due to local effects and do not necessarily represent the conditions of the open ocean. As an alternative approach, bulk-rock Cr/Co and V/Co ratios in oxic pelagic clays are explored here as tracers of redox-evolution of the global oceans on the million year timescale. Bulk-rock Cr/Co and V/Co ratios are always much lower than that of riverine inputs. One of the explanations for this phenomenon is that Cr and V are probably sequestered in anoxic environments on oceanic margins whereas the open ocean appears to be the major sink for Co. Based on this mass balance approach, it seems that, in theory, it should be possible to use these elemental ratios as potential tracers of redox state in the ocean. However, here we were not able to confirm this possibility. Bulk-rock Cr/Co and to a lesser extent V/Co ratios in several pelagic clay sites in the Pacific show a rise from ~25 Ma ago to the present. While this secular change in bulk ratios might suggest an increase in oxygenation of the Pacific Ocean, the changes can be simply explained by an increase in detrital component, such as wind-blown dust or volcanic ash, or equivalently, by a proportional decrease in hydrothermal inputs. In order to minimize influence on the change caused by detrital component, we tried to calculate hydrogenous component in our samples by subtracting out a detrital component that has a constant composition, which in reality is likely to vary significantly and the value is not known well. Besides variation in the composition of the detrital component, there are other explanations including diagenetic effects and changes in the composition of the dissolved inputs into rivers with time. We conclude that although Cr/Co and V/Co hold promise as potential redox-sensitive tracers, interpreting their signals is presently frustrated by many complicating factors