Elemental settling and burial fluxes in the Cariaco Basin

The vertical flux of particles and their interactions within the ocean, together with the global ocean circulation, control the water-column distribution of a wide range of elements. Such processes condition patterns of their deposition and burial within marine sediments. Our understanding of these processes can be used to hindcast environmental conditions in the past from the sedimentary record. Here we report the major, minor and trace element compositions of a small sub-set of samples of sinking material collected at four depths (225, 410, 810, 1200m) in the Cariaco Basin (10.5° N, 64.67° W), which is anoxic below ~275m depth. We compare these data with the composition of the most recent basin floor sediment at the same location. The sediment trap samples examined represent material collected during both a high- and a low-productivity season, during a short-period flood event in coastal Venezuela, and immediately following an earthquake that induced a gravity flow. Our results show that the lithogenous composition of all four groups of trap samples and the basin floor sediment is uniform. This indicates that the source of siliciclastic detritus is largely from the local rivers draining the coastal region to the south of the Cariaco station. Enrichments of Ba relative to a model lithogenous background are found in the opal-rich high productivity samples from all depths; enrichments of Ag, Cd, Cu, Ni and Zn, directly associated with the flux of phytoplankton organic matter from the photic zone, are found not only in the high productivity samples, but also in the low productivity samples that have higher CaCO3 contents and lower organic matter and opal contents. In addition, modest particle and sediment enrichments of some redox-sensitive trace elements that are known to be removed from solution and sequestered in suboxic and anoxic sediments, namely Cr, V and Re are observed. However, barely detectable enrichments of U and Mo are present in all trap samples. In contrast with the trap sample compositions, the modern laminated basin-floor sediment exhibits significantly higher contents of Re, Mo and U than a solely lithogenous contribution, confirming studies that have found greater sub-sea floor fixation of these elements. Finally, the high settling flux of Ba observed in the high productivity trap samples is not preserved in the bottom sediment.