Radionuclides as indicators of sedimentary processes in abyssal Caribbean sediments

Abstract

Concentrations of 210Pb and 239,240Pu were measured in triplicate sub-cores from three different sedimentary environments of the Venezuela Basin. 239,240Pu profiles gave bioturbation rate coefficients from 500 to 900 cm 2 ka −1. Bioturbation rate coefficients determined from 239,240Pu distributions were not consistent with those derived from a two-layer mixing model applied to the 210Pb concentration data. A four-layer mixing model established that, at two of the sites, mixing was most rapid from 2 to 7 cm, indicating that macrofauna at these depths are more effective bioturbators. These findings are discussed with reference to the animals found at each site. Approximately 20% of the 239,240Pu that has fallen to the surface of Caribbean Sea is present in the sediments. Excess 210Pb inventories varied from 30 to 110% of the estimated 210Pb in the overlying water column. Sediments from the hemipelagic, turbidite, and pelagic carbonate sites contained 121, 108, and 40% of the estimated flux of 210Pb to the sea floor. Higher 210Pb inventories at the hemipelagic and turbidite sites could have resulted from the recent incorporation of shelf sediments rich in 210Pb. Alternatively, high inventories may indicate that 210Pb, produced in deep waters, is effectively scavenged by the cycling of reduced metals near the sediment—seawater interface. Iron and manganese are found in reduced forms near the sediment—seawater interface at these sites.