Accumulation rates and sources of sediments and organic carbon on the Palos Verdes shelf based on radioisotopic tracers (Cs-137, Pu-239,Pu-240, Pb-210, Th-234, U-238 and C-14)

We report here bioturbation and sediment accumulation rates determined from replicate sediment cores at four different sampling sites on the Pales Ver des shelf, Southern California, using bomb fallout and natural radionuclide s (Cs-137, Pu-239,Pu-240, Pb-210, Th-234, and C-14), along with supporting measurements of organic carbon (OC), porosity and granulometry. Present-day particle reworking, on time scales of several months, is restricted to the upper 3 cm, with rates ranging from 13 to 200 cm(2)/year, as deduced from 234Th,, profiles. There is little evidence that particle reworking reached depths significantly greater than 5 cm. Post-1963 (or post-1971) sediment a ccumulation rates ranged from 0.7 to 1.4 g/cm(2)/year (equivalent to 1.1-1. 8 cm/year for surficial sediments), as calculated from Pu and Cs isotope pr ofiles, with Little change over time or distance from the outfall. Lateral transport of older sediment and multiple sediment sources on the Pales Verd es shelf is suggested from radiocarbon measurements on foraminifera and bul k sedimentary organic matter at two sampling sites, which showed variable, old and refractory sources of OC, Pre-1953 sediments accumulated at rates t hat were at least 0.4 g/cm(2)/year (greater than or equal to 0.3 cm/year), based on Pb-210(xs) dating. Given the abundance of sediment sources to the Pales Verdes shelf, the high sedimentation rates, and shallow particle mixe d layers, contaminant-enriched layers should continue to move deeper into t he sediments. (C) 2001 Elsevier Science B.V. All rights reserved.