Evidence of modern fine-grained sediment accumulation in the Monterey Fan from measurements of the pesticide DDT and its metabolites

The documented presence of the pesticide DDT (1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane) and its metabolites DDE (1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene), and DDD (1,1-dichloro-2,2-bis(p-chlorophenyl)ethane), collectively referred to as ΣDDT, in sediments from the Monterey Submarine Canyon, Monterey Fan Channel and Monterey Fan indicates that during the present sea-level highstand the Monterey Fan has been an active depositional system. The presence of ΣDDT in marine sediment is an indication of the recent arrival of terrigenous fine-grained sediment because DDT was only applied on land in California between 1944 and 1972, and is being transported to the ocean in the fine-grained sediment fraction of rivers. ΣDDT was found in 81 out of 82 cores collected in water depths deeper than 3000 m in the Monterey Fan Channel and in the Monterey Fan, within a geographical extent of 250 km to the south and 210 km to the west of the Monterey Canyon head, with surface concentrations averaging 6 and 3 ng g− 1, in the upper and lower Monterey Fan, respectively. This is the first documentation of ΣDDT presence in ocean sediments below 3000 m water depth worldwide. The estimated total amount of ΣDDT in the fan is consistent with the amount of ΣDDT discharged by the Salinas and Pajaro Rivers since DDT started being used in California, suggesting that current fine-grained sediment discharge from these rivers has been accumulating in the fan. The ratio DDE/ΣDDT, an indication of the extent of DDT degradation, provides a first-order proxy for the relative age of the sediments. DDT is found along the thalweg of the Monterey Fan channel as far as 215 km from the canyon head, indicating recent fine-grained sediment transport along the channel. In contrast, only DDE was detected in sediments on the fan, which suggests that the fan does not accumulate fine-grained sediments at a constant rate.