Continuous and Episodic Modern Sediment Accumulation on Monterey Fan: Evidence from Ddt, 137Cs and Excess 210Pb

The mode and magnitude of fine-grained sediment accumulation on the Monterey Fan off the California central coast was investigated using pesticide concentrations and radioactive tracer profiles in sediment cores. DDT is a man-made pesticide that was used extensively in central California between 1945 and 1970. As such, its presence in marine sediments is a telltale sign of a modern sedimentation age. DDT and its metabolites, DDE and DDD, (collectively referred to as DDTr) were measured in fifty-five ~20cm-long sediment cores collected from the surface of the Monterey Fan up to 250 km to the south and 210 km to the west of the Monterey Canyon head, and in four transects across the Monterey Canyon channel at maximum water depths of 3160, 3380, 3580, and 3880 meters. Profiles of excess ²¹⁰Pb (²¹⁰Pb_xs) and ¹³⁷Cs were measured in 5 cores from the Fan to estimate recent sedimentation rates. Detectable levels of DDTr were observed in all but one of these cores, with DDTr concentrations characteristically highest at the surface and decreasing with depth. The area-normalized and depth-integrated DDTr content measured in all the cores in the Fan and in the deepest two channel transects was geographically fairly homogenous, with no statistical relationship between DDTr inventory and distance from the main channel crossing the Fan. The total sediment mass deposited on the Fan over the last 60 years, inferred from the total inventory of DDTr present in the area surveyed, is consistent with the amount of sediment delivered by the Salinas River over the same time period. ²¹⁰Pb_xs activities are fairly homogeneous within an uppermost layer of variable thickness (4.6-8cm) and decrease exponentially below it, but these exponential decreases are often interrupted by horizons with constant or increased ²¹⁰Pb_xs activity. Moreover, the coexistence of variable DDTr concentrations with homogeneous ²¹⁰Pb_xs activities in the top sediment indicates that the uniformity of ²¹⁰Pb_xs is not due primarily to bioturbation but rather the result of deposition of thick layers with constant ²¹⁰Pb_xs. These data indicate that fine-grained sedimentation on the Monterrey Fan appears to have a dual modality consisting of sustained slow accumulation at a modest rate during quiescent periods interrupted by episodes of rapid accumulation during discrete events. Median sedimentation rates during quiescent periods are 0.07 cm yr^-1, but over the 110 year period of detectable ²¹⁰Pb_xs, which includes both slow continuous sedimentation and rapid accumulation events, the median overall sedimentation rate, calculated from the deepest detectable ²¹⁰Pb_xs samples, is 0.19 cm yr^-1. On a per mass basis, sedimentation on the Fan has been equally divided between periods of continual slow accumulation and episodes of fast accumulation. The rapidly deposited layers on the Monterey Fan are inferred to result from the injection of fluvial sediments onto the Fan during episodic and exceptionally large floods of the Salinas River.