MODERN SEDIMENTATION ON THE CENTRAL CALIFORNIA CONTINENTAL SHELF

Nearshore waters of central California support a marine ecosystem that is both ecologically and economically important. While it is unclear how this system will respond to climate change over the next century, some insight may be gained by understanding the system's response to changes over the past several decades. This study investigates modern sedimentation processes preserved in the Monterey Bay continental shelf sediments at decadal time scales using a pair of short (<40 cm) multicore sediment cores that were collected from the bay (PS1410-11MC) and adjacent open shelf (PS1410-04MC). Age control for the cores was established using ²¹⁰Pb geochronology, and the records cover the past ~350 years. Sediment accumulation rates increase from the late 1960s to the late 1990s from < 1 mm/yr to > 4 mm/yr. Preliminary analysis of total export productivity in PS1410-11MC (inferred from the Br/Cl ratio) shows a decrease from the late 17^th Century to the early 20^th Century, but carbonate export (inferred from the Ca/Al ratio measured by core-scanning XRF) increase through time. Because sediment mixing is minimal in this core as estimated from the ²¹⁰Pb profile, we also observed decadal-scale oscillations in total productivity, sediment accumulation, and to a lesser extent carbonate export, possibly related to ENSO cycles. Modern changes to the central California marine ecosystem are detected by the presence of Pseudo-nitzschia australis in the surface sediments, a toxic diatom that became prevalent in coastal California waters following the 1997-1998 El Nino. The timing of the significant increases in sediment accumulation rates through the 1970s and 1980s is potentially consistent with a shift in the Pacific Decadal Oscillation (PDO) Index from a cool phase (negative PDO) to a warm phase (positive PDO). Collectively, these results suggest that Monterey Bay shelf sediments are sensitive indicators of ocean conditions and ecosystem variability, and that the recent sedimentary record can be useful as a baseline for evaluating current and future impacts from climate change.