VARIABILITY IN MODERN SEDIMENTATION ALONG THE CENTRAL CALIFORNIA CONTINENTAL SHELF: INVESTIGATING ANTHROPOGENIC AND CLIMATIC DRIVERS IN IMPACTING SHELF STRATIGRAPHY

The continental shelf is a dynamic zone within the continental margin, where sedimentation integrates terrestrial, oceanic, and biological processes with climatic impacts. Anthropogenic influences add further complexity to these sediment dynamics, and parsing the relative impact of human vs climatic impacts may improve our insight into future changes. This study investigates modern sedimentation processes preserved in Monterey Bay continental shelf sediments using multiple short (~15-35 cm) sediment cores that were collected in 2014 and 2015. Sedimentological characteristics (e.g. bulk density, grain size, relative elemental abundances) were analyzed, and age control was established using ²¹⁰Pb/¹³⁷Cs geochronology. Two of the cores have complete chronologies spanning ~150 years. Results show sediment accumulation rates increasing from < 1 mm/yr to > 4 mm/yr during the late 1960s through the late 1990s. Within this trend, data indicate decadal-scale oscillations in total productivity, sediment accumulation, and, to a lesser extent, carbonate export. These shorter-term oscillations may be related to El Nino Southern Oscillation (ENSO) cycles. The likely influence of ENSO in this area is supported 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. Concurrent with the increased accumulation rates is an increase in proportion of sand in the sediment. The timing of these increases is consistent with a shift in the Pacific Decadal Oscillation (PDO) from a cool to a warm phase through the 1970s and 1980s. This period also followed the peak in dam construction on California rivers, and coincides with increased coastal erosion. We suggest these observed changes in shelf sedimentation might result from increased coastal erosion, which may be driven by reduced sediment supply to the coast from rivers due to dams, and/or increased ocean storm during the warm phase PDO with more frequent and intense El Nino events. These results show that Monterey Bay shelf sediments are sensitive archival records of environmental variability resulting from both climatic and anthropogenic impacts on interannual to decadal time scales, and may be useful for evaluating future changes in the region.