Decadal-scale changes to the central California coastal ocean recorded in shelf sediments

The nearshore waters of central California support a marine ecosystem that is important both ecologically and economically. How this system responds to climate change over the next century remains unresolved, but some insight may be gained through understanding the system's response to changes over the past several decades and centuries. This study seeks to investigate the decadal-scale record of recent changes preserved in the Monterey Bay shelf sediment. Multiple short sediment cores were collected from the bay and adjacent open shelf areas, and analyzed using a suite of sedimentological and geochemical techniques. Detailed analysis of PS1410-11MC, approximately covering the past 350 years, shows a significant increase in the rate of sediment accumulation in the late 1970s through the 1980s. This is consistent with a shift in the Pacific Decadal Oscillation (PDO) from a cool phase (negative PDO) to a warm phase (positive PDO). While sediment accumulation rates increase over time, preliminary analysis of total productivity indicators (inferred from the Br/Cl ratio measured by core-scanning XRF) shows a decrease from the late 17^th century to the early 20^th century, but total carbonate productivity (inferred from the Ca/Al ratio) increases through time. In addition, because sediment mixing is minimal in the core, we observed decadal-scale oscillations in total productivity, sediment accumulation, and to a lesser extent carbonate productivity possibly related to ENSO cycles. The influence of ENSO can be supported by the presence of Pseudo-nitzschia australis in the surface sediments, a toxic diatom that was prevalent in southern California waters following the 1998 El Nino. These results suggest that this ecosystem is influenced by longer-term trends related to PDO cycles and climate change, with shorter-term variability likely related to ENSO. Ultimately, these findings highlight the need for anticipating changes to the California Current upwelling system on a variety of time scales, and from a variety of drivers.