Detrital terrestrial sediments preserved in near-shore marine basins bear distinctive geochemical identifiers that can be used to identify the on-shore sediment sources and sediment routing through time. Santa Barbara Basin (SBB), offshore of southern California, USA, contains a well-known, continuous, high-resolution Holocene flood record that can provide insights into the frequency and changes in on-shore sources across time for such events. Here SBB-adjacent stream bed sediments are characterized using mineralogical, elemental, and radiogenic strontium and neodymium isotopic compositions. Modern and Holocene SBB flood deposits and Last Glacial Maximum (LGM) sediments were similarly analyzed. The Southern Slopes of the Santa Ynez Mountains and Topatopa Mountains account for ∼85% of SBB Holocene flood deposit sediments, as calculated from Sr-Nd isotope mixing models. During the LGM sea level low stand, the Southern Slopes contribution increased (to ≥90%), while relative sediment contribution from Santa Clara River diminished. This loss was likely compensated, however, by increased sediment flux from the Southern Slopes and the Channel Islands.

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