PRODUCTION VS. PRESERVATION: A GEOCHRONOLOGICALLY-CONSTRAINED TEST OF THE MONTEREY HYPOTHESIS FOR MIOCENE CLIMATE CHANGE

The Miocene Monterey Formation was deposited in basins along the western margin of North America, and is the primary source and reservoir rock for California’s hydrocarbon resources. Organic carbon-rich intervals in the Monterey Formation of the Santa Barbara Basin have been associated with Miocene climate change. Specifically, the Monterey hypothesis posits that organic carbon abundance in the Monterey Formation reflects changes in primary productivity associated with global cooling, the development of a stronger thermocline, and enhanced upwelling in the eastern Pacific. This model predicts that organic carbon burial in the Monterey Formation was coeval with global positive carbon isotope excursions (including the eponymous “Monterey Event”) and a cooling trend from the Miocene climatic optimum. Here, we develop a sedimentation rate model constrained with U/Pb zircon geochronology, and pair these data with total organic carbon (TOC) and XRF data at ~30-cm resolution. TOC abundance generally follows an inverse relationship with sedimentation rate, suggesting that preservation, rather than primary production, controls TOC abundance in the Monterey Formation of the Santa Barbara Basin. Peaks in TOC mass accumulation rate do not temporally coincide with the positive carbon isotope excursions of the Monterey Event. When normalized for sedimentation rate, productivity proxies show little correlation with TOC abundance through high-TOC intervals. However, clay proxies show a strong positive relationship, suggesting a link between clay abundance and organic carbon preservation. Our data suggests that preservation, rather than primary productivity, drove TOC abundance in the Monterey Formation strata of the Santa Barbara Basin.