EARLY EOCENE CARBON ISOTOPE EXCURSIONS IN RED ALGAL LIMESTONES FROM THE SANTA SUSANA FORMATION AND SIERRA BLANCA LIMESTONE, SOUTHERN CALIFORNIA

Environmental and stable isotopic studies demonstrate that the Paleocene-Eocene Santa Susana Formation and laterally equivalent Sierra Blanca Limestone of Southern California are interpreted to record short-term hyperthermal events overprinted on the general long-term warming trend of the early Paleogene. These episodes of abrupt climate change, as recognized in the deep-sea foraminiferal record, are manifested in California as sudden bursts of shallow water carbonate precipitation often on isolated tectonic highs, with reefs dominated by coralline red algae, notably Lithothamnium sp. Lithothamnium, which generally flourishes within the margins of the habitable zone for typical reef builders. Although there is no obvious trend in abundance or latitude of reefs into and across the Paleocene-Eocene Thermal Maximum (PETM) and the early Eocene Climatic Optimum (EECO), the Lithothamnium fraction of these reefs increases globally at ~57-54 Ma. A ~40 m thick limestone unit within the Santa Susana Formation, located in the Santa Monica Mountains, records a sharp negative (~4‰) carbon isotope excursion (CIE) interpreted to be associated with the PETM. This CIE is between sandstone units with Paleocene mollusks and those with early Eocene gastropods. Red algal carbonate dominates the section following the CIE, suggesting the promotion of such algal reefs after the PETM. The Sierra Blanca Limestone is more widely exposed and better studied than the limestone of the Santa Susana Formation. A PETM-like CIE has not been found at two of the thickest sections of Sierra Blanca Limestone, the type locality in the San Rafael Mountains and a locality in the Santa Ynez Mountains. A longer-lived and larger CIE (~5‰) is observed through ~2.5 m of limestone in the middle of the Santa Ynez Mountains section. We interpret the Sierra Blanca CIE to be correlative with the multi-million year long EECO. Oxygen isotope trends are inconclusive due to diagenetic and post-depositional thermal overprints. The Santa Susana-Sierra Blanca reefs record increased carbonate reef development during a period of both deep sea and continental shelf carbonate dissolution. Possible explanations include greater Ca/Mg, or greater Ca delivery in the shallow water caused by increased weathering of Ca-rich anorthosites in the San Gabriel Mountains.