256-13 Stable isotopic data from Pliocene-Pleistocene gastropods indicate long-term stability of shallow water abiotic conditions in southern California, interrupted by late Pleistocene cooling

Session: Life and Environments Through Time and Space: Multi-Record Approaches to Stratigraphic Paleobiology, Part I

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Abstract:

The marine fossil record of southern California includes a diverse and well-preserved molluscan fauna of Pliocene and Pleistocene age. Among the most frequently and abundantly encountered taxa in these shallow marine sediments is the extant gastropod Turritella cooperi P. P. Carpenter, 1864. To better characterize the abiotic conditions associated with Pliocene and Pleistocene deposits in this region we use T. cooperi as paleoceanographic data loggers. We employed stable carbon and oxygen isotope sclerochronology on twelve specimens of this species from six stratigraphic units (in descending age), the, Pico, Niguel, Fernando, and Santa Barbara formations and the San Pedro Sand and Palos Verdes Sand, and across three sedimentary basins,  Santa Barbara, Ventura, and Los Angeles  Basin.

Mean oxygen isotopic values became slightly heavier over time throughout the region, however when adjusted for expected variation in seawater δ¹⁸O this does not necessarily indicate long-term change in seawater temperatures. Early Pliocene temperatures between ~14 and 17 degrees are  consistent with earlier findings of seawater temperatures around 15° C in the Miocene. Using interglacial seawater δ¹⁸O values we are able to detect ~4° C cooling into the end of the Pleistocene, with temperatures ~11° C, consistent with interpretations of these deposits as interglacial intervals. Present-day surface seawater temperatures are ~17° C (with ~4° C in temperature difference between 0 and 100 m water depth), suggesting long-term stability of conditions, interrupted by late Pleistocene cooling.

Most specimens are likely weakly to moderately influenced by seasonal upwelling, based on  negative correlations between carbon and oxygen isotope excursions. The strength of this correlation did not vary with stratigraphic age or geographic position, suggesting fairly consistent influence of upwelling conditions throughout the basin.

Notably, it is uncommon for so many conspecific specimens to be sclerochronologically analyzed at high-resolution. Our findings for Turritella cooperi agree with results from ten Torcula altilira specimens from the Gatún Formation in Panama in observing consistent maximum ages (life span) and similar growth patterns (size at estimated age) among specimens. This supports the validity of life history data from turritellid species with fewer published sclerochronologies. The abundance of Turritella cooperi in the fossil record, including many turritellid dominated assemblages, compared with the relative rarity of the species in the modern, warrants further investigation.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9278

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