176-10 Coastal catastrophe and cooked clams: A landslide provides unexpected insights to the taphonomy of shallow marine ecosystems

Session: Laws of the Grave: Advances in Taphonomy Across the Paleontologic Record

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Abstract:

The historic Portuguese Bend Landslide, located on the Palos Verdes Peninsula of California, reactivated in 2023, resulting in rapid land movement that impacted roads, utilities, and residential areas. The coastline along the margin of the landslide was also heavily affected, and continues to rapidly evolve, with significant changes to substrate availability, shoreline profile, and habitat distribution. Notably, a once intertidal and shallow subtidal rocky habitat was rapidly uplifted and exposed as underlying soft mudstone and bentonite was deformed and bulged upwards. The result was a significant die-off of now stranded low tide and subtidal benthic communities. Concurrently, a new wave-cut platform, comprising soft mudstone and bentonite formed, although this feature has already experienced significant erosion, threatening even the earliest stages of recovery in rocky intertidal communities.

The physical characteristics of this shoreline and the diversity and composition of rocky intertidal communities on it have been monitored over a twelve-month period through an array of transects and quadrats across the current intertidal zone. More than 2000 photoquadrats provide monthly insight into changing habitat characteristics and organism richness and coverage (live and dead) during this period of ecological decline and reorganization. An additional 15 plots are located on boulders that were formerly located in the low tide and shallow subtidal zones, but now exposed to the elements. These have been surveyed periodically using both digital and LiDAR imaging, in addition to a one-time bulk sampling of biological remains (halos) that had accumulated around the base of each boulder. Collectively, these data permit analysis of the transition from a biocoenosis to thanatocoenosis to taphocoenosis in this coastal setting.

While rocky shore assemblages are generally thought to have poor representation in the fossil record, they are frequent and well-preserved in the Quaternary of California. However, little is documented of their fidelity to once living communities. Insights from this unexpected natural experiment are pertinent to interpretation of Quaternary fossil assemblages, and reveal the degree to which the fidelity of diversity and ecological function is lost through taphonomic processes.

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

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