174-8 Coralline Algae Bioconstructions in the Early Eocene Greenhouse

Session: Environmental Instability During Greenhouse Periods: Impact on Terrestrial and Marine Ecosystems

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Anthropogenic climate change poses a serious threat to reefs. Under thermal stress, many reef-building corals are susceptible to the loss of photosymbionts, or bleaching, which often leads to death. Modern reefs are dominated by corals, but reefs can be built by other organisms including coralline algae, foraminifera, bivalves, and sponges. Continued climate change may shift the abundance and composition of reefs, altering marine ecosystems. While the nature of these changes remains uncertain, the fossil record of reefs during ancient climate change episodes may provide insight to how modern reefs will respond to anthropogenic change.

Reef crises have been associated with ocean warming and acidification during ancient hyperthermals throughout Earth’s history. The early Paleogene was the warmest time in the Cenozoic and was characterized by increasing temperatures from the early Paleocene through the early Eocene, culminating in the Early Eocene Climate Optimum. Coralgal reefs were restricted to low then mid latitudes as temperature increased in the Paleocene and disappeared almost entirely by the early Eocene. Climate-induced changes in geographic range, abundance, and composition of reefs during the early Eocene may be indicative of how reefs respond to ocean warming and acidification in general.

The Sierra-Blanca Limestone in Santa Barbara County, California, is a rare early Eocene bioconstruction built predominantly by free-living coralline algae called rhodoliths. In this study, I use the Sierra-Blanca Limestone as a case study for bioconstruction on the west coast of North America in the early Eocene greenhouse.

Sierra-Blanca rhodoliths are primarily composed of coralline genera Mesophyllum, Lithothamnion, and Sporolithon interlayered with bryozoans, indicating a cool water carbonate setting. Grainstone to rudstone facies and spheroidal morphology of rhodoliths indicate a high-energy environment with regular water movement. A lack of siliciclastic sediment suggests rhodoliths accumulated on an offshore bathymetric high beyond the reach of terrestrial sedimentation.

Despite widespread ocean warming and acidification in the early Eocene, refugia existed where environmental conditions allowed the proliferation of reef-like rhodolith bioconstructions. In the future, similar offshore bathymetric highs along the west coasts of North and South America may be suitable for the development of large rhodolith beds, preserving marine diversity as coral reefs continue to decline.

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

doi: 10.1130/abs/2025AM-8958

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