THE SIERRA BLANCA LIMESTONE: A CASE STUDY FOR NORTH AMERICAN CARBONATE BIOCONSTRUCTION IN THE EARLY EOCENE GREENHOUSE

The effects of anthropogenic climate change pose a serious threat to reefs. Many reef-building corals contain zooxanthellate photosymbionts which provide up to 90% of their energy. For these corals, thermal stress-induced loss of photosymbionts, or bleaching, is of particular concern and often leads to death. Although modern reefs are dominated by corals, reefs can also be built by other organisms including coralline algae, larger benthic foraminifera, bivalves and sponges. Continued climate change may alter 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 increase understanding of how modern reefs will respond to continued anthropogenic change.

Generally, reef crises are associated with ocean warming and acidification during ancient hyperthermals throughout Earth’s history. The early Paleogene was the warmest time in the Cenozoic and is characterized by a steady increase in temperature from the early Paleocene through the early Eocene, culminating in the Early Eocene Climate Optimum. Coral 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, and therefore may provide insight into how reefs will respond to continued anthropogenic warming and acidification.

The Sierra Blanca Limestone in Santa Barbara County, CA, is a rare example of an 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 North American bioconstruction in the early Eocene greenhouse, using thin section and polished slab analyses to understand its structure and composition. Preliminary data suggest deposition on an offshore topographic high devoid of siliciclastic sedimentation as indicated by the taxonomic composition and growth forms of rhodoliths and associated fauna. Further analysis of this bioconstruction may provide additional insight into changes in reef building induced by continued anthropogenic CO₂ emissions.