Coralline algae, calcifying red algae of the orders Corallinales, Hapalidiales and Sporolithales, are important components of many marine ecosystems, providing stability and protection from high energy waves in coral reefs, and providing shelter, substrate and nutrients for many organisms across a range of marine environments. Corallines are thought to be amongst the most vulnerable marine organisms in the face of climate change, as their high-Mg calcite mineralogy makes them especially vulnerable to ocean acidification. Corallines also require light to perform photosynthesis and are therefore vulnerable to siliciclastic sedimentation which can reduce illumination and bury corallines. Lab based acidification experiments show most coralline species respond negatively to elevated CO2 conditions, but the nature and magnitude of effects vary between species, and the rate of experimental CO2 increase is much faster than natural rates, making it difficult to use experimental data to predict how corallines respond to acidification on time scales of thousands to millions of years. While most coralline groups are not directly affected by temperature changes, global warming can alter the hydrosphere, increasing precipitation intensity and therefore terrestrial runoff in some coastal areas. Increased runoff can directly lower illumination in the water column and can also cause phytoplankton blooms that significantly reduce illumination. The earth’s climate and ocean conditions have changed greatly throughout the Cenozoic, with significant effects on many marine organisms including corals, benthic foraminifera, bivalves, sponges, and many other groups. The response of coralline algae to Cenozoic climate change, however, has received less research attention despite their importance in marine ecosystems.
This dissertation uses the fossil record to assess how coralline algae responded to different types of Cenozoic environmental change at three different scales. Chapter 1 looks at coralline response to rapid warming, acidification and sediment influxduring the Paleocene-Eocene Thermal Maximum (PETM) in northeastern India and Tibet. The PETM is the largest hyperthermal of the Cenozoic, where a massive release of CO2 caused widespread ocean acidification and warming of ~5-8 °C over ~10,000 years. At both study locations, coralline abundance is high in the pre-PETM and remains high into the PETM onset. Corallines then disappear abruptly as carbonate sedimentation rapidly ends under heavy siliciclastic input. The results of this chapter suggest that coralline algae may be more tolerant of acidification, but less tolerant of sedimentation, than previously thought.
Chapter 2 uses the early Eocene Sierra Blanca Limestone in Santa Barbara County, California, as a case study for rhodolith bed construction in the early Eocene greenhouse. Elevated temperatures and CO2 in the early Eocene prevented rhodolith beds from developing in most areas, so the Sierra Blanca Limestone provides rare insight into the environmental conditions allowing a robust coralline deposit to form in a seemingly hostile ocean. The lithology, taxonomic assemblages, and characteristics of Sierra Blanca rhodoliths suggest they developed on an offshore uplifted block along an upwelling coastline, allowing rhodoliths to thrive in relatively cool water beyond the reach of siliciclastic sedimentation.
Chapter 3 examines what controls the partitioning of coralline algae into reef and non-reef habitats on carbonate platforms during the Paleogene and Neogene. Coral reefs are significantly impacted by climate change during this interval, but coralline algae are more resilient. Coralline algae occupy non-reef settings through the studied interval and their abundance in non-reef environments shows little variation. As coral reefs develop, corallines persist in non-reef environments and expand into reefs where they play a crucial role in cementing the framework. When coral reefs collapse, corallines do not take over reef building, but persist and even expand in non-reef settings, often constructing rhodolith beds that serve some of the same ecological roles as coral reefs.
Together, these chapters highlight the ability of coralline algae to adapt to changing environmental conditions despite their assumed vulnerability. The discrepancies between lab-based experiments and paleontological observations suggest that the rate of change is vitally important: corallines can adapt to even significant change if it occurs slowly enough but are less able to adapt to rapid environmental change.
Hassel K., 2026. The response of coralline algae to cenozoic environmental change. PhD thesis, University of California Santa Cruz, 89 p. Thesis (access restrict).
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