Some 7,000 years ago, the Dominican Republic's Lake Enriquillo basin was a sheltered bay lined by 100 km of fringing coral reef. After a river delta blocked the entrance to the bay, the sea evaporated, leaving a salty lake 42 meters below sea level. Eventually, storm channels carved through the exposed fossil reefs. Exploring the towering walls of solid fossil coral reef is as close as scientists can get to traveling back in time to an age of pristine coral reefs.

A hallmark of the Anthropocene is the speed at which humans leave a permanent stamp on Earth's geological record. Major environmental changes that usually play out over tens of thousands of years happen during human generations. The decline of Caribbean reefs is a striking example.

Decline has happened so quickly that a clear picture of healthy reef communities even a century ago eludes us. Filling this knowledge gap is as essential as understanding how reef remnants have proven resilient to pollution, overfishing, disease, ocean warming and acidification. Once scientists and conservationists sort out what it takes for reef communities to reverse course, they’ll need a yardstick to measure the success of restoration.

Building that tool is one goal of STRI staff scientist Aaron O’Dea’s Baseline Caribbean project, which recently wrapped up an intensive two-week field trip to two spectacularly preserved fossil reef sites.

O’Dea and his team — consisting of STRI fellows Erin Dillon and Mauro Lepore and O’Dea Lab manager Félix Rodríguez — collected dozens of bulk samples from the exposed Dominican fossil reefs. They focused on sites that had well-preserved sediments holding tiny fossils that help reconstruct the composition of the whole reef community many thousands of years ago. These include the ear bones of fish (otoliths), the spiny skeletal parts of sponges (spicules), microscopic protozoans (foraminifera), and, as part of a new research technique being pioneered by Dillon, shark scales (dermal denticles).

Shark Clues

Using these tiny and hard-to-find denticles (a 10-kg bag of sediment may only contain 25 of them), Dillon hopes to discover what shark communities on healthy Caribbean reefs looked like. As an apex predator, sharks play a key role in shaping reef ecosystems, and denticles are the best available clue to reconstructing pre-human Caribbean shark baselines.

To assess whether shark communities have shifted over time, the team collected samples from modern reefs that were comparable to the fossil reefs. During the field trip, Dillon and Lepore took reef sediment samples from the degraded reefs of Samaná Bay, which is ecologically and geographically similar to Lake Enriquillo before it was isolated from the Caribbean.

The Dominican Republic samples expand the O’Dea lab’s project from Bocas del Toro, Panama, where they have been working to establish a quantitative coral reef baseline with which to compare the ecology and health of Caribbean reefs during the age of humans.