One of the problems of trying to work out what coral reefs were like before human impact is that the reefs of the past are underneath the reefs of today.
One way to get to these reefs is by coring through the reef and producing a timeline of the coral reef as it grew. This is great for small and common fossils, such as molluscs and fish teeth, but the core itself is only 10cm in diameter and so very large animals (such as large coral heads) and very rare fossils (such as shark dermal denticles) are not captured. This means the complexity of the coral reefs is often missed.
One way to get to the whole reefs of the past, and capture the full variation and natural complexity of the reefs, is wait for an excavation to dig out and drain the modern reef, exposing the fossil reef underneath. In Bocas del Toro we had the good fortune to be able to explore one such massive excavation – see the video here.
The Enriquillo Lake in the Dominican Republic was originally a marine embayment, flanked by hills on either side and open to the ocean to the east. The coral reefs fringing these hills are those we are studying. Around 4,000 years the River Yaqui del Sur deposited a large delta right in front of the entrance to the bay, sealing it off from the open sea. Bbecause the area sits in a rain shadow, evaporation exceeds precipitation and the lake began to dry out. Eventually the lake level dropped to below 40m below sea level and became hypersaline. With the reefs exposed above the lake level, storm channels started to cut through the ancient reefs and it is in these small canyons that we get to see the incredible full sections of coral reefs that grew between 6,000 and 9,000 years ago.
Recently, the lake level has started to rise again and nobody is sure why. Many theories have been proposed from climate change to natural variability. Whatever the cause, the impact on the local communities around the lake is devastating as it eats up the little arable land they have, and drowning the roads that encircle the lake. The government seems to be taking steps forwards helping and have started building a new road 20m or so above the lake level.
The problem for us is that if the lake continues to rise at the steep rate of recent years, within a decade these spectacular fossil reefs could once again be under water, and unavailable for study.