On July 5th, I entered Science heaven, literally ☺ As a result of a collaboration lead by Rich Aronson and Lauren Toth at Florida International University (FIT), we published our research findings in the journal Science (http://www.sciencemag.org/content/337/6090/81.short).
In this study, we show that coral reefs in the Tropical Eastern Pacific had collapsed for about 2500 years. This disaster, which started 4000 years ago, was driven by increased intensity and variability of the so-called El-Niño Southern Oscillation (ENSO) and was a Pacific-wide phenomenon. The same natural cycle is still ongoing today and periodically causes great harm by dramatically changing weather patterns, which often leads to severe droughts, floodings and sea surface temperature anomalies that can cause catastrophic coral mortalities. It currently occurs every five to seven years but is predicted to increase in frequency and intensity due to global climate change, which makes our study so important and alarming.
We used 6m long aluminum tubes (i.e. rain gutters) that we pushed into the reef to extract push-cores (vertical samples of reef substrate; I’ll post some fieldwork pics later). These cores consist mostly of sediments and coral fragments that we dated, using mostly radiocarbon methods. Since reefs tend to grow constantly upwards, the cores allow us to look back into the history of these reefs, up to 6900 years in this case. We can thus determine the species composition of a reef at specific points in time. Moreover, we can draw conclusions about past environmental conditions by looking at the preservation of coral fragments: well preserved coral fragments indicate that the coral skeleton was quickly buried in the rapidly growing reef after the coral died while poorly preserved fragments indicate erosion and abrasion due to prolonged exposure.
Applying these techniques to push cores from the Pacific coast of Panama, we noticed that all reefs we sampled virtually stopped growing 4000 years ago and the altered species composition and the degraded condition of the few coral fragments we found from this time told us that environmental conditions had deteriorated. Approximately 1500 years ago, corals resumed vertical growth, the original species composition re-established itself and the coral fragments we found were in much better shape again. This temporal absence of reef growth is in line with reports from Costa Rica, Australia and Japan, where similar dramatic anomalies have been reported. This indicates that our observations were not locally restricted but part of a Pacific-wide disaster.
The good news is that reefs came back after 2500 years of demise, i.e. they have a remarkable ability to recover. Importantly, this recovery took place in the pristine conditions 1500-2000 years ago, with no man-made pollution, overfishing, global climate change and ocean acidification. But if we can provide reefs with ocean environments they can thrive in, there is hope for a substantial recovery of even the most severely degraded coral reefs.
Science podcast interview with lead author Rich Aronson: