Pursuing underwater pipe dreams: collecting sediment cores from Moorea’s coral reefs

Written by Erin Dillon

What do you get when you combine a three-meter-long piece of aluminum irrigation pipe, a fence post driver, an old pipe clamp, and a motley crew of scientific divers? Quite a lot, it turns out.

We were twelve meters deep on a fringing reef along the northern coast of Moorea, a French Polynesian island in the South Pacific, braving bad weather and blinding plumes of swirling silt to unravel the history of these reefs. We hammered relentlessly, trying to hold the pipe perfectly upright as it crept, centimeter by centimeter, deeper into the sediment (Fig. 1). Excitement coursed through usas the pipe slid further and further down into the matrix of sediment and dead branching coral. We held our breaths (figuratively speaking; don’t worry, diving safety officers), hoping that the core barrel would not hit a large coral head and get stuck. We hammered and hammered, the air left in our tanks being the only limiting factor.

Figure1. Pounding the core barrel into the substrate, surrounded by clouds of silt.Photo credit: Aaron O’Dea

Pounding the core barrel into the sand was challenging, but pulling it out was even harder. We curled ourselves awkwardly around the pipe and twisted and tugged until finally the core was free. This core would give us a chronology of the reef over the last several hundred years, pre-dating modern monitoring efforts and major human impact in the region.

After over 20 hours of dive time as well as significant lifting, paperwork, and logistical maneuvering, we tallied up our spoils – around 650kg of sand. These samples were then shipped on pallets over 6700km to our lab in California, where they will be processed and carefully examined with a microscope to yield a tiny treasure trove of shark dermal denticles, fish teeth, otoliths, urchin spines, sponge spicules, foraminifera, and coral fragments – each piece a window into the past. Together, these remnants, contextualized by high precision coral dating, allow us to reconstruct how these coral reefs have changed over time alongside Moorea’s human history. I’ll be spending my days counting and classifying the shark dermal denticles (Dillon et al. 2017) to explore how shark communities shifted in size and composition over time on islands with different levels of human impact and settlement histories (Moorea as compared to Tetiaroa and Rangiroa). At the same time, some of my colleagues in the lab will be teasing apart patterns in the number and types of fish otoliths and teeth, particularly those belonging to important reef herbivores. Our coring work on Moorea is one piece of this larger puzzle, but it all begins with countless meters of pipe, ambition, and a lot of heavy lifting.

References

Dillon E, Norris R, O’Dea A (2017) Dermal denticles as a tool to reconstruct shark communities. Mar Ecol Prog Ser 566:117–134. http://dx.doi.org/10.3354/meps12018.

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