A sediment core inspired road trip


As coral reefs accrete upward, their histories become buried beneath the surface. One way to unravel this history is to visit areas where the older reef has been exposed by excavations, road cuts, or storm channels. This is what we did in Bocas del Toro, Panamá and in the Dominican Republic’s Enriquillo Basin. In most places, however, living reef or mangroves grow on top of the Holocene reef. To access the underlying history, we can insert several-meter-long sediment cores into the substrate. While this technique samples a sliver of the surface area of a reef, it has the unique advantage of looking back in time. Depending on the rate of reef accretion and sedimentation, sediment cores can travel back several hundred to several thousand years with high temporal resolution. The remnants of the animals and plants captured in these cores help paint a picture of the reef in the past and how it has changed over time.

Today, I had the good fortune of sampling a gravity core extracted from Palmyra Atoll’s lagoon by a team of USGS scientists. In addition to being home to numerous sharks, Palmyra has an interesting human history despite never being settled. The atoll was originally discovered in 1802 when an American ship was blown off course in a storm. In 1862, Palmyra was claimed by the Kingdom of Hawaii, and it later became a U.S. territory following Hawaii’s annexation and subsequent statehood. At that time, it was privately owned by the Fullard-Leo family. During WWII, the U.S. Navy took over and drastically altered the atoll for use as a naval air facility. Afterwards, it was occasionally frequented by yachters despite its remote location – a nearly 1000-nautical-mile sail southwest of Hawaii. From 2000-2001 onwards, the atoll has been under the protection of the Nature Conservancy and U.S. Fish and Wildlife Service as a National Wildlife Refuge, and a handful of scientists and donors visit each year.

The 3.2m-long sediment core that I sampled covers nearly 650 years of history on the atoll, capturing its pristine state prior to discovery in addition to the last 200 years of sparse human occupation. We will use the core to begin investigating whether shark baselines on the atoll are naturally dynamic and whether the abundance and diversity of sharks changed after the arrival of humans. Palmyra has been the location of many shark tagging and survey studies over the last 12 years since the creation of the Palmyra Atoll Research Consortium (PARC), and it is considered to be a relatively untouched, healthy reef ecosystem. This new historical time series will help us understand whether its condition has fluctuated over time before these monitoring efforts began.

This opportunity was certainly worth the 10-hour round-trip road trip from Santa Barbara to Menlo Park, which is located about 40 minutes southeast of San Francisco. In addition to picking up the core from the sample storage refrigerator on the USGS campus, my secondary aim was to use their facilities to slice it into 64 5cm-long segments, each representing about 10 years of reef history. The core was composed of very fine, silty material, so cutting through it was not difficult with the help of some metal utensils and a meter stick. Furthermore, the challenging task of cutting the core in half through the PVC piping had already been accomplished. I carefully measured and cut each section and placed them into individual labeled plastic bags. After several hours of meticulous slicing and record keeping, I had amassed quite the pile of samples and was caked in a thin layer of dry mud.

Coring Caribbean reefs

Alongside our work examining the relative abundances of coral and mollusc skeletons, shark dermal denticles, sponge spicules and fish otoliths in large tracts of exposed mid-Holocene reefs, we have also been collaborating with Drs. Katie Cramer and Richard Norris (Scripps) to explore sequential changes in Caribbean reef ecosystems by extracting reef matrix cores in Belize and Bocas del Toro, Panama. We collected more than twenty 6m long aluminium cores from several lagoonal fringing reefs that capture reef ecosystem conditions going back the last couple of thousand years.

See more about the approach in this little video