One of the most unique and innovative aspects of IODP Expedition 389 is the opportunity to collect data from extremely old corals.
“The coral samples we’re getting from this expedition are fascinating because they span the last half million years,” says Ana Prohaska, a microbiologist and paleogenetic specialist. “In my work with ancient DNA, we’ve never done this before – the oldest samples of coral that provided DNA were from 6,000 years ago – and now we can potentially go much further back in time.”
Determining how much further back in time is the tricky part. For corals that are less than 50,000 years old, the team can use traditional radiocarbon dating (carbon-14) methods.
But many of the corals are significantly older than 50,000 years. For these samples, the team utilizes uranium-thorium dating.
Just like a human body builds bones as it grows, when a coral makes its skeleton, it incorporates elements from its natural environment. Because uranium naturally occurs in seawater, it becomes part of the coral’s skeleton. After the corals die, the uranium in their skeletons starts to decay into thorium. By examining the concentration of the different isotopes of uranium and thorium scientists can determine the age of the corals.
“We don’t yet know how old our oldest core is, but we anticipate recovering several hundred thousand years,” says Andrea Dutton, a geochemist on the Expedition 389 team. “We know we have a wide range of ages, from roughly 15,000 years ago to several hundred thousand years.”
Many of the reef cores include particularly thick and well-preserved deposits that represent specific time intervals. The science team anticipates that these cores will give them a clearer picture of the timing of events and fill large gaps in existing climate records.
“People really want to know how sea level change will affect humanity,” Dutton says. “This dating allows us to get closer to looking at changes that are more relevant to human time scales.”
Before they can begin the process of dating, our scientists must screen each individual core to determine its level of preservation.
“If the original coral skeleton material has been altered, it will mess up our age calculation,” Dutton says. “We have to work with pristine material.”
To conduct this quality control, members of the science team take a bit of material, grind it up with a mortar and pestle, spread it onto slide, and look at it under X-ray diffraction. This process allows them to determine the minerology, which can establish its level of integrity.
For example, aragonite converts to calcite over time. “If you see that conversion, you know something has changed,” Dutton says. “You wouldn’t analyze a sample full of calcite.”
After completing this close examination of the minerology, the geochemical analysis can begin – an intricate process that can take anywhere from one to two weeks.
But it’s worth the wait.
“Studying a coral that is a quarter of a million years old is a new frontier,” says Ed Hathorne, a geochemist on the Expedition 389 team. “This really hasn’t been done before.”
Blog of IODP Expedition 389: Hawaiian Drowned Reefs 