As the sun dims over the Pacific’s glassy Solomon Sea, Guillaume Iwankow dons his diving gear and descends from the research schooner Tara into a motorized dinghy. His goal is to bring back a core, an arm’s-length sample of a coral colony that chronicles decades of its lifetime.
About 10 minutes after the dinghy leaves Tara, its motor slows. It’s so shallow here that reef fish dart about just inches from the surface. Iwankow, a scientific dive master for the Tara Expeditions Foundation, is looking over the coral for the biggest, most pristine example he can find of Porites lobata — a round, yellow-green coral species that often grows so large that it looks like the contents of Godzilla’s cranium. Coral colonies are made up of soft-bodied animals called polyps, which (with the help of symbiotic algae) secrete thin layers of the mineral calcium carbonate. Over time, these annual layers accumulate one atop another in a hard mass that makes up the coral skeleton.
Once he’s located the ideal Porites, Iwankow presses the three-inch-wide circular blade of his coring tool into the surface of the coral. There’s a faint whirring noise as the blade sinks into the skeleton, and clouds of coral dust billow in the surrounding water. After penetrating through all the coral layers, Iwankow taps the corer’s barrel this way, then that, to dislodge the base of the sample he’s cut, which is typically about a foot and a half long. He repeats that process twice more through the same hole, then swims back up and deposits the lengths of core sample —about four feet long in their entirety — in the dinghy. Only the surface of the Porites formation contains live polyps, so after drilling, the coral formation should continue to grow in the shallows, unharmed and without interruption.
Marine science expeditions like this one collect all kinds of biological samples, from ocean water to reef fish to coral microbes. But coral cores are distinct from the rest. They are organic time capsules, containing records of local pollution, geology, temperature and reef health that go back hundreds or thousands of years. As researchers refine the surprising methods available to extract this kind of information from coral skeletons, the advice for climatologists, geochemists and paleontologists who want to plumb the oceans’ past has increasingly become: Look to the cores. “I call them natural reef history books,” said Janice Lough, a climatologist and coral core expert at the Australian Institute of Marine Science. “They’ve got lots of stories to tell.”
Drilling Into Ocean History
Like forensic detective work, coral coring has become a reliable way to add detail and credibility to theories about past events — or to prove that they happened at all. It’s easy to forget that no one was even sure corals set down annual growth rings until the 1970s. That was when a team of University of Hawaii geophysicists visited Enewetak Atoll in the South Pacific.
Enewetak was an unassuming island with an unusual history: The United States tested nuclear bombs there on various dates in the 1940s and ’50s. The Hawaii researchers were curious to see whether coral skeletons near Enewetak would show evidence of this radioactivity. If the coral core layers contained radioactive elements with a known half-life, it would be possible to calculate almost exactly when each growth ring was made. “They took a slice of a massive colony, put it on [light-]sensitive paper in a darkroom for a month, and they saw a series of radioactive bands,” Lough said. The spacing of the bands on the paper hinted that there might be more to discover within the hidden structure of the coral, suggesting a further test was in order. “They got in touch with the local doctor and said, ‘Would you mind X-raying our coral slice?’”
When the coral slices were put in the X-ray scanner, a distinctive series of light and dark growth rings became visible, reflecting the density of the calcium carbonate that made up the coral skeleton. Dating the radioactive elements in the skeleton revealed that a double set of rings was laid down each year: a larger, more porous ring and a narrower, denser ring. In a 1972 Science paper, the researchers dubbed the cores “coral chronometers,” alluding to their utility as natural timepieces. Other scientists have since reported that corals lay down larger growth rings during wet seasons when temperatures are more moderate, and smaller rings during dry seasons when conditions are more extreme.
Coral species grow between 0.3 and 10 centimeters per year, but a general rule of thumb is that a 100-centimeter-long core sample, for instance, supplies a record of about 100 years of that coral’s history. Often it is the most recent 100 years, but not always. Fossilized corals may contain sequences of growth rings that date back as far as the last interglacial period, more than 100,000 years ago. X-ray scans are still used today to assess the relative density of coral growth rings, which reflects the climatic conditions at the time the rings were created. But marine scientists have worked steadily to discover the significance of other coral core properties as well.
One of the richest stores of data inside a core, coral detectives are finding, is its year-by-year record of trace elements in ocean water. Coral polyps take in ocean water to extract minerals they need to build their skeletons, so each carbonate layer contains tiny amounts of whatever was in the water when the layer was created. While coral growth rings are “not as nice and tight as tree rings, due to the complex internal shape of the skeleton,” said Gregory Webb, a University of Queensland paleontologist, “they do record the chemistry of the water they grow in.”
Tests of coral core composition, therefore, allow scientists to chart levels of many different compounds in an ocean zone from one year to the next. This can yield insights into planetary processes that seem to have little to do with coral. Marine scientists at China’s Guangxi Key Laboratory recently deduced the strength of East Asian winter monsoons over the past 150 years by measuring levels of rare earth elements, such as lanthanum and cerium, in each layer of a Porites coral core. These rare earth elements come from swirls of dust deposited during the winter storms, so the elements’ prevalence is a reliable gauge of storm intensity.
