Deep in the subtropical Pacific, one of the world's longest-lived animals has been documenting the ocean's history.

Called the Hawaiian gold coral, the organism lives in treelike colonies about 1,300 feet below the ocean's surface. Scientists have dated pieces of its skeleton going as far back as 3,000 years.

Now, those pieces, collected in a series of deep-sea dives, are being used by scientists to prove that changes in ocean chemistry are linked with changes in the climate.

"It turns out there is a very distinct change in our records, and it begins about 150 years ago," said Owen Sherwood, a geologist who conducted research on these corals as a postdoctoral fellow at the University of California, Santa Cruz, and who is now at the University of Colorado, Boulder.

That change, Sherwood continued, "coincides with two major things. One is the end of the Little Ice Age, and the other is the beginning of the Industrial Revolution."

Sherwood and his fellow researchers, who published an article on the research Sunday in the journal Nature, were careful to note that their research is unable to distinguish between whether the changes are due to natural climate changes such as the Earth's exit from the Little Ice Age, or human-caused warming.

"We can't answer that question," said Tom Guilderson, a staff scientist at Lawrence Livermore National Laboratory and a co-author of the paper.

Like trees, which add rings of growth as they grow, the corals add layers to their protein-based skeleton over time. What's in those layers depends on what's in their food.

"These corals feed on particles that rain down from the surface layers [of the ocean], and they incorporate that material into their skeleton," Sherwood said.

Exploring mysteries of the deep
In order to collect the coral samples, Guilderson traveled deep into the ocean inside human-occupied submersible vehicles called Pisces 4 and Pisces 5, operated by the Hawaii Undersea Research Laboratory. Like the famous submersible Alvin, these vehicles can take passengers deep into the ocean.

"We would either track along contours to look at the fauna at a certain depth horizon, or we could clamber up ravines and up walls and up escarpments looking for specific features where deep-sea corals are known to be found," Guilderson said.

Once a promising specimen was found, the pilot would grab it with a robotic arm. The scientists are careful to collect mostly dead corals, although they will take some live samples, as well.

Then, back in the laboratory, the researchers analyze the layers of coral skeleton for their chemical makeup.

In this case, what the scientists were looking for were changes in the ratio of two isotopes of nitrogen.

One type of isotope signature indicates that the nitrogen is primarily upwelling from deep in the ocean. Scientists have theorized that as the ocean has warmed, it has become more stratified, which may limit the amount of upwelling that occurs.

That's where cyanobacteria come in. Living on the ocean's surface, these microorganisms can take nitrogen from the atmosphere and convert it to nitrate, a form that plankton can use. But nitrogen in this form will have a different isotope signature than the nitrogen that comes from the ocean depths.

That is the distinct shift Sherwood and Guilderson saw when they analyzed the chemical makeup of Hawaiian gold coral skeletons over the past 1,000 years. About 150 years ago, the source of nitrogen, which is used as food by primary producers like plankton, shifted from the ocean depths to the atmosphere.

Change at the base of the food web
"What's groundbreaking about our research is that there's been this huge change at the base of the food web that corresponds with these climate drivers," Sherwood said.

The area where Sherwood and Guilderson see these changes is called the North Pacific Subtropical Gyre, an area of circular ocean currents that is often called the largest contiguous ecosystem on Earth.

Satellite observations have indicated that this and other gyres, areas of rotating ocean currents, have been growing larger, and this research also supports that finding, Guilderson said.

"That increase in the gyre [size] has probably been going on for a little bit longer than people had previously thought," he said.

Although the changes in the coral skeleton compositions correlate with the exit of the Little Ice Age and the start of the Industrial Revolution, Guilderson was careful to note that correlation does not equal causation.

The researchers also posited another mechanism behind the change in nitrogen isotopes, that dust blowing from Asia fertilized cyanobacteria in the ocean, leading them to proliferate.

Guilderson said he expected that although this may have played a partial role, if it were more significant, there would have been a different trend in the isotopes they saw in the corals.

In order to determine whether human-caused climate change is behind the shift in where the nitrogen is coming from, scientists will have to run ocean circulation models that include elevated levels of greenhouse gases and compare those with models run without those human-linked climate changes, Guilderson said.

Now that the research is published, Guilderson said he could talk with some colleagues about conducting that type of experiment.

The researchers also plan to use their samples to look further back in time beyond 1,000 years to see whether they can see other changes in the coral skeleton chemistry, Sherwood said.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500