Coral Proxy Data Shows Ocean Thermocline (Cool/Warm Water Boundary) Rising Since 1970s

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Specifically, the researchers were interested in how the boundary layer between the warmer, shallow water and the colder deeper water -- the thermocline -- has changed. But directly measuring that over time and across a broad area is impossible. Instead, they used the soft corals as a substitute, a proxy, for determining how the thermocline rose and fell over time. Working with samples taken from corals at five meters (16.4 feet), 85 meters (279 feet) and 105 meters (345 feet), they analyzed the chemical structure of the coral skeleton that had built up over time.

Slices cut through the base “trunk” of the coral revealed concentric circles resembling tree rings which showed the growth of the “trunk” over time. By analyzing material from the rings from the outer surface inward to the center, they assembled a growth record covering the last century or so.

They were looking for the ratio between two isotopes of nitrogen in the material as a clue to how the thermocline rose or fell over time. Warmer waters above the thermocline contained lower levels of nutrients resulting in organic matter with a high ratio of nitrogen-15 to nitrogen-14 isotopes. Waters below the thermocline held higher levels of nutrients resulting in organic matter with a low ratio of nitrogen-15 to nitrogen-14 isotopes. As the polyps in the coral consumed the organic matter over the years, the nitrogen isotopes were chemically recorded in the coral. By comparing the nitrogen-15 to nitrogen-14 ratios in samples from the three depths, they could draw a picture of how the thermocline moved.

“Over several decades, specifically since the mid- to late-1970s, the records show that the mean depth of the thermocline has been getting shallower,” Williams said. The researchers did a similar analysis of the ratio of two carbon isotopes in the coral samples as well. “I think it’s fair to say that the carbon isotope record supports this interpretation,” Grottoli said. “It’s another piece of evidence backing our conclusions.”

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http://www.redorbit.com/news/science/1930637/coral_records_show_ocean_thermocline_rise_with_global_warming/index.html?source=r_science