Using deep-sea corals collected north of Hawaii, California scientists have determined climate change has produced a long-term rise in nitrogen in the Pacific Ocean.
Nitrogen in the North Pacific has increased by about 20 percent since the mid-1800s and the trend appears to be continuing, according to a study published Dec. 15 in the journal Nature.
Scientists can look for clues to past conditions in the oceans by studying seafloor sediments, collected in cylindrical cores several feet long.
But that approach is not very useful for the past several thousand years. Sediment accumulates so slowly in the open North Pacific that the past 12,000 years or so, a span of time called the Holocene epoch, is represented by less than 4 inches of sediment which has been stirred up by organisms living on the seafloor.
So the California scientists turned to deep-sea corals, including the Hawaiian golden coral, collected by the Hawai‘i Undersea Research Laboratory’s Pisces V submersible.
These so-called "proteinaceous" corals can live for thousands of years, making them probably the world’s longest-living animals. They deposit layers of protein-rich skeleton that record the chemical signatures of their food: plankton material that rains down from the surface.
"They’re like living sediment traps, recording long-term changes in the open ocean that we can’t see any other way," co-author Matthew McCarthy, professor of ocean sciences at the University of California, Santa Cruz, said in a statement Dec. 15.
The focus of the study was an area called the North Pacific gyre, a ringlike system of ocean currents north of the tropics and the planet’s largest contiguous ecosystem. Tapping the chemical information locked in organic skeletal layers, the scientists used the corals as detailed recorders of changes at the base of the open-ocean food web over the past 1,000 years.
"In the marine environment, the two major sources of nitrogen are dissolved nitrate, which is more concentrated in the subsurface and deep water and is brought to the surface by upwelling and mixing, and nitrogen fixation by specialized micro-organisms that are like the legumes of the sea," said researcher Owen Sherwood, formerly with UC Santa Cruz and now at the University of Colorado Boulder.
But as sea surface temperatures rise, there is more stability in the ocean’s various layers, called the water column, and less upwelling.
The phytoplankton community at the surface, the base of the food web, needs nitrogen to turn dissolved organic carbon into cells for growth. With less nitrogen available from the depths, the phytoplankton became increasingly dominated by nitrogen-fixing cyanobacteria, which are able to use the nitrogen gas absorbed by surface waters from the atmosphere.
The deep-sea corals showed an increase of 17 percent to 27 percent in nitrogen from that latter source since the middle of the 19th century after a millennium of only minor fluctuations. That coincides with the start of the Industrial Revolution, first powered by greenhouse-gas producing coal fires.
The scientists can tell the difference between the two nitrogen sources by comparing the ratios of the isotopes N-14 and N-15, a novel technique pioneered at Santa Cruz.
The results fit with recent observations of changes in the gyre, once considered a static, nutrient-poor desert.
"This also has very significant implications about how we understand, and perhaps, can better predict effects of global warming in the Pacific, but also likely in other subtropical regions," co-author Tom Guilderson, a scientist at the Lawrence Livermore National Laboratory, said in a statement Monday.
Guilderson has been collecting deep-sea corals for more than a decade to study them for clues to past oceanographic and environmental conditions.
