Using deep sea corals gathered near the Hawaiian Islands, a Lawrence Livermore scientist in collaboration with UC Santa Cruz colleagues have determined that a long term shift in nitrogen content in the Pacific Ocean has occurred as a result of climate change.
Overall nitrogen in the North Pacific Ocean has increased by about 20 percent since the mid 1800s, -- a shift, similar to major paleoceanographic transitions in the sedimentary record --and this long-term change appears to be continuing today, according to a study published Dec. 15 edition of the journal, Nature.
The study combines a unique compound-specific isotope technique (largely being pioneered at University of California, Santa Cruz), measured in deep-sea proteinaceous corals, to reconstruct changes in biogeochemical cycles from the North Pacific subtropical ringlike system of ocean currents: the largest contiguous biome in the world.
Using chemical information locked in organic skeletal layers, the team used these ancient corals as detailed recorders of changes at the base of the open Pacific food web over the last 1,000 years. This represents the first detailed biogeochemical records for the planet's largest contiguous ecosystem
"The timing of the change -- at the start of the anthropogenic era (post-industrial, and coming out of the Little Ice Age) -- begs the question as to whether there is more than just a correlation between our record and, for example, northern hemisphere temperatures and increased dust deposition due to land use change," said Tom Guilderson, a Livermore scientist working at the Laboratory's Center for Accelerator Mass Spectrometry.
The study, authored by UCSC's Owen Sherwood and Matt McCarthy and LLNL's Guilderson, is a collaborative project to develop novel isotopic measurements, and use them in long-lived deep sea proteinaceous corals.
Colonial corals can live for multiple millennia, making them likely the world's longest living animals. They deposit layers of protein rich skeleton that record the chemical signatures of the constant rain of detrital plankton material from the ocean's surface (the corals' food), making these animals akin to "living sediment traps," according to Sherwood.
However, unlike sediment traps, the corals provide highly detailed records not for decades but for up to thousands of years. This aspect is critical in places like the open Pacific where sediment accumulation is so slow that it is essentially impossible to get meaningful data for recent centuries.
"We reconstructed highly detailed records of how ocean biogeochemical systems have changed through the Holocene and, in particular, to understand what the 'baselines' really were before instrumental record, and dramatic anthropogenic changes began," Guilderson said.
The results may fundamentally change how we understand open ocean ecosystem stability, Sherwood said. The Pacific contains the largest contiguous ecosystem on the planet. Time series data near Hawaii have shown dynamic decadal scale variability. But the new records from deep corals now show that the decadal-scale time series changes are really only small oscillations superimposed on a dramatic long-term shift at the base of the Pacific ecosystem.
"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," Guilderson said.
The team used isotopes of nitrogen in both the bulk skeleton and in individual amino acids and noticed a significant change in nitrogen isotope ratios in the coral skeleton since the end of the Little Ice Age (about 1850). The observed change is not due to changes in trophic level but reflects changes in the nitrogen source at the base of the food-web. Nitrogen in the ocean and just like in many gardens is usually a limiting nutrient. Plants (phytoplankton) require nitrogen to turn dissolved inorganic carbon into plant material and start the biological cycle.
"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 microorganisms that are like the legumes of the sea," explained first author Owen Sherwood, who worked on the study as a postdoctoral researcher at UCSC and is now at the University of Colorado in Boulder.
The results indicate an increase in nitrogen fixation of 17 percent to 27 percent since the end of the Little Ice Age. This implies that the surface waters are becoming less well mixed with the waters below them and more stratified. The obvious linkages with anthropogenic climate change are increased sea surface temperatures, which increase stabilization of the water column, and changes in the overlying surface winds. A corollary to this, and given the location of the samples, is an expansion of the gyre itself.
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