Thus, two independent factors--the trophic level and the original source of the nitrogen--determine the nitrogen isotope ratio in an organism. McCarthy's lab developed a technique that can separate these two factors by analyzing individual amino acids--the building blocks of proteins. It turns out that the isotope ratios of some amino acids remain unchanged as they move up the food web, while other amino acids become enriched in nitrogen-15 with each trophic transfer.
"Amino acid analysis decouples the two effects so we can see their relative magnitudes," McCarthy said. "What we're seeing in the central Pacific is a major shift at the base of the food web."
The extent of the change is dramatic: a 17 to 27 percent increase in nitrogen-fixation since about 1850, after almost a millennium of relatively minor fluctuations. "In comparison to other transitions in the paleoceanographic record, it's gigantic," Sherwood said. "It's comparable to the change observed at the transition between the Pleistocene and Holocene Epochs, except that it happens an order of magnitude faster."
These and other recent results are changing scientists' notions about the stability of open ocean gyres such as the North Pacific Subtropical Gyre, which is the largest contiguous ecosystem on the planet. These open ocean gyres were once considered relatively static, nutrient-deprived "deserts." In the 1980s, however, scientists began regularly monitoring oceanographic conditions at deep-water station ALOHA near Hawaii, revealing a surprising amount of variability.
"Instead of relatively constant ocean deserts, time-series data has shown dynamic decadal-scale changes," McCarthy said. "Our new records from deep-sea cora
|Contact: Tim Stephens|
University of California - Santa Cruz