"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 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, Boulder.
The shift revealed in the coral record--from an ecosystem supported by nitrate coming up from deeper waters to one supported more by nitrogen-fixing organisms--may be a result of the North Pacific Subtropical Gyre expanding and becoming warmer, with more stable layering of warm surface water over cooler subsurface water. This increased "stratification" limits the amount of nutrients delivered to the surface in nutrient-rich subsurface water.
Scientists have observed warming and expansion of the major mid-ocean subtropical gyres in the past few decades and have attributed this trend to global warming. The new study puts these observations in the context of a longer-term trend. "It seems that the change in nitrogen sources, and therefore possibly large-scale shifts in ocean conditions, switched on at the end of the Little Ice Age and it is still continuing today," McCarthy said.
A key innovation in nitrogen isotope analysis was crucial to this study. Nitrogen-15 is a minor stable isotope of nitrogen, and the ratio of nitrogen-15 to nitrogen-14 is widely used to trace different sources of nitrogen. The nitrogen fixed by cyanobacteria in surface water, for example, has a different isotope ratio fr
|Contact: Tim Stephens|
University of California - Santa Cruz