Nitrogen isotopes of coral skeleton-bound organic matter and a 20-year record from Central Equatorial Pacific

We have developed a sensitive method for measuring nitrogen isotopes of coral skeleton-bound organic matter, requiring 5 mg of chemically cleaned coral skeleton powder and yielding a standard deviation for measured d15N of ~0.3%. This method allows for the generation of high-resolution d15N records of coral skeleton-bound organic matter, which we seek to advance as a proxy for the ocean nitrogen cycle in the past. A 20-year coral d15N record (1977-1997) from the Central Equatorial Pacific (Line Islands, Kiribati (2°N, 157°W)) yields an average d15N of 13%, which is similar to the d15N of mixed layer nitrate in this region but 6% higher than the subsurface nitrate being upwelled in this region. The 21 samples over this 20 year period yield a range of only 1.5% (12.5-14.0%), suggesting no substantial change of the mixed layer nitrogen pool over this period. There is some evidence for a lower d15N during El Nino events, but this requires testing by extension of the record and higher resolution sampling. If robust, this d15N decrease may reflect an increased reliance of corals on particulate organic nitrogen and recycled forms of nitrogen relative to nitrate during El Nino events, when mixed layer nitrate concentration declines in the Central Equatorial Pacific.