Title: Ice core measurements of the isotopic composition of nitrate: new results and interpretation
Author: Steig, E.J., Hastings, M.G., Alexander, B., Jarvis, J.C. and Kunasek, S.C.
Periodical: American Geophysical Union, Fall Meeting 2006, abstract #U32A-06
Abstract: Several fundamental questions about the magnitude of natural variability in the global nitrogen cycle, and the impact of human activities in the last century, remain open. For example, lakes around the world show significant declines in organic nitrogen isotope ratios but it remains unclear to what extent this reflects changed nitrate sources or in situ changes in lake biogeochemistry. It also remains unknown whether atmospheric NOx mixing ratios were significantly different during the last glacial period; because atmospheric NOx abundances influence methane oxidation chemistry, this may have implications for closure of the global methane budget. While ice core measurements of nitrate concentration demonstrate that human activity has at least doubled the concentration of nitrogen oxides in the atmosphere in the last century, more quantitative use of such data has proven difficult due to the variety of possible nitrate sources, the complex atmospheric chemistry, and the potential for post-depositional change. The utility of ice cores in elucidating past changes in the global nitrogen cycle may be greatly enhanced through the analysis of nitrate isotope ratios. Our work on ice cores is coupled with direct atmospheric measurements of gas-phase HNO3 and its precursors, and global modeling of isotope variations in reactive nitrogen species, reported elsewhere at this meeting. Here, we report on our ongoing analyses of nitrogen and oxygen isotope ratios from ice cores at South Pole, the WAIS Divide (site of the new US drilling effort in West Antarctica), GISP2, and a new 100-meter core from Summit, Greenland. Snow pit data demonstrate that changes in oxygen isotope ratios (??18O and ??17O [= ??17O -0.52*??18O]) in nitrate can be related directly to changes in the ratio of gas phase ("daytime") vs. aqueous phase ("nighttime") chemistry in the production of nitrate in the atmosphere. Nitrogen isotope ratios (??15N) are also affected by photochemistry, but variations in ??15N are dominated by variations in nitrate source. The central Greenland cores suggest that ??15N declines and that both ??17O and ??18O increase (on average) in the last century. These changes reflect both the differing isotope ratios of HNO3 sources (i.e., industrial vs. biological NOx production), and the increase in atmospheric ozone. While long term ??17O measurements of nitrate are not yet available from the deep cores, a significant increase in ??18O in glacial compared with interglacial samples from the GISP2 core is best explained by a greater contribution of aqueous phase chemistry to nitrate production and/or higher ozone concentrations in the glacial atmosphere. Changes in ??15N, on the other hand, are not readily explained by photochemistry, implying significant changes in nitrate sources to Greenland on glacial-interglacial timescales. In particular, the high ??15N during the glacial period in GISP2 is highly suggestive of an enhanced contribution from biogenic sources. We note, however, that significantly higher ??15N may also reflect post- deposition nitrate losses, as observed in samples from low accumulation sites such as South Pole and Dome C. Samples from the WAIS Divide site -- with snow accumulation rate comparable to that at GISP2 -- will be important in testing this interpretation.
Year: 2006