ABSTRACT

Global rainfall patterns have changed over the last 35 years, particularly in the tropics, where high-precipitation regions have become wetter, and low-precipitation regions have become drier. In addition, subtropical dry zones have expanded poleward by some 125-250 miles. The underlying causes of these changes are not well understood, owing to the complexity of the climate system and an imperfect understanding of tropical precipitation mechanisms. However, both heuristic arguments and state-of-the-art climate models suggest that these trends will continue if greenhouse gases continue to accumulate in the atmosphere faster than they can be absorbed by the ocean and land biosphere. Confidence in these projections could be improved with a longer precipitation record, particularly over the low-latitude oceans. Unfortunately, the start of the precipitation record over the ocean began just 35 years ago, coinciding with the satellite weather era. Thus the only way to extend the tropical rainfall record back in time is through indirect means. The purpose of this project is to develop a new approach to reconstruct tropical and subtropical rainfall rates from hydrogen and carbon isotope ratios in lipids produced by mangrove trees inhabiting intertidal regions throughout the tropics and subtropics. The research team will continue to engage local government agencies and non-governmental organizations in rapid ecological assessments of pristine and anthrpogenically-impacted systems, and will conduct workshops on mangroves and climate for Micronesian school teachers. Funding also supports education and training of a PhD student and provides research and field experiences for several undergraduates at the University of Washington.

This new method for determining rainfall is based on the sensitive biochemical and biophysical responses of mangrove trees to salinity, and the resultant impact on the hydrogen and carbon isotope composition of lipids in mangrove leaves. Recent studies have shown that when salinity increases, 2H/1H fractionation in mangrove lipids systematically increases, and 13C/12C fractionation systematically decreases. From these empirical relationships, salinity and the 2H/1H ratio of growth water can be determined from measurements of the 2H/1H and 13C/12C ratios of taraxerol, a leaf lipid produced by Rhizophora (red) mangroves. Those data will be used to calculate the 2H/1H ratio of precipitation, from which precipitation rates can be determined, owing to the anti-correlation between precipitation 2H/1H ratios and rainfall amount in the tropics. Taraxerol will be extracted from previously-collected mangrove swamp sediments from six US-affiliated Micronesian islands across 2,800 km of ocean that span modern mean annual rainfall rates of 6 to 14 mm d-1. The 2H/1H ratio of precipitation, salinity, and rainfall rates will be determined for: (1) the last 10-20 years from 0-2 cm sediments and compared to instrumental data, (2) the 20th century from 2~10 cm sediments to test whether satellite-derived precipitation since ~1990 is higher than the 20th century mean, and (3) the early (1400-1600 AD) and late (1600-1800 AD) Little Ice Age to test whether precipitation was lower in the western tropical North Pacific during that time.

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