Quantitative Salinity & Water D/H from Paired H & C Isotopes in Mangrove Lipids

University Of Washington, Seattle WA

Investigators

Abstract

Global patterns of rainfall have been changing over the last half-century, impacting the lives of people worldwide. A full understanding of these changes has not yet emerged, but they have been linked to a warming of global temperatures in response to the buildup of greenhouse gases in the atmosphere. Among the more widespread changes in precipitation have occurred in the tropics, where wet regions have been getting wetter and dry regions drier. It remains unknown whether the changes in tropical rainfall observed over the last 30 years are unusual when viewed on a multi-century or multi-millennial time scale. As yet it is therefore not possible to say whether recent precipitation trends are outside the range of natural variability. Any such conclusion will require records of rainfall from the tropics extending back in time a thousand years or more. But at present there are few, if any methods to accurately reconstruct rainfall in most locations on the planet prior to the advent of rain gauges and satellites, especially in the maritime tropics. The goal of this project is to develop a method for determining the salinity and deuterium-to-hydrogen ratio of water, two parameters very closely linked to rainfall in the tropics, from the chemistry of mangrove lipids that can be recovered from ocean and lake sediment cores. Mangroves are remarkable trees that have adapted to living in saltwater. They inhabit the intertidal zones of the global tropics and subtropics. The approach of the proposed study is based upon the investigator's recent observation that the ratio of both deuterium (2H) to hydrogen (1H) and carbon-13 (13C) to carbon-12 (12C) in the lipids of mangrove leaves changes systematically when the salinity of the water in which they live changes. This study will calibrate that response in multiple species of mangroves by growing them in the laboratory at a range of different salinities. The investigator will then test whether those calibrations are consistent when applied to multiple mangrove species inhabiting the shorelines of saline lakes on the tropical island of Palau that span a wide range of salinities. Because the hydrogen isotope composition of mangrove lipids incorporates the deuterium-to-hydrogen ratio of the water in which the mangroves live, as well as the salinity of that water, and the carbon isotopes only respond to the salinity, it is possible to calculate both the salinity and the hydrogen isotope ratio of the coastal water. From these two parameters one can make good estimates of rainfall, since both the salinity and the deuterium-to-hydrogen ratio of coastal water decreases when rainfall increases. When measured on mangrove lipids extracted from sediment cores taken in coastal settings and maritime lakes it will be possible to reconstruct rainfall estimates back in time, thus providing the long-term view needed to conclude whether recent trends in tropical rainfall are outside the range of natural variability. This project is supported jointly by the Marine Geology and Geophysics Program in the NSF Division of Ocean Sciences.