Investigation of magnesium isotope fractionation during basalt weathering

Cornell University, Ithaca NY

Investigators

Abstract

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5). We propose a study of magnesium isotope variations produced by weathering processes and biogeochemical cycling in the surface environment of Hawaii. Recent work has demonstrated Mg isotopic variations among fresh rocks, soils, stream, spring and ground waters in silicatehosted watersheds, and between natural carbonates and water. Other recent experimental work has demonstrated Mg isotope fractionation during uptake by higher plants. However, the magnitudes, mechanisms, and even direction of Mg isotopic fractionation in the surface environment remain poorly constrained. We hypothesize that weathering related fractionation is a function of the type of secondary minerals produced. We will utilize a series of climate gradient sites at two locations in the Hawaiian Islands where significant changes in secondary mineral assemblages arise because of climatic variation. While all sites have similar basalt substrate lithology, dominant secondary minerals range from Mg-carbonates and halloysite at the dry Kohala sites, to allophane at wetter sites. In the SE Koolau Range, smectite gives way to halloysite and kaolinite with increasing rainfall. We will analyze rocks, soils and soil extracts, rain, stream and ground waters, and leaf tissues from these sites, to characterize 26Mg variations during basalt alteration and biogeochemical cycling. We will also synthesize clay minerals and measure 26Mg on reactant and product phases, to better constrain mineralogical controls on Mg isotope fractionation. The project will be carried out in close collaboration with Dr. Nathalie Vigier of the CRPG, Nancy, France, who has developed protocols for 26Mg analysis in these kinds of samples. Scientific merit: This project will provide critical data for understanding the mechanisms of Mg isotope fractionation in the surface environment, and so develop the use of this new tool to trace sources and pathways of Mg in weathering and biogeochemical processes. The cycle of Mg is closely linked to the cycles of carbon and silicon at multiple times scales, and is of first order importance. The project will be transformative in that it will enable the use of 26Mg as a new and powerful quantitative tool in studying biogeochemical cycles. Broader impacts: Previous studies of tracer systems at our sites in Hawaii have had substantial impact in the scientific community because of the value of Hawaii as a natural laboratory and model system, and we expect this to be similar. Secondly, we will develop a strong international collaboration with a promising young French researcher and her students, who are funded independently to work with us. We also will gain access for our students to state of the art MCICP- MS instrumentation not available in the US. US graduate students will visit French laboratories, and French students will visit our labs and Hawaiian field sites. Third, this project will interface closely with Cornell?s undergraduate field program in Hawaii. Students will interact with project researchers and work on aspects of this project while in Hawaii. We will sponsor at least two senior thesis projects associated with the project.