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The central goal of this research project is to constrain the origin of the vast amounts of organic carbon (OC) deposited in floodplain sediments of the Rio Beni system, a river that drains the Andes Mountains.  To do so, my research group is using the hydrogen and carbon isotopic signatures of two suites of organic compounds, lignin phenols and plant-wax lipids, to determine the elevation at which the OC was produced and what types of plants it derives from.  We have divided this project into two parts: the first establishes the signatures of OC from various environments in this system via analysis of plant samples collected along a large gradient of elevations, and the second uses analysis of floodplain sediments to reconstruct the origins of the deposited carbon.

            During the second year of this project (9/09-8/10), my research group extracted both suites of compounds from all 30 of the plant samples.  These analyses yielded information about the type of carbon present, and the compounds will soon be analyzed for their isotopic signatures at another facility.  Lignin phenol concentration data yielded clear and distinct signatures for grass, leaf and wood tissues in this region.  Additionally, we have compound-specific carbon-13 results for plant-wax lipids from a subset of 10 plant samples spanning almost 5000 meters of elevation.  These first compound-specific isotopic results are quite promising: the lipids show progressive enrichment of ¹³C with increasing elevation.  This suggests that the central methodological hypothesis of this proposal is true: these isotopes should serve as effective tracers of the original elevation of plant material deposited downstream.  Further analyses should flesh out the relationship and allow firmer determination of the isotope-elevation correlation.  Overall, our work on the first part of the project is complete; we are now waiting for results from the isotopic analytical facilities.

For the second part of the project, we have processed 13 floodplain sediment samples deposited at the base of the Andes according to particle size by sieving the sediments at 63 um.  This will allow us to analyze the geochemically distinct larger and smaller fractions separately from each other to determine how the origins of these two fractions differ.  Bulk elemental and isotopic analyses suggest that, as found in other systems, the larger fraction is enriched in carbon, and probably contains more discrete plant material, whereas the smaller fraction appears to be dominated by minerals and mineral-associated OC.  Thus far, we have extracted lignin from a third of the size fractionated sediments and have begun to extract the lipid compounds from these same samples.  Lignin analysis indicates that the sedimentary lignin derives from a mixture of plant and woody tissues, with very little input from grasses.  This suggests that most of the OC deposited on the floodplain derives from mid- and lower elevation forests, not from the grasslands prominent at the higher elevations.  We also have analyzed the lipids extracted from the <63um fraction of one sediment sample for their ¹³C composition.  Comparison of these results with those from the 10 plant samples analyzed suggests that these lipids derive mostly from mid-elevations.  We will finish isolating the compounds from the sediment samples by the end of the fall, taking advantage of an extension of this grant.  Likewise, isotopic analysis of these compounds should be complete by December or January. 

All laboratory analyses for this project should be complete by the end of the fall 2010, and the main goal of my spring 2011 sabbatical is to turn these results into one or several publications.  I hope to submit an initial manuscript for publication by May 2011.

Two undergraduate students worked part-time on this project during the school year (5-10 hr/wk) and full-time during the summer, one of whom was new to the project and the other continuing her work from the previous summer.  These students were involved in all aspects of the research funded by PRF, which provided outstanding research opportunities for both students, both of whom were excellent research students.  One student has completed her work on the project, but the other, Sarah Knudsen, will continue her work and write a senior thesis on her aspects of this project this spring (2011).  I also plan to send her to the Spring 2011 ACS National Meeting to present her work on the lipids of this river system.