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Background and Introduction

            PRF funding facilitated the collection of sediment samples from Lake Malawi (Malawi, Africa) in January 2009.  During this sampling expedition three undergraduates and two graduate students accompanied the PI to Lake Malawi, where sediment cores were collected via multi-corer aboard the Malawi Fisheries vessel R/V Ndunduma.  Specifics of sample collection, transport back to Minnesota, and sample storage are outlined in the previously submitted annual review. 

In the past year the undergraduate students who were involved with sample collection have completed their basic core analyses, which included grain size analysis, diatom abundance, bulk organic carbon and nitrogen content, bulk ¹³C and ¹⁵N signals, and core scanning on an X-ray fluorescence (XRF) scanner at 1mm resolution to obtain concentrations of a suit of major elements. They have subsequently begun to compile their data into a manuscript for publication. 

PRF funded PhD student Brittany Kruger is currently in the process of extracting specific compound classes from the sediment samples in preparation for compound specific δ¹³C and Δ¹⁴C analysis.  She has applied for and been awarded a Graduate Student Internship at the National Ocean Sciences Accelerator Mass Spectrometry Facility (NOSAMS) (Woods Hole, MA) for δ¹³C and Δ¹⁴C analysis of these samples in early 2011.  This award will enable her travel to the NOSAMS facility during which she will learn the preparative column gas chromatography (PCGC) techniques necessary to isolate specific compounds and subsequent radiocarbon analysis methods.

Methods

            Current analysis is focused on isolating n-alcohols from Lake Malawi sediment samples, which will be separated (at the NOSAMS facility) into individual alcohols to be analyzed for δ¹³C and Δ¹⁴C.  Isolation of n-alcohols from samples first entails extraction of the total lipid fraction from freeze-dried and ground sediments.  Freeze drying and lipid extraction instrumentation were checked to ensure they would not impart a C14 signal to the samples due to pump oil vapor flow or leaching from plastic tubing, etc. 

Once obtained, the lipid fraction is then separated into three fractions based on neutrality, and the neutral fraction is again separated into three fractions based on polarity.  The resulting polar fraction contains the n-alcohols of interest.  Before analysis these n-alcohols must then be derivatized to make them less polar and therefore more amenable to gas chromatography.   Samples are injected on a GC/MSD for peak confirmation and on a GC/FID for compound abundance calculation prior to analysis for carbon isotope and radiocarbon content.

Upon completion of n-alcohol isolation, sediment samples will then be further treated to extract phenols, which are derived from terrestrial vegetation. Finally, sediment samples will undergo a final analysis to isolated ‘protokerogen’, or the uncharacterized organic component preserved in sediments.  These biomarkers will also undergo δ¹³C and Δ¹⁴C analysis after abundances are quantified.

Projected Timeline

Isolation of n-alcohols from samples is the current primary focus and is expected to be completed by the end of this year; in time for the scheduled visit to the NOSAMS facility in late January/early February 2011.  During this visit to NOSAMS we aim to obtain δ¹³C and Δ¹⁴C signals of select individual n-alcohols from all samples, and values for as many phenol/protokerogen samples as time allows.  Samples not analyzed on the currently planned trip to NOSAMS will be subsequently submitted for δ¹³C and Δ¹⁴C quantification.   Nearly all lab work and sample analysis is expected to be completed by the summer of 2011.

Discussion

            By looking at the abundances of the sedimentary organic carbon (OC) compounds outlined above in combination with their δ¹³C and Δ¹⁴C signatures, we will obtain insight into the individual carbon cycling regimens of these biomarkers in Lake Malawi.  Furthermore, by isolating these individual compounds for analysis, we will be able to better understand the sources of carbon which is ultimately preserved in the lake sediments.  For example, the ratio of short chain to long chain n-alcohols in the sediment provides a measure of whether more of the organic carbon was terrestrially or aquatically produced.  Similarly, analyzing individual phenols contained in the sediments indicates particular groups of terrestrial vegetation that were likely the parent material for that carbon.  Through these analyses we expect to obtain one of the most thorough and complete views produced to date of sediment OC sources in an African rift lake.