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Introduction

We had a successful field program on Lake Malawi in January 2009 aboard the Malawi Fisheries research vessel, R/V Ndunduma.  PRF funds were leveraged with funds from other sources to enable two graduate students and three undergraduate students from the University of Minnesota Duluth to accompany the PI on the expedition.  The cruise entailed sailing from the port of Monkey Bay, Malawi at the south end of the lake, approximately 300 miles north to the mouth of the Songwe River on the border of Malawi and Tanzania.  A transect of coring stations was occupied along a depth transect southwards from the river mouth, recovering multi-cores in water depth intervals of 20 m from 80 m near the river delta to 220 m offshore.  Two additional multi-cores were recovered at about 300 m and 400 m depths.

All three undergraduate students who participated in the field work are carrying out basic analyses of the sediment cores, funded by the University of Minnesota’s UROP (Undergraduate Research Opportunities Program).  One student is analyzing sediments for grain size and fossil diatom assemblages.  Another student is analyzing the sediment for bulk organic carbon and nitrogen isotopes.  The third student is analyzing the cores by scanning X-ray fluorescence, for major and minor element abundances and stratigraphy.

The Ph.D. student who is funded by this PRF grant, Brittany Kruger, participated in the field program and is currently developing her organic geochemical laboratory skills, which involve solvent extraction and separation of organic compound classes for subsequent analyses for C13 and C14.  Brittany will be ready to undertake the proposed analyses of the Lake Malawi sediments within another month.

Methods

An Ocean Systems Multi-corer was used to recover sediment in northern Lake Malawi. The core sites were surveyed by high-resolution seismic reflection profiling, and positions were obtained by gps navigation.  The multi-corer simultaneously recovers four, 10-cm diameter cores that are typically 50 cm long.  The core tubes are mounted on a quadripod that is lowered to the lake floor and inserts the cores slowly into the sediment by a gravity-driven, hydraulically damped mechanism.  This enables recovery of the sediment-water interface with minimal disturbance.  The upper 3 cm of one of the cores at each site were extruded in the field at 0.5 cm intervals and immediately frozen in pre-cleaned glass jars.  The remainder of this core and one additional full core from each site were capped, labeled and sealed in the field, to be air freighted back to Minnesota.  

Once back in Minnesota, the cores were taken to the LacCore National Lake Sediment Core Repository at the University of Minnesota Minneapolis, where they were scanned for magnetic susceptibility and gamma bulk density.  The cores were then split lengthwise into working and archive halves.  The archive halves were imaged digitally and the working halves were sub-sampled for initial analyses for water content, smear slide (microscopic) analysis, bulk organic carbon and nitrogen, and total inorganic carbon.  The cores and subsamples were then transported to the Large Lakes Observatory at UMD for subsequent analyses.

Grain size analyses entail pre-digestion of the sediment with hydrogen peroxide to remove organic matter and with sodium hydroxide to remove biogenic silica, prior to analysis with a Bekman Coulter laser grain size analyzer.  Bulk organic matter abundance and isotopic composition is determined with an elemental (C, N) analyzer attached to an isotope ratio mass spectrometer.  Total carbon and total inorganic carbon were analyzed by coulometry.  Diatom assemblage composition is determined by microscopic examination of smear slides.  Abundances of approximately 16 major and minor elements in the sediment cores is being determined by scanning X-Ray Fluorescence (XRF).

Various compound groups are being analyzed for δ13C and Δ14C after lipid extraction and separation into polar and apolar compound groups.  We are in the early stages of the extractions and separations.

Results to Date

Mean grain size of the sediment drops from about 18 microns to 9 microns in a water depth range from 80 to 220 m.  This is accompanied by a slight increase in %TOC (from 2.0 to 2.9%) and an accompanying slight drop in δ13C of the bulk organic matter, from -22.5 to -24 per mil and in δ15N, from 3.3 to 1.3 per mil.  C/N ratio rises on average with increasing water depth, from about 10.5 at the nearshore site to about 9.5 at 220 m.  

Discussion

The suite of sediment cores collected on Lake Malawi extend from near the mouth of a major river delta in 80 m water depth, down the slope and out onto the basin floor in about 400 m depth.  This transect extends through the chemocline of Lake Malawi, which separates the oxygenated water above 200 m depth to the anoxic waters below.  The excellent preservation of uppermost surface sediment recovered by the multi-corer allows us for the first time to quantify the changes observed in the organic matter that accompany this dramatic shift in water depth and ambient chemical conditions.  The direction of change in all of the parameters analyzed to date is as we would expect along the coring transect.  However now we have not only confirmed the anticipated direction of change but also quantified the extent of the change.  This information will be useful for interpreting these signals in the Lake Malawi drill cores that were recovered in 2005 (maximum length approximately 380 m, representing the past ca. 500,000 years).  The drill cores provide evidence of lake level shifts of several hundred meters, with the drill sites at times having been in water depths much shallower than today.

Isotopic analyses of organic compound classes are yet to be initiated, but the preparatory work is well underway.  We expect the results of these analyses will significantly improve our understanding of the relative contributions of terrestrial and aquatic sources to the bulk organic matter in the sediments that have accumulated in the past few hundred years.