Author: Modesta Magazi
Mentor: Mike Soreghan
Introduction
Lake Tanganyika is a 9-12 ma old (Cohen et al., 1993) rift lake situated in the western branch the east Africa rift valley. The geomorphology of the lake is controlled by tectonic activity associated with rifting which also controls stream morphology (Soreghan and Cohen, 1996). There are a large number of different streams that bring fresh water to the lake; for my study I focused on two streams, the Ngelwa stream in a deforested watershed and the Kalande stream in a forested watershed. Both streams are situated in the eastern part of the lake, north of Kigoma (Tanzania)
The proportion of organic matter and grain size in lake Tanganyika reflect processes of primary productivity and wave energy, all of which can be related to depth and climate (Jimenez, 2005). The amount of organic matter largely depends on water depth but also productivity and preservation (Cohen,
2003). The character and distribution of allochthonous organic matter in the sediments are controlled by fluvial geomorphology, drainage size, source area lithology and tectonics structure of the deposition basin and climate. The main purpose of my study is to explore the relationship of organic matter and grain size from the depth of 60m to 160m in Lake Tanganyika offshore of the deforested (Ngelwa) and forested
(Kalande) drainages. The goal is to test whether there is a relationship between the amount of organic matter and grain size distribution offshore of the two deltas caused by the different levels of deforestation.
Methodology
The R/V Echo was utilized to obtain samples at different depths offshore of the two drainages.
Sample locations are given in the report by E. Astley of this volume (Nyanza 2006). A ponar grab sample was used in all areas to collect sediments at each depth, about 13 samples collected and were placed in
Whirl paks for analysis in the labs; the following analyses were done for all samples.
Total Organic Matter
Loss on ignition was used to determine the amount of organic matter of each of the grab samples; crucibles to be used were precombusted at 900°C for 30 minutes to clean them and then were stored in airtight bags. Afterward, the empty crucibles were pre-weighed and about five grams of sample sediment was placed in the crucibles and the combined weight was taken. The samples were heated in a dry oven at
60°C over night to remove water content and then weighed, then combusted in muffle furnace at 550°C for two hours to remove organic matter; upon cooling were weighed to determining the amount of organic carbon, again the samples were heated at 900°C for two hours and weighed for determination of TIC. The percentage of each substance was determined by using the following formulas:
H
2
O%=(Wet sample wt. - dry sample wt.) / Wet sample wt.
TOC%=(Dry sample wt. - burned sample wt. (at 900° C) / dry sample wt.
TIC%=(Burned sample wt. (at 550°C) - burned sample wt. (at 900°C) / burned sample (at 550°C)
Laser Analysis for Fine Grain Size <63µm
Wet sieving was completed for all samples; afterward the sediments were allowed to settle in covered buckets over night and then the water was siphoned off until only 250ml of water remained in the bucket. The sediment and water was transferred to a beaker and this was allowed to settle. Then the beakers

were shaken to homogenize all the suspended sediments and exactly 10ml of water mixed with sediment was collected by syringe into a centrifuge tube for laser particle analysis.
Prior to grain size analysis, three different chemicals were used to treat the sediment in the centrifuge tube: HCl, NaOH and H
2
O
2
. Concentrated HCl was used to remove calcite from the sediments by adding it to the centrifuge tube and heating in a water bath for 10 minutes followed by centrifuging for
10 minutes then adding distilled water and centrifuging again. A 50% H
2
O
2
solution was used to remove organic matter by adding it directly to the centrifuge tube and heating for an hour in a water bath and then centrifuging for 10 minutes, followed by distilled water rinse and centrifuging again. Finally, NaOH was used to remove biogenic silica material by adding it directly to the centrifuge tube for 1 hour in a hot water bath. After all the chemical processes were completed, the samples were centrifuged two times with distilled water to clean the samples. From the remaining sediment-water slurry, 1 ml of sample was taken after shaking the centrifuge tube and the sub-sample was added to a cleaned beaker containing 100 ml of distilled water. This beaker was used in the Spectrex Laser Particle Counter (PC_2000) instrument to determine the mean grain sizes of each sample.
Smear Slides
A smear slide was prepared for each untreated sample by placing a small amount of sediment on a clean slide with a metal spatula and placing them in a drying oven for about 5 minutes. When dry, a drop of mounting medium was added over the sediment and a cover slip was placed on top and then they were cured in the sun. Afterward they were viewed using the polarizing microscope at 40x magnification to determine the relative abundance of aragonite crystals, shells, algae and charcoal.
Results
The samples were collected from two different areas, i.e Kalande (forested) area and Ngelwa
(deforested) area, and were analyzed together for statistical significance. Due to their difference in both spatial distribution and depositional environment, the data were treated separately for regression analysis to determine the relationships between the percent organic matter and grain size and depth.
The data collected from Kalande (Figure 1) shows a weak negative correlation between median grain size and total organic carbon (r 2 =0.500). That is, the finer average grain size of the sediment correlates with a higher content of organic matter, but note that the amount of organic matter becomes more variable as grain size increases. Within the Kalande area, however, the percent of total organic carbon correlates poorly (r 2 =0.0003) with depth (Figure 2). Total inorganic carbon shows a positive correlation with depth (r 2 =0.4489; Figure 3) but correlates poorly with median grain size.
The data from Ngelwa shows a positive correlation between water depth and organic carbon
(r 2 =0.404; Figure 4) but the percent total organic carbon correlates poorly with median grain size.
However, in the data from Ngelwa, the percent inorganic carbon is highly correlated with grain size
(r 2 =0.8095; Figure 5) but shows a weaker correlation with depth.
Smear slide analysis shows less content of diatoms, high content of algae and aragonite in forested Kalande area, but those from the Ngelwa area shows high content in diatom and less in algae.
Discussion
Both Ngelwa (deforested) and Kalande (forested) streams show similar changes in total inorganic carbon content, because the calcite crystals are transported as fine sediment like silt and clay, and deposited accordingly with decreases in wave and current energy, generally in the deeper depths of the lake.
But for organic matter the two regions show different trends. The amount of organic matter in the area can vary because of increased production within the lake or increased transport of terrestrial organic matter into the lake. In the Kalande area organic matter shows good agreement with depth because of the presence of trees in Kalande watershed, which adds potentially more terrestrial organic sediment to the lake. The Ngelwa stream, which is situated in a deforested area, shows a lesser trend, possibly because there is less terrestrial matter that gets transported into the lake.

According to Cocguyt’s (1999) analysis of modern diatoms assemblages in lake Tanganyika, two of the most important factors governing diatom distribution are depth and presence of clastic material in the area. In my study, depth was measured using an Echo-sounder and all the samples were collected at high depth from 60m to 160m. The Ngelwa samples contain a higher content of diatoms compared to the
Kalande samples. The Ngelwa stream is in a deforested area and therefore there is a high influence of clastic material in the area, which brings nutrients for diatoms to grow. It is possible that the area is also influenced by higher wave energy, which would transport the sediment rich in diatom content from shallow to deeper parts of the lake. Finally, within the Ngelwa samples, there is also a lower abundance of carbonates, which could dilute the diatoms content. The Kalande stream, however, runs through a forested area and thus could transport large amounts of terrestrial organic sediment into the lake; there is also a lower amount of clastic sediment and higher amount of organic matter, which could dilute the diatom content.
Grain size is not a factor that relates to percent organic matter in these samples, because the
Kalande area shows some correlation between organic matter and grain size while the grain size is >0.33 um, but in Ngelwa, the organic matter correlates poorly with grain size while the grain size is >0.23 um which suggests that the Ngelwa area consists of finer grain sizes than the Kalande area. This analysis does not support the hypothesis that there is a direct correlation between grain size and organic content in this area.
Inorganic carbon, however, appears to correlate better with grain size, presence of clastic sediment and high wave energy in the Ngelwa samples. But in the Kalande samples, inorganic carbon correlates poorly with grain size but grain size shows a poor correlation with depth in this area. There appears to be less algal material in the Ngelwa region, and therefore the algae do not contribute to the organic content.
Inorganic carbon proved to be the best predictor of depth in the two areas, because TIC shows a correlation with water depth in both areas: i.e. Kalande (r 2 =0.4489) and Ngelwa (r 2 =0.3606). Therefore from this study it appears that TIC is the most robust indicator of water depth.
Acknowledgements
I am glad to thank the almighty God for guiding me during my research process, and thanks for the good mentorship I received from my mentor Mike Soreghan during grab sampling and sample analysis in the labs. More thanks for the University of Dar-es-Salaam for giving me a chance to join the Nyanza project for my fieldwork, thanks for Geo team for their cooperation during grab sampling. Lastly thanks for captain Chande and Chata for crewing the R/Vecho during sampling process, thanks for TAFIRI and the
University of Arizona for finding this research.
References
Cohen; A.S, 2003, Paleolimnology: The History and Evolution of lake systems: Oxford Press.
Currano, E., 2003, When a geology project becomes chemistry: the carbonate vs. diatom content of surface sediments from Lake Tanganyika, Nyanza Project Annual Report, p. 33.
Jimenez, G., 2005, Distribution of organic matter and carbonate minerals in surface sediments of Tafiri Bay and the Luiche Platform, Lake Tanganyika: Implications for paleoclimate reconstruction, Nyanza
Project Annual Report, p. 59.
Soreghan, M.J., and Cohen, A.S., 1996, Textural and compositional variability across littoral segments of
Lake Tanganyika: The effect of asymmetric basin structure on sedimentation in large rift lakes:
American Association of Petroleum Geologists Bulletin, v. 80 p. 382-409.
Prothero, D.R., and Schwab F., 1996, An Introduction to Sedimentary Rocks and Stratigraphy, W.H
Freeman and Company.
Wetzel, R.G., 1975, Limnology. Philadelphia: W.B.saunders Company

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