Sediment distribution and analysis of TAFIRI Bay and Luiche River platform, Lake Tanganyika, East
Africa: a prediction of hydrodynamic transport and wave-influenced deposition on clastic sedimentation
Students:
R. J. Hartwell—Fayetteville-Manlius High School, Manlius, NY USA
Florence Daudi—Dept. of Geology, University of Dar es Salaam, Tanzania
Mentors:
Dr. Kiram Lezzar—University of Arizona
Dr. Jim Russell—Brown University
Introduction / Purpose of Study
Lake Tanganyika (approximately 3o-4o S, 29o-31o E, elevation 773 meters), an ancient and deep, freshwater
lacustrine system of the western branch of the East African Rift Zone, holds many sedimentological secrets relating
to past climate that are waiting to be uncovered. Recent studies have estimated the age of Lake Tanganyika to be
approximately 9-12 million years old and as such is believed to contain sediment records dating to that age (Cohen
et. al., 1993). The following study, however, investigates recent sedimentation patterns and grain size distributions
across TAFIRI Bay (4.88o S, 29.62 o E) and the Luiche River platform (4.93 o S, 29.62 o E) (as inferred from surface
grab samples) to estimate sedimentation patterns resulting from internal hydrodynamic forces, wind / wave energy
and distances from the shoreline. A similar research study by Soreghan et al. (1996) will be used to compare
sedimentary conditions within the same general study site. By understanding how present-day sedimentation
patterns affect the distribution of clastic particles within protected and unprotected bays, we can hope to reconstruct
paleo-depositional conditions using similar analyses on past and future sediment cores recovered from the lake.
Sediment grain size distribution is directly related to the velocity of moving water; the velocity of the water is
indirectly related to depth; therefore, grain size is indirectly related to water depth. Sediments composed of sandsized particles can be deposited by relatively higher energy water, which is indicative of near-shore, beach type
littoral environments, whereas silt-sized sediments more likely accumulate in farther-shore, lower energy & deeper
water settings (Yuretich, 1979). However, sedimentation driven by varying water energies is influenced by the
presence and speed of blowing winds, which in turn affects both wave frequency and magnitude (Johnson, 1980).
High frequency shifts in wind speed, direction, and resultant wave propagation caused by storm events distribute the
different sediment size classes across different water depths (Johnson, 1980).
The purpose of this investigation is to demonstrate modern-day sedimentation patterns as they are related to
locations of varied sloping lake floor topography that are near and far from wind-driven, high-energy wave action,
such as in the exposed Luiche River platform and the head-land protected TAFIRI Bay.
Study Sites
The TAFIRI Bay and northern Luiche River platform were both selected as potential sites for high-resolution grain
size research based on recommendations by past Nyanza Project field scientists and reconnaissance work conducted
in the field. Each site was chosen because of similar yet contrasting environments of deposition. TAFIRI Bay,
approximately 4 km2 in size and 110 m deep, is moderately well protected from wind and wave action during both
the wet and dry seasons by the Nondwa Point to the north and the Bangwe Peninsula to the south. The Luiche
platform, approximately 582 km2 and 253 m deep, is geographically near TAFIRI Bay, yet is exposed to southerly
and northerly wind / wave energy in the dry and wet seasons, respectfully.
Five bathymetric transects in TAFIRI Bay were completed early during the field research period (Smith, Nyanza
Project 2005 Annual Report/NPAR05) and transects 3 and 4 were selected for sediment grab sample recovery based
on their interesting depth-slope profiles. The northern Luiche platform grab sample transect discussed in this study
was proposed based on field work conducted in 2002 (Morgan; Lewis, NPAR02) to complement earlier
sedimentological observations of those deposits.
Analytical Methods
Coarse fraction grain size analysis
Samples along the TAFIRI Bay shoreline were retrieved at 0 m water depth (17 July 05) while sediment grab
samples were collected using a small Ponar grab sampler between 5-25 m water depth within the bay itself (23 July
05). Deeper grab samples were taken in TAFIRI Bay along Transects 3-4 (Smith, NP 2005 Report) between 0-109
m (18-19 July 05) onboard the R/V Echo. Additional surface sediment samples were recovered from the Luiche
River Platform at different water depths ranging between 0-253 m, and as with all previous sediments, those from
the Luiche were stored in Whirl Pak bags for later grain size analysis back in the laboratory (Figure 1).
Samples were prepared for grain size analysis using a U.S. standard 63 micron-sized sieve in order to separate the
fraction finer than 63 microns (for fine grain size analysis) and coarser than 63 microns (for sieve analysis). The
samples that were taken at water depths of 20 meters and less were dried in an oven at 105 degrees centigrade,
sieved by using mesh sizes of 500, 425, 250, 125 and 63 microns, and percentages were calculated based on total
mass of the sieved sample. Cumulative %, mode, median, mean, skewness, kurtosis and standard deviation were
calculated in each class weight in EXCEL in order to determine the distribution of each grain size.
Fine fraction grain size analysis
For the grain size fraction finer than 63 microns, samples were initially pretreated with 1.0M of hydrochloric acid
(HCl) in a boiling water bath for ten minutes to remove inorganic carbon compounds, followed by rinsing twice with
centrifugation for twenty minutes each. Then samples were pretreated with 0.5M hydrogen peroxide (H2O2) in a
boiling water bath for one hour and then followed by two rinses and centrifugation for twenty minutes to remove
organic matter. Thereafter, samples were pretreated with 0.5M sodium hydroxide (NaOH) in a boiling water bath
for one hour to remove biogenic silica, leaving behind clastic particles only. Samples were analyzed for grain size
characterization using the Spectrex Laser Particle Counter. This system measures volumetric grain size between 192 microns by analyzing in situ the angle of refraction of a laser beam that is aimed through a blemish-free container
holding the sediments diluted with distilled water.
Statistical analysis
Statistical analyses were performed using the computer program JUMP to correlate grain size distributions with
other factors such as depth, distance from shoreline, and slope. Percentage of each grab sample coarser than 63
microns was directly compared to the depth and distance from the shoreline at which the sample was recovered.
Before relationships between grain sizes and slope could be investigated, the slope of the lake floor along all
sampling transects was calculated for TAFIRI Bay (Figures 4 & 5) and the Luiche Platform (Figures 6 & 7). Each
transect was broken into four slope classes (except for the North Luiche depth transect, which was separated into
three slopes classes since no grab samples were recovered in slope area S4-Figure 6) and each grab sample was
assigned its slope based on where it was taken along the transect. Slope values were then compared with grain size
distributions using statistical methods (Figures 8 & 9). In all situations after statistical analysis, r2 and Probability >
F values were used to assess the strength of correlations between grain size and depth, distance from shoreline, and
slope (Table 1).
Limnological hydrodynamic analysis
The following mathematical equations (Komar & Miller, 1973, 1975) were used to create the predictive tools that
suggest where sediments of different sizes should be deposited along the lake bottom, depending on wave energy
and water depth:
(1) ( ρ Um2 ) / ( ρs – ρ ) g D = 0.46 π ( do / D ) ¼
(2) ( ρ
Um2
) / ( ρs – ρ ) g D = 0.13 (do / D )
½
...if D > 0.5 mm
... if D < 0.5 mm
The formula above incorporates sediment grain density (ρs=2.6 g/cm3), water density (ρ=1.0 g/cm3), acceleration
due to gravity (g=9.8 m/s), the maximum horizontal water velocity (Um) and horizontal sediment displacement (do)
along the lake floor. The latter two variables were calculated using additional formula manipulations from Johnson
(1980) and were used in this study as averages of wave velocity and dependants of depth where the wave energy is
expected to influence sediment transport and deposition.
Results
As a first order data analysis, results were broken up by location: 1) transects 3 & 4 of TAFIRI Bay and 2) North /
South Luiche sampling transects. The percentage of the recovered sediment sample that was coarser than 63µ is
plotted with respect to depth for each location (Figures 2 and 3). In TAFIRI Bay, transects 3 & 4 follow the same
general pattern: samples are very coarse (90-100%) from the beach (0 m) to approximately 50 m water depth,
thereafter becoming nearly all finer than 63µ between 50 m to 75 m depth (10% coarse, 90% fine). From 75 m to 95
m, samples in both TAFIRI transects coarsen slightly but remain relatively fine-grained for the rest of the sampling
transect (Figure 2).
The Luiche Platform transects are similar to the TAFIRI Bay rough grain sizes in that the northern and southern
samples are relatively coarse at shallow depths (0-50 m) but abruptly fine around 50 meters water depth in the
southern transect (4% coarse, 96% fine) and around 70 meters depth in the northern transect (20% coarse, 80% fine)
(Figure 3). Both transects show a sudden peak in coarseness approximately 30 meters above the location of their
finest values but gently fine throughout the remaining samples. The only portion of the Luiche samples that does
not correlate roughly with each other between the percentages of sample coarser than 63µ is the upper 15 meters of
the northern transect where samples are much coarser (83-98%) than the southern transect (60-98%) (Figure 3).
Slopes (S1-S4) that were calculated for transect 3 and 4 of TAFIRI Bay, as well as both transects of the Luiche study
site, range between -0.921 (S2) to -0.215 (S4) in transect 3 (Figure 4), -0.1130 (S3) to -0.0264 (S1) in transect 4
(Figure 5), -0.0360 (S2) to -0.0199 (S3) in the northern Luiche (Figure 6), and -0.0207 (S4) to 0 (S3) in the southern
Luiche (Figure 7).
Statistical comparisons between the percentages of coarse-grained sediments recovered by grab sampling and depth,
distance from shoreline, and slope of the lake floor show varying degrees of correlation between each other. The r2
and Probability > F values for the relationships between coarse particle distribution and depth, shoreline distance,
and slope are listed from Figure 8 A-F & Figure 9 A-F in Table 1 below:
% Coarse with…
Depth
Distance from Shore
Slope
r2
Prb> F
Prb> F
Prb> F
r2
r2
Transect 3 (TAFIRI)
0.786
<.0001
0.803
<.0001
0.081
0.287
Transect 4 (TAFIRI)
0.876
<.0001
0.481
0.009
0.807
<.001
North Luiche
0.602
0.005
0.611
0.005
0.650
0.003
South Luiche
0.159
0.158
0.037
0.509
0.239
0.076
Table 1: r2 and Probability > F (Prb>F) values for TAFIRI and Luiche transects of % coarse-grained samples with
depth, distance from shoreline, and slope of lake floor. r2 values approaching 1.0 and Probability > F values less
than 0.0001 indicate the strongest correlations between variables.
Finally, results of the limnological hydrodynamic analysis (based on calculations solved from Johnson, 1980) that
predict the smallest sediment particle that can be deposited as a function of wind speed, fetch of the lake, water
depth, and wave height are listed in Table 2. Wind speeds of 5.5 m/s and 11 m/s were used, as well as fetch
distance of 80,450 meters and deep water depth of 580 meters; fetch distances applied to the equations varied from
650,000 meters to 3218 meters but are not all shown in the table below (only results from fetch of 80,450 meters):
10m
20m
30m
40m
50m
60m
70m
80m
Size (µ); 5.5 2.86
1.89E-5
1.24E-10
8.08E-16
5.29E-21
3.46E-26
2.26E-31
1.48E-36
m/s wind
Size (µ); 11 7266.4 92.2
0.863
0.0078
6.99E-5
6.28E-7
5.64E-9
5.07E-11
m/s wind
Table 2: The smallest size that would be predicted to be deposited based on calculations involving wind speed,
wave-energy, and other factors vs. depth. Results are provided only to 80 m as a result of the untrustworthiness of
the variables input into the calculations, as well as the calculations themselves in terms of their usefulness in the
sites studied in this investigation.
Discussion
As a first-order summary of the grain size distributions within a selected study site, it is simple to analyze the
percentage of the sample that is coarser than 63 microns and compare it to the percentage of the same sample that is
finer than 63µ. Results of TAFIRI Bay and Luiche samples generally show that grain sizes become finer with depth
(Figures 2 & 3). Since depth and distance from shoreline are directly related, grain size is indirectly related to
distance from the shoreline. This interpretation holds true for sediments studied by Soreghan et. al. (1996). In
TAFIRI Bay there is a slight coarsening with depth in both transects that may be attributed to an underwater canyon
near depth transect 4. It appears that grab samples TB22 and TB23 lie near a channel levee system of a deeper
underwater canyon (Smith, NPAR05). Since levees are the outside edges of relatively coarse underwater sediment
flow channels, these slightly coarser grab samples seem to complement the bathymetric interpretations made of
TAFIRI Bay.
The Luiche Platform samples are similar to those recovered in TAFIRI Bay, but the upper 15 meters of the northern
and southern transect do not correlate with each other (Figure 3). Since winds commonly blow from the south in the
dry season (April—September), wave energy is higher in the shallow areas (0-15 m) of the northern Luiche as there
are no headlands to block the energetic waves. Therefore, coarser sediments are transported and deposited by this
higher energy water. The results from the percentage coarse vs. depth from the North Luiche indicates this waveinfluenced effect on grain sizes.
Interestingly, when both sets of results are compared between the TAFIRI and Luiche study areas, all sediment
seems to fine at approximately 50 meters water depth. Jimenez, NPAR05 documented a sudden change in carbonate
values at the same 50 meter depth in TAFIRI Bay and the Luiche platform. Again these results are expected to
correlate since aragonite (measured by Jimenez, NPAR05 carbon/carbonate study) is very fine (~2 microns).
Smith, NPAR05 interprets that the base of a cliff and shell lag exists at approximately 50 meters water depth in
TAFIRI Bay, which may account for such a peak in aragonite values.
Jimenez also documented a decrease in aragonite concentrations in the Luiche sample areas at approximately 100
meters water depth; grain size thus is expected to increase with the absence of aragonite at this depth, which is
confirmed by the grain size results in Figure 3. Morgan, NPAR02 interpreted underwater sediment channels
flowing to the north and south around the Luiche delta lobe. It is believed that grab samples recovered at 84 m
(Northern Luiche—LP21) and 100 m (Southern Luiche—LP10) contain part of a relatively coarse-grained levee
system surrounding these underwater sediment channels.
Based on statistical analyses, TAFIRI Bay transects 3 and 4 show strong correlations of particle distribution with
depth and distance from the shore (Figure 8 A, B, D, E & Table 1; r2=0.706-0.803; Prb>F=<0.001). Within this bay
the correlations are strong (except for slope) since grain size distribution is closely linked to water depth and
distance from the shore; hence grain size generally decreases with increasing water depth and distance from the
shore. In the Luiche Platform transects, the correlation between distribution of particles is weaker compared to that
of TAFIRI Bay, but they are still statistically significant (except for slope) (Figure 9 A, B, D, E & Table 1; r2=0.6020.650; Prb>F=0.003-0.076). This may be attributed to tectonic activity forcing the bottom of lake in the North
Luiche to have complex slope characteristics; hence varying distributions of sediment particles. This is more
applicable for the intensively-faulted TAFIRI bay and less so for the South Luiche River platform. However, the
North Luiche seems to be intensively faulted as well and slopes there are very different from South Luiche. This
helps elucidate the reasons for weak correlations between sediment grain sizes with depth, distance from shoreline,
and slope. For all transects in TAFIRI Bay and Luiche Platform, slope does not consistently determine grain size
deposition since r2 and Prb > F values are so low (Figures 8-9 & Table 1; r2=0.037-0.611; Prb>F=0.005-0.509).
Lack of correlation between slope and grain size distribution was not unexpected, however, since similar lake floor
slopes may exist in shallow water as well as at depth.
Results of the limnological hydrodynamic analysis to predict size distributions with depth using mathematical
equations (Table 2) from Johnson, 1980 and Komar & Miller, 1973, 1975 have very limited value in this grain size
study. Different variables of wind speed, fetch distance of the lake, and deep water depth were applied in order to
match predicted grain sizes with measurements obtained from this study. However, with reasonable values input as
variables to the equations, predicted grain sizes were either too high or too low (Table 2). These inaccuracies may
be results of 1) incorrect variable values of wind speed, deep water depth, & fetch, or 2) these calculations may not
be applicable to the locations in our study site. Since TAFIRI Bay is protected by headlands to the north and south,
other unknown (not investigated in this study) hydrodynamic variables may be influencing sedimentation patterns in
the bay. The only potentially valuable piece of information obtained from these calculations is that wave-energy
tends to influence sedimentation by size only in the upper 25 meters of the water column (at average wind speeds of
5.5 m/s; deeper during higher-wind/storm events). Below this depth, the calculations suggest that waves do not
control grain size distributions along the lake bottom, with the exception of high-wind / storm events.
Conclusions
1) Coarse grains are deposited in shallow water where there is high wave energy from blowing winds. Finer
grains are deposited in deeper, lower energy water that is not affected by wind and wave energy.
Depositional areas that are protected by headlands favor fine-grained sedimentation closer to shore,
whereas those exposed to high-energy wind and waves are dominated by coarser-grained deposits.
2) Grain size is related to TIC (aragonite) distribution throughout these study sites. Aragonite is very finegrained (~2 microns) so a direct relationships between grain size and aragonite is expected. Where
aragonite is not abundant, coarse grain sizes seem to dominate.
3) Grain size distribution is dependent on depth, depth is dependent (in the absence of tectonic influence) on
distance from shoreline, therefore size distribution is indirectly related to distance from shore. Size
distribution appears to be independent of slope of the lake bottom since similar slopes may occur in shallow
water near shore environments as well as ones far off shore and at greater depths.
4) Statistical analyses suggest that the TAFIRI Bay transects provide strong and reliable correlations between
grain size deposition vs. depth and distance from the shoreline. Similar correlations also exist in the Luiche
Platform transects, but are weaker and less consistent than those in TAFIRI Bay. However, r2 and
Probability > F values suggest that there are very weak correlations between grain size and slope, if any
correlation exists at all, at both study sites.
5) Predictive calculations for sediment sizes deposited as a mathematical method of wind speed and wave
height are not reliable for TAFIRI Bay and Luiche Platform, either a result of inaccurate or inconsistent
variables such as wind speed and effective fetch of the lake.
6) Understanding the present day distribution of grain sizes at the current lake level can aid in the
reconstruction of paleo-lake environments by showing where near shore vs. offshore environments were
once located, hence past lake level stands, based on sediment core records recovered from the lake.
Additionally, being able to locate potential fine-grained petroleum source rocks as well as coarse-grained
reservoir deposits is beneficial to the oil-industry and world economy.
Future Work & Recommendations
Now that grab samples have been recovered and present-day grain size distributions investigated in TAFIRI Bay and
the Luiche River Platform, the next step should include extensive sediment coring using grain sizes from this study
(as well as TOC, TIC, ostracode, and diatom analysis, NPAR05) as proxies for paleo-lake level conditions . Using a
laser particle analyzer for this preliminary study is the best approach to analyzing grain size characterizations for this
lake. However, future projects would benefit markedly by using an analyzer capable of measuring a broader range
of grain sizes (beyond 1-92 microns) so sieve analysis could be eliminated.
Acknowledgements
We thank our mentors Drs. Kiram Lezzar and Jim Russell for providing their insight and guidance through the
course of project. Florence provides special thanks to Dr. Hudson Nkotagu and the University of Dar es Salaam for
assisting her participation in the Nyanza Project. RJ recognizes the Fayetteville-Manlius school district for financial
and professional assistance this summer. Appreciation to Christine Gans for her amazing ability to save the day on
numerous occasions and thanks to Mupape, Chata, and Captain Chale for research help on board the R/V Echo.
Recognition is given to Gwyn Smith, Gloria Jimenez, and Libin Zhang for assistance with data analysis.
Participants of the Nyanza Project 2005 credit TAFIRI for providing work space during the summer and the
National Science Foundation for funding the geological and paleoclimatological aspects of the project (NSF ATM0223920).
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