Fossil shell accumulations in

I: Styles of deposition across Kigoma Bay and the Luiche River Platform

II: Constraining recent environmental change using comparative taphonomic analysis

Student Participants: Mike McGlue (U. of Arizona) and John Mischler (Augustana College) Secondary School Teacher Participant: R.J. Hartwell (Fayetteville-Manlius High School) Project Mentors: Andy Cohen, Kiram Lezzar (U. of Arizona), and Ellinor Michel (NHM)

Introduction

Fossil-rich sedimentary deposits are important and conspicuous components of the rock record that often hold chronostratigraphic significance (Kidwell et al., 1986; Van Wagoner et al., 1989). A thorough understanding of the processes and mechanisms that control the formation of fossil-rich deposits in modern environments is central to a guided and meaningful interpretation of ancient strata. In this study, we investigate modern analogs of fossil- rich lacustrine carbonate deposits using examples from Lake Tanganyika, East Africa. Lake Tanganyika, the world’s second largest extant ancient lake, houses one of the most diverse suites of freshwater carbonates ever documented (Cohen and Thouin, 1987). Accordingly, Lake Tanganyika provides an unparalleled site to investigate the development of tropical, shallow water fossil-rich carbonate accumulations.

Lacustrine depositional systems are dynamic and sedimentary processes in lakes operate at different frequencies and amplitudes than marine systems (Bohacs et al., 2000). For paleoclimate-themed research, the rift lakes of East Africa are particularly valuable because of the long, high-resolution archives of tropical climate change housed in their sedimentary sequences (Cohen et al., 2000). Lake Tanganyika is well suited for studies aimed at investigating environmental change on timescales ranging from 102 – 106 yrs, due in part to its great antiquity, depth and breadth of depositional environments For example, paleorecords of sediment mass accumulation and ostracod biodiversity reveal the effects of historical land use change while lithofacies patterns, diatom assemblages and seismic reflection profiles reveal major lake level lowstands in phase with high latitude glaciations (Scholz and Rosendahl, 1987; Gasse et al., 1989; Lezzar et al., 1996; Cohen et al., 1997; Alin et al., 2002). Because bathymetry and water chemistry strongly influence the nature of carbonate sedimentation, biostratinomic processes and early diagenesis, shell-rich accumulations and their fossil preservation in Lake Tanganyika’s littoral zone could provide an important index of recent environmental change. Our objectives also have consequences for paleoclimate studies seeking deeper temporal resolution. As the practice of continental scientific drilling expands, lacustrine fossil shell accumulations may take on new significance as event horizons Figure 5: Location Map of 2005 Fossil Shell and chronostratigraphic markers in deep time. Developing modern Accumulation Study Area; HB = Hilltop Beach; analogs of ancient fossil shell accumulations also has application to KB = Katabe Bay; MB = Muzungu Beach; NLP = natural resource exploration, as Cretaceous-aged rift lake coquinas Northern Luiche Platform; UB = Ulombola Bay form economic hydrocarbon reservoirs offshore both Brazil and West Africa (Bertani and Carrozi, 1985; Abrahao and Warme, 1990).

Our study for the Nyanza 2005 field season is exploratory and centers on two key issues. First, we evaluate the depositional style of fossil-rich accumulations found in the region of Kigoma Bay and the Luiche River Delta. Second, we use semi-quantitative taphonomic analyses on target taxa (Caelatura bivalves) to infer environmental change based on fossil preservation. Prior studies dealing with fossil accumulations in Lake Tanganyika have focused primarily on general facies relationships and on the paleobiology of endemic gastropods (Cohen and Thouin, 1987; Cohen, 1989; Tiercelin et al., 1992; Soreghan and Cohen, 1996). Cohen (1989) concluded that shell beds in Lake Tanganyika form due to winnowing associated with small-scale (< 30 m) changes in lake level, based on time-averaging and on morphometric variations between fossil and living Paramelania domani. Our work seeks to complement and extend these studies through detailed analyses on shell rich accumulations in the region of Kigoma Bay.

Methods

We studied fossil shell accumulations in Lake Tanganyika using lake-bottom sediment samples collected by SCUBA divers or from the R/V Echo using a standard Ponar grab sampler. We identified sampling sites through a search of existing literature and by examining bathymetric profiles (e.g. Kinyanjui, 2002; Wheeler, 2004). SCUBA field techniques included the collection of box-cores and bulk samples. Lithified samples were collected using a sledgehammer. Sediment samples and fossil shells used in this study were collected along depth transects (steeply sloping environments) and along isobaths (gently sloping environments). Sampling sites and relevant bathymetric information are listed in Table 1.

Site Name Site Description Depth Range Relative Slope Analysis Hilltop Beach Base of rocky headland; sample transect trending W 17 – 22 m Moderate (8%) DS, TA Katabe Bay (KB) Base of rocky headland; sample transect trending 13 – 21 m Moderate DS, TA WSW into Katabe Bay (11%) Muzungu Beach Wave dominated cobble beach along Bangwe Point; 10 – 15 m High DS, TA (MB) sample site at N end of beach (16 %) N. Luiche R iver Large semi-exposed bay south of Kitwe Point; 9 m (Site 1) Low DS, TA Platform (NLP) sample sites in center of bay and along beach; grab 4 m (Site 4) (< 3%) sample near Kitwe 29 m (GB) S. Luiche River Central small exposed bay N of Ulombola Point; 10 m Low DS, TA Platform sample sites in center of bay (< 2%) (Ulombola Ba y) Tafiri Bay (TB) Central small protected bay N of Hilltop Point; 46 m (GB) Moderate DS, TA sample site at base of fault scarp (10%) Table 1: Sample sites for fossil shell accumulations study, Nyanza Project 2005; GB = grab sample from R V Echo; DS = depositional style; TA = taphonomic analysis

Upon collection, sediment samples were placed in labeled well log bags and transported with care to limit post retrieval damage to the dataset. In the laboratory, sediments were analyzed for the purpose of classifying style of deposition following the methods of Kidwell and others (1986) and Kidwell and Holland (1991; Table 2). Descriptive terminology follows the classification of modern lake sediments provided in Schnurrenberger et al. (2003). This analysis relied heavily on observations made from field photos and on box cores, which provide an undisturbed snapshot of sedimentary facies as they exist on the lake bottom. In order to approximate grain size variability, we homogenized bulk sediment from each site and wet sieved a representative amount to 63 µm. The sieved fractions were subsequently dried at 110 oC for 24 hours and weighed to yield a relative coarse – fine ratio.

To address the objectives of our taphonomy study, we selected Caelatura bivalves as our target taxa due to their abundance in the fossil shell deposits in Lake Tanganyika’s littoral zone (Kinyanjui, 2002; personal observations). Bivalves of the Caelatura are shallow, infaunal, filter feeders that inhabit mixed substrates (Leloup, 1953; Coulter, 1991). To facilitate rapid analysis while retaining important environmental differences in taphonomic signature, we considered only adult Caelatura in the size fraction above 4 mm (after Kidwell et al., 2001). In order to stabilize fossil preservation (damage) profiles, we attempted to analyze at least 100 Caelatura shells from each sampling site. The taphonomic indices and scoring system used in the analysis are summarized in Table 3 (after Brett and Baird, 1986; Kidwell et al., 2001). The scoring process was standardized through collaboration among multiple scorers until a reasonable training set was established. These data were then analyzed in MS Excel. Due to an apparent observer bias that strongly skewed data trends, the analyses from only two observers were included in the final results. Data trends between two observers confirmed consistent scoring and two samples from each multi-sample site (Katabe Bay, Hilltop Beach, N. Luiche Platform and Ulombola Bay) were analyzed. The results from these sites were averaged in order to smooth any remaining observers biases from the data. Care was taken to insure that the averaged scores were consistent with each other in order to produce an accurate taphonomic characterization for each site. Other study sites (Muzungu Beach, N. Luiche Platform 5, Tafiri Bay) utilized only a single sample for taphonomic analyses. Results from all taphonomic analyses were tabulated in MS Excel and the highest degree of damage for each taphonomic index was analyzed for first order trends. Principal component analyses (PCA) were conducted to reveal further variance in damage patterns.

Metric Classifications Description Method of Analysis Deposit • Autochthonous Describes if present have been Comparison of sample to • Para-autochthonous transported out of habitat reference collection • Allochthonous Taxonomic • Monotypic Describes number of species present Composition • Polytypic Close Packing • Densely Packed Grain support or matrix support; Visual inspection of diver box • Loosely Packed Dunham-type classification core • Dispersed Biofabric • Plane of Bedding: azimuth direction 3D arrangement of grains Visual inspection of diver box • X-section: concordant, perpendicular, core stacked, edgewise, oblique, imbricated Size Sorting • Well Sorted Trends in size of bioclastic grains Visual inspection of bulk • Bimodal sample • Poorly Sorted Geometry • Stringer, pavement, lens, wedge, pod, Arrangement/orientation of deposit in Visual inspection of UW clump, bed plan view; influenced by bathymetry, photo; leverage bathymetric currents/waves, bioturbation data Internal Structure • Simple Type and style of bedding; simple has Visual inspection of diver box • Complex monotonic trend (i.e. fining or core coarsening up)

Table 2: Summary of metrics used to classify style of deposition for fossil shell accumulations. Modified after Kidwell et al. (1986) and Kidwell and Holland (1991).

Results

I. Style of Deposition

Major observations regarding the style of deposition at each sampling location (Figure 1) are compiled below.

• Hilltop Point: Sampling along a depth transect in water ranging from 17 – 22 m deep, we encountered an expansive, para-autochthonous bed of unconsolidated whole and fragmented bioclasts and gravel-sized, sub- rounded sandstone lithoclasts overlying lithified coquina ledges and stromatolites at the base of a slope. Isolated clumps of gastropod shells were present within bioturbated sections of the bed. Sediments in the deposit are densely packed and lacked preferred arrangement of skeletal material in both cross section and bedding plane surfaces. The maximum measured bed thickness was 16 cm, though the bed gives the strong visual impression of lateral heterogeneities due to the underlying stromatolites. Beds exhibited a complex vertical internal structure; a lateral increase in fine-grained material was apparent down depositional dip in deeper water (> 24 m). Multiple, poorly sorted mollusk shells were present in the deposit, including Caelatura (dominant), Pleiodon, Neothauma, Vinundu (+ live), Lavigeria, Paramelania (+ live), Reymondia and Mysorelloides and Syrnolopsis (+ live). Coarse grain sizes constitute 98% of this deposit by weight.

Taphonomic 0 = 1 = 2 = Moderate 3 = Comments Variable No Damage Low Damage High Damage Damage Fragmentation Whole Whole but Chipped Large shell Environmental energy articulate disarticulated angular edges, fragment shell but still whole Bioerosion Absent ------Present Bioerosion by crabs or fungal endoliths Abrasion Shiny Slight irregular wear Loss of some Pitted, eroded External wear; original on edges; most ornaments; dull hydrodynamics luster; no original wear ornamentation Encrustation None Patchy Partial Complete Total coverage by all taxa – Coverage Coverage algae, calcareous micro- organisms Oxidation Staining Original Limited pink patches Dark pink to Deep red Redox conditions color light red Reduction Staining Original Limited light gray Medium gray to Black Redox conditions; burial color patches dark gray

Table 3: Summary of taphonomic variables used to detect environmental signals in L. Tanganyika’s fossil shell accumulations.

• Katabe Bay: Sampling along a depth transect in water ranging from 13 – 21 m deep, we encountered cemented clumps of stromatolite encrusted fossil Pleiodon shells (long axis 8 – 14 cm long) along the top of a steep slope. At the base of this slope, we encountered an expansive bed of unconsolidated, para-autochthonous, poorly- sorted bioclasts mixed with gravel-sized, sub-rounded sandstone lithoclasts. Isolated clumps of gastropod shells were present within bioturbated sections of the bed. The bioclasts in this bed are densely packed and lack preferred orientation. Bed thickness varied along the sampling transect, reaching a maximum of 12 cm near the 20 m isobath. Both the vertical and lateral internal structure of the deposit is complex, due in part to its association with adjacent and underlying stromatolites. Multiple skeletal types were present in the deposit, including Caelatura (dominant), Neothauma, Vinundu (+ live), Lavigeria, Paramelania, Reymondia, Mysorelloides and Syrnolopsis (+ live). Coarse grain sizes constitute 97% of this deposit by weight.

• Muzungu Beach: Sampling along the 15 m isobath revealed clumps of Pleiodon, Neothauma and Paramelania shells cemented together in the hollows formed within stromatolite reefs. Numerous Neothauma (~ 50%) shells display a preferred orientation, trending parallel to the shoreline (long axes aligned north). In deeper water (~ 20 m), fossil rich beds, dominated by Caelatura shells and well-rounded, gravel-sized lithoclasts, form in association with a stromatolite reef, down depositional dip of the coarse sandstone cobbles of the foreshore area. Coarse grain sizes constitute 98% of this deposit by weight. Bioclasts within the beds are polytypic and para- autochthonous (except Neothauma). Common genera represented in our samples include: Lavigeria, Reymondia, Paramelania, Vinundu and Syrnolopsis.

• Northern Luiche Platform: Four types of fossil shell accumulations were observed along the Northern Luiche Platform. At Ujiji Beach, two large ridges of allochthonous, polytypic fossil material were sampled. The larger of the two reached 120 cm wide, 25 m long and 6 shells thick. Neothauma dominate, but other genera such as Caelatura, Spekia and Melanoides are also present. The shells in this deposit are abraded and in some cases partially dissolved or stained. Offshore, shallow subaqueous shell-rich deposits (~ 1 m water depth) display normal grading, with large Neothauma shells (maximum 2 shells thick) capped by Caelatura valves, shell hash and wave-rippled sand. These deposits are loosely packed and do not display a preferred biofabric. The lateral extent of this type of accumulation is not well constrained; our sampling area covered a region ~ 3 m2. Shelly accumulations encountered at dive depths (4 m and 10 m) were para-autochthonous, polytypic beds dominated by Caelatura and Neothauma . Bioclasts were loosely packed in an algae-rich mud matrix; mud matrix constituted 24% of these deposits by weight. While laterally expansive, the precise extent of the shelly accumulations found at both Sites 1 and 4 could not be estimated from our sampling. Examination of a box core from Site 1 did not reveal any preferred biofabric. The internal structure is simple, grading from less shell rich mud (~ 8 cm below lake bottom) into dense layers of Caelatura and Neothauma shells. North of the dive sites, a grab sample near Kitwe Point recovered highly encrusted, strongly reduced, well sorted bioclasts. This deposit is polytypic; Neothauma (dominant), Vinundu, Paramelania, and Chytra shells are present, as well as fragments of a branching sponge. Coarse grain sizes constitute 84% of this deposit by weight.

• Ulombola Bay: We encountered a fossil shell-rich accumulation at 10 m water depth. The deposit was a para- autochthonous, polytypic bed dominated by Caelatura and Neothauma fossils; Syrnolopsis, Vinundu and Lavigeria shells were also present in lesser amounts. The maximum measured bed thickness was 11 cm and bedding was internally complex. Bioclasts were loosely packed in an algae-rich mud matrix; mud matrix constituted 23% of these deposits by weight. Fish nest craters, filled with Neothauma shells and live sponges were also present within the bed. While laterally expansive (> 15 m along the 10 m isobath), the precise extent of the shelly accumulation found at Ulombola could not be estimated from our sampling.

• Tafiri Bay: Grab sampling in Tafiri Bay recovered a fossil rich deposit 46 m below modern lake level at the base of a sharp fault scarp. The sample appeared para-autochthonous, polytypic and poorly sorted. Paramelania shells dominate, with Vinundu, Lavigeria, Caelatura and Syrnolopsis making up a lesser component. Most of these fossils lack significant post mortem modification. Coarse grain sizes constitute 95% of this deposit by weight.

Discussion:

I. Depositional Style

Based on our classification system, the dense fossil rich accumulations in the region of Kigoma Bay and the Luiche River Platform appear to fall within four facies types: 1) gravelly molluscan hash; 2) muddy molluscan hash; 3) molluscan hash and 4) stromatolitic boundstone (Table 5). Gravelly molluscan hash facies occurs adjacent to fault-controlled headlands. These environments of deposition are distinguished by their steep slopes and high wave energy. We interpret these deposits to form as the result of gravity flows, owing to their dense packing, massive bedding, poor sorting, and weak biofabric. The presence of rounded lithoclasts in these deposits suggests that waves, acting as an eroding agent on Pre-Cambrian aged Kigoma quartzites and Manyovu Redbeds outcropping along headlands, also play a role in the development of gravelly molluscan hash. Gravity flows can initiate through a number of mechanisms, but we suggest the most probable cause for the development of this facies type is recent lake level change. Proxy data suggests that lake level has fluctuated with high frequency over the past 500 years (Cohen et al., 2005). Dropping lake level induces a flux in hydrostatic pressure, causing gravitational instabilities in sediments occupying accommodation space atop sloped platforms. For example, during the climatic fluctuations associated with the Little Ice Age, several regression events forced basin-ward shifting of the shoreline by ~120 m west of Hilltop point, exposing the steep slope to sub-aerial processes. Due to their strong degree of taphonomic overprinting, it seems plausible that the bioclasts in shelly debrites are time averaged, and could reflect aggradation following successive changes of base level during this time. We cannot fully discount seasonal contrasts in discharge and lateral currents as a contributing influence on the formation of gravelly molluscan hash, but these mechanisms are much less compelling given the sedimentological data and the distance from influent rivers. Allochthonous shells are included in this facies, but these are, at least in part, extrinsically biogenic, and serve as nest building material for fishes. In addition to gravelly molluscan hash, stromatolitic boundstone facies was observed at each of the steeply sloped, high wave energy sampling sites. These deposits are well indurated and commonly form in ~ 10 – 13 m water depth. Our observations suggest that only those bioclasts with the longest taphonomic half lives, such as the robust surf clam Pleiodon spekii and the viviparid gastropod Neothauma tanganyicense , are commonly incorporated into stromatolitic boundstone facies.

Muddy molluscan hash facies typifies the fossil rich accumulations located within ~7 km of the Luiche River Delta (Northern and Southern Luiche Platform dive sites and NLP5 grab sample). The presence of an algal- rich mud matrix (> 15% by weight) and loose packing of bioclasts are the key diagnostic features of this facies (Figure 2). In addition, species richness decreases in this Slope vs. Sediment Coarse Fraction (> 63 microns) facies in comparison to the gravelly molluscan hash; Caelatura and Neothauma shells are clearly the dominant bioclasts. We 18 interpret this facies to, in part, reflect the dynamic nature of the ) 16 % (

deltaic region. Deltas in Lake Tanganyika, due to their high

e 14 t i 12 nutrient supply and varied sedimentary environments, support S e l

p 10 numerous species of living mollusks and other fauna m 8

Sa (Kinyanjui, 2002). However, due to the processes of t 6 a

e 4 distributary channel avulsion, lobe switching, and p o 2 Sl progradation/incision during base level lowering, environments 0 70 80 90 100 of deposition near the delta mouth can be starved of clastic Weight % Coarse sediment input or be subject to winnowing and erosion. Because hardpart production in these systems is typically high, sedimentation rates probably exert the strongest control on the Figure 6: Slope vs. Sediment Coarse Fraction of samples formation of fossil shell accumulations in deltaic analyzed in this study. environments. As a result, it is likely that muddy molluscan hash forms as a function of these dynamics. Deltaic sedimentation also limits post mortem modification to shell material, as partial or complete burial by fine-grained matrix retards biostratinomic processes. This relationship hold true for every site sampled with the exclusion of NLP5 GB1, the shells of which show increased external wear and fragmentation (Figure 3). We suggest that these shells are transported by wind driven lateral currents (dry season) and build up adjacent to Kitwe Point, which acts as a baffle to southerly flow. This mechanism accounts for both the decreased proportion of fine siliciclastic matrix as well as the increased instances of high encrustation and fragmentation damage.

Examples of pure molluscan hash facies are limited over our study area. Visually, the sediments of this facies type approach the ideal “shell lag coquina”: a high weight percent of coarse bioclasts lacking associated fine- grained matrix. The best example of pure molluscan hash facies comes from the base of a fault scarp in Tafiri Bay, where recently dead, deep water species accumulate and are likely winnowed by bottom currents. Other fault controlled coquinas have been observed along accommodation zone margins in Lake Tanganyika (Soreghan and Cohen, 1996). Nearshore depositional processes along the N. Luiche River Platform also produce pure molluscan hash facies, in the form of beach bars. The origin and persistence of these features seems most plausibly linked to wave energy and climate, respectively. Wave energy along the exposed section of the N. Luiche River Platform can be relatively high, especially during the dry season (Weier and Smith, this volume), and numerous Neothauma shells are carried by traction and pile up at along the lake shore. However, transport and subaerial exposure strongly abrades and dissolves shell carbonate. As a result, we suggest that these deposits have a limited taphonomic half- life unless they are buried subsequent to formation by a relative rise in lake level. Additionally, shallow water tempestites, common in the modern setting, are probably reworked and transported through time, and ultimately deposited as beach bars or lateral lags adjacent to headlands.

Indeed, the facies associations we observe near faulted headlands and open platforms differ from the idealized nearshore carbonate facies model proposed by Cohen and Thouin (1987). Our data imply that in these faulted headland environments, facies assemblages are commonly mixed siliciclastic-carbonate, with nearshore dominance of cobble and gravel-sized siliciclastic material (0 – 10 m) grading into stromatolitic boundstone (10 – 15 m), gravelly molluscan hash (15 – 22 m) and finally a stromatolite reef (> 22 m water depth) before grading into fine grained sediments further offshore (see Daudi and Hartwell, this volume). Along open platforms, the nearshore environment is characterized by beach sands and pure molluscan hash (0 – 2 m) grading into muddy molluscan hash (3 – 10 m) before grading into fine-grained sediments further offshore (see Daudi and Hartwell, this volume).

Facies Environment Taphonomic Interpretation Example Type of Signal Deposition Gravelly Littoral zone; ++ F Sedimentological concentration; Molluscan moderate to high ++ E strongly influenced by Hash slope; adjacent to + A gravitational flows; may occur as a fault controlled + O result of lake level lowering headlands (pressure changes)

Muddy Littoral zone; ++ R Biogenic/Sedimentological Molluscan low slope; + + E, F concentration; may reflect death Hash proximal to (distal lags assemblages enhanced by deltas only) winnowing (lake level change; erosion) or sediment starvation (channel avulsion); lateral transport forms distal lags; tempestites

Pure Molluscan Littoral to sub- + + R Biogenic/Sedimentological Hash littoral zone; (a only) concentration; reflects a) death moderate to low + A (b only) assemblages concentrated along slope fault controlled benches b) high energy nearshore bars

Stromatolite Littoral zone; + + E Biogenic concentration; strongly Boundstone moderate-to-high influenced by microbial slope activity/reef building

Table 5: Facies descriptions and interpretations of fossil rich deposits in the study region.

II. Comparative Taphonomy

Upon analysis of the data, first order trends are readily apparent (Figure 3). A principle components analysis conducted on data from each site with high damage revealed two distinct groups as well as two outliers. The steeply sloping headland sites (Hilltop, Katabe Bay, Muzungu Beach) show a distinct separation from those sites with gentle slopes located in semi-exposed/exposed bays (Northern Luiche Platform Sites 1 and 4, Ulombola Bay site). The Northern Luiche Platform Site 5 and Tafiri Bay Site 5 are clear outliers among the data clusters; both sites share some characters with the headland sites, such as high frequencies of fragmentation and encrustation, as well as other characteristics with the bay sites. NLP5 is most separated from the rest of the data by its high reduction value, and to a lesser extent, its high encrustation value. Variation in the TB5 site results from its high frequency of intense fragmentation and also its significantly low frequency of encrustation.

When focusing on the two distinct data groups, it is readily apparent that one group consists of samples collected at steeply sloping headland sites and the other encompasses the gently sloping embayments. Fossil preservation patterns between these groups are dramatically different. Specifically, different frequencies of intense fragmentation, encrustation, and reduction explain most of the variance between the environments. Abrasion values, while a less significant influence, also influence variation between the datasets. The headland sites are generally defined by relatively high frequency fragmentation, abrasion, encrustation, and oxidation values while those clustered around the Luiche delta have larger reduction values (Figure 4). An exception to these general patterns occurs in shells from NLP1 (Figure 1). Oxidation values for this site are anomalously high for a gently sloping exposed bay setting (Figure 4). Our data show that redox staining in gently sloping environments is dominated by reduction blackening. One explanation for this anomalous value is that it could be an artifact of the scoring system. Our training set exercise revealed that operator differences in evaluating degree of staining intensity might have introduced this artifact into our categorical dataset.

The taphonomic data presented in this study clearly discriminate between the steeply sloping headlands and the gently GENERAL TRENDS AMONG SITES sloping sites proximal to the Luiche

E 70 delta. L

P 60 MB 2 Elevated M 50 KB AVE SA HT AVE F 40

O NLP1 AVE T 30 N NLP4 AVE E 20 C ULB AVE 10 NLP5 1 PER 0 %F %A %E %O %R Figure 3:Damage frequency by site and taphonomic variable. F=fragmentation, A=abrasion , E= encrustation, O=oxidation stTaining,APH R=redONOMuctionIC sVtaining.ARIA SeeBL textE for details.

fragmentation and abrasion values found at MB, HT, and KB may be explained by a combination of the steeply sloping environment as well as the intense wave energy commonly concentrated on a faulted headland versus a bay. The shells at the three headland sites appear to have been subject to rigorous hydrodynamic conditions and transport among cobbles and stromatolites. Our observations of living Caelatura bivalves, though non-systematic, indicate that these clams live throughout the cobbled nearshore area, both above and on the steep slopes at MB, KB, and HT, although their observed abundances do not correlate with the volume of shells present in the base of slope gravelly molluscan hash facies. So while it is clear that modern processes move some Caelatura shells down slope under the force of gravity, it is also clear that paleo-processes have aided the development of gravelly molluscan hash at both HT and KB. This assertion is supported by the preservation of shells and by the thickness and areal extent of the deposit. While age data is not presently available, damage states of Caelatura shells suggest time averaging. As a result, we suspect large debris flows, perhaps initiated by changes in base level, may be one of the mechanisms responsible for fossil shell concentration at these sites. The stromatolite reefs at KB and HT also appear to play an important role in defining the deposit. For example, at MB, the slope continually falls steeply and there is no large stromatolite reef to baffle gravity flow, as at KB and HT. Therefore, the shells at MB are only able to collect in relatively small pockets between stromatolites. In this environment, Caelatura are readily destroyed, while larger, more robust shells belonging to other taxa collect and become cemented between the stromatolites.

The low fragmentation and abrasion values for NLP1, NLP4, and Figure 4: PCA describing variance between steeply ULB suggest these shells have not been transported a great distance and are sloping and gently sloping sites. See text for details. not under an especially rigorous hydrodynamic/gravitational regime as would be expected from the gently sloping bay environment of these sites.

The fragmentation value for NLP5 is anomalous for a site near the Luiche River delta mouth (~ 11 km). Confounding the abrasion signal at this site is the high frequency of encrustation, which obscures the surface ornamentation of most shells. The fragmentation frequency suggests that these shells may have been transported along shore and are accumulating along the southern margin of the Kitwe headland (Figure 1). This massive movement of Caelatura is corroborated by the large amount of these and other shells found along the beach in this area. Also, the bulk sample at this site contained less muddy matrix (85:15 coarse-fine ratio). Such a sample character lends credence to the interpretation of lateral transport across the bay and accumulating along the side of the point.

The high encrustation values found among MB, KB, and HT may be explained by the metabolic activity of reef building microbialites. Stromatolites, as previously mentioned, are relatively common at MB, KB, and HT. Stromatolites grow as continual encrusting layers covering a surface. The environmental conditions found between 15 m and 20 m at these headland sites seem to be ideal for stromatolite growth, and therefore, may also be suitable for other taxa of encrusters. These sites, characterized by good circulation and a relatively meager sediment influx, provide conditions suitable for the growth of vast reefs of mushroom shaped stromatolites. It is possible that those shells found around the delta, encased at least partially in a muddy matrix, do not spend enough time exposed on the lake bottom for encrusting communities (other than sponges) to thrive. It is only at NLP5, where it is known that the substrate is mostly coarse bioclasts exposed at the lake bottom, that encrustation returns to a high level. This high encrustation value may also be a function of depth considering that NLP5 was taken 20 m below the other delta sites.

The environmental conditions at MB, KB, and HT lead to relatively high oxidation values (Figure 4). The shells are constantly being reworked due to the energy of the system. Elevated wave energy ensures that the bottom waters stay suitably oxygenated so that there will be an adequate amount of oxygen for oxidation to continue operating on the shells. The percent organic matter is also relatively low in Tafiri bay (Jimenez, this volume). Decaying organic matter consumes oxygen, so the less organic matter present in the water column the more oxygen available for oxidation (J. Russell, personal communication). Such favorable conditions for oxidation explains the lack of reduction at these sites.

The oxidation values for the delta sites are more puzzling. They are generally lower compared to the headland sites. In fact, the deltaic environment seems to be generally reducing. This may be due to the larger amount of organics contained in the waters compared to Tafiri Bay (Jimenez, this volume). These organics contribute to an oxygen poor environment in which manganese begins to oxidize instead of iron (Cohen, 2002). This produces the black staining found on most of the shells of this region. Despite this environment, the oxidation value for NLP1 is comparable with those in Tafiri Bay. NLP1 is the site located closest to the mouth of the Luiche River. This oxidation value may be explained by the bogs contained in the delta itself. This environment is commonly associated with ample ferric iron. This iron, in conjunction with iron derived from bedrock seeps, enters the lake through the river and supplies ample iron to oxidize the shells closest to the river (Wetzel, 2001).

IV. Conclusions and Future Work

The results of our study indicate a wide variety of fossil shell-rich accumulations exist in Lake Tanganyika’s littoral zone. Common facies encountered include: gravelly molluscan hash, muddy molluscan hash, pure molluscan hash and stromatolite boundstone. Shell rich facies are widespread and expansive features of Lake Tanganyika’s littoral zone, and because previous studies indicate that fossil mollusks can persist for thousands of years at or near the lake bottom (Cohen, 1989), we suggest that these facies play an important role in taphonomic feedback in the lake. Many organisms (sponges, cichlid fishes, other mollusks) use the unique substrate created by these shelly deposits as habitat, and some even utilize larger bioclasts in brooding strategies. An important aspect of future research using data collected this summer will be to further investigate the relationship between fossil mollusk preservation and taphonomic feedback in Lake Tanganyika. The results of our comparative taphonomic work indicate that there is a relationship between fossil preservation and environment of deposition. We are especially intrigued by patterns of encrustation, which may ultimately be useful in reconstructing paleo-water depth. Future work is needed to complete our project. Very little is known about the modern ecology of both Caelatura bivalves and Neothauma tanganyicense gastropods, the two dominant fossil bioclasts we encountered. Such work would be helpful in extending our results and interpretations, and provide interesting projects for future Nyanza students. Acknowledgments

We thank Andrew Cohen, Kiram Lezzar, Ellinor Michel and Jon Todd for their guidance, suggestions and expertise during the completion of this project. Jim Russell and Lindsay Powers also provided much needed insights and assistance during the final stages of the project. Christine Gans is thanked for help in the field and in the lab. George Kazumbe, Issa Petit and Mupape Mukli are thanked for diving and logistics support in the field. This research was funded by the National Science Foundation (ATM-0223920).

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