Paleoenvironmental Implications of Time-Averaging and Taphonomic Variation of Shell Beds in Lake Tanganyika, Africa

PALAIOS, 2020, v. 35, 49–66 Research Article DOI: http://dx.doi.org/10.2110/palo.2019.037 PALEOENVIRONMENTAL IMPLICATIONS OF TIME-AVERAGING AND TAPHONOMIC VARIATION OF SHELL BEDS IN LAKE TANGANYIKA, AFRICA EMILY K. RYAN,1 MICHAEL J. SOREGHAN,1 MICHAEL M. MCGLUE,2 JONATHAN A. TODD,3 ELLINOR MICHEL,3 DARRELL S. KAUFMAN,4 5 AND ISMAEL KIMIREI 1University of Oklahoma, School of Geology and Geophysics, Norman, Oklahoma 73019, USA 2University of Kentucky, Department of Earth and Environmental Sciences, Lexington, Kentucky, 40506, USA 3The Natural History Museum, Departments of Earth Sciences and Life Sciences, London, SW7 5BD, United Kingdom 4School of Earth and Sustainability, Northern Arizona University, Flagstaff, Arizona, 86011, USA 5Tanzania Fisheries Research Institute, P.O. Box 90, Kigoma, Tanzania email: [email protected] ABSTRACT: The lake bottom along structural platforms in Lake Tanganyika, Africa, is carpeted with numerous large shell beds, known to be of late Holocene age, but of uncertain assemblage process. The shell beds may be the result of sedimentological (physical) assembly processes, or biological processes, or both. Previous work focused on the distribution of shell-rich facies, and showed time averaging of the surficial shell bioclasts over the last ~ 1600 calendar years BP. We focus on an extensive shell deposit along a deltaic platform in Kungwe Bay, Tanzania and examine time- averaging and taphonomy of Neothauma tanganyicense shells to constrain sedimentological and biological processes forming concentrations of shells. New radiocarbon dating indicates that Neothauma shells are time-averaged over the last ~ 3000 calendar years. Younger shells predominate shallow-water and exhibit unimodal age distributions, while shells from deeper-water exhibit a broader age distribution. Taphonomic results indicate that water depth and distance from the delta river mouth influence shell abrasion and encrustation with more encrustation developing away from sediment input points. Shells with black coatings and reddish-orange oxidation patinas suggest local burial and exposure. The age-frequency distributions of the shells suggest production rates of the shells vary over time and with water depth, tracking climatically driven lake-level changes (e.g., Little Ice Age, ~ 100–650 BP). In addition, age- distributions suggest that (1) mixing of different populations are more prevalent along the steeper deltaic slopes, and (2) recent decreasing production rates may reflect anthropogenic land-use change and attendant sedimentation, which has implications for Neothauma itself, and for organisms that are obligate occupants of the shell beds. These results suggest both climatic and depositional processes play unique roles in the distribution and accumulation of shell beds in Lake Tanganyika, which informs interpretation of similar paleoenvironments in the geologic record. INTRODUCTION calendar years BP. Nevertheless, the mode of formation, maintenance, and the degree of time averaging of these shell bed deposits are neither well Shell beds in modern aquatic environments consist of hard parts that understood nor constrained. accumulate after death of individuals in successive animal populations. The snail, Neothauma tanganyicense Smith, 1880 (hereafter, Neo- They are analogues for fossilized shell-rich deposits, revealing processes of thauma), is the largest bioclast within the shell beds, but is ecologically formation, and can also be used as proxies for recent environmental and poorly understood (McGlue et al. 2010). Recent surveys of the shell beds anthropogenic change (Kidwell et al. 1986; Fursich¨ and Aberhan 1990; in northern and central Lake Tanganyika (McGlue et al. 2010; Soreghan Kidwell and Bosence 1991: Tomaˇsovych´ et al. 2006; Kidwell 2013; 2016; Busch et al. 2018) suggest that living Neothauma are rare-to-absent Kidwell and Tomaˇsovych´ 2013). Lake Tanganyika in the East African Rift within the extensive shell bed deposits comprised of dead Neothauma contains extensive shell beds that are noteworthy for two reasons: (1) the shells. Live-dead agreement exists in environments where the majority of shell beds host a range of taxa, some of which are restricted to these beds dead shells within the accumulation belong to the living groups in the same including species of crabs, sponges, mollusks and a diverse group of habitat (Kidwell 2001). In regions acutely impacted by human-induced cichlid fish (Coulter 1991; Sato and Gashagaza 1997; Marijnissen et al. changes, studies have shown poor live-dead agreement in marine (Kidwell 2009; Genner et al. 2007; Erpenbeck et al. 2011), and (2) live gastropods 2007, 2009), coastal, and non-marine environments (Brown et al. 2005; of the taxon whose shells make up the majority of the bioclasts are mostly Erthal et al. 2011; Michelson and Park 2013). This may reflect local absent from the shell bed substrate in many parts of the lake. These shell extinctions, reduced rate of shell production, geographic range shifts, or beds form a crucial interface between the bio- and geosystems of the lake evolutionary differences between modern organisms and past populations as they record past environmental and ecological change and also play a that formed accumulated assemblages (Kidwell 2013; Tomaˇsovych´ and part in the adaptation and evolution of those endemic species that inhabit Kidwell 2017). Alternatively, poor live-dead agreement can occur through them (McGlue et al. 2010). McGlue et al. (2010) investigated shell-rich largely biological negative taphonomic feedbacks if shell production and facies, the environments in which they developed, and documented that the accumulation decreases future survival of organisms in the habitat (Kidwell surficial bioclastic deposits are time-averaged over at least the last ~1600 and Jablonski 1983). Finally, it may be that the shell beds of Lake Published Online: February 2020 Copyright Ó 2020, SEPM (Society for Sedimentary Geology) 0883-1351/20/035-49 Downloaded from https://pubs.geoscienceworld.org/sepm/palaios/article-pdf/35/2/49/4938541/i0883-1351-35-2-49.pdf by Kathleen Huber on 28 February 2020 50 E.K. RYAN ET AL. PALAIOS Tanganyika each comprise one or more relict deposits formed through sedimentologic-taphonomic processes, such that extant mollusk populations and active sedimentary processes contribute little to their formation and instead, the shell beds record past depositional and environmental changes within the lake. In order to discriminate among differing processes that are acting upon the shell accumulations and to develop a model for their formation, we (1) document spatial variation in taphonomic overprinting of Neothauma shells, and (2) derive frequency distributions of calibrated radiocarbon ages of Neothauma shells, sampled from across one extensive shell accumulation on a deltaic platform of Lake Tanganyika (Kungwe Bay, Tanzania). The taphonomic variation, coupled with variation in the range and shape of the age frequency distributions affords the opportunity to test among different models of shell bed formation (Tomaˇsovych´ et al. 2016). For example, did the shell beds form in response to lake-level fluctuations and attendant winnowing and sediment bypassing, by temporal or spatial changes in shell production rate, changes in shell destruction rate, or some combination of these processes? This study provides additional constraints on the distribution, spatial variability, and formative processes of the shell beds within Lake Tanganyika and builds on earlier studies that documented the distribution (Cohen 1989; Soreghan and Cohen 1996) and taphonomic state of the shell beds (McGlue et al. 2010). Constraining the processes that formed these shell beds will, in turn, lead to a better understanding of the genesis of the shell accumulations in this and other tropical lakes and provide a measure of recent ecological and environmental change recorded by the time- averaging of the shells. This study also provides constraints on the formation and variability of shell accumulations that are common components of ancient lacustrine deposits, such as the Cretaceous of offshore Brazil and West Africa (Abrahao and Warme 1990; McHargue 1990; Bracken 1994; Carvalho et al. 2000; Chinelatto et al. 2018) and the Paleogene of eastern China (Li et al. 1981; Bertani and Carrozi 1985). BACKGROUND Lake Tanganyika is the largest tropical lake and the second deepest lake in the world (Coulter 1991). Located in the East African Rift, the lake is bordered by four countries (Burundi, the Democratic Republic of the Congo, Tanzania, and Zambia) (Fig. 1). Cohen et al. (1993) estimated the FIG. 1.—Location of Lake Tanganyika in Africa (inset) and the major structural age of Lake Tanganyika between 9 and 12 Ma. The lake occupies a series features, location of shell-beds assemblages, and study area. Modified from of half-graben basins bounded by high-angle, normal faults along which Soreghan (2016). the polarity of the footwall alternates along strike (Fig. 1) (Rosendahl et al. 1986; Sander and Rosendhal, 1989; Chorowicz 2005). Lake Tanganyika’s and Thouin 1987; Soreghan and Cohen 1996; McGlue et al. 2010). The tectonic geometry impacts variations in sediment input processes, thicknesses of the surficial shell beds are poorly constrained, but do not nearshore environments, lake bathymetry, the size of adjacent watersheds seem to exceed 20 cm (McGlue et al. 2010). There is an association and the width of the shallow platforms on which the shell beds occur between the location

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