Ostracode Taphonomy from Modern Shell Beds in Lake Tanganyika, East Africa

Abstract

Lake Tanganyika (LT), in the East African Rift, contains extensive shell beds that are noteworthy because the gastropods composing most of the LT shell beds are absent from the living in-situ assemblages of organisms hosted by the shell bed substrate. Understanding the geologic and biologic processes that formed these beds has the potential to improve our understanding of fossil shell beds in East Africa. The shell beds may also record recent lake-wide environmental change from climate or watershed impacts, which may be evident through community and taphonomic changes in the populations of shelly invertebrates and so the research seeks to examine whether shell bed substrates have experienced significant anthropogenic impacts that have altered the living assemblages. We investigated total abundance and taphonomic properties of ostracode fossils from shell beds at two sites in LT, one near the Luiche River Delta (a highly deforested site along the northern Tanzanian coast) and another near the Mahale Mountains National Park, an area of substantially lower human population density, 157 km to the south of the Luiche site. In laymen terms, ostracods are microscopic crustaceans that thrive in lacustrine environments and serve as very good indicators for past paleo-environmental and paleochemical reconstructions (Cohen, Nielsen 1986). We studied ostracode assemblage because of its sensitivity to chemical and physical changes in the environment. Ostracods indicate changes in shell beds. The analysis was done by sampling each site along 8 depth transects at the Luiche site and 7 transects at the Mahale site at water depths of 3, 6, 9, 12, 15, 20, 30, 40, 50, and 60 meters. We analyzed total abundance, %valves vs. carapace, % adult vs. juvenile, % carbonate coated, reduction stained, oxidation stained, and %broken vs intact valves for ostracode shells. Results of LT show that there is a decrease in proportion of broken valves, slight decrease in carbonate staining, and an increase in reduction staining and proportion of whole carapaces with increasing water depth. Results of MT show that there is a decrease in broken valves and whole carapaces and an increase in adults, carbonate coated and reduction stained ostracodes with depth. The implications of this study towards paleo-limnology is that the pattern and post-mortem alterations record environmental conditions in shell bed area which can be used to further research in the study of ostracods and environmental changes.