Fossil Ostracodes from Lake Mobutu (Lake Albert): Palaeoecologic and Taphonomic Implications

Two deep water cores from Lake Mobutu (= Lake Albert) were analyzed for fossil ostracode content to determine palaeohydroclimatic conditions for the lake during the Late Quaternary. Low diversity ostracode assemblages with highly erratic frequency patterns were collected from both cores, spanning a time interval 13 000 BP to the present. Ostracodes dated at approximately 13 000 BP indicate shallow (3-8 m depth), moderately alkaline-saline water conditions at an open lake site currently 47 m deep. Above this horizon, progressive deepening and the establishment of a profundal, fresher water fauna is indicated. Depositional hiatuses, possibly due to deep water erosion, make the dating of this progressive deepening difficult to establish, but it certainly predates 7 500 BP. Three taphonomic associations are recognized from the ostracode assemblages found. Ostracode sand lag assemblages are life assemblages of unabraded, articulated fossils formed in relatively shallow water conditions, where fines are winnowed out and refractory organics are oxidized, leaving behind a highly concentrated ostracodal sand. Profundal life assemblages form in deep water, where food availability is low. They are characterized by very low density accumulations of unbroken and often articulated valves, reflecting a lack of transport and low initial population densities. Profundal mixed assemblages are transported admixtures of both shallow and deep water ostracode species, which probably form as a result of bottom current transport. They are recognized by the co-occurrence of species with very different habitat requirements. Fossil material in these assemblages is normally broken or abraded, and valves are mostly disarticulated.