RADIOCARBON DERIVED TIME AVERAGING OF SHELL BED ACCUMULATIONS IN LAKE TANGANYIKA: EVIDENCE FOR ANTHROPOGENIC IMPACT ON A UNIQUE HABITAT

Shell beds in modern environments are increasingly used to infer anthropogenic change in community structure. In regions acutely impacted by increases in human-induced sedimentation or nutrient flux, the living organisms can differ from the shelly accumulations that reflect the previous generations, reflecting local extinctions and/or range shifts. Studies have suggested this anthropogenic signal can also be recorded in the age distribution of a random sampling of shells from modern shell-bed accumulations, where the age distribution of shells would exhibit a pre-modern age mode, with the mode marking the time at which the anthropogenic impacts began.

Lake Tanganyika (LT), which lies in the East African Rift, contains extensive shell beds on the modern lake floor in shallow-water (10-50 m) at multiple locations in the lake. The LT shell beds are dominated by a single gastropod (Neothauma tanganyiscense) and form an important habitat for organisms that colonize the shell beds, including other gastropods, ostracodes, sponges and cichlid fish (members of the Lamprologini tribe). We sampled four widely spaced shell beds in water depths of 9 and 20 m, randomly selecting 60+ shells per site for radiocarbon dating. After correcting for the old-carbon reservoir of the lake and calibrating ages, the shells exhibit multi-modal age distributions with up to 3000 years of time-averaging. Shells collected from all shallow-water sites exhibit similar age peaks and shells from the deep-water sites all exhibit similar older ages. The similar age spectra among widely separated sites, suggest lake-level, and likely sediment influx, controlled the rate of shell production at a given site in the past. But in three out of four sites, the corrected, calibrated ages contain few to no shells with ages < 100 cal yr BP. The lack of young/modern ages in three of the four sites suggest that human impacts in the form of enhanced sedimentation through land-use change is impacting shell production in these areas. This has important implications for habitat conservation because with enhanced sedimentation, shell beds are actively being blanketed, and the increased sediment input appears to be decreasing the production of new shells and threatening the ecosystem that depends on them.