GEOCHEMICAL AND SEDIMENTOLOGICAL SIGNALS OF BOTTOM WATER OXYGEN DYNAMICS FROM LAKE TANGANYIKA (ZAMBIA)

Lake Tanganyika is a large, ancient freshwater body with exceptional biodiversity, located in the East African Rift System. Lake Tanganyika’s size and longevity make its sedimentary record useful for understanding environmental changes over time and space. Lake Tanganyika is known to be susceptible to global warming given its tropical location. Over 12 million people living within Lake Tanganyika’s watershed rely on fishing for their income and nutrition. Changes to limnological function driven by climate could have serious implications for human health and biodiversity conservation.

This study focuses on using geochemical properties of modern lake bottom samples (n=81) and short sediment cores (n=8) to understand changes in bottom water oxygen in Nkamba and Kasaba Bays (Zambia). Grab samples and core tops provide a snapshot of sediment characteristics deposited under modern conditions. Short cores were collected along a bathymetric transect from 44 to 267 m and allow us to explore temporal variability in lake floor oxygen over the past century using ²¹⁰Pb chronology. The analytical work includes redox-sensitive trace metals (Mn, Fe, Mo, Pb), grain size analysis, total organic and inorganic carbon content (TOC and TIC), total sulfur, and stable isotopes (C, N).

Preliminary results from surface sediments (<100 m water depth) indicate low-intermediate TOC (mean 2.66 wt.%; range 0.12 to 4.55 w.t %), whereas δ¹⁵N_org is generally low (mean -0.71 ‰; range -4.02 to 0.62 ‰); both variables tend to increase in deeper water. TIC displays low-intermediate values (mean 1.69 wt. %; range 0.17 to 4.39 w.t %) that generally decrease with increasing water depth. Grain size analysis revealed that sand is the most common particle size class in the modern sediment samples (mean 52.82 %; range 3.40 to 97.52 %). In many shallow water (10-40 m depth) samples, percentages of both sand and TIC are elevating, which can be linked to the presence of carbonate shell-producing mollusks in these sites. Sediment core top samples from deeper water (~231m) display much higher TOC values (up to ~ 8.44 wt. %), notionally a result of much lower lake floor oxygen and more favorable conditions of organic matter preservation. Ongoing work will focus on a detailed analysis of the core geochemical and sedimentological datasets to better constrain if the history of the oxycline position can be detected and reconstructed.