Date Available

1-10-2025

Year of Publication

2025

Document Type

Master's Thesis

Degree Name

Master of Science (MS)

College

Arts and Sciences

Department/School/Program

Earth and Environmental Sciences (Geology)

Advisor

Dr Mike McGlue

Abstract

Lake Tanganyika is a large, ancient freshwater body with exceptional biodiversity that is located in the East African Rift System. Lake Tanganyika’s size and longevity make its sedimentary record useful for understanding environmental changes that cross spatial and temporal boundaries. 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 therefore have serious implications for human health and biodiversity conservation. This study focuses on using geochemical properties of modern lake bottom samples and sediment cores 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 (~44 to 267 m) and allow us to explore temporal variability in lake floor oxygen in the recent past. Both lake floor samples and cores were analyzed for their composition, sedimentology, and geochemistry.

Geochemical signatures from surface sediments indicate low-intermediate TOC (mean 2.66 wt.%; range 0.12 to 4.55 w.t %), whereas δ15Norg 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 shows 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 relatively high, which can be linked to the presence of carbonate shell-producing mollusks in these sites. The prevalence of laminations and thin beds increases in depositional environments in water > ~170 m deep. Fine-grained sediments with abundant diatoms and amorphous organic matter accumulate in those offshore environments, whereas mollusk bioclasts are very rare or absent. Turbidites occasionally occur in the deepwater cores, most likely owing to rift structural influences on bathymetric slopes. Gravity flows transporting coarse detritus and O2 into the anoxic profundal zone stand out in magnetic susceptibility, particle size distributions, and redox-sensitive trace element vertical profiles. The modern oxycline position at the study site is tentatively inferred to be between ~175 and 125 m water depth. Sediment core top samples from deeper water 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.

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