Influence of depth and mixing regime on sedimentation in a small, fluctuating tropical soda lake

Verschuren, Dirk

Limnol. Oceanogr., 44(4), 1999, 1103-1113 | DOI: 10.4319/lo.1999.44.4.1103

ABSTRACT: The historical sediment record of Lake Sonachi (Kenya) was used to study the influence of lake depth and mixing regime on patterns of sedimentation in a small, fluctuating tropical soda lake. Lake Sonachi last desiccated completely in the early nineteenth century and has fluctuated between 3- and 18-m lake depth over the past 115 yr. A freeze-core of offshore sediments describes recent lake history as a succession of meromictic episodes, represented by varved or subannually laminated muds, and holomictic episodes, represented by more coarsely layered muds. Two interbedded horizons of colloidal amorphous silica dated to a period of rising lake level after a prolonged lowstand were deposited by abiogenic, pH-driven precipitation from the water column and represent the instanta-neous sequestering of an estimated 63 mg liter-1 or possibly >50% of the lake¬ís dissolved-silica reservoir. Changes in offshore sedimentation and inferred bottom dynamics over time indicate that sediment resuspension and focusing in Lake Sonachi occur mostly during infrequent events of deep circulation between the average mud deposition boundary depth at ~2 m and the chemocline depth at 4-5 m; wind-driven sediment redistribution across the lake floor is important only at lake depths of ≤3 m. Dry sediment accumulation has varied between 86 and 620 g m-2 yr-1 over the past 175 yr, with no relationship to lake depth but, on average, lower rates during meromixis (199 +/- 90 g m-2 yr-1) than during holomixis (349 +/- 152 g m-2 yr-1). Net organic carbon accumulation offshore varied between 0 and 92 g m-2 yr-1, with no significant relationship to either lake depth or mixing regime at the time of deposition. Sedimentary organic carbon content (5.7-26.9%) is negatively correlated with bulk sediment accumulation; 73% of this variation is accounted for by the clastic dilution of sedimented planktonic algal production with low-organic littoral sediments redeposited offshore.

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