Depositional history of Lake Chala (Mt. Kilimanjaro, equatorial East Africa) from high-resolution seismic stratigraphy

<p>Sediments deposited in Lake Chala constitute a high-resolution archive of past climate and environmental change in equatorial East Africa spanning two glacial-interglacial cycles. To correctly interpret the proxy records it contains, it is crucial to understand the evolution of lacustrine sedimentation in this volcanic crater basin since its formation on the lower south-eastern slope of Mt. Kilimanjaro. A dense grid of 37 km high-resolution seismic-reflection profiles allowed the reconstruction of the depositional history of Lake Chala. The seismic-stratigraphic sequence comprises sixteen distinct and finely-stratified units (U1-U16, youngest to oldest), grouped into five major depositional stages. Depositional stage I (U16, ~243-198 ka) marks the initiation of sedimentation in the originally ring-shaped depositional area surrounding two central tuff cones emerging from the basin floor and is characterized by a high rate of sediment accumulation and frequent occurrence of mass-wasting events (MWEs) under conditions of a relatively low lake-surface level compared to today. Depositional stage II (U15-U12, ~198-114 ka) represents the onset of basin-wide sedimentation above the central tuff cones, implying a higher position of the lake surface, less sediment focusing, and a shift to more strictly hemipelagic sedimentation. Multiple large-scale slope failures occurred around the basin periphery accompanying the progressive rise in lake level. Depositional stage III (U11-U8, ~114-97 ka) represents the development of a relatively flat lake floor under a significantly lower lake level, with evidence for strong sediment focusing implying accelerated sediment accumulation in central bottom areas. Depositional stage IV (U7-U4, ~97-20 ka) is again characterized by largely undisturbed hemipelagic sedimentation under mostly high lake-level conditions. However, frequent occurrence of mass-wasting events (MWEs) after ~48 ka resulted in the development of a longer, more gentle bottom slope towards the basin center. Depositional Stage V (U3-U1, ~20 ka BP to Present) represents the most recent period of basin evolution, during which the frequent occurrence of basin-focused sedimentation under a fluctuating lake surface level contributed to the establishment of the present-day very broad and flat basin floor of Lake Chala. Extrapolation of sedimentation rates established for the uppermost part of the sediment sequence, supplemented with basin-morphometric inferences derived from the successive depositional stages, yields an estimated age of ~243 ka for the oldest sediments in Lake Chala.</p>

Effects of hypolimnetic aeration on the quantity and quality of settling material in a eutrophied dimictic lake

Effects of hypolimnetic aeration (pumping of epilimnetic water into the hypolimnion) on the quantity of settling material in eutrophied Lake Vesijärvi, Finland were studied by comparing spatially comprehensive gross sedimentation rates as dry and organic matter prior to aeration activity and during two aerated years. Possible changes in the organic matter (as loss on ignition, LOI), carbon (C) and nitrogen (N) contents and changes in the C/N ratio of the settling material and surface sediment were quantified. Thermal stratification broke up earlier due to aeration and was followed by sedimentation peaks. The absolute amount of dry and organic matter as well as C and N settling to the lake bottom were significantly higher in the aerated years. Increased sedimentation rates were especially pronounced in the deep zones indicating enhanced sediment focusing. Increased sedimentation of C and N reflected higher primary production during the aerated years, which most likely was associated with increased temperature and turbulence and the subsequent regeneration and recycling of nutrients in the water body. Aeration seemed to slightly enhance degradation, but contrary to its ultimate aim, it failed to decrease the phosphorus content of the water column and deposits of organic material in the deep zones of the lake.

Maize pollen concentrations in Neotropical lake sediments as an indicator of the scale of prehistoric agriculture

We evaluated the potential of maize pollen concentrations in lake sediment profiles to serve as indicators of the extent of prehistoric agriculture in neotropical lake basins using records from a network of five sediment cores recovered from Laguna Zoncho, Costa Rica. The watershed of this small (0.75 ha) lake in the Diquís archaeological region has a c. 3000 year history of prehistoric agriculture and subsequent forest recovery, as documented through previous studies of pollen, charcoal, diatoms, and phosphorus fractions in a single core recovered from the center of the lake. In our new network of cores, we compared maize pollen concentrations with two independent proxies for the scale of agriculture in the same cores: abundance of organic matter (OM), which is an indicator of soil erosion, and bulk sediment stable carbon isotope ratios of organic matter (δ13COM), which reflect the proportion of forested and cleared land within the watershed. In none of the five cores did maize pollen concentrations correspond with either OM or δ13COM, suggesting that sedimentary maize pollen concentrations are not sensitive to the scale of maize agriculture in small neotropical watersheds. We found maize pollen in relatively high concentrations in two of the four cores taken near the lakeshore, but the others contained little or no maize pollen. The core from the center of the lake consistently recorded maize pollen, a finding that we attribute to sediment-focusing processes.