Lake Chala

Sediments on the bottom of Lake Chala, a 92-m deep crater lake on the border of Kenya and Tanzania near Mt. Kilimanjaro, contain a uniquely long and continuous record of past climatic and environmental change in the easternmost equatorial Africa. Supported in part by the International Continental Scientific Drilling Programme (ICDP), the DeepCHALLA project has now recovered this sediment record down to 214.8 m below the lake floor, with 100% recovery in the uppermost 121.3 m (the last ~160 kyr BP) and ~85% recovery in the older part of the sequence, down to the lowermost distinct reflector identified in the seismic stratigraphy. This acoustic basement represents a ~2 m thick layer of coarsely laminated, diatom-rich organic mud mixed with volcanic sand and silt deposited ~250 kyr ago, overlying an estimated 20-30 m of unsampled lacustrine deposits representing the early stages of lake development. Down-hole logging produced profiles of in-situ sediment composition that calibrate with an absolute depth-scale, both the recovered cores and the seismic stratigraphy. Over ~70% of the recovered sequence is finely laminated (varved), especially in the upper 72 m (encompassing the last ~90 kyr). All other sections display at least dm -scale lamination, demonstrating continuity of a pelagic-type depositional environment throughout the lake history. The sequence is occasionally interrupted by several fine-grained turbidites (<1 to 100 cm thick), each clearly bracketed between a non-erosive base and diatom-laden cap. Moreover, the sedimentary record is interrupted by 30 visible tephra layers (2-9 mm thick). Tie points between sediment markers and main seismic reflectors support a preliminary age model inferring a near-constant rate of sediment accumulation over the last glacial cycle (140 kyr BP to present). In addition to this great time span, the exquisite temporal resolution of Lake Chala sediments provides a valuable opportunity to study past tropical climate dynamics at both short (inter-annual to decadal) and long (glacial-interglacial) time-scales. Moreover, it offers the opportunity to study the influence of climate change on the region’s freshwater resources, the functioning of terrestrial ecosystems, and the history of the African landscape in which modern humans (our species, Homo sapiens) originally evolved and have lived ever since.

Lead PI: Prof. Dirk Verschuren (University of Gent, Belgium).