Microbial mats, located along the margins of hot-spring pools and out¯ow channels at Lake Bogoria, Kenya, are commonly silici®ed forming friable laminated crusts. Columnar microstromatolites composed of silica and calcite are also forming at several springs in sites of oscillating water level or spray. Silici®cation of the microbes involves impregnation of organic tissue by very ®ne amorphous silica particles and encrustation by small (< 2 lm) silica spheroids. Rapid silici®cation of the microbes, which may begin while some are still alive, can preserve sheaths and in some examples, the ®laments, capsules and cells. Although this provides evidence of their general morphology, the biological features that are required for taxonomic identi®cations are commonly poorly preserved.The silica precipitation results mainly from evaporative concentration and rapid cooling of spring waters that have been drawn upward through the mats and microstromatolites by capillary processes. Almost all the silica at the Loburu springs nucleates on microbial substrates. This af®nity of silica for functional groups on microbial surfaces contributes to the rapid silici®cation of the microbes and their preservation in modern and ancient cherts.

International audienceNew intermediate-resolution, normal-incidence seismic reflection profiles from Lake Tanganyika's central basin capture dramatic evidence of base-level change during two intervals of the late Pleistocene. Four seismically-defined stratigraphic sequences (A­D) tied to radiocarbon-dated sediment cores provide a chronology for fluctuating environmental conditions along the Kalya Platform. Stacked, oblique clinoforms in Sequence C are interpreted as prograding siliciclastic deltas deposited during a major regression that shifted the paleo-lake shore ~21 km towards the west prior to ~106 ka. The topset-to-foreset transitions in these deltas suggest lake level was reduced by ~435 m during the period of deposition. Mounded reflections in the overlying sequence are interpreted as the backstepping remnants of the delta system, deposited during the termination of the lowstand and the onset of transgressive conditions in the basin. The youngest depositional sequence reflects the onset of profundal sedimentation during the lake level highstand. High amplitude reflections and deeply incised channels suggest a short-lived desiccation event that reduced lake level by ~260 m, interpreted as a product of Last Glacial Maximum (32­14 ka) aridity. Paleobathymetric maps constructed for the two interpreted regressions reveal that despite the positive lake-floor topography created by the Kavala Island Ridge Accommodation Zone, Lake Tanganyika remained a large, mostly connected water body throughout the late Pleistocene. The results of this analysis further imply that Lake Tanganyika is the most drought resistant water body in the East African tropics, and may have acted as a refuge for local and migrating fauna during periods of prolonged aridity