Lithological and biological features of a fossiliferous tufa in the Kapthurin Formation, Baringo, Kenya, reveal the presence of a lush wetland in a semiarid environment during the Middle Pleistocene (ca 500 ka) in this portion of the East African Rift Valley. Four geological sections, each between 3 m and 8 m in thickness, exposed over a distance of 0AE5 km, reveal a 1 to 2 m thick paludal tufa which is composed of three carbonate beds, two dark grey silty claystones and a reddish-brown silty palaeosol. High resolution stratigraphic analysis, carbonate petrography, stable isotope and elemental geochemistry, clay mineralogy and fossil remains (molluscs, ostracods, diatoms and charophytes) reveal a ground water-fed system that fluctuated in depth and periodically disappeared altogether. Oxygen isotope ratios (d 18 O) of tufa matrix range from )4AE5& to )8AE0& (Vienna Pee Dee Belemnite) and become more positive up section, indicating the decreasing influence of fault-related fluids and increasing residence time or freshness of wetland water, rather than evaporative enrichment. This spring was situated on a lake margin during low lake levels, thrived during periods of increased ground water input and was ultimately replaced by an alkaline lake. The wetland would appear to have existed during a cool interval within the generally warm Marine Isotope Stage 13 or perhaps during the warm second half of Marine Isotope Stage 13. The ground water source of this wetland arose through a fault system. Thus, the position of the tufa deposit is controlled structurally but the timing and duration of the wetland system may have been influenced by both climatic and tectonic factors.

The Olduvai Basin (3u S), situated just west of the East African Rift System in northern Tanzania, contains a twomillion-year record of paleoclimate and paleoenvironmental change, as well as a rich archive of vertebrate paleontology (including hominins). Milankovitch precession cycles (, 20 kyr) regulate the mean annual precipitation (250-700 mm/yr), and the , 2500 mm/yr evapotranspiration results in a negative hydrologic balance. Despite persistent aridity, extensive deposits of freshwater limestone punctuate the stratigraphic record. Between 2 and 1 Ma, Lake Olduvai occupied the basin, and its sediments are a proxy for climate-driven cycles. Three limestones (1.84, 1.80, and 1.36 Ma in age), which formed within lakemargin floodplains adjacent to the lake, were studied to determine their depositional environment using field relations, sedimentary structures, fossils, petrography, and stable-isotope and major-element geochemistry. The three limestones are similar in that they contain peloidal micrite, siliciclastic detrital grains, and rhizoconcretions. Abundant faunal remains (gastropods, ostracods, Charaphytes, and fish) indicate that ponded water was relatively fresh and alkaline.Geochemical and stable-isotope data indicate two types of groundwater-sourced carbonate-producing waters: a deepersourced fluid that was enriched in iron and manganese due to extended water-rock interactions, and a shallower groundwater that traveled through alluvial-fan deposits. Regional faults tapped the deeper groundwater, producing carbonate at spring sites, while seeps associated with basinward changes in alluvial-fan slopes drew on shallower groundwater sources. Isotope compositions indicate that fault-related waters experienced some evaporation as water moved away from the spring sites while compositions of the seep-related carbonate remained relatively constant. Pedogenic alteration and meteoric calcite cementation affected the carbonate when the spring and seep sites dried out. Secondary strontium-rich dolomite precipitated within the limestones during burial under paleo-Lake Olduvai sediments and fluids during periods of lake expansion.Integrating these data within the geological context of regional paleoclimatic and local environmental change indicates that the freshwater carbonates formed periodically when the conditions were just right, i.e., a ''Goldilocks Effect.'' Carbonates near the basin center formed from groundwater flowing under a hydraulic head from faults or fractures during the falling limbs of Milankovitch cycles when the lake was in recession. Carbonates near the basin margin formed from an increased rate of groundwater seepage that occurred only on the rising limbs of cycles. In both contexts, the continuous flow of groundwater with surface evaporation and CO 2 degassing optimizes the conditions for limestone formation in this arid environment. These results help explain the formation of freshwater limestones in the rift basin, link the carbonates to specific portions of Milankovitch cycles, and document th...

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Multiple climate proxies indicate episodic changes in moisture levels within an ∼1 Ma duration (early−mid Pliocene) interval. Limestones within the Opache Formation, Calama Basin, Atacama Desert region, Chile, contain evidence for wetter and drier periods on short time scales. Proxies include carbonate lithological changes, paleontology (stromatolites, oncolites, gastropods, ostracods and diatoms), O and C stable isotopes, geochemistry, and mineralogical changes (aragonite, calcite, Mg-calcite, dolomite and gypsum) throughout a 30 m stratigraphic section. Stromatolite fossil cyanobacteria dark and light laminations and mesohaline to hypersaline diatom species suggest Pliocene annual seasonality. Short-term changes between wetter and drier conditions indicate that at least this part of the Atacama region was experiencing relatively rapid early−mid Pliocene climate instability. The predominance of limestone in the Opache Formation, in contrast to the 1500 m of Oligocene-Miocene siliciclastic conglomerates and sandstones, interpreted as arid climate alluvium, that underlie it, indicates a shift from arid or hyperarid climate to a semi-arid climate. Semi-arid conditions promoted limestone deposition in a shallow lacustrine-palustrine environment. In this setting, events such as storms with associated surface water flow, erosion, siliciclastic sand, gravel, and intraclast deposition, coupled with significant biological activity, represent sedimentation during more humid periods in a shallow lacustrine depositional environment. In contrast, limestone characterized by mudcracks, Navicula diatoms, and vadose syndepositional cementation, reflect periods of enhanced evaporation, water shallowing, and episodic desiccation, characteristic of a palustrine depositional system. These facies shifts, in conjunction with geochemical and isotopic proxy evidence, yield a sedimentary record of wetter and drier climate shifts.