Freshwater Limestone In An Arid Rift Basin: A Goldilocks Effect

Abstract

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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