Subtle signatures of seeps: Record of groundwater in a Dryland,DK, Olduvai Gorge, Tanzania

Abstract

Few proxies exist to identify aridity in the depositional record, although drylands cover ca 30% of the modern continental surface. New exposures in a siliciclastic and carbonate sequence in an arid to hyperarid basin at Olduvai Gorge, Tanzania provide a unique multi-proxy record of a 1Á85 Ma landscape that was exploited by early humans. The 2 m thick sequence of clastics and carbonates that are exposed along a 450 m outcrop records climate change over a single precession (dry-wet-dry) cycle. Siliciclastic data (sedimentary structures, grain size, mineralogy) and biological data are combined with data for a 10 to 35 cm thick limestone (stable isotopes, elemental geochemistry, petrography) to generate a depositional facies model for a site DK (Douglass Korongo) on this dry rift basin landscape. This site was situated on a low gradient, distal portion of a volcaniclastic alluvial fan. The clastics are intercalated distal alluvial fan sandy silts and lake clays that accumulated in a low energy environment. Groundwater discharge and the alkaline springs and seeps during wet-to-dry change in climate made a freshwater carbonate-rich environment. Bedded lithofacies (a lime mudstone with fossils) were deposited in shallow standing (spring-fed) pools, while nodular lithofacies with calcite spherulites indicate permanently saturated ground (seeps). Both environments experienced similar diagenesis, that is, the precipitation of authigenic barite from supersaturated groundwater, desiccation and pedogenesis, and late-stage calcite precipitation. Compositional and isotopic data suggest that a fresh groundwater-fed system was available to early humans even during dry intervals of the precession cycle.

Characteristics, Main Impacts, and Stewardship of Natural and Artificial Freshwater Environments: Consequences for Biodiversity Conservation

“…Human evolution is uniquely tied to springs, as demonstrated in the paleohydrological reconstruction of the Olduvai Gorge in Kenya [150], and springs have played an important role in human history (e.g., [151,152]). Despite their value to humans, springs and the aquifers that support them are globally threatened by human activities.…”

Pollen, People and Place: Multidisciplinary Perspectives on Ecosystem Change at Amboseli, Kenya

“…Wetland deposits on low gradient landscapes exposed in 1.85 million year old rock at Olduvai Gorge, Tanzania, suggest that permanently saturated ground from groundwater seepage can persist through arid climate periods and fill with shallow standing water during wetter climates (Ashley et al, 2016). At Olduvai, these landscape features are associated with bone deposit evidence suggesting continued potential for exploitation by mammal and protohuman populations (Magill et al, 2012a,b;Ashley et al, 2016).…”

Widespread freshwater carbonate in the Olduvai Basin, a precursor to a major eruption in the East African Rift System

“…Variation in nodule type, from calcite rosettes to micrite with halos, suggests variability in groundwater‐related formation processes. For example, calcite rosettes may be associated with a high and fluctuating groundwater table (Ashley et al , ; ) while Mn/Fe halos indicate changing redox conditions (Retallack, ). Playa lake shoreline fluctuations are associated with Milankovitch‐driven climate change (Ashley, ; Magill et al , ) and precipitation changes would also be reflected in water table fluctuations resulting in variable groundwater saturation and redox conditions.…”