Carbonates as evidence for groundwater discharge to the Nile River during the Late Pleistocene and Holocene

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The African Humid Period (AHP, ~15–5 ka) is the latest in a series of Saharan pluvials studied using carbonates. Evidence from desert paleolakes indicate that water tables rose across the region caused by increased precipitation during these wet periods and that this higher groundwater discharged at the surface creating a wetter environment beyond what increased seasonal precipitation could support alone. Carbonate cemented material and sediments have been noted at key archaeological sites such as Khartoum Hospital, and al-Khiday and at site distributions along Wadi Howar. In the deserts west of the Nile, extensive surface discharge of groundwater is the accepted generator of paleolakes and contributed to wadi flow. In contrast, studies in the Nile Valley have focused on the river itself as the source for any moisture to explain documented effects of a higher water table. Carbonate morphology and oxygen isotope composition of carbonate samples are used to develop a conceptual model demonstrating their accumulation in areas near the Nile River as a result of a regional rise in groundwater driven by greater precipitation. Morphological evidence shows a variety of formative environments at key topographic locations, indicating water availability above modeled contemporary Nile River flood levels. New results presented here from oxygen isotopes show locations dominated by evaporative enrichment at higher elevations, a transition to stable isotopic conditions at lower elevations, and ultimately a return to more enriched values at the lowest elevations that would have undergone intermixing of discharging groundwater and floodwater from the Nile. For areas beyond the river's flood extent or an associated river-fed alluvial aquifer, the only perennial moisture source capable of maintaining an isotopic equilibrium in a monsoonal climate would be groundwater. Results coincide with isotopic patterns and values observed in carbonates from studied paleolakes in the desert. This model provides an explanation for the carbonate accumulation at archaeological sites — while offering greater understanding of contemporary water availability for vegetation, animals, and human populations — and whether the Nile River functioned as the dominant water source for cultural groups in the valley during the early to mid-Holocene, as it did for the later agricultural populations.