Hydroclimatic and geochemical palaeoenvironmental records within tufa: A cool-water fluvio-lacustrine tufa system in the Wadi Dabsa volcanic setting, western Saudi Arabia

Abstract

A series of uncharacterised tufa deposits within the lowland volcanic provinces of western Saudi Arabia are investigated here in order to account for their presence and to examine their utility as a palaeoenvironmental archive. The Wadi Dabsa basin, in the Harrat Al Birk, is part of the Saharo-Arabian region, where there is increasing evidence for multiple hominin dispersals during windows of reduced aridity during the last ~400 ka of the Late Quaternary. This first and detailed petrological and geochemical characterisation of these tufa carbonates is particularly important for palaeoenvironmental and climatological reconstruction at a site that contains a major concentration of Early and Middle Stone Age artefacts. We aim to determine the nature of deposition, to answer whether these are cool, freshwater tufa or hydrothermal travertine, and to establish what the tufa settings, stratigraphies and facies reveal about the hydroclimatic regime in this semi-arid environment.A widespread lacustrine-paludal and fluvial tufa system is preserved, as well as topographically-controlled tufa barrages and cascades. Observable sections, exposed by incision, reveal a thickness of tufa deposits of 3 to 6 m in places, which contain two packages of tufa-cemented conglomerate facies. This allows us to provide a conceptual model for the deposition of semi-arid tufa, based on existing type sites in the Naukluft Mountains of Namibia and the Ebro Basin of Spain. The varied topography within this volcanic harrat provides a setting at Wadi Dabsa that combines the low-lying lacustrine-paludal model from Spain with the steeper and higher-energy model from Namibia. The two tufa-cemented bedload units and the evidence for post-depositional erosion of other tufa facies indicates a shifting hydroclimatic regime, with episodes during which the basin contained water and there was steady flow in the fluvial channels, alternating with episodes of high-energy flashy flows.Geochemical analysis using 87Sr/86Sr isotopic ratios (0.7049 to 0.7056 with one sample at 0.7062) and stable carbon isotopes (−12.9 to −6.3‰) reveal that these are cool-water tufas were deposited from meteoric water that has interacted with both metamorphosed limestone in the Asir Escarpment and the mafic metavolcanic bedrock of the Wadi Dabsa Basin, without a geothermal influence. These tufa are low-concentration‑magnesium calcite, with traces of quartz (to a maximum of 3%) and other silicates. Some samples have ≤7.5% palygorskite, which along with petrological evidence for microbial filaments suggests the importance of biomineralisation by cyanobacteria within some of these tufa. Petrological analysis shows the fabrics are dominated by micrite cements with some microspar, and very little post-depositional alteration. Where samples are banded, this reflects variation in calcite crystal size as opposed to any striking variations in mineralogical composition. Oxygen stable isotopic composition (−14.6 to −1.9‰) indicates deposition from an isotopically more depleted rainfall source than the modern day, noting additional corrections for unknown past temperature and any other fractionation effects are needed. Depleted rainfall sources in this region may relate to either a far-travelled Atlantic moisture source or a higher-intensity Indian Ocean monsoon source, or some combination.