Holocene playas as sedimentary evidence for recent climate change in the presently hyperarid western desert, Egypt

Abstract

Integrated lithostratigraphic and geochronologic studies of Umm Akhtar Playa, a newly discovered dry lake located at ≈ N 22° 36' E 30° 18' in s. Egypt, indicate the accumulation and rapid desiccation of a sizeable (>65 km²) standing water body during the Late Quaternary. An optical date of 15,120 cal yr on basal aeolian sediments indicates that the deflation basin was mantled with aeolian sands during the Late Pleistocene. Overlying Holocene fluvio-lacustrine muds and gravels attests to the former availability of ponded runoff waters at this now-hyperarid locality. Playa waters were deep and persistent enough to create beach berms composed of well-rounded pebbles, and to sustain cultural activities of Neolithic pastoralists. Twenty-two radiocarbon dates bracket 'wet' phases and concurrent prehistoric human occupation from ≈ 8915-8580 ¹⁴C yr BP (9925-9544 cal yr BP) and ≈ 7105-5955 BP (7878-6799 cal yr BP). Increasing amounts of sand were incorporated into the playa after 7500 ¹⁴C yr BP; two optical dates constrain the timing of enhanced aeolian activity from 6720-6250 cal yr. These optical data comprise the only existing absolute ages for the emplacement of aeolian sediments during the rapid transition from wet (playa) to dry (sands) conditions in the Holocene. Playa desiccation after 5955 ¹⁴C yr BP (6799 cal yr BP) was marked by cultural abandonment, evaporite precipitation, dune migration, and deflation. The AMS dates from Umm Akhtar Playa fall in the same range as published radiocarbon data from contemporary sites and playas in Egypt and northern Sudan; the main period of enhanced surface water storage from 8100-6000 BP is linked to the intensified Afro-Asian monsoon forced by cyclical astronomical variations. The record from the Arba'in Desert of Egypt and northern Sudan lags the Northern Hemisphere seasonal insolation maximum (centered at 10,400 BP) and the greatest frequency of African lake highstands, (9500-8500 BP) and may result from biogeophysical feedback mechanisms involved with the 'recycling' of water within the continental interior. Sub-millenial variability apparent in the composite record from Egypt likely reflects the influence of atmospheric-oceanic dynamics throughout the Holocene.