THE GEOMORPHOLOGICAL EFFECTS OF DIURNAL FOG PRECIPITATION ON HYPERARID DUNES, NAMIBIA

Fog is the principal mode of precipitation in the hyper-arid Namib Desert. It is an important resource for humans, plants and animals. The influence of the fog regime in aeolian systems is not well understood, despite the significant volume of moisture (and salts) that can precipitate. This paper presents data from a field campaign in Namibia that measured the diurnal-scale cycling of fog precipitation on dunes. Data was collected on moisture and temperature at the surface, in the shallow sub surface (<5cm) to 50 cm depth over 5 consecutive diurnal cycles under conditions of fog precipitation. Dune samples were analysed for salt and grain size.

Fog collectors deployed at the dune surface show daily variation in total moisture receipts between 28% and 58% weight. Our data show that while there is some limited effect of dune aspect on the receipt of moisture, the fog moisture penetration front is shallow, detected only in the upper 2 cm of mobile sand. Natural drying of moisture from the dune surface occurred rapidly each day during the sample period.

Dune crestal sands had between 7 and 33 ppm cations and 7 and 52 ppm anions. Pilot samples indicate a decrease in concentration with distance from the coast. There is significant variation in the concentrations of salts in the dune and interdune sediments. This reflects an enrichment of interdune sediments by the high local water table.

A geomorphic threshold was crossed on the dunes where extensive shallow slumping of slopes when >55% and <58% (weight) moisture precipitation on the dunes occurred. Meso and micro fissuring of the moist surface sand was extensive on the steeper dune slopes with the development of large slump areas (1-4 m wide) with concomitant sand blocks, overthrusts and tangential fracture systems. The failure plane developed between the moistened surface and dry underlying sand. Sediment transport during wind gusts may have played a role in destabilizing slopes.

On North Polar dunes on Mars, slumping on steep slopes have been identified. These features are identified in HiRISE image data and appear to be forming in current climate conditions at inter-annual timescales. The origin of these martian dune morphologies are explored using the Namib dunes as analogs.