Temporal changes in moisture distribution in sandstones near Petra, Jordan

Most of weathering processes are connected to moisture presence and flow that affects salt transport and crystallization. However, knowledge of moisture distribution and capillary flow in areas with cavernous weathering forms is scarce. Honeycombs and tafoni, common cavernous weathering forms, occur on different types of rocks and in different climatic conditions, but in arid environments such as south Jordan, tafoni are clearly actively evolving and abundant in the local sandstones, both on natural outcrops and on ancient carved monuments such as in the historic city of Petra.

The depth of the evaporation front was measured in 3 sites with tafone near Petra in Jordan in November 2018, December 2019 and December 2021 (in a cold and relatively wet period of a year). The first site (A) is a tafone situated 5 m below the sub-horizontal surface allowing infiltration. It is facing to the north.The second site (B) is located at the foot of a 50 m high rock cliff. This tafone is facing to the south.The rock is fractured, so it likely allows faster infiltration.The third site (C) is a tafone situated at the foot of the rock cliff, 15 m below the top, facing to the southeast.

The evaporation front is the boundary within the rock that separates the dry layer usually and the capillary zone, and its depth has a major effect on weathering as salts accumulate and crystallize here. The depth of the evaporation front was measured by the ‘uranine-probe’ method, inside tafoni (6 measuring points) and in visors i.e. the thin rock separating the tafone hollows (5 measurement points). We compared the measured depths of the evaporation front with the period of time since the last precipitation event. In 2018, only 14 days elapsed from the significant 83 mm precipitation event, in 2019 only 33 days elapsed from single 244 mm rain event, while in 2021 there were just 2 mm of rain followed by 316 days of no rain (very dry period).

At the site A, the evaporation front was not detected in any measurement, as it was deeper than 10 cm, meaning that evaporation strongly dominates over inflow from sandstone massive. At the site B, the evaporation front was at nearly constant depth at all visits (the average 75 mm, time oscilation only +-5%).  At the site C, there is the largest fluctuation in the depth of the evaporation front. The greatest depth of the evaporation front (average 52 mm) was measured in 2018. In 2019 the average depth of the evaporation front was 42 mm. In contrast, in 2021, the depth of the evaporation front was only 24 mm below the surface. It is clear from the measured data that the depth of the evaporation front does not correspond to antecedent precipitation. From this we can conclude that water does not respond to individual precipitation events, but changes in water reserves are probably controlled by longer cycles or by evaporation demand, rather than rain.

This research was funded by the Charles University Grant Agency (GAUK - 265421).