Biogenic Deterioration of Nari (Calcrete)
Nari (calcrete) is a hard rock compared to the soft rock below it. For a comprehensive discussion of this rock the reader should consult Wieder et al., (1993) and Itkin et al., (2012). One of the most common rock types carrying Nari rock is chalk (Fig. 18.1.1). The past inhabitants of the Judean foothills (Shefela), near Bet Govrin, dug or quarried caves in the chalk for different purposes; they relied on the Nari layer as a ceiling that protected the opened cavity. Nari was formed through continental processes on top of rocks of marine origin. Hence, the Nari follows local topography and not the marine features (such as bedding). The boundary of the Nari layer is clear and sharp (Fig. 18.1.2), due to its impact on the vegetation. The soft substratum of the valleys among the hills supports annual plants (Fig. 18.1.2) whereas shrubs and trees prevail in the area where Nari outcrops occur.
There is a biological advantage for the shrubs and trees on the Nari. Their roots penetrate into crevices and soil pockets in the Nari. The annual plants, being successful in the valleys, out-compete the shrubs and trees by desiccating the soil in the first summer and thus preventing their establishment. The annuals roots do not penetrate deep parts of the Nari layers. The high correlation between the distribution of shrubs and Nari outcrops led me to present the bio-deterioration of Nari to my students. The processes of weathering are in fact plant succession on the Nari, leading to its deterioration. Our recommended place for displaying the following chapter is hills and valleys some half a kilometer SW of Sdot Micha.
18.1 The Nari crust and its deterioration
The “upper Nari” (Fig. 18.1.3, no. 2) occurs above the chalk and is covered by a hard crust up to 10 mm thick. In places where the upper Nari was broken a few years ago by human activity, the Nari crust becomes prominent due to its low water-holding capacity and the absence of microorganisms (Fig. 18.1.4). The upper Nari, situated below the crust, is relatively soft, has high water-holding capacity and becomes populated by lichens, algae, fungi, cyanobacteria and mosses (Fig. 18.1.4-18.1.6), a short time after exposure. Some of these organisms develop dark pigments that assist them in withstanding solar radiation.
When mosses grow on a layer of soft Nari, as below the crust (Fig. 18.1.6, center), they trap dust efficiently and increase the water reserves at the rock surface, thus intensifying local rock dissolution. When the undisturbed Nari is exposed for a long time, it becomes populated by microorganisms that typically grow under the local climatic and microclimatic conditions. In the Shefela, epilithic lichens (= growing above the rock; Fig. 18.1.7) occur on the Nari crust of the north-facing slopes. The center of the lichen in Fig. 18.1.7 was consumed by snails and slugs. However, before this event took place, the contact of the lichen with the crust increased rock dissolution by rain-water: water that passed through the lichen’s body and was enriched by various materials released by the lichens, caused enhanced dissolution of the rock. It is hard to evaluate the entire lichens succession because no such study was ever conducted in Israel. The Nari outcrops support endolithic lichens as well (= developed inside the upper layer of the rock; Figs. 18.1.7-18.1.9).
For a simplified way of differentiating epilithic- from endolithic lichens, wet the lichen with some water and scratch it with your finger nail; if a green layer is revealed you touched epilithic lichen; by scratching you removed a thin layer of fungi occurring above the algae. If you have to use metal or flint to scratch and reveal the green layer, you are dealing with endolithic lichens. The upper stone layer with the endolithic lichens is softer than the other parts of the rock. This softer, thin layer is constantly consumed by snails and slugs when the rock is wetted in winter. Consuming the upper lichen parts with the soft rock takes place in sites that have already been consumed by snails. As a result, [useful_plants_f9 typical ”ornamentation” of the rock takes place] (Fig. 18.1.9). The activity of the endolithic lichen inside the rock leads to weakening of the bond between rock particles at the surface and deeper layers of the rock. Thus, rain drops easily splash away rock particles.
I do not have direct estimation of the weathering speed by endolithic lichens, the organisms consuming them, and the increased weathering by the splashing of rock particles by rain drops. In a study I conducted on walls of hard limestone in Jerusalem I arrived at an estimation of the weathering rate as 1 mm per 200 years. Until studied, the weathering rate of the Nari crust may be estimated as similar.