Using portable OSL reader to obtain a time scale for soil accumulation and erosion in archaeological terraces, the Judean Highlands, Israel

Abstract

Current conventions on the fate of agricultural bench terraces in the mountainous Mediterranean climatic zones, assume that after abandonment terraces undergo rapid degradation and soil loss. First walls crumble, followed by soil washed out of breaches in the walls by runoff, resulting in rapid erosion of soil from the slopes downhill into the streams. Additionally, soil erosion may explain why previous optically stimulated luminescence (OSL) dating of terrace soils in the Judean Highlands, Israel, found mostly young soils dating to the past 700 years, and only occasionally were older ages obtained for soils at the very base of these terraces. In contrast, observations made in the same region show that slopes with degraded terraces appear to still retain much soil even though only faint remains of the terraces exist. To test if terraces and soils indeed erode entirely and how long this might take, a relatively smooth hill slope showing remains of highly degraded sets of terraces was studied. Samples were collected from excavated pits for full OSL dating, in addition to samples densely collected for OSL measurements using a portable OSL reader (PR). Air-born photogrammetry was used to obtain high spatial resolution Digital Elevation Model. Results show that the main body of terraces was first built ∼800 years ago and maintained until 175-100 years age when they were abandoned and subsequently degraded. Since then 30–45% of soil volume was lost to erosion, however steady-state was reached at a relatively high slope of 65%, with stabilization by vegetation. The thick soil present on most of the slope suggests that after the first stage of rapid degradation the slopes reache equilibrium, most likely due to vegetation that reduces direct soil erosion, so most of the soil is retained on the slope. Finally, the PR allows for a much more nuanced understanding of terrace soil history.