Water reservoirs and cisterns: effects of manual digging on sediment bleaching for OSL dating and the potential use of pollen from infill deposits as an environmental archive in a desert environment

Recently we showed that construction, maintenance and abandonment of cisterns and reservoirs in the Negev desert environment (southern Israel) can be dated by means of optically stimulated luminescence (OSL) (e.g. Junge et al. 2023). This article addresses two issues that stemmed from our previous research: a) looking into the challenge of OSL dating in light of incomplete bleaching due to short transport distances or limited daylight exposure of sediments moved by people and/or flash floods, and b) the potential of infill deposits within cisterns to serve as paleo-environmental archives. In the first part of the study, we simulated the construction of an open reservoir by manually digging a small experimental pit into the desert surface. We then dated the sediment by OSL before and after manual digging, to test how much luminescence signal is lost during the anthropogenic movement of sediment. We compared this to infill from a neighbouring ancient reservoir, naturally accumulated by flash floods since its abandonment. Again, we dated the material before and after manual digging, to reconstruct how sediments are bleached during the use and maintenance of a cistern. In the second part of the study, we explored the potential of pollen to serve as an environmental proxy, from infill sediments found within water reservoirs and cisterns. Our experiments show that sediments moved by anthropogenic activity have OSL signals that are reduced significantly but not always to the zero-level required for accurate dating, resulting in an age overestimation of a few tens of years. The naturally moved sediments, transported during flash floods into the cistern, also showed incomplete bleaching of a few tens of years. Pollen was found preserved in the infill sediments of the cistern, however, the pollen spectrum included grains from Casuarina and Eucalyptus, non-native plants introduced to Israel only ca. 130 years ago. These pollen grains occur throughout much of the stratigraphic sequence that according to OSL dating accumulated in the last 200–600 years or so. We propose that these pollen grains moved down-profile by bioturbation and along cracks that formed during drying cycles of the cistern. Overall, the OSL signal of infill sediments in abandoned cisterns is affected by incomplete bleaching in the order of tens of years while young pollen may be found in deposits that are a few hundred years older, due to vertical movement. Therefore, caution should be taken when studying young infill deposits in cisterns, both for age determination using OSL and for pollen as an environmental proxy.