Distinguishing biogenic from geogenic anhydrites in ash: a case study of Iron Age Qumran

A white layer composed of anhydrite, a calcium sulfate mineral, was identified in an archaeological context within a cave near the Dead Sea, radiocarbon dated to the Iron Age period. Under ambient conditions, anhydrite typically hydrates to gypsum, rendering its retention a notably rare phenomenon in an archaeological setting. This prompted questions regarding both its preservation and origin. Certain plants, such as tamarisk, are known to produce calcium sulfate hemihydrate, which dehydrates to anhydrite when burned. Similarly, geogenic gypsum converts to anhydrite upon heating, as occurs in gypsum plaster production. Based on this, we hypothesized that the archaeological layer formed as a result of thermal processes. In this study, we demonstrate that it is possible to distinguish between pyrogenic anhydrite derived from biological versus geological sources. To do so, we conducted controlled heating experiments on geological and synthetic calcium sulfates, as well as on various tamarisk tissues, including bark, green branches, and woody parts. These experiments enabled us to quantify changes in chemical composition, morphology, and atomic-scale structural disorder caused by heating. The structural disorder was assessed using a novel infrared spectroscopic approach we developed, which tracks changes in anhydrite vibrational bands. Our results indicate that the thick anhydrite layer in Qumran Cave 49 originated from the burning of green tamarisk branches circa 2800 years ago, providing new constraints on hydration processes in hyper-arid conditions. This conclusion is supported by evidence of disordered crystallinity, elevated sodium content, and irregular particle morphology of the crystals. The methodology presented here offers a new avenue for identifying the origin of calcium sulfate materials in archaeological contexts, including gypsum plaster found at sites distant from geological gypsum sources. It can help determine whether local biological sources were exploited or if geological materials were acquired through trade.