The aqueduct of Gerasa – Intra-annual palaeoenvironmental data from Roman Jordan using carbonate deposits

Abstract

Calcium carbonate (CaCO3) deposits from Roman aqueducts are an innovative archive to obtain local high-resolution palaeoenvironmental and archaeological data in interdisciplinary studies. Deposits from one of the aqueducts of the Roman city of Gerasa provide a record of 59 years during the 1st to 3rd centuries CE, divided into three sequences separated by plaster layers. Annual carbonate layers show an alternation of sparite, formed in winter, and micrite, formed in summer. Brown bands at the base of many sparite layers probably correspond to large rainstorms in early winter. A fine lamination present in the brown bands may be diurnal in origin. Stable isotope and trace element data confirm annual layering, indicate strongly variable flow rate in the aqueduct and show truncations that may have been associated with drying up of the channel in some years. The trace element pattern is typical of a relatively small aquifer with a rapid response to precipitation. The trace element composition changes abruptly from the first to the second carbonate sequence, suggesting that a spring was added to increase the flow rate. Deformation twins in calcite crystals at the top of the second sequence may be due to earthquake damage after 48 years of use. The presence of abundant clay in the carbonate sequence, especially in the third sequence, suggests seismic damage to the channel. The channel was usually replastered after damage. The aqueduct went out of use sometime after the mid-2nd to mid-3rd century CE. The carbonate archive stores key information on groundwater quantity and composition and indirectly on air temperature, rainfall, extreme environmental events and land use at sub-annual resolution.