Person responsible:

Katharina Elena Schmitt studied geology/palaeontology in Mainz, where she graduated in 2013 in the working group for analytical and applied palaeontology. In 2019, she defended her dissertation on "Early Cretaceous shoal water carbonates from the Central Apennines: sedimentology, chemostratigraphy and sclerochemistry" at Leibniz University, Hannover. Afterwards she established a stable isotope laboratory at the University of Ferrara (Italy) and managed it for 1.5 years. Back in her former research group at the University of Mainz, her research focus for this project is on the reconstruction of the sub-seasonal to decadal climate in the older and recent past, the palaeoecology of molluscs and the influence of climate change on the evolution of the Earth.

Methods applied:

The focus of this sub-project is the reconstruction of seasonal and multi-annual climate conditions by means of snail sclerochronology, i.e., the study of the physical and chemical variations in the accretionary hard tissues of invertebrates. The shells of snails, and more in general mollusks, are build via semi-continues accretion of tiny calcium carbonate layers. These layers are usually formed on sub-daily to daily bases and their widths, stable isotopic composition as well as minor and trace element concentration are strictly related to the environmental conditions. Therefore, the analysis of such records along the length of a shell can provide information on the habitat conditions to which the organism was exposed. In many aspects, sclerochronology is analogous to dendrochronology, the study of annual tree rings. However, thanks to the employed ultra-high-resolution (µm-scale) analytical techniques, shell sclerochronology can open windows into the climate of the past at a nearly-daily temporal resolution.

The sub-project Malacology focuses mainly on the sclerochronological analysis of ancient freshwater and land snail shells. Similar to the modern situation, the red-rimmed melania (Melanoides tuberculata) is among the most common freshwater snail in the Bronze Age strata of Central Oman.  Because this species is fully aquatic, its occurrence is of particular interest, since it documents the presence of air-exposed water bodies in a generally water-constricted habitat. Shell stable carbon (δ13C) and oxygen isotope (δ18O) records of these snails can be used to gain information on temperatures and the origin of water sources. Although it may seem counterintuitive, land snails can also prosper in arid and desert regions. Among others, the Subulinidae Zootecus Insularis is probably the most common terrestrial gastropod of Central Oman Holocene strata.  Unlike, the aquatic species, the geochemical composition of this land snail provides limited information on the environmental temperature, however, its δ18O records can be used to reconstruct rainfall and air humidity conditions. Additionally, the shell δ13C signal of Z. insularis is strictly related to the presence of C3 and C4 plants, which, in turn, are valuable indicators for edaphic (soil-related) moisture.

Geochemical analysis (in particular the δ13C and δ18O records) is particularly challenging since the studied shells rarely exceed one centimeter in length and are usually less than 0.5 mm thick. For this purpose, very small amounts of carbonate powder (~50µg) are carefully extracted from shells using a dental hand drill mounted under a microscope.  Powders are then dissolved in acid and the liberated CO2 gas is analyzed using an Isotope Ration Mass Spectrometer. Stable isotopic data are then combined with micro growth increment analysis to place the geochemical information in a temporal context. Other analytical tools are employed in the project, such as crystal fabric investigation by mean of scanning electron microscopy, in-situ geochemical LA-ICP-MS analysis, and Raman Spectrometry, to gather additional insights on both the life-history traits of the studied archive as well as on environmental forcing.

The sclerochronological analysis of mollusks from archeological sites represents an unprecedented opportunity to gain more knowledge on how Bronze Age communities were able to prosper under severe climate conditions of Central Oman. At the same time, this study will provide an extraordinary record of the low-latitude mid-Holocene climate and the behavior of the intertropical climate zone during the last thousands of years. Such data are fundamental to understand ancient seasonal and interannual climate dynamics, i.e., the environmental variability on the scale of human perception.

Interim results:

The first isotope data analyses in the subfield of malacology could be used to reconstruct some palaeoenvironmental conditions. The δ13C values of the terrestrial snails show that C4 (grasses) plants were predominant in the region during the Early Bronze Age, ruling out large-scale agriculture. However, this changed from the Late Islamic period onwards, as snails fed mainly on C3 plants (trees and crops) from then on. With the δ18O values of the terrestrial species, it was possible to reconstruct a highly variable paleo precipitation regime in the Al-Khashbah region. Thus, one shell showed a monsoonal/cyclonic precipitation pattern originating from the Indian Ocean, while all other shells showed the precipitation signature of air masses originating from the north.

The aquatic snails, from the ditches at Al-Khashbah indicate that the water must have been permanent and flowing. The attenuated seasonal δ18O values indicate that the snails lived in waters with a strong environmental buffering effect, which could have resulted from continuous freshwater recharge with constant temperature and/or constant δ18O-water signal, so flowing groundwater at the bottom of the ditches is very likely.