The significance of aragonite in the interpretation of the microscopic archaeological record

Abstract

Aragonite is one of the metastable polymorphs of calcium carbonate and is commonly found at archaeological sites in the form of mollusk shells and speleothems. Other occurrences include corals, fish otoliths, seed endocarps, wood ash, and lime plaster. These materials contain microscopic embedded information of fundamental importance for the assessment of the state of preservation of the archaeological record, the reconstruction of paleoenvironments, the identification of pyrotechnological processes, and the establishment of absolute chronologies. However, this information can be used only if the aragonite found at archaeological sites preserves its pristine structure and chemistry. Aragonite is metastable at ambient conditions and tends to dissolve and recrystallize into secondary aragonite or calcite, a process that entails the loss of its original elemental and isotopic signature. Therefore, well‐grounded archaeological interpretations depend on the correct identification of aragonite and the careful characterization of its basic properties. This article reviews the dissolution processes of different types of aragonite, the methods used for their identification, and the archaeological information that can be extracted. Particular emphasis is given to the discussion of key issues regarding absolute dating and postdepositional processes of archaeological sediments, and to a detailed overview of the recently observed pyrogenic aragonite.

Thermal Influences on Shells: an Archaeological Experiment from the Tropical Indo-pacific

“…In addition, the mineralogical preservation of aragonitic shells does not necessarily imply that the microstructure is unaltered (Toffolo, 2021), which is again evidenced in the SEM images (Fig. 9).…”

“…Studies by ), Milano et al (2016), and Villagran (2014 for example have made important first steps towards understanding the various effects that different cooking methods, temperatures, and durations have on particular shell species; however, microstructural alterations also need to be explored. Studies also show that at normal earth surface pressure, together with carbonation at temperatures above 600 °C, pyrogenic aragonite in the shape of acicular crystals can form (Toffolo, 2021;Toffolo et al, 2017;Toffolo & Boaretto, 2014). These acicular crystals were present on some of the experimental samples that underwent flames and 600 °C temperatures but also on a couple of the coals and 400 °C samples.…”

“…These acicular crystals were present on some of the experimental samples that underwent flames and 600 °C temperatures but also on a couple of the coals and 400 °C samples. As the process of formation of pyrogenic aragonite is still ambiguous, it is possible that other factors besides temperature are at play, such as microstructure, relative humidity, and CO 2 pressure (Toffolo, 2021).…”

“…10) showed a wide range of variation between the different methods for each species. By determining the presence of aragonite in shells through XRD and FTIR at an archaeological site, it can be assumed that preservation is good (Stein, 1992;Toffolo, 2021;Weiner, 2010). Conversely, shells containing some or entirely made of calcite indicate recrystallisation and a poor state of preservation (Weiner, 2010:160); however, whether recrystallisation was due to taphonomic processes after site formation or due to thermal processes is difficult to distinguish.…”

Temperature Estimates of Lower Miocene (Burdigalian) Coastal Water of Southern India Using a Revised Otolith “Clumped” Isotope Paleothermometer

“…Assuming fish otoliths form under equilibrium conditions, their stable isotope ratios of carbon (δ 13 C) and oxygen (δ 18 O) may therefore be used for the estimation of temperatures of their habitats (Fay, 1984; Morris & Kittleman, 1967). Being monomineralic (aragonitic), they can be screened for diagenetic effects based on the determination of the occurrence of secondary calcite using bulk mineralogical analyses (such as XRD or FTIR) and assessment of the degree of local structural ordering using micro Raman techniques (Toffolo, 2021). Previous studies employing the δ 18 O composition of otoliths demonstrated that the accretionary growth layers is in equilibrium with the water in which the host fish lives (Kalish, 1991b; Patterson et al., 2013; Thorrold et al., 1997).…”