Stable Isotope Geochemistry

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 aims at reviewing 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 post-depositional processes of archaeological sediments, and to a detailed overview of the recently observed pyrogenic aragonite.

Between 5 and 4 thousand years ago, crippling megadroughts led to the disruption of ancient civilizations across parts of Africa and Asia, yet the extent of these climate extremes in mainland Southeast Asia (MSEA) has never been defined. This is despite archeological evidence showing a shift in human settlement patterns across the region during this period. We report evidence from stalagmite climate records indicating a major decrease of monsoon rainfall in MSEA during the mid- to late Holocene, coincident with African monsoon failure during the end of the Green Sahara. Through a set of modeling experiments, we show that reduced vegetation and increased dust loads during the Green Sahara termination shifted the Walker circulation eastward and cooled the Indian Ocean, causing a reduction in monsoon rainfall in MSEA. Our results indicate that vegetation-dust climate feedbacks from Sahara drying may have been the catalyst for societal shifts in MSEA via ocean-atmospheric teleconnections.

Bone collagen stable carbon isotope values for the inland riverine Roonka Flat archaeological population of southeastern South Australia indicate that prehistoric Aboriginal diets were based on local C3-derived terrestrial foods throughout the Late Holocene. Whereas 91% of the sample had values indicative of a predominantly C3-based
terrestrial diet (mean d13C= - 20.0%, N=41), the remaining 9% had values indicating a mixed diet of C3- and C4-derived terrestrial foods and marine foods (mean d13C = - 16.1%,  N=4). There were no significant differences between mean d13C values for adult males, adult females, and subadults. These bone collagen d13C values suggest that there was limited hunter-gatherer mobility between the inland riverine site and adjacent coastal and arid interior habitats during the Late
Holocene. Bone collagen stable carbon isotope comparisons with the Early Holocene Roonka Flat population could not be made due to extensive collagen degradation in these older interments.

This paper focuses on the geochemical composition and isotope geochemistry of brines in the Cenomanian-Tur-onian carbonate Mishrif reservoir of southern Iraq. Main dissolved constituents, trace elements, δ2H and δ18O, 87Sr/86Sr, mineral saturation indices and thermodynamic calculations were investigated in formation waters from the Mishrif Formation to obtain a better understanding of brine evolution and diagenetic effects over geological time. Previous published δ11B data were also reinterpreted as a geothermometer tool. The results are compared with previous published data for local oilfields and coeval formations in the Arabian Gulf. The Mishrif brine has a marine origin and is diagenetically modified to Ca-excess and Na-deficit. Formation waters are quartz supersatu-rated and are in equilibrium with chalcedony and calcite-dolomite in the temperatures range of 50-75°C, which is also confirmed by calcite-water oxygen isotope fractionation and δ11B geothermometer. The potential role of clays in conditioning brine chemistry during diagenetic processes was highlighted by activity diagrams; in particular , their adsorption/exchange effect on sodium could explain the lower temperature obtained by the Na/Li geothermometer (42 ± 6°C). The δ2H and δ18O values show that oxygen isotope composition of the brines was isotopically more affected by interaction with limestone during diagenesis than seawater evaporation. The main effect is an 18O-enrichment on the brine starting from the SMOW value. Locally, dilution by present-day meteoric water was also detected (Rumaila South), which is shifted towards the local meteoric water line. The strontium isotope ratios range from 0.707713 to 0.707749 and correspond to a marine strontium of late Cenomanian-early Maastrichtian age, except for the Majnoon sample, which shows a more radiogenic value (0.708043). Radiogenic strontium and gypsum and anhydride saturation indices of the Majnoon sample could indicate the contribution of calcium and sulphate from the strontium-rich sulphate minerals of the Cambrian salt domes occurring in the oilfields of southern Iraq. The higher manganese concentration (4 mg/l) and the slightly higher temperature inferred by geothermometers (up to 74°C) in comparison with present-day could indicate that the Majnoon brine is a hot fluid, probably related to a deeper structure such as the Zagros Foredeep Fault.

A collagen extraction procedure was performed on cortical bone samples from 86 prehistoric human interments excavated from a sand dune at the Roonka Flat archaeological site near Blanchetown, South Australia. The interments date from the Early Holocene to the period of
European contact, ca. A.C. 1840. Carbon and nitrogen concentrations in extracts were used to assess collagen preservation. Over 83% (15/18) of the Early Holocene sample and 34% (23/68) of the Late Holocene sample were regarded as poorly preserved on the basis of low carbon
and/or nitrogen concentrations (C <5% and/or N < 0.5%). Approximately 67% (32/48) of the remaining sample showed good collagen preservation (C > 13% and N >4.8%) and the other 33% (16/48) was included in an intermediate preservation category. A comparison of
the good, intermediate, and poor preservation categories shows a stepwise trend toward higher atomic C/N ratios and more positive d I3C values as samples become less well preserved. In general,
carbon and nitrogen concentrations in extracts are lower and C/N ratios are higher in the Early Holocene versus Late Holocene sample, whereas stable carbon isotope values do not vary with duration of burial. Because extract C/N ratios, d I3C values, and yields are extremely variable, as independent criteria they are not useful in identifying degrees of bone collagen preservation. The determination of carbon and nitrogen concentrations in acid insoluble extracts provides an initial screening procedure to select bone samples for radiocarbon or stable isotope analysis.

Cortical bone samples were collected from marsupial and eutherian herbivores at five field sites along a 1275-km south–north transect from temperate coastal to arid interior South Australia in order to address variability in stable carbon isotope composition. Collection sites were located along the eastern border of the state. Mean annual rainfall along the transect ranges from 700–800 mm at coastal Mount Gambier to 150–175 mm at Cordillo Downs in the north-east corner of the state. Bone collagen carbon isotope values become more positive towards the
arid north in relation to increasing quantities of C4 grasses. Thus, stable carbon isotope analysis of bone specimens provides a method to address dietary selection and dietary variability in Australian herbivores. In addition, isotopic analyses of archaeological and palaeontological bones and teeth can be used to address changes in Quaternary
climate and vegetation distributions in Australia.

Bone collagen stable carbon and nitrogen isotope analysis of prehistoric human remains recovered from various known localities in southeastern South Australia provide isotopic signatures that distinguish the following geographic regions: the coastal Coorong, the coastal Murray River Mouth, the coastal Murray River Mouth, Swanport (Lower Murray River), and Roonka (Upper Murray River).  These regional isotopic signatures are employed to determine geographic origin of unprovenanced Aboriginal skeletal remains curated by the South Australian Museum.  Nearly 85% of the unprovenanced sample (77/91) could be assigned to a particular geographic zone on the basis of isotopic values, and a further 13% (12/91) were assigned to areas intermediate between two geographic zones.  Only two of the 91 individuals possessed anomalous isotopic values in relation to the standard values derived from known geographic localities.  Isotopic analysis provides an independent means to address geographic origin of skeletal remains that can supplement other methods, e.g. metric, non-metric, and DNA analysis.