Red Sea palaeoclimate: stable isotope and element-ratio analysis of marine mollusc shells

The southern Red Sea coast is the location of more than 4,200 archaeological shell midden sites. These shell middens preserve archaeological and climatic archives of unprecedented resolution and scale. By using shells from these contexts, it is possible to link past environmental information with episodes of human occupation and resource processing. This chapter summarises current knowledge about the marine gastropod Conomurex fasciatus (Born 1778) and discusses its use in environmental and climatic reconstruction using stable isotope and elemental ratio analysis. It offers a review of the most recent studies of shell midden sites on the Farasan Islands, their regional importance during the mid-Holocene, theories about seasonal use of the coastal landscape, and preliminary results from new methods to acquire large climatic datasets from C. fasciatus shells

Calibration of hydroclimate proxies in freshwater bivalve shells from Central and West Africa

Freshwater bivalve shell oxygen and carbon stable isotope ratios (d18O, d13C) may act as recorders of hydroclimate (e.g., precipitation-evaporation balance, discharge) and aquatic biogeochemistry. We investigate the potential of these hydroclimate proxies measured along the growth axis of shells collected from the Oubangui River (Bangui, Central African Republic) and the Niger River (Niamey, Niger). Biweekly water samples and in situ measurements collected over several years, along with daily discharge data from both sites allowed a direct comparison with proxies recorded in the shells. Data from a total of 14 unionid shells, including three species (Chambardia wissmanni, Aspatharia dahomeyensis, and Aspatharia chaiziana), confirmed that shells precipitate carbonate in oxygen isotope equilibrium with ambient water. Because water temperature variations were small, shell d18O values (d18Oshell) also accurately record the seasonality and the range observed in water d18O (d18Ow) values when calculated using an average temperature. Calculated d18Ow values were in good agreement over the entire record of measured d18Ow values, thus d18Oshell records can be reliably used to reconstruct past d18Ow values. Discharge and d18Ow values from both rivers fit a logarithmic relationship, which was used to attempt reconstruction of past hydrological conditions, after calculating d18Ow values from d18Oshell values. A comparison with measured discharge data suggests that for the two rivers considered, d18Oshell data are good proxies for recording discharge conditions during low(er) discharge levels, but that high discharge values cannot be accurately reconstructed due to the large scatter in the discharge-d18Ow relationship. Moreover, periods of bivalve shell growth cessation due to high turbidity or air exposure should be taken into account. While d13C values of dissolved inorganic carbon in both rivers showed clear seasonality and correlated well with discharge, most of the shells analyzed did not record these variations adequately, likely due to the complication of vital effects including the variable contribution of metabolic CO2. Thus, tropical African unionid d18Oshell values can be used to reconstruct d18Ow values with high confidence to provide insight on past hydroclimate such as precipitation-evaporation balance and periods of low discharge.status: publishe

Archaeology and Sclerochronology of Marine Bivalves

International audienceIn a rapidly changing world, maintenance of the good health of the marine environment requires a detailed understanding of its mechanisms of change, and the ability to detect early signals of a shift away from the equilibrium state that we assume characterized it before there was any significant human impact. Given that instrumental measurements of the oceans go back no further than a few decades, the only way in which we can assess the long-term baseline variability that characterizes the pre-perturbation equilibrium state of the marine environment is by the use of proxy records contained in stratified or layered natural archives such as corals, fish otoliths and bivalve mollusc shells.In this chapter we will look at the ways in which the environmental signals recorded in the shells of bivalve molluscs can be used to shed light on marine variability both in the present and over past centuries and millennia, and specifically how they can be used to study marine climate, the marine environment and the economic and cultural history of the relationship between humans and the oceans.The chapter is divided into two parts: section one describes the morphological, geochemical and crystallographic techniques that are used to obtain information from the shells, while section two covers the use of bivalve shells in a wide range of applications, including ecosystem services, environmental monitoring, archaeology, climate reconstruction, and climate modeling