Profiles of trace elements and stable isotopes derived from giant long-lived Tridacna gigas bivalves: Potential applications in paleoclimate studies

Abstract

This study investigates the environmental and biological controls on trace element partitioning and stable isotope composition of modern giant long-lived bivalves (Tridacna gigas) with the aim to use these archives for paleoclimatic reconstructions. Firstly, the intra-shell variability is studied by measuring time equivalent profiles in the different shell layers characterised by different growth rates. Secondly, the inter-site variability is studied by comparing profiles derived from three modern specimens collected in sites across the Indo-Pacific region characterised by different ranges of temperature and productivity. These results show that δ18O profiles are highly reproducible across the shell regardless of significantly different growth rates. Shell δ18O is primarily controlled by water δ18O and temperature. Comparison of intra shell Mg/Ca profiles shows a clear and systematic partitioning where inner layer Mg/Ca values are a least 2-3 times higher than outer layer and hinge areas. Inner layer Mg/Ca shows seasonal oscillations but superimposed on an ontogenetic trend with increasing values and increasing amplitude Mg/Ca oscillations with age. The Sr/Ca profiles do not show clear reproducible seasonal trends in the different shell zones. It is concluded that Mg/Ca and Sr/Ca profiles appear to reflect a combination of biological and environmental controls that will need to be disentangled before using these proxies in paleoclimatic studies. Finally, intra shell Ba/Ca profiles are reproducible in great detail for all modern specimens studied. Inter-site comparison shows that the amplitude and the timing of the Ba/Ca peaks appear to reflect the timing and the amplitude of the chlorophyll peaks associated with phytoplankton blooms at each locality making this tracer a potential paleoproductivity indicator. Crown