Bivalve shells as paleo-proxy archives: assessment of the usefulness as environmental and climatic recorders in the Southern Hemisphere
thesisposted on 28.03.2022, 15:49 by Herath Mudiyanselage Dilmi Vindya Herath
In contrast to other skeletal bio-carbonates, the potential of freshwater bivalves as environmental and climate archives has been less explored, despite the fact that freshwater bivalves are well-known to record environmental variables such as temperature, precipitation, primary productivity, and river discharge in multiple ways which facilitates the combination of numerous proxies that strengthens the proxy records. Although there are a large number of marine proxy studies in the southern hemisphere, the number of similar studies in freshwater environments are comparatively limited, creating a considerable lack of understanding on the past continental climate in the region. In this thesis, freshwater bivalves are studied from three different localities at similar latitudes in the southern hemisphere, namely New Zealand, Australia and South America, in order to identify the potential of five different species as new environmental proxy archives. The results show that physical and chemical characteristics in three studied species; Echyridella menziesii , Alathyria profuga and Diplodon chilensis patagonicus have the possibility to be used as proxies of numerous environmental and climatic variables. Echyridella menziesii and Alathyria profuga exhibit the suitability of using shell oxygen isotope ratio as a temperature proxy since temperature reconstructions using these proxies agree well (± 2.5 and ± 3.5 °C respectively) with instrumental temperature data. Similarly, Sr/Ca in Echyridella menziesii and Diplodon chilensis patagonicus can function as a proxy of ambient temperature as both exhibit significant correlations (r > 0.6, p < 0.05) with instrumental temperature. More importantly, this study introduces the use of two different empirical equations for summer and winter which improves the reliability in summer temperature reconstructions. Further, all three species exhibit distinct growth variations in responding to regional climatic phenomena such as the Southern Annular Mode and the El Niño southern oscillation. Both Echyridella menziesii and Diplodon chilensis patagonicus show a distinctive reduction in growth during El Niño years contrary to tree ring data from these regions. Finally, fossil Diplodon chilensis patagonicus shells are used to evaluate the environmental conditions during medieval times (1200 - 1350 cal. yr. BP) and identified the medieval climatic anomalies as variations in Sr/Ca ratios in the shells supporting similar findings in other paleoclimate proxy archives in Argentinean Patagonia. Additionally, this provides new estimates for lifespans and growth rates for all studied species. This thesis delivers three new proxy archives which are widespread in the southern hemisphere and have the potential to improve available proxy networks, thus enhancing the precision of spatially resolved continental-scale temperature reconstructions.