Freshwater Mussels as Paleoenvironmental Indicators

The shells of marine bivalves contain a wealth of information that can be used in paleoenvironmental studies. Conventional stable isotope (e.g., C and O) and trace element profiles (e.g., Mg/Ca, Sr/Ca, Ba/Ca) provide insights into past oceanographic conditions and processes with unparalleled spatial and temporal resolution. By extending these records to freshwater environments, an important link is established between terrestrial and aquatic environments that permits continental climate to be evaluated.

The geochemical profiles of freshwater bivalves are more complex to interpret than their marine counterparts because of the relatively wide range of environmental conditions encountered in fluvial and lacustrine environments. For example, the oxygen isotope composition of meteoric waters varies temporally and spatially in ways that can complicate paleotemperature estimates. In addition, physiological and ecological effects on the geochemistry of shell carbonate are not well know for many freshwater bivalves, and preservation potential may be so poor that shells disintegrate before they are incorporated in the fossil record.

Nevertheless, substantial gains are being made in the development of freshwater bivalve records as a continental indicator of climate. Notable among these are studies that use shells to define pre-industrial conditions for fluvial and lacustrine environments impacted by anthropogenic activities. In addition, new proxies are being developed that take advantage of emerging technologies such as laser-ablation inductively coupled mass spectrometry, secondary ion mass spectrometry and synchrotron microprobe x-ray fluorescence. Other techniques such as elemental analysis-isotope ratio mass spectrometry (pyrolysis/combustion-IRMS), and gas/liquid chromatography-IRMS are permitting whole new classes of isotope profiles to be generated (e.g., H-, N- and S-isotopes) from the organic matrix of shell. Combining cutting edge advancements in technology with a better understanding of the biology and ecology of freshwater bivalves will undoubtedly place these shell archives on a firm footing as a robust continental paleoenvironmental indicator.