HYDROGEN ISOTOPES AS A COMPLIMENTARY GEOCHEMICAL RECORD OF ENVIRONMENTAL VARIATION IN THE SHELLS OF FRESHWATER BIVALVES

The incremental growth bands of the freshwater bivalve Elliptio complanata were analyzed for hydrogen isotopic ratios to test whether the organic fraction of the shell represents a historical record of environmental change independent of the mineral fraction.  Hydrogen isotope ratios (dD) in terrestrial systems most notably respond to the dD of precipitation and to evaporation.  Therefore dD values of river water should display a seasonal (rainy season) signal as well as indicate periods of drought.  Research on the stable isotopes of mollusk shells has focused on carbon and oxygen in the mineral fraction where there is no hydrogen, yet freshwater bivalve shells are, by weight, up to 5% hydrogen rich organic material or conchiolin.  This proteinaceous material should retain original dD values due to the large proportion of non-exchangeable C-H bonds.

To test the correlation between climate, water dD, and shell dD, we collected E. complanata from streams on the Savannah River Site in South Carolina in the summer of 2003.  We chose two streams for which we had monthly water samples for the previous five years and which displayed different dD values.    The water samples were analyzed for hydrogen and oxygen isotope ratios.  Shells from each site were mechanically cleaned to remove exterior contaminants and the periostracum and then drilled along growth lines.  The samples were then analyzed separately for dD and dO.  The water data show a 20 ‰ offset in mean dD between sites with site 1 showing an average annual variation of ~10 ‰ and site 2, ~20 ‰.  Initial shell data indicate similar intrasite variability of 10 ‰ and 20 ‰, but no offset in dD between sites.  In addition there is interannual variation in dD values.  2001 and 2002 water samples show dampened annual variation corresponding to abnormally dry years in the region.  Detection of dD cycles in freshwater bivalves may provide corroboration of dO and dC seasonal signals as well as provide a potential archive of recent regional climatic conditions.