ASSESSING THE REPRODUCIBILITY AND POTENTIAL OF HIGH RESOLUTION TRACE ELEMENT PROFILES IN AN ARAGONITIC BIVALVE (SAXIDOMUS GIGANTEUS) FOR ENVIRONMENTAL RECONSTRUCTION

Elemental ratios archived in biogenic aragonite can be used as a record of past environmental conditions. While there are numerous studies on corals, bivalves have received less attention despite their widespread occurrence and well preserved fossil shells. The shell chemistry of two Saxidomus giganteus from Puget Sound (Washington, USA) has been investigated under the premise that if there is an environmental control (either direct or indirect) on a proxy, then it should be similar between two individuals that grew in the same environment. Data from these two shells were then compared with a third shell that grew under different environmental conditions (Kodiak Island, Alaska, USA). High resolution Sr/Ca, Mg/Ca, Ba/Ca, and U/Ca profiles in two S. giganteus shells that grew at the same location in Puget Sound vary in a similar fashion. Ratios of Pb/Ca were more dissimilar, but were generally low. The similarities between the two shells may indicate an environmental control on the chemical composition of the shell. However, similar ratios and variations were also noted in the Alaskan shell, despite the different environmental conditions at this location. Therefore, relating the variations in these elemental ratios to environmental parameters is not straightforward. Similarities in the ontogenic decreases in Sr/Ca from the three shells highlight that biological processes dominate in the incorporation of Sr into the shell. Ratios of Mg/Ca, which have been proposed as a temperature proxy, were similar between shells from Puget Sound, but were in the same range in the Alaskan shell, which grew in cooler waters. Ratios of Ba/Ca displayed sharp episodic peaks, which were very well correlated between the two Puget Sound specimens, but could not be correlated to either salinity or phytoplankton production. These results illustrate the difficulties in using trace elements as proxies for environmental conditions and the need for a mechanistic understanding of trace element incorporation into bivalve shell carbonate.