VARIABLE BIVALVE SHELL MICROSTRUCTURE PRESERVATION IN THE CEBADA AND GRACIOSA MEMBERS OF THE CAREAGA SANDSTONE (PLIOCENE, SANTA MARIA, CA) HIGHLIGHTS NEED TO SEM ALL SPECIMENS PRIOR TO ISOTOPIC ANALYSIS

Carbon, oxygen, and strontium isotopic composition of marine mollusk shells have been shown to change greatly from the primary isotopic signal as preservation of microstructure decreases (Cochran et al., 2010), stressing the importance of assessing preservation before isotopically analyzing samples. Preservation can vary widely within a single outcrop due to changes in depositional environments and substrate types through time, ultimately complicating isotope curves if all specimens are not well preserved. Coarser grained sediments are expected to have poorer preservation of microstructure than fine grained sediments due to various taphonomic factors including, but not limited to, increased dissolution and diagenesis of aragonite from infiltrating water through larger interstitial spaces. The Pliocene Careaga Sandstone of southern California has the potential for differing preservation between the Cebada and Graciosa Members. Grain size between these two members differs; the Cebada Member consists mainly of fine-grained to very fine-grained sand, while the Graciosa Member is characterized by coarse-grained sand or sandstone with stringers of coarse gravel and conglomerate (Woodring, 1948). In this study, bulk sediment samples were collected from the Graciosa and Cebada Members and were screen-washed, picked, and sorted. Ten specimens each of the bivalves Anadara trilineata and Cyclocardia occidentalis were selected from each member. Preservation of each specimen was assessed using scanning electron microscopy (SEM) and ranked using the preservation index established by Knoll et al. (2016). Preservation of microstructure was compared within species between the two members as well as between species within each member. Preliminary results show differing preservation between two specimens of Anadara trilineata within the Cebada fine-grained member. These results stress the importance of SEMing all specimens prior to isotopic analysis as superficially similar specimens of the same species from comparable gross lithologies exhibit notably different preservation and can result in different isotopic signals.