Sclerochronological and geochemical study of modern and ancient Semele corrugata from North Coastal Peru
Shell structure, growth patterns and oxygen isotope distributions were analyzed in Semele corrugata, a common bivalve mollusk from the Pacific coast of South America. Analyzed shell samples were collected from near the city of Chimbote, Peru (9°04' S, 78°35' W) and the archaeological site, Huaca Prieta (7°55' S, 79°18' W), in order to assess if their oxygen isotope records can be used to determine season of capture and/or serve as a useful climate proxy for seasonal water temperature variation. X-ray diffraction analysis on a modern shell determined the mineralogy of S. corrugata to be aragonite. Scanning electron microscope (SEM) imaging revealed 3 distinct shell layers, defined as the outer, inner and innermost layer to be composite prismatic, crossed lamellar and complex crossed lamellar structure, respectively. Different growth patterns were observed in SEM images as well including annual growth bands, lunar-daily lines and non-periodic interrupting growth checks. Specimens were micromilled at high resolution parallel to growth increments on the outer shell layer through ontogeny for oxygen isotope analysis. The resulting δ18O data did not consistently display sinusoidal curves, which commonly represent seasonality in other species and habitats. Attempts to assign season of capture estimates to these shell δ18O profiles were only accurate in 57% of the shells, caused by the combination of shell growth cessations and short-term environmental variations. Consequently, S. corrugataδ18O profiles are not useful for season of capture or seasonal paleoclimate studies. The data suggest future analysis may be improved by analysis of ontogenetically younger shells (within 0-2 years old). Such young shells may yield more useful δ18O profiles, because the shells have faster growth rates and are thus able to preserve more detailed time-series data. Furthermore, time series δ18Owater and temperature data collected from the shell habitat are needed at the collection site in order to quantitatively assess the fidelity of these δ18O data and ascertain why the seasonal distribution of δ18O is not more sinusoidal.