Variation in isotopic composition (C, O and Sr) of Holocene Mactra isabelleana (Bivalvia) from the coast of Buenos Aires Province, Argentina

The ¹³C/¹²C and ¹⁸O/¹⁶O ratios of carbonate (incremental growth bands and whole shells) of the bivalve Mactra isabelleana collected from Holocene (including modem) littoral deposits along the coastal area of Buenos Aires Province, close to the Rio de la Plata river, reflect different environments which stayed relatively constant throughout the Holocene. Differences in ¹³C/¹²C and ¹⁸O/¹⁶O ratios along the growth axis of two shells from the modern estuarine and marine situations constrain the differences between the two types of environment. The data show a marked cyclicity from both sites but the increase in river flow during the winter is reflected by very low δ¹⁸O and δ¹³C values in the estuarine shell. In contrast the shell from the marine site has fewer cycles with lower amplitude which are consistent with seasonal temperature variations away from the influence of river input. The differences in the isotopic cyclic variations between the marine and estuarine sites average out in the whole shell analysis to produce different mean compositions. Wholeshell analysis on a large number of individuals from both geographically and stratigraphically different sites identified three groups of data. Group I shells have a range in δ¹³C and δ¹⁸O values consistent with coastal marine waters. Group 2 shells are ¹⁸O-deficient with respect to coastal marine water and include the modern samples from the estuarine environment. Group 3 shells have the lowest Ad¹³C values and are indicative of lagonnal environments. Variation in C, O and Sr isotope ratios of individual shells occurs within these groups due to the complex interplay of environmental and biological controls; therefore a large number of isotopic determinations are needed to characterize these types of assemblages.