Seasonal stable isotope variations of the modern Amazonian freshwater bivalve Anodontites trapesialis

Abstract

In a floodplain lake of the Amazon River near the city of Iquitos, northeastern Peru, a one-year monitoring experiment was conducted during which water samples and living bivalves (Anodontites trapesialis) were collected with the aim to investigate seasonal d18O variation in and fractionation between bivalve aragonite and host water. Both host water and molluscan growth increments show more than 8xseasonal variation in d18O. In the floodplain lake under study the d18O variation of the water is controlled by contrasting dry and wet season evaporation^precipitation regimes. Molluscan d18O appears to be in equilibrium with the host water. Although an approximately 4.0x offset occurs, d13C records of water and bivalves are in good agreement, suggesting that both d18O and d13C of the shells of freshwater bivalve A. trapesialis are good recorders of (palaeo-)environmental conditions. The d13C of Dissolved Inorganic Carbon (DIC) is governed by plant growth and/or by changes in aquatic chemistry, affecting the DIC pool.