Seasonal oxygen isotope variations in freshwater bivalve shells as recorders of Amazonian rivers hydrogeochemistry

Freshwater bivalve shell oxygen isotope values (δ¹⁸O_S) may act as a recorder of river δ¹⁸O variations that can then be interpreted in terms of hydrology (e.g. precipitation-evaporation balance, precipitation and river discharge patterns). We investigated the potential of this proxy measured across the hinge of South American unionid shells: Anodontites elongatus collected in Peru and A. trapesialis in Brazil. The isotopic signatures were reproducible between individuals of the same species. A. trapesialis clearly showed a strong δ¹⁸O_S cyclicity in accordance with its growth patterns while A. elongatus presented less clear δ¹⁸O_S with lower amplitude. We confirm that the deposition of successive growth lines and increments is annual, with growth line corresponding to the wet season. Also, we suggest that low amplitude of δ¹⁸O_S in the A. elongatus shells indicates a habitat close to the river while large amplitude of δ¹⁸O_S cycles observed in A. trapesialis shells would reflect a floodplain lake habitat, seasonally disconnected from the river and thus subjected to higher seasonal fluctuations in water δ¹⁸O. Considering these promising first results, future studies could be directed towards the use of fossil shells to reconstruct the past and present hydrological and geochemical conditions of the Amazon.