Dominance of reworked fossil shells in modern estuarine environments: Implications for paleoenvironmental reconstructions based on biological remains

DE FRANCESCO, CLAUDIO G.; HASSAN, GABRIELA S.

Palaios

SEPM Society of Sedimentary Geology

Año: 2008 vol. 23 p. 14 - 23

0883-1351

Death assemblages from contemporary marginal marine settings carved into ancient shell deposits are composed of fossil shells exhumed by currents and tides, and shells derived from living populations. The understanding of the bias produced by such a mixing process would be of interest for studies that use modern death assemblages as analogues of similar past habitats. In order to evaluate the magnitude of reworking and redeposition of fossil shells in modern environments, a taxonomic (composition, abundance, and richness) and taphonomic (taphofacies) study was carried out in the Mar Chiquita coastal lagoon, Argentina (37°40’S, 57°20’W). The nature and extent of reworking was explored along a gradient in tidal energy from the outer to the inner reaches of the coastal lagoon. Results indicated that modern death assemblages are mostly composed of fossil (late Holocene) reworked shells and that reworking varies along a gradient in tidal energy, being higher in the outer reaches of the coastal lagoon, where tidal action is more significant. Temporal mixing in the coastal lagoon appears to be associated with condensation (remanié) rather than with a subtle mixing of shells as occurs in time-averaged deposits. This reworking process leads to an abundance of old shells in modern death assemblages, which has negative consequences for their utilization as modern analogues of past lagoons. Multidisciplinary studies involving different biological indicators might have to worry about this kind of bias in order not to achieve an erroneous inference of the Quaternary evolution of coastal lagoons.