Can molluscan assemblages give insights into Holocene environmental changes other than sea level rise? A case study from a macrotidal bay (Marennes–Oléron, France).
Résumé
During the Late Holocene, the rate of sea level rise decreased and climate changes, hydrodynamic processes or anthropogenic impacts became predominant parameters governing the sedimentary infill of estuarine environments. The aim of this study is to describe the response of past benthic mollusc communities to these forcing factors. Mollusc skeletal remains were sampled from three 8000, 5500 and 2600 year-long sedimentary records in the Marennes–Oléron Bay (Atlantic Coast, France), where environmental changes have previously been identified thanks to a combination of very high resolution seismic profiles and sedimentological data. Molluscan assemblages successfully record environmental changes, even at the smallest temporal scale. They provide relevant information regarding bathymetry, salinity, hydrodynamics, oxygen content and sediment supply. In the 8000 year-long record, the transition from an upper to a lower intertidal mudflat community provides evidence of a transgressive phase that occurred between 8000 and 7600 years BP. In the 5500 year-long record, an oyster bank developed on the slopes of a tidal channel has been recovered. The composition of the fossil community and the careful examination of skeletal remains suggest that the channel was hydraulically inefficient, in response to the sediment infilling of its drainage area. In the 2600 year-long record, a soft-bottom, shallow-water, subtidal mollusc community, similar to that found nowadays in the Marennes–Oléron Bay, has been recovered from both lower mixed sand-mud and upper muddy units separated by a major regional unconformity dated to about 1000 years BP. This sharp transition is recorded by a few rare species, and was responsible for the local extinction of the bivalve Lepton squamosum. However, the abundance of the most common species and the quantity of fragmented shells remain stable, which may indicate that mud drape deposition was related to an increase in mud supply rather than a decrease in water energy. The level of detail of the palaeoenvironmental reconstructions varies, depending on the ecological requirements of the species. Stenotypic indicator species provide accurate, quantitative information, whereas eurytypic species add undesired variability to the assemblages. Since fossil molluscs are easy to collect and to identify and provide long-term, smoothed records of environment variations, this study suggests that they can be used prior to any other biological proxy, as a first step into reconstructions of Holocene coastal environments.
Domaines
Paléontologie
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