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Biogenic structures formed by the activity of macro-and meiobenthic organisms exert a great influence on the physical, chemical, and biological properties of sediments. Although bioturbation processes are recognized as ecologically important, the characterization and quantification of biogenic structures produced by benthic organisms remain difficult. Several techniques (e.g., resin casting, X-radiography) were previously used to quantify these structures. However, due to a lack of definition, entire burrows, including fine structures created by meiobenthic and small macrobenthic species, could not be identified with precision. Here, we show how axial tomodensitometry (CT scan) can be used to visualize biogenic structures and organic matter in three dimensions with high resolution. Further, a simple method for calculating the space occupied by biogenic structures at given depths is developed, based on comparisons of the densities of biogenic structures with those of surrounding sediments. Other potential uses of the CT scan technique in studying benthic ecology are discussed.

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Comparison of glacially derived clastic inputs in high altitude proglacial lake Bramant (Western French Alps) with measured North Atlantic Oscillation winter (NAOw) index reveals an inverse correlation between ad 1884 and 1968 at the pluridecadal timescale (20–25 years). This reflects periodical variations in snow accumulation over Lake Bramant catchment area partly influencing the glacier mass balance in the watershed. Further comparisons with reconstructed NAOw index since ad 1500 highlight spatial and temporal variations of the pluridecadal NAOw influence on this alpine climate, especially at the end of the ‘Little Ice Age’. In addition, wavelet analysis of continuous proxies of clastic sedimentation over the last 4150 years indicates significant pluridecadal variability at frequencies compatible with the NAO (30 years), while periods centered at 60–70 years could also be linked to the North Atlantic Ocean–atmosphere internal variability (Atlantic Multidecadal Oscillation (AMO)). The influence of the North Atlantic deep water production on the regional alpine climate is also suggested by a significant 550 yr cycle of clastic inputs since 2800 cal. BP. Coupling between the North Atlantic Ocean and the atmosphere seems therefore to play a fundamental role on glacier mass balance and climate during the late Holocene in the western Alps.

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