The analysis of lacustrine sediments, humus layers and bedrock organic matter in the Peyssier watershed (Hautes-Alpes, France), points out to the occurrence of Mesozoic OM mainly composed with opaque particles in present lacustrine sediments and humus layers. This result shows that opaque particles found in recent sediments and soils must not be systematically interpreted as degradation products of present or recent biologic production (oxidation or combustion). Moreover, the contribution of reworked material in the soil organic stock entails that the total amount of organic matter in soils not only depends on the net primary production. It suggests that past fluvial and lacustrine OM not only gives evidence of past vegetal cover but can testify of the bedrock that feed the detrital supply.

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