[1] The mesostructure (millimeter to micrometer scale) of clay-rich sedimentary rocks is generally characterized by a connected fine-grained clay matrix embedding coarser nonclay minerals. We use the Callovo-Oxfordian clay-rich rock formation (France) to illustrate how mesostructure influences solute transfer in clay-rich rocks at larger scales. Using micrometer resolution imaging techniques (SEM and micro-CT) major mineral phases (clay matrix, carbonates, tectosilicates, and heavy minerals) were mapped both in two dimensional (2-D) and three dimensional (3-D) at the mesoscale. Orientation and elongation distributions of carbonate and tectosilicate grains measured on mineral maps reveal an anisotropic mesostructure relative to the bedding plane, in agreement with the geological history of the sedimentary rock. Diffusion simulations were performed based on the 3-D mineral maps using a random walk method thus allowing direct computation of mesoscopic scale-related diffusion anisotropy and tortuosity. Considering an isotropic clay matrix, simulated diffusion anisotropy (1.11-1.26) was found lower than the one experimentally measured on macroscopic samples (1.5 to 2), due to the anisotropy feature of pores within the clay matrix. The effects of the mineral content variations on diffusion properties were then investigated by numerical modifications of a mineral map combined with diffusion simulations. Evolution of the tortuosity and diffusion anisotropy with the clay matrix content were successfully interpreted by the Koponen percolation/diffusion model, whereas the Archie approach fails to reproduce diffusion properties at low clay contents. A comparison of fitting parameters with those obtained experimentally indicates that diffusion coefficient variations observed at a large scale could be mainly controlled by the mesostructure.

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MacroporeEarthworm behaviour X-ray tomography is used increasingly to study the macroporosity resulting from earthworm activity. However, macropores are not the only features visible on the images; other zones resulting from bioturbation by earthworms can be detected due to differences in greylevels. Four different soil cores were incubated with two earthworm species at two different densities (4 or 8 adults of the endogeic species Allolobophora chlorotica or 2 or 4 adults of the anecic species Aporrectodea nocturna). A fifth core without earthworms was used as a control. After six weeks, the cores were analysed by X-ray tomography using a medical scanner. The 3D earthworm burrow systems were reconstructed and a new and specific algorithm was used to determine other bioturbated zones (BZ) that were physically influenced by the earthworms. Expected differences in the structure of the burrow systems between the endogeic and anecic species were observed: the A. chlorotica burrows were narrower and more numerous, more branched and less vertical. When the earthworm density doubled, the volume and length of the A. chlorotica burrow system increased whereas no increase was observed for A. nocturna. The BZ, which were located in the upper section of the cores, represented almost the same volume as the macropores. These zones tended to be located further from the burrows in the A. chlorotica cores: 50% of the voxels corresponding to BZ were at a distance greater than 4 and 5.5 mm from the closest macropore for A. nocturna and A. chlorotica, respectively. Three processes may have contributed to form these zones, which are characterised by increased soil density: (i) soil compaction around the burrows during burrow creation, (ii) cast deposition in the burrows (burrow backfilling) and (iii) crushing of casts on the burrow walls (so called cutanes). The longer distance between BZ and macropores in A. chlorotica cores suggests that the proportion of burrow backfilling is higher for the endogeic species compared to the anecic species. If we assume that BZ located further than 10 mm from any burrow are actually burrows backfilled with casts, the volume of burrow backfilled in our study ranged from 14 to 18% for A. chlorotica and from to 8 to 10% for A. nocturna.