The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing the burden, to Department of Defense, Washington Headquarters Services, Directorate for Information 14. ABSTRACT The mechanics of uncemented soft sediments during bubble growth are not widely understood and no rheological model has found wide acceptance. We offer definitive evidence on the mode of bubble formation in the form of X-ray computed tomographic images and comparison with theory. Natural and injected bubbles in muddy cohesive sediments are shown to be highly eccentric oblate spheroids (disks) that grow either by fracturing the sediment or by reopening preexisting fractures. In contrast, bubbles in soft sandy sediment tend to be spherical, suggesting that sand acts fluidly or plastically in response to growth stresses. We also present bubble-rise results from gelatin, a mechanically similar but transparent medium, that suggest that initial rise is also accomplished ABSTRACT idization, e.g., gravity flows, during some natThe mechanics of uncemented soft sediments during bubble growth are not widely ural disturbances have suggested that such understood and no rheological model has found wide acceptance. We offer definitive ev-sediments can act fluidly or plastically in reidence on the mode of bubble formation in the form of X-ray computed tomographic sponse to stress. Past mechanical models of images and comparison with theory. Natural and injected bubbles in muddy cohesive bubbles in these sediments have visualized the sediments are shown to be highly eccentric oblate spheroids (disks) that grow either by bubbles as essentially spherical (e.g., Wheeler, fracturing the sediment or by reopening preexisting fractures. In contrast, bubbles in soft 1988; Sills et al., 1991), with the implication, sandy sediment tend to be spherical, suggesting that sand acts fluidly or plastically in intentional or not, that the surrounding mediresponse to growth stresses. We also present bubble-rise results from gelatin, a mechan-um reacts fluidly or plastically to their growth ically similar but transparent medium, that suggest that initial rise is also accomplished and rise. Scientists and engineers have develby fracture. Given that muddy sediments are elastic and yield by fracture, it becomes oped an impressive understanding of bubble much easier to explain physically related phenomena such as seafloor pockmark forma-growth in fluids, and a vast literature covers tion, animal burrowing, and gas buildup during methane hydrate melting.the topic (e.g., Clift et al., 1978; Lohse, 2003). However, we show here that muddy sediment Keywords: bubbles, mud, fracture, methane.does not respond mechanically either a...

Ferrihydrite, which is known to form in the presence of oxygen and to be stabilized by the adsorption of Si, PO4 and SO4, is ubiquitous in the fine-grained fractions of permeable reactive barrier (PRB) samples from the U.S. Coast Guard Support Center (Elizabeth City, NC) and the Denver Federal Center (Lakewood, CO) studied by high-resolution transmission electron microscopy and selected area electron diffraction. The concurrent energy-dispersive X-ray data indicate a strong association between ferrihydrite and metals such as Si, Ca, and Cr. Magnetite, green rust 1, aragonite, calcite, mackinawite, greigite and lepidocrocite were also present, indicative of a geochemical environment that is temporally and spatially heterogeneous. Whereas magnetite, which is known to form due to anaerobic Fe0 corrosion, passivates the Fe0 surface, ferrihydrite precipitation occurs away from the immediate Fe0 surface, forming small (<0.1 microm) discrete clusters. Consequently, Fe0-PRBs may remain effective for a longer period of time in slightly oxidized groundwater systems where ferrihydrite formation occurs compared to oxygen-depleted systems where magnetite passivation occurs. The ubiquitous presence of ferrihydrite suggests that the use of Fe0-PRBs may be extended to applications that require contaminant adsorption rather than, or in addition to, redox-promoted contaminant degradation.

Chondroitin sulphate proteoglycans (CSPGs) are up-regulated in the CNS after injury and inhibit axon regeneration mainly through their glycosaminoglycan (CS-GAG) chains. We have analysed the mRNA levels of the CS-GAG synthesizing enzymes and measured the CS-GAG disaccharide composition by chromatography and immunocytochemistry. Chondroitin 6-sulfotransferase 1 (C6ST1) is up-regulated in most glial types around cortical injuries, and its sulphated product CS-C is also selectively up-regulated. Treatment with TGFalpha and TGFbeta, which are released after brain injury, promotes the expression of C6ST1 and the synthesis of 6-sulphated CS-GAGs in primary astrocytes. Oligodendrocytes, oligodendrocyte precursors and meningeal cells are all inhibitory to axon regeneration, and all express high levels of CS-GAG, including high levels of 6-sulphated GAG. In axon growth-inhibitory Neu7 astrocytes C6ST1 and 6-sulphated GAGs are expressed at high levels, whereas in permissive A7 astrocytes they are not detectable. These results suggest that the up-regulation of CSPG after CNS injury is associated with a specific sulphation pattern on CS-GAGs, mediating the inhibitory properties of proteoglycans on axonal regeneration.

STEM CELLS 2008;26:798 -809 Disclosure of potential conflicts of interest is found at the end of this article.