No abstract

Abstract. A new type of gas chimney exhibiting an unconventional linear planform is found. These chimneys are termed Linear Chimneys, which have been observed in 3-D seismic data offshore of Angola. Linear Chimneys occur parallel to adjacent faults, often within preferentially oriented tier-bound fault networks of diagenetic origin (also known as anisotropic polygonal faults, PFs), in salt-deformational domains. These anisotropic PFs are parallel to salt-tectonic-related structures, indicating their submission to horizontal stress perturbations generated by the latter. Only in areas with these anisotropic PF arrangements do chimneys and their associated gas-related structures, such as methane-derived authigenic carbonates and pockmarks, have linear planforms. In areas with the classic isotropic polygonal fault arrangements, the stress state is isotropic, and gas expulsion structures of the same range of sizes exhibit circular geometry. These events indicate that chimney's linear planform is heavily influenced by stress anisotropy around faults. The initiation of polygonal faulting occurred 40 to 80 m below the present day seafloor and predates Linear Chimney formation. The majority of Linear Chimneys nucleated in the lower part of the PF tier below the impermeable portion of fault planes and a regional impermeable barrier within the PF tier. The existence of polygonal fault-bound traps in the lower part of the PF tier is evidenced by PF cells filled with gas. These PF gas traps restricted the leakage points of overpressured gas-charged fluids along the lower portion of PFs, hence controlling the nucleation sites of chimneys. Gas expulsion along the lower portion of PFs preconfigured the spatial organisation of chimneys. Anisotropic stress conditions surrounding tectonic and anisotropic polygonal faults coupled with the impermeability of PFs determined the directions of long-term gas migration and linear geometries of chimneys. Methane-related carbonates that precipitated above Linear Chimneys inherited the same linear planform geometry, and both structures record the timing of gas leakage and palaeo-stress state; thus, they can be used as a tool to reconstruct orientations of stress in sedimentary successions. This study demonstrates that overpressure hydrocarbon migration via hydrofracturing may be energetically more favourable than migration along pre-existing faults.

A comprehensive study of seep carbonates at the top of the organic-rich Maastrichtian to Danian Moreno Formation in the Panoche Hills (California) reveals the mechanisms of generation, expulsion, and migration of biogenic methane that fed the seeps. Two selected outcrops show that seep carbonates developed at the tip of sand dykes intrude up into the Moreno Formation from deeper sandbodies. Precipitation of methane-derived cements occurred in a succession of up to 10 repeated elementary sequences, each starting with a corrosion surface followed by dendritic carbonates, botryoidal aragonite, aragonite fans, and finally laminated micrite. Each element of the sequence reflects three stages. First, a sudden methane pulse extended up into the oxic zone of the sediments, leading to aerobic oxidation of methane and carbonate dissolution. Second, after consumption of the oxygen, anaerobic oxidation of methane coupled with sulfate reduction triggered carbonate precipitation. Third, progressive diminishment of the methane seepage led to the deepening of the reaction front in the sediment and the lowering of precipitation rates. Carbonate isotopes, with d 13 C as low as -51‰ Peedee belemnite, indicate a biogenic origin for the methane, whereas a one-dimensional basin model suggests that the Moreno Formation was in optimal thermal conditions for bacterial methane generation at the time of seep carbonate precipitation. Methane pulses are interpreted to reflect drainage by successive episodes of sand injection into the gas-generating shale of the Moreno Formation. The seep carbonates of the Panoche Hills can thus be viewed as a record of methane production from a biogenic source rock by multiphase hydraulic fracturing. A U T H O R S

In vertebrates, butyrylcholinesterase (BChE T ) and the T splice variant of acetylcholinesterase (AChE T ) consist of a catalytic domain of approximately 500 residues, followed by C-terminal tail (t) peptides [1,2]. These peptides of 41 and 40 residues, respectively, contain seven strictly conserved aromatic residues, including three evenly spaced tryptophans, and a cysteine located at position )4 from the C-terminus. The t peptide plays an important role in the biosynthesis of cholinesterases, particularly their folding and export. For example, it has been shown that it induces the misfolding of a significant fraction of newly synthesized AChE polypeptides, and that this effect depends on hydrophobicity since it was maintained when the aromatic residues were replaced by leucines. The t peptide also reduces export, as indicated by the fact that Butyrylcholinesterase (BChE) and the T splice variant of acetylcholinesterase that is predominant in mammalian brain and muscles (AChE T ) possess a characteristic C-terminal tail (t) peptide. This t peptide allows their assembly into tetramers associated with the anchoring proteins ColQ and PRiMA. Although the t peptides of all vertebrate cholinesterases are remarkably similar and, in particular, contain seven strictly conserved aromatic residues, these enzymes differ in some of their oligomerization properties. To explore these differences, we studied human AChE (Aa) and BChE (Bb), and chimeras in which the t peptides (a and b) were exchanged (Ab and Ba). We found that secretion was increased by deletion of the t peptides, and that it was more efficient with a than with b. The patterns of oligomers were similar for Aa and Ab, as well as for Ba and Bb, indicating a predominant influence of the catalytic domains. However, addition of a cysteine within the aromatic-rich segment of the t peptides modified the oligomeric patterns: with a cysteine at position 19, the proportion of tetramers was markedly increased for Aa(S19C) and Ba(S19C), and to a lesser extent for Bb(N19C); the Ab(N19C) mutant produced all oligomeric forms, from monomers to hexamers. These results indicate that both the catalytic domains and the C-terminal t peptides contribute to the capacity of cholinesterases to form and secrete various oligomers. Sequence comparisons show that the differences between the t peptides of AChE and BChE are remarkably conserved among all vertebrates, suggesting that they reflect distinct functional adaptations.Abbreviations AchE T , T splice variant of acetylcholinesterase; BChE, butyrylcholinesterase; DEPQ, 7-[(diethoxyphosphoryl)oxy]-1-methylquinolinium iodide; Nbs 2 , 5,5¢-dithiobis(2-nitrobenzoic acid); PRAD, proline-rich attachment domains; t, tail.

No abstract

No abstract

Abstract. The mechanisms that govern the vertical growth of seep carbonates were deciphered by studying the sedimentary architecture of a 15 m thick, 8 m wide column of limestone encased in deep-water marl in the middle Callovian interval of the Terres Noires Formation in the SE France Basin. The limestone body, also called “pseudobioherm”, records intense bioturbation, with predominant traces of the Thalassinoides/Spongeliomorpha suite, excavated by decapod crustaceans. Bioturbation was organized in four tiers. The uppermost tier, tier 1, corresponds to shallow homogenization of rather soft sediment. Tier 2 corresponds to pervasive burrows dominated by large Thalassinoides that were later passively filled by pellets. Both homogenized micrite and burrow-filling pellets are depleted in 13C in the range from −5 ‰ to −10 ‰. Tier 3 is characterized by small Thalassinoides that have walls locally bored by Trypanites; the latter represent tier 4. The diagenetic cements filling the tier-3 Thalassinoides are arranged in two phases. The first cement generation constitutes a continuous rim that coats the burrow wall and has consistent δ13C values of approximately −8 ‰ to −12 ‰, indicative of bicarbonate originating from the anaerobic oxidation of methane. In contrast, the second cement generation is dominated by saddle dolomite precipitated at temperatures >80 ∘C, at a time when the pseudobioherm was deeply buried. The fact that the tubes remained open until deep burial means that vertical fluid communication was possible over the whole vertical extent of the pseudobioherm up to the seafloor during its active development. Therefore, vertical growth was fostered by this open burrow network, providing a high density of localized conduits through the zone of carbonate precipitation, in particular across the sulfate–methane transition zone. Burrows prevented self-sealing from blocking upward methane migration and laterally deflecting fluid flow. One key aspect is the geometric complexity of the burrows with numerous subhorizontal segments that could trap sediment shed from above and, hence, prevent their passive fill.

Abstract. A new type of gas chimney exhibiting unconventional linear planform has been observed on 3D seismic data offshore Angola, and is termed Linear Chimneys. These chimneys occur in the shallow buried hemipelagic succession which was affected by syn-sedimentary remobilisation processes related to hydrocarbon migrations. Linear Chimneys are oriented parallel to the adjacent faults, within preferentially oriented tier-bound fault networks of diagenetic origin (also known as anisotropic Polygonal Faults, PFs) in the salt-deformational domain. These anisotropic PFs are parallel to salt-tectonic-related structures indicating their submission to horizontal stress perturbations generated by the latter. Only in anisotropic PF areas chimneys and their associated gas-related structures, e.g. methane-derived authigenic carbonates and pockmarks, show linear planforms. In areas without anisotropic PFs where the stress state is isotropic, gas expulsion structures of the same range of sizes exhibit circular geometry. In areas experiencing a transitional stress field, Linear Chimneys follow the trend of weak anisotropic PFs rather than the nearby tectonic structures. Therefore, the development of Linear Chimneys is interpreted to have been predominantly affected by the anisotropic stress field of PFs. The initiation of polygonal faulting formed 40 to 80 m below the seafloor and predates Linear Chimneys. The majority of Linear Chimneys nucleated at the lower part of the PF tier below the impermeable, upper portion of PFs, where gas accumulation was facilitated by a regional impermeable barrier. The permeable part of polygonal fault-bound traps is evidenced by PF cells filled with gas. These PF gas traps restricted the leakage points of overpressured gas-charged fluids to occur along the lower portion of PFs and hence, controlling the nucleation sites of chimneys. Gas leaking along the lower portion of PFs pre-configured the spatial organisation of chimneys. Anisotropic stress fields of tectonic and polygonal faults couple with partial impermeability of PFs determined directions of gas migration, linear geometry of chimneys, long term migration pathways and successive leaking events. Methane-related carbonates that precipitated above Linear Chimneys inherited the same linear planform geometry, both structures record the timing of gas leakage, the orientation of palaeo stress and thus can be used as a tool of stress reconstruction in sedimentary successions.