What makes seep carbonates ignore self-sealing and grow vertically: the role of burrowing decapod crustaceans

The mechanisms that govern the vertical growth of seep carbonates were deciphered by studying the sedimentary architecture of a 15m thick, 8m 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 13 C 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 δ 13 C 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.