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

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.

Methane Seepage Caused by Gas Hydrate Dissociation in the Mid‐Okinawa Trough Since the Last Glacial Maximum

“…The abundance of bicarbonate allows the fast precipitation of aragonite (Raiswell, 1988). As this precipitation continued, the permeability of the carbonate crust decreased, which was previously described as the self-sealing process (Blouet et al, 2020;Hovland, 2002), and this caused the downward advection of sulfate from seawater to be more likely to bypass the carbonate-paved areas and to enter sediment pores and maintain the SD-AOM (Figure 3b). The pores within the recently formed carbonates might not be completely clogged at this stage (14-12 ka).…”

THE TRACE FOSSILTISOA SIPHONALISIN ITS TYPE AREA—CHARACTERISTICS AND ENVIRONMENTAL SIGNIFICANCE

“…Tisoa is a trace fossil. In fact, methane-charged fluids can vent through burrows because methane is not toxic (e.g., Kamens and Stern 1973), and concretions may form around such burrow-related conduits of methane-charged fluids (e.g., Wetzel 2013;Wiese et al 2015;Blouet et al 2017;Blouet et al 2021bBlouet et al , 2021c. Depending on the amount of bicarbonate and Ca 2þ ions as well as growth time, small, elliptical or long, tubular concretions may form (see Friren 1870;Breton 2006).…”