Thomas H. Naehr

Chapopote Asphalt Volcano may have been generated by supercritical water

Asphalt volcanoes and lava-like flows of solidified asphalt on the seafloor (Figure 1) were first discovered and described by MacDonald et al. [2004]. The flows covered more than one square kilometer of a dissected salt dome at abyssal depths (∼3000 m) in the southern Gulf of Mexico. “Chapopote” (93°26′W, 21°54′N) was one of two asphalt volcanoes they discovered. MacDonald et al. determined that the apparently fresh asphalt must initially have flowed in a hot state, and subsequently chilled, contracted, and solidified, much in the same way as normal lava does on the surface of the Earth. The two asphalt-volcanoes discovered occur at the apex of salt domes that pierce through the seafloor. These “piercement salt domes,” known as the Campeche Knolls, are pertinent features of the deep Campeche Sedimentary Basin, which has a sediment thickness of about 10 km. According to conventional theory [Vendeville and Jackson, 1992], piercement salt domes represent “salt diapirs” that have risen up, due partly to density contrasts between salt and clay/sand from the “mother salt” located between 7 and 10 km below seafloor. A salt diapir is a vertical body of sub-surface salt, which is most often circular in cross section, is one to several kilometers in diameter, and can be 8–10 km high.

Comparison of Archaeal and Bacterial Diversity in Methane Seep Carbonate Nodules and Host Sediments, Eel River Basin and Hydrate Ridge, USA

Anaerobic oxidation of methane (AOM) impacts carbon cycling by acting as a methane sink and by sequestering inorganic carbon via AOM-induced carbonate precipitation. These precipitates commonly take the form of carbonate nodules that form within methane seep sediments. The timing and sequence of nodule formation within methane seep sediments are not well understood. Further, the microbial diversity associated with sediment-hosted nodules has not been well characterized and the degree to which nodules reflect the microbial assemblage in surrounding sediments is unknown. Here, we conducted a comparative study of microbial assemblages in methane-derived authigenic carbonate nodules and their host sediments using molecular, mineralogical, and geochemical methods. Analysis of 16S rRNA gene diversity from paired carbonate nodules and sediments revealed that both sample types contained methanotrophic archaea (ANME-1 and ANME-2) and syntrophic sulfate-reducing bacteria (Desulfobacteraceae and Desulfobulbaceae), as well as other microbial community members. The combination of geochemical and molecular data from Eel River Basin and Hydrate Ridge suggested that some nodules formed in situ and captured the local sediment-hosted microbial community, while other nodules may have been translocated or may represent a record of conditions prior to the contemporary environment. Taken together, this comparative analysis offers clues to the formation regimes and mechanisms of sediment-hosted carbonate nodules.