Peculiarities of mud volcanism in Lake Baikal

Baikal is the largest and oldest freshwater lake on Earth. This syn-rift thick sedimentary basin hosts a large variety of present-day geological sedimentary processes, among which focused fluid seepage, mud volcanism and gas hydrate accumulation, that manifests a relationship with hydrocarbon systems in the basin. Offshore mud volcanism is well known to be one of the geological phenomena that is often associated with the presence of gas hydrates all around the World. The almost ubiquitous coupling of these processes may be a key to understand distinctive “Baikalian” mud eruption process and the resulting mud volcanic deposits. So far twenty-two mud volcanoes (MVs) have been identified in different areas of the lake, and the identification of new structures, the formation processes, and their roots depth represent challenging topics for researchers. During the last five years numerous gas hydrate-bearing features with positive circular morphology have been identified at the bottom of the lake and these represent ideal candidates to be considered as mud volcanoes.

Most mud volcanoes worldwide are characterized by the presence of mud breccia. This melange of erupted sediments consist of clayey-silty-sandy matrix mixed with clasts of different sizes and lithologies representing mainly well-lithified fragments of the different formations pierced and brecciated through the MV feeder channel. Most of the offshore MVs share these characteristics, and this criteria has been used as an unambiguous evidence to classify new MV structures. A similar approach is hardly applicable for some of the investigated structures of Lake Baikal. Seismic images reveal that the conduits of these MV candidate structures are typically shallow rooted (<0.5 km blf). The lake is a deep sedimentary basin that has been characterized by high sedimentation rates since almost 25 Ma resulting in thick deposits that are unlithified in its top part. Therefore, the recovered sediment cores are barren by the typical presence of mud breccia lithified clasts which are characteristic for “classic” mud volcanic breccia.

Here we report a set of multidisciplinary studies (including petrography, geochemistry and tomography) conducted on mud breccia cores collected from several MVs of the Baikal. Sediment core observations revealed the presence of semi-lithified clayey clast seemingly broadly distributed in the structureless sediments. Petrography studies of the individual clast reveal that they differ in mineralogical composition, and their poor lithification indicates that they originate from shallow sediments. The otherwise invisible internal structure of the cored sediments has been studied with CT-scan. Results confirm that the recovered cores contain numerous semi-lithified clasts displaying different X-ray absorption and thus mineralogical content. The absence of sedimentary structures and a completely chaotic matrix indicates a vigorous mechanism (i.e. typically ongoing in mud volcano conduits) able to amalgam different lithologies. Geochemical analyses of the sediments pore gas show the presence of prevalently microbial methane further supporting microbial reactions that commonly occur at relatively shallow depths. Results are integrated in the regional geological context and combined with geophysical data to explain the mechanisms of eruption and the peculiar sedimentary texture that differs from the traditional MVs worldwide.