Diatom Assemblages from the Daginsky Mud Volcano Sediments (Eastern Sakhalin) and Their Implication

Diatom assemblages in mud volcanoes are quite rare and are poorly studied. The finding of a rich diatom flora in the sediments of the Daginsky Mud Volcano (DMV), located in the tidal zone of the Nyisky Bay of the Sea of Okhotsk, is of interest to study the conditions for the formation of host sediments in the zone of influence of various extreme factors, such as outflows of gases and water–mud mass, thermal springs, and tides. For this purpose, the taxonomical composition of diatom assemblages and concentration of diatoms in DMV sediments were studied. A comparison was made with the diatoms of geothermal springs of the Russian Far East associated with volcanic activity, and the stratigraphic occurrence of the found extinct diatoms was analyzed, which is important for determining their source and the age of the mud volcano roots. Diatom assemblages from DMV sediments are mainly characterized by the predominance of Metascolioneis tumida, Paralia sulcata, Odontella aurita, Pinnunavis yarrensis, Petroneis marina, Cocconeis scutellum, and Navicula digitoradiata. They consist of diatoms of different biotopes and extinct species. The diversity and abundance of brackish water and marine species indicates the predominant influence of sea waters on the formation of DMV sediments. The diverse freshwater species were mainly introduced into sediments with river runoff, but it is likely that some of these, such as the cosmopolitan alkaliphilic species, are inhabitants of geothermal springs. The presence of extinct species from the underlying Neogene sediments from where they were carried with gas–water–mud masses is the most typical for diatom assemblages of the DMV.