A unique find of the earliest multicellular algae in the Lower Proterozoic (2.45 Ga) of the Kola Peninsula

Abstract

The weathering crusts are markers of continental sedimentation. The question of land colonization by bacteria is interesting in itself and studies in this field are rather fruitful. An active role of bacteria in the for� mation of ancient weathering crusts has already been shown and land colonization by bacteria is proposed to start at the onset of the formation of sedimentary rocks, i.e., at the very beginning of the geological record (1-3). In the summer of 2007, within the framework of the International Continental Drilling Program (ICDP) headed by V. Melezhik and A. Lepland (Geological Survey of Norway), borehole 5018_1A was drilled in the Monchegorskii region (central part of the Kola Peninsula), on the western flank of the Lower Protero� zoic ImandraVarzuga Riftogenic Belt. The borehole (which is situated 67.88° N, 33.07° E, see the database FARDEEP: http://fardeep.icdponline.org) dis� closed the volcanogenic sedimentary strata of the Sumian-Sariolian age. Within 186.69-193.72 m of depth, weathering crusts on metabasalts of the Upper Kukshinskaya Sub� formation of the Lower Proterozoic are discovered; this corresponds to the Sariolian Superhorizon of the Lower Proterozoic according to the regional strati� graphic chart. These weathering crusts are overlain by the Lower Seidorechensk Subformation and it is believed that volcanites of the Kukshinskaya Forma� tion have undergone exogenic weathering before the deposition of the Seidorechensk Formation (4). According to refined data, metarhyodacites from the upper part of the Seidorechensk Formation section are dated by the UPb method on zircon as 2.448 ± 8 Ga (4). Since our samples come from weathering crusts underlying the lowest part of the Lower Seidorechensk Subformation, they are older than 2.448 Ga. Bacterial paleontological studies of these rocks were performed using a Zeiss EVO 50 scanning elec� tron microscope, with an xray Inca Oxford (350) microanalyzer, and CamScan�4, with a Link�860 microanalyzer. Fossil microorganisms recorded in these weather� ing crusts are rather diverse. These are threadlike microorganisms with traces of division into cells, bio� films, and, which is particularly interesting, abundant remains of very highorganized forms, presumptively microscopical algae, which come from 188.11 m of depth.