This is an EAGER project to support fieldwork targeting 360 million year old rocks in western Mongolia. The field trip will be led by Dr. Yarinpil Ariunchimeg, who is a senior scientist at the Mongolia Academy of Sciences in Ulaanbaatar. The goal of the fieldwork is to examine these ancient outcrops to locate rocks that are full of fossils that expose a boundary in time called the Frasnian/Famennian boundary as well as other key intervals that show important evolution and extinction events. The investigators hope to expand their knowledge of the key biotic and geochemical events during this time interval including the Kellwasser and Hangenberg oceanographic events, that reflect the possible depletion of oxygen in the oceans.
The Devonian was the time of rapidly increasing diversification of tree-like vegetation on land, the development of root systems and the formation of soil, which dramatically changed atmospheric CO2 and climates. A number of biodiversity crises, presumably associated with the climate change, occurred in the Middle and Late Devonian including the Givetian / Frasnian event, the Frasnian / Famennian event and the Devonian / Carboniferous event. Taxonomic severity ranking of the 11 largest Phanerozoic biodiversity crises place the F/F event at number six with a taxonomic loss of 35%, the Givetian / Frasnian event, at number seven with a taxonomic loss of 30% and the D/C event at number eight with a loss of 28%. Although many Devonian-Carboniferous sections contain evidence of these extinction and anoxic events, I am a participant in working groups conducting fieldwork in the Central Asian Orogenic Belt not only to expand the global scale of such studies, but also to examine stratigraphic sections deposited on volcanic island arc systems which may have served as refugia in the aftermath of major biotic extinction events. As a Co-Leader of IGCP 596 I helped organize and participate in a field workshop in western Mongolia in the summer of 2012. My participation was funded by the EAGER grant. The goal was to locate fossiliferous sections that expose the stage boundaries from the Eifelian / Givetian boundary (Middle Devonian) to the Devonian / Carboniferous boundary to expand our knowledge of the key Devonian biotic and geochemical events such as the Frasnian / Famennian extinction event and the Ka?ák, Kellwasser and Hangenberg oceanographic events. During the field workshop, we examined sections in the Nariinhar Formation, Baruunhuurai Formation and Samnuuruul Formation from the Baruunhuurai Terrane. Our preliminary data indicate that the Eifelian to Frasnian Baruunhuurai Formation in the study area was deposited in a shallow marine nearshore setting with regionally dispersed limestone lenses produced by locally restricted fossil communities and intermittent volcanically derived sediment. The lithological content of the sequence shows characteristics of both areas in NW-China during the Eifelian, but seems to have a closer relation to the eastern Junggar region during the Givetian. The Late Famennian Samnuuruul Formation is dominated by volcanically derived fine grained clastic deposition on a shallow shelf interspersed with sporadic deposition of limestone in the lower and middle part. The upper part of the formation consists of a series of sandstones perhaps deposited as a series of offshore bars or coastal barrier bars that grade upwards into a terrestrial coal producing sequence. The Samnuuruul Formation shares many biotic elements with the Hongguleleng Formation in western Junggar region of China, but seems to occupy a somewhat different sedimentary regime. The Hongguleleng Formation appears to represent a deepening sequence from a volcaniclastic sandstone to conglomerate dominated sedimentation at the top of the underlying unit followed by shallow water carbonates intercalated by siltstones overlain by radiolarian rich maroon and green shales. Although sedimentation across the D/C-boundary appears to be conformable at the type locality of the Hongguleleng Formation, no sedimentological evidence of the Hangenberg Event is present. The Samnuuruul Formation contains a black shale interval in a stratigraphic position consistent with the Hangenberg Event. In contrast, the Late Famennian interval of the Ke’ankuduk Formation in eastern Junggar region consists of a non-carbonatic terrestrial development. As a result finding similar aged formations with strikingly different patterns of sedimentation between the sections in western and eastern Junggar regions and the Samnuuruul Formation in western Mongolia is not surprising, but the resulting geographic isolation provided the proper conditions to support ecological refugia, particularly in the aftermath of the Frasnian / Famennian event. A manuscript with our initial results has been accepted for publication in the peer reviewed journal Episodes (www.episodes.co.in/), a publication of the International Union of Geological Sciences.
