DEEP-WATER FIRST OCCURRENCES OF EDIACARA BIOTA PRIOR TO THE SHURAM CARBON ISOTOPE EXCURSION IN THE WERNECKE MOUNTAINS, YUKON, CANADA

Ediacara-type macrofossils are known to appear as early as ~570 Ma in deep-water slope facies of the Avalon Terrane of Newfoundland. Whether they had their evolutionary origins in this setting (as opposed to simply being recorded there first), and the timing of their origination relative to the Shuram carbon isotope excursion (CIE), remains open questions. Our ability to assess whether this deep-water origination is a genuine reflection of evolutionary succession, an artifact of an incomplete stratigraphic record, or a bathymetrically controlled biotope is fundamentally limited by a lack of geochronological constraints and incomplete exposures of connected shallow and deep-water environments preserved within Ediacaran basins. The Ediacaran Rackla Group in the Wernecke Mountains of NW Canada represents a well-exposed shelf-to-slope depositional system for understanding the spatiotemporal and environmental context of Ediacara-type organisms’ stratigraphic occurrences. New sedimentological and carbonate carbon isotopic data from the Rackla Group establish a stratigraphic framework correlating shelfal strata in the Goz Creek area to lower slope deposits of the Rackla belt area. Furthermore, the Shuram CIE is demonstrably present in shallow shelf to slope deposits and provides a robust regional and global chronostratigraphic marker that links these two regions. New fossil discoveries have also been found in slope deposits of the Nadaleen Formation (formerly ‘June beds’), below the Shuram CIE in the Rackla Belt, but are absent in coeval shelfal strata in the Goz Creek region. In contrast, new Ediacara-type fossils found above the Shuram CIE in the Blueflower Formation occur in both slope and shelf deposits. Critically, this is the first time the presence/absence of Ediacara-type fossils within a single basin has been observed with temporal control on both shallow- and deep-water environments. The presence of the Ediacara biota solely in deep-water environments before the Shuram CIE supports earlier hypotheses that Ediacaran organisms evolved in the deeper ocean, potentially a reflection of temperature-induced hypoxia in a warm, low pO₂ global Ediacaran surface ocean. Furthermore, these data provide the first unambiguous evidence of Ediacara biota occurring before the Shuram CIE.