Early neoproterozoic marine redox conditions recorded in black shale from the little Dal Group, Northwest Territories, Canada

Abstract

Black shale in the Little Dal Group (ca. <817 Ma), Mackenzie Mountains Supergroup (<1005 Ma; >779 Ma), was deposited during the early Neoproterozoic, and is one of the few known black shale deposits from this crucial time in Earth’s evolutionary history. Relative iron enrichment (FeT/Al) and conventional iron speciation (DOP), along with enrichment in molybdenum, total sulphur, and total organic carbon, were studied. Iron systematics (FeT/Al >0.5 and DOP <0.80) indicate ferruginous, anoxic, and possibly oxic bottom-water conditions over the time of deposition of the entire black shale unit. The enrichment factors of several of the authigenic redox-sensitive trace elements (U, Mo, V) are strongly correlated, and appear to be related to both the FeT and the organic carbon content of the black shale. Molybdenum enrichment (<10 ppm) is limited, which is in very good agreement with data from Mesoproterozoic black shales, but is much lower than Mo enrichments in Paleozoic black shales (typically >100 ppm). Several black muddy siltstones yielded similar results, but authigenic iron was greatly overwhelmed by siliciclastic sedimentation. These new data support the theory that ocean bottom-waters returned from sulphidic to ferruginous prior to development of oxygenated conditions in the Ediacaran open ocean. This study documents a predominantly open-marine basin that was characterised by ferruginous conditions, similar to Archean and early Paleoproterozoic conditions, with brief intervals when oxic conditions developed.