Early neoproterozoic marine redox conditions recorded in black shale from the little Dal Group, Northwest Territories, Canada
Date
2014-08-11
Authors
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Publisher
Laurentian University of Sudbury
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.
Description
Keywords
black shale deposits, Little Dal Group, Mackenzie Mountains Supergroup, Neoproterozoic