Trace metal records of regional paleoenvironmental variability in Pennsylvanian (Upper Carboniferous) black shales

A critical look at iron paleoredox proxies: New insights from modern euxinic marine basins

“…Work in other ancient sediments also suggests that neither an oxic iron shuttle nor enrichment by physical sorting of particulates is a significant factor in controlling sediment iron chemistry in the ocean (e.g., Werne et al, 2002;Cruse and Lyons, 2004). In basins characterized by abrupt transitions between oxic/suboxic and euxinic deposition, as suggested independently by benthic macrofaunal ecologies, sulfur isotopes, and molybdenum concentrations, the sediments show correspondingly abrupt shifts in Fe T /Al ratios.…”

“…Secondary enrichments are known to occur, particularly in association with abrupt transitions in depositional conditions (e.g., marine versus nonmarine, normal marine versus euxinic marine). As a consequence of these environmental changes, sediments with very different properties are juxtaposed, and the resulting concentration gradients result in localized enrichments in iron sulfides and potentially Fe HR (Passier et al, 1996(Passier et al, , 1997Lyons et al, 2003;Cruse and Lyons, 2004;Jørgensen et al, 2004). Although a source of potential paleoenvironmental ambiguity, secondary effects are typically easily distinguished from primary euxinic deposition by their lack of lamination, highly localized mineralization, increased proportion of euhedral pyrite morphologies, and diagnostic d 34 S values, including highly 34 S-enriched samples (Middelburg, 1991;Middelburg et al, 1991;Sternbeck and Sohlenius, 1997;Wilkin and Arthur, 2001;Lyons et al, 2003;Jørgen-sen et al, 2004).…”

The Iron Biogeochemical Cycle Past and Present

“…However intermediate values are not necessarily diagnostic of restricted bottom water conditions but can also occur where there is exposure to high concentrations of dissolved sulphide in normal marine conditions, or from high rates of sedimentation (sufficiently high to dilute the additional reactive iron) under euxinic conditions (Werne et al, 2002;Cruse and Lyons, 2004;Lyons and Severmann, 2006). Rapid deposition swamps the iron enrichment (see Section 4) and continued exposure to dissolved sulphide has relatively little effect because the reactive iron pool is exhausted (Lyons, 1997;Hurtgen et al, 1999).…”

Geochemical constraints on the origin of Doushantuo cap carbonates in the Yangtze Gorges area, South China

“…Hydrothermal vent fluids are enriched in Eu and some transition metals (e.g. Fe, U, and Cu) (Cruse and Lyons, 2004;Edmonds and German, 2004). Prolonged input of hydrothermal fluids would effectively enhance these element concentrations in the deep ocean.…”