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The late-stage "ferruginization" of the Ediacara Member (Rawnsley Quartzite, South Australia): Insights from uranium isotopes.

Abstract：

:The paleoenvironmental setting in which the Ediacara Biota lived, died, and was preserved in the eponymous Ediacara Member of the Rawnsley Quartzite of South Australia is an issue of long-standing interest and recent debate. Over the past few decades, interpretations have ranged from deep marine to shallow marine to terrestrial. One of the key features invoked by adherents of the terrestrial paleoenvironment hypothesis is the presence of iron oxide coatings, inferred to represent the upper horizons of paleosols, along fossiliferous sandstone beds of the Ediacara Member. We find that these surficial oxides are characterized by (234 U/238 U) values which are not in secular equilibrium, indicating extensive fluid-rich alteration of these surfaces within the past approximately 2 million years. Specifically, the oxide coatings are characterized by (234 U/238 U) values >1, indicating interaction with high-(234 U/238 U) fluids derived from alpha-recoil discharge. These oxides are also characterized by light "stable" δ238/235 U values, consistent with a groundwater U source. These U isotope data thus corroborate sedimentological observations that ferric oxides along fossiliferous surfaces of the Ediacara Member consist of surficial, non-bedform-parallel staining, and sharply irregular patches, strongly reflecting post-depositional, late-stage processes. Therefore, both sedimentological and geochemical evidence indicate that Ediacara iron oxides do not reflect synsedimentary ferruginization and that the presence of iron oxides cannot be used to either invoke a terrestrial paleoenvironmental setting for or reconstruct the taphonomic pathways responsible for preservation of the Ediacara Biota. These findings demonstrate that careful assessment of paleoenvironmental parameters is essential to the reconstruction of the habitat of the Ediacara Biota and the factors that led to the fossilization of these early complex ecosystems.

journal_name

Geobiology

journal_title

Geobiology

authors

Tarhan LG,Planavsky NJ,Wang X,Bellefroid EJ,Droser ML,Gehling JG

doi

10.1111/gbi.12262

subject

Has Abstract

pub_date

2018-01-01 00:00:00

pages

35-48

issue

eissn

1472-4677

issn

1472-4669

journal_volume

pub_type

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The late-stage "ferruginization" of the Ediacara Member (Rawnsley Quartzite, South Australia): Insights from uranium isotopes.