Abstract
The Ediacaran age (ca. 570 Ma) Shuram excursion, a ca. 12‰ depletion in δ 13 C carb , may record a dramatic oxidation of marine sediments associated with a reorganization of Earth’s carbon cycle closely preceding the rise of large metazoans. However, several geochemical indicators suggest it may instead record secondary processes affecting the sediments such as post-depositional alteration. The stable isotopic composition of iron incorporated within carbonates (δ 56 Fe carb ) reveals an anomalous 56 Fe-depletion (down to -1.05‰) in strata containing the Shuram excursion, while the underlying and overlying strata have crustal δ 56 Fe carb values. These depleted δ 56 Fe carb data during the Shuram excursion reflect incomplete reduction of iron oxides, limited by low ambient organic carbon contents. This elevated pulse of sedimentary iron oxides would consume the majority of the limited pool of organic carbon and therefore would give rise to very low net organic carbon burial during a time of enhanced detrital delivery of oxidized iron to the sediments. These results imply a syndepositional origin for the Shuram excursion, which represents a shift in the redox composition of Earth’s sedimentary shell toward more oxidizing conditions, perhaps removing a long-lived buffer on atmospheric oxygen.
Highlights
Metazoa first appeared in the fossil record during the Ediacaran Period
Several aspects of the Shuram excursion (most notably, frequent covariation of δ13Ccarb and δ18Ocarb isotope ratios and the lack of a similar excursion in organic carbon isotopes (δ13Corg) in strata with low total organic carbon (TOC) abundance), have resulted in suggestions that this stratigraphic pattern instead resulted from post-depositional processes, ranging from interaction with meteoric fluids [Knauth and Kennedy, 2009, Swart, 2008] to latestage burial diagenesis [Derry, 2010a,b]
Each of these alteration hypotheses fails to explain the widespread occurrence of the Shuram excursion during mid-Ediacaran time, as these diagenetic processes are intrinsically local—and not temporally restricted to mid-Ediacaran age strata
Summary
Metazoa first appeared in the fossil record during the Ediacaran Period. A variety of paleoenvironmental proxy records suggest a possible association of metazoan evolution with a perturbation in the redox structure of the atmosphere and ocean basins [Canfield et al, 2007, Fike et al, 2006, McFadden et al, 2008]. Several aspects of the Shuram excursion (most notably, frequent covariation of δ13Ccarb and δ18Ocarb isotope ratios and the lack of a similar excursion in organic carbon isotopes (δ13Corg) in strata with low total organic carbon (TOC) abundance), have resulted in suggestions that this stratigraphic pattern instead resulted from post-depositional processes, ranging from interaction with meteoric fluids [Knauth and Kennedy, 2009, Swart, 2008] to latestage burial diagenesis [Derry, 2010a,b]. In that this hypothesis is fundamentally driven by a global and synchronous change in the redox composition of the sediments (toward a more oxidizing bulk composition) at the time of deposition, resulting in a sedimentary reactor that was preconditioned to produce a widespread δ13Ccarb excursion of similar magnitude in basins around the world [Calver, 2000, Fike et al, 2006, Kaufman et al, 2007, McFadden et al, 2008]
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