Publication Date:
2022-05-25
Description:
Author Posting. © The Author(s), 2016. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Geobiology 15 (2017): 211–224, doi:10.1111/gbi.12222.
Description:
Records of the Ediacaran carbon cycle (635 to 541 million years ago) include the
Shuram excursion (SE), the largest negative carbonate-carbon isotope excursion in
Earth history (down to -12 ‰). The nature of this excursion remains enigmatic given the
difficulties of interpreting a perceived extreme global decrease in the δ13C of seawater
dissolved inorganic carbon (DIC). Here, we present carbonate and organic carbon
isotope (δ13Ccarb and δ13Corg) records from the Ediacaran Doushantuo Formation along
a proximal-to-distal transect across the Yangtze Platform of South China as a test of the
spatial variation of the SE. Contrary to expectations, our results show that the
magnitude and morphology of this excursion and its relationship with coexisting δ13Corg
are highly heterogeneous across the platform. Integrated geochemical, mineralogical,
petrographic, and stratigraphic evidence indicates that the SE is a primary marine
signature. Data compilations demonstrate that the SE was also accompanied globally by
parallel negative shifts of δ34S of carbonate-associated sulfate (CAS) and increased
87Sr/86Sr ratio and coastal CAS concentration, suggesting elevated continental
weathering and coastal marine sulfate concentration during the SE. In light of these
observations, we propose a heterogeneous oxidation model to explain the high spatial
heterogeneity of the SE and coexisting δ13Corg records of the Doushantuo, with likely
relevance to the SE in other regions. In this model, we infer continued marine redox
stratification through the SE but with increased availability of oxidants (e.g., O2 and
sulfate) limited to marginal near-surface marine environments. Oxidation of limited
spatiotemporal extent provides a mechanism to drive heterogeneous oxidation of
subsurface reduced carbon mostly in shelf areas. Regardless of the mechanism driving
the SE, future models must consider the evidence for spatial heterogeneity in δ13C
presented in this study.
Description:
We thank the National Key Basic Research Program of China (Grant
2013CB955704) and the State Key R&D project of China (Grant 2016YFA060104) as well
as the NSF-ELT program and the NASA Astrobiology Institute (TWL) for funding.
Keywords:
Ediacaran carbon cycle
;
Doushantuo Formation
;
Shuram excursion
;
Spatial heterogeneity,
;
Surface-ocean oxygenation
Repository Name:
Woods Hole Open Access Server
Type:
Preprint
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