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Modeling the Shuram δ13C Excursion as the onset of calcium carbonate biomineralization by acritarchs
  • Elizabeth Bailey
Elizabeth Bailey
University of California, Santa Cruz, University of California, Santa Cruz

Corresponding Author:[email protected]

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The Shuram excursion, an extremely negative and prolonged carbonate δ13C anomaly, is recorded globally in late Ediacaran shallow-water marine sedimentary sequences and coincided with a time of increasing ocean oxygenation. Marine planktonic microorganisms with organic cell walls, known as acritarchs, were common in the Ediacaran Period. The onset of the Shuram excursion occurred during a time of rapid acritarch cell wall diversification, and the excursion’s resolution was followed by the appearance of macroscopic calcium carbonate biomineralizing metazoans in the late Ediacaran. Accordingly, I consider that the Shuram excursion may be attributed to sedimentary accumulations of material sourced from weakly calcified acritarchs, which primitively biomineralized their cell walls by precipitating 13C-enriched marine carbonate onto 13C-depleted organic carbon. Decay of this organic carbon to authigenic carbonate after burial may have produced the low δ13Ccarb values. The carbon isotope mass balance of global carbon cycle reservoirs permits this effect to produce a Shuram excursion of any duration. An initial organic fraction of ~0.6 is required in the acritarch-derived phase, in agreement with fossil evidence that the earliest biomineralized structures contained a major fraction of organic carbon. I will discuss how fossil, petrographic and geochemical observations are consistent with this hypothesis. During the excursion, a peak globally averaged organic carbon burial fraction of ~0.4 is predicted. Burial sequestration of organic carbon in organic-rich biomineralized calcium carbonate could account for the rise in oxygen associated with the Shuram excursion.