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\begin{document}

\title{Modeling the Shuram \selectlanguage{greek}δ\selectlanguage{english}13C Excursion as the onset of calcium carbonate
biomineralization by acritarchs\selectlanguage{english}}



\author[1,1]{Elizabeth Bailey}%
\affil[1]{University of California, Santa Cruz}%


\vspace{-1em}



  
  \date{June 13, 2024}


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\begin{abstract}
The Shuram excursion, an extremely negative and prolonged carbonate \selectlanguage{greek}δ\selectlanguage{english}13C
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 \selectlanguage{greek}δ\selectlanguage{english}13Ccarb 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 \textasciitilde{}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 \textasciitilde{}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.%
\end{abstract}%



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