25th International Meeting on Organic Geochemistry IMOG 2011

O-35
Biomarker evidence for the Late Neoproterozoic deep-water
oxygenation
Chunjiang Wang 1 , Maoyan Zhu 2
1 Laboratory of Geochemistry and Environmental Sciences, China University of Petroleum, Beijing, China,
2 Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing, China
(corresponding author:wchj333@126.com)
An important increase in atmospheric oxygen appears
to have taken place during the late Neoproterozoic
period [1, 2] . This increase may have stimulated the
evolution of macroscopic multicellular animals [1, 3] and
may have led to oxygenation of the deep ocean [4] .
Carbon, oxygen and sulfur isotopic data are widely
discussed for understanding the nature and timing of
Neoproterozoic oxidation, while the molecular
geochemical studies are limited, with an exception of
that abundant sedimentary 24-isopropylcholestanes
detected in the Huqf Supergroup (South Oman) may
indicate the presence of Metazoa (demosponges)
even before the end of the Marinoan glaciation (
635Myr ago) [5] . Here we present the results of a highresolution
molecular geochemical research on the
Doushantuo Formation at the Jiulongwan Section,
South China, which may have provided new evidence
for the stepwise oxygenation of the deep ocean. The
homologue of methylated steranes is mainly
composed of 3-me-steranes, 4-me-steranes and
dinosteranes. This sterane composition pattern
indicates that high diversity of alga appeared after
Marinoan glaciation and persisted over most of the
Ediacaran period (635~551 Myr) in the South China
Sea, and contributed greatly to the increased
atmospheric and marine oxygen. However, the 24iso-propylcholestanes
and 24-n-propylcholestanes
are of extremely low concentration or generally under
detection limit by GC-MS-MS method. In contrast with
the enrichment of 24-isopropylcholestanes in the Huqf
Supergroup, the absence of 24-iso-propylcholestanes
in the Doushantuo Formation may indicate that the
distribution or evolution of demosponges were
different worldwide. Hopanoids predominantly occur
in aerobic bacteria, such as methanothrophs,
heterotrophs, and cyanobacteria. Thus, the relative
abundance of hopanes in sediments is directly related
to relative contribution of the aerobic bacteria. We
have found two episodes of hopane enrichment
during the Ediacaran period, which just coincide with
the two major C-isotopic negative excursions of
carbonates. The covariation of hopane abundance
and C-isotopic negative excursion is suggested as a
robust evidence for the deep-water oxygenation.
YSJ-Ds-2B-4
414.4 > 98.1
1.13e5
414.4 > 217.2
2.35e5
414.4 > 231.2
1.81e6
3-m-
4-m-
Dinost.
400.4 > 217.2
3.54e6
Fig. 1. MRM GC-MS ion chromatograms of C29–C30
steranes in the typical argillaceous dolostone from the
Doushantuo Formation, South China.
References
[1] Fike D A, Grotzinger J P, Pratt L M, Summons R E (2006) Nature
444, 744–747
[2] Canfield D E, Poulton S W, Narbonne G M (2007) Science 315,
92–95.
[3] McFadden K A, et al. (2008), Proc. Natl. Acad. Sci. USA 105,
3197–3202.
[4] Rothman D H, Hayes J M, Summons R E. (2003) Proc. Natl.
Acad. Sci. USA 100, 8124–8129.
[5] Love G D, et al. (2009) Nature 457, 718–721.
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