25th International Meeting on Organic Geochemistry IMOG 2011

O-07
The syngeneity of Precambrian sterane biomarkers
Amber Jarrett, Jochen Brocks
Research School of Earth Sciences, Australian National University, Canberra, Australia (corresponding
author:amber.jarrett@anu.edu.au)
Precambrian oceans and the organisms inhabiting
them were fundamentally different than today. In
modern oxygenated oceans algae such as diatoms,
dinoflagellates and coccolithophorids are the
dominant primary producers. Precambrian oceans
were hypothesised to be primarily anoxic and/or
sulfidic with eukaryotic organisms constrained to the
upper mixed zone in the water column [1]. Primary
productivity was controlled by single-celled organisms
such as cyanobacteria and various groups of primitive
algae, possibly Rhodophyta and Chlorophyta (red and
green algae respectively). The Proterozoic body fossil
record is patchy. From the late Mesoproterozoic to
early Neoproterozoic (~1,200 - 850 Ma) simple
microfossils of eukaryotes are present in low
abundance but begin to significantly diversify from the
middle of the Neoproterozoic (ca. 700Ma) to the
beginning of the Phanerozoic (542 Ma) [2].
Organic geochemistry has become a common
method for filling the gaps in this body fossil record.
The extraction and identification of molecular fossils
have been used to make far reaching assumptions on
the ecology and evolution on early life, such as the
first appearance of eukaryotes [3]. The most common
biomarkers for eukaryotic organisms are the steranes
cholestane (C27), ergostane (C28) and stigmastane
(C29). Previous studies have shown that the relative
concentration of sterane homologues changes
systematically through the Phanerozoic, with a
generalized increase in C28/C29 coinciding with the
shift in primary producers in the ocean [4]. However, it
is unusual that published sterane ratios from older
sequences (~1,700 - 635 Ma) are usually
indistinguishable to the Phanerozoic, despite the fact
that eukaryotic microfossils from this interval appear
to me primordial algal groups, not secondary and
tertiary endosymbiotic algae of Phanerozoic oceans.
In this study we completed a detailed re-assessment
of 50 Proterozoic sediments from the Amadeus and
Officer Basins of Australia over key intervals such as
the ‗Snowball Earth‘ events, the Acraman Impact,
through the Ediacaran to the Precambrian-Cambrian
boundary. Our methodology consisted of removing
exterior surfaces from a sample which could
potentially be exposed to anthropogenic
contaminants. We compared the exterior and interior
fractions, systematically investigating the permeability
of rock samples to hydrocarbon infiltration by
identifying polyethylene by-products such as BAQCs
and cyclopentanes [5].
The results of our study are surprising. We found 85%
of our samples only contain steranes of
anthropogenic origin. However, 15% had a clean
interior and yielded indigenous Precambrian steranes.
The indigenous sterane ratios appear to be more
compatible with the body fossil record. Our data
suggest that eukaryotes in the Mesoproterozoic were
not significant. This is then followed by the
appearance of red algae (predominance of C27) in the
early Neoproterozoic, and then green algae
(predominance of C29) in the late Neoproterozoic,
broadly consistent with the body fossil record.
References:
[1] Falkowski & Knoll, 2007, Academic Press
[2] Butterfield 2000, Paleobiology, 26(3), 386-404
[3] Brocks et al. 1999, Science, 285(5430), 1033-1036
[4] Schwark & Empt 2006, Paleoceanography
paleoclimatology, paleoecology 240, 225-236
[5] Brocks et al. 2008, Geochim. Cosmochim. Acta
72, 871-888
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