25th International Meeting on Organic Geochemistry IMOG 2011

O-59
Black shale formation by microbial mats, lacking steraneproducing
eukaryotes in the Late Mesoproterozoic Taoudeni
Basin (1.1 Ga; Mauritania)
Martin Blumenberg 1 , Joachim Reitner 1 , Sascha Doering 2 , Walter Riegel 1 , Linda C. Kah 3 ,
Geoffrey J. Gilleaudeau 3 , Volker Thiel 1
1 Geoscience Center, Geobiology Group, Georg-August-University Göttingen, Göttingen, Germany,
2 Wintershall Holding GmbH, Kassel, Germany, 3 Department of Earth & Planetary Sciences, University of
Tennessee, Knoxville, United States of America (corresponding author:martin.blumenberg@geo.unigoettingen.de)
The composition of organisms thriving in Proterozoic
oceans is still controversial. Recently the early
emergence of 4-desmethyl sterane-producing modern
algae, dated back to at least 2.7 Ga [1], was seriously
questioned [2]. Likewise, the idea of algae as
important primary producers in earlier Proterozoic
Oceans was rejected [3], but the number of settings
studied so far is still limited.
Here we examine organic molecular components
within 1.1 Ga [4] black shale from Mauritania. By 1.1
Ga, algal components are well established [5], but
their ecosystem contribution is unknown. Within Late
Mesoproterozoic strata of the Taoudeni Basin
(Mauritania), various black shales were deposited in
the Touirist Formation. These shales partially contain
more than 20% organic carbon and show a very low
maturity. Hydrocarbon biomarkers in these black
shales were analysed for palaeoenvironmental
reconstruction. Microfacies and biomarkers support
stratigraphic interpretation of a shallow marine (likely
within wave base), high productivity-, low oxygen
setting with benthic mats as key-players in the
accumulation of organic matter. High TOC/S-ratios
and the occurrence of rearranged hopanes suggest
that euxinic conditions were not prevalent in these
environments, but do not preclude deeper-water
euxinia. The lack of steranes indicates that modern,
eukaryotic algae were of minor importance or even
absent, consistent with data from older
Mesoproterozoic settings [3]. However, palynological
survey of black shales from the Touirist Formation
revealed the presence, although in low abundance, of
simple acritarchs. Inputs of highly aromatic
biopolymers typical of those Mesoproterozoic
arcritarchs or exopolymeric substances may account
for unusually high aromaticity of biomarker extracts.
High amounts of hopanes and mid-chain branched
alkanes suggest that the benthic mats were strongly
influenced by cyanobacteria. Furthermore, the
presence of 2,3,6-trimethyl arylisoprenoids points at
contributions of organic matter from anoxygenic
phototrophic bacteria.
However, low concentrations of these compounds
and stable carbon isotope signatures of biomarkers
argue against major photic zone anoxia in these
depositional settings. More likely is the role of
anoxygenic phototrophs as part of the benthic
microbial mat communities, or the presence of other,
yet unknown, bacterial sources for arylisoprenoids.
Hopanes are very abundant in the samples from the
Touirist Formation. Since hopanes are common in
nitrogen fixing (cyano)bacteria, nitrogen-limited
conditions may have prevailed in the Taoudeni basin
at 1.1 Ga.
Our results support ideas of widespread nitrogen
deficiency that emerged in Mesoproterozoic oceans
resulting from enhanced denitrification rates in anoxic
deep waters [6]. Cyano- and other phototrophic
bacteria, as well as simple acritarchs were able to
cope with N-limited conditions, as well as associated
limitation of bioavailable trace metals, much better
than modern algae with their higher demands of
bioavailable nitrogen and essential trace metals [7].
[1] Brocks J.J., Logan, G.A., Buick, R., Summons, R.E. (1999)
Science 285, 1033-1036
[2] Rasmussen B., Fletcher, I.R., Brocks, J.J., Kilburn, M.R. (2008)
Nature 455, 1101-1104.
[3] Brocks, J. (2009) Geochim. Cosmochim. Acta 73, A161
[4] Rooney, A.D., Selby, D., Houzay, J.-P., Renne, P.R. (2010) Earth
Planet. Sci. Lett. 289, 486-496.
[5] Javaux, W.J., Knoll, A.H., Walter, M.R. (2004) Geobiology 2, 121-
132.
[6] Zerkle, A.L., House, C.H., Cox, R.P., Canfield, D.E. (2006)
Geobiology 4, 285-297.
[7] Anbar, A.D, Knoll, A.H. (2002) Science 297, 1137-1142.
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