School of Engineering and Science - Jacobs University

MICHAEL BAU AND BRIAN ALEXANDER
81
Preservation of primary REE patterns without Ce anomaly
during dolomitization of Mid-Paleoproterozoic limestone and the
potential re-establishment of marine anoxia immediately
after the“Great Oxidation Event”
Michael Bau and Brian Alexander
Geosciences and Astrophysics Program, School of Engineering and Sciences,
International University Bremen, P.O. Box 750561, D-28725 Bremen, Germany
email: m.bau@iu-bremen.de; b.alexander@iu-bremen.de
© 2006 March Geological Society of South Africa
ABSTRACT
Comparison of shallow-water limestone and silicified dolomite from the Mid-Paleoproterozoic Mooidraai Formation, Transvaal
Supergroup, South Africa, shows that the primary rare earth element (REE) distribution of these pure marine sedimentary carbonates
has been preserved during dolomitization and silicification. Both lithologies display REE (and Y) patterns closely resembling those
of present-day seawater, i.e. they show enrichment of the heavy relative to the light REE, positive anomalies of La, Gd and Lu, and
super-chondritic Y/Ho ratios. However, these shallow-water carbonates lack any Ce anomalies, indicating that in the Mid-
Paleoproterozoic the redox-level of surface water in the Griqualand-West sub-basin of the Kaapvaal Craton did not allow for
oxidation of Ce(III). The absence of Ce anomalies from shallow water Mooidraai carbonates indicates a return to marine anoxia
immediately after large amounts of marine sedimentary Mn oxides had been deposited in a highly oxygenated marine environment
in the underlying Hotazel Formation. This suggests that the “Great Oxidation Event” in the Paleoproterozoic was a transition period
characterized by strong fluctuations of the redox level of the Earth’s surface ocean.
Introduction
Interpretation of trace element distribution and isotope
ratios in Precambrian sedimentary carbonates suffers
from the fact that these rocks may occur as (sometimes
silicified) dolomites. Field evidence, such as
dolomitization fronts, suggests that the dolomite is not a
primary seawater precipitate and that dolomitization
occurred during diagenesis, but in most cases such
evidence is missing. Hence, conclusions based on the
trace element and isotope compositions of dolomites are
often questioned. This is unfortunate, since Neoarchean
and Paleoproterozoic carbonates such as those from the
Transvaal Supergroup in South Africa and from
the Hamersley Group in Australia, for example, should
have recorded potential changes in the seawater
composition before and after the “Great Oxygenation
Event” and during episodes of low-latitude glaciation in
the Paleoproterozoic (for recent summaries of early
Precambrian atmosphere-hydrosphere evolution see,
e.g. Holland, 2004, Canfield, 2005 and Catling and Claire,
2005).
The rare earths and yttrium (REY) hosted in marine
sedimentary carbonates can be used as proxies for the
REY distribution in ambient seawater, since the partition
coefficients between carbonate and seawater do not
show major differences within the REY series (Terakado
and Masuda, 1988; Zhong and Mucchi, 1995; Webb and
Kamber, 2000; Tanaka et al., 2004). A positive Eu
anomaly, for example, may reveal the presence of a
high-temperature hydrothermal REY component even in
shallow seawater and the presence or absence of
a Ce anomaly may indicate an oxygenated or
anoxic atmosphere-hydrosphere system, respectively.
Understanding the impact of dolomitization on the REY
distribution in marine sedimentary carbonates, therefore,
might enable us to significantly improve our knowledge
of the chemical evolution of the atmospherehydrosphere
system across the Archean-Proterozoic
boundary and in the early Proterozoic. The topic of REY
mobility during dolomitization had been addressed,
amongst others, by Banner et al. (1988) and Nothdurft
et al. (2003), for example, who studied sedimentary
carbonates of Phanerozoic age. However, their
conflicting conclusions do not prove or disprove the
suitability of REY in Precambrian dolomites as proxies
for REY in ambient seawater, but highlight the need for
further study.
Here, we compare the REY distribution in marine
limestone and in silicified dolomite from the Paleoproterozoic
Mooidraai Formation in the Postmasburg
Group of the Transvaal Supergroup, South Africa.
We show that dolomitization did not modify the primary
REY distribution of the marine sedimentary carbonates,
suggesting that these dolomites still provide information
on the REY distribution in Precambrian seawater.
We also show that neither the limestones nor the
dolomites from the Mooidraai Formation display
significant Ce anomalies and, hence, do not support the
assumption of the existence of strongly oxygenated
surface seawater during Mid-Paleoproterozoic
“Mooidraai times”.
Geology
The Mooidraai Formation occurs within the uppermost
part of the Neoarchean to Paleoproterozoic Transvaal
Supergroup, Northern Cape Province, South Africa.
SOUTH AFRICAN JOURNAL OF GEOLOGY, 2006,VOLUME 109 PAGE 81-86