Environmental Engineering Reference
In-Depth Information
Atlantic (e.g. [81]), Jurassic black shales in Europe (e.g. [75, 77, 83]) or De-
vonian and Silurian rocks from Europe and North America (e.g. [36, 85]).
Additional biomarker evidence for paleoredox conditions during sedimenta-
tion includes the lycopane/C
31
n
-alkane ratio (e.g. [82]). Assumed to be derived
from a photoautotrophic organism, high abundances of lycopane (with a char-
acteristic carbon isotope signature) in modern and ancient sediments deposited
from anoxic water column conditions attest to its preferential preservation under
oxygen-deficient conditions. This selective preservation results in an increase
of the lycopane/C
31
n
-alkane ratio, which provides a respective proxy signal
for palaeoxicity.
4. ANOXIA IN THE GEOLOGICAL PAST
4.1 The Precambrian World
Qualitatively, the notion of an anoxic world during the early part of Earth's
history (Fig. 1) is based in part on our understanding that the early atmosphere
was largely reducing, containing CO, NH
3
,H
2
, all inorganic compounds from
which the early building blocks of life could have been generated [47]. With
regard to quantitative aspects, two strongly opposing models have been pro-
posed in the past: a three stage model with the consecutive oxygenation of the
atmosphere, surface water and deep water [29, 39] and an invariable abundance
of atmospheric oxygen for the past 4 Ga, possibly within 10 % of the present
days atmospheric level [51].
This longstanding controversial discussion whether or not the Archean
and early Paleoproterozoic ocean-atmosphere-system contained substantial
amounts of free oxygen has recently moved forward by a new “piece of evi-
dence”, notably the discovery of mass-independent sulphur isotope fractiona-
tion, recorded in sedimentary sulphide and sulphate. Independent of the precise
mechanisms it appears that the photochemical dissolution of sulphur dioxide
is the principal cause for mass-independent sulphur isotope fractionation. The
fact that such signals have been measured in near-surface sedimentary sulphide
and sulphate indicates an “oxygen free” atmosphere. Otherwise, oxidation of
reduced atmospheric sulphur species, rainout of these as sulphate and subse-
quent homogenization of this signal in the ocean would have resulted in the loss
of such a signature. However, a very distinct record of mass-independent sul-
phur isotopes exists [7, 26, 48, 52], thereby pointing to an atmospheric oxygen
abundance of
<
10
−
5
PAL [57]. Evidence for significant mass-independent sul-
phur isotope fractionation is absent in sedimentary rocks younger than 2.32 Ga,
based on a sulphur isotope study of sedimentary pyrite from the Transvaal Su-
pergroup, South Africa [7]. This in turn suggests, that the atmospheric oxygen
abundance increased at 2.32 Ga, possibly from
<
10
−
5
to 10
−
2
PA L O
2
.
