Environmental Engineering Reference
In-Depth Information
ANOXIA THROUGH TIME
Harald Strauss
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We s t f alische Wilhelms-Universit at M unster, Geologisch-Pal aontologisches Institut, Correnss-
trasse 24, 48149 M unster, Germany
Abstract
The rock record provides unequivocal evidence for multiple times in Earth history
during which the entire global ocean or parts of it were characterized by severe
oxygen-deficiency. Evidence includes geological and paleontological observa-
tions but also diverse geochemical fingerprints such as trace element abundances,
organic geochemical markers or various isotope records, all of which are diagnos-
tic for water column anoxia. The duration of such episodes of oxygen-deficiency
ranges from a few thousand to millions of years. In fact, considering the pro-
posed temporal sequence of oxygenation of the atmosphere-ocean system during
Earth's early history with the ventilation of the deep ocean not earlier than 1 Ga
ago, it appears that the modern oxic world represents an exceptional state of the
atmosphere-ocean system on our planet during its 4.5 Ga years history.
Keywords:
Oceanic anoxic events, proxy signals, earth system evolution
1. INTRODUCTION
Earth scientists, specifically those studying the low temperature sedimentary
realm, define the term “anoxia” as an environmental situation in which the
abundance of free molecular oxygen is at (or very near) zero. Thereby, the
attribute “anoxic” is applied to all scales, ranging from the interstitial water in
porous sediments to parts of the oceanic or lacustrine water column and all the
way up to the redox-state of the world's ocean-atmosphere system.
In contrast to anoxia, oxic conditions reflect the presence of free oxygen.
Quantification of O
2
-abundance for the geological past frequently refers to a
percentage of the present day atmospheric level of oxygen (PAL O
2
).
Somewhat more loosely defined in terms of its O
2
-abundance are redox-
states such as suboxic or dysoxic. However, common sense tells us that these
attributes characterize an environmental situation between oxic and anoxic
conditions.
Finally, the term “anoxia” is frequently utilized interchangeably with the
term “euxinic”. Euxinic conditions, however, refer to a water column, which
contains dissolved hydrogen sulphide.
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