Environmental Engineering Reference
In-Depth Information
Evidence for a stepwise increase in the atmospheric oxygen abundance can
be further obtained from long-term isotope records. Secular variations in the
isotopic compositions of carbon, oxygen, strontium and sulphur in seawater
are believed to faithfully reflect the evolution of the ocean-atmosphere system.
While the strontium isotopic composition records temporal variations in the
proportional inputs from continental weathering and high-temperature basalt-
seawater reactions at mid-ocean ridges, carbon and sulphur isotopes react to
biologically mediated redox reactions, resulting in changes of the fractional
burial of reduced (organic C, pyrite S) vs. oxidized (carbonate C, sulphate S)
compounds. Finally, oxygen isotopes are a proxy signal for climatic changes.
Respective isotope records have been determined with high temporal reso-
lution for the Phanerozoic [38, 87] and with substantially less detail also for
the Precambrian [45, 80, 84]. The seawater strontium isotopic evolution clearly
displays two periods with a steep increase, between 3.0 and 2.0 Ga and be-
tween 0.8 and 0.55 Ga, respectively. Major tectonic rearrangements, resulting
in an increasing contribution of radiogenic strontium, are the likely cause for
this pattern [28]. The first time interval represents a period in Earth history
of significant growth of continental crust whereas the Neoproterozoic shift in
87
Sr/
86
Sr reflects the break-up of Rodinia. Both processes would result in a net
increase of near-shore shelf habitats. Interestingly, the same time intervals also
display an overall
13
C enriched carbonate carbon isotope signature [32, 46],
sharply punctuated by shifts to negative δ
13
C
carbonate
values during intervals of
glaciation. Mass-balance considerations suggest that these
13
C enriched values
reflect an increase in the fractional burial of organic carbon [32] and/or alterna-
tively a prolonged residence time of organic matter in the water column [70].
An increase in the fractional burial of organic carbon, however, translates into
an increasing contribution of oxygen to the atmosphere, resulting in the pro-
posed stepwise oxygenation of Earth's atmosphere [22]. Less well constrained,
but still discernible, are shifts in the sulphur isotope record. In particular the
terminal Neoproterozoic and early Cambrian display a strongly positive δ
34
S
signature of seawater sulphate [84]. Again, mass balance considerations sug-
gest a higher fractional burial of reduced (biogenic) sulphur, and consequently
less oxygen demand.
Recently, a new model for Proterozoic ocean chemistry [18], notably the
drawdown of dissolved Fe from Proterozoic oceans as iron sulphide, has been
proposed. Sulphide sulphur isotope data from Mesoproterozoic successions
have been provided as evidence for a proposed anoxic if not sulphidic Paleo-
and Mesoproterozoic deep ocean [60, 79]. It was suggested that oxygenation of
deep ocean waters occurred not earlier than the early Neoproterozoic, consistent
with earlier views on the emergence of sulfur disproportionation [19].
