116
J. 1. Hedges
tial to modulate the rate of O2 generation on a global scale. Because surface area delivery to coastal zones is ultimately controlled by continental uplift and O2 availability to the ocean floor is directly dependent upon the contemporary atmospheric
concentration, a tectonic transmission and geochemical governor for a global O2 control apparatus appear to be in place.
4.5
Maintaining Atmospheric O 2 within Safe Bounds
The geochemical challenge of maintaining relatively constant (within a factor or two)
atmospheric concentrations of O2 (mean residence time -4 million years) over the last
600 million years has been recognized for several decades (Broecker 1970; Van Valen
1971). Twenty-five years ago, Walker (1974) pointed out that due to nearly complete
remineralization of reduced elements during sedimentary rock weathering, the inferred global O 2 control mechanism must operate on the source side within depositing marine sediments. The first comprehensive attempts to quantitatively model reservoir sizes and exchange rates among major redox-active elements in the Earth's surface (exogenic) cycle came in the mid 1970S (e.g. Holland 1973; Garrels and Perry 1974;
Garrels et al. 1976). The latter authors demonstrated mathematically that the network
of biogeochemical exchanges among the major reduced and oxidized forms of carbon (DC and CO 2 + CaC0 3 ) and sulphur (SO~- and FeS2) at the Earth's surface appears
to embody "effective feedback mechanisms" for controlling atmospheric COz and Oz.
For example, they pointed out that an increase in continental erosion rates would decrease the partial pressure of atmospheric Oz, which would lead (through gas exchange)
to lower O 2 concentrations in sea water and (presumably) to more efficient sedimentary organic matter preservation (Fig. 4.1). The eventual system response would be a
net release of more Oz to partially offset the initial atmospheric decrease. The functional relationships used in this model to determine the exchange fluxes of O2 and
the various C and S forms, however, were largely empirical or presumed, with weak
and incomplete mechanistic underpinnings.
It was also recognized about this time that large excursions in Oz and CO2 could be
avoided if the Earth's exogenic redox cycles were confined largely among a select set
of solid reactants and products. The overall global redox reaction can be represented
as the reduction of carbon in carbonate and dolomite with sulphur in pyrite, to produce organic matter, iron oxide and sulphate (Garrels and Perry 1974).
4 FeS2 + CaC03 + 7 CaMg(C03h + 7 SiOz + 15 H20
H 15 CHzO + 8 CaS04 + 2 Fez03 + 7 MgSi03
(4.2)
This formulation bypasses the intermediary role of Oz and CO 2 in photosynthesis/respiration and weathering and has the budgetary advantage that all the major electron
exchangers occur predominantly in the rock reservoir. The masses of these redox-sensitive minerals in sedimentary rocks are so huge that any imbalance of the reaction
network could rapidly change the comparably small amounts of sulphate and bicarbonate dissolved in the ocean, as well as of O2 and CO2 in the atmosphere. An illustration of the geological processes linking the key redox forms in Eq. 4.2 is given in
J. 1. Hedges
tial to modulate the rate of O2 generation on a global scale. Because surface area delivery to coastal zones is ultimately controlled by continental uplift and O2 availability to the ocean floor is directly dependent upon the contemporary atmospheric
concentration, a tectonic transmission and geochemical governor for a global O2 control apparatus appear to be in place.
4.5
Maintaining Atmospheric O 2 within Safe Bounds
The geochemical challenge of maintaining relatively constant (within a factor or two)
atmospheric concentrations of O2 (mean residence time -4 million years) over the last
600 million years has been recognized for several decades (Broecker 1970; Van Valen
1971). Twenty-five years ago, Walker (1974) pointed out that due to nearly complete
remineralization of reduced elements during sedimentary rock weathering, the inferred global O 2 control mechanism must operate on the source side within depositing marine sediments. The first comprehensive attempts to quantitatively model reservoir sizes and exchange rates among major redox-active elements in the Earth's surface (exogenic) cycle came in the mid 1970S (e.g. Holland 1973; Garrels and Perry 1974;
Garrels et al. 1976). The latter authors demonstrated mathematically that the network
of biogeochemical exchanges among the major reduced and oxidized forms of carbon (DC and CO 2 + CaC0 3 ) and sulphur (SO~- and FeS2) at the Earth's surface appears
to embody "effective feedback mechanisms" for controlling atmospheric COz and Oz.
For example, they pointed out that an increase in continental erosion rates would decrease the partial pressure of atmospheric Oz, which would lead (through gas exchange)
to lower O 2 concentrations in sea water and (presumably) to more efficient sedimentary organic matter preservation (Fig. 4.1). The eventual system response would be a
net release of more Oz to partially offset the initial atmospheric decrease. The functional relationships used in this model to determine the exchange fluxes of O2 and
the various C and S forms, however, were largely empirical or presumed, with weak
and incomplete mechanistic underpinnings.
It was also recognized about this time that large excursions in Oz and CO2 could be
avoided if the Earth's exogenic redox cycles were confined largely among a select set
of solid reactants and products. The overall global redox reaction can be represented
as the reduction of carbon in carbonate and dolomite with sulphur in pyrite, to produce organic matter, iron oxide and sulphate (Garrels and Perry 1974).
4 FeS2 + CaC03 + 7 CaMg(C03h + 7 SiOz + 15 H20
H 15 CHzO + 8 CaS04 + 2 Fez03 + 7 MgSi03
(4.2)
This formulation bypasses the intermediary role of Oz and CO 2 in photosynthesis/respiration and weathering and has the budgetary advantage that all the major electron
exchangers occur predominantly in the rock reservoir. The masses of these redox-sensitive minerals in sedimentary rocks are so huge that any imbalance of the reaction
network could rapidly change the comparably small amounts of sulphate and bicarbonate dissolved in the ocean, as well as of O2 and CO2 in the atmosphere. An illustration of the geological processes linking the key redox forms in Eq. 4.2 is given in