Chapter 4
Sedimentary Organic Matter Preservation and
Atmospheric O 2 Regulation
J. 1. Hedges
4.1
Introduction
The global cycles of organic carbon (OC) and molecular oxygen (0 2 ) are inextricably
linked by the fact that both substances are uniquely produced and destroyed in equimolar amounts by photosynthesis and respiration. These opposing processes have been
balanced on the geochemical equivalent of a knife's edge for at least the last 600 million years, over which O2 dependent metazoans are continuously represented in the
geologic record. Throughout this period, O2 release from the preservation of organic
matter (-50 wt% OC) in marine sediments has been closely compensated for by simultaneous uptake of O 2 during weathering of organic matter (and reduced inorganic
minerals) in continental rocks. In this way, the atmospheric reservoir of O 2 has been
maintained within the relatively narrow concentration range (±50%), above which
runaway vegetation fires occur and below which anoxia becomes deadly to multicellular life (Berner 1989). Given that the mean residence time of atmospheric O2 is four
million years with respect to contemporary sedimentary OC burial, over the last
600 million years the Earth's global O2 control system has had roughly 300 chances to
fail on either the side of conflagration and anoxia - and has not (Walker 1974; Watson
et al. 1978; Garrels et al. 1976; Jones and Chaloner 1991). Such an extended planetary
winning streak, at least from a human perspective, suggests the existence of an effective control system for global-scale cycling of bioactive elements for at least the most
recent eighth of Earth history (Van Valen 1971; Petsch and Berner 1998).
This chapter lays forth the hypothesis that a tectonically-driven "mineral conveyer
belt," modulated by a shifting "organic carbon compensation depth" along continental margins, has provided the basis for sensitive atmospheric O 2 control over Phanerozoic time (0-0.6 billion years B.P.). In this scenario, flow of mineral debris from
weathering mountains to depositing marine sediments physically links these geographically separate sinks and sources of O2 on a geologically short time scale. In an
enmeshed cycle, "excess" organic matter depositing in marine sediments is oxidized
by downward mixing of surface ocean waters, whose initial oxygen content is directly
proportional to the current atmospheric O2 concentration. A small fraction of this
depositing organic matter, however, escapes remineralization to be preserved in sediments, thereby releasing just enough O2 to compensate globally for weathering of continental rocks. This combination of mineral mass balance modulated by negative-feedback control during sedimentary organic matter preservation along continental margins appears to exhibit the capacity, sensitivity, and response time required to provide the Earth's hidden O 2 safety net.
Sedimentary Organic Matter Preservation and
Atmospheric O 2 Regulation
J. 1. Hedges
4.1
Introduction
The global cycles of organic carbon (OC) and molecular oxygen (0 2 ) are inextricably
linked by the fact that both substances are uniquely produced and destroyed in equimolar amounts by photosynthesis and respiration. These opposing processes have been
balanced on the geochemical equivalent of a knife's edge for at least the last 600 million years, over which O2 dependent metazoans are continuously represented in the
geologic record. Throughout this period, O2 release from the preservation of organic
matter (-50 wt% OC) in marine sediments has been closely compensated for by simultaneous uptake of O 2 during weathering of organic matter (and reduced inorganic
minerals) in continental rocks. In this way, the atmospheric reservoir of O 2 has been
maintained within the relatively narrow concentration range (±50%), above which
runaway vegetation fires occur and below which anoxia becomes deadly to multicellular life (Berner 1989). Given that the mean residence time of atmospheric O2 is four
million years with respect to contemporary sedimentary OC burial, over the last
600 million years the Earth's global O2 control system has had roughly 300 chances to
fail on either the side of conflagration and anoxia - and has not (Walker 1974; Watson
et al. 1978; Garrels et al. 1976; Jones and Chaloner 1991). Such an extended planetary
winning streak, at least from a human perspective, suggests the existence of an effective control system for global-scale cycling of bioactive elements for at least the most
recent eighth of Earth history (Van Valen 1971; Petsch and Berner 1998).
This chapter lays forth the hypothesis that a tectonically-driven "mineral conveyer
belt," modulated by a shifting "organic carbon compensation depth" along continental margins, has provided the basis for sensitive atmospheric O 2 control over Phanerozoic time (0-0.6 billion years B.P.). In this scenario, flow of mineral debris from
weathering mountains to depositing marine sediments physically links these geographically separate sinks and sources of O2 on a geologically short time scale. In an
enmeshed cycle, "excess" organic matter depositing in marine sediments is oxidized
by downward mixing of surface ocean waters, whose initial oxygen content is directly
proportional to the current atmospheric O2 concentration. A small fraction of this
depositing organic matter, however, escapes remineralization to be preserved in sediments, thereby releasing just enough O2 to compensate globally for weathering of continental rocks. This combination of mineral mass balance modulated by negative-feedback control during sedimentary organic matter preservation along continental margins appears to exhibit the capacity, sensitivity, and response time required to provide the Earth's hidden O 2 safety net.