CHAPTER 4 . Sedimentary Organic Matter Preservation and Atmospheric O2 Regulation
119
of O2 in a short period of time (Fig. 4.11). Because the only long term source of P is
rock weathering, and the ocean reservoir is relatively small and can be depleted in a
matter of thousands of years (Broecker and Peng 1982), phosphate serves as a sensitive tectonic throttle for bioactive element cycling.
Another important constraint on O2 overproduction is that sulphate and carbon
dioxide reduction convert insoluble sedimentary organic matter to dissolved sulphide
and methane. In many coastal zones, these two reduction products diffuse upward into
oxic zones where they remove the same amount of O 2 that was generated when ilie
parent organic matter was photosynthesized (Berner 1982). At extremely rapid sediment accumulation rates, this mechanism breaks down because sulphide and methane are buried faster than they can diffuse upward. Such conditions are rare, however,
and at sediment accumulation rates less than approximately 1 g cm- 2 yr- 1 greater than
95% of the generated sulphide is diffusively lost and oxidized at the ultimate expense
of atmospheric O2 (Morse and Berner 1995).
A recent modelling effort involving a detailed sulphur isotope record for Cenozoic
sea water sulphate preserved in sedimentary barite (Payton and Arrigo 2000) demonstrated that the stable sulphur and carbon isotopic records are not consistent wiili
atmospheric O2 control solely by burial of pyrite S and organic matter (Fig. 4.10). The
difficulty with stable-isotope based models in general may be traced to their extreme
sensitivity to the isotopic compositions of the C and S rock reservoirs (Fig. 4.10). The
almost universal assumption in these models that isotopic fractionations have remained constant over the Phanerozoic as C and S are transferred among the major
geologic reservoirs may simply not be realistic (Payton and Arrigo 2000). Additional
mechanisms for constraining atmospheric O2 content over geologic time may well exist,
and in fact be necessary.
4.6
The Mineral Conveyer Belt and Sedimentary Afterburner
The "mineral conveyer belt" model for control of atmospheric O 2 (Fig. 4.12) combines
the concept of mass balance in the weatlIering/deposition cycle (e.g. Van Capellen and
Ingall1996) with a variant of Broecker's (1970) early concept for Oz-controlled marine sedimentary preservation. The assumption that mineral transport from weathering rocks to coastal marine sediments occurs wiili little net change in organic matter loading is based on the observation that most marine sediments deposited on
upper continental margins exhibit a relatively uniform surface area "loading" of
0.5-1.0 mg OC m- 2 • Because the weight percentages of OC in ancient shales and nearshore fine-grained marine sediments are similar (Hunt 1996), this concentration factor has not changed greatly over the Phanerozoic. A key assumption of ilie conveyer
belt model is that sedimentary rocks weather to primary particles that exhibit a surface area comparable to the mineral grains from which the rocks were originally
formed. This assumption is critical only for the clay and silt fractions of sediments
(primarily shales), which account for most of the total buried surface area (Keil et al.
1994a; Bergamaschi et al. 1997). The assumption that mineral surface area controls the
maximal burial potential of sediments at a given stage of oxic degradation is consistent with field observations (e.g. Figs. 4.3 and 4.9) and does not conflict with previous inferences that phosphate may be co-limiting.
119
of O2 in a short period of time (Fig. 4.11). Because the only long term source of P is
rock weathering, and the ocean reservoir is relatively small and can be depleted in a
matter of thousands of years (Broecker and Peng 1982), phosphate serves as a sensitive tectonic throttle for bioactive element cycling.
Another important constraint on O2 overproduction is that sulphate and carbon
dioxide reduction convert insoluble sedimentary organic matter to dissolved sulphide
and methane. In many coastal zones, these two reduction products diffuse upward into
oxic zones where they remove the same amount of O 2 that was generated when ilie
parent organic matter was photosynthesized (Berner 1982). At extremely rapid sediment accumulation rates, this mechanism breaks down because sulphide and methane are buried faster than they can diffuse upward. Such conditions are rare, however,
and at sediment accumulation rates less than approximately 1 g cm- 2 yr- 1 greater than
95% of the generated sulphide is diffusively lost and oxidized at the ultimate expense
of atmospheric O2 (Morse and Berner 1995).
A recent modelling effort involving a detailed sulphur isotope record for Cenozoic
sea water sulphate preserved in sedimentary barite (Payton and Arrigo 2000) demonstrated that the stable sulphur and carbon isotopic records are not consistent wiili
atmospheric O2 control solely by burial of pyrite S and organic matter (Fig. 4.10). The
difficulty with stable-isotope based models in general may be traced to their extreme
sensitivity to the isotopic compositions of the C and S rock reservoirs (Fig. 4.10). The
almost universal assumption in these models that isotopic fractionations have remained constant over the Phanerozoic as C and S are transferred among the major
geologic reservoirs may simply not be realistic (Payton and Arrigo 2000). Additional
mechanisms for constraining atmospheric O2 content over geologic time may well exist,
and in fact be necessary.
4.6
The Mineral Conveyer Belt and Sedimentary Afterburner
The "mineral conveyer belt" model for control of atmospheric O 2 (Fig. 4.12) combines
the concept of mass balance in the weatlIering/deposition cycle (e.g. Van Capellen and
Ingall1996) with a variant of Broecker's (1970) early concept for Oz-controlled marine sedimentary preservation. The assumption that mineral transport from weathering rocks to coastal marine sediments occurs wiili little net change in organic matter loading is based on the observation that most marine sediments deposited on
upper continental margins exhibit a relatively uniform surface area "loading" of
0.5-1.0 mg OC m- 2 • Because the weight percentages of OC in ancient shales and nearshore fine-grained marine sediments are similar (Hunt 1996), this concentration factor has not changed greatly over the Phanerozoic. A key assumption of ilie conveyer
belt model is that sedimentary rocks weather to primary particles that exhibit a surface area comparable to the mineral grains from which the rocks were originally
formed. This assumption is critical only for the clay and silt fractions of sediments
(primarily shales), which account for most of the total buried surface area (Keil et al.
1994a; Bergamaschi et al. 1997). The assumption that mineral surface area controls the
maximal burial potential of sediments at a given stage of oxic degradation is consistent with field observations (e.g. Figs. 4.3 and 4.9) and does not conflict with previous inferences that phosphate may be co-limiting.