CHAPTER 4 . Sedimentary Organic Matter Preservation and Atmospheric O2 Regulation
121
dases) to degrade recalcitrant, carbon-rich substances such as lignin (Emerson and
Hedges 1988), the specific mechanisms involved are presently unknown. Thus, the inferred "oxic effect" should be regarded as a general phenomenon that characteristically occurs in the presence of molecular O2, but does not necessarily involve this
chemical species directly.
What is abundantly clear, however, is that deep open ocean sediments contain
(Fig. 4.3) and bury (Fig. 4.2) very little organic matter (Premuzic et al. 1982). Where
extensive off-shore gradients in sedimentary OC have been analyzed in detail, sharp
decreases have been observed along lower (>2000 m) continental margins (Hedges
and Keil1995; Van der Weijden et al.1999; Graneshram et al.1999). These trends, viewed
within the context of an oxygen exposure model, suggest the existence of an "organic
carbon compensation depth" (OCCD), much in parallel to the carbonate compensation depth (CCD) that has been recognized for decades on sea floor promontories
(Broecker and Peng 1982). In both instances, slow external erosion appears to compete with sediment burial to determine preservation patterns that exhibit relatively
sharp boundaries determined by bottom water conditions. In the case of the OCCD,
however, the sharpness of the boundary and hence the sensitivity of the feedback
mechanism may be enhanced by the sharp increase in OC preservation per unit surface area that typically occurs (Fig. 4.3) when bottom water O2 falls to low values as
are now seen along the Peruvian and NW Mexican margins. Although no attempt is
made here to numerically model the combined effects on atmospheric O2 concentrations that might result from a mineral conveyer belt operating in conjunction with a
marine sedimentary "afterburner" (Fig. 4.12), such quantitative simulations should be
possible. To understand the past expressions and the future functions of this proposed
addition to the global safety net, it will be necessary to learn more about how organic
matter becomes associated with sedimentary minerals and by what mechanism oxic
degradation proceeds on land and in the ocean.
Acknowledgements
Cindy Lee and Kenia Whitehead made helpful comments on drafts of this chapter.
References
Baldock JA, Skjemstad JO (2000) Role of the soil matrix and minerals in protecting natural organic
materials against biological attack. Org Geochem 31:697-710
Bergamaschi BA, Tsamakis E, Keil RG, Eglinton TI, Mont!uc;on DB, Hedges JI (1997) The effect of grain
size and surface area on organic matter, lignin and carbohydrate concentrations and molecular compositions in Peru Margin sediments. Geochim Cosmochim Acta 61:1247-1260
Berner RA (1982) Models for carbon and sulfur cycles and atmospheric oxygen: Application to Paleozoic
geologic history. Am J Sci 287:177-196
Berner RA (1987) Burial of organic carbon and pyrite sulfur in ilie modern ocean: Its geochemical and
environmental significance. Am J Sci 282:451-473
Berner RA (1989) Biogeochemical cycles of carbon and sulfur and their effect on atmospheric oxygen
over Phanerozoic time. Palaeogeogr Palaeoclimatol Palaeoecol 73:97-122
Berner RA (1999) Atmospheric oxygen over Phanerozoic time. Proc Nat! Acad Sci 96:10955-10957
Berner RA, Canfield DE (1989) A new model for atmospheric oxygen over Phanerozoic time. Am J Sci
289:333-361
Betts IN, Holland HD (1991) The oxygen content of ocean bottom waters, the burial efficiency of organic carbon, and the regulation of atmospheric oxygen. Palaeogeogr Palaeoclimatol Palaeoecol
97:5-18
121
dases) to degrade recalcitrant, carbon-rich substances such as lignin (Emerson and
Hedges 1988), the specific mechanisms involved are presently unknown. Thus, the inferred "oxic effect" should be regarded as a general phenomenon that characteristically occurs in the presence of molecular O2, but does not necessarily involve this
chemical species directly.
What is abundantly clear, however, is that deep open ocean sediments contain
(Fig. 4.3) and bury (Fig. 4.2) very little organic matter (Premuzic et al. 1982). Where
extensive off-shore gradients in sedimentary OC have been analyzed in detail, sharp
decreases have been observed along lower (>2000 m) continental margins (Hedges
and Keil1995; Van der Weijden et al.1999; Graneshram et al.1999). These trends, viewed
within the context of an oxygen exposure model, suggest the existence of an "organic
carbon compensation depth" (OCCD), much in parallel to the carbonate compensation depth (CCD) that has been recognized for decades on sea floor promontories
(Broecker and Peng 1982). In both instances, slow external erosion appears to compete with sediment burial to determine preservation patterns that exhibit relatively
sharp boundaries determined by bottom water conditions. In the case of the OCCD,
however, the sharpness of the boundary and hence the sensitivity of the feedback
mechanism may be enhanced by the sharp increase in OC preservation per unit surface area that typically occurs (Fig. 4.3) when bottom water O2 falls to low values as
are now seen along the Peruvian and NW Mexican margins. Although no attempt is
made here to numerically model the combined effects on atmospheric O2 concentrations that might result from a mineral conveyer belt operating in conjunction with a
marine sedimentary "afterburner" (Fig. 4.12), such quantitative simulations should be
possible. To understand the past expressions and the future functions of this proposed
addition to the global safety net, it will be necessary to learn more about how organic
matter becomes associated with sedimentary minerals and by what mechanism oxic
degradation proceeds on land and in the ocean.
Acknowledgements
Cindy Lee and Kenia Whitehead made helpful comments on drafts of this chapter.
References
Baldock JA, Skjemstad JO (2000) Role of the soil matrix and minerals in protecting natural organic
materials against biological attack. Org Geochem 31:697-710
Bergamaschi BA, Tsamakis E, Keil RG, Eglinton TI, Mont!uc;on DB, Hedges JI (1997) The effect of grain
size and surface area on organic matter, lignin and carbohydrate concentrations and molecular compositions in Peru Margin sediments. Geochim Cosmochim Acta 61:1247-1260
Berner RA (1982) Models for carbon and sulfur cycles and atmospheric oxygen: Application to Paleozoic
geologic history. Am J Sci 287:177-196
Berner RA (1987) Burial of organic carbon and pyrite sulfur in ilie modern ocean: Its geochemical and
environmental significance. Am J Sci 282:451-473
Berner RA (1989) Biogeochemical cycles of carbon and sulfur and their effect on atmospheric oxygen
over Phanerozoic time. Palaeogeogr Palaeoclimatol Palaeoecol 73:97-122
Berner RA (1999) Atmospheric oxygen over Phanerozoic time. Proc Nat! Acad Sci 96:10955-10957
Berner RA, Canfield DE (1989) A new model for atmospheric oxygen over Phanerozoic time. Am J Sci
289:333-361
Betts IN, Holland HD (1991) The oxygen content of ocean bottom waters, the burial efficiency of organic carbon, and the regulation of atmospheric oxygen. Palaeogeogr Palaeoclimatol Palaeoecol
97:5-18