CHAPTER 4 ' Sedimentary Organic Matter Preservation and Atmospheric O2 Regulation
109
One of the most widely observed patterns in the composition of marine sediments
is that their organic carbon content increases with decreasing mean particle size within
deposits of a given depositional regime (Bordovskiy 1965; Premuzic et al. 1982), This
relationship was once thought to occur because particles of organic matter and finer
mineral grains are similarly sorted during transport and because the supply rate of
oxidizing agents to organic debris is more restricted in clay-rich deposits. This earlier
notion was fundamentally changed by the finding that organic matter and coexisting
sedimentary minerals could not be physically separated hydrodynamically or with
heavy liquids (e.g. Mayer et al. 1993; Keil et al. 1994a). Recognition that most sedimentary organic matter is directly associated with mineral grains has been one of the most
formative conceptual breakthroughs in the study of organic matter distribution and
preservation.
Mayer (1994a,b) demonstrated that the weight percentage of organic carbon
(wt% DC) is directly correlated to the specific surface area (SA) of bulk sediments and
soils, ratl1er than simply to texture. Moreover, he observed that coastal marine sediments depositing outside deltas typically exhibit ~C/SA ratios in the range of
0.5-1.0 mg DC m- 2 • This range corresponds to an organic carbon "loading" similar to
that expected for average protein molecules adsorbed to mineral surfaces in a single
layer, and was thus referred to as a "monolayer-equivalent" concentration. At the time,
Mayer (1994a,b) stressed that this term refers to an equivalent concentration and not
a mechanism, because the type and uniformity of organic molecule association with
sedimentary minerals was unknown. Subsequent characterizations of mineral/organic
associations by microscopic techniques (e.g. Ransom et al. 1998), gas adsorption energetics (Mayer 2000) and electron spectroscopy (Furukawa 2000) indicate that most
sedimentary organic matter associated with minerals occurs in clumps rather than
being distributed evenly in complete monolayers. This finding confirms intimate organic-mineral associations down to the nanometre scale (Furukawa 2000), but indicates that the molecules involved are not simply sorbed and thus may not have necessarily once been dissolved. Parallel studies in soils indicate that organic association is
selective, dependent upon the surface characteristics of the mineral and often involves
multivalent cation bridging (Kaiser and Guggenberger 2000; Baldock and Skjemstad
2000). The observation that organic matter separated from the mineral matrix of soils
(Nelson et al.1994) and sediments (Keil et al.1994b) becomes orders of magnitude more
reactive, also points toward a physical protection mechanism that would explain observed patterns in long-term preservation.
The wide range of ~C/SA ratios observed in marine sediments from contrasting
depositional regimes (Fig. 4.3), however, indicates that surface area is not the only
determinant of sedimentary DC preservation rates (Hedges and KeiI1995). In comparison to typical continental margin deposits, sediments accumulating beneath anoxic bottom waters usually have higher ~C/SA loadings (1-5 mg DC m- 2 ), whereas
those deposited under the open ocean (depth> 2000 m) almost always have lower
ratios «0.5 mg DC m- 2 ). Singling out the reasons for this nearly universal pattern is
difficult, because many complex processes and properties covary in modern depositional settings. Primary production rates, water column depth, accumulation rates and
varying degrees of biological mixing (bioturbation) and pumping (irrigation) are
among frequently suggested causative factors. Particularly strong evidence has been
presented that wt% DC increases with sediment accumulation rate (Heath et al. 1977;
109
One of the most widely observed patterns in the composition of marine sediments
is that their organic carbon content increases with decreasing mean particle size within
deposits of a given depositional regime (Bordovskiy 1965; Premuzic et al. 1982), This
relationship was once thought to occur because particles of organic matter and finer
mineral grains are similarly sorted during transport and because the supply rate of
oxidizing agents to organic debris is more restricted in clay-rich deposits. This earlier
notion was fundamentally changed by the finding that organic matter and coexisting
sedimentary minerals could not be physically separated hydrodynamically or with
heavy liquids (e.g. Mayer et al. 1993; Keil et al. 1994a). Recognition that most sedimentary organic matter is directly associated with mineral grains has been one of the most
formative conceptual breakthroughs in the study of organic matter distribution and
preservation.
Mayer (1994a,b) demonstrated that the weight percentage of organic carbon
(wt% DC) is directly correlated to the specific surface area (SA) of bulk sediments and
soils, ratl1er than simply to texture. Moreover, he observed that coastal marine sediments depositing outside deltas typically exhibit ~C/SA ratios in the range of
0.5-1.0 mg DC m- 2 • This range corresponds to an organic carbon "loading" similar to
that expected for average protein molecules adsorbed to mineral surfaces in a single
layer, and was thus referred to as a "monolayer-equivalent" concentration. At the time,
Mayer (1994a,b) stressed that this term refers to an equivalent concentration and not
a mechanism, because the type and uniformity of organic molecule association with
sedimentary minerals was unknown. Subsequent characterizations of mineral/organic
associations by microscopic techniques (e.g. Ransom et al. 1998), gas adsorption energetics (Mayer 2000) and electron spectroscopy (Furukawa 2000) indicate that most
sedimentary organic matter associated with minerals occurs in clumps rather than
being distributed evenly in complete monolayers. This finding confirms intimate organic-mineral associations down to the nanometre scale (Furukawa 2000), but indicates that the molecules involved are not simply sorbed and thus may not have necessarily once been dissolved. Parallel studies in soils indicate that organic association is
selective, dependent upon the surface characteristics of the mineral and often involves
multivalent cation bridging (Kaiser and Guggenberger 2000; Baldock and Skjemstad
2000). The observation that organic matter separated from the mineral matrix of soils
(Nelson et al.1994) and sediments (Keil et al.1994b) becomes orders of magnitude more
reactive, also points toward a physical protection mechanism that would explain observed patterns in long-term preservation.
The wide range of ~C/SA ratios observed in marine sediments from contrasting
depositional regimes (Fig. 4.3), however, indicates that surface area is not the only
determinant of sedimentary DC preservation rates (Hedges and KeiI1995). In comparison to typical continental margin deposits, sediments accumulating beneath anoxic bottom waters usually have higher ~C/SA loadings (1-5 mg DC m- 2 ), whereas
those deposited under the open ocean (depth> 2000 m) almost always have lower
ratios «0.5 mg DC m- 2 ). Singling out the reasons for this nearly universal pattern is
difficult, because many complex processes and properties covary in modern depositional settings. Primary production rates, water column depth, accumulation rates and
varying degrees of biological mixing (bioturbation) and pumping (irrigation) are
among frequently suggested causative factors. Particularly strong evidence has been
presented that wt% DC increases with sediment accumulation rate (Heath et al. 1977;