Organic Matter Burial in Sediments

At present, sedimentary organic matter burial occurs in swamps, lakes, reservoirs, estuaries, and in the open marine environment. The ultimate sources of the organics are land vegetation and marine phytoplankton. Also, soil organic matter, which is intimately associated with clay minerals, is eroded and transported to the sea by rivers (Hedges et al., 1994). A major question is how much of the total global burial is of marine or nonmarine origin. Recent work has shown that organic burial on land is much higher than previously recognized, especially as a result of human activities (Dean and Gorham, 1998; Stallard, 1998). In marine sediments, quantitatively separating marine from terrrestrial origin is difficult, but qualitative methods are available (e.g., Prahl et al., 1994). There is no doubt that more organic matter is carried to the sea than is buried there (Berner, 1982), which means that an appreciable proportion of terrestrially derived organics are destroyed in seawater (Hedges et al., 1997).

Within the oceans the locus of organic matter deposition and burial has been shown to be mainly in river deltas and on other areas of the continental shelves (Berner, 1982). Although sediments under areas of upwelling and high productivity are very rich in organic matter, the total burial in these areas is small compared to that in the organic-poor sediments of deltas and other near-shore depositional areas. Rates of sediment deposition in deltas, such as those of the Ganges/Brahmaputra and Amazon rivers, are so large that, even with low organic carbon concentrations, the global deltaic and shelf burial flux of carbon is more than 80% of all marine burial (table 3.1).

A major question is how the rates of global organic matter burial have varied in the geologic past. Knowledge of past global organic burial rates is essential for modeling the evolution of both atmospheric oxygen and carbon dioxide (see chapters 5 and 6). A key question is what were the factors that brought about increased or decreased organic matter sedimentation and preservation, the two steps leading to organic burial. For the nonmarine environment, there is no doubt that the rise and evolution of land plants brought about greater organic sedimentation (see next section). For the marine environment, there is some disagreement concerning the relative roles of sedimentation and preservation, both at present and in the past. There is no doubt that in the present ocean, more than 99% of organic matter sedimented to the sea floor becomes reoxidized, and not buried (Holland, 1978; Berner, 1989). However, it is the small fraction that becomes buried that controls O2 production and CO2 consumption, and this proportion could have varied in the past.

Table 3.1. Present (prehuman) burial fluxes of organic carbon in marine sediments.

Sediment type

Flux (1012 mol/year)

Deltaic-shelf sediments

4.4

Biogenous sediments underlying regions

of high productivity

0.5

Shallow-water carbonate sediments

0.2

Pelagic sediments (not overlain by regions

of high productivity)

0.2

Total

5.3

Values represent modern surface sedimentation rates and are corrected for 20% loss of carbon during early diagenesis and an anthropogenic contribution of about 50%. Data from Berner (1982).

Values represent modern surface sedimentation rates and are corrected for 20% loss of carbon during early diagenesis and an anthropogenic contribution of about 50%. Data from Berner (1982).

Increased burial over geologic time could have been brought about by (1) increased global sedimentation of total solids (e.g., Berner and Canfield, 1989); (2) increased global biological productivity and increased organic sedimentation due to higher levels of dissolved phosphorus added to the oceans by weathering (e.g., Holland, 1994; Guidry and Mackenzie, 2000); (3) increased preservation due to development of anoxic bottom waters in a highly stratified ocean (Van Cappellen and Ingall, 1996); and (4) increased preservation due to a low oxygen content of the atmosphere and the oceans (Lasaga and Ohmoto, 2002). As with all other geologic phenomena, a combination of these factors could have been operative. For example, Van Cappellen and Ingall (1996) call for both increased preservation and increased productivity by the preferential release to seawater of phosphorus from sediments buried in anoxic bottom waters.

Organic production via photosynthesis in the present ocean is nutrient limited, and the principal nutrients are nitrogen and phosphorus, although iron may be limiting in some polar waters (Falkowski and Raven, 1997). Over geologic time there is disagreement whether phosphorus (e.g., Holland, 1978, 1994) or nitrogen (Falkowski, 1997) is the principal limiting nutrient. Settling this issue is important because the concentrations of the two elements in seawater are controlled by different processes. Nitrogen levels reflect a balance between nitrogen fixation, the conversion of atmospheric N2 to nitrogen compounds accessible to organisms, and denitrification, the reduction of dissolved nitrate (the principal form of nitrogen in seawater) back to N2. Weathering of rocks is unimportant in the nitrogen cycle. Phosphorus, in contrast, is minimally involved in atmospheric cycling, and the overall level of phosphorus in the ocean is a function of the riverine input from weathering of phosphate minerals and output by the burial of organic and inorganic phosphorus in sediments (Holland,

1978). An important additional factor is that photosynthesis can occur only in water shallow enough for the penetration of light. Falling dead organic debris is broken down to dissolved N and P via microbial respiration at depth and this, along with photosynthesis in shallow waters, leads to gradients in N and P with lower values at the surface and higher values at depth (Falkowski and Raven, 1997). Thus, vertical oceanic circulation, such as coastal upwelling, plays a role in supplying nutrients to shallow waters and fueling organic production and eventually burial in bottom sediments.

Because organic matter burial involves the production of oxygen and the consumption of CO2, and marine organic burial is controlled largely by nutrient availability, considerable attention has been given to the cycles of phosphorus and nitrogen as to how they could have affected the burial of organic matter over geologic time (Holland, 1994; Van Cappellen and Ingall, 1996; Falkowski, 1997; Lenton and Watson, 2000; Wallmann, 2001). All of the P and N cycle hypotheses call for negative feedback mechanisms that limit fluctuations in the rates of organic burial. For Wallmann (2001), CO2 consumption via organic burial is tied directly to the supply of phosphate to the oceans from the weathering of silicate, carbonate, and organic matter because phosphorus is associated with all three groups.

The various proposed nutrient feedback mechanisms can be represented in terms of a systems analysis diagram (figure 3.1). Positive responses are represented by plain arrows; negative responses are represented by arrows with bullseyes. Any loop with an odd number of bullseyes represents negative feedback or stabilization. Consider feedback loop A, one favored by Wallmann (2001) and Hansen and Wallmann (2003). Global warming due to elevated CO2 brings about accelerated phosphate weathering and transport of P to the sea, leading to an increase in aqueous nutrient P. This in turn leads to greater organic carbon burial and greater CO2 consumption, with the overall process producing negative feedback. The phosphorus-controlled feedback of Holland (1994), Colman and Holland (2000), and Van Cappellen and Ingall (1996) is shown by feedback loop B. An increase in atmospheric and oceanic O2 leads to greater burial of phosphorus adsorbed on ferric oxides (FeP), which reduces the amount of aqueous nutrient P available for organic production. The lower concentration of nutrient P leads to less organic burial and a diminution in O2 production, thus completing the feedback cycle. Then there is cycle C, favored by Falkowski (1997). Higher O2 leads to diminished nitrogen fixation, leading to less organic matter production and burial and less O2 production. Finally, there is an indirect interaction between atmospheric O2 and CO2. For example, a decrease in nutrient P in seawater, due to a rise in O2, leads to decreased organic burial and higher CO2.

There are periods of time during the Phanerozoic when it is believed that large portions of seawater became anoxic. Short-term (few million

Feedback Short Term Organic Carbon Cycle

Figure 3.1. Systems analysis diagram for the effects of nutrients on O2, CO2, and the organic subcycle. Arrows with bullseyes represent negative response; arrows without bullseyes represent positive response. A complete cycle (loop) with an odd numbers of bullseyes means negative feedback and stabilization; a complete cycle with an even number of bullseyes, or no bullseyes, means positive feedback and (normally) destabilization. The three loops shown are all for negative feedback and stabilization of the atmospheric gases.

Figure 3.1. Systems analysis diagram for the effects of nutrients on O2, CO2, and the organic subcycle. Arrows with bullseyes represent negative response; arrows without bullseyes represent positive response. A complete cycle (loop) with an odd numbers of bullseyes means negative feedback and stabilization; a complete cycle with an even number of bullseyes, or no bullseyes, means positive feedback and (normally) destabilization. The three loops shown are all for negative feedback and stabilization of the atmospheric gases.

year) anoxic ocean events (AOEs) during the Mesozoic have been well documented (Arthur et al., 1985; Jenkyns, 1988; Arthur and Sageman, 1994). A much longer anoxic period occurred during the early Paleozoic, when the black shales of the graptolite facies were deposited (Berry and Wilde, 1978; Wilde, 1987) and the average ratio of organic carbon to pyrite sulfur buried in sediments globally was unusually low (Berner and Raiswell, 1983). This is shown in figure 3.2. Low organic C/pyrite S is characteristic of deposition in anoxic bottom waters—in other words, in euxinic environments characterized by the pyrite-rich present day deep Black Sea sediments (Berner and Raiswell, 1983). (The prominent maximum in C/S of figure 3.2 is discussed below in "Land Plant Evolution.") If euxinic conditions lead to greater preservation of organic matter, then increased global anoxicity could have led to greater organic burial rates during these anoxic periods. Greater burial during the AOEs is suggested by increases in 813C of seawater indicating greater removal of light carbon in organic matter (e.g., Arthur et al., 1985). Also, there is a good correlation between high global organic burial rate and times of formation of oil-source rocks (Berner, 2003).

There is an argument concerning whether anoxic waters preserve organic matter better than oxic waters. Pederson and Calvert (1990) have claimed that greater burial of organic matter is due more to higher pro-

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Responses

  • CONNOR
    How to complete large portions in short time?
    8 years ago
  • achille
    How much carbon is tied up as buried organic matter?
    8 years ago
  • tom
    What is the ratio of carbon buried as carbonate to carbon buried as organic matter in todays oceans?
    8 years ago
  • alasdair sutherland
    What is the sediments buried organic matter?
    7 years ago
  • ANNE
    IS OXYGEN INVOLVED IN BURIAL OF ORGANIC MATTER?
    6 years ago
  • Yerusalem
    What causes an increase in buried organic carbon?
    4 years ago
  • senait
    Is burying organic matter better for the environment?
    2 years ago
  • stephanie pulliam
    Is burying organic matter eco friendly?
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