The Rock Abundance Method

The rock abundance method (Berner and Canfield, 1989), for Fbg and Fbp, is based on original global sedimentation rates of terrigenous sediments (sandstones and shales) and their organic carbon and pyrite sulfur contents. (Most sedimentary organic matter and pyrite is found in shales.) For this purpose terrigenous deposition over time is divided into three major categories (Ronov, 1976): coal basin sediments, noncoal continental deposits (mainly redbeds), and marine sediments. This leads to the expressions:

Fbp = [O2(0)/O2](fmarSmar + fcbScb + frb SA) Ft (6.15)

where fraction of total terrigenous sedimentation deposited as marine sediments, coal basin sediments, and redbeds mean organic carbon and pyrite (+ organic) sulfur content of marine sediments, coal basin sediments, and red-beds respectively

mar cb rb

mar cb rb mar cb rb

Ft = total global rate of terrigenous sedimentation

02(0)/02 = ratio of oxygen mass at present to that at some prior time.

Changes in the relative proportions fmar, fcb, frb of each type bring about changes in global rates of burial of organic C and pyrite S (the terms for redbeds are dropped because they contain negligible organic C and pyrite S). The term 02(0)/02 is added to include the inverse effect of 02 levels on the burial of pyrite (Berner and Raiswell, 1983). The fraction of terrigenous sediments as coal basin sediments and as redbeds is shown in figure 6.2 as a function of time. Because the average organic carbon content of coal basin sediments (mainly dispersed at subeconomic levels) are about 3-5 times higher than that for average marine terrigenous sediments and because redbeds contain essentially no organic carbon or pyrite sulfur, these plots are a qualitative measure of 02 production rate versus time. High proportions of coal basin sediments mean high 02 production, and high proportions of redbeds means low 02 production.

To complete their calculations of 02 versus time, Berner and Canfield (1989) assumed that the weathering of organic C and pyrite S was a direct function of physical erosion rate. This is in keeping with the find-

Berner Rock Abundance
Figure 6.2. Plots versus time of the fraction of terrigenous sediments (sandstones and shales) present as coal basin sediments and as other terrestrial deposits (mainly redbeds). Data from Ronov (1976).

ings on present-day organic matter and pyrite weathering discussed in chapter 3. Because global erosion equals global sedimentation, this provides a strong negative feedback against variations in O2 due to variations in organic C burial accompanying variations in total global sedimentation. Also, to provide additional negative feedback against excessive O2 variation, Berner and Canfield introduced the concept of rapid recycling. In rapid recycling, the organic C and pyrite S in younger sediments are assumed to weather faster than in older ones that have become buried and sheltered from the atmosphere. As a simple first-order approach, sediments were divided into rapidly weathering "young" sediments with a mean age of 100 million years and all other slowly weathering "old" sediments with greater ages.

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