Progress Report What Is the Cumulative Biotic Enhancement of Weathering

If the biotic enhancement of weathering factor of vascular plants, lichens, and bryophytes is one order of magnitude greater than primitive land biota of the Precambrian, an estimate consistent with field, experimental studies, and GEOCARB modeling, then only another factor of 10 is required of the latter over an abiotic regime to give B ~100, the order of magnitude we infer from inverting the Archean/early Proterozoic record from modeling the carbonate-silicate cycle, assuming a high surface temperature (50-70°C) scenario and a carbon dioxide greenhouse (to be discussed in chapter 7). An

QUANTIFYING THE BIOTIC ENHANCEMENT OF

WEATHERI

N G 97

TABLE 6-3 .

Computed Effective Biotic Enhancement Factors

computed Be values

R E (x: 0

0.1

0.5

1)

1000 1000 1000

9.91

2

1

100 1000

91.7

19.8

10

10 1000

526

182

100

100 1000 100

1.0

0.2

0.1

100 100

9.2

2.0

1

10 100

53

18

10

The model calculations illustrate that, except for a low assumed effect on weathering intensity from abiotic regolith (E), the computed effective biotic enhancement factor (Be) declines markedly with modest increases in abiotic regolith coverage (x). We have argued here that both relatively small abiotic regolith coverage and weathering intensity are likely as a result of water and wind erosion on bare abiotic land surfaces, implying that relatively high effective biotic enhancement factors should be used in modeling. Perhaps geomorphologic data and theory will provide more insight on the likely regolith weathering effect on abiotic land surfaces.

additional order of magnitude for the net enhancement on land colonized by primitive biota over abiotic appears to be plausible given the range of potential effects on each scale.

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