Quantifying the Biotic Enhancement Factor on Present Earth Surface

The biotic enhancement of weathering factor (B) is defined in a quantitative way as follows: B is how much faster the silicate weathering carbon sink is under biotic conditions than under abiotic conditions at the same atmospheric carbon dioxide level (pCO2) and surface temperature (Schwartzman and Volk 1989). Although some biotically related effects may locally or temporarily reduce chemical weathering (e.g., macropores in soils, microbial coatings shielding grains), as already mentioned in chapter 5, the net biotic role is likely to increase global denudation rates. There is a consensus that B > 1, if only for biotic elevation of pCO2 in soils (possibly significant even before vascular plants from microbial respiration; see Keller and Wood 1993). Just how much bigger is still under contention. We have argued that B is probably on the order of 1oo or even greater. Estimates to date have mainly come from field and laboratory studies of recent weathering.

An indication of the possible magnitude of B comes from the ratio (about 1ooo) of the rate of chemical weathering in temperate/tropical soil to that of a computed abiotic two-dimensional bare rock rate at similar temperatures and runoff conditions (Schwartzman and Volk 1989), although several caveats are in order (e.g., the likely extent of soil cover and the contribution from porous volcanics under abiotic conditions). This computed ratio of 1ooo deserves closer examination. Chemical denudation rates of common silicate rocks in temperate/tropical soils range from 1o—3 to 1o—1 mm yr— 1 with typical values of around 1o—2 mm yr— 1 These rates have been estimated from studies of water chemistry of rivers. Two-dimensional dissolution rates of common rock-forming CaMgFe silicates in these rocks are needed for comparison. At the pH corresponding to present atmospheric pCO2 in equilibrium with water at 25°C (5.66, 3 X 1o—4 bars, respectively) the experimental rates are mostly around 1o—5 to 1o—6 mm yr—1 (hornblende, augite, bronzite, and olivine; the olivine value is from one of the few experiments made on natural unground crystals, from Grandstaff 1986). Diopside, another common CaMg silicate, has a rate that may be as high as 1o—3 8 mm yr— 1 but an artifact effect may be relevant here (data from Brantley and Chen 1995b). Factors tending to make the ratio ofnatural soil weathering to two-dimensional bare rock a minimum include the following:

1. The calculation for the two-dimensional bare rock surface assumes continuous flushing with undersaturated water; in nature a bare rock or coarse regolith would not be in continuous contact with water, thereby reducing the computed dissolution rate by the fraction oftime there is water contact.

2. The artifact effect in laboratory experiments that measure dissolution rates; grinding ofpowders may raise measured dissolution rates one or more orders of magnitude as a result of the creation of deformation and dislocations that preferentially dissolve during the dissolution experiment (Petro-vich 1981a, 1981b; Eggleston etal. 1989). Inparticular,Egglestonetal. (1989) found that dissolution rates ofdiopside powders were an order ofmagnitude less than initial rates when determined 8 months after laboratory grinding. Is this decline exponential? That is, should we expect in general that silicate powders have experimental dissolution rates that are two to three orders of magnitude higher than for natural weathering under similar pH and temperature regimes? Some experimental data apparently lead to an answer of "no" to this question; no significant difference between dissolution rates was found for some freshly ground silicates and natural mineral powders obtained from sieving soils (Drever and Clow 1995). More studies on aging powders should clarify whether this artifact effect is important. Perhaps the effect is more important for some silicates than others.

Abiotic rock rates for common silicate mineral compositions on the order of 10~5 mm yr_1 or less is supported by the following studies of natural weathering:

1. A rate of weathered rind development of less than 3 X 10 mm yr_1 can be inferred from Jackson and Keller's (1970) study on recent Hawaiian basalt flows (this from an area of moderate to heavy rainfall of 127 to 191 cmyr-1).

2. Rates less than 5 X 10~5 mm yr_1 are derived from a study of rind development on andesitic and basaltic clasts from the western United States from soils up to 500,000 years old (Colman and Pierce 1981).

Note that in both cases, some microbial involvement is probable, making the rates probable maximums for abiotic rates.

On the other hand, several factors would make the biotic soil/abiotic two-dimensional bare rock rate less than 1000:

1. Some regolith (broken-up rock) would be likely be present on an abiotic Earth surface, particularly in the early Precambrian, when more active volcanism likely ejected volumes offine ash and more frequent impacts continually pulverized the land surface. What the average or steady-state regolith cover would be (or its thickness and grain size) is a matter for conjecture (further discussion of the possible implications of an abiotic regolith effect is given in the Appendix). However, fine particles would be expected to be rapidly eroded by runoffand wind erosion, leaving a rocky surface, similar to many desert areas. Regolith would probably not form at all in mountainous regions, precisely those sites where the most rapid chemical denudation now apparently occurs because of a combination of mass movement exposing fresh rock and vegetation-anchoring soils. Furthermore, the process of Ostwald ripening, the preferential dissolution of smaller grains relative to larger because of the greater surface area per volume of the former, also may contribute to a reduction of reactive surface area/land area (Steefel and Van Cap-pellen 1990).

2. Weathering in permeable sediments and volcanics as well as in joints doubtlessly would occur in an abiotic surface regime. However, the role of microbially induced dissolution in these systems may be quite significant now and in the past. Recent research indicates a significant biotic enhancement ofdissolution, particularly for volcanic glass, even in marine conditions (Staudigel et al. 1995; Thorseth et al. 1992, 1995a, 1995b). Furthermore, the carbon source for bicarbonate production in many sediments is likely kero-gen; thus, the weathering reaction in this case and eventual deposition of carbonate in the ocean may actually constitute a net source ofcarbon dioxide to the atmosphere, rather than a sink. In any case, this flux of bicarbonate from aquifers is relatively small compared with that carried by the much greater discharge of rivers to the ocean.

In light of the latter factors, the ratio of 1000 may well be an upper limit to B. The lower limit for B, the present cumulative biotic enhancement of weathering on the Earth surface over the abiotic rate at the same temperature and atmospheric carbon dioxide level, is likely close to about 100 (however, even a factor of 10 is quite an enhancement).

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