biogenic weathering

biogenic weathering Over geological time-spans (more than 10 000 years) silicate weathering reactions have controlled the movement of carbon dioxide between the atmosphere and the oceans, and during the past 400 million years the animals and plants of a region (the biota) have played an important role in the process. The weathering of calcium and magnesium silicates is the primary ‘sink’ for atmospheric carbon dioxide (CO₂) over geological time. James Lovelock considers that amplification of weathering caused by the appearance of plants and animals in the Precambrian almost certainly caused a decrease in atmospheric CO₂ levels. Although the cause and effect linkages are fairly straightforward, the precise magnitude of the shift from abiotic to organically mediated weathering is uncertain. It has been argued that the effect of biotic enhancement is to increase weathering by at least one order and perhaps more than three orders of magnitude. This uncertainty about biotic enhancement is a critical problem in models of early climate, since large-scale shifts in atmospheric CO₂ levels would probably, because of the greenhouse effect, result in temperature excursions. A minor biotic enhancement of weathering would imply that global temperature changed relatively little with the colonization of land by animals and plants. A high biotic weathering enhancement would, on the other hand, point to a substantial lowering of temperature. For example, if biotic weathering were ten (or a hundred) times faster than abiotic weathering, then the Earth would be 15 °C (or 30 °C) warmer than at present. The order-of-magnitude uncertainty in the biotic effect thus makes it difficult to model global habitability over geological time. It is thus important to determine the order of magnitude of the acceleration effect on biogenic weathering by soil microorganisms, lichens and vascular plants. This is, however, not a straightforward question to answer: the factors that control the weathering of the Earth's crust are complex, often coupled, and as a result are understood at the field scale only in a semiquantitative sense. Soils rich in organic matter commonly have high CO₂ pressures and contain abundant organic acids, and are commonly warmer than soils that are not; soils exposed to heavy rainfall commonly have high organic activity. All these conditions would result in accelerated weathering. Weathering rates increase with temperature, ambient moisture, and organic activity, but the derived dependencies are somewhat approximate because no two weathering basins are the same in a mineralogical, hydrological, or biological sense. Measurements of abiotic versus biogenic weathering suggest that abiotic weathering is proportional to rainfall and that biogenic weathering is far more sensitive to rainfall. Abiotic weathering is therefore limited by the availability of weathering solutions, and the weathering is further enhanced in the presence of the biota because of the effects of three possible factors: the ability of organic acids to form complexes; the extended retention of moisture in the pore walls because of the presence of a microflora; and the effect of the biota in increasing the surface area that is being weathered.

K. Vala Ragnarsdottir