Using Rock Dust Helps To Control Atmospheric CO2 Buildup and the Greenhouse Effect

by Donald J. Supkow, PhD

Introduction

The atmospheric CO2 content has varied significantly during the past 150,000 years oscillating between about 200 and 300 ppm (Barnola et al, 1987). However, during the period between 1959-1989, the atmospheric CO2 content has risen from about 315 to 350 ppm (Keeling et al, 1989), with 1995 projected to average about 360 ppm. The modern rate of atmospheric CO2 buildup, therefore, is many times greater than previous rates.

The scientific community estimates that global temperatures are currently rising at the rate of about 0.45 degrees C. per century (Balling, 1992). There is still some controversy regarding the rate of global temperature rise and whether global temperatures may actually be decreasing or about to start decreasing (Hamaker et al, 1982).

Public perception is that the global temperature rise is caused by the burning of fossil fuels and forests which in turn is raising the atmospheric CO2 content faster than natural processes can remove the excess CO2 from the atmosphere. The concept that global temperature rises are caused by atmospheric CO2 build up is referred to as the Greenhouse Theory (Arrhenius, 1896; Chamberlin, 1898; Plass, 1956; Budyko and Ronov, 1979; Fischer, 1984). Other factors which are believed to be contributing to global warming are deforestation, desertification, urbanization and the buildup of greenhouse gases other than CO2 (Balling, 1992). In modern times atmospheric scientists have developed sophisticated computer models which attempt to predict how much global temperatures will rise at various assumed rates of atmospheric CO2 buildup (Krause et al., 1989; Hough ton et al., 1990, Annex).

Scientists world wide express concern as to how global climates may change as a result of the atmospheric CO2 buildup (Mintzer, 1992). Anticipated changes include overall global warming, global cooling, or dramatic changes in weather patterns such as increased storm violence or shifting rainfall belts. Most environmental scientists generally agree, however, that atmospheric CO2 buildup is not good for the environment. Scientists and environmental activists generally propose that one way to reduce the rate of atmospheric CO2 buildup (and presumably thereby reduce the rate of global warming) is to reduce the rate of burning of fossil fuels (Leggett, 1990; Ehrlich and Ehrlich, 1991; Mintzer, 1992). This paper will present evidence based on published data which indicates that it is technically feasible to halt the present buildup of atmospheric CO2 content without reducing the existing rate of fossil fuel consumption.

The Global Carbon Cycle

The pioneering work of Berner (1991) has shown in a mathematically rigorous manner that under natural conditions the atmospheric CO2 content is controlled primarily by the rate of weathering of silicate rocks. Berner (1991) showed that there are about 15 factors which control the atmospheric CO2 content but indicated that the primary control factor is the rate of weathering of silicate rocks, such as granite, basalt, gneiss and extrusive volcanic rocks. Silicate rocks contain significant quantities of calcium and magnesium, As the silicate rocks weather, the calcium and magnesium combine with atmospheric CO2 to ultimately form calcium carbonate and magnesium carbonate (Urey, 1952; Berner, 1991) which is deposited primarily in the oceans in the form of limestone and dolomite deposits. The CO2 is ultimately returned to the atmosphere by volcanic eruptions.

Since the silicate rocks weather and remove CO2 from the atmosphere, this means that the faster the silicates rocks weather, the faster CO2 is removed from the atmosphere thereby causing the CO2 content of the atmosphere to decrease (Berner, 1991). Berner (1991) gives several factors which control the rate of weathering of silicate rocks, the primary factors being the total land area exposed above sea level and the type of vegetation growing on the land. Under this scenario, it is obvious that with a lower sea level, the total land area of the continents above sea level is larger and therefore the area of silicate rocks exposed to weathering is greater. Vegetation growing on the land produces organic acids (Berner, 1991) which percolate downward and increase the rate of weathering of the underlying silicate rocks situated within the zone of percolating ground water. Berner (1991) indicates that the rate, of weathering increased in geologic time because of: (1) the evolution of vascular plants which produce more organic matter and organic acids than primitive plants such as moss and lichens, and climaxed with 2) the evolution of angiosperms broad leafed trees) which produce a prodigious quantity of organic debris which falls to the ground, decomposes and then produces significantly greater quantities of organic acids which weather the underlying silicate rocks it a faster rate than plants such as mosses, ferns and gymnosperms (needle bearing trees).

The angiosperm trees not only (1) serve as a mechanism to increase the rate of weathering of silicate rocks, thereby releasing calcium and magnesium which combine with atmospheric CO2 to ultimately form limestone and dolomite deposits in the oceans, they also (2) serve as a mechanism to remove CO2 from the atmosphere and lock it up in the form of organic debris which is buried in marine sedimentary deposits which ultimately form fossil fuels (Berner, 1991). The quantity of carbon stored in trees and surface vegetation (560 billion tons) and soil (1400 billion tons) is significant, but is small compared to the quantity of organic carbon stored in marine sediments (12,000,000 billion tons) (Hoffert, 1992). Although trees recycle much organic matter, part of the carbon which is fixed by trees in the form of leaves and other plant parts is eroded from the soil every year and reaches the ocean in the form of organic debris (Berner, 1991).

Increasing the Weathering Rate of Silicate Rocks

Under the natural scenario modeled by Berner (1991), the silicate rocks are weathered by organic acids which percolate downward from the overlying surficial soil zone. It is therefore easy to visualize that unweathered silicate rocks at depth below the land surface have the highest content of calcium and magnesium and that weathered rocks (including weathered alluvial soils) at and near the land surface have the lowest content of calcium and magnesium. This weathering process reaches its climax in tropical rainforest areas which develop laterite soils that are virtually devoid of calcium and magnesium. One can therefore visualize an historical process whereby the evolution of a rainforest is accompanied by the evolution of a laterite soil and a deepening of the interface between unweathered and weathered silicate bedrock. Because laterite soils have a high clay content and therefore a relatively low hydraulic conductivity, one can intuitively expect that the rate of weathering of the underlying silicate bedrock will tend to decrease with time as residual clay soil accumulate overlying the unweathered bedrock.

Considering the natural scenario of silicate rock weathering, one can visualize that: (1) the greatest potential rate of silicate rock weathering exists within residual top soil at the land surface because the top soil has the highest content of organic matter and microbial activity, which, according to Berner (1991), increases the rate of weathering of silicate rocks compared to underlying zones, and (2) under natural conditions the top soil zone has the lowest actual rate of silicate rock weathering because the top soil contains weathering byproducts rather than unweathered silicate bed rock.

The global rate of silicate rock weathering can therefore be increased artificially by adding finely pulverized silicate rock (commonly called rock dust in Remineralize the Earth) to vegetated soil at the land surface world wide. Weathering of the pulverized silicate rock in the soil will then extract CO2 from the atmosphere and lock it up in the formation of calcium carbonate and magnesium carbonate deposits thereby reducing the atmospheric CO2 green house effect.

The average igneous rock contains 3.49 % Mg0 and 5.08% CaO (Pettijohn, 1948). The weathering of 1 ton of pulverized igneous rock when added to soil will therefore lock up 0.0327 ton of atmospheric carbon. The burning of fossil fuels and forests contributes about 3.4 billion tons of carbon to the atmosphere annually (Oeschger and Mintzer, 1992). It would therefore require the weathering of about 106 billion tons of pulverized silicate rock per year to remove the 3.4 billion tons of excess carbon from the atmosphere each year. With a total global land area of about 33 billion acres (exclusive of Antarctica and Greenland) this would require adding a maximum average of about 3.2 tons of pulverized silicate rock per acre per year world wide. Obviously, adding pulverized silicate rock to desert areas would not be very effective because of the scarcity of precipitation and plant life in deserts. Therefore areas of rapid vegetation growth such as tropical rainforests would require higher application rates to maintain the annual 3.2 tons/acre average rate. Such a project of adding pulverized silicate rock to soils world wide at an average rate of about 3.2 tons/acre is feasible from an economic and a technical view point. For starters, pulverized silicate rock is available as a low cost byproduct of rock quarries world wide. In New Jersey, U.S.A., pulverized basalt (locally called stone dust or mill ends) costs about $10.00/ton delivered within about 50 miles of the basalt quarries. When this supply of rock quarry byproducts is used up, crushed stone can be pulverized at an incremental cost.

In reality, significantly less than 3.2 tons of pulverized silicate rock per acre of soil would be required to be added each year to halt the buildup of atmospheric CO2 because the weathering of the silicate minerals in the top soil releases trace elements which increase the productivity of tree and plant growth (Hamaker et al, 1982), sometimes by as much as a factor of four times. The increase of plant productivity would increase the storage of carbon in vegetation living above ground and in the underlying soils as well as increase the rate at which organic debris is deposited in ocean sediments. When factoring in the increased productivity of plant life, it can therefore be estimated that the average amount of pulverized silicate rock required world wide to halt the buildup of atmospheric CO2 is in the range of 0.8 to 3.2 tons per acre, assuming a halt to world wide deforestation. Increases in the value of agricultural crops and timber products resulting from adding pulverized silicate rock to soils would offset part of the cost of applying the pulverized silicate rock to the soils world wide. When other benefits of soil remineralization are factored in, such as saving dying forests, reducing world wide hunger and disease, averting disastrous climatic changes, and saving precious top soil, the cost of worldwide soil remineralization can be repaid many times over.

Acknowledgements

The author is indebted to Prof. B. H. Wilkenson of the University of Michigan Department of Geological Sciences for introducing me to the work of Robert A. Berner which forms the rigorous basis for demonstrating that adding pulverized silicate rock to soils can control atmospheric CO2 Special thanks to Don Weaver for reviewing the manuscript.

References