Introduction

The Paleozoic Era, which extended from about 540 million years ago to about 240 million years ago, was a time of proliferation of plant and animal species. The early part, the Cambrian Period, was hot and humid but temperatures declined in the Devonian Period. It was also in that period that land plants developed leading to extensive forestation during the Carboniferous Period. One of the most important climatic episodes of the Paleozoic Era was the glaciation that occurred about 285 million years ago, the Carboniferous-Permian Glaciation. This episode requires some explanation.

A depletion of the CO₂ in the atmosphere to sufficiently low levels and the subsequent reduced greenhouse effect could account for a cooling which would lead to the glaciation. The depletion of CO₂ would generally have to come from an accelerated rate of net absorption. The development of deep-rooted plants preceded the glaciation so that opens the possibility of a causal connection with the cooling. Robert A. Berner feels he has found the process that connects the development of deep-rooted plants and the increased absorption of CO₂.

Berner asserts that deep rooted plants increase the rate of weathering of silicate rocks because of the following phenomena:

• Rooted plants keep moisture in contact with rock surfaces by holding clayey soil over rock surfaces.
• Plant rootlets secrete organic acids that chemically react with the rocks leading to calcium and magnesium ions and carbonate ions which ultimately produce carbonate rocks, thus reducing the CO₂ in the atmosphere.
• The decay of plant material produces carbonic acid that also chemically reacts with the silicate rocks and ultimately produces carbonate rocks, also reducing the CO₂ in the atmosphere.

This process is an alternative to the more naive attempt to explain the depletion of CO₂ by plants through photosynthesis and the burial of organic matter. This theory is not satisfactory because the buried organic material dates from a time after the CO₂ declined.

Berner created a mathematical model of this process, which he called GEOCARB. This model with parameter values estimated by Berner predicts a decline of CO₂ as a result of increased coverage of upland area by deep rooted vegetation. The results of the GEOCARB modeling by itself would probably not be very convincing. The modeling results need corroboration from empirical data. In the case of climatology empirical data is often difficult to obtain but Berner found two sources of corroborating empirical evidence for his thesis.

Experimental Evidence of Deep-Rooted Plants Accelerating Chemical Weathering of Silicate Rocks

Berner cites in footnotes to his article two experiments which confirm the deep-rooted plants vastly increase the rate of breakdown of silicate rocks by carbonic ions. The first experiment was by B.T. Bormann and associates at Hubbard Brook, New Hampshire. The researchers measured the amount of calcium lost in drainage and biomass for two soil parcels, one covered with moss and lichens and the other covered with pine trees. The calcium loss from pine covered soils was ten times that of soil covered with moss and lichens. The second experiment was by Berner himself and K. Mouton. They compared the calcium and magnesium loss from adjacent plots in western Iceland, one covered with trees and the other with bare ground or moss and lichens. They found the ratio of the loss from tree covered soil to be five to ten times higher than that of soil which is bare or covered with moss and lichens.

Empirical Evidence Concerning Paleo-Levels of CO₂

Berner sought empirical confirmation of a decline in the atmospheric CO₂ levels in the Devonian Period (400 to 360 million years ago) before and during the Carboniferous-Permian Glaciation. He found it from two sources, carbon isotope ratios and fossil plant leaf evidence. Carbon atoms usually consist of nuculei containing six protons and six neutrons, but a small proportion of carbon nuculei contain seven neutrons instead just six. The ratio of the rarer ¹³C to the more common ¹²C in limestone is affected by plants.

Another source of data for estimating the paleo-CO₂ levels is the density of plant stomata. Stomata are the tiny holes in leaves which let in CO₂. These holes also let out moisture bringing the risk of plant dehydration. For a given combination of atmospheric CO₂ levels, temperature and humidity there is an optimal density of stomata. A plant variety that has the optimal density of stomata has a decisive survival advantage over plant varieties with non-optimal stomata density. Therefore the plant varieties within a species that will be dominant are the ones with the optimal stomata densities. By comparing paleo stomata densities with current ones reserchers can make some estimate of the relative CO₂ levels in past times comparaed with the present. There are two problems with this method of estimating paleo-CO₂ levels. First, the stomata density depends not just on CO₂ levels and second, many of these fossil plants are now extinct. Research have tried to implement this method by using the closest living relative plant species. This introduces another element of possible error into the estimates could make those estimates virtually worthless.

The graph below is reproduced from the graph in Berner's article and shows the empirical estimates of the ranges of CO₂ relative to the present without distinguishing among the various researchers and their methods of estimation. The only clear result is a relatively low level of CO₂in the period of about 350 million years ago to about 275 million years ago.

Berner asserts that his GEOCARB model predicts the results shown in the graph below, but he apparently has not released the details and parameter estimates for GEOCARB for other researchers to confirm his results. It is very difficult to see how his model as he describes it in general could produces the ups and downs of short term fluctuations in the CO₂ levels. Normally such models produce smooth curves and are considered successfully if they predict the proper trends. It is a major accomplishment of such models to predict a reversal of trends. Berner gives measures of uncertainty for the estimates but does not tell how these limits are arrived at other than saying they are based upon sensitivity analysis. But an analysis of the sensitivity of the model results to variation in parameters tells us nothing unless we have measures of the uncertainties of the parameters. Usually for dynamic models the uncertainties grow with the distance from the base case. This is not the case with Berner's uncertainty estimates. Berner's uncertainty estimates seem to be simple multiples of GEOCARB result.

References:

• Berner, Robert A. (1997): The Rise of Plants and Their Effect on the Weathering and Atmospheric CO₂. Science, 276, 544-545.
• Berner, Robert A. (1998): The carbon cycle and CO₂ over Phanerozoic time: the role of land plants. Philosophical Transactions of the Royal Society of London, B 353, 75-82.
• Berner, Robert A. (1999): A New Look at the Long-term Carbon Cycle. GSA Today, 9, 1-6.
• McElwain, Jennifer C. and William G. Chaloner (1995): Stomata Density and Index of Fossil Plants Track Atmospheric Carbon Dioxide in the Palaeozoic. Annals of Botany 76, 389-395.
• Retallack, Gregory J. (1997): Early Forest Soils and Their Role in Devonian Global Change. Science, 276, 583-585.