Geology, Vol. 1 [of 3]

by Thomas C. Chamberlin; Rollin D. Salisbury

(1) Changes in the composition of the atmosphere.

The succession of modifications which the atmosphere has undergone from time to time through the action of life will be discussed as the earth’s history is followed in the second volume. It may suffice here to note briefly the chief ways in which the atmosphere has probably been modified by the agency of life, not only as regards its quantity but also as regards the proportions of its constituents.

The consumption and restoration of carbon dioxide.—As the fundamental food of the organic world, carbon dioxide has suffered enormous consumption in the course of the geological ages, and is now reduced to the very small proportion of .0004 or .0003 of the whole. At the outset it was probably one of the most abundant constituents; possibly even the chief one. It has been partially restored, concurrently with its consumption, by animal respiration, by certain classes of plant action, and by combustion and other forms of inorganic combination. This restorative action has been incomplete at all known stages of the earth’s history, and hence there has been constant loss of carbon dioxide. The inorganic processes which have also profoundly affected both the consumption and restoration of carbon dioxide are here neglected and discussed elsewhere.

The freeing and consumption of oxygen.—The oxygen of the atmosphere is actively consumed by animals and by plants, but on the other hand, it is set free abundantly by green plants, and hence its amount has probably fluctuated from time to time according to the state of balance between the organic processes of its production, and those of its consumption. The consumption of oxygen by organic processes is, however, little more than a reversal of the previous process by which it was set free; for instance, green plants in forming their food set free the oxygen of the carbon dioxide used for the purpose. When the organic substance so formed is ultimately consumed through plant or animal action or by inorganic means, an equivalent amount of oxygen reunites with the carbon to again form carbon dioxide. And so if the whole of the organic matter is returned to the inorganic state, no more oxygen is consumed than had been before set free in the process of forming the organic matter. But, as a matter of fact, a large amount of organic matter has not gone back completely to the inorganic state, and this residue constitutes a factor of no small importance in the geological record.

The organic residue.—There is a certain portion of vegetation that is not consumed by animals or by other plants, and that escapes combustion and all kinds of ordinary decay, and this constitutes a part of the organic residue. Animals never completely oxidize all the organic matter they take into their systems; their bodies never entirely consume themselves. A like statement may be made respecting those plants that feed on organic matter. That which animals and plants leave unoxidized is indeed more or less preyed upon by other animals and plants, and relatively little escapes final reoxidation, but there is a remnant, and this constitutes another part of the organic residue. The more conspicuous forms of the organic residue are found in the mucks, peats, lignites, coals, organic oils, and gases, but in addition there is not a little disseminated organic matter in nearly all the sedimentary rocks; in the aggregate, this probably amounts to more than the distinct organic deposits.

The meaning of the organic residue.—All the unoxidized, or incompletely oxidized, carbon in the organic residue implies that oxygen has previously been separated from this residual carbon by plants and given to the atmosphere, and hence has been a source of atmospheric enrichment in oxygen. The amount thus contributed is equal to that which is required to restore the residual carbon to its original state of oxidation. So, in a similar way, the unoxidized hydrogen in the organic hydrocarbons and like compounds implies that oxygen has been separated from the hydrogen of water and given to the atmosphere, and hence this also is a source of atmospheric enrichment in oxygen. It seems safe, therefore, to conclude that the action of life, taken as a whole, has increased the free oxygen of the atmosphere.

While not here under consideration, it is not to be forgotten that inorganic processes involving the same atmospheric constituents have been in operation concurrently with the organic processes, and that they have also affected the amounts and proportions of the atmospheric constituents. Rocks have been oxidized in greater or less measure at the expense of the atmospheric oxygen, and hence when the total atmospheric problem is considered, there arises the question whether the amount of oxygen in the atmosphere has been increased or diminished during geological history, when the balance is struck between the inorganic and the organic actions. The probabilities seem to us to strongly favor the view that organic action has preponderated, and that the oxygen has been increased beyond its primitive amount, but that it has fluctuated during known geological history. The reasons for this view will appear in the historical chapters.

The disintegration of the crystalline rocks and the solution of limestone have consumed much carbon dioxide, and this is to be added to the loss through organic action. On the other hand, there are inorganic processes that supply carbon dioxide, and hence when the larger problem of the atmosphere is raised, the factors become so complicated that their consideration is best deferred to the historical chapters. This passing reference may stand us in good part lest we forget, for the moment, the inorganic factors in the atmospheric problem.

The more inert factor.—Nitrogen in the free state is relatively inert chemically, and it does not appear that it can be used directly by the higher plants and animals in appreciable amounts. Certain bacteria, and perhaps certain algæ[288] and other low forms of plants, have the power of using free nitrogen, and this is a principal way in which it is put within the reach of higher plants. Nitrogen is also combined in small quantity in the atmosphere by electric action, and thus made available for plants. On account of the inertness of nitrogen and of the relatively limited amount required for organic purposes, the nitrogen of the atmosphere has been less consumed than the carbon dioxide. Besides this, the nitrogen compounds are very decomposable, and are very generally and completely returned to their original state. Deposits of nitrates or other nitrogenous compounds are relatively rare.

It is obvious that if there is any considerable source of supply concurrent with this slight loss, the amount of nitrogen in the atmosphere must have been increasing. We have seen that volcanoes give forth considerable quantities of nitrogen, and that this may be a real addition to the atmosphere, and not merely a return of the atmospheric nitrogen that had been carried down previously by underground-water. It has also been noted that crystalline rocks contain occluded nitrogen, which is doubtless freed by their disintegration. It is, therefore, not improbable that the nitrogen of the atmosphere has been increasing, both actually and relatively.