Certaines formes de sédimentation calcaire quaternaires sont-elles dues à des déséquilibres géochimiques périodiques?

Abstract

Very striking is the occurrence of limestone deposits in various parts of the world which can be dated at the end of the various Quaternary glaciations: limestone crusts (“caliche”) of subtropical dry countries (Mediterranean countries, Mexico), travertines, limestone breccias and conglomerates in alluvial deposits, phreatic precipitation of limestone, beach rock (tropical and subtropical coasts with a dry season) and coral reefs. The sedimentary process involving the bulk of these types of sedimentation is seriously slowed down now, even totally stopped (limestone hard pans and crusts). This is interpreted as the consequence of a world-wide geochemical disequilibrium due to climatic changes and glaciation. The following model is proposed: 1. (1) During glacial growth: 1.1. (a) A large volume of CO 2 is imprisoned in glaciers. A higher percentage of carbon dioxide occurs in ocean water as result of the diminution of temperature. 1.2. (b) A serious decrease of the total biomass in the world resulting from climate change returns a certain amount of CO 2 to the atmosphere through organic matter. 2. (2) During glacial recession: 2.1. (a) Melting glaciers return a large volume of CO 2 directly into the air, and sea water as well. Presently, an abnormal percentage of carbon dioxide has been measured in the atmosphere in western Greenland, near the glacier margin. 2.2. (b) The growth of the biomass as a result of more favourable climates both in oceans and on the land results in the incorporation of higher quantities of carbon dioxide in living beings (shells, corals, algae, trees, grass), for a shorter or longer duration. Physical phenomena, such as changes in ocean temperatures and changes in the glacial balance occur simultaneous with biological changes as a result of the effect of glacial oscillations upon climate. This results in geochemical disequilibriums caused by insufficiencies of adjustment in the processes quoted above. The duration of the disequilibriums is short: only some millenia. Geomorphic processes, which are a negative rètroaction, act towards a reduction of these equilibriums. The existence of a higher content of CO 2 in the atmosphere favours mobilization of limestone because of more intense weathering. Continental waters have a higher carbonate content and precipitation is easier where favourable conditions exist. The result is the formation of limestone hard pans, breccias, travertines, etc. Where these condition do not exist, the continental waters reach the oceans and contribute to a higher carbonate content of sea water. This results in a better growth of corals and shells and in the construction of atolls and beachrock reefs during the Flandrian transgression. At the end of the glacial recession the forming of carbonates is precipitated in the oceans by the rise of sea-water temperature. The rhythmic paroxysms of carbonate sedimentation during the Quaternary Period are a form of regulation of the CO 2 content of the atmosphere which have prevented excessive changes of this content as a consequence of glaciations and climatic changes.