Title : How subsurface waters record the earth’s history
Abstract:
Author supposes that formation waters inherit chemical composition of surface water from sedimentary basin. Salinity of ancient basins is a reason to explain the formation water (sedimentary water) composition in many cases. Hydrochemical analysis of lithological sections for every basin by excluding the factors in connection with infiltration and evaporation gives the possibilities to compare the waters of “sedimentary origin” in the geological time and space. The principal tendencies are the decreasing in Cl and Ca content during geological time and difference in water salinity between Northern and Southern Hemispheres.
Data on the distribution of strong brines (up to 200 g/l and above) in the Precambrian rocks of the Canadian, Scandinavian shields, crystalline massifs of the East European and Siberian platforms are widely known. Against this background, the data on the chemical composition of waters in the crystalline massifs of the African continent, where mainly fresh water is distributed, appear to be a sharp dissonance. Comparative study of these continents suggests that the geological history of the African continent has been particularly favourable for the accumulation of fresh and, in some cases, ultra-fresh (mineralization less than 0.1 g/l) waters and their preservation, despite the harsh arid conditions of the region. The snowball Earth hypothesis is suitable to explain this situation. Later, the accumulation of fresh water on the African crystalline massifs occurred on the Ordovician glaciation in the period 440-420 million years ago. During this period, the areas of Gondwana, located in the Southern Polar zone, were covered with powerful glacial deposits. Hypersaline brines in sedimentary basins of the Northern Hemisphere are opposite to the fresh waters of African crystalline massifs and their peripheral sedimentary basins. Statistical hydrochemical parameters of the Northern American and Eastern European Platforms show their similarity. Decreasing in mineralization (from 300 to 200 g/l) was in connection with melting of ice and global evaporation processes that occurred on the Late Paleozoic time. Salt deposition works as a machine that removes salts from the hydrosphere.
Mesozoic hydrosphere was lager in volume than Paleozoic and water salinity was decreased dramatically. Salinity of formation waters are 30-150 g/l, Ca content is 5-10 g/l. Extended Mesozoic sedimentary basins are comfortable to study the influence of hydrothermal injection into sedimentary sections. Formation waters composition of Cenozoic Basins reflexes the diversity of sedimentary basins. Salinity of many basins is close to the Ocean salinity (35 g/l).
Formation waters record the processes of poles migration, climate changes and volume of hydrosphere. Transition of water to ice - removing fresh water from the hydrosphere and, in the contrary, opposite processes: origin of brines and later carbonate and salt deposition –removing Cl and Ca from the seawater. These global events have important implications not only for modelling of the chemical history of the ocean and sedimentary basins, but also for the explanation of extinction periods, and the many aspects of geology that depend on the composition of the oceans and their peripheral basins.
