Environment subaerial

The causes for the different site of hydrothermal activity (submarine and subaerial environment) could be considered in terms of tectonic and geologic evolution of this metallogenic province from middle Miocene to Pleistocene. 

It is generally accepted that Kuroko deposits formed under the submarine environment, while polymetallic vein-type deposits in central and Northwest Japan (Ashio, Tsugu, Kishu, Obira, etc.) under the subaerial environment. 

In contrast, in Southwest Japan, polymetallic veins (so-called xenothermal-type deposits in the sense of Buddington (1935) or subvolcanie hydrothermal type in the sense of Cissartz (1928, 1965) and Schneiderhohn (1941, 1955) occur. Examples of these deposits are Ashio, Tsugu, Kishu and Obira. All these vein-type deposits have formed at middle Miocene age in western part of Tanakura Tectonic Line under subaerial environment. In these deposits, many base-metal elements (Sn, W, Cu, Pb, Zn) and small amounts of Au and Ag are concentrated. These deposits are associated with felsic volcanic and plutonic rocks along the Median Tectonic Line (MTL) or south of MTL. 

The deposits have been formed under the subaerial environment. No contribution of seawater and low /s2 and /02 conditions are consistent with the geologic environment. 

As noted already, the formation of polymetallic vein-type deposits and Kuroko deposits occurred under the subaerial and submarine environments, respectively, at nearly the same time (middle Miocene). 

It was shown in previous chapters that intense hydrothermal activities occurred in the Neogene age in and around the Japanese Islands under the submarine and subaerial environments. In this chapter the influence of these hydrothermal activities on the seawater chemistry, and the global geochemical cycle are considered. 

For instance, polymetallic vein-type deposits formed under the subaerial environment influenced by igneous and sedimentary components. 

Matthew RK (1968) Carbonate diagenesis Equihbration of sedimentary mineralogy to the subaerial environement coral cap of Barbados, West Indies. J Sed Petrol 38 1110-1119 McCulloch MT, Esat T (2000) The coral record of last interglacial sea levels and sea surface temperatures. ChemGeol 169 107-129... 

U and Th concentrations in secondary deposits precipitated from solution generally reflect relative abundances in the hydrosphere. Uranium is co-precipitated with CaCOs in subaerial environments on exsolution of CO2 (or evaporation), while the immediate daughter products are essentially absent. This represents extreme chemical fractionation of parent and daughter isotopes within the hydrosphere. 

The continuous availability of trillions of independent microreactors greatly multiplied the initial mixture of extraterrestrial organics and hydrothermal vent-produced chemicals into a rich variety of adsorbed and transformed materials, including lipids, amphiphiles, chiral metal complexes, amino add polymers, and nudeo-tide bases. Production and chiral amplification of polypeptides and other polymeric molecules would be induced by exposure of absorbed amino adds and organics to dehydration/rehydration cydes promoted by heat-flows beneath a sea-level hydro-thermal field or by sporadic subaerial exposure of near-shore vents and surfaces. In this environment the e.e. of chiral amino adds could have provided the ligands required for any metal centers capable of catalyzing enantiomeric dominance. The auto-amplification of a small e.e. of i-amino adds, whether extraterrestrially delivered or fluctuationally induced, thus becomes conceptually reasonable. 

The Plio-Peistocene activity is by far the best studied (Beccaluva et al. 1998 Trua et al. 1998). It took place during Lower-Middle Pliocene (4.9 to 3.5 Ma) and Upper Pliocene-Lower Pleistocene (2.4 to 1.5 Ma). The earlier products are principally submarine and are represented by volcano-clastic products, pillow lavas and minor subaerial lava flows. The Upper Pliocene-Lower Pleistocene activity erupted lava in subaerial to shallow marine environments (Di Grande et al. 2002). 

There were times on our planet when the barren dryness of uninhabited continents sharply contrasted with the densely populated sea. The continental lithosphere was then essentially represented by rock surfaces of different types. Sedimentary rocks were rare, if not absent. As rock materials became exposed to the subaerial environment at the Earth s surface, they encountered a whole range of environmental challenges such as temperature fluctuations, water, unbuffered cosmic and solar irradiation and atmospheric gases and solids instead of dissolved species. These influences resulted in rocks undergoing alterations in material properties leading to erosion and breakdown into ever-smaller particles and constituent minerals, formation of sandy sediments, and mineral soils (Ehrlich, 1996). Primordial terrestrial environments can therefore be visualized as a freshly exposed and only slightly physically pre-weathered rock surface. 

The strong survival potential of the rock biofilm indicates that the permanent presence of water is not the most essential attribute for the evolution and spread of life (Reysenbach Cady, 2001 Costerton Stoodley, 2003). The existence of a microbial biofilm on the rock surface is more determined by the interactions of the organisms with the mineral substrate. Lack of water even over several years is tolerated by rock biofilms. However sporadic the supply might be in subaerial conditions, some water from rain, snow, ice, dew, or fog is always present in terrestrial environments. However, for chemoorganotroph life forms the mineral substrate harbours additional difficulties as it is, or almost immediately becomes, deficient in organic matter, as nutrients and energy resources reach this habitat mainly from the atmosphere as particulates and volatile matter. This difficulty is partially overcome by the presence of EPS, which significantly increase the residence time of air-borne particles on any rock surface. 

Subaerial rock communities metabolize under conditions of limited water availability and high solar and cosmic irradiation levels and can be found even on desert rocks and at high altitudes. In these places, they have found an ideal environment that allows for a stressful but less competitive existence. In more favourable conditions, these communities are quickly succeeded by more developed but less stress-tolerant symbiotic lichen or... 

A concise picture of how life prospers on a subaerial rock surface may retrospectively explain how primary terrestrial environments were conquered with the help of biogeochemical adaptations. Adaptation mechanisms of modern rock inhabitants may provide important clues for understanding the processes of early land colonization. 

---