Archean eon

Past depletions in dissolved inorganic phosphate may have also limited nitrogen-fixation and primary production due to high adsorption of phosphate by iron-rich minerals. Indeed, based on adsorption isotherms in banded iron formations, sea-water phosphate concentrations in the Archean have been suggested to have been more than ten fold lower than the present values (0.15-0.6 pM versus the modem value of 2.3 pM) (Bjermm and Canfield, 2002). Thus, present restrictions of phosphate on N2 fixers in some oligotrophic areas such as the western subtropical Atlantic (Mills et ai, 2004) and in the tropical Pacific (Moutin et al., 2007) may have correlations in the paleoceanographic records of the late Archean eon. 

North America was not formed in one piece, or at one time, the way a cake is baked from batter. Various parts of it were formed all over the world, at various times over four billion years, and were brought together and assembled into one continent by the endless process of plate tectonics. What is now called North America began to form in the first two and one-half billion years of Earth s history, a period of time called the Archean eon. 

In the late Archean eon, the plates of Earth s crust may have continued to move at a relatively high speed. Evidence of these wild times can be found in the ancient core of North America. The scars of tectonic events appear as rock outcrops throughout the part of northern North America called the Canadian Shield. One example of this kind of scar, a greenstone belt, may be the mangled remains of ancient island arcs or rifts within continents. Gold and chromium are found in the greenstone belts, and deposits of copper, zinc, and nickel. Eorma-tions of iron ore also began to form in the Archean eon, and fossils of microscopic cyanobacteria-the first life on Earth—are found imbedded in them. 

Archean eon—The interval of geologic time from 3.8 billion years ago to 2.5 billion years ago. 

What is the most difficult research job in all of science It is the task of deducing the history of Earth s evolution in its first two billion years, during the Hadean eon from 4.5 to 4.0 Ga (IGa = 10 years before present) and the Archean eon from 4.0 to 2.5 Ga. Detectives of ancient rocks are faced with a lack of clues to investigate. Most of Earth s surface is covered by water. The cores of the continents are probably underlain by rocks older than 2.5 Ga but they are hidden by a veneer of younger sediments. Even in localities where Hadean-Archean rocks are readily available, alteration by subsequent geologic events has obscured the record of conditions under which they were formed. It is revealing to compare and contrast the detailed information available in a textbook of Earth s modem ecologies with the sparse collection of facts accepted by disciplinary consensus for the Hadean-Archean eras. 

The heat flow from Earth s interior was substantially greater during the earlier Precambrian (e.g. Lambert 1976). During the past 3.0 billion years, radioactive decay of the elements El, Th and k has been the principal source of this heat, therefore their decay over time has caused global heat flow to decline. Heat flow 3.0 billion years ago has been estimated to have been 2.2 times its modern value (Turcotte 1980), therefore the midocean ridge mantle CO2 flux was substantially larger during the Archean Eon (discussed later). 

Recycling of old oceanic crust (option 2 above) is the most attractive alternative because subduction of oceanic crust into the mantle is an important tectonic process that has been occurring on the Earth since the Archean Eon. 

Archean Eon The period of time between the occurrence of the oldest rocks on the Earth (around 4000 million years ago prior to which is called the Hadean Eon) and the base of the Proterozoic eon at about 2500 million years ago. 

L. Margulis, Symbiosis in Cell Evolution Microbial Communities in the Archean and Proterozoic Eons, Freeman, New York, 1993. 

Precambrian era—The combined Archean and Proterozoic eons, the first four billion years of Earth s history. 

Archean Terrestrial geologic eon that extends from the Hadean (3.8 Ga) until... 

Metasedimentary rocks formed in these two eons extending from 590 to 4,000 Ma B.P., but differ both in lithological abundances and geochemistry, for instance their Sr/ Sr ratios in carbonates are <0.703 for the Archean and <0.704 for the post-Archean. As might be expected almost all such rocks have undergone extensive metamorphism. Carbonates have been used to evaluate the history of the isotopic composition of seawater. As postdepositional alteration is... 

Proterozoic Eon The period in Earth history between the Archean and the Phanerozoic eons (2500-542 million years... 

Also motivated by the desire to constrain further the history of oxidation of the atmosphere and oceans, Frei et al. [37] determined the isotopic compositions of Cr in banded iron formation (BIF) samples, representing the Archean (>2.5 Ga ago) and Proterozoic (2.5 Ga to 542 Ma ago) Eons. They anticipated that isotopic variations in those rocks would correlate with changes in global redox conditions over time because of previous work reporting isotope ratio variations correlated with the Cr oxidation state in natural samples [38, 39], experiments [40, 41], and theoretical calculations [42]. 

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