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Archean Aeon ##BEST##

By the Neoarchean, plate tectonic activity may have been similar to that of the modern Earth, although there was a significantly greater occurrence of slab detachment resulting from a hotter mantle, rheologically weaker plates, and increased tensile stresses on subducting plates due to their crustal material metamorphosing from basalt into eclogite as they sank.[15][16] There are well-preserved sedimentary basins, and evidence of volcanic arcs, intracontinental rifts, continent-continent collisions and widespread globe-spanning orogenic events suggesting the assembly and destruction of one and perhaps several supercontinents. Evidence from banded iron formations, chert beds, chemical sediments and pillow basalts demonstrates that liquid water was prevalent and deep oceanic basins already existed.

archean aeon

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The Archean atmosphere is thought to have nearly lacked free oxygen; oxygen levels were less than 0.001% of their present atmospheric level,[20][21] with some analyses suggesting they were as low as 0.00001% of modern levels.[22] However, transient episodes of heightened oxygen concentrations are known from this eon around 2,980-2,960 Ma,[23] 2,700 Ma,[24] and 2,501 Ma.[25][26] The pulses of increased oxygenation at 2,700 and 2,501 Ma have both been considered by some as potential start points of the Great Oxygenation Event,[24][27] which most scholars consider to have begun in the Palaeoproterozoic.[28][29][30] Furthermore, oases of relatively high oxygen levels existed in some nearshore shallow marine settings by the Mesoarchean.[31] The ocean was broadly reducing and lacked any persistent redoxcline, a water layer between oxygenated and anoxic layers characterised by a strong redox gradient, that would become a feature in later, more oxic oceans.[32] Despite the lack of free oxygen, the rate of organic carbon burial appears to have been roughly the same as in the present.[33] Due to extremely low oxygen levels, sulphate was rare in the Archean ocean, and sulphides were produced primarily through reduction of organically sourced sulphite or through mineralisation of compounds containing reduced sulphur.[34] The Archean ocean was enriched in heavier oxygen isotopes relative to the modern ocean, though δ18O values decreased to ones comparable to those of modern oceans over the course of the later part of the eon as a result of increased continental weathering.[35] 041b061a72


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