Chapter 8 Precambrian Earth and Life History— Hadean and Archean The Precambrian lasted for 4 b.y., 88% of estimated geologic time. No rocks are known from the first 640 million years of geologic time, though evidence suggests their existence.

Hadean Early Proterozoic Archean Middle Late Precambrian Terminology in North America Phanerozoic

about 4.6 billion years ago Earth was a rapidly rotating, hot, barren, waterless planet, bombarded by comets and meteorites, with no continents, widespread volcanism yielded a thin atmosphere of volcanic gases, unable to shield from cosmic and UV radiation. As Earth cooled, water vapor in atmosphere condensed, infilling topographic lows as new oceans. 3

Hadean crust was likely thin, unstable, and ultramafic. Rotation of the Earth is an hypo- thesized origin of vertical convection currents within the upper portions of the molten outer mantle. As Earth cooled, more crustal material solidified. Each time ultramafic and mafic crust was subducted, a little more silica and aluminum was “liberated” by partial melting. Re-erupted andesitic lavas helped “build” the earliest island arc systems. Collisions of island arc systems “built” the nuclei of earliest continents. Oldest zircons in metamorphic rocks 4.2 b.y. only remnants of earliest igneous rocks. Oldest rocks include 3.96 b.y. Acasta Gneiss in Canada, other rocks in Montana. 4

Continuation of subduction and partial melting further contributed intermediate and later felsic rocks to the growing continents. Exposed Precambrian rocks are referred to as “Precambrian shields”. Surrounding the shields are platforms of covered Precambrian rocks. Shield + platform = craton. Most cratons have experienced little deformation since the Precambrian ended. Exposed North American craton = Canadian Shield. Canadian shield and adjacent platform consists of smaller cratons and Early Proterozoic “deformation belts” composed of plutonic, sedimentary, and metamorphic rocks. 5

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Primary Archean rock associations – Greenstone Belts & Granite-Gneiss complexes. Subduction complexes become Granite- Gneiss complexes. Island Arc and Back-Arc Systems become Greenstone belts. 7

Idealized Greenstone se- quence – volcanics most common in lower and middle units. Low grade metamorphism altered some Fe – Mg minerals to chlorite. Underwater volcanism yields pillow lavas while pyroclastics are usually erupted above sea level. Ultramafic flows suggest much higher (300 0 C) mantle and surface temperatures. 8

Many of the sedimentary rocks are successions of graywacke (sandstone with abundant clay and rock fragments) and argillite (a slightly metamorphosed mudrock). Other sedimentary rocks include turbidites showing cross-bedding and graded bedding; and quartz sandstones and shale, suggesting delta, tidal-flat, barrier-island and shallow marine deposition. Accretion of Greenstone Belts & Granite-Gneiss Complexes onto cratons = Archean-type Crustal Evolution. Was “completed” in South Africa – 3 b.y. ago and “completed” in North America 2.95 to 2.45 b.y. ago. 9

Accepted model of Greenstone volcanism and sedimentation occurred in back-arc extensional basins that later closed during compression. During compression – back-arc is folded, faulted, & intruded by rising plutons. Sea of Japan – modern example. 10

Archean Plate Tectonics in contrast to later on… Plates likely moved faster due to residual heat from Earth’s origin, radiogenic heat, faster rotation - magma was generated more rapidly. Rapid movement of plates – continents grew rapidly along margins (continental accretion) as plates collided with island arcs and other plates. Deformation belts between cratons offer evidence. Also, ultramafic lava flows were more common due to the higher temperatures. There is little evidence of passive margin sedimentation during Archean. By end of Archean – 30% to 40% of present crust is thought to have existed. 11

Last major Late Archean event = Deformation of the southern Superior craton, was part of a more extensive orogeny that formed the Superior and Slave cratons and other cratons now in the older parts of the Canadian shield. Archean Atmosphere and Hydrosphere Modern Atmosphere N %H 2 O0.1 to 4.0% O CO % Ar 0.93O % Ne Others Earliest atmosphere may have been hydrogen & helium, lost to space prior to magnetic field. 12

How the Rock Cycle gave birth to the Water Cycle. Water vapor released from lava flows, when Earth cooled sufficiently, water vapor con- densed, filled-in low areas (late Hadean?). 13

Development of magnetic field and gravity helped atmosphere to accumulate. In addition to water vapor, early atmosphere included carbon dioxide, methane (CH 4 ), Ammonia (NH 3 ), nitrogen gases, sulfur gases, etc.. Gases + rainwater = acids, which began to chemically weather the exposed lava flows. Earliest oceans were likely fresh water, but very acidic. Minerals had not weathered sufficiently to contribute salts to oceans. Evidence suggests the lack of free oxygen (O 2 ) due to oxygen’s reactivity, i.e., reducing conditions prevailed as is shown in sediments. Conditions likely remained constant through the Archean. Extent of Archean oceans difficult to estimate. 14

Two processes which release free oxygen into the atmosphere: Photochemical dissociation – UV radiation separated water molecules releasing oxygen. When certain levels of oxygen are reached, UV radiation forms ozone, which in-turn blocks UV radiation, thus this process accounts for a small amount of oxygen. Photosynthesis - Appearance of simple single- celled plants and photosynthetic bacteria began converting atmospheric CO 2 and releasing O 2 as a byproduct. These two processes probably produced no more than 1% of today’s free oxygen level, by the end of the Archean. 15

Life in the Archean Fossil Bacteria from Archean rocks to 3.5 billion years old. Carbon isotope ratios in rocks in Greenland (3.85 billion years old) suggest presence of life. Minimum requirements for our definition of life = metabolism & reproduction. Within hosts, viruses are alive, but are dormant outside of hosts. In 1924, Russian biochemist, A.I. Oparin, postulated that life originated when Earth’s atmosphere had little or no free oxygen. Oxygen is damaging to Earth’s most primitive living organisms, some types of bacteria must live where free oxygen is not present. 16

The origin of life has 2 requirements: A source of appropriate elements for organic molecules – volcanic gases and dissolved minerals. Energy sources to promote chemical reactions – Naturalism (UV, lightning) or Creation? All organisms are composed mostly of carbon (C) hydrogen (H), nitrogen (N), oxygen (O) All of which were present in Earth’s early atmo- sphere as: Carbon dioxide (CO 2 ), water vapor (H 2 O), nitrogen (N 2 ), possibly methane (CH 4 ), and possibly ammonia (NH 3 ) 17

These elements can combine to form simple compounds, i.e., the building blocks of life. During the late 1950s Stanley Miller synthesized several amino acids by circulating gases approximating the early atmosphere in a closed glass vessel. Other experiments & results are discussed pp. 145 – 146. Experiments are controlled, nature is not. Oldest known organisms – 3.0 b.y. stromatolites, layers of photosynthetic cyanobacteria that grow as sediment grains are trapped on the sticky surface. Archean organisms were procaryotic, simple cells with no nucleus nor membrane enclosed organelles. 18