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ARCHEAN ENVIRONMENTS De La Mare, W.K., 1997

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ARCHEAN ENVIRONMENTS De La Mare, W.K., 1997 34 ARCHEAN ENVIRONMENTS de la Mare, W.K., 1997. Abrupt mid-twentieth-century decline in Antarctic distinctions in part based on whether or not rocks have survived. sea ice extent from whaling records. Nature, 389,57–60. The term “Hadean” is also based on the preconception that de Vernal, A., and Hillaire-Marcel, C., 2000. Sea-ice cover, sea-surface Earth was so hot from formation and bombardment by meteor- salinity and halo-/thermocline structure of the northwest North Atlantic: Modern versus full glacial conditions. Quat. Sci. Rev., 19,65–85. ites that the atmosphere was dominated by steam and rock deb- Gersonde, R., and Zielinski, U., 2000. The reconstruction of late quaternary ris, and solid material would have been pulverized or melted antarctic sea-ice distribution – The use of diatoms as a proxy for sea- (Cloud, 1972). Detrital zircon crystals (ZrSiO4) from this per- ice. Palaeogeogr. Palaeoclimateol. Palaeoecol., 162, 263–286. iod challenge this preconception (Compston and Pidgeon, Gersonde, R., Crosta, X., Abelmann, A., and Armand, L.K., 2005. Sea sur- 1986; Mojzsis et al., 2001; Peck et al., 2001; Wilde et al., face temperature and sea ice distribution of the last glacial. Southern Ocean – A circum-Antarctic view based on siliceous microfossil 2001; Valley et al., 2002; Cavosie et al., 2004, 2005), and pro- records. Quat. Sci. Rev., 24, 869–896. vide evidence for proto-continental crust and low temperatures Gloersen, P., Campbell, W.J., Cavalieri, D.J., Comiso, J.C., Parkinson, C.L., at the surface of the Earth during the period 4.4–4.0 Ga. and Zwally, H.J., 1992. Arctic and Antarctic sea ice, 1978–1987: Satellite passive-microwave observations and analysis. NASA Spec. Publ. SP-511, National Aeronautics and Space Administration, 4.57 to 4.45 Ga: the hot early earth Washington D.C., pp. 290. Hodell, D.A., Shemesh, A., Kanfoush, S., Crosta, X., Charles, C.D., and The most widely-cited age of the Earth is ca. 4.55 Ga as deter- Guilderson, T., 2000. Abrupt cooling of Antarctic surface waters and mined by Claire Patterson from lead isotopes in primitive sea ice expansion in the South Atlantic sector of the Southern Ocean meteorites and terrestrial sediments (Patterson, 1956). This is at 5000 cal yr bp. Quatern. Res., 56, 191–198. the age of condensation of solid material from the solar nebula Horner, R.A., 1985. Ecology of sea ice microalgae. In Horner, R.H. (ed.), (since revised to 4.56–4.57 Ga), and dates the origin of the – Sea Ice Biota. Boca Raton, Florida: CRC Press, pp. 83 104. material from which Earth began to accrete. Morales-Maqueda, M.A., and Rahmstorf, S., 2002. Did Antarctic sea-ice expansion cause glacial CO2 decline? Geophys. Res. Lett., 29(1): There are no recognized terrestrial samples from the first 1011. doi: 10.1029/2001GL013240. 150 Myr of Earth’s history, thus constraints on surface condi- Schweitzer, P.N., 1995. Monthly Averaged Polar Sea-Ice Concentration. tions are inferred from isotopic data and the geologic history Virginia: U.S. Geological Survey Digital Data Series. of the Moon. The currently accepted model of Moon formation Shemesh, A. Hodell, D.A., Crosta, X., Kanfoush, S., Charles, C.D., and calls for impact of the Earth with a planet-sized bolide, causing Guilderson, T., 2002. Sequence of events during the last deglaciation separation of the Moon from Earth’s mantle (Canup and in Southern Ocean sediments and Antarctic ice cores. Paleoceanogra- phy, 17(4): 1056. doi: 10.1029/2000PA000599. Righter, 2000). This model explains the orbit of the Moon Stephens, B.B., and Keeling, R.F., 2000. The influence of Antarctic sea ice and its geochemical similarity to Earth. The oldest Moon rocks on glacial-interglacial CO2 variations. Nature, 404, 171–174. have ages ca. 4.49 Ga, constraining the age of the catastrophic Zielinski, U., and Gersonde, R., 1997. Diatom distribution in Southern impact (Canup and Righter, 2000) and in agreement with W Ocean surface sediments (Atlantic sector): Implications for paleoenvir- isotopes, which suggest formation of the Moon within 30 Myr onmental reconstructions. Palaeogeogr. Palaeoclimateol. Palaeoecol., ’ 129, 213–250. of the Earth s accretion (Halliday, 2000). Heat from meteorite impacts, residual heat from the Earth’s accretion and core formation, and high heat production from Cross-references radioactive elements likely made the surface unsuitable for Albedo Feedbacks the formation of oceans and continents during the beginning Antarctic Glaciation History of Earth’s history. For example, estimates of heat production Arctic Sea Ice from the early Earth’s store of radioactive elements are on the CLIMAP order of 3–6 times that of today (Pollock, 1997). Conditions Cryosphere on the surface may have been extreme enough to form magma Diatoms Last Glacial Maximum oceans until the waning of large impacts and the dissipation Marine Biogenic Sediments of heat into space (cf. Jones and Palme, 2000). Dissipation of Paleoclimate Modeling, Quaternary heat could have resulted in rapid and turbulent mantle con- Radiolaria vection well into the Archean (Pollock, 1997). The last large Transfer Functions terrestrial impact could have caused the resetting (and presum- ably planet-scale mixing) of xenon and other isotope systems at 4.45 Æ 0.05 Ga (Zhang, 2002). It is possible that a slightly older impact formed the Moon. No terrestrial materials appear ARCHEAN ENVIRONMENTS to have survived this last catastrophic event. A primary research objective in the study of Archean rocks The cool early earth: evidence from 4.40 to 4.00 Ga (>2.5 Ga) has been to determine conditions on the surface of detrital zircons the Earth during the first two billion years of its history. Recent In the early 1980s newly developed ion microprobe technology advances in isotope geochemistry and geochemical microanaly- was used to identify individual zircon crystals older than sis have allowed new insights into the earliest few hundred 4.0 Ga (Froude et al., 1983; Compston and Pidgeon, 1986). million years of Earth’s history, and have pushed back the time These crystals were eroded from their original host rocks, trans- at which the first life could have emerged. ported by water ca. 3 Ga, and are now found in metasedimentary The earliest subdivision of the Archean Eon beginning with rocks of the Narryer Gneiss Complex (Western Australia). The ori- Earth’s accretion at 4.56 Ga and extending to ca. 4.0–3.8 Ga, ginal 4.0 Ga igneous rocks are not known to have survived ero- has been called both the Priscoan (“previous time”) Era and sion and recycling. These Zircon crystals provide the only direct Hadean (“Hell-like”) Era. These divisions of geologic time evidence for conditions at the surface of the Earth 4.0 Ga, as no are based on the inherent bias of the rock record, making other samples of solid material are known from this time period. ARCHEAN ENVIRONMENTS 35 Recently, a crystal dated at 4.4 Ga was discovered (Wilde et al., 2001), providing a glimpse of conditions within 160 Myr of the Earth’s accretion. Zircons from Western Australia are evidence for 4.4–4.0 Ga rocks similar to modern continental crust. Mineral inclusions in the 4.4 Ga zircon include SiO2 (Peck et al., 2001; Wilde et al., 2001; Cavosie et al., 2004), indicating crystallization from an evolved, silica-saturated magma. Similar conditions have been inferred for other Archean magmas based on inclusions from other 4.2–4.0 Ga zircons (Maas et al., 1992). Trace element compositions (including rare earth elements) of these zircons are also elevated relative to the mantle, consistent with crystal- lization from an evolved magma (Maas et al., 1992; Peck et al., 2001). All of these lines of evidence suggest the existence of igneous rocks similar to those of modern continental crust. Isotopic analysis of these 4.4–4.0 Ga crystals has provided new constraints on surface conditions of the Earth (Peck et al., Figure A23 Preserved Archean crust (2.5 Ga), (after Bowring and 2000, 2001; Mojzsis et al., 2001; Wilde et al., 2001; Cavosie et al., Housh, 1995) and localities >3.8, mentioned in text. 2005). Oxygen isotope ratios (18O/16O) of these crystals are ele- vated relative to the primitive ratios of the Earth’s mantle and the Moon (Valley, 2003). Elevated oxygen isotope ratios in terrestrial Gneiss Complex, and 4.0 Ga detrital zircons that show deri- materials are most typically caused by low-temperature interac- vation from a mantle which had already produced crust earlier tion between rocks and water at the surface of the Earth (i.e., in its history ( 4.3 Ga; see Jacobsen, 2003 and references hydrothermal alteration or low-temperature mineral formation). therein). Taken with the continental affinities of the Jack Hills Magmas with elevated oxygen isotope ratios are formed by melt- detrital zircons, there is now ample evidence for continental ing or assimilation of this material. The possible presence of crust between 4.4 and 4.0 Ga. Most of this original crust was liquid water calls into question some widely assumed conditions eroded and/or recycled back into the mantle, perhaps aided of the “hell-like” nature of the early Earth (Valley et al., 2002). by meteorite bombardment of the early Earth. Timing of meteorite bombardment Emergence of continental crust Meteorite bombardment of the Earth during the period 4.4–3.8 Ga The first igneous rocks on Earth were most likely basaltic is known from the cratering record of other bodies in the or komatiitic, with similarities to modern oceanic crust (e.g., inner solar system, but the terrestrial record has been erased Taylor, 1992). Lack of preservation of this crust may be due to by plate tectonics and erosion.
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