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34 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. , 389,57–60. The term “” is also based on the preconception that de Vernal, A., and Hillaire-Marcel, C., 2000. Sea-ice cover, sea-surface 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 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 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 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 and terrestrial (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 (Halliday, 2000). Heat from 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 during the beginning Antarctic Glaciation ’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 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 the Earth during the first two billion 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 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 Complex (Western ). 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”) 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 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 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. On the Moon (where impact melting and recycling back into the mantle. The timing and events have been dated) the time-integrated flux of impacts development of continental crust is a fundamental question as is estimated through crater density, addition of material to to surface conditions of the early Earth. The oldest recognized lunar crust, and the extent of impact stirring (Arrhenius and continental crust is 4.03 Ga granitic gneiss preserved in the Lepland, 2000; Hartmann et al., 2000). Acasta Gneiss Complex, Northwest Territories, Canada. These The terrestrial meteorite flux over time is controversial and strongly deformed rocks constitute <20 km2 of the 4.0–2.6 Ga has not been uniquely determined. Presumably meteorite bom- Slave . Other remnants of early (3.8 Ga) crust are lim- bardment would have been at its peak early in the Solar Sys- ited to a few thousand square kilometers in the Uivak tem’s history, during and preceding extraction of the moon at of Labrador, the Qianxi and Anshan Complexes in China, the ca. 4.5 Ga. The waning of impacts is not well constrained, Napier Complex of Antarctica, and the Isaq Gneiss Complex although many lunar samples of impact glass yield a group- and Aasivik Terrane of southwestern (Figure A23). ing of 3.9 Ga ages called the “” Sedimentary rocks ca. 3.5–3.0 Ga in age contain some of the (e.g., Cohen et al., 2000). If the rate of impacts has decreased oldest recognized terrestrial material: 4.4–3.2 Ga detrital zircons steadily through time, then the high rate of impacting at in the Narryer Gneiss Complex, detrital zircons as old as 4.0 Ga 3.9 Ga suggests even more impacts earlier. from the Wyoming Province, USA, and detrital zircons as old as However, recent dating of lunar impact glasses supports an 3.85 Ga from metasediments in the Qianxi complex (see alternate hypothesis: that the cluster of 3.9 Ga ages represents Nutman et al., 2001). The Archean rock record is more complete a renewal of impact intensity caused by deflection of materials for rocks ca. 3.5 Ga and younger. from the outer Solar System (Cohen et al., 2000). If the Earth The small amount of early Archean crust can either be inter- was subjected to a spike of meteorite bombardment ca. 3.9 Ga, preted as the surviving pieces of once extensive continental then the 4.4–4.0 Ga interval might have been relatively tranquil, crust or as a reflection of the low rate of continental crust pro- consistent with cooler surface temperatures inferred from zircons duction in the early Archean. This dichotomy is reflected in crystallized during this period (see Valley et al., 2002). different crustal growth models developed over the last The terrestrial record of bombardment is equivocal. There 30 years: some propose rapid generation of Archean crust are no reported descriptions of shock features in 4.4–3.8 Ga (most of which would be recycled), while others call for initi- zircons or rocks, as would be expected if they had been ally slow growth. Studies of middle Archean sediments support involved in large impacts. Metasedimentary rocks from south- this latter view (e.g., Nutman et al., 2001), as recycled early western Greenland (3.8–3.7 Ga) do not contain impact-related Archean rocks constitute a minor component. This contrasts sedimentary structures (i.e., surge deposits, see Arrhenius and with geochemical signatures from the Acasta Gneiss, the Itsaq Lepland, 2000), nor has measurable extraterrestrial iridium 36 ARCHEAN ENVIRONMENTS been found. However, evidence has been presented for an ele- vated extraterrestrial tungsten isotope signature in Isua metase- diments of Greenland (Schoenberg et al., 2002), similar to evidence for extraterrestrial chromium isotopes associated with 3.2 Ga South African beds of impact glass (Kyte et al., 2003). The timing of the early bombardment of Earth is poorly constrained and limited by the rocks available for study. How- ever, indirect geologic and geochemical evidence for liquid water during this period argues for at least transient periods of quiescence during bombardment. It is possible that tranquil periods allowed the precipitation of oceans between 4.4 (the first evidence for continental crust) and 3.9 Ga (the late heavy bombardment). Oceans A critical parameter for evaluating surface conditions on Earth is whether or not liquid water is a stable phase, a function of both temperature and vapor pressure of the atmosphere. Supra- crustal rocks in the Itsaq Gneiss Complex (the Isua and Akilia Figure A24 Oxygen isotope ratio of zircon through time (after Valley Island localities) are the earliest known rocks deposited in et al., 2005). Oxygen isotope ratios 6% are indicative of interaction water: 3.8–3.7 Ga pillow lavas and chemical sediments (Myers between liquid water and rocks at the surface of the Earth. Note and Crowley, 2000). The age and ultimate origin of rocks of the that the signature of a liquid hydrosphere extends to samples as old Akilia Island locality are controversial, but the Isua lithologies as 4.4 Ga. are widely accepted as having formed under submarine condi- tions (below wave base according to Fedo et al., 2001). The next The constancy of isotope ratios suggests that surface temperatures oldest unequivocal evidence for submarine deposition, although from 4.4 to 4.0 Ga were similar to those of the later Archean. The indirect, is the abundance of metasediments 3.5 Ga world- increase in values starting at the Archean- boundary wide. Volcanogenic massive sulfide deposits are also common marks an increased involvement of mature supracrustal rocks in rocks 3.4 Ga and could have formed at a variety of ocean (with a water-rock interaction signature) in the genesis of igneous depths (including deep water >1,000 m; Rasmussen, 2000). rocks, but it is not certain if this indicates increased rates of recy- There are critical gaps in the rock record from 3.8 Ga and cling or more highly evolved compositions (Peck et al., 2000). between 3.7 and 3.5 Ga, a period during which life is believed The temperature of Archean oceans has been controversial to have originated. A proxy for the presence of water is the because 3.8–2.5 Ga chemical sediments have low oxygen iso- geochemical signature of water-rock interaction associated with tope ratios compared to younger rocks (see Perry, 1967; Perry Archean detrital zircons (Peck et al., 2001; Mojzsis et al., 2001; and Lefticaru, 2003). This has been taken to indicate any (and Wilde et al., 2001; Valley et al., 2002; Cavosie et al., 2005). all combinations) of the following factors: (a) Archean ocean The elevated oxygen isotope ratios in these 4.4–4.0 Ga zircons water temperatures of up to 80 C, (b) lower oxygen isotope are consistent with low-temperature exchange between rocks ratios of Archean ocean water, and (c) later alteration of original and water. On the modern seafloor, hydrothermal alteration isotope ratios in ancient samples. The oxygen isotope ratio of ca. 250 C produces similar oxygen isotope ratios, but there the ocean seems to have been buffered at or near present levels is more uncertainty as to the conditions of water-rock inter- since at least 3.6 Ga, as predicted by models and observed in action during the Archean. Lower temperature estimates are hydrothermally altered rocks (see Muehlenbachs, 1998). In plausible if: (a) oxygen isotope ratios of the oceans have slowly addition, careful sampling in 3.5 Ga rocks has constrained risen over time (e.g., Perry, 1967), (b) meteoric waters instead the influence of post-depositional alteration. Although the oxy- of seawater were involved in Archean water-rock interaction, gen isotope ratio of the ocean has certainly fluctuated within or (c) hydrothermal alteration was less efficient than today a relatively small range and some chemical sediments may have (Valley et al., 2002). Steam and liquid water could have co- been altered, the overall evidence points towards ocean tempera- existed at temperatures 250 C, especially given the likeli- tures up to ca. 40 C higher than those today as late as 3.5 Ga hood of periodic meteorite impacts. However, steam alone is (Knauth and Lowe, 2003; Perry and Lefticaru, 2003). an inefficient mechanism for exchange between water and What was the thermal history of the hydrosphere during the rocks and a steam atmosphere the size of modern oceans would Archean? During the period of accretion and heaviest bombard- require temperatures outside of the oxygen isotope constraints ment of the Earth (4.5 Ga) all water must have been vapor or from the detrital zircons (Valley et al., 2002). dissolved in silicate melts. Oxygen isotope evidence from 4.2 The oxygen isotope record from zircons records the compo- to 4.0 Ga zircons indicates a liquid or liquid-steam hydrosphere sitions of magmas from 4.4 Ga to the present (Figure A24). 250 C. This is consistent with predictions of rapid cooling of This record includes 4.4–4.0 and 3.6–3.2 Ga detrital zircons, the surface of the Earth (1–10 Myr) shortly after accretion and and zircons in igneous rocks from 3.6 Ga. The Archean during waning bombardment (Pollack, 1997; Zahnle et al., zircons came from igneous rocks that had oxygen isotope ratios 1988; Sleep et al., 2001). Late heavy bombardment of Earth ranging from mantle-like to elevated values, similar to Phaner- at 3.9 Ga could have vaporized Earth’s oceans, but beginning ozoic rocks but with a more limited spread (Valley, 2003). at 3.6–3.8 Ga there is good geological evidence for a stable These similarities would not be expected if older rocks had hydrosphere. It has been suggested that in order for oceans to formed in the absence of water and only younger ones had remain liquid rather than freezing, the lower luminosity of the interacted with water (see also Campbell and Taylor, 1983). during the Archean may have been offset by high levels ARCHEAN ENVIRONMENTS 37 of greenhouse gases (Sagan and Chyba, 1997), or that oceans Canup, R.M., and Righter, K., 2000. Origin of the Earth and Moon. may have been partially covered by ice and warmed by sub- Tucson: University of Arizona Press, 555p. marine volcanism and periodic impacts (Bada et al., 1994). Cavosie, A.J., Wilde, S.A., Liu, D., Valley, J.W., and Weiblen, P.W., 2004. Internal zoning and U-Th-Pb chemistry of the Jack Hills detrital zir- cons: a mineral record of early Archean to magmatism. Evidence and controversy for early life Precam. Res., 135, 251–279. Cavosie, A.J., Valley, J.W., Wilde, S.A., and EIMF, 2005. Magmatic d18Oin The need to understand the timing of and conditions for the origin 4400–3900 Ma detrital zircons: A record of the alteration and recycling of of life is one of the principle motivations for assessing the early crust in the early Archean. Ear. Plan. Sci. Lett. 235, 663–681. Archean environment. The primary requirements for Cloud, P., 1972. A working model of the primitive earth. Am. J. Sci., 272, of life are organic molecules, a source of energy, and liquid water. 537–548. Organic molecules could have been delivered to Earth by carbo- Cohen, B.A., Swindle, T.D., and Kring, D.A., 2000. Support for the lunar naceous meteorites and comets, and energy was available from cataclysm hypothesis from lunar meteorite impact melt ages. Science, 290, 1754–1756. the Sun and terrestrial volcanism. The precipitation of liquid Compston, W., and Pidgeon, R.T., 1986. Jack Hills, Evidence of more very water from an early steam-rich atmosphere was thus the final old detrital zircons in Western Australia. Nature, 321, 766–769. ingredient for an environment where life could have evolved dur- Fedo, C.M., Myers, J.S., and Appel, P.W.U., 2001. Depositional setting ing the period 4.4–4.0 Ga, perhaps to be extinguished by large and paleogeographic implications of earth’s oldest supracrustal bolide impacts or during the late heavy bombardment at 3.9 rocks, the >3.7Ga Isua , West Greenland. Sed. Geol., 141/142,61–77. Ga. Theoretical limits of the habitability of the early Earth are Froude, D.O., Ireland, T.R., Kinny, P.D., Williams, I.S., Compston, W., thoroughly discussed elsewhere (see Nisbet and Sleep, 2001), Williams, I.R., and Myers, J.S., 1983. Ion microprobe identification of but here we focus on evidence for early life from the rock record. 4,100–4,200-Myr-old terrestrial zircons. Nature, 304, 616–618. The earliest evidence for life is a distinctive carbon isotope Halliday, A.N., 2000. Terrestrial accretion rates and the origin of the moon. signature from metamorphosed sedimentary rocks of southwes- Ear. Plan. Sci. Lett., 176,17–30. tern Greenland. Low carbon isotope ratios (13C/12C) in reduced Hartmann, W.K., Ryder, G., Dones, L., and Grinspoon, D., 2000. The time-dependent intense bombardment of the primordial earth/moon sedimentary carbon are commonly the result of isotope fractiona- system. In: Canup, R.M., and Righter, K. (eds.), Origin of the Earth tion by biological pathways, and thus are taken as a “fingerprint” and Moon. Tucson: University of Arizona Press, pp. 493–512. of ancient life even in the absence of preserved (Schi- Jacobsen, S.B., 2003. Lost terrains of early Earth. Nature, 421, 901–903. dlowski, 2001). While some studies have been controversial, Jones, J.H., and Palme, H., 2000. Geochemical constraints on the origin of the best evidence for early life comes from southwest Greenland: the Earth and Moon. In: Canup, R.M., and Righter, K. (eds.), Origin low carbon isotope ratios of particles in clastic sedimen- of the Earth and Moon. Tucson: University of Arizona Press, pp. 197–216. tary rocks from 3.8 to 3.7 Ga rocks at Isua (Rosing, 1999). These Knauth, L.P., and Lowe, D.R., 2003. High Archean climatic temperature rocks are preserved in low-strain zones, were clearly waterlain, inferred from oxygen isotope geochemistry of in the 3.5 Ga and the graphite may represent original organic detritus. Swaziland Supergroup, South . Geol. Soc. Am. Bull., 115, The earliest evidence for life are 3.5 Ga microscopic 566–580. structures in the Apex of Australia as well as numerous Kyte, F.T., Shukolyukov, A., Lugmair, G.W., Lowe, D.R., and Byerly, G.R., 2003. Early Archean spherule beds: Chromium isotopes confirm origin occurrences of (mounds produced by photosyn- through multiple impacts of projectiles of carbonaceous chondrite type. thetic microbes). These microstructures are interpreted as being , 31,283–286. the remains of various Archean microbes (Schopf, 1993). Maas, R., Kinny, P.D., Williams, I.S., Froude, D.O., and Compston, W., This claim of diversity of life has been challenged by Brasier 1992. The earth’s oldest known crust: A geochronological and et al. (2002), based on morphology of the features as well as geochemical study of 3900–4200 Ma old detrital zircons from Mt. a hydrothermal origin of the host-rock. However, the kero- Narryer and Jack Hills, Western Australia. Geochim. Cosmochim. Acta, 56, 1281–1300. gen-like Raman spectra and low carbon isotope ratios are Mojzsis, S.J., Harrison, T.M., and Pidgeon, R.T., 2001. Oxygen-isotope strong evidence of biologic activity (Schopf et al., 2002). evidence from ancient zircons for liquid water at the earth’s surface Furthermore, abundant stromatolites are found in associated 4,300 Myr ago. Nature, 409, 178–181. rocks. The association of with seafloor hydrother- Muehlenbachs, K., 1998. The oxygen isotopic composition of the oceans, mal systems has been recognized in rocks as old as 3.5 Ga sediments and the seafloor. Chem. Geol., 145, 263–273. (Van Kranendonk, 2001), supporting the hypothesis that life Myers, J.S., and Crowley, J.L., 2000. Vestiges of life in the oldest Greenland rocks? A review of early Archean geology in the Godthabsfjord originally evolved near submarine vents or springs in early region, and reappraisal of field evidence for >3850 Ma life on Akilia. Pre- Archean oceans (e.g., Nisbet and Sleep, 2001). cam. Res., 103, 101–124. Nisbet, E.G., and Sleep, N.H., 2001. The habitat and nature of early life. Nature, 409, 1083–1091. William H. Peck and John W. Valley Nutman, A.P., Friend, C.R.L., and Bennett, V.C., 2001. Review of the old- est (4400–3600 Ma) geological and mineralogical record: Glimpses of the beginning. Episodes, 24,93–101. Bibliography Patterson, C.C., 1956. Age of meteorites and the earth. Geochim. Cosmo- Arrhenius, G., and Lepland, A., 2000. Accretion of moon and earth and the chim. Acta, 19, 157–159. emergence of life. Chem. Geol., 169,69–82. Peck, W.H., King, E.M., and Valley, J.W., 2000. Oxygen isotope perspective Bada, J.L., Bigham, C., and Miller, S.L., 1994. Impact melting of frozen on crustal growth and maturation. Geology, 28,363–366. oceans on the early earth: Implications for the origin of life. Proceed- Peck, W.H., Valley, J.W., Wilde, S.A., and Graham, C.M., 2001. Oxygen ings of the National Academic Science, 91, 1248–1250. isotope ratios and rare earth elements in 3.3 to 4.4 Ga Zircons: Ion Bowring, S.A., and Housh, T., 1995. The earth’s early evolution. Science, microprobe evidence for high d18O continental crust in the early 269, 1535–1540. Archean. Geochim. Cosmochim. Acta, 65, 4215–4229. Brasier, M.D., Green, O.R., Jephcoat, A.P., Kleppe, A.K., Van Kranendonk, Perry, E.C. Jr., 1967. The oxygen isotope chemistry of ancient cherts. Earth M.J., Lindsay, J.F., Steele, A., and Grassineau, N.V., 2002. Questioning Planet. Sci. Lett., 3,62–66. the evidence for earth’s oldest fossils. Nature, 416,76–81. Perry, E.C., and Lefticariu, L., 2003. Formation and geochemistry of Pre- Campbell, I.H., and Taylor, S.R., 1983. No water, no – no oceans, cambrian cherts. In: Holland, H., and Turekian, K. (eds.), Treatise on no continents. Geoph. Res. Lett., 10, 1061–1064. Geochemistry, Amsterdam: Elsevier, pp. 99–113. 38 ARCTIC SEA ICE

Pollock, H.N., 1997. Thermal characteristics of the Archaean. In: deWit, M., system because its reflectivity plays a role in the energy budget and Ashwal, L. (eds.), Greenstone Belts. Oxford: Oxford University ’ – at the Earth s surface and because it forms a barrier between Press, pp. 223 232. the atmosphere and the ocean, thus reducing the exchange of Rasmussen, B., 2000. Filamentous microfossils in a 3,235-million--old volcanogenic massive sulphide. Nature, 405, 676–679. heat, moisture and gases. Moreover, a portion of the Arctic Rosing, M.T., 1999. 13C-depleted carbon microparticles in >3700-Ma sea- sea ice is exported to the North Atlantic Ocean, where it floor sedimentary rocks from West Greenland. Science, 283, 674–676. becomes a source of freshwater that can interfere with deep Sagan, C., and Chyba, C.F., 1997. The early faint Sun paradox: Organic water formation and thus with the thermohaline circulation pat- shielding of ultraviolet-labile greenhouse gases. Science, 276, 1217–1221. tern. Recent data on Arctic sea ice derived from direct observa- Schidlowski, M., 2001. Carbon isotopes as biogeochemical recorders of life over 3.8 Ga of earth history: Evolution of a concept. Precam. Res., 106, tions and satellite imagery indicate large interannual variations 117–134. and suggest a decreasing trend in both area and thickness dur- Schoenberg, R., Kamber, B.S., Collerson, K.D., and Moorbath, S., 2002. ing the last decades of the twentieth century. Longer-term data Tungsten isotope evidence from approximately 3.8-Gyr metamorphosed on a time scale ranging from 102 to 106 years available from sediments for early meteorite bombardment of the Earth. Nature, 418, historical archives and from deep-sea sedimentary records – 403 405. reveal large fluctuations in the average extent of sea ice during Schopf, J.W., 1993. Microfossils of the early Archean Apex Chert: New evidence of the antiquity of life. Science, 260, 640–646. the Quaternary, from more reduced to much more extended Schopf, J.W., Kudryavtsev, A.B., Agresti, D.G., Wdowiak, T.J., and Czaja, covers than at present, generally as a result of hemispheric- A.D., 2002. Laser-Raman imagery of Earth’s earliest fossils. Nature, scale climate changes. For example, historical information indi- 416,73–76. cates that sea ice expanded in the subpolar North Atlantic from Sleep, N.H., Zahnle, K., and Neuhoff, P.S., 2001. Initiation of clement sur- the Medieval Warm Period to the Little Ice Age. At another face conditions on the earliest earth. Proceedings of the. National. time scale, biological and sedimentary tracers of sea ice Academy of. Science, 98, 3666–3672. Taylor, S.R., 1992. Solar system evolution: A new perspective. New York: demonstrate that it was significantly more widespread than at Cambridge University Press, 307pp. present in both the North Atlantic and North Pacific during Valley, J.W., 2003. Oxygen isotopes in zircon. In Zircon (eds. Hanchar, J., the Last Glacial Maximum, whereas the middle and Hoskins, P). Rev. Miner. Geochem., 53, 343–385. warm episode seems to have been marked by reduced sea ice Valley, J.W., Peck, W.H., King, E.M., and Wilde, S.A., 2002. A cool early cover in the Arctic Ocean and circumpolar seas. The . Geology, 30, 351–354. Valley, J.W., Lackey, J.S., Cavosie, A.J., Clechenko, C., Spicuzza, M.J., initial formation of the sea ice cover in the Northern Hemi- Basei, M.A.S., Bindeman, I.N., Ferreira, V.P., Sial, A.N., King, E.M., sphere during the late remains an open question, Peck, W.H., Sinha, A.K., Wei, C.S., 2005. 4.4 of Crustal but the development of quasi-permanent pack ice in the Arctic Maturation: Oxygen Isotope Ratios of Magmatic Zircon. Ocean probably occurred during the late . Van Kranendonk, M.J., 2001. Volcanic degassing, hydrothermal circulation and the flourishing of early life on Earth: new evidence from the c. Sea ice formation 3460 Ma , Pilbara, Western Australia. Earth System Processes Programmes with Abstracts,91–92. Sea ice is a general term that comprises several types of ice ori- ginating from the freezing of marine waters, which occurs at a Wilde, S.A., Valley, J.W., Peck, W.H., and Graham, C.M., 2001. Evidence from detrital zircons for the existence of continental crust and oceans on water temperature of about 1.8 C, depending upon salt con- the earth 4.4 gyr ago. Nature, 409, 175–178. tent. The term “sea ice” excludes icebergs, which are massive Zahnle, K.J., Kasting, J.F., and Pollack, J.B., 1988. Evolution of a steam ’ 74 – pieces of floating freshwater ice that were calved from the front atmosphere during earth s accretion. Icarus, ,62 97. of continental glaciers. The freezing process of marine water Zhang, Y., 2002. The age and accretion of the earth. Earth Sci. Rev., 59, 235–263. differs from that of freshwater because it involves downward migration of brines resulting from the segregation of sea salts and ice crystals composed of almost pure water. It is a rela- Cross-references tively complex process from a thermodynamic viewpoint since Atmospheric Evolution, Earth ice crystal formation induces an increase in the salt content of Bolide Impacts and Climate the surrounding seawater, thus increasing the density and Carbon Isotopes, Stable decreasing the freezing point of this water. Dating, Radiometric Methods Sea ice formation is a consequence of the cooling of marine Isotope Fractionation water in response to low air temperatures. It also depends upon Oxygen Isotopes the structure of the water masses, since the vertical density gra- Stable Isotope Analysis dient actually determines the depth of the mixed surface layer, which must cool before the surface can reach the freezing point (e.g., Barry et al., 1993). A shallow stratification in the upper ARCTIC SEA ICE water column and the presence of a low-salinity (thus, density) surface water layer are important parameters for the formation of sea ice. In the Arctic Ocean, a mixed layer with salinities often below 30% at the surface, overlies a generally warmer Introduction and more saline (>34.5%) water layer originating from the Sea ice occupies a large part of the world ocean (about 7%) and Atlantic. The low salinity of the mixed layer in the Arctic covers most of the Arctic Ocean A25(Figure ), where itOcean results from freshwater inputs, notably from Eurasian averages about three meters in thickness. Sea ice undergoes rivers (Yenessei, Ob, Lena, Kolyma; 1,700 km3 yr1) and very large seasonal variations in its areal extent in response to the Mackenzie River (260 km3 yr1) (Carmack, 1998). changes in solar insolation. In circumpolar regions of the In the Arctic Ocean, a large part of the sea ice formation Northern Hemisphere, its coverage rises from approximately occurs over shallow continental shelves surrounding the basin 7 to 15 million2 from km summer to winter (see A2Figure6). ( Figure A25 ), notably in the Siberian and Laptev Seas, where Sea ice is an extremely important component of the climate the shelf is particularly wide and sea surface salinity is