Retallack 2007 Proserpina principle - University of Oregon

planetesimals formed early during the formationof the solar system. As primeval soils ofthe solar system, they were similar in theirsmectites, salts, and carbonates to the soils ofMars, which are probably paleosols relict froma time of free water at the martian surface untilat least 2,500 Ma (Retallack, 2001a).A second interpretation of carbonaceouschondrites is as primary condensates of thesolar nebula. By this view, their hydrolyticalteration is due to melting in cometary nucleiduring close passes with the Sun, or due totransient heating events by shock waves orcollisions (McSween, 1999). Other carbonaceouschondrites show metamorphic alterationwith minerals similar to those in Earthformed during deep burial under elevatedtemperatures and pressures (Brearley, 1999).Like soils and paleosols on Earth and Mars,carbonaceous chondrites demonstrate the greatantiquity of hydrolytic weathering in diluteacidic solutions, presumably of carbonic acidderived from water vapor and CO 2 . These remainthe principal gases released from volcanoes,and soils remain important buffers forthis environmental acid.5.18.3.2 Archean–PaleoproterozoicGreenhouse PaleosolsDespite predictions that Precambrian paleosolswould be thin, rocky, and dominated byphysical weathering (Schumm, 1956), hundredsof Precambrian paleosols now described havebeen found to be thick, clayey, deeply weathered,and in some cases with possible traces oflife, so that chemical and biological weatheringcan be traced back almost to the beginning ofthe suitably preserved sedimentary rock recordon Earth (Rye and Holland, 2000). The oldestknown profile interpreted to be a paleosolshows alteration to depths of 50 m on granitesunconformably underlying the 3,500 Ma sedimentarysuccession of the Warrawoona Groupin northwestern Western Australia (Buicket al., 1995). Corundum ores in the 3,500 MaAldan Schists of Siberia have been interpretedas metamorphosed, deeply weathered bauxites(Serdyuchenko, 1968).The Jerico Dam paleosol of South Africa(3,000 Ma; Grandstaff et al., 1986), the Prontopaleosol of Canada (2,450 Ma; Mossman andFarrow, 1992), the Hokkalampi paleosol ofFinland (2,200 Ma; Marmo, 1992), a varietyof paleosols associated with the HekpoortBasalt of South Africa (2,100 Ma; Yang andHolland, 2003), and the Sheigra paleosol ofScotland (1,000 Ma; Figures 3 and 4) have beenRecord of Past Soil and Global Change 9subjected to exceptionally detailed geochemicaland petrographic analyses. Along with manyother Precambrian paleosols reviewed by Ryeand Holland (1998), these paleosols revealthe antiquity and thoroughness of hydrolyticweathering during the Precambrian. Eventhen, rock and sediment were under relentlessacid attack, which leached base cations (especiallyCa 2 þ ,Mg 2 þ , and Na þ ), and left thick,clayey soil.It is likely that at least back to 3,500 Ma, theprincipal environmental acid driving this hydrolyticreaction was carbonic acid dissolved inrain and groundwater (Holland, 1984), as is thecase in soils today (Nahon, 1991). Much soilCO 2 may also have come from respiring organisms,which also could have contributedorganic acids. Nitric and sulfuric acid mayhave been locally important in soils developedon particular parent materials, but nitrogenand sulfur salts are so far unreported in Precambrianpaleosols, unlike modern soils ofmine dumps (Borden, 2001), and hypothesizedmodern soils on Mars (Bell, 1996; Farquharet al., 2002) and Venus (Barsukov et al., 1982;Basilevsky et al., 1985).This view of the likely acids involved in formingPrecambrian soils on Earth is supportedby the isotopic composition of carbon, nitrogen,and sulfur in sedimentary organic matter,carbonates, sulfates, and sulfides, which aresurprisingly similar to their modern counterpartsback to 3,500 Ma, and unlike meteoriticor mantle values (Schidlowski et al., 1983; DesMarais, 1997; Canfield and Teske, 1996).Evidence for life in Precambrian soils comesfrom isotopic studies of organic carbon withinpaleosols. Microlaminated chips in the 2,765Ma Mt. Roe paleosol of Western Australiahave extremely depleted carbon isotopic compositions(d 13 C org 40%). Isotopic fractionationof carbon to this degree is only known inmethanogens and methanotrophs (Rye andHolland, 2000). These chips could be fragmentsof pond scum rather than a true soil microbiota.Organic matter in the 2,560 Ma Schagenpaleosols of South Africa is not nearly as depleted( 16% to 14% d 13 C org ) as organicmatter in overlying marine sediments ( 35% to30% d 13 C org ). Interpretation of the carbon inthis paleosol as the signature of a hypersalinemicrobial soil community is compatible withshallow dolocretes and other features of thepaleosols (Watanabe et al., 2000).Normal isotopic values for soil organic matter( 25% to 27% d 13 C org ) have been reportedfrom Precambrian paleosols as well(Mossman and Farrow, 1992; Retallack andMindszenty, 1994). Virtually all Precambrianpaleosols have a very low content of organic