Retallack 2007 Proserpina principle - University of Oregon

10 Soils and Global Change in the Carbon Cycle over Geological Timecarbon comparable with that of well-drainedpaleosols of the Phanerozoic. If life had beenpresent in the Early Precambrian paleosols,they would have become carbonaceous in theabsence of a decomposing microbiota of actinobacteriaand of fungi and metazoans duringthe later Precambrian. Isotopic evidence thussuggests that methanogenic, hypersaline, normal,and decompositional microbes were presentin Precambrian paleosols. Other evidencefor life in Precambrian paleosols includes microfossils(1,300 Ma; Horodyski and Knauth,1994), microbial trace fossils (2,200 Ma; Retallackand Krinsley, 1993), chemofossils(2,900 Ma; Prashnowsky and Schidlowski,1967), plausible megafossils (2,900 Ma; Hallbaueret al., 1977; Retallack, 1994b), and theimpressive thickness and soil structure of Precambrianpaleosols (3,500 Ma; Retallack, 1986,2001a; Buick et al., 1995; Gutzmer and Beukes,1998; Beukes et al., 2002). Life and its byproductssuch as polysaccharides may havebeen soil binders, like molasses applied to acornfield (Foster, 1981), protecting soils fromphysical weathering so that biochemical weatheringcould proceed.The likely existence of microbial mats at thesoil surface considerably complicates the use ofpaleosols as indicators of ancient atmospheres(Ohmoto, 1996). Tropical rainforest soils nowhave soil CO 2 levels up to 110 times that of theatmosphere, because of high levels of soil respirationby termites and microbes of an abundantsupply of soil organic matter that forms ina living membrane separating the subsoil fromthe atmosphere (Brook et al., 1983; Colin et al.,1992). Nevertheless, modeling by Pinto andHolland (1988) makes it unlikely that microbialscums of the Precambrian were as productiveand effective membranes as rainforests.The observation that Precambrian paleosolswere chemically weathered to an extent comparablewith rainforest soils today probablyindicates much higher levels of CO 2 in the atmosphereat that time (Holland, 1984). Theextent of this greenhouse is poorly constrained,but the apparent lack of siderite in paleosolssuch as the Hekpoort and Mt. Roe paleosolshas been used by Rye et al. (1995) to argue thatCO 2 concentrations could not have been morethan ca. 100 times present levels before the riseof oxygen at ca. 2,100 Ma.Siderite is common in Phanerozoic wetlandpaleosols (Ludvigsen et al., 1998) in which respiredsoil CO 2 exceeded this level. Thus, theestimate of Rye et al. (1995) of no more than100 times present levels of soil CO 2 also is acap on soil respiration and biological productivityduring the Precambrian (Sheldonet al., 2001). The contribution of CH 4 to theatmospheric greenhouse effect was probablyalso much higher than at present, becauseit was necessary to maintain planetary temperaturesabove that of the freezing of water atthe time of a faint young Sun (Kasting, 1992;Pavlov et al., 2000).5.18.3.3 Proterozoic Icehouse PaleosolsThe oldest known periglacial paleosolsare from the 2,300 to 2,400 Ma Ramsay LakeFormation of Ontario, Canada (Young andLong, 1976; Schmidt and Williams, 1999). Theyhave prominent ice wedges, which are stronglytapering cracks filled originally with ice, butnow with massive or horizontally layered sandand claystone breccia. Modern ice wedges formin climates with a mean annual temperatureof 4to 8 1C, coldest month temperatures of25 to 40 1C, warmest month temperaturesof 10–20 1C, and a mean annual precipitationof 50–500 mm (Williams, 1986; Bockheim,1995). Periglacial paleosols of the Late Precambrian(600–1,000 Ma) in Scotland, Norway,and South Australia include sand wedges(Figure 7), which indicate an even drier andmore frigid climate: a mean annual temperatureof 12 to 20 1C, a mean cold-monthtemperature of 35 1C, a mean warm-monthtemperature of 4 1C, and mean annual precipitationof 100 mm (Williams, 1986). Some ofthe Late Precambrian glaciations were remarkablein extending to very low latitudes, as indicatedby the paleomagnetic inclination ofglaciogene sediments, and have been dubbedSnowball Earth events (Kirschvink, 1992;Hoffman et al., 1998; Schmidt and Williams,1999). Between and before these Precambrianepisodes of periglacial paleosols and associatedglaciogene sediments there is no evidence offrigid conditions, so that the alternation ofglobal icehouse and greenhouse paleoclimatesis ancient indeed.These climatic fluctuations could be attributedto changes in solar luminosity, volcanicdegassing, or ocean current reorganization withcontinental drift (Barley et al., 1997; Dalziel,1997), but paleosols reveal that these ice ageswere also times of change in the atmosphereand life on land. Highly ferruginized pisoliticlateritic paleosols first appear in the geologicalrecord at 2,200–1,920 Ma in South Africa(Gutzmer and Beukes, 1998; Beukes et al.,2002). The lateritic paleosols are part of acomplex erosional landscape with a variety ofpaleosols of significantly different geologicalages, including mildly oxidized (Retallack,1986; Maynard, 1992) and chemically reduced