Retallack 2007 Proserpina principle - University of Oregon

Foreman, 1992), indicating modest carbonstorage in soil organic matter and carbonatecompared, e.g., with modern savanna grasslandsoils (de Wit, 1978). The Ordovician paleosolsstudied so far show unusually high soil respiration,considering their probable low levels ofprimary productivity. They also formed at atime estimated from sedimentary mass balancemodels as the steamiest greenhouse period ofall Phanerozoic time, with B16 times thepresent atmospheric levels of CO 2 (Berner andKothavala, 2001). The carbon budget ofknown Ordovician paleosols would have contributedto this greenhouse.5.18.3.5 Terminal Ordovician IcehousePaleosolsPeriglacial paleosols, unknown in Cambrianand Early to Middle Ordovician rocks, arefound again in latest Ordovician (Hirnantian)rocks. Periglacial paleosols are best documentedin South Africa, where patterned ground andsand wedges are common in red beds of thePakhuis Formation (Daily and Cooper, 1976).The ice sheets extended over much of Africa(Ghienne, 2003).The causes of this ice age areespecially enigmatic, because volcanic activityincreased through the Ordovician and the continentswere dispersed (Bluth and Kump, 1991),thus working against cold Late Ordovicianpoles. Mass balance models make the Ordovicianice age seem particularly enigmatic, becausethey predict atmospheric CO 2 levels 16times PAL (Berner and Kothavala, 2001). Thismay be an artifact of the 10 Ma spacing of datapoints in the model, blurring the o10 Ma durationof the ice age that is indicated by carbonisotopic data (Brenchley et al., 1994). Studies ofcarbonate isotopic compositions from paleosolswithin the glacial interval are needed to reexaminethis question. Also needed is an examinationof paleosols within this interval forevidence of fossil mosses, which would havebeen more deeply rooted than liverworts andso have accelerated weathering and carbonsequestration. Rare Late Ordovician mosslikemegafossils (Snigirevskaya et al., 1992) andspores (Nøhr-Hansen and Koppelhus, 1988)support indications from cladistic analysis(Kenrick and Crane, 1997) for a latest Ordovicianorigin of mosses.5.18.3.6 Siluro-Devonian GreenhousePaleosolsRoot traces of vascular land plants appear inSilurian paleosols, but until the Early Devonian,Record of Past Soil and Global Change 13root traces are small and shallow within theprofiles (Figure 9b). The earliest known vascularland plants of the Middle and Late Silurianlacked true roots. Instead, they had stems thatran along the surface and just beneath the surfaceof the soil as runners and rhizomes furnishedwith thin unicellular root hairs (Kenrickand Crane, 1997). Plant bioturbation in soilsonly extended down to a few centimeters, butburrows of millipedes reached more deeply, andin some soils were more abundant than planttraces (Retallack, 1985). In addition to detritivorousand perhaps also herbivorous millipedes(Retallack, 2001c), Late Silurian soil faunasincluded predatory centipedes and spiderliketrigonotarbids (Jeram et al., 1990). Fungalhyphae and spores in Silurian and Devonianrocks indicate proliferation of chytrids and otherfungi (Sherwood-Pike and Gray, 1985; Taylorand Taylor, 2000).Early Devonian paleosols have abundanttraces of true roots, including woody tap rootsof a variety of land plants (Elick et al., 1998).Root traces reached tens of centimeters downinto paleosols, extending greatly the depth ofthe active rhizosphere and its associatedmucigel of microbes. Among the numerousroots of Early and Middle Devonian paleosols,the burrows of soil fauna are less prominent(Figure 9b). Devonian soils also have higherclay content and are more deeply weathered ofbases than Silurian or Ordovician soils (Figure9a). They have isotopically lighter pedogeniccarbon, closer to the isotopic composition ofcoexisting organic carbon, than Silurian andOrdovician paleosols (Mora et al., 1996; Moraand Driese, 1999). Within the parameters of thepedogenic carbonate palaeobarometer of Cerling(1991), these data indicate declining atmosphericlevels of CO 2 from the Silurian intothe Devonian (Figure 9c). Consumption of atmosphericCO 2 by increased hydrolytic weathering,and burial of carbon in limestone andorganic matter during the Silurian and Devonianhas been widely interpreted as an instanceof atmospheric global change induced by theevolution of life (Retallack, 1997b; Berner,1997; Algeo and Scheckler, 1998).5.18.3.7 Late Devonian to Permian IcehousePaleosolsPeriglacial paleosols and glaciogene sedimentaryfacies unknown in Silurian and Earlyto Middle Devonian appear in the latestDevonian, and remain locally common inCarboniferous and Permian rocks, especiallywithin the Gondwana supercontinent, then positionednear the south pole (Figure 10; Krull,