P. F. Hoffman et al. and N. Christie-Blick et al. discuss Hoffman et al.'s paper that "developed a modified 'snowball Earth' hypothesis (2) to explain the association of Neoproterozoic low-latitude glaciation with the deposition of 'cap carbonate' rocks bearing highly depleted carbon isotopic values (δ13C ≤ −5‰). According to Hoffman et al., the ocean became completely frozen over as a result of a runaway albedo feedback, and primary biological productivity collapsed for an interval of geological time exceeding the carbon residence time (greater than 105 years). During this interval, continental ice cover is inferred to have been thin and patchy owing to the virtual elimination of the hydrological cycle. 

Christie-Blick, Nicholas

Kennedy, Martin J.

Sohl, Linda E.

Science

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Columbia University Academic Commons

Considering a NeoproterozoicSnowball EarthPaul F. Hoffman et al. (1) developed a mod-ified “snowball Earth” hypothesis (2) to ex-plain the association of Neoproterozoic low-latitude glaciation with the deposition of “capcarbonate” rocks bearing highly depleted car-bon isotopic values (d13C # 25‰). Accord-ing to Hoffman et al., the ocean becamecompletely frozen over as a result of a run-away albedo feedback, and primary biologi-cal productivity collapsed for an interval ofgeological time exceeding the carbon resi-dence time (.105 years). During this inter-val, continental ice cover is inferred to havebeen thin and patchy owing to the virtualelimination of the hydrological cycle.These ideas are worthy of serious scruti-ny, and we would like to discuss geologicaldifficulties not addressed in an earlier Letterto the Editor (3). Climatological consider-ations suggest that snowball conditionswould have developed gradually, probablyover a span of 106 to 107 years. The wide-spread distribution of Sturtian (;750 to 700Ma) and Marinoan (;600 to 575 Ma) (4, 5)glacial deposits, in places thousands ofmeters thick (6), as well as evidence for$160 m drawdown of sea level for the Mari-noan event (7), indicate a vigorous hydrolog-ical cycle, with marked erosion beneath largeice sheets and deposition under generallytemperate conditions at and adjacent to icemargins (8). Catastrophic termination ofsnowball conditions interpreted by Hoffmanet al. requires the bulk of these sediments tohave accumulated before or during the inter-val in which the ocean was frozen becauserapid melt out from any residual glacial ice,even thick glacial ice, would have left only aveneer of glacial and glacially derived sedi-ments. However, gradual retreat of the icefront is recorded in many areas by tens tohundreds of meters of glacial deposits, insome cases with abundant outwash sediments(7–9). The time scale for this retreat is con-servatively estimated in Australia as .104 to105 years, and most likely .105 years, on thebasis of reversals in magnetic polarity inMarinoan outwash sandstones (10). This sce-nario suggests that if the ocean surface werecompletely frozen, it must have become un-frozen well before the end of glaciation.If highly depleted carbon isotopic valuesof cap carbonates are the result of the col-lapse of primary productivity, then maximumdepletion of the ocean as a whole ought todate from the time at which the ocean wasfrozen. However, in Namibia (1, 5), isotopicdepletion increases up section from the baseof the cap carbonate (a trend that is typical ofMarinoan cap carbonates) (5, 11). Hoffman etal. ascribe this trend to isotopic fractionationassociated with the hydration of atmospheri-cally derived CO2 in the surface ocean, withdepletion returning to bulk oceanic values asthe amount of CO2 in the atmosphere subsid-ed from ;0.12 to 0.001 bar. This interpreta-tion requires the ocean to have remainedeffectively lifeless for an unduly long spanafter snowball conditions had ceased—com-parable to the duration of Marinoan deglacia-tion in Australia, including whatever timewas needed for the drawdown of CO2 bycontinental weathering (104 to 106 years?)(12) and for deposition of the cap carbonates(,105 years) (13).Nicholas Christie-BlickLinda E. SohlDepartment of Earth and EnvironmentalSciences andLamont-Doherty Earth Observatory ofColumbia University,Palisades, NY 10964–8000, USAE-mail: ncb@ldeo.columbia.eduMartin J. KennedyExxon Production Research Company,Post Office Box 2189,Houston, TX 77252–2189, USAReferences and Notes1. P. F. Hoffman, A. J. Kaufman, G. P. Halverson, D. P.Schrag, Science 281, 1342 (1998).2. J. L. Kirschvink, in The Proterozoic Biosphere, J. W.Schopf and C. Klein, Eds. (Cambridge Univ. Press,Cambridge, 1992), pp. 51—52.3. G. S. Jenkins and C. R. Scotese, Science 282, 1644(1998).4. K. V. Evans, K. Lund, J. N. Aleinikoff, C. M. Fanning,Geol. Soc. Am. Abstr. Programs 29 (no. 6), 196(1997); A. J. Kaufman, A. H. Knoll, G. M. Narbonne,Proc. Natl. Acad. Sci. U.S.A. 94, 6600 (1997); S. A.Bowring and D. H. Erwin, GSA Today 8 (no. 9), 1(1998); P. F. Hoffman, A. J. Kaufman, G. P. Halverson,ibid., p. 1; B. Z. Saylor, A. J. Kaufman, J. P. Grotzinger,F. Urban, J. Sediment. Res. 68, 1223 (1998).5. M. J. Kennedy, B. Runnegar, A. R. Prave, K.-H. Hoff-mann, M. A. Arthur, Geology 26, 1059 (1998).6. M. J. Hambrey and W. B. Harland, Eds., EarthÕs Pre-Pleistocene Glacial Record (Cambridge Univ. Press,Cambridge, 1981); Palaeogeogr. Palaeoclimat.Palaeoecol. 51, 255 (1985); N. Eyles, Earth-Sci. Rev.35, 1 (1993).7. N. Christie-Blick, Brigham Young Univ. Geol. Studies23, 1 (1997).8. M. B Edwards, Sedimentology 22, 75 (1975); NorgesGeol. Unders. Bull. 394, 1 (1984); N. Christie-Blick, inGlacial-Marine Sedimentation, B. F. Molnia, Ed. (Ple-num, New York, 1983), pp. 703—776; M. Deynoux,Palaeogeogr. Palaeoclimat. Palaeoecol. 51, 97(1985); N. M. Lemon and V. A. Gostin, in The Evolu-tion of a Late Precambrian—Early Palaeozoic Rift Com-plex: The Adelaide Geosyncline, J. B. Jago and P. S.Moore, Eds. (Geological Society of Australia, Spec.Publ. 16, Sydney, Australia, 1990), pp. 149—163; G. M.Young and V. A. Gostin, in Glacial Marine Sedimen-tation; Paleoclimatic SigniÞcance, J. B. Anderson andG. M. Ashley, Eds. (Geological Society of America,Spec. Publ. 261, Boulder, CO, 1991), pp. 207—222.9. In marginal areas of sedimentary basins, most glacialsediment accumulates during retreat of the ice sheet[C. H. Eyles, N. Eyles, A. D. Miall, Palaeogeogr. Palaeo-climat. Palaeoecol. 51, 15 (1985); N. Eyles and C. H.Eyles, in Facies Models: Response to Sea LevelChange, R. G. Walker and N. P. James, Eds. (GeologicalAssociation of Canada, St. JohnÕs, Canada, 1992), pp.73—100; J. M. G. Miller, in Sedimentary Environments:Processes, Facies and Stratigraphy, H. G. Reading, Ed.(Blackwell, Oxford, UK, ed. 3, 1996), pp. 454—484].10. L. E. Sohl, Geol. Soc. Am. Abstr. Programs 29 (no. 6),195 (1997); L. E. Sohl, N. Christie-Blick, D. V. Kent,Geol. Soc. Am. Bull., in press.11. M. J. Kennedy, J. Sediment. Res. 66, 1050 (1996).12. The high concentration of dissolved CO2 would atÞrst have precluded the precipitation of carbonate inthe ocean.13. M. J. Kennedy and N. Christie-Blick, paper presentedat SEPM-IAS Research Conference in Sicily, Italy, 15to 19 September 1998.14. We thank L. A. Derry and J. Lynch-Stieglitz for re-views, and W. S. Broecker, P. E. Olsen and S. R.Hemming for discussions. This comment was writtenfollowing a seminar presented by P. F. Hoffman atLamont on 15 January 1999. We thank Hoffman forthe vigorous discussion that his visit inspired. Ourresearch in Neoproterozoic geology has been sup-ported by NSF grants EAR 92-06084, EAR 94-18294,and EAR 96-14070.9 February 1999; accepted 26 March 1999Response: Christie-Blick et al. point out thatNeoproterozoic glacial deposits in Australiaand North America differ in many respectsfrom those we reported in Namibia (1). Thiscalls for a modification of one statement inour characterization of a snowball Earth, soas to account for geological observations inareas of contrasting paleogeography. In addi-tion, Christie-Blick et al. question our inter-pretation of the carbon isotopic records inNamibia (1, 2).In our original report (1), we inferred thatwhen the world ocean was covered by sea ice(as a result of runaway ice-albedo feedback),“continental ice cover was thin and patchybecause of the virtual elimination of the hy-drologic cycle” (1). Unlike the Ghaub glaci-ation in Namibia, on which we based ourreport, Neoproterozoic glacial deposits inAustralia and North America are locally thick(.1 km), fill incised paleovalleys (,150 mdeep), contain faceted and striated stones,have associated outwash deposits, and recordas many as six magnetic polarity reversals (3,4, 5). These features indicate that substantialamounts of flowing ice existed on land fortime scales of 106 years. Some of this flowingice may be ascribed to conventional glacia-tion preceeding a snowball Earth, given thatice lines must reach ;35° latitude before anice-albedo runaway can occur (6), but theMarinoan glacial deposits in Australia cannotbe accounted for in this way because theyformed near sea level at ,8° paleolatitude (5,7). In fact, a limited hydrologic cycle wouldstill exist in a snowball Earth because ofsublimation of sea ice in the tropics and slowaccretion of ice at higher elevations (depend-ing on lapse rate) and higher latitudes. GivenT E C H N I C A L C O M M E N Twww.sciencemag.org SCIENCE VOL 284 14 MAY 1999 1087athe estimated bounds of 4 and 30 My for theduration of a snowball Earth (1), net accretionrates as low as 1.0 or 0.1 mm/year, respec-tively, would suffice to form glaciers 3 to 4km thick, which would flow gravitationallyand transport sediment. Direct glacial deliv-ery of sediment to the ocean would accountfor the predominance of subaqueous outwashdeposits (4). Sections of tidal rhythmites in-terpreted to have accumulated in shallow wa-ter near the paleo-equator (8) are remarkablyundisturbed by wave action, suggesting thatwaves were damped by sea ice (9). On theother hand, ice-free land area is indicated bythe presence of aeolian dune fields and peri-glacial sand-wedge polygons (10). The exis-tence of both ice-covered and ice-free landsurfaces points to a complex interplay be-tween sublimation, accretion and lateralflowage of ice under changing climatic con-ditions attending the progressive buildup ofatmospheric CO2 in a snowball Earth.The Ghaub glaciation in Namibia lacksthick glacial deposits, incised paleovalleys,faceted and striated stones, and outwash de-posits. These features can be attributed to thefact that this area was part of a vast, shallow-water, tropical platform (1), lacking high-lands on which ice would be subaerially ac-creted in a snowball Earth. The glacial depos-its are derived from the directly underlyingplatformal carbonates and consist of debrisadvected upward from the sea bed by ground-ed sea ice. Sublimation at the surface andfreezing at the base drove continual upwardadvection of the ice in which the debris wasentrained. The debris was released mostlywhen the sea ice dissipated at the end of thesnowball period, although some could havebeen released earlier as atmospheric concen-trations of CO2 rose causing sea ice to thin. Incontrast, the older Chuos glaciation in Nami-bia occurred at a time of tectonic instabilityand significant topography. The Chuos gla-cial deposits are locally thick (.1 km), fillincised paleovalleys (,180 m deep), containfaceted and striated stones derived from distalsources, including crystalline basement, andare associated with outwash deposits (1, 11).Both the Chuos and Ghaub glaciations inNamibia have cap carbonates and negativecarbon-isotope anomalies (the prime subjectsof our report), for which Christie-Blick et al.do not offer an alternative explanation to asnowball Earth.Christie-Blick et al. also question our in-terpretation of low carbon isotopic (d13C)values in the cap carbonate above the glacialdeposits, asserting that they require the oceanto be essentially lifeless for an extended timeperiod after snowball conditions had ceased.The d13C value of marine carbonate reflectsthe relative amounts of carbonate carbon andorganic carbon burial in sediments. In ourhypothesis, the low d13C values reflect highrates of carbonate precipitation resultingfrom intense chemical weathering in theextreme greenhouse conditions following themelting of sea ice. If the rate of alkalinitydelivery to seawater, and hence carbonateaccumulation, was very high, recovery ofbiological productivity could be instanta-neous after the deglaciation, and reach levelseven greater than modern, but still not af-fect significantly the d13C values of the capcarbonates.Paul F. HoffmanDaniel P. SchragDepartment of Earth andPlanetary Sciences,Harvard University,20 Oxford Street,Cambridge, MA 02138, USAE-mail: hoffman@eps.harvard.eduReferences1. P. F. Hoffman, A. J. Kaufman, G. P. Halverson, D. P.Schrag, Science 281, 1342 (1998).2. M. J. Kennedy, B. Runnegar, A. R. Prave, K.-H. Hoff-mann, M. A. Arthur, Geology 26, 1059 (1998).3. W. V. Preiss, The Adelaide Geosyncline (Geol. Surv.South Australia Bull. 53, Adelaide, 1987); G. M. Youngand V. A. Gostin, Geol. Soc. Am. Bull. 101, 834(1989); I. A. Dyson and C. C. von der Borch, inIncised-Valley Systems: Origin and Sedimentary Se-quences, R. W. Dalrymple, R. Boyd, B. A. Zaitlin, Eds.(Society of Sedimentary Geology, Spec. Publ. 51,Tulsa, OK, 1994), pp. 209—222; M. Levy, N. Christie-Blick, P. K. Link, in ibid., pp. 369—382.4. G. M. Young and V. A. Gostin, Precambrian Res. 39,151 (1988); M. N. Lemon and V. A. Gostin, in TheEvolution of a Late Precambrian—Early Palaeozoic RiftComplex: The Adelaide Geosyncline, J. B. Jago and P.S. Moore, Eds. (Geological Society of Australia, Spec.Publ. 16, Sydney, Australia, 1990), pp. 149—163.5. L. E. Sohl, N. Christie-Blick, D. V. Kent, Geol. Soc. Am.Bull., in press.6. G. R. North, R. F. Cahalan, J. A. Coakley, Jr., Rev.Geophys. Space Phys. 19, 91 (1981).7. P. W. Schmidt and G. E. Williams, Earth Planet. Sci.Lett. 134, 107 (1995).8. G. E. Williams, J. Geol. Soc. London 146, 97 (1989).9. R. W. Dalrymple, personal communication.10. M. Deynoux, Palaeogeogr. Palaeoclimat. Palaeoecol.39, 55 (1982); G. E. Williams and D. G. Tonkin,Australian J. Earth Sci. 32, 287 (1985); J.-N. Proustand M. Deynoux, in The EarthÕs Glacial Record: FaciesModels and Geodynamic Evolution, M. Deynoux et al.,Eds. (Cambridge Univ. Press, Cambridge, 1994), pp.121—144; G. E. Williams, Aust. J. Earth Sci. 45, 733(1998).11. K.-H. Hoffmann and A. R. Prave, Communs. Geol.Surv. Namibia 11, 47 (1996); P. F. Hoffman, A. J.Kaufman, G. P. Halverson, GSA Today 8 (no. 9), 1(1998).12. We thank R. B. Alley, W. S. Broecker, and R. W.Dalrymple for insightful comments.8 March 1999; accepted 26 March 1999T E C H N I C A L C O M M E N T14 MAY 1999 VOL 284 SCIENCE www.sciencemag.org1087a

Considering a Neoproterozoic Snowball Earth

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