Christie-Blick 1999-Science-SB Comment

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Considering a Neoproterozoic Snowball EarthNicholas Christie-Blick, et al. Science 284, 1087a (1999);DOI: 10.1126/science.284.5417.1087a The following resources related to this article are available online atwww.sciencemag.org (this information is current as of August 12, 2008 ): Updated information and services, including high-resolution figures, can be found in the onlineversion of this article at: http://www.sciencemag.org/cgi/content/full/284/5417/1087a This article cites 11 articles, 7 of which can be accessed for free: http://www.sciencemag.org/cgi/content/full/284/5417/1087a#otherarticles This article appears in the following subject collections: Geochemistry, Geophysics http://www.sciencemag.org/cgi/collection/geochem_phys Information about obtaining reprints of this article or about obtaining permission to reproducethis article in whole or in part can be found at: http://www.sciencemag.org/about/permissions.dtl Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by theAmerican Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright1999 by the American Association for the Advancement of Science; all rights reserved. The title Science is aregistered trademark of AAAS. Downloaded from www.sciencemag.org on August 12, 2008 T ECHNICAL C OMMENTG. M. Ashley, Eds. (Geological Society of America,Spec. Publ. 261, Boulder, CO, 1991), pp. 207—222.9. ...

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• Langue: English

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Considering a Neoproterozoic Snowball Earth Nicholas ChristieBlick,et al. Science284, 1087a (1999); DOI: 10.1126/science.284.5417.1087a

The following resources related to this article are available online at www.sciencemag.org (this information is current as of August 12, 2008 ):

Updated information and services,including highresolution figures, can be found in the online version of this article at: http://www.sciencemag.org/cgi/content/full/284/5417/1087a

This articlecites 11 articles, 7 of which can be accessed for free: http://www.sciencemag.org/cgi/content/full/284/5417/1087a#otherarticles

This article appears in the followingsubject collections: Geochemistry, Geophysics http://www.sciencemag.org/cgi/collection/geochem_phys

Information about obtainingreprintsof this article or about obtainingpermission to reproduce this articlein whole or in part can be found at: http://www.sciencemag.org/about/permissions.dtl

Science(print ISSN 00368075; online ISSN 10959203) is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 1999 by the American Association for the Advancement of Science; all rights reserved. The titleScienceis a registered trademark of AAAS.

T E C 1 A I C A 7 C O 8 8 E A T CSOaiLeTiOM E AeSpTSetTSzSiI COSwHElEETtN Paul F. Hoffmanet al.(1 Marinoan cap carbonates) ( ) developed a mod- 5, 11). Hoffmanet ified “snowball Earth” hypothesis (2) to ex- al.ascribe this trend to isotopic fractionation plain the association of Neoproterozoic low- associated with the hydration of atmospheri- latitude glaciation with the deposition of “cap cally derived CO in the surface ocean, with 2 carbonate” rocks bearing highly depleted car- depletion returning to bulk oceanic values as 13 bon isotopic values (d C#2the amount of CO5‰). Accord-in the atmosphere subsid- 2 ing to Hoffmanet al., the ocean becameed from;0.12 to 0.001 bar. This interpreta- completely frozen over as a result of a run-tion requires the ocean to have remained away albedo feedback, and primary biologi-effectively lifeless for an unduly long span cal productivity collapsed for an interval ofafter snowball conditions had ceased—com- geological time exceeding the carbon resi-parable to the duration of Marinoan deglacia- 5 dence time (.tion in Australia, including whatever time 10 years).During this inter- val, continental ice cover is inferred to havewas needed for the drawdown of CO by 2 4 6 been thin and patchy owing to the virtualcontinental weathering (10to 10years?) elimination of the hydrological cycle.(12) and for deposition of the cap carbonates 5 These ideas are worthy of serious scruti-(,10 years)(13). ny, and we would like to discuss geological Nicholas Christie-Blick difficulties not addressed in an earlier Letter Linda E. Sohl to the Editor (3). Climatological consider- Department of Earth and Environmental ations suggest that snowball conditions Sciencesand would have developed gradually, probably Lamont-Doherty Earth Observatory of 6 7 over a span of 10 to 10years. The wide- Columbia University, spread distribution of Sturtian (;750 to 700 Palisades, NY 10964– 8000,USA Ma) and Marinoan (;600 to 575 Ma) (4, 5) E-mail: [e-mail] glacial deposits, in places thousands of Martin J. Kennedy meters thick (6), as well as evidence for Exxon Production Research Company, $160 m drawdown of sea level for the Mari- Post Office Box 2189, noan event (7), indicate a vigorous hydrolog- Houston, TX 77252–2189, USA ical cycle, with marked erosion beneath large ice sheets and deposition under generally temperate conditions at and adjacent to ice R9f9R9N69a aN8 toN9a margins (8). Catastrophic termination of -. P. A. CdYYbSc( 6. E. FShYbSc( B. .P CSaiWefdc( 9. P. LU[eSZ(JPiSbPS150( -01. (-95.) snowball conditions interpreted by Hoffman .. E. G. FiefU[iic]( icLVS PfchSfczciP .icgdVSfS( E. W. et al. requires the bulk of these sediments to LU[dpY ScV 8. FaWic( ;Vf. (8SbTeiVZW Ncii. PeWff( have accumulated before or during the inter- 8SbTeiVZW( -9.)(p. 2-Ð2.. val in which the ocean was frozen because 0. B. L. EWc]icf ScV 8. K. LUdgWfW(JPiSbPS151( -311 (-95.) rapid melt out from any residual glacial ice, 1. F. O. ;iScf( F. GhcV( E. I. 6aWici]dYY( 8. H. ASccicZ( even thick glacial ice, would have left only a GScY. JcP. -a. -Oghf. PfcUfNag19(cd. 3)( -93 veneer of glacial and glacially derived sedi- (-94); 6. E. FShYbSc( 6. C. Fcdaa( B. H. ISeTdccW( PfcP. FNhY. -PNR. JPi. M.J.-.93( 33,, (-94); L. 6. ments. However, gradual retreat of the ice 7dkeicZ ScV 9. C. ;ekic(GJ- LcRNm5(cd. )9( - front is recorded in many areas by tens to (-95;) .P A. CdYYbSc( 6. E. FShYbSc( B. P. CSaiWefdc( hundreds of meters of glacial deposits, in iOiR.( p. -; 7. R. LSmade( 6. E. FShYbSc( E. P. BedgizcZWe( A. NeTSc(J. JSRiaSbh. ISg.65( -..0 (-95.) some cases with abundant outwash sediments 2. H. E. FWccWVm( 7. KhccWZSe( 6. K. PeSiW( F.)C. CdYY) (7–9). The time scale for this retreat is con- bScc( H. 6. 6eg[he(GScYcUm16( -,29 (-95). 4 servatively estimated in Australia as. 10 to 3. H. E. CSbTeWm ScV W. 7. CSeaScV( ;Vf.(:NfhVÕg PfS, 5 5 10 years,and most likely.10 years,on the PYSighcPSbS GYNPiNY ISPcfR(8SbTeiVZW Ncii. PeWff( 8SbTeiVZW( -95-;)PNYNScUScUf.PNYNScPYiaNh. basis of reversals in magnetic polarity in PNYNScSPcY.40( .22 (-952); I. ;maWf(:NfhV,JP.i IS.v Marinoan outwash sandstones (10). This sce- 24( - (-90). nario suggests that if the ocean surfacewere 4. I. 8[eifgiW)7aiU](.fiUVNa YcibU Mb.vi GScY. JhiRiSg 12( - (-94.) completely frozen, it must have become un- 5. H.7 ;VkSeVf(JSRiaSbhcYcUm11( 42 (-942);FcfUSg frozen well before the end of glaciation. ScY. MbRSfg. .iYY.293( - (-951;) I. 8[eifgiW)7aiU]( ic If highly depleted carbon isotopic values GYNPiNY,ENfibS JSRiaSbhNhicb( 7. A. HdaciS( ;V. P(aW) chb( IWk Pde]( -950)(p. 4,0Ð443; H. 9Wmcdhl( of cap carbonates are the result of the col- PNYNScUScUf. PNYNScPYiaNh.PNYNScSPcY.40( 94 lapse of primary productivity, then maximum (-952;) I. H. GWbdc ScV O. 6. Bdfgic( icLVS :vcYi, depletion of the ocean as a whole ought to hicb cT N CNhS PfSPNaOfiNbÐ:NfYm PNYNSczciP IiTh Cca, date from the time at which the ocean was dYSl: LVS -RSYNiRS GScgmbPYibS( E. 7. ESZd ScV P. L. HddeW( ;Vf. (BWdadZiUSa LdUiWgm dY 6hfgeSaiS( LpWU. frozen. However, in Namibia (1, 5), isotopic PhTa. -3( LmVcWm( 6hfgeSaiS( -9,)( .p-19 Ð-30; B. H. depletionincreasesup section from the base PdhcZ ScV O. 6. Bdfgic( icGYNPiNY ENfibS JSRiaSb, of the cap carbonate (a trend that is typical of hNhicb;PNYScPYiaNhiP JiUbÞiPNbPS( E. 7. 6cVWefdc ScV

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B. H. 6f[aWm( ;Vf. (BWdadZiUSa LdUiWgm dY 6bWeiUS( LpWU. PhTa. .3-( 7dhaVWe( 8J( -9-)(p. .,4Ð.... 9. DcbSeZicSa SeWSf dY fWVibWcgSem TSficf( bdfg ZaSUiSa fWVibWcg SUUhbhaSgWf VheicZ eWgeWSg dY g[W iUW f[WWg [8. C. ;maWf( I. ;maWf( 6. 9. HiSaa(PNYNScUScUf.PNYNSc, PYiaNh.PNYNScSPcY.40( -2 (-952); I. ;maWf ScV 8. C. ;maWf( icANPiSg EcRSYg: ISgdcbgS hc JSN CSvSY CVNbUS( K. B. WSa]We ScV I. .P ESbWf( ;Vf. (BWdadZiUSa 6ffdUiSgidc dY 8ScSVS( Lg. Ed[cÕf( 8ScSVS( -9.)(p. 40Ð-,,; E. H. B. HiaaWe( icJSRiaSbhNfm :bvifcbaSbhg: PfcPSggSg, ANPiSg NbR JhfNhiUfNdVm( C. B. KWSVicZ( ;V. (7aSU]kWaa( JlYdeV( NF( WV. 0( -93)( .p121 Ð 151]. -,. G.;. Ld[a(GScY. JcP. -a. -Oghf. PfcUfNag19(cd. 3)( -92 (-94); G. ;. Ld[a( I. 8[eifgiW)7aiU]( 9. O. FWcg( GScY. JcP. -a. .iYY.( ic peWff. --. H.E. FWccWVm(.J JSRiaSbh. ISg.6( -,2, (-93.) -.. M[W [iZ[ UdcUWcgeSgidc dY ViffdaiWV 8J kdhaV Sg . Þefg [SiW peWUahVWV g[W peWUpigSgidc dY USeTdcSgW ic g[W dUWSc. -0. H.E. FWccWVm ScV I. 8[eifgiW)7aiU]( pSpWe peWfWcgWV Sg L;PH)D6L KWfWSeU[ 8dcYWeWcUW ic LiUiam( DgSam( -2 gd -9 LWpgWbTWe -95. -1. WW g[Sc] G. 6. 9Weem ScV E. GmcU[)LgiWZaigz Yde eW) iiWkf( ScV W. L. 7edWU]We( .P ;. JafWc ScV L. K. CWbbicZ Yde VifUhffidcf. M[if UdbbWcg kSf keiggWc YdaadkicZ S fWbicSe peWfWcgWV Tm P. A. CdYYbSc Sg GSbdcg dc -2 ESchSem -9. WW g[Sc] CdYYbSc Yde g[W iiZdedhf VifUhffidc g[Sg [if iifig icfipeWV. Jhe eWfWSeU[ ic IWdpedgWedzdiU ZWdadZm [Sf TWWc fhp) pdegWV Tm ILA ZeScgf ;6K 9.),3,51( ;6K 91)-5.91( ScV ;6K 93)-1,4,. 9 AWTehSem -;9SUUWpgWV .3 HSeU[ -9

Response: Christie-Blicket al. point out that Neoproterozoic glacial deposits in Australia and North America differ in many respects from those we reported in Namibia (1). This calls for a modification of one statement in our characterization of a snowball Earth, so as to account for geological observations in areas of contrasting paleogeography. In addi- tion, Christie-Blicket al.question our inter- pretation of the carbon isotopic records in Namibia (1, 2). In our original report (1), we inferred that when the world ocean was covered by sea ice (as a result of runaway ice-albedo feedback), “continental ice cover was thin and patchy because of the virtual elimination of the hy- drologic cycle” (1). Unlike the Ghaub glaci- ation in Namibia, on which we based our report, Neoproterozoic glacial deposits in Australia and North America are locally thick (.1 km), fill incised paleovalleys (,150 m deep), contain faceted and striated stones, have associated outwash deposits, and record as many as six magnetic polarity reversals (3, 4, 5). These features indicate that substantial amounts of flowing ice existed on land for 6 time scales of 10 years. Some of this flowing ice may be ascribed to conventional glacia- tion preceeding a snowball Earth, given that ice lines must reach;35° latitude before an ice-albedo runaway can occur (6), but the Marinoan glacial deposits in Australia cannot be accounted for in this way because they formed near sea level at,8° paleolatitude (5, 7). In fact, a limited hydrologic cycle would still exist in a snowball Earth because of sublimation of sea ice in the tropics and slow accretion of ice at higher elevations (depend- ing on lapse rate) and higher latitudes. Given

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the estimated bounds of 4 and 30 My for the duration of a snowball Earth (1), net accretion rates as low as 1.0 or 0.1 mm/year, respec- tively, would suffice to form glaciers 3 to 4 km thick, which would flow gravitationally and transport sediment. Direct glacial deliv- ery of sediment to the ocean would account for the predominance of subaqueous outwash deposits (4). Sections of tidal rhythmites in- terpreted to have accumulated in shallow wa- ter near the paleo-equator (8) are remarkably undisturbed by wave action, suggesting that waves were damped by sea ice (9). On the other hand, ice-free land area is indicated by the presence of aeolian dune fields and peri- glacial sand-wedge polygons (10). The exis- tence of both ice-covered and ice-free land surfaces points to a complex interplay be- tween sublimation, accretion and lateral flowage of ice under changing climatic con- ditions attending the progressive buildup of atmospheric CO in a snowball Earth. 2 The Ghaub glaciation in Namibia lacks thick glacial deposits, incised paleovalleys, faceted and striated stones, and outwash de- posits. These features can be attributed to the fact 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 underlying platformal carbonates and consist of debris advected upward from the sea bed by ground- ed sea ice. Sublimation at the surface and freezing at the base drove continual upward advection of the ice in which the debris was entrained. The debris was released mostly when the sea ice dissipated at the end of the snowball period, although some could have been released earlier as atmospheric concen-

T E C 1 A I C A 7 C O 8 8 E A T trations of CO rose causing sea ice to thin. In 2 contrast, the older Chuos glaciation in Nami- bia occurred at a time of tectonic instability and significant topography. The Chuos gla- cial deposits are locally thick (.1 km), fill incised paleovalleys (,180 m deep), contain faceted and striated stones derived from distal sources, including crystalline basement, and are associated with outwash deposits (1, 11). Both the Chuos and Ghaub glaciations in Namibia have cap carbonates and negative carbon-isotope anomalies (the prime subjects of our report), for which Christie-Blicket al. do not offer an alternative explanation to a snowball Earth. Christie-Blicket al.also question our in- 13 terpretation of low carbon isotopic (d C) values in the cap carbonate above the glacial deposits, asserting that they require the ocean to be essentially lifeless for an extended time period after snowball conditions had ceased. 13 Thed C value of marine carbonate reflects therelativeamounts of carbonate carbon and organic carbon burial in sediments. In our 13 hypothesis, the lowd C values reflect high rates of carbonate precipitation resulting from intense chemical weathering in the extreme greenhouse conditions following the melting of sea ice. If the rate of alkalinity delivery to seawater, and hence carbonate accumulation, was very high, recovery of biological productivity could be instanta- neous after the deglaciation, and reach levels even greater than modern, but still not af- 13 fect significantly thed C values of the cap carbonates. Paul F. Hoffman Daniel P. Schrag Department of Earth and Planetary Sciences,

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Harvard University, 20 Oxford Street, Cambridge, MA 02138, USA E-mail: [e-mail]

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