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Climatic Cycles During a Neoproterozoic “Snowball” Glacial Epoch

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Climatic Cycles During a Neoproterozoic “Snowball” Glacial Epoch Climatic cycles during a Neoproterozoic “snowball” glacial epoch Ruben Rieu Repsol YPF, Exploration & Production, C/Orense 34, 28020 Madrid, Spain Philip A. Allen* Department of Earth Science and Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK Michael Plötze Institute for Geotechnical Engineering, ETH-Zürich, Schafmattstrasse 6, CH-8093 Zürich, Switzerland Thomas Pettke Institute for Geological Sciences, University of Berne, Baltzerstrasse 1-3, CH-3012 Bern, Switzerland ABSTRACT is enhanced during humid and warm climate The profound glaciations of the Neoproterozoic Cryogenian period (ca. 850–544 Ma) rep- conditions. Low CIA values, on the other hand, resent an extreme climatic mode when, it is claimed, Earth was fully or almost completely indicate the near absence of chemical weather- covered with ice for millions of years. We show that the geochemistry and mineralogy of fi ne- ing and consequently might refl ect cool and/or grained Neoproterozoic sedimentary rocks in Oman are best explained by climatic oscillations arid conditions. Since clay minerals form dur- that drove variations in the intensity of chemical weathering on contemporary land surfaces. ing progressive chemical weathering, largely The cold climate modes of the Cryogenian were therefore cyclical, punctuated with well- at the expense of plagioclase and potassium defi ned warm-humid interglacial periods. The hydrological cycle and the routing of sediment feldspar, with quartz being relatively stable, the were active throughout the glacial epoch, which requires substantial open ocean water. This ratio quartz/(quartz + K-feldspar + plagioclase) reconstruction represents a signifi cantly different target for numerical climate models at this (which is knows as the mineralogical index of critical time in the evolution of Earth’s biosphere. alteration [MIA]; see supplementary methods in the GSA Data Repository1) is also expected Keywords: snowball Earth, climate, weathering, glaciation, Neoproterozoic, Fiq, Oman. to be infl uenced by the intensity of chemi- cal weathering (Johnsson, 1993; Nesbitt et al., INTRODUCTION et al., 2004). The cap carbonate passes up grada- 1996; Nesbitt and Markovics, 1997). The possibility that Earth was repeatedly tionally into the marine shales and sandstones of Changes in clay mineral composition and completely frozen for periods of several mil- the Masirah Bay Formation (Allen and Leather, abundance may also refl ect variability in source lions of years in the Neoproterozoic (1000– 2006). The assemblage of glacial diamictites, rocks and/or hydrodynamic sorting during sedi- 544 Ma) (Hoffman et al., 1998) is currently debris-fl ow deposits, turbiditic sandstones, ment transport. To ensure a well-mixed prov- among the most interesting and controversial hemipelagic shales, and wave-rippled shoreface enance and to minimize the effects of hydrody- topics in Earth history. There is considerable sediments is thought to be end-Cryogenian in namic sorting, this study is limited to mudstone debate, however, as to whether the boundary age (Brasier et al., 2000; Allen et al., 2004). A beds and the mudstone matrices of diamictites. conditions required for Earth to enter or exit a low paleolatitude for Oman has been proposed Subtle grain-size differences between the mud- “snowball” state were reached (Crowley et al., (Kempf et al., 2000; Kilner et al., 2005). The stone samples may exist, however. MIA values 2001; Lewis et al., 2004; Pierrehumbert, 2004). Fiq Formation in Oman is an ideal test-bed for are particularly useful in this case, since they Additionally, a growing body of sedimen- the validity of key aspects of the snowball Earth are largely unaffected by sorting and abrasion tary (e.g., Leather et al., 2002; Kellerhals and hypothesis, and whatever can be learned about (Nesbitt et al., 1996; Nesbitt and Young, 1996). Matter, 2003) and paleobiological (Olcott et al., the snowball-type end-Cryogenian glaciation in In addition, the absence of a positive correlation 2005; Corsetti et al., 2006) evidence suggests Oman is likely to be of generic importance in between LOI (loss on ignition) and CaO content less severe freezing than envisaged in the snow- assessing Neoproterozoic climate change. implies that higher Ca contents refl ect incorpo- ball Earth hypothesis and that open continental ration of a higher proportion of less weathered shelves or equatorial oceans may have existed BULK MINERALOGY AND material and not of carbonate (Fig. DR3 [see even at times of glacial climax (Chandler and ELEMENTAL COMPOSITION AS AN footnote 1]). Sohl, 2000; Hyde et al., 2000; Crowley et al., INDEX OF CLIMATE CHANGE We analyzed 76 bulk samples from continu- 2001). It is crucial to remove some of these If severe climatic changes took place in the ous sections in the western Jabal Akhdar (Fig. 1; uncertainties if the Cryogenian climatic mode is Neoproterozoic era, a record of these changes Fig. DR1) that record the end-Cryogenian to be used as an example of the Earth system at would be expected to be preserved in the bulk glaciation and its direct aftermath, using its climatic limit (Hoffman and Schrag, 2002). mineralogical and chemical compositions of laser-ablation–inductively coupled plasma–mass The Huqf Supergroup of Oman provides critical the associated siliciclastic rocks, which depend spec trometry (ICP-MS) for trace elements (32 evidence for the dynamics of glaciation during on the intensity of chemical weathering in the samples), X-ray fl uorescence (XRF) for major the Cryogenian. source areas (Nesbitt and Young, 1982; Nesbitt elements, and, Rietveld analysis of X-ray dif- The Huqf Supergroup of Oman crops out in et al., 1996). Therefore, changes in the chemical fraction (XRD) spectra to obtain quantitative three main areas (Fig. 1). In the Jabal Akhdar of and mineralogical compositions of sedimentary mineralogical compositions (Tables DR1 and northern Oman, the Huqf Supergroup contains a rocks can potentially be used as a proxy for cli- DR2). Analytical procedures are described in relatively thick (at least 1.5 km) succession (Fiq mate change. the supplementary information (see footnote 1). Formation) of glacigenic and nonglacial marine The chemical index of alteration (CIA) is Corrections of CIA values for carbonate-derived sedimentary rocks that fi lled a fault-bounded potentially useful to evaluate changes in cli- Ca were <0.1 CIA unit (<0.02 wt% CaO). Cor- basin formed by continental extension, overlain mate (Nesbitt and Young, 1982; McLennan by a thin (<8 m) cap carbonate, which has carbon et al., 1993; Fedo et al., 1995; Nesbitt et al., 1GSA Data Repository item 2007074, supple- isotopic values depleted in 13C and is known as 1996; Scheffl er et al., 2003). High CIA values mentary information on methods and additional geo- 2+ chemical data, is available online at www. geosociety. the Hadash Formation (Leather et al., 2002; Allen refl ect the removal of mobile cations (e.g., Ca , org/pubs/ft2007.htm, or on request from editing@ + + Na , K ) relative to stable residual constituents geosociety.org or Documents Secretary, GSA, *E-mail: [email protected]. (Al3+, Ti4+) during chemical weathering, which P.O. Box 9140, Boulder, CO 80301, USA. © 2007 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, April April 2007; 2007 v. 35; no. 4; p. 299–302; doi: 10.1130/G23400A.1; 3 fi gures; Data Repository item 2007074. 299 Predominant lithologies A Iran IRAN B may cause signifi cant deviations from these Jabal Akhdar Huqf Dolomite Diamictite ideal pathways (Fedo et al., 1995; Nesbitt et al., ARABIAN GULF 23 10 Fara Fm. Limestone v v Extrusive 1996; Nesbitt and Markovics, 1997). UAE Oman GULF OF OMAN PC/C Buah Fm. Saudi Arabia Buah (542 ± 0.3 Ma) Shale / siltstone Basement Fm. Shuram Fm. MUSCAT Sandstone Shuram RESULTS U.A.E. Khufai Fm. Yemen Jabal Fm. Throughout the Fiq and Masirah Bay Forma- Akhdar Khufai Masirah Bay Fm. sampled 500km tions, there are signifi cant compositional and NAFUN GROUP ARA Hadash sections Mas.Bay Hadash Fm. Mirbat cap carbonate Halfayn Fm. Shareef Fm. SAUDI SULTANATE Hadash (ca. 635 Ma) mineralogical variations (Figs. 2 and 3). When ARABIA OF (ca. 802 Ma) Marsham Fm OMAN (Upper Member) plotted in A-CN-K and Qtz-Pl-Kfs space, the Neoproterozoic outcrops data defi ne trends roughly parallel to the A-CN Sedimentary rocks20o 30 Huqf Arkahawl Fm. area 20° N 24° N HUQF SUPERGROUP (Middle Member) Basement and Pl-Qtz boundaries (Fig. 2), suggesting vari- ability in the extent of chemical weathering of Fiq Formation cap carbonate MIRBAT GROUP MIRBAT ARABIAN m k the sediment in the source area (Fedo et al., SEA 1 ABU MAHARA GROUP ABU MAHARA (Mirbat Sandstone Formation) Ayn Fm. (Lower Member) 1995; Nesbitt et al., 1996). Before interpreting SALALAH 100 km Saqlah Mirbat area v v v unconformity Ghubrah the data in terms of climatic variations, the infl u- YEMEN 54° E58 00 58° E 712± 1.6 Ma 726 ± 0.4 Ma ence of potential changes in grain size, hydro- Figure 1. Main outcrop areas of Neoproterozoic basement and sedimentary rocks in Oman dynamic sorting, provenance, and diagenetic (A) and stratigraphic framework of Huqf Supergroup (B). UAE—United Arab Emirates. alteration on the composition of the sediments must be evaluated. The fi rst-order trend in CIA with stratigraphic rections for phosphate-derived Ca were typi- space (Fig. 2), the position of incipiently weath- position is also seen in the mineralogical matu- cally <1.5 CIA units (rarely up to 5). ered samples strongly depends on source rock rity (MIA) of the Fiq and Masirah Bay depos- When plotted in A-CN-K (Al2O3–CaO + lithology. In both compositional and miner- its (Fig. 3), as would be expected if the relative N2O–K2O) space (Fig. 2), sediments produced alogical space, however, changes due only to enrichment in clay minerals (high CIA) resulted by intense chemical weathering appear in posi- increased chemical weathering cause samples from increased alteration of feldspars rather tions commensurate with high values of CIA to move roughly parallel to the A-CN or P-Q than from hydrodynamic sorting.
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