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Climatic cycles during a “snowball” glacial epoch

Ruben Rieu Repsol YPF, Exploration & Production, C/Orense 34, 28020 Madrid, Spain Philip A. Allen* Department of 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 The profound glaciations of the Neoproterozoic 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 - covered with ice for millions of . 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 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: , 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 spectrometry (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. This suggests (80–100), whereas incipiently weathered sedi- boundary, resulting in higher values of both that the fi rst-order trend in CIA is not controlled ments plot near the feldspar join (CIA of 50–70). CIA and MIA. Diagenetic alteration, changes in by the hydrodynamic sorting mechanism. In Qtz-Pl-Kfs (quartz–plagioclase–K-feldspar) provenance, and, in A-CN-K space, grain size The composition of samples that have been little affected by chemical weathering (low CIA, Fig. 2B) suggests a granodioritic composition of sediment sources, which is in agreement with other major-element and trace-element contents that suggest a constant granitic to granodioritic source (Figs. DR4 and DR5 [see footnote 1]) (Taylor and McLennan, 1985; McLennan et al., 1993; Fedo et al., 1997), and with felsic source terrains indicated by clast lithologies in diamic- tites (granite, rhyolite, and volcanic tuff in addi- tion to sedimentary clasts; Allen et al., 2004). Subtle variations in source rock composition may have caused some spread in sediment com- positions, but the lack of correlation between CIA and provenance indicators (e.g., Th/Sc, Al/Ti, Zr/Ti) (Taylor and McLennan, 1985; McLennan et al., 1993; Fedo et al., 1997) (Fig. DR4) demonstrates that the fi rst-order changes in CIA cannot be explained by provenance changes alone. Importantly, the main trends in CIA are largely unaffected by the lithological facies of the beds sampled, and therefore they are clearly superimposed on the detailed sedi- Figure 2. Mineralogical and compositional variations of diamictite matrix and mudstone mentary architecture of the succession. Since samples of Fiq (black dots) and Masirah Bay (open circles) Formations. A: Q-P-K (quartz– CIA values do not coincide with facies changes plagioclase–K-feldspar) mineralogical space (mineralogical index of alteration [MIA]). B: A-CN-K (Al O –CaO + Na O–K O) compositional space (chemical index of alteration [CIA]). (Fig. 3), we can rule out the dominance of prov- 2 3 2 2 enance changes or changes in depositional envi- MIA = [quartz/(quartz + K-feldspar + plagioclase) ] × 100 and CIA = [Al2O3 /(Al2O3 + K2O + Na2O + CaO*)] × 100. In B, arrow parallel to A-CN boundary is ideal chemical weathering trend of ronment on CIA values. granodioritic bedrock, which may be shifted toward K-apex (dashed arrow) due to potassium Relatively low Zr/Sc ratios (McLennan et al., metasomatism (illitization). Correction for potassium metasomatism is made by projecting 1993) (Fig. DR4e [see footnote 1]) indicate that data points back onto ideal weathering pathway from K-apex. Projection of data points back onto P-K join suggests source rocks of granodioritic composition on average. Qtz—quartz; Fiq and Masirah Bay rocks are the products of Pl—plagioclase; Kfs—K-feldspar; Cpx—clinopyroxene; Hbl—hornblende; Sm—smectite; a fi rst-order cycle from erosion to deposition Bt—biotite; Ms—muscovite; Kln—kaolinite, Chl—chlorite. without polycyclic reworking, although the high

300 GEOLOGY, April 2007 CIA (uncorrected: dashed line) CIA (uncorrected: dashed line) would be favored by relative sea-level fall or by A 55 65 75 85 B 65 75 85 a change to wetter climate following glaciation.

cap carbonate cap carbonate Increased cycling of sediment during periods MB Fm. 1600 MB Fm. T6 of higher tectonic activity in rifts, resulting in glacial glacial lower CIA values, would normally be associ-

Fiq F7 F7 ated with increased footwall topography and 70 80 40 60 80 100 generally increasing water depths in hanging- 1200 CIA (corrected: solid line) MIA wall depo centers. The genetic stratigraphy of the T5 F6 Fiq Forma tion (Leather et al., 2002; Allen et al., Legend: 2004) and Masirah Bay Formation (Allen and T4 glacial 800 F5 Cap carbonate Leather, 2006), however, suggests an increase T3 Diamictites in CIA values associated with transgression Stratigraphic height (m)

Fiq Formation T2 Sandstones and a decrease of CIA values associated with F3 glacial Shales, siltstones, shallowing- and coarsening-up trends into gla- thin sandstones cially infl uenced deposits (Fig. 3). Although 400 No exposure Dropstones small-scale variations in CIA values observed T1 F3 Diamictite unit between transgressive surfaces T3 and T5 (Fig. 3) increasing chemical weathering incr. chemical weathering T1 Transgression seem to follow the trends expected to be associ- 0 m ated with base-level variation, the major trends in 60 70 80 6080 100 CIA (corrected: solid line) MIA CIA are not closely linked to paleowater-depth variations, and where there is a correlation (as Figure 3. Variations in chemical and mineralogical indexes of alternation (CIA and MIA) above diamictite F6; Fig. 3), rapid deepening is with stratigraphic height for section in Wadi Sahtan (A) and critical section across glacial- associated with an increase in CIA values rather postglacial transition at Hadash, Wadi Mistal (B). Numbering of diamictites and fl ooding sur- faces is after Leather et al. (2002) and Allen et al. (2004). Errors in CIA due to uncertainties in than a decrease. We therefore rule out base-level major-element concentrations are <1.5% (<1 CIA unit). Two sigma error bars are indicated for change as the mechanism responsible for the MIA values. MB Fm.—Masirah Bay Formation. major trends in CIA values, and we are confi dent that the fi rst-order trends in CIA are driven by variations in the intensity of chemical weather- proportion of sedimentary clasts (average 40%) a similar fi rst-order trend is revealed, consisting ing associated with climate change. Such climate suggests that inheritance of a previous weath- of three intervals during which chemical weath- changes also infl uenced sea levels through the ering history of the sediment is possible. Such ering was reduced as indicated by relatively low build-up and melting of continental ice. inheritance is supported by the fact that CIA CIA and MIA values. Reduced chemical weath- Since no chronometer for the climatic cycles values in glacially infl uenced deposits are never ering in these intervals is in agreement with in the Fiq Formation is available, their duration as low as would be expected for sediments pro- the presence of distinctive sedimentary facies is unknown. A typical sediment accumulation duced solely by mechanical erosion. However, (diamictites, dropstone-bearing laminites) that rate in rift basins of 0.1–0.2 mm y–1 implies minor amounts of chemical weathering also suggest a cold climate. These intervals alternate that cycles comprising 200–500 m of stratigra- may have occurred in glaciofl uvial and other with units that are characterized by relatively phy would represent 1–5 m.y., and that the Fiq periglacial environments. high CIA and MIA values and that lack evidence glacial epoch lasted a total of 10 m.y. or more. During burial diagenesis, potassium meta- for any glacial infl uence during sedimentation, We do not know the forcing mechanism or somatism may strongly change the bulk compo- which are interpreted to represent interglacial internal system dynamics for climatic cyclicity sition, and consequently the CIA, of sediments periods. Because the lowermost diamictite unit on this time scale. However, we note that a simi- (Fedo et al., 1995). Potassium metasomatism is (F1) is not preserved in the sampled sections, lar, dynamic climatic regime of multiple glacia- suspected to have infl uenced the sedimentary an older glacial period than that represented tions of short duration (<5 m.y.) alternating with rocks studied because the samples that deviate in this section by the lowermost unit (F3) may longer periods of globally warmer interglacial from the ideal weathering trend are enriched in have existed (Allen et al., 2004). Importantly, or nonglacial conditions within a long icehouse potassium, which is supported by the observa- the end of the entire glacial epoch corresponds epoch is now the preferred view of the late tion of potassium-rich overgrowths on quartz to a major increase in CIA and MIA values in Paleozoic glacial epoch (Scheffl er et al., 2003; and feldspar minerals (Fig. DR2) and the com- the lowermost Masirah Bay Formation. These Fielding et al., 2006), rather than a single pro- mon presence of illite in the studied samples. To values are the highest found in the succession tracted glaciation between 320 and 265 Ma. allow for a maximum possible effect of potas- (CIA > 80, MIA = 100). sium metasomatism on CIA values, we applied It is possible that the observed compositional IMPLICATIONS FOR CRYOGENIAN a correction assuming that the ideal weathering and mineralogical variations in the Fiq Forma- CLIMATE EXTREMES trend originated from a granodioritic source tion and lowermost Masirah Bay Formation are The recognition of climatic cycles embedded rock composition (Fig. 3). due to variations in chemical weathering that within the Fiq glacial succession is important for are unrelated to climate (Johnsson, 1993). For the evaluation of climatic extremes during the VARIATIONS IN CHEMICAL example, the residence time of material exposed Neoproterozoic, since it is problematic for such WEATHERING DURING A to chemical weathering while stored in continen- cycles to have been generated on a completely SNOWBALL EPOCH tal basins depends on catchment size and gov- frozen Earth characterized by a hydrological Both uncorrected CIA values and those cor- erning tectonics. As catchments enlarge, more shutdown or much-reduced water cycle driven rected for a maximum amount of potassium sediment is stored in trunk streams and alluvial by sublimation. Consequently, the importance metasomatism are plotted as a function of their valleys. Evacuation of stored and chemically for climatic reconstruction rests on the precise stratigraphic height in Figure 3. In both cases, weathered sediment from catchment valleys points in time within the glacial epoch when

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302 GEOLOGY, April 2007