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84 PRESERVATION OF PRIMARY REE PATTERNS WITHOUT CE ANOMALY Figure 4. Plot of Ce SN /Ce SN * vs. Pr SN /Pr SN * that allows to quantify Ce and La anomalies. The Mooidraai carbonates show Pr SN /Pr SN * ratios close to unity, i.e. similar to those of shales, which demonstrates the absence of negative Ce anomalies. Ce SN /Ce SN * ratios below unity indicate the presence of positive La anomalies in the Mooidraai carbonates, as commonly found in seawater and marine chemical sediments. The absence of significant negative Ce anomalies suggests that surface seawater during “Mooidraai times” was too reducing to allow for oxidation of Ce(III). For further explanation see text. Data from Tsikos et al., 2001 (1); Bau et al., 1999 (2); Bau et al., 2003, and M. Bau, unpublished data (3); Taylor and McLennan, 1985 (4, 5). the non-redox-sensitive REY together with the lack of siliciclastic detritus also preclude deposition of the Mooidraai Formation in coastal waters in close proximity to a land mass, but rather support the drowned-platform environment suggested by Beukes (1983). The lack of clear positive Eu SN anomalies in PAASnormalized REY distribution patterns demonstrates the absence of a high-temperature hydrothermal REY component from shallow seawater during “Mooidraai times”. This is different from what is seen in Neoarchean carbonates, such as those from the Lower Transvaal Supergroup, South Africa (Kamber and Webb, 2001), or from the Hamersley Supergroup, Australia (M. Bau, unpublished data), and in marked contrast to what is observed in iron formation from the lower Transvaal Supergroup, that give strong evidence for a blacksmoker-type hydrothermal REY source (Bau and Dulski, 1996; Bau et al., 1997). We emphasize that there exist no significant Ce SN anomalies in the Mooidraai limestones or dolomites. As discussed previously (Bau and Dulski, 1996; Kamber and Webb, 2001), the commonly used equation to quantifiy Ce SN anomalies (Ce SN /Ce SN * = Ce SN /(0.5La SN +0.5Pr SN )) must not be applied in studies of marine chemical sediments due to the presence of positive La SN anomalies that suggest Ce SN * values that are much too high. A plot of Ce SN /Ce SN * vs. Pr SN /Pr SN * (Figure 4; Pr SN * = 0.5Ce SN +0.5Nd SN ; for details on this approach see, for example, Bau and Dulski, 1996; Kamber and Webb, 2001) clearly demonstrates that neither the limestones nor the dolomites from the Mooidraai Formation are characterized by distinct negative Ce SN anomalies (Figure 4). The Mooidraai carbonates, although free from detrital aluminosilicates, show Pr SN /Pr SN * ratios that are considerably smaller than those of Lower Carboniferous limestones from Ireland and England, for example, that display negative Ce SN anomalies (Bau et al., 2003; M. Bau, unpublished data). Their Pr SN /Pr SN * ratios are, however, very similar to those typical of shales (Figure 4). The absence of Ce SN anomalies from the dolomites does not result from the dolomitization process but is a primary feature of the chemical sediment and, therefore, reflects the absence of any Ce SN anomaly from Mooidraai seawater. We emphasize that the Mooidraai REY data are yet another example of how neglecting the existence of positive La SN anomalies in seawater and in some marine chemical sediments can lead to false claims of the presence of negative Ce SN anomalies. Using a Pr SN /Pr SN * ratio of >1.1 as the positive criterion for a negative Ce SN anomaly, it appears that except for a few individual analyses of samples from iron-formations and cherts in India (e.g., Kato et al., 2002, and references therein), there exist no published data from Archean or Early Paleoproterozoic chemical sediments that show primary Ce SN anomalies. The absence of distinct negative Ce SN anomalies from the Mooidraai carbonates, however, is surprising considering that the Mooidraai Formation was deposited immediately above the Hotazel Formation. The Hotazel Formation contains the first and hence oldest significant deposits of marine sedimentary Mn oxides in the geological record. The precipitation of these sedimentary Mn oxides represents a major event in Earth’s history as the Mn oxide deposits are widespread and several meters thick (for a recent discussion, see Schneiderhan et al., 2006). These Mn oxides are the prot-ore for the Mn ore in the Kalahari Manganese Field, the largest deposit of sedimentary Mn oxides on Earth. The precipitation and, in particular, the preservation of such a huge amount of marine sedimentary Mn oxides requires a highly oxygenated depositional environment (e.g., Kirschvink et al., 2000; Kirschvink and Weiss, 2002; Kopp et al., 2005). The redox level necessary to stabilize significant amounts of Mn(IV) oxides, however, is significantly higher than that needed to oxidize Ce(III) and to form Ce anomalies. Hence, it comes as no surprise that, indeed, negative Ce SN anomalies occur in manganese formation from the Hotazel Formation (Figure 3). Their stratigraphic position above the Mn-oxidesbearing Hotazel Formation and, in particular, above the SOUTH AFRICAN JOURNAL OF GEOLOGY
MICHAEL BAU AND BRIAN ALEXANDER 85 ~2.32 Ga-old Timeball Hill Formation that does not show mass-independent sulphur isotope fractionation (Bekker et al., 2004), indicates that the deposition of the Mooidraai carbonates post-dates the “Great Oxygenation Event”. The lack of specific Ce depletion in Mooidraai seawater, therefore, suggests that after the deposition of the Hotazel Formation the redox-level of seawater in the Griqualand-West sub-basin had returned to a lower level insufficient to oxidize Ce(III). Apparently, Mn(II) oxidation had consumed the emerging oxygen reservoir and biogenic oxygen production was not yet able to balance the additional oxygen demand. Thus, local marine anoxia was re-established during Mid- Paleoproterozoic Mooidraai times. It remains to be seen whether this retreat to marine anoxia in the Mid-Paleoproterozoic was a worldwide phenomenon or whether it was confined to the Griqualand-West sub-basin of the Kaapvaal Craton. Nevertheless, the REY data from the Mooidraai limestones and dolomites suggest that even the oxygenation of the surface water of the Earth’s oceans did not follow a unidirectional trend from generally anoxic to generally oxic conditions. Rather, oxygenated surface environments became progressively more abundant until the previously exotic “oxygen oases” had grown and become more persistent before they eventually represented the normal state of the system. Hence, it might be more apt to refer to the oxygenation of the Earth’s surface system in the Paleoproterozoic as the “Great Oxygenation Period”. Such a soft rather than sharp transition is also more in line with, for example, the contemporaneous formation of oxic and anoxic paleosols (Rye and Holland, 1998, Yang and Holland, 2003), the observation of abundant hydrothermal mantle Os in pyrites that do not show mass-independent sulphur isotope fractionation (Bekker et al., 2004; Hannah et al., 2004), and low Mo concentration and unfractionated Mo isotope ratios in the Transvaal Supergroup (Siebert et al., 2004). Conclusions Comparison of marine shallow-water limestone and silicified dolomite from the Mid-Paleoproterozoic Mooidraai Formation, Transvaal Supergroup, South Africa, demonstrates that the REY distribution of the marine sedimentary carbonate was preserved during dolomitization and silicification. With one exception, both lithologies display all the details of the REY distibution in present-day seawater, such as positive anomalies for La, Gd and Lu, and a super-chondritic Y/Ho ratio. However, the Mooidraai carbonates lack the negative Ce anomaly that indicates oxidation of Ce(III) in the Earth’s surface system. It appears that after the deposition of Mn oxides in the Hotazel Formation, which is indicative of a highly oxygenated supergene environment, conditions again became sigificantly less oxic. This suggests that during the transition period from a rather reducing to an oxygenated atmospherehydrosphere system in the Paleoproterozoic (the “Great Oxygenation Period”) the redox-level of the Earth’s surface ocean fluctuated between reducing and oxic. Acknowledgements We gratefully acknowledge many fruitful discussions of Precambrian geochemistry and geology with H.D. Holland, J. Gutzmer, and J. Kasting. The paper also benefited from constructive reviews by A. Lepland, G. Shields, and J. Gutzmer. In particular, however, we want to thank Nic Beukes for his advice, help, and kind hospitality during many years of collaborative work. His efforts have without doubt stimulated worldwide interest in and promoted South African chemical sediments as a valuable source of information on the chemical evolution of the Earth’s atmospherehydrosphere system. References Aharon, P. (2005) Redox stratification and anoxia of the early Precambrian oceans: Implications for carbon isotope excursions and oxidation events. Precambrian Research, 137, 207-222. Banner, J.K., Hanson, G.L. and Meyers, W.J. (1988) Rare earth element and Nd isotopic variations in regionally extensive dolomites from the Burlington-Keokuk Formation (Mississippian): Implications for REE mobility during carbonate diagenesis. Journal of Sedimentary Petrology, 58, 415-432. Bau, M. and Dulski, P. (1996) Distribution of yttrium and rare-earth elements in the Penge and Kuruman Iron-Formations, Transvaal Supergroup, South Africa. Precambrian Research, 79, 37-55. Bau, M., Höhndorf, A., Dulski, P. and Beukes, N.J. (1997) Sources of rareearth elements and iron in Paleoproterozoic iron-formations from the Transvaal Supergroup, South Africa: Evidence from neodymium isotopes. Journal of Geology, 105, 121-129. Bau, M., Romer, R., Lüders, V. and Beukes, N.J. (1999) Pb, O, and C isotopes in silicified Mooidraai dolomite (Transvaal Supergroup, South Africa): Implications for the composition of Paleoproterozoic seawater and “dating” the increase of oxygen in the Precambrian atmosphere. Earth and Planetary Science Letters, 174, 43-57. Bau, M., Romer, R., Lüders, V. and Dulski, P. (2003) Tracing element sources of hydrothermal mineral deposits: REE and Y distribution and Sr-Nd-Pb isotopes in fluorite from MVT deposits in the Pennine Orefield, England. Mineralium Deposita, 38, 992-1008. Bekker, A., Holland, H.D., Wang, P.-L., Rumble, D. III, Stein, H.J., Hannah , J.L., Coetzee, L.L. and Beukes, N.J. (2004) Dating the rise of atmospheric oxygen. Nature, 427, 117-120. Beukes, N. J. (1983) Palaeoenvironmental setting of iron-formations in the depositional basin of the Transvaal Supergroup, South Africa. In: A.F. Trendall and R.C. Morris (Editors), Iron-formation: Facts and Problems. Developments in Precambrian Geology, 6, 131-209. Buick, I.S., Maas, R. and Gibson, R. (2001) Precise U-Pb titanite age constraints on the emplacement of the Bushveld Complex, South Africa. Journal of the Geological Society London, 158, 3-6. Canfield, D.E. (2005) The early history of atmospheric oxygen: Homage to Robert M. Garrels. Annual Reviews Earth and Planetary Sciences, 33, 1-36. Catling, D.C. and Claire, M.W. (2005) How Earth’s atmosphere evolved to an oxic state: A status report. Earth and Planetary Science Letters, 237, 1-20. Hannah, J.L., Bekker, A., Stein, H.J., Markey, R.J. and Holland, H.D. (2004) Primitive Os and 2316 Ma age for marine shale: implications for Paleoproterozoic glacial events and the rise of atmospheric oxygen. Earth and Planetary Science Letters, 225, 43-52. Holland, H.D. (2004) The geological history of seawater. In: H.D. Holland and K.K. Turekian (Editors), Treatise of Geochemistry, Elsevier, Amsterdam, The Netherlands, 6, 583-625. Kamber, B.S. and Webb, G.E. (2001) The geochemistry of late Archaean microbial carbonate: Implications for ocean chemistry and continental erosion history. Geochimica et Cosmochimica Acta, 65, 2509-2525. Kato Y., Kano T. and Kunugiza, K. (2002) Negative Ce Anomaly in the Indian Banded Iron Formations: Evidence for the Emergence of Oxygenated SOUTH AFRICAN JOURNAL OF GEOLOGY
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Iron sedimentation and the neodymiu
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The process(es) by which IFs were d
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This thesis represents original and
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viii
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goal of the research is to discern
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Figure 1. Map of southern Africa sh
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appropriate to briefly discuss: 1)
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100 10 MORB 1 0.1 raw data (mg/kg)
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10 -2 hydrothermal fluid 10 -3 10 -
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seawater and returned to the sedime
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100 MCO + 3 + M(CO 3 )- 2 rare eart
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net effect of carbonate complexatio
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feature was not recognized in early
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sound geochemical basis for anomalo
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deviation of the 143 Nd/ 144 Nd rat
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available, it should be possible to
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modern and Archean oceans, as well
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Bau M., Möller P., and Dulski P. (
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Elderfield H. and Greaves M.J. (198
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Haley B.A., Klinkhammer G.P. (2003)
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Lee J.H. and Byrne R.H. (1993) Comp
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Piepgras D.J. and Wasserburg G.J. (
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Tosiani T., Loubet M., Viers J., Va
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Abstract The benefits of inductivel
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List of Figures Figure 1. Sample de
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1. Introduction The advent of induc
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Figure 1. Flow-chart diagram of the
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during decomposition (SiF 4 and CO
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Fractionation between the three iso
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cps, then 3000 cps must be subtract
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variety of ways. These range from r
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Table 3. Change in HFSE concentrati
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0.90, corresponding to an anomalous
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10 2 JDo-1 reference values 10 1 IF
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Figure 5. Diagram depicting various
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20 18 basalt BHVO-2 (n=5) run preci
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The method precision for elements p
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Two approaches may be utilized to d
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For the more recent, full 32 elemen
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isotopic data for geologic CRMs, av
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Table 4. Interferences observed for
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value uncertainty (as %RSD) is repr
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1.50 FeR-4 (n=2) 1.25 JUB / referen
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1 IF-G/shale 0.1 0.01 La Ce Pr Nd P
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consistent (App. 1), and similar to
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1.50 SGR-1b (n=3) 1.25 JUB / refere
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1.50 JCh-1 (n=3) 1.25 JUB / referen
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1.50 JDo-1 (n=5) 1.25 JUB / referen
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determinations in carbonate rocks r
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1.50 JMn-1 (n=4) 1.25 JUB / referen
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eference value. The result is that
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eference value and the measured JUB
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the 32 analyzed elements, with Ti b
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Govindaraju K. (1995) 1995 working
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Appendix 1. Analytical data Appendi
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Appendix 1. Literature reference va
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Appendix 1 continued. Data in mg/kg
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the concentration of the major elem
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Potential interferences on monoisot
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1500 0.5 M HCl 4.0 interference on
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4000 0.40 0.20 3500 0.17 mg/kg 0.18
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interference on 95 Mo in sample sol
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