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Neoproterozoic-Cambrian Biogeochemical Evolution$ CHAPTER 10 Neoproterozoic-Cambrian Biogeochemical Evolution$ Galen P. Halverson1, Matthew T. Hurtgen2, Susannah M. Porter3 and Alan S. Collins1 Contents 10.1. Introduction 351 10.2. Tectonics and Palaeoclimate 352 10.3. The Carbon Isotope Record 354 10.3.1. Inorganic carbon 354 10.3.2. Organic carbon 356 10.4. The Strontium Isotope Record 356 10.5. The Sulphur Isotope Record 357 10.6. Ediacaran-Cambrian Palaeobiology 359 10.7. Late Neoproterozoic-Cambrian Ocean Redox 362 10.7.1. An early Ediacaran oxygen pulse? 362 10.7.2. Transition to the Palaeozoic 363 10.8. Reorganisation of the Marine Carbon Cycle 364 10.9. Conclusions 365 Acknowledgements 365 10.1. Introduction The Proterozoic-Phanerozoic transition stands out as one of the greatest turning points in Earth’s history (Knoll and Walter, 1992). The beginning of the Phanerozoic is of course best known for the diversification of animal life, even though the first Metazoa appeared in the fossil record certainly by about 570 Ma (Narbonnne and Gehling, 2003), and possibly as early as about 632 Ma (Yin et al., 2007) 2 tens of millions of years prior to the onset of the Cambrian explosion. This ca. 100 million year window of early animal evolution and diversification coincided with profound biogeochemical changes to Earth’s surface environment, beginning with the end of the last snowball Earth event (Hoffman et al., 1998) at 635 Ma (Hoffmann et al., 2004; Condon et al., 2005) and ending with full oxygenation of the oceans. These environmental changes, in turn, overlapped and almost certainly interacted with the tectonic upheaval that broke up the last vestiges of the Rodinian supercontinent and gave birth to Gondwana (Derry et al., 1992a; Collins and Pisarevsky, 2005). The reorganisation of the biosphere during this time is manifested not only in the fossil record, but also in the various sedimentary proxies for seawater chemistry and redox state. For example, the most extreme negative d13C anomaly in marine carbonates in Earth’s history occurred sometime in the middle of the Ediacaran Period (Le Guerroue´ et al., 2006; Fike et al., 2006), while the Precambrian-Cambrian boundary interval is distinguished by large and relatively rapid fluctuations in marine d13C, long-lived peaks in seawater sulphate d34S values (e.g., Shields et al., 2004) and 87Sr/86Sr (Derry et al., 1992a; Halverson et al., 2007a; Shields, 2007b), a decrease in the relative abundance of reactive iron in shales (Canfield et al., 2007, 2008; Shen et al., 2008c), and a transient anomaly in d95/97Mo (Wille et al., 2008). Likewise, the abundance in seawater of a host of bioessential elements likely shifted at this time in response to the increased oxidation state of the ocean2atmosphere system $Halverson, G.P., Hurtgen, M.T., Porter, S.M., Collins, A.S. 2009. Neoproterozoic-Cambrian Biogeochemical Evolution. In: Gaucher, C., Sial, A.N., Halverson, G.P., Frimmel, H.E. (Eds.): Neoproterozoic-Cambrian Tectonics, Global Change and Evolution: a focus on southwestern Gondwana. Developments in Precambrian Geology, 16, Elsevier, pp. 3512365. 1 Tectonics, Resources, and Exploration (TRaX), Geology & Geophysics, School of Earth & Environmental Sciences, University of Adelaide, North Terrace, Adelaide SA 5005, Australia. 2 Department of Earth and Planetary Sciences, Northwestern University, 1850 Campus Dr., Evanston, IL 60208, USA. 3 Department of Earth Science, University of California at Santa Barbara, Santa Barbara, CA 93106, USA. Developments in Precambrian Geology, Volume 16 r 2010 Elsevier B.V. ISSN 0166-2635, DOI 10.1016/S0166-2635(09)01625-9 All rights reserved. 351 352 Galen P. Halverson et al. (Anbar, 2008). Determining the timescale and global synchronicity of biogeochemical events and linking them, as either drivers or consequences, of climatic, tectonic, and biological events, is an active area of research, with new data emerging from all corners of the globe. Areas of vigorous research include biogeochemical evolution during and in the aftermath of the end-Cryogenian (i.e., Elatina or Marinoan) snowball glaciation (Kasemann et al., 2005; Hurtgen et al., 2006; Elie et al., 2007; Le Hir et al., 2009); the tempo and driver of the presumed late Neoproterozoic oxygenation event (Canfield, 1998; Kennedy et al., 2006; Fike et al., 2006; Canfield et al., 2007) and their connection with the origin of the Metazoa (e.g., Peterson and Butterfield, 2005; Sperling et al., 2007; McFadden et al., 2008; Shen et al., 2008c); and the timing, duration, and causes of Ediacaran-Cambrian d13C anomalies (e.g., Kirschvink and Raub, 2003; Rothman et al., 2003; Maloof et al., 2005; Saltzman et al., 2005; Le Guerroue´ et al., 2006; Fike et al., 2006). The purpose of this chapter is to review the record of biogeochemical evolution spanning the Precambrian- Cambrian boundary. Such a narrative must be made within the context of contemporaneous tectonic, biological, and climatic change, but because each of these topics is treated in greater detail elsewhere in this volume, they will only be briefly reviewed here to lay the groundwork for the remainder of the chapter. Biogeochemical change is measured predominantly via chemostratigraphic and fossil records, and to a lesser extent, in changes in the style and distribution of chemical precipitates from the ocean. Here we will focus on traditional chemostratigraphic records 2 marine d13C, d34S, and 87Sr/86Sr 2 within the context of the biogeochemical evolution of the oceans and the related evidence for progressive oxygenation of Earth’s surface environment and the reorganisation of the marine carbon cycle. 10.2. Tectonics and Palaeoclimate The distribution of the continents and their interactions affect the nature of continental chemical weathering, atmospheric and oceanic circulation, and the magnitude of planetary albedo. Therefore, continental geography exerts a first-order control on global climate. The generally low-latitude arrangement of continents in the late Neoproterozoic (Kirschvink, 1992b; Li et al., 2008b; Figure 10.1) would have favoured a cool climate for multiple reasons. First, silicate weathering rates are greater in the tropics, meaning p levels would have been lower than at CO2 times when the continents were more evenly dispersed across the globe (Marshall et al., 1988; Worsley and Kidder, 1991). Second, because continents are more reflective than the open ocean, their low-latitude position during the Neoproterozoic would have increased the globally averaged surface albedo and decreased Earth’s energy budget (Kirschvink, 1992b). These effects would have been amplified by glaciation, which would have exposed more reflective (relative to seawater) continental shelves to weathering (Hoffman and Schrag, 2002). Finally, high organic productivity and elevated erosion rates would have enhanced organic carbon burial (Maloof et al., 2006), which would have further reduced atmospheric p . CO2 Figure 10.1 Palaeomagnetically viable plate tectonic reconstructions for the Gondwanan hemisphere in Ediacaran and Cambrian times, after Collins and Pisarevsky (2005). Neoproterozoic-Cambrian Biogeochemical Evolution 353 Despite the recognition that the remnants of the Rodinian supercontinent largely resided in the low latitudes (Meert and Torsvik, 2004; Li et al., 2008b), global palaeogeography for the late Cryogenian, Ediacaran, and Early Cambrian is poorly constrained. What is becoming clear in the recent literature, however, is that the mid-late Neoproterozoic was not a time of large, Asian-scale, continents as was previously thought (McWilliams, 1981; Stern, 1994; Dalziel, 1997), but instead was dominated by smaller, Australian-scale, continents that either rifted off Rodinia or were never a part of Rodinia, and came together along an intricate network of Ediacaran to Cambrian orogens that stitched Gondwana (Meert, 2003; Collins and Pisarevsky, 2005; Li et al., 2008b; Pisarevsky et al., 2008; Figure 10.1). These orogens, which have historically been lumped together as ‘Pan-African’ belts, record a global change in plate kinematics at B6502630 Ma, when a series of arcs amalgamated to Neoproterozoic continents [in particular, Azania, and much of the Arabian2Nubian Shield to the eastern margin of the Congo and Saharan cratons (Collins and Pisarevsky, 2005), and the Goia´s arc to the Congo2Sa˜o Francisco continent (Baldwin and Brown, 2008)]. The collisions of the Congo2Sa˜o Francisco continent with Amazonia (and Parana´) and the Saharan continent with the Congo2Sa˜o Francisco also appear to have occurred at this time (Figure 10.1). The second major series of tectonic events recorded in these orogens are the B5602520 Ma final amalgamation orogenies that occurred as Kalahari collided with both the south side of the Congo2Sa˜o Francisco continent and the Rio de la Plata continent, India collided with the combined Azania/Congo2Sa˜o Francisco continent, and Australia/Mawson and India collided along the Pinjarra Orogen. Between these two periods of mountain building, Laurentia rifted off Amazonia, forming the Iapetus Ocean (Cawood, 2005). The previously postulated latest Ediacaran-Cambrian supercontinent Pannotia (cf. Powell, 1995) is thus negated. The significantly modified view of Neoproterozoic palaeogeography (Figure 10.1) has important implications for the role of the continents in modulating or even driving Neoproterozoic-Cambrian climatic and biogeochemical change. For instance, the arrangement of Gondwanan landmasses in many smaller continents rather than a few large continents would have exposed a greater
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