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Palaeoproterozoic Supercontinents and Global Evolution: Correlations from Core to Atmosphere

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Palaeoproterozoic Supercontinents and Global Evolution: Correlations from Core to Atmosphere Palaeoproterozoic supercontinents and global evolution: correlations from core to atmosphere S. M. REDDY1* & D. A. D. EVANS2 1The Institute for Geoscience Research, Department of Applied Geology, Curtin University of Technology, GPO Box U 1987, Perth, WA 6845, Australia 2Department of Geology and Geophysics, Yale University, New Haven, CT 06520-8109, USA *Corresponding author (e-mail: [email protected]) Abstract: The Palaeoproterozoic era was a time of profound change in Earth evolution and represented perhaps the first supercontinent cycle, from the amalgamation and dispersal of a poss- ible Neoarchaean supercontinent to the formation of the 1.9–1.8 Ga supercontinent Nuna. This supercontinent cycle, although currently lacking in palaeogeographic detail, can in principle provide a contextual framework to investigate the relationships between deep-Earth and surface processes. In this article, we graphically summarize secular evolution from the Earth’s core to its atmosphere, from the Neoarchaean to the Mesoproterozoic eras (specifically 3.0–1.2 Ga), to reveal intriguing temporal relationships across the various ‘spheres’ of the Earth system. At the broadest level our compilation confirms an important deep-Earth event at c. 2.7 Ga that is mani- fested in an abrupt increase in geodynamo palaeointensity, a peak in the global record of large igneous provinces, and a broad maximum in several mantle-depletion proxies. Temporal coinci- dence with juvenile continental crust production and orogenic gold, massive-sulphide and porphyry copper deposits, indicate enhanced mantle convection linked to a series of mantle plumes and/or slab avalanches. The subsequent stabilization of cratonic lithosphere, the possible development of Earth’s first supercontinent and the emergence of the continents led to a changing surface environment in which voluminous banded iron-formations could accumulate on the conti- nental margins and photosynthetic life could flourish. This in turn led to irreversible atmospheric oxidation at 2.4–2.3 Ga, extreme events in global carbon cycling, and the possible dissipation of a former methane greenhouse atmosphere that resulted in extensive Palaeoproterozoic ice ages. Following the great oxidation event, shallow marine sulphate levels rose, sediment-hosted and iron-oxide-rich metal deposits became abundant, and the transition to sulphide-stratified oceans provided the environment for early eukaryotic evolution. Recent advances in the geochronology of the global stratigraphic record have made these inferences possible. Frontiers for future research include more refined modelling of Earth’s thermal and geodynamic evolution, palaeomagnetic studies of geodynamo intensity and continental motions, further geochronology and tectonic syntheses at regional levels, development of new isotopic systems to constrain geochemical cycles, and continued innovation in the search for records of early life in relation to changing palaeoenvironments. Supercontinents occupy a central position in the et al. 1984). These eustatic effects can influence long term processes of the Earth system. Their global climate by changing global albedo and rates amalgamations result from lateral plate-tectonic of silicate weathering (a sink for atmospheric motions manifesting mantle convection, and their carbon dioxide) on exposed versus drowned disaggregation, although thought by some to be continental shelves (Nance et al. 1986; Marshall externally forced by plume or hotspot activity (e.g. et al. 1988). Chemical weathering of pyrite and Storey et al. 1999), is widely considered to result organic carbon in mountain belts is a major influ- from their own thermal or geometric influences ence on the atmospheric oxygen budget as is their upon such convection (Gurnis 1988; Anderson burial in marine sediments. These processes are 1994; Lowman & Jarvis 1999; Vaughan & Storey in turn dependent on global tectonics (Berner 2007; O’Neill et al. 2009). Continental collisions 2006). Climatic changes, plus geographic patterns compress land area and thus are expected to lower of ocean circulation, will exert a profound influ- sea level; fragmentations involve crustal thinning ence on the biosphere (Valentine & Moores and generate young seafloor, and therefore 1970). Microbes play an important role in concen- should raise sea level (Fischer 1984; Worsley trating low-temperature mineral deposits through From:REDDY, S. M., MAZUMDER, R., EVANS,D.A.D.&COLLINS, A. S. (eds) Palaeoproterozoic Supercontinents and Global Evolution. Geological Society, London, Special Publications, 323, 1–26. DOI: 10.1144/SP323.1 0305-8719/09/$15.00 # Geological Society of London 2009. 2 S. M. REDDY & D. A. D. EVANS near-surface redox reactions (e.g. Dexter-Dyer Evolution of the core et al. 1984; Labrenz et al. 2000) and many mineral deposits are associated with specific tec- Today, the metallic core of the Earth consists of a tonic environments related to broader super- solid inner part, with radius of about 1200 km, sur- continent cycles (Barley & Groves 1992; Groves rounded by a liquid outer part with outer radius of et al. 2005). about 3500 km. As such, it occupies about one-sixth The connections illustrated above are based on of the Earth’s volume, and dominates the planetary concepts or a few well-constrained examples. The concentration of the siderophile elements Fe and Ni. actual record of supercontinents on Earth is not It is generally accepted that the core was originally yet well enough known to verify or modify the entirely fluid, and that through time the solid inner models in deep time. Prior to Pangaea (approxi- core nucleated and grew as the Earth cooled gradu- mately 0.2520.15 Ga) and Gondwana-Land ally (Jacobs 1953; Stevenson 1981; Jeanloz 1990). (0.5220.18 Ga), the possible configurations and However, the rate of this inner-core growth is even existence of Neoproterozoic Rodinia (c. highly uncertain and depends upon various esti- 1.020.8 Ga) are intensely debated (Meert & mates of radioactive element (mainly K) concen- Torsvik 2003; Li et al. 2008; Evans 2009). Prior to trations in the core versus mantle and crust Rodinia, an earlier supercontinental assemblage at (Nimmo et al. 2004; Davies 2007). 1.921.8 Ga has been suggested. Although only pre- Recent estimates of the age of inner core nuclea- liminary and palaeomagnetically untested models of tion, on a theoretical basis constrained by the geo- this supercontinent have been published (Rogers & chemical data, are typically about 1 Ga (Labrosse Santosh 2002; Zhao et al. 2002, 2004), its assembly et al. 2001; Nimmo et al. 2004; Butler et al. 2005; appears to have followed tectonic processes that are Gubbins et al. 2008). Much discussion on this remarkably similar to those of the present day topic is confounded with discussions of the intensity (Hoffman 1988, 1989). This supercontinent is of Earth’s ancient geomagnetic field. This is referred to by various names (e.g. Columbia, because two of the primary energy drivers of the Nuna, Capricornia) but here, and below, we refer geodynamo are thought to be thermal and compo- to it as Nuna (Hoffman 1997). sitional convection in the outer core due to inner It is unclear whether Nuna’s predecessor was a core crystallization (Stevenson et al. 1983). Early large supercontinent, or whether it was one of attempts to determine the palaeointensity of several large, but distinct coeval landmasses Earth’s magnetic field, which is among the most (Aspler & Chiarenzelli 1998; Bleeker 2003). laborious and controversial measurements in geo- Nonetheless, numerous large igneous provinces, physics, suggested an abrupt increase in moment with ages between 2.45 and 2.2 Ga, perforate the near the Archaean2Proterozoic boundary (Hale world’s 35 or so Archaean cratons and could rep- 1987). Subsequent refinements to techniques in resent an episode of globally widespread continental palaeointensity (e.g. the single-crystal technique rifting at that time (Heaman 1997; Buchan et al. applied by Smirnov et al. 2003) have generated 1998; Ernst & Buchan 2001). mixed results, but several of the measurements indi- Given that the Palaeoproterozoic era is defined cate a strong field in the earliest Proterozoic chronometrically at 2.5–1.6 Ga (Plumb 1991), it (Fig. 1a). More traditional palaeointensity tech- thus encompasses one or more episodes, perhaps niques complete the later Proterozoic time interval cycles, of global tectonics. As enumerated by the with results of generally low palaeointensity following examples, these tectonic events coincide (Macouin et al. 2003). There is currently no with fundamental changes to the Earth as an inte- systematic test among the various palaeointensity grated system of core, mantle, lithosphere, hydro- techniques, so absolute Precambrian palaeointensity sphere, atmosphere, and biosphere. Understanding values remain ambiguous. In addition, the large these changes requires the integration of seemingly apparent increase in palaeointensity at c. 1.2 Ga disparate geoscience disciplines. One pioneering (Fig. 1a) is underpinned by sparse data that lack review of this sort (Nance et al. 1986) has been some standard reliability checks (Macouin et al. followed by an incredible wealth of precise geo- 2004). However, we tentatively explore the possi- chronological data constraining ages within the bility that all the available records provide reliable Palaeoproterozoic geological record. In this estimates of ancient geomagnetic field strength. chapter we have compiled the currently available Given that there
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