The Ecology of the Cambrian Radiation - Andrey Zhuravlev - Chapter 5

05-C1099 8/10/00 2:05 PM Page 90 CHAPTER FIVE Toni T. Eerola Climate Change at the Neoproterozoic-Cambrian Transition Varangerian and lower Sinian glacial deposits are found in Argentina, Uruguay, MatoGrosso(Brazil),Namibia,Laurentia,andprobablysouthernBrazil,whichwere all situated close together during Neoproterozoic-Cambrian times. According to con-tinental paleoreconstructions, glacial deposits of these regions, together with those of Scotland, Scandinavia, Greenland, Russia, Antarctica, and Australia, formed the Varangerian-Sinian Glacial Zone of the supercontinent Rodinia. Tectonic activity associated with the amalgamation of Rodinia and Gondwana was probably related to the origin of these deposits, as in the case of mountain glaciers that formed in uplifted areas of fragmenting or colliding parts of this supercontinent. In such cir-cumstances, the Pan-African and Brasiliano orogenies and the site of opening of the Iapetus Ocean would have been in key positions. However, some paleomagnetic re-constructions locate these regions near the South Pole, where glaciers could have formed even in the absence of tectonic events. In this case, the change to warm cli-mate and the evolutionary explosion of the Cambrian could have been due to rapid shift of continents to equatorial latitudes, although these changes might also have been triggered by supercontinent breakup. These events are reflected in the isotopic records of strontium and carbon, which provide some of the best available indicators of the climatic and environmental changes that occurred during the Neoproterozoic-Cambrian transition. They also appear to reveal the occurrence of a discrete cold period in the Cambrian: the disputed lower Sinian glaciation. INTRODUCTION The Neoproterozoic-Cambrian transition was characterized by ophiolite formation (Yakubchuk et al. 1994), the formation and breakup of supercontinents (e.g., Bond etal.1984),theCambrianevolutionaryexplosion(Moores1993;Knoll1994),andin-tense climatic changes, among which the most important might be considered glacia- 05-C1099 8/10/00 2:05 PM Page 91 CLIMATE CHANGE AT THE NEOPROTEROZOIC-CAMBRIAN TRANSITION 91 Figure 5.1 Time distribution of glaciogenic sedimentary rocks, showing their sporadic nature and possible relationship with supercontinentality. Source: Modified from Young 1991. tions (e.g., Hambrey and Harland 1985) and the shift from Neoproterozoic icehouse to Cambrian greenhouse conditions (Veevers 1990; Tucker 1992). Atleast10majorglacialperiodshavebeenrecordedpriortothePleistocene(Young 1991; Eyles 1993) (figure 5.1). Probably the most extensive and enigmatic of these occurred during the Neoproterozoic and at the beginning of the Cambrian, at ~900– 540Ma(HambreyandHarland1985;Young1991;Eyles1993;MeertandVanderVoo 1994). There are signs of four Neoproterozoic-Cambrian glacial periods (figures 5.1– 5.2): the Lower Congo (~900 Ma), the Sturtian (~750–700 Ma), the Varangerian (~650–600 Ma), and the lower Sinian (~600–540 Ma) (Hambrey and Harland 1985; Eyles1993;MeertandVanderVoo1994).Thereare,however,alsoproposalsforonly two (Kennedy et al. 1998) or even five (Hoffman et al. 1998a; Saylor et al. 1998). This chapter presents a brief overview of Neoproterozoic-Cambrian climate changes and events, with emphasis on the Varangerian and lower Sinian glacial peri-ods and the subsequent global warming in the Cambrian (see also chapters in this vol-ume by Brasier and Lindsay; Seslavinsky and Maidanskaya; Smith; and Zhuravlev). PALEOMAGNETIC RECONSTRUCTIONS AND GLACIERS The application of paleomagnetic investigations to research into the Neoproterozoic has yielded important findings. It is now recognized that continental drift may have been faster than at present (Gurnis and Torsvik 1994) and that glaciers might have formed at sea level even in low latitudes (e.g., Hambrey and Harland 1985; Schmidt 05-C1099 8/10/00 2:05 PM Page 92 92 Toni T. Eerola Figure 5.2 Locations of some glaciogenic deposits formed during the 1000–540 Ma interval. Source: Modified from Meert and Van der Voo 1994. and Williams 1995), implying a significant climatic paradox (Chumakov and Elston 1989). The glacial interpretation of many Neoproterozoic deposits was questioned by Schemerhorn (1974). Many factors have been presented to explain the generation of glaciers at low latitudes (see Meert and Van der Voo 1994), such as the incorrect in-terpretation of paleolatitudes due to remagnetization (e.g., Gurnis and Torsvik 1994); global glaciation, i.e., “the snow-ball Earth” (Kasting 1992; Kirschvink 1992; Hoff-man et al. 1998b); astronomical causes, such as modification of the obliquity of the earth’s rotation (Williams 1975; Schmidt and Williams 1995); and tectonic causes, such as the formation of mountain glaciers in rift and collisional zones of supercon-tinents (Eyles 1993; Eyles and Young 1994; Young 1995). According to Dalziel et al. (1994) and Gurnis and Torsvik (1994), continents were situated close to the southernpole during the Vendian (figure 5.3), in which case con-tinental glaciation would be expected. Meert and Van der Voo (1994) argued, how-ever, that continents occupied middle latitude position at that time. SUPERCONTINENTS, CORRELATIONS, AND THE VARANGERIAN–LOWER SINIAN GLACIAL ZONE Glacial horizons are often treated as the best markers for stratigraphic correlation (e.g., Hambrey and Harland 1985; Christie-Blick et al. 1995), although this has been contested by Chumakov (1981). Varangerian glacial deposits, ~600 Ma (figure 5.3), seem to be correlative in Namibia (Numees Formation, Gariep Group), in Laurentia (e.g., Gaskiers and Ice Brook formations; Eyles and Eyles 1989; Young 1995), and 05-C1099 8/10/00 2:05 PM Page 93 CLIMATE CHANGE AT THE NEOPROTEROZOIC-CAMBRIAN TRANSITION 93 Figure 5.3 Reconstruction of the Neopro-terozoic supercontinent Rodinia, at ~600 Ma (modified from Dalziel et al. 1994) and its coeval glaciogenic record: the Varangerian– Lower Sinian Glacial Zone (cf. Eerola and Reis 1995; Young 1995). Deposits of Antarctica (Stump et al. 1988) and Australia (Schmidt and Williams 1995) are also included (cf. Eerola 1996). possibly also in the Santa Bárbara Basin, Rio Grande do Sul State, southern Brazil (Eerola 1995, 1997; Eerola and Reis 1995). Coeval glacial deposits in the present-day North Atlantic region have also been related to these (e.g., Hambrey 1983). Glacial formations of similar age are also found in Mato Grosso and Minas Gerais, Brazil (Uh-lein et al. 1999), western Brazil (Alvarenga and Trompette 1992), Argentina (Spalletti and Del Valle 1984), and possibly Uruguay (F. Preciozzi, pers. comm., 1994) (see figures 5.2 and 5.3). Evidences for lower Sinian cold climate are found in West Gondwana (Schwarzrand Subgroup, Nama Group in Namibia [Germs 1995]; and the Taoudenni Basin in West Africa [Bertrand-Sarfati et al. 1995; Trompette 1996]) and in China and Kazakhstan (Hambrey and Harland 1985). A glacial deposit of Cam-brian age has been tentatively identified in the Itajaí Basin, Santa Catarina State, southern Brazil (P. Paim, pers. comm., 1996), but the origin and age have still to been confirmed. 05-C1099 8/10/00 2:05 PM Page 94 94 Toni T. Eerola Given that Laurentia and Fennoscandia were situated close to South America in Neoproterozoic-Cambrian times, forming the supercontinent Rodinia (e.g., Bond et al. 1984; Dalziel et al. 1994; Young 1995) (figure 5.3), extensive glaciation is pos-sible (Meert and Van der Voo 1994). Such connections may play an important role in paleogeographic reconstructions. According to the paleogeography of Dalziel et al (1994), the glacial formations at 600 Ma constituted a continuous zone that can be traced from Svalbard, through Scandinavia, Greenland, and Scotland, to eastern Laurentia and western South Amer-ica (Young 1995) (figure 5.3). Eerola and Reis (1995) and Eerola (1996) called this zone the Varangerian-Sinian Glacial Zone, on the basis of the ages of the glacial de-posits, and suggested that the zone appears to continue to Mato Grosso, Argentina, probably to Uruguay,southernBrazil, Namibia, Antarctica (Nimrod area, Stump etal. 1988), and Australia (Marinoan glacial deposits; e.g., Schmidt and Williams 1995). The tectonics of Rodinia probably had a strong influence on the generation and dis-tribution of these glacial deposits (Eyles 1993; Moores 1993; Young 1995). DEBATE ON THE SEDIMENTARY RECORD OF NEOPROTEROZOIC GLACIATIONS Although the existence of Neoproterozoic glaciations is widely accepted, there have been authors who have questioned the concept with reference to some particular de-posits, for instance, the Bigganjargga tillite in northern Norway (figure 5.4) (Crowell 1964; Jensen and Wulff-Pedersen 1996), some parts of the basal Windermere Group in Canada (Mustard 1991), and the Schwarzrand Subgroup of the Nama Group in Namibia (P. Crimes, pers. comm., 1995; Saylor et al. 1995). The whole concept of the NeoproterozoicglaciationwasputindoubtbySchemerhorn(1974)andrecentlycriti-cized by P. Jensen (pers. comm., 1996). The problem is that in the case of some Neo-proterozoicdeposits,thesimplepresenceofdiamictiteshasbeenconsideredsufficient proof of glacial origin (Schemerhorn 1974; Eyles 1993; Jensen and Wulff-Pedersen 1996). Distinguishing between the results of glacial and other processes is a difficult task, both in ancient sequences (Chumakov 1981) and in more recent deposits—for in-stance, in alluvial fan facies (Carraro 1987; Kumar et al. 1994; Marker 1994; Owen 1994; Hewitt 1999), especially in volcanic settings (Ui 1989; Eyles 1993), and even when glacial influence is evident (Vinogradov 1981; Clapperton 1990; LeMasurier et al. 1994). The problem is that a variety of processes can generate deposits that may easily be confused with those of glaciation (e.g., Crowell 1957; Eyles 1993; Bennett et al. 1994). This is especially true in relation to diamictites (figure 5.5), which could also result from mud flows, debris flows, lahars, debris-avalanches, or meteorite im-pacts in many different environments (Crowell 1957, 1964; Ui 1989; Rampino 1994) and are not, in themselves, climatic indicators (Crowell 1957, 1964; Heezen and Hol- ... - tailieumienphi.vn