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Paleomagnetic Polarity Reversals in Marinoan (Ca. 600 Paleomagnetic polarity reversals in Marinoan (ca. 600 Ma) glacial deposits of Australia: Implications for the duration of low-latitude glaciation in Neoproterozoic time Linda E. Sohl* Department of Earth and Environmental Sciences and Lamont-Doherty Earth Observatory of Nicholas Christie-Blick} Columbia University, Palisades, New York 10964 Dennis V. Kent Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964, and Department of Geological Sciences, Rutgers University, Piscataway, New Jersey 08854 ABSTRACT surface of the Earth. Supercontinents assembled, (CLIMAP Project Members, 1976) have as- then broke apart with apparent great rapidity sumed steep thermal gradients and general cool- A paleomagnetic investigation of Marinoan (Moores, 1991; Dalziel, 1991, 1995; Hoffman, ing of oceanic surface water at middle to high lat- glacial and preglacial deposits in Australia was 1991; Powell et al., 1993). The first complex itudes, with little or no change in tropical ocean conducted to reevaluate Australia’s paleo- metazoan organisms appeared (e.g., Cowie and surface temperatures. Some researchers have geographic position at the time of glaciation Brasier, 1989; Lipps and Signor, 1992; Bengtson, even proposed that the average equatorial sea- (ca. 610–575 Ma). The paleomagnetic results 1994; Grotzinger et al., 1995; Xiao et al., 1998), surface temperature has remained within 1 °C of from the Elatina Formation of the central perhaps spurred by enhanced carbon burial that its present level for billions of years (Fairbridge, Flinders Ranges yield the first positive regional- yielded significant amounts of oxygen (Knoll, 1991). Only in the past few years has evidence of scale fold test (significant at the 99% level), as 1992, 1994). There were two broad intervals of cooling of the tropics come to light, and the cal- well as at least three magnetic polarity inter- widespread and extended (and possibly repetitive) culated temperature changes are only a few de- vals. Stratigraphic discontinuities typical of glaciation in the Neoproterozoic Era: the Sturtian grees in magnitude (Beck et al., 1992; Stute et al., glacial successions prevent the application of a glacial interval (ca. 780–700 Ma) and the 1995; Broecker, 1996; Patrick and Thunell, 1997). magnetic polarity stratigraphy to regional cor- Varanger glacial interval (which includes the In light of Pleistocene climate modeling, the relation, but the positive fold test and multiple Marinoan glaciation in Australia and Ice Brook occurrence of low-latitude glaciation in Neopro- reversals confirm the previous low paleolati- glaciation in Canada, ca. 610–575 Ma; see Knoll terozoic time seems bizarre; however, it is the rar- tude interpretation of these rocks (mean D = and Walter, 1992; Bowring and Erwin, 1998). ity of low-latitude glaciation in the Precambrian α 214.9°, I = –14.7°, 95 = 12.7°, paleolatitude = These glacial intervals are unlike any in Phanero- that is strange. After all, models of stellar evolu- 7.5°). The underlying preglacial Yaltipena For- zoic Earth history because of their great severity. tion suggest that 600 Ma, the sun’s luminosity mation also carries low magnetic inclinations Paleomagnetic evidence from glacial strata sug- should have been 3%–6% lower than present lev- α (mean D = 204.0°, I = –16.4°, 95 = 11.0°, paleo- gests that continental-scale ice sheets advanced els (e.g., Gough, 1981). Under those conditions, latitude = 8.4°), suggesting that Australia was over land at or near sea level and equatorward of producing an extremely frigid climate should located at low paleolatitude at the onset of 20° latitude (Sturtian of North America; Park, have been simple. However, the existence of wa- glaciation. The number of magnetic polarity in- 1994, 1997; Marinoan of Australia; Schmidt et al., ter-lain sedimentary deposits dating back to 3.9 tervals present within the Elatina Formation 1991; Schmidt and Williams, 1995). On only one Ga indicates that temperatures were warm enough and the Elatina’s lithostratigraphic relation- other occasion in Earth history, the Paleoprotero- for running water, a problem known as the “faint ship to other Marinoan glacial deposits suggest zoic Huronian glaciation (ca. 2.2 Ga), may glacia- young sun” paradox (Sagan and Mullen, 1972; that glaciation persisted at low latitudes in Aus- tion have been equally severe (Evans et al., 1997; Newman and Rood, 1977). To compensate for re- tralia for a minimum of several hundreds of Williams and Schmidt, 1997). duced solar luminosity, researchers have postu- thousands to millions of years. Paleoclimatologists accustomed to Pleis- lated a supergreenhouse effect, caused by elevated tocene glacial conditions tend to greet word of levels of greenhouse gases such as methane and INTRODUCTION low-latitude glaciation with polite disbelief, be- CO2, that declined with the passage of time until cause the Pleistocene climate models that have the atmosphere achieved a composition similar to The Neoproterozoic Era (1000–543 Ma) marks guided thinking about climatic processes have that of the present day (Sagan and Mullen, 1972; a time of unusual and profound changes on the never needed to address such an extreme case of Newman and Rood, 1977; Owen et al., 1979; global cooling. Most climate models for glacial Kasting, 1992). (Note that an “effective” level of *E-mail: [email protected]. intervals since the days of the CLIMAP Project CO2 is often used as a replacement for other Data Repository item 9965 contains additional material related to this article. GSA Bulletin; August 1999; v. 111; no. 8; p. 1120–1139; 12 figures; 3 tables. 1120 PALEOMAGNETIC POLARITY REVERSALS, MARINOAN GLACIAL DEPOSITS, AUSTRALIA greenhouse gases in climate modeling [Washing- rocks normally associated with warm, arid, or laminated siltstones contain dropstones that are ton, 1992].) Estimates of normal atmospheric tropical settings. Redbeds, carbonates, and/or inferred to have been ice rafted (Crowell, 1964; CO2 levels ca. 600 Ma are around 840 ppm evaporites are commonly found stratigraphically Ovenshine, 1970; Vorren et al., 1983) rather than (Carver and Vardavas, 1994), compared to the beneath the glacial rocks, and the end of glacial transported by some other means (e.g., biological preindustrial value of ~280 ppm. As Kasting deposition for both the Sturtian and Varanger in- rafting or gravitational processes; Bennett et al., (1992) pointed out, having to account for low-lat- tervals is typically marked by the presence of thin, 1994; Menzies, 1995). Signs of erosion beneath itude glaciation at sea level in a supergreenhouse laterally persistent (over hundreds of kilometers), ice sheets are preserved, particularly around world presents a rather difficult problem. laminated, buff to pink dolomites now called cap basin margins, as rare, glacially striated pave- The entire Proterozoic climate scene may carbonates (because they cap the glacial deposits; ments (Biju-Duval and Gariel, 1969), in some seem too far removed from present-day condi- Roberts, 1976; Williams, 1979; Deynoux, 1985; cases associated with valleys tens to hundreds of tions to be of any interest to paleoclimatologists Young, 1992; Kennedy, 1996). Third, early paleo- meters deep (Christie-Blick, 1983; Edwards, working in more recent geologic intervals. How- magnetic studies of rocks associated with (but not 1984; Karfunkel and Hoppe, 1988; Sønderholm ever, the physics of climatic processes does not from) Varanger-age glacial deposits in Greenland and Jepsen, 1991). Periglacial features, such as change—only the boundary conditions do. While and Norway yielded low paleolatitudes (Harland sand wedges and frost-heaved blocks, are also great strides have been made in improving our and Bidgood, 1959; Bidgood and Harland, 1961). found in connection with a number of the glacial understanding of the various forcings and feed- The widespread distribution of the glacial depos- deposits (e.g., Nystuen, 1976; Deynoux, 1982; backs that control climate today and in the recent its, their unusual relationship with carbonates and Williams and Tonkin, 1985; Zhang, 1994). Con- geologic past (e.g., CLIMAP Project Members, other arid-climate deposits, and the paleomag- sidering the extent and regional stratigraphic con- 1976; Hays et al., 1976; Hansen and Takahashi, netic data led Harland (1964a) to postulate that text of those deposits, it is clear that debris flows 1984; Broecker and Denton, 1989), the role that somehow, the entire Earth had become glaciated or meteorite impacts cannot explain the Neopro- these perturbations play in long-term climate during an interval he referred to as “the great In- terozoic deposits (Young, 1993). Suggestions trends and the possible range of climate variabil- fra-Cambrian glaciation” (see also Chumakov that the deposits are glacially derived but were ity is still poorly understood. Understanding the and Elston, 1989; Kirschvink, 1992). As the field produced by glacial activity in an alpine setting circumstances under which the Earth’s climate of paleomagnetism developed and methods for (Eyles, 1993) cannot account for all circum- can be compelled to change from unusual measuring and analyzing paleomagnetic rema- stances, because several examples were clearly warmth to extreme cold, as well as the time nence improved, additional studies were under- deposited at some distance from features with scales over which various processes interact, may taken in rocks associated with certain glacial units significant topographic relief
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