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Evidence from Ice Ages, Isotopes, and Impacts

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Evidence from Ice Ages, Isotopes, and Impacts IT’S TIME!—RENEW YOUR MEMBERSHIP & SAVE 15% CELEBRATE GSA’S 125TH AnnIVERSARY OCT O BER 2013 | VOL. 23, NO. 10 A PUBLICATION OF THE GEOLOGICAL SOCIETY OF AMERICA® Evolution of Earth’s climatic system: Evidence from ice ages, isotopes, and impacts OCTOBER 2013 | VOLUME 23, NUMBER 10 SCIENCE ARTICLE GSA TODAY (ISSN 1052-5173 USPS 0456-530) prints news and information for more than 25,000 GSA member read- ers and subscribing libraries, with 11 monthly issues (April/ 4 Evolution of Earth’s climatic system: May is a combined issue). GSA TODAY is published by The Evidence from ice ages, isotopes, and Geological Society of America® Inc. (GSA) with offices at 3300 Penrose Place, Boulder, Colorado, USA, and a mail- impacts ing address of P.O. Box 9140, Boulder, CO 80301-9140, USA. Grant M. Young GSA provides this and other forums for the presentation of diverse opinions and positions by scientists worldwide, Cover: Laminated argillites of the Paleoproterozoic Gow- regardless of race, citizenship, gender, sexual orientation, ganda Formation in the Huronian outcrop belt, Ontario, religion, or political viewpoint. Opinions presented in this publication do not reflect official positions of the Society. Canada. Fine (dark) and coarse layers are thought to rep- resent seasonal freeze-thaw cycles, but paleomagnetic © 2013 The Geological Society of America Inc. All rights reserved. Copyright not claimed on content prepared results indicate a tropical setting. Note striking occur- wholly by U.S. government employees within the scope of rence of pairs of identical light layers, possibly represent- their employment. 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Young, Dept. of Earth Sciences, University of Western took place in the Ediacaran Period. In contrast to previous glacia- Ontario, London, Ontario, Canada N6A 5B7; [email protected] tions, these ice sheets developed in high latitudes and many follow mountain building episodes. During the Ediacaran Period, ABSTRACT Earth’s climatic zonation and controls appear to have undergone a Multiple glaciations took place near the beginning and end of radical change that persisted throughout the Phanerozoic Eon. 13 the Proterozoic Eon. Neoproterozoic (Cryogenian) glacial depos- The change may coincide with the world’s greatest negative d C its are more widespread than those of older (Paleoproterozoic) excursion, the Shuram event, here interpreted as the result of a glacial episodes. Paleomagnetic results suggest that most Protero- very large marine impact that decreased the obliquity of the zoic glaciogenic rocks were deposited at low paleolatitudes. Some ecliptic, causing the Earth’s climatic system to adopt its present contain enigmatic evidence of strong seasonal temperature varia- configuration. Attendant unprecedented environmental reorgani- tions, and many formed at sea level. These attributes inspired both zation may have played a crucial role in the emergence of complex the snowball Earth hypothesis and the high obliquity theory, but life forms. only the latter explains strong seasonality at low latitudes. The INTRODUCTION Proterozoic glaciations may have been triggered by drawdown of There is evidence of local glaciation in Archean times (at ca. atmospheric CO2 during enhanced weathering of elevated super- continents. Multiple glaciations resulted from a negative feedback 2.9 Ga), and several Paleoproterozoic (Huronian) glaciations are loop in the weathering system that ended when the superconti- recorded in North America, NW Europe, South Africa, Western nent broke apart. A radical reorganization of the climatic system Australia, and Asia (Young, 1973, 2013; Ojakangas, 1988; Tang Figure 1. Schematic depiction of some aspects of Earth’s climatic history. Note the two great Proterozoic glacial episodes (at left), each followed by an atmospheric “oxidation event.” The “barren billion” has little evidence of glaciations or iron formations. Two contrasted climatic regimes are depicted in the central part of the diagram. During most of the Proterozoic eon, glaciations appear to have been concentrated in low latitudes, possibly as a result of Earth’s high obliquity (Williams, 2008). OBER 2013 OBER 2013 T A radical change occurred near the beginning of the Phanerozoic eon, when a putative large marine impact brought about a major shift in the orientation of the world’s C 13 O spin axis, resulting in a new climatic zonation that still exists today. The impact is inferred from the Shuram d C anomaly (right side; modified from fig. 1 of Halverson et al., 2010) and associated stratigraphic and sedimentological evidence. Note that the Shuram anomaly and impact (age after Bowring et al., 2009) is depicted as younger Y | A than the Gaskiers glaciation (age after Mason et al., 2013, their fig. 3) but has been considered older by some. See text for full explanation. G.O.E.—great oxidation event; N.O.E.—Neoproterozoic oxidation event; G—glacial episode. GSA TOD GSA Today, v. 23, no. 10, doi: 10.1130/GSATG183A.1. 4 and Chen, 2013). Among Neoproterozoic glaciations (Fig. 1), the PRECAMBRIAN GLACIAL DEPOSITS IN TIME AND SPACE most convincing and widespread are the Sturtian (730–705 Ma) The distribution of glaciogenic rocks (Hambrey and Harland, and the Marinoan (663–635 Ma), although geochronological 1981) (Fig. 1, left side) suggests that younger Proterozoic glacia- problems remain. Between ca. 2.2 Ga and 730 Ma, there is little tions were more widespread than their older counterparts. This evidence of glaciation (but see Williams, 2005). The temporal dis- may be due, in part, to growth of continental crust with time, pro- tribution of Precambrian glacial deposits is shown in Figure 1. viding a larger substrate for extensive ice sheets and increasing the Possible relationships to the supercontinental cycle (Fischer, 1984; preservation potential of their deposits. Worsley et al., 1984; Bleeker, 2004) were discussed by Young Glacial deposits present great challenges to geochronologists, (2013). For full documentation of ancient glacial occurrences, see but, in some cases, ages can be obtained from interbedded tuffs Hambrey and Harland (1981), Eyles (1993), Deynoux et al. (1994), and lavas, or they may be bracketed between dates from older and Crowell (1999), Chumakov (2004), Fairchild and Kennedy (2007), younger rocks. and Arnaud et al. (2011). The dearth of glacial deposits in the early part of Earth’s history IDENTIFICATION OF GLACIAL DEPOSITS is surprising because of the less radiant young Sun (Sagan and Mullen, 1972; Sackmann and Boothroyd, 2003), but the surface of Physical criteria for the identification of ancient glacial activity are the early Earth was probably warmed by atmospheric greenhouse relatively simple, but few are unequivocal. These include widespread gases (notably CO2). Archean supracrustal rocks attest to the pres- diamictites—conglomerates with a variety of clast types “floating” ence of abundant surface water, but glacial deposits are rare. The in a matrix of rock flour (Fig. 2A). A second criterion is the presence Paleoproterozoic Huronian Supergroup (2.
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