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Absolute Chronology for Major Pleistocene Advances of the Laurentide Ice Sheet

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Absolute Chronology for Major Pleistocene Advances of the Laurentide Ice Sheet Absolute chronology for major Pleistocene advances of the Laurentide Ice Sheet Greg Balco1* and Charles W. Rovey II2 1Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709, USA 2Geography, Geology, and Planning, Missouri State University, Springfi eld, Missouri 65897, USA ABSTRACT netically reversed till that overlies colluvium We used a 26Al-10Be burial isochron method to date the glacial stratigraphic section in Mis- and bedrock residuum; (2) the magnetically souri, USA, that records the largest advances of the Laurentide Ice Sheet. This permits an reversed Moberly Formation; and (3) the normal improved comparison of terrestrial and marine records of glaciation. The fi rst recorded advance polarity Fulton, Columbia, and Macon members of the Laurentide Ice Sheet reached 39°N, near the extreme southern limit of North Ameri- of the McCredie Formation (Fig. 1; Rovey and can glaciation, 2.4 Ma. The next advance to this latitude took place near the beginning of the Tandarich, 2006). Regional loess and soil stra- mid-Pleistocene transition, 1.3 Ma, and three more took place from 0.75 to 0.2 Ma. There is no tigraphy indicates that all fi ve predate marine evidence that the Laurentide Ice Sheet advanced south of ~45°–47°N between 2.4 and 1.3 Ma. isotope stage (MIS) 6 (ca. 0.15 Ma) (Rovey, This chronology: (1) shows that North American continental glaciation postdated Cordilleran 1997). Although glacial sequences typically alpine glaciation; (2) is consistent with the hypothesis that both of these events were threshold contain unconformities, several features of the responses to tropical cooling; (3) is consistent with the hypothesis that the fi rst advance of the Missouri sequence indicate that all ice advances Laurentide Ice Sheet was glaciologically anomalous due to the presence of deformable preglacial into this region are recorded by tills. First, a regolith; (4) is not consistent with the hypothesis that this deformable regolith persisted until the dense network of boreholes and extensive expo- mid-Pleistocene transition; and (5) indicates that the increase in global ice volume at the mid- sure due to active clay mining permits compre- Pleistocene transition was at least in part the result of a more extensive Laurentide Ice Sheet. hensive access to the entire section. Second, although not all tills are observed at all sections, INTRODUCTION cult to correlate over large areas, because of the tills always occur in the same sequence. Third, The defi ning feature of Earth’s climate during discontinuous nature of glacial deposition. Here paleosols are nearly always present between the past 2–3 m.y. is the presence of ice sheets we use cosmogenic-nuclide burial dating to date tills, indicating that tills were rarely signifi cantly on the northern continents. However, nearly all the sequence of glacial deposits that record the eroded by later ice advances. paleoclimate records for this period, that under- southernmost Pleistocene advances of the Lau- pin models that seek to explain the initial growth rentide Ice Sheet into the central United States. 26AL-10BE BURIAL DATING and periodic advance of these ice sheets, are This clarifi es the history of the Laurentide Ice We used two different methods of cosmo- drawn from marine sediments. This refl ects the Sheet and provides new constraints on ice extent genic-nuclide burial dating. These rely on pairs contrast between the continuous and well-pre- that can help to guide Pliocene–Pleistocene cli- of trace nuclides that are produced at a fi xed served nature of marine sediments and the dis- mate–ice sheet models. ratio by cosmic-ray bombardment of minerals continuous and diffi cult to date terrestrial glacial exposed at the Earth’s surface, but have differ- stratigraphic record. Global ice volume can be MISSOURI TILL SEQUENCE ent half-lives. Here we use 26Al and 10Be, which reconstructed from marine geochemical records, The glacial stratigraphic section in central are produced in quartz at 26Al:10Be = 6.75:1 but, with the exception of the most recent glacia- Missouri, USA, includes fi ve lithologically and have half-lives of 0.705 Ma and 1.39 Ma, tion, it is rarely possible to know where individ- and magnetostratigraphically distinct tills that respectively. When quartz undergoes surface ual ice sheets were located. This in turn makes record advances of the Laurentide Ice Sheet to exposure, its 26Al and 10Be concentrations con- it diffi cult to understand processes responsible near its most southern extent at 39°N. These form to the production ratio. If it is then bur- for Pliocene–Pleistocene climate and ice sheet tills include: (1) the Atlanta Formation, a mag- ied deeply enough to halt the cosmic-ray fl ux, change. For example, (1) the latitudinal extent of ice sheets controls their susceptibility to sum- Ma SMS92A Sieger CQ mer melting, but cannot be determined from 0 WL3 pit C Ma marine geochemical records, and (2) one cannot C IOWA 5 NF06 FU02 Ma Musgrove 10 C C distinguish, from marine records alone, whether pit F 8 9 F 10 WB19 F F variability in total ice volume refl ects variability Mo F F 15 PF2 in the geographic extent of ice sheets, variabil- 5 6 7 M Mo Mo i Mo Mo s s ILLINOIS 20 i Mo 40 ity in their thickness, or simply which ice sheets Mo s SMS92A s Pendleton i p p Thickness (m) Conklin existed during particular ice volume excur- 25 pit 4 3 i A R Quarry, IA MISSOURI . Mo sions (e.g., Raymo and Huybers, 2008). These SP 30 A Generalized till stratigraphy WB19 Aurora PF2 NF06 issues could be resolved by absolute dating of 1 2 (central Missouri) Mbr. PP 35 MP WL3 Loess Macon mbr. (Ma) FU02 s the extensive glacial stratigraphic records on McCredie M i s o u Silt/clay Columbia mbr. (C) Winthrop r i R. 40 Sand Fm. Mbr. the polar continents. However, this has not been Fulton mbr. (F) 12 Paleosol Till Moberly Fm. (Mo) Alburnett (bar height reflects Fm. 11 38 possible because Pliocene–Pleistocene glacial B horizon thickness) Bedrock Atlanta Fm. (A) 92 W 90 sedimentary sequences in North America and Eurasia are both diffi cult to date, because vol- Figure 1. Stratigraphic nomenclature for glacial sediments in Missouri, site locations, and stratigraphy of clay pits and boreholes investigated in this work (mbr.—member; Fm.— canic ashes are rare in cratonic areas, and diffi - forma tion]]. Correlation of Missouri stratigraphy to that at Conklin Quarry, Iowa (IA), is also shown. Numbers correspond to site numbers in Table 1. Dotted line on map shows southern *E-mail: [email protected]. limit of glacial deposits. © 2010 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, September September 2010; 2010 v. 38; no. 9; p. 795–798; doi: 10.1130/G30946.1; 3 fi gures; 1 table; Data Repository item 2010224. 795 26Al and 10Be decay and the 26Al/10Be ratio AGES bia till have a mean age of 0.22 ± 0.16 Ma, and diverges from the production ratio. Till-paleo- We dated the lowest till, the Atlanta For- one on the uppermost Macon till yields an age sol sequences imply that quartz in paleosols mation, at two sites where it overlies bedrock of 0.21 ± 0.18 Ma. The large uncertainties in underwent surface exposure during soil forma- residuum. Four simple burial ages from one these youngest ages refl ect the growth of rela- tion, and then was buried by emplacement of site agree with two from another and yield a tive uncertainties with decreasing burial age, the overlying till. Thus, 26Al and 10Be concen- mean age of 2.42 ± 0.14 Ma (Table 1). This at a rate controlled by the production rates and trations in paleosol quartz can be used to date till and underlying sediment are magnetically half-lives of the nuclide pair (Balco and Shuster, overlying tills. reversed (Rovey et al., 2006), so must postdate 2009). The 26Al/10Be pair is not well suited to The simplest approach to 26Al-10Be burial the 2.6 Ma Matuyama-Gauss reversal. The till dating sediments younger than 0.4 Ma. These dating applies when a sample has undergone a was most likely emplaced during MIS 96–100 two tills were most likely emplaced during MIS two-stage history including a single period of (2.43–2.54 Ma). We dated the second-lowest 8–12 (0.25–0.5 Ma). surface exposure and subsequent burial. In this till, the Moberly Formation, at two sites. Four Rovey and Kean (1996) proposed correla- case, 26Al and 10Be measurements on a single simple burial ages from a site where it overlies tions between the Missouri till sequence and a sample yield a unique burial age (Granger, bedrock residuum agree with an isochron age sequence of at least fi ve widely recognized tills 2006). We refer to an age thus determined as at a site where it overlies the Atlanta till, yield- to the north in Nebraska and Iowa (Fig. 1) on a “simple” burial age. This situation applies at ing a mean age of 1.307 ± 0.089 Ma (Table 1; the basis of paleomagnetism, till composition, several of our sites where a paleosol formed in Fig. 2). At some sites the Moberly displays a and stratigraphic order. The Atlanta till resem- place from underlying bedrock is overlain by parting or weak weathering zone separating tills bles reversed tills called C by Boellstorff (1978) till. However, the parent material of most paleo- of identical composition, suggesting the pos- or R2 by Roy et al. (2004). At one site a C till sols in this study is till.
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