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Chronology of Taylor Glacier Advances in Arena Valley Produced As a working hypothesis, we suggest that the disjointed out- crop pattern of Sessrurnir and Asgard tills, the cross-cutting \1I linear ground flutes, and the Nibelungen drift-and-hollow se- iW quence represent a single glacial event postdating deposition of the Sessumir and Asgard tills. We argue that northeast- trending flutes and inferred transport path of Nibelungen drift wtr suggest subglacial modification beneath a young episode (post- dating deposition of Asgard till) of northeast-flowing ice. As such, these features are consistent with an origin beneath northeast-flowing overriding ice which egulfed the western As- gard Range (Marchant et al. 1990; Sugden et al. 1991). We thank Thomas Fenn, Garth Hirsch, and Charles Lager- born for excellent assistance in the field and D.E. Sugden for kindly reviewing this paper. The U.S. Navy provided helicopter support. This work was funded by the Division of Polar Pro- grams of the National Science Foundation. Figure 3. Photograph of hand-dug section cut across surface con- tact of Sessrumir and Asgard tills at point indicated in figure 2. Note inclined strata within the Asgard till (light colored unit) trun- cated at the present ground surface. References Ackert, R.P., Jr. 1990. Surficial geology and stratigraphy in Njord Valley, Nibelungen Valley shows strong evidence of an early phase western Asgard Range, Antarctica: Implications for late Tertiary glacial his- of temperate overriding(?) glaciation followed by a later phase tory. (Masters Thesis, University of Maine, Orono, Maine.) of predominantly cold-based(?) erosive glaciation(s). Sessrumir Denton, G.H., M.L. Prentice, D.E. Kellogg, and T.B. Kellogg. 1984. till represents an early phase of wet-based glaciation in Nibe- Late Tertiary history of the Antarctic Ice Sheet: Evidence from the lungen Valley. Far-traveled siltstone erratics within this unit Dry Valleys. Geology, 12, 263-267 imply northeast-flowing temperate glaciation of the western Marchant, DR., G.H. Denton, and D.E. Sugden. 1990. Surficial geol- ogy of Sessrumir Valley, western Asgard Range, Antarctica: Impli- Asgard Range (Marchant et al. 1990). Younger, predominantly cations for late Tertiary ice-sheet overriding. Antarctic Journal of the cold-based (although locally erosive) glaciations are inferred U.S., 25(5), 53-55. from the Nibelungen drift-and-hollow sequence, highly eroded Sugden, D.E., G.H. Denton, and D.R. Marchant. 1991. Subglacial melt- and dissected till outcrops, and linear ground flutes superim- water Channels, Sessrumir Valley, Western Asgard Range, Antarc- posed on Asgard and Sessrurnir tills. tica. Geographiska Annaler, 73A, 109-121. McMurdo Sound area of Antarctica with the goal of constrain - Chronology of Taylor Glacier ing the glacial history of the region. In the 1989-1990 and 1990- advances in Arena Valley 1991 field seasons, numerous samples of glacial moraines and using in situ other unconsolidated surface deposits, exposed bedrock, and lava flows were collected. Our initial efforts concentrated on produced cosmogenic helium-3 measurements of in situ produced cosmogenic helium-3 in samples from an extremely well-exposed sequence of boulder- belt moraines deposited by the Taylor Glacier in lower Arena EDWARD J BROOK, MARK D. Kuiz, and ROBERT P. ACKERT, JR. Valley (figure 1). Complementary measurements of in situ pro- duced beryllium-10 and aluminum-26 have also been made in Department of Marine Chemistry and Geochemistry a subset of these samples (Brown et al. 1991; Brook et al. in Woods Hole Oceanographic Institution press). Woods Hole, Massachusetts 02543 The moraines in lower Arena Valley are believed to have formed during expansions of the Taylor Glacier and adjacent GEORGE H. DENTON east antarctic plateau in global interglacial periods. Correlations with uranium-thorium dated deposits in middle Taylor Valley (Hendy et al. 1979; Denton et al. 1989), based on soil develop- Department of Geological Sciences and ment and other weathering characteristics, suggest that the Institute for Quaternary Studies "Taylor II" and "Taylor III" moraines were deposited during University of Maine interglacial oxygen isotope stages 5 and 7, respectively (approx- Orono, Maine 04469 imately 100,000 and 200,000 years ago). These moraines have been used to reconstruct ice-surface profiles for the Taylor Gla- cier at these times (Denton et al. 1989). Within Arena Valley, Isotopes produced in rocks by cosmic rays offer a promising however, there are no direct age constraints for these deposits. new tool for studying exposure histories of geologic surfaces. Quartz mineral separates from sandstone and granite boul- We are applying these techniques in the McMurdo Dry Valleys- ders in the moraines were analyzed for helium isotopes at 82 ANTARCTIC JOURNAL 0 è4_3 Figure 1. Aerial photograph of Arena Valley and Taylor Glacier showing the outer limits of the moraine sequence discussed in the text. The informal nomenclature, "Taylor II" through "Taylor lVb" of Denton et al. (1989) is used. Note that the Taylor "Ill" deposit consists of two single ridges superimposed on the older "Taylor lVa" drift. All samples were collected from the outermost moraine loop of each sequence. Photo is an enlargement of a portion of an aerial photograph (TMA 2485, Frame 149) available from the U.S. Geological Survey. Location is approximately 77050S 161 00E. Scale is approximately 1:13,500. 1991 REVIEW 83 Woods Hole Oceanographic Institution using mass-spectro- high erosion rates on individual boulders, boulders shifting on metric techniques described elsewhere (Kurz et al. 1987). Also the moraine, the possible input of material into the moraines analyzed were two samples of Quartermain Drift, a Taylor Gla- by rock-fall from valley walls, and loss of helium-3 by diffusion. cier deposit that is stratigraphically older than the Arena Valley For example, the youngest two samples in the "Taylor IVY moraines and is exposed directly south of the "Taylor IVb" sequence (79,000 ± 25,000 and 179,000 ± 56,000 years) may have moraine limit. An additional sample collected from a boulder fallen from sandstone outcrops in cliffs above the moraine on the surface of the Taylor Glacier near Arena Valley was (Brook et al. in press). Anomalously old ages could be a re- analyzed to examine the production of cosmogenic nuclides sult of an inherited signal from exposure to cosmic rays prior during glacial transport. to deposition in the moraine. The outlying age of Exposure ages (figure 2) were calculated using the measured 654,000±202,000 years in the "Taylor III" moraine sequence helium-3 concentrations and a helium-3 production rate of may be an example of this, and represent a boulder from the 191 ± 59 per gram per year (Brook and Kurz in preparation). "Taylor IVa" deposit that was later incorporated into the "Taylor This production rate is derived from helium-3 measurements III" moraine. The sample collected from the Taylor Glacier sur- in young (less than 1,000-year-old) carbon-14-dated lava flows face had an exposure age of 9,000 years, suggesting that pro- from Hawaii (Kurz et al. 1990). The uncertainty in the produc- duction of helium-3 during glacial transport is minimal relative tion rate is large because of the small concentrations of cos- to the ages of the moraines in Arena Valley. Beryllium-10 and mogenic helium-3 in these samples. The 1-sigma uncertainty aluminum-26 data for a subset of these samples (Brown et al. in the absolute exposure age is, as a result, approximately 30 in press; Brook et al. in press) are generally concordant with percent, but because the analytical uncertainty for individual helium-3 data, but suggest some loss of helium-3 due to dif- measurements is normally 5-10 percent, relative ages can be fusion. determined more precisely. Accuracy of exposure-age calcula- The average exposure ages for "Taylor II" and "Taylor III" tions should improve as production rate calibration improves. (figure 2) are consistent with deposition during isotope stages Because loss of helium-3 has been observed from some 5 and 7, and mean exposure ages for "Taylor IVa" and "lVb" quartz samples the exposure ages are minimum estimates of suggest minimum ages of 337,000 years and 1.2 million years, the time the sample has been exposed to cosmic rays (Brook respectively. The mean age may not be the best estimate of true and Kurz in press). In addition, because the cosmic-ray flux is age, however, and because all of the factors leading to the attenuated with depth in the rock, erosion will also lower the distribution of ages observed in figure 2 are not well under- exposure age. Erosion rates for these samples, estimated from stood, these preliminary conclusions need to be tested with beryllium-10 and aluminum-26 data (Brown et al. 1991) are low, additional data. however (approximately 4-7 x 10b centimeters per year). The The "Taylor IVb" limit is approximately 250 meters above the affect on exposure ages is relatively minor; for example, with present Taylor Glacier, giving a maximum thickening, relative an erosion rate of 4 x 10- centimeters per year, ignoring ero- to today, for this part of the Taylor Glacier throughout most of sion in the exposure-age calculation underestimates the true Quaternary time. The two Quartermain Drift samples yielded age by only 5 percent in 2 million years and the effect is less significantly different helium-3 ages, 1.2 ± 0.3 and 2.3 ± 0.7 mil- significant for younger samples. lion years, and beryllium-10 data indicate an age of approxi- Figure 2 shows that each deposit exhibits a range of exposure mately 3 million years for this deposit (Brown et al. in prepa - ages. The distribution of ages is probably caused by several ration). The discrepancy is probably due to loss of helium-3 factors.
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