Late Quaternary volcanic activity in Marie Byrd Land: Potential 40Ar/39Ar-dated time horizons in West Antarctic ice and marine cores T. I. Wilch* Department of Geological Sciences, Albion College, Albion, Michigan 49224 W. C. McIntosh Department of Earth and Environmental Science, New Mexico Institute of Mining and Technology, and New Mexico Bureau of Mines and Mineral Resources, Socorro, New Mexico 87801 N. W. Dunbar New Mexico Bureau of Mines and Mineral Resources, Socorro, New Mexico 87801 ABSTRACT More than 12 40Ar/39Ar-dated tephra lay- ies is to determine the basal age of the ice sheet. ers, exposed in bare ice on the summit ice cap The 1968 Byrd Station ice core in the West Late Quaternary volcanic activity at three of Mount Moulton, 30 km from their inferred Antarctic Ice Sheet has an inferred basal age of major alkaline composite volcanoes in Marie source at Mount Berlin, range in age from 492 only 74 ka (Hammer et al., 1994). The exten- Byrd Land, West Antarctica, is dominated by to 15 ka. These englacial tephra layers provide sive lateral flow of ice from the ice divide area explosive eruptions, many capable of deposit- a minimum age of 492 ka for the oldest iso- to Byrd Station may have removed or disturbed ing ash layers as regional time-stratigraphic topically dated ice in West Antarctica. This much of the basal ice record. Consequently, the horizons in the West Antarctic Ice Sheet and well-dated section of locally derived glacial ice 74 ka date of the Byrd core provides only a in Southern Ocean marine sediments. A total contains a potential “horizontal ice core” minimum age of the West Antarctic Ice Sheet. of 20 eruptions at Mount Berlin, Mount record of paleoclimate that extends back The locations of the future WAISCORES at Takahe, and Mount Siple are recorded in lava through several glacial-interglacial cycles. The Siple ice dome and the central ice divide mini- and welded and nonwelded pyroclastic fall de- coarse grain size and density of the englacial mize lateral ice-flow problems and will theoret- posits, mostly peralkaline trachyte in compo- tephra (mean diameters 17–18 mm, densities ically permit sampling of the oldest ice of the sition. The eruptions, dated by the 40Ar/39Ar 540–780 kg/m3), combined with their distance West Antarctic Ice Sheet. On the basis of ice- laser-fusion and furnace step-heating meth- from source, indicate derivation from highly flow models, geophysical data, and accumula- ods, range in age from 571 to 8.2 ka. explosive Plinian eruptions of Mount Berlin. tion histories, Nereson et al. (1996) predicted Tephra from these 40Ar/39Ar-dated Marie that both planned WAISCORES could contain Byrd Land eruptions are identified by geo- INTRODUCTION climate information dating to the previous in- chemical fingerprinting in the 1968 Byrd Sta- terglacial. An age of >125 ka would nullify the tion ice core. The 74 ka ice-core record con- The West Antarctic Ice Sheet, the world’s hypothesis that the ice sheet collapsed during tained abundant coarse ash layers, with model only remaining marine ice sheet, is considered the last interglacial. The determination of a ice-flow ages ranging from 7.5 to 40 ka, all of by many to be inherently unstable and prone basal age using common stratigraphic tech- which were previously geochemically corre- to catastrophic collapse and melting (e.g., niques (sedimentation rates or oxygen isotope lated to the Mount Takahe volcano. We iden- Hughes, 1973; Mercer, 1978; MacAyeal, 1992; correlations) can be complicated by shearing tify a one-to-one geochemical and age correla- Bindschadler et al., 1998; Oppenheimer, 1998). and poor preservation near the base of the ice tion of the youngest (ca. 7.5 ka) tephra layer in A 6 m rise in sea level, equivalent to the volume sheet, as was found in recent Greenland ice the Byrd ice core to an 8.2 ± 5.4 ka (2σ uncer- of water locked up in the West Antarctic Ice Sheet cores (e.g., Dansgaard et al., 1993). In this pa- tainty) pyroclastic deposit at Mount Takahe. (Drewry et al., 1982), occurred during the ca. 125 per we document a precise 40Ar/39Ar chronol- We infer that the 20–30 ka tephra layers in the ka isotopic stage 5e interglacial and has been at- ogy of late Quaternary explosive volcanism in Byrd ice core actually were erupted from tributed to West Antarctic Ice Sheet collapse West Antarctica that can be correlated to tephra Mount Berlin, on the basis of age and geo- (Scherer, 1991). The possibility of catastrophic layers in the 1968 Byrd ice core. The volcanic chemical similarities. If products of these collapse of the West Antarctic Ice Sheet has stim- record offers the potential of an independent youngest, as well as the older 40Ar/39Ar-dated ulated extensive glaciological and geophysical chronology of future WAISCORES and marine eruptions are identified by geochemical fin- research, including two planned deep ice cores in core stratigraphies through a critical period of gerprinting in future ice and marine cores, the West Antarctic Ice Sheet (WAISCORES), climate history. they will provide the cores with independently one at Siple Dome, near the Ross ice shelf, and a Tephra produced by large explosive volcanic dated time horizons. second at the central ice divide, about 200 km eruptions in West Antarctica can be widely dis- east of Byrd Station (Fig. 1) (Bindschadler, persed over oceans and ice sheets and trapped *E-mail: [email protected]. 1995). One objective of the WAISCORES stud- within accumulating sediment or snow layers, Data Repository item 9981 contains additional material related to this article. GSA Bulletin; October 1999; v. 111; no. 10; p. 1563–1580; 13 figures; 3 tables. 1563 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/111/10/1563/3383088/i0016-7606-111-10-1563.pdf by guest on 29 September 2021 WILCH ET AL. ° 180 W 170°W 160°W 150°W 140°W 70°S ° 75 S Large map area Mount Berlin volcano 0 250 500 Late Pleistocene to active Mount Moulton km 3478 m a.s.l. Late Pleistocene englacial tephra site ~3000 m a.s.l. 0 1000 East West km 80˚S Mount Siple volcano Ross Late Pleistocene Ice Shelf 3110 m a.s.l. E D J9 Mount Waesche englacial tephra WAISCORES site ° Siple Dome site 110 W C Mount Takahe volcano Late Pleistocene to Holocene 85˚S B Byrd Station WAISCORES 3460 m a.s.l. Transantarctic A Mountains planned UpB inland site 70°S 100°W CASERTZ subglacial Marie Byrd Land volcano Volcanic Province Ice West Divide Late Quaternary major volcano Whitmore Pre–Late Quaternary major volcano Mtns. Antarctica Minor volcano South Pole Station Ice core site Ice shelf Ice stream 90¡ 85°S 80°S Figure 1. Map of Marie Byrd Land, West Antarctica (Drewry, 1983), showing major volcanoes (triangles) and minor volcanoes (asterisks) and several sites important to research of the West Antarctic Ice Sheet. Volcano study sites are designated by large bold type. The Transantarctic Mountains bisect the continent and form the divide between the mostly terrestrial East Antarctic Ice Sheet and the mostly marine-based West Antarctic Ice Sheet. Five ice streams (A–E) account for about 90% of ice drained from the West Antarctic Ice Sheet (Hughes, 1977). UpB desig- nates the base camp on Ice Stream B. Byrd Station is the 1968 Byrd ice-core drill site. The dashed arrow is the estimated ice-flow vector of 25 ka ice in the Byrd ice core. CASERTZ designates the Corridor Aerogeophysics of Southeastern Ross Transect Zone, the area of an inferred active subglacial volcano (Blankenship et al., 1993). J9 is the 1977–1979 drill site of the Ross Ice Shelf Project. a.s.l.—above sea level. forming instantaneously deposited time-strati- Byrd Station ice core in the West Antarctic Ice trated in the 14–20 ka range and coarse ash (me- graphic horizons. These horizons, when dated, Sheet (Kyle et al., 1981; Palais et al., 1988). dian grain sizes to 60 µm) concentrated in the can provide independent tests of chronologies The 1968 Byrd Station ice core provides the 20–30 ka range. The fine ash and coarse ash lay- based on other methods. In Marie Byrd Land, most complete existing record of Marie Byrd ers were interpreted as products of phreato- West Antarctica, 18 large alkaline volcanoes Land volcanism. The more than 2000 tephra magmatic and magmatic eruptions, respectively (2300–4000 m above sea level) occur as layers recovered from the ice core were too (Palais et al., 1988). Only 24 of the tephra units, nunataks in or as islands adjacent to the West sparse and too fine (median grain sizes mostly one from the 7.5 ka layer and the remainder Antarctic Ice Sheet (Fig. 1). On the basis of geo- <20 µm) to be directly dated by existing meth- from the 20–30 ka layers, were geochemically chemical compositions, Marie Byrd Land vol- ods (Palais et al., 1988). Available chronology, analyzed and were all correlated to Mount canoes have been interpreted as likely sources based in large part on ice-flow models, indicates Takahe, situated about 250 km from the calcu- of fine-grained ashes recovered from Eltanin that the Byrd tephra layers were erupted be- lated site of tephra deposition (Fig. 1) (Kyle et al., deep-sea piston cores in the southern Pacific tween 13 and 40 ka; there is one isolated 7.5 ka 1981; Palais et al., 1988). The Eltanin tephra Ocean (Shane and Froggatt, 1992) and the 1968 layer. The tephra layers include fine ash concen- layers were dated stratigraphically to late Qua- 1564 Geological Society of America Bulletin, October 1999 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/111/10/1563/3383088/i0016-7606-111-10-1563.pdf by guest on 29 September 2021 LATE QUATERNARY VOLCANIC ACTIVITY, MARIE BYRD LAND, ANTARCTICA ternary time (<60 ka) and correlated geochemi- Takahe are the subject of a forthcoming paper.