Late Quaternary Volcanic Activity in Marie Byrd Land: Potential 40Ar/39Ar-Dated Time Horizons in West Antarctic Ice and Marine Cores

Home , Marie Byrd Land, Mount Berlin, Mount Siple, Mount Takahe

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.

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° 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 designates 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-

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ternary time (<60 ka) and correlated geochemi- Takahe are the subject of a forthcoming paper. probe (Palais et al., 1988) were unavailable for cally to Marie Byrd Land volcanoes, located In addition, late Pleistocene tephra samples reanalysis. more than 1500 km from core sites (Shane and were obtained from a bare ice area on the sum- Froggatt, 1992). No specific correlations were mit ice cap of Mount Moulton, located 30 km 40Ar/39Ar Dating Methods made between the Eltanin tephra and Marie east of Mount Berlin. Byrd Land source volcanoes. Recent advances in 40Ar/39Ar geochronology, There is no previous documentation of Geochemistry Methods including laser heating, lower extraction line widespread explosive late Quaternary erup- blanks, and higher resolution mass spectrome- tions at source volcanoes in Marie Byrd Land. Previous geochemical analyses reported by ters, have led to successes in dating anortho- LeMasurier and Rex (1991) characterized out- Palais et al. (1988) include electron microprobe clase- and sanidine-bearing volcanic rocks as crops at the late Quaternary volcanoes as com- major element data from Byrd core coarse ash young as Holocene age (e.g., Hu et al., 1994; posed mostly of lavas and noticeably lacking layers (n = 11) and X-ray fluorescence (XRF) van den Bogaard, 1995; Chen et al. 1996; Renne volcanic ash. Four of the Marie Byrd Land vol- analyses of whole-rock samples from selected et al., 1997). Most attempts to date young felsic canoes, Mount Berlin, Mount Takahe, Mount outcrops at Mount Takahe (n = 15) and Mount volcanic rocks have focused on averaging the Siple, and Mount Waesche, were listed as “pos- Berlin (n = 2). Whole-rock powders from results of many laser-fusion analyses of single sibly or probably active” by LeMasurier (1990, Mount Berlin (n = 41) and Mount Siple (n = 2) sanidine crystals from the same sample. The Fig. B.I.). Previous indications of recent vol- were analyzed for major and trace elements by principal advantage of the laser method is that canic activity in coastal Marie Byrd Land in- XRF at the University of Keele, U.K., and New contamination by older xenocrysts can be rec- clude steaming ice towers at Mount Berlin Mexico Institute of Mining and Technology. ognized and eliminated from the mean age re- (LeMasurier and Wade, 1968), englacial basal- Obsidian and pumice glass fragments from sult (LoBello et al., 1987; Deino and Potts, tic tephra layers near Mount Waesche (Smellie Mount Berlin (n = 2), Mount Moulton englacial 1990; Chen et al., 1996). The primary disadvan- et al., 1990), and trachytic pyroclastic rocks at tephra (n = 5), and Mount Takahe (n = 3) and tage of the laser method is that small isotopic the summit of Mount Takahe that were geochem- feldspars from four Mount Moulton englacial signal sizes from young samples may be near ically correlated to tephra layers in the Byrd ice tephra samples were analyzed for major ele- mass spectrometer detection limits, resulting in core (Kyle et al., 1981; Palais et al., 1988). Active ments and F, Cl, and P by electron microprobe large analytical uncertainties in isotopic mea- subglacial volcanism has also been inferred in at Arizona State University. Major element sured ages. In contrast to the laser-fusion southern Marie Byrd Land on the basis of aero- compositions of a number of glass fragments method, the more conventional furnace step- geophysical surveys (Blankenship et al., 1993). were determined using a JEOL 8600 electron heating approach has rarely been used in recent Previous attempts at conventional K-Ar dating of microprobe at Arizona State University. An ac- studies of young volcanism. There are two main exposed summit crater rocks, potential source celerating voltage of 15 kV and beam current of advantages in the step-heating method: (1) large volcano deposits for ash layers in ice and marine 15 nA were used for the analysis. ZAF recalcu- samples can be analyzed, resulting in small an- cores, yielded imprecise age estimates of <100 ka lation procedures were used. In order to mini- alytical uncertainties; (2) the step-heating ap- (LeMasurier and Rex, 1983; LeMasurier, 1990). mize volatilization of Na, the beam was en- proach permits assessment of basic assumptions In this paper we present new field evidence larged to a diameter of 10 µm, and Na was about sample homogeneity, closed-system con- from Mount Berlin and Mount Siple that sug- included in the first set of analyzed elements. ditions, and trapped argon compositions (e.g., gests abundant late Quaternary explosive activ- Analytical error is estimated to be ±1%, based Esser et al., 1997). The disadvantage of the step- ity of Marie Byrd Land volcanoes. Field, geo- on replicate analysis of alkaline glass reference heating approach is that problems of xenocrys- chemical, and 40Ar/39Ar age data chronicle 20 materials KN-18 and KE-12 (Devine et al., tic contamination cannot be confidently cor- late Quaternary (571–8.2 ka) trachytic to 1984). Many of the lava and welded and non- rected and in the worst case might remain basanitic eruptions of Mount Berlin, Mount welded pyroclastic fall samples contained abun- undetected (LoBello et al., 1987). Given ade- Siple, Mount Takahe. Eruptive units from dant microlites and were unsuitable for micro- quately large samples of uncontaminated vol- Mount Berlin and Mount Takahe volcanoes are probe analyses. canic alkali feldspar phenocrysts, the furnace correlated by age and geochemistry to tephra Samples from Mount Berlin (n = 5) and Mount single-crystal step-heating method potentially layers in the Byrd Station ice core. Takahe (n = 3) were analyzed for trace elements results in isotopic measurements and ages that using a Cameca IMS 3f ion microprobe at Ari- are more precise and as accurate as those deter- METHODS zona State University. A 1–2 nA mass-analyzed mined by the laser-fusion method. Estimates of primary beam of 16O- was focused to a 10 µm accuracy can be obtained by checking the Field work and sampling for this study was spot. Secondary ion intensity for trace elements 40Ar/39Ar ages with stratigraphy or by compar- carried out at Mount Takahe during the was calibrated against NBS-610, a sodium- and ing them to ages determined by other dating 1985–1986 austral summer field season and at silica-rich glass containing ~500 ppm of 61 trace methods. Mount Berlin and Mount Siple during the elements (values used for calibration are mainly We dated 52 samples by 40Ar/39Ar methods 1993–1994 field season. Because of ice cover, from Hollocher and Ruiz, 1995). Observed repro- from Mount Berlin and Mount Moulton (n = outcrops at the >3000-m-high Marie Byrd Land ducibility of secondary standards suggests that 44), Mount Takahe (n = 6), and Mount Siple (n volcanoes are extremely limited. Most outcrops precision of analyses is ±10%. = 2) volcanoes (Fig. 1). Sample ages were de- on Mount Berlin were sampled and are in- Microprobe major and trace element data termined based on a total of 95 analyses: 70 by cluded in this study. Only samples from the were also obtained from a composite sample of furnace step-heating and 25 by laser fusion summit crater area of Mount Takahe and Mount glass shards from five tephra layers in the Byrd methods. With the exception of 11 basanitic to Siple are included in this study; the chronology Station ice core. The tephra layers are from core trachytic groundmass concentrates from Mount and glaciovolcanic record of mafic outcrops depths of 1377, 1415, 1457, 1710, and 2006 m. Berlin, all of the dated samples are anorthoclase near the bases of Mount Siple and Mount Samples previously analyzed by electron micro- phenocrysts derived from trachytic and phono-

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litic lavas and pyroclastic rocks. Anorthoclase is K-Ar age of the sample, is a mean of the incre- dynamics (column height and eruption rate) a high-potassium, anhydrous, low-diffusivity mental ages, each weighted by the percent 39Ar rather than changes in magma composition. feldspar, ideal for 40Ar/39Ar dating. Coarse- in each step. An age plateau is defined by three Other lithofacies recognized at study sites in- grained (0.4–3 mm diameter) anorthoclase crys- or more contiguous incremental ages that com- clude englacial tephra layers, mafic scoria de- tals were separated from crushed rock samples pose >50% of total 39Ar released and agree posits, ash-rich pyroclastic breccia and associ- using standard magnetic, heavy-liquid, and within 2 σ (Fleck et al., 1977). The furnace ated accretionary lapilli-bearing tuff, a distal hand-picking techniques and treated for 5–10 plateau ages are averages of the concordant in- fiamme-rich tuff, and abundant dense lavas. min with 15% hydrofluoric acid to remove ad- cremental ages weighted by the inverse of vari- Several englacial pumiceous tephra layers occur hering glass. Sample purity of anorthoclase sep- ance. Results from a small number of analyses in a stratigraphically coherent horizontal section arates was >99%. For the 10 aphyric trachyte- did not meet plateau criteria and maximum of bare ice on the summit of Mount Moulton. phonolite and mafic lava samples from Mount mean ages were calculated from subjectively se- We infer that these englacial tephra layers re- Berlin, groundmass concentrates (0.4–0.8 mm lected contiguous incremental ages that are sulted from explosive eruptions of Mount Berlin diameter grain size) were separated magneti- nearly concordant. Uncertainties on plateau and volcano, located 30 km to the west. Two out- cally. Mafic samples were treated with 10% hy- mean ages are calculated using formulas in crops of mafic scoria and lava deposits on the drochloric acid for 5 min. Samson and Alexander (1987). All analytical north flank of Mount Berlin are attributed to Complete analytical methods, data tables, fur- uncertainties are reported at a ±2 σ confidence monogenetic cinder cone eruptions. The ash- nace age spectra, and laser relative probability level, except those shown on the ideogram plots rich pyroclastic breccia and accretionary lapilli- spectra are available (Appendices 1–5) from the at a ±1 σ confidence level. bearing tuff, which form part of the youngest se- GSA Data Repository.1 Most samples were irra- Mean laser-fusion ages were calculated by quence in the Mount Berlin summit crater wall, diated in the D-3 position of the reactor at the Nu- weighting individual crystal ages by the inverse are attributed to a hydrovolcanic eruptive phase. clear Science Center, Texas A & M University, of the variance. Individual ages included in the An isolated outcrop of densely welded (fi- for 2 hr (average neutron flux yielded ~0.0001 mean age met MSWD (mean squared weighted amme-rich) tuff on the southeast flank of Mount J/hr). The irradiation-induced thermal neutron deviation) criteria as described by Mahon Berlin is interpreted as an ignimbrite, one of the 40K(n, p) 40Ar interference reaction was mini- (1996). Uncertainties on laser mean ages are rare examples in Marie Byrd Land. mized by boron shielding at the Texas A & M re- calculated using formulas of Samson and actor. Fish Canyon Tuff sanidine (FCT-1, 27.84 Alexander (1987). Geochemistry Results Ma; Deino and Potts, 1990) was used as a neutron flux monitor. Individual flux monitor crys- RESULTS Major and trace element data from micro- tals (6 sanidine crystals per monitor position) and probe analyses of Mount Takahe and Mount 21 Marie Byrd Land samples (6–52 anorthoclase Field Observations Berlin samples are listed in Table 2 (a complete

crystals per sample) were fused with a CO2 laser. set of chemical data is available; see footnote 1). Bulk separates (50–200 mg) of 39 of the Nonwelded and welded trachytic pyroclastic Data from microprobe analyses of 11 glass samples were step heated in a resistance fur- breccias and tuff breccias, composed of shards from the Byrd ice-core composite sam- nace and single crystals from 21 of the samples pumiceous bombs, obsidian clasts, and lithic ple are included in Appendix 6 (see footnote 1).

were fused by a CO2 laser. Replicate analyses fragments, were ubiquitous in crater-rim expo- Because the sample is a composite of mixed age were made of many samples by both furnace sures (Fig. 2; Table 1). The lithic fragment com- tephra, analytical data were not averaged and and laser techniques. After laser or furnace ponents included trachytic lava and hypabyssal one-to-one correlations between tephra layers heating, argon isotopes were measured on a clasts, compositionally similar to juvenile and source volcano rocks are not possible. Mass Analyzer Products (MAP) 215–50 mass pumice and obsidian clasts. Typical breccia de- Major element oxide percentages, except

spectrometer at the New Mexico Geochrono- posits were moderately sorted and crudely to Na2O, are slightly higher in microprobe data than logical Research Laboratory. Typical blanks well bedded, with beds mantling topography. in XRF data. These differences are attributed to (including mass spectrometer backgrounds) at Inferred welding processes varied from weak volatilization and loss of sodium during electron 40 39 38 37 36 masses Ar, Ar, Ar, Ar, and Ar were 80, agglutination to load pressure compaction, with microprobe analyses that resulted in lower Na2O 0.2, 0.1, 0.2, and 0.6 x 10–17 mol for furnace fiamme aspect ratios as low as 1:10. Clastic tex- contents (1.85%–2% lower in microprobe data analyses and 8, 0.2, 0.06, 0.2, and 0.1 x 10–17 tures are nearly obliterated in agglutinates that compared to XRF data) and apparent gains in mol for laser analyses. The sample ages were were transformed into clastogenic lavas. Clasto- other oxides. Because of the Na loss problem, it corrected for blank, background, mass discrim- genic lavas exhibit characteristics of rheomor- is best to make direct comparisons between ma- ination, radioactive decay, and interfering reac- phism, including recumbent folds. Lithofacies jor element data obtained by the same technique. tions. The decay constants and isotopic abun- characteristics, such as pumice and/or lithic Figure 3 shows that whole-rock samples are dances used are those suggested by Steiger and content and degree of welding, varied dramati- dominantly trachytes, with minor phonolites. Jaeger (1977). cally over short vertical distances but were uni- The trachytes are mostly peralkaline, with per-

Integrated and plateau ages were calculated form over lateral distances. Unit thicknesses alkaline indices (molecular [Na2O + K2O] for samples analyzed by the furnace step-heating range from 1 m at Merrem Peak crater to >70 m /Al2O3) as high as 1.48, and include both pan- method. The integrated age, equivalent to the at the summit crater wall of Mount Berlin. We telleritic and comenditic trachytes (LeBas et al., interpret these pyroclastic breccias as proximal 1986). Although most samples are classified as 1GSA Data Repository item 9981, supplemental pyroclastic fall deposits. Welding may have trachytes, the chemical data cluster into discrete tables and figures, is available on the Web at been facilitated by high temperatures and low populations that are consistent with the sample http://www.geosociety.org/pubs/ftpyrs.htm. Re- quests may also be sent to Documents Secretary, viscosities of peralkaline magmas (Mahood, ages. Compositions of both Mount Berlin and GSA, P.O. Box 9140, Boulder, CO 80301; e-mail: 1984). Changes in welding and lithofacies char- Mount Takahe eruptions have varied with time, [email protected]. acteristics are attributed to changes in eruption although no systematic changes are recognized.

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D C

Figure 2. Mount Berlin volcano. (A) View looking west to the 2-km-diameter Mount Berlin summit crater. A 150 m thickness of two welded pyroclastic fall units overlain by a thin non-welded deposit is exposed in the crater wall. Several fumarolic ice towers occur on the western rim of the crater (light-colored bumps). (B) Partly agglutinated pyroclastic fall deposit in summit crater wall of Mount Berlin contains abundant slightly flattened light-colored pumiceous bombs. (C) A 12 × 12 cm rock slab cut from welded pyroclastic fall deposit at Merrem Peak crater contains flattened pumice and abundant lithic clasts. (D) View of nonwelded englacial tephra layer at Mount Moulton with Mount Berlin summit 30 km to the west.

Electron microprobe analyses of alkali feldspar shown in Figures 4, 5, and 6. In most cases, the analytical uncertainties (Fig. 4). The mean 2 σ un- samples from four englacial tephra layers at 40Ar/39Ar analyses produced normally distributed certainty is ±9.5 ka for all furnace-heated samples Mount Moulton indicate similar Ca-poor (%CaO laser ages and flat and furnace age spectra that are and ±30 ka for all laser-heated samples. The vari- <0.3) anorthoclase compositions. In thin section, reproducible, consistent with stratigraphic posi- able, lower precision age determinations of some the anorthoclase feldspars are euhedral, with sim- tion, and interpreted as geologically meaningful laser samples are attributed to the small 40Ar and ple Carlsbad twins and rare tartan twins. We infer eruption ages. Exceptions include three samples 36Ar content in each young crystal and to variabil- that all of the alkali feldspars in the trachytes and from the Mount Moulton tephra sequence that ity in system blank. The most precise laser ages phonolites are anorthoclase rather than sanidine. produced discordant ages that are inconsistent have similar uncertainties (±2–3 ka) to the most with stratigraphy. In most replicate analyses, both precise furnace ages (Fig. 4; Table 1). 40Ar/39Ar Geochronology Results the furnace and laser ages are reproducible among Most samples analyzed by the single-crystal different irradiation packages (Table 1; Figs. 4, 5, laser-fusion method produced a normally dis- Geochronology results are summarized in and 6). Although the furnace and laser mean ages tributed population of individual crystal ages, Table 1, and representative examples of the data overlap at 2 σ, the laser ages tend to have higher with a few or no older outliers (Fig. 5). Two of

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WCM93-22 laminated pumice, top anorth NM-32 (5014) lm 7 37 ± 12 of upper welded unit NM-32 (5014) fp 9 100.0 27.1 ± 3.1 NM-36 (5347) fp 11 100.0 25.2 ± 3.1 Mean 3 26.5 ± 3.7 WCM93-17 pumiceous rheomorphic anorth NM-36 (5360) fp 12 100.0 25.0 ± 5.9 tuff, upper welded unit WCM93-21 welded fall, below anorth NM-36 (5346) fp 12 100.0 27.7 ± 3.7 WCM93-22 NM-36 (5346-02) fp 5 65.4 24.3 ± 2.9 Mean 2 25.6 ± 3.9 WCM93-23 east side, lower welded anorth NM-32 (5015) lm 6 34 ± 13 unit NM-32 (5015) fp 8 84.2 25.5 ± 4.9 NM-36 (5363) fp 4 38.4 33 ± 11 Mean 3 27.5 ± 6.4 WCM93-15 spatter lava with anorth NM-32 (5013) lm 8 28 ± 11 cognate xenoliths NM-32 (5013) fp 9 79.7 23.8 ± 2.9 NM-36 (5344) fp 13 100.0 25.8 ± 4.2 Mean 3 24.6 ± 2.9 Mount Berlin summit crater fumarolic cave lava Mean 1 10.3 ± 5.3 Mount Berlin summit crater non-welded fall deposit Mean 1 18.2 ± 5.8 Mount Berlin summit crater welded fall deposits Mean 5 25.5 ± 2.0

Mount Berlin–Merrem Peak crater WCM93-130 1 m thick pumiceous anorth NM-15 (1299) lm 9 161 ± 32 phonolite fall NM-36 (5367) fp 12 100.0 141.4 ± 5.7 WCM93-123 black/yellow fall (southeast) anorth NM-15 (1297) lm 9 161 ± 26 WCM93-128 trachyte bomb anorth NM-32 (5011) fp 10 100.0 183.8 ± 4.6 (west-northwest) WCM93-129 pumiceous trachyte anorth NM-15 (1298) lm 7 174 ± 17 fall NM-32 (5012)fp 10 100.0 185.3 ± 4.6 WCM93-133 welded trachyte fall anorth NM-32 (5007) fp 10 100.0 181.9 ± 4.8 (northeast) NM-32 (5008) fp 10 100.0 191.0 ± 7.5 Mean 2 185 ± 9.1 WCM93-139 welded clastogenic anorth NM-32 (5001) fp 10 100.0 182.1 ± 4.6 flow NM-32 (5001) lm 8 237 ± 28 WCM93-140 welded clastogenic anorth NM-36 (5351) fp 11 100.0 179 ± 14 flow Merrem Peak non-welded fall deposit Mean 1 141.4 ± 5.7 Merrem Peak welded fall deposit Mean 5 183.6 ± 2.9

Mount Berlin–flank trachyte deposits WCM93-011 near-vent trachyte anorth NM-34 (5182) lm 9 295 ± 36 lava (northeast) NM-37 (5345) fp 11 100.0 231 ± 11 WCM93-152 welded trachyte anorth NM-34 (5190) lm 10 257 ± 43 ignmibrite (southeast) NM-36 (5341) fp 11 100.0 228 ± 12 Mount Berlin–flank deposits mean age Mean 2 229.8 ± 8.3

Mount Berlin–Mefford Knoll mafic deposits WCM93-001 basanite cinder cone gms-bas NM-33 (5138) fp 9 100.0 211 ± 35 NM-37 (5397) fp 5 57.2 197 ± 49 Mean 2 207 ± 31 WCM93-004 basanite cinder cone gms-bas NM-33 (5139) fp 6 68.4 213 ± 28 NM-37 (5398) fp 8 100.0 213 ± 36 Mean 2 213 ± 22 WCM93-008 hawaiite flow levee gms-haw NM-33 (5140) fp 10 100.0 207 ± 80 Mefford Knoll mafic deposits mean age Mean 3 211 ± 18

Mount Berlin–Merrem Peak (southwest) trachyte lava WCM93-134 trachyte lava anorth NM-34 (5186) lm 8 450 ± 175 WCM93-127 foliated trachyte lava anorth NM-32 (5029) fp 12 100.0 454 ± 41 WCM93-125 foliated trachyte lava anorth NM-32 (5020) fp 11 96.2 573 ± 15 WCM93-126 clastogenic trachyte lava anorth NM-32 (5009) fp 6 65.7 566 ± 12 NM-32 (5010) fp 10 100.0 563 ±13 Mean 2 565 ± 9.4 WCM93-137 trachyte lava anorth NM-32 (5023) fp 7 82.6 576 ± 13 WCM93-138 clastogenic trachyte lava anorth NM-32 (5030) fp 11 100.0 580 ± 40 WCM93-135 clastogenic phonolite anorth NM-34 (5187) lm 19 594 ± 18 NM-36 (5349) fp 12 100.0 560 ± 20 Mean 2 579 ± 36 WCM93-009 trachyte flow levee gms-tr NM-33 (5141) fp 5 100.0 589 ± 28 (northwest)

Mount Berlin–southwest flank trachyte lava 2 Mean 2 454 ± 40 Mount Berlin–southwest flank trachyte lava 1 Mean 6 571.0 ± 8.9

1568 Geological Society of America Bulletin, October 1999

TABLE 1. LATE QUATERNARY MARIE BYRD LAND VOLCANOES 40Ar/39Ar AGE SUMMARY (Continued) Sample Description Sample type* Irrad.# (L#)† Method§ n# %39Ar** Age†† ± 2 σ§§ (ka) Mount Berlin–Brandenberger Bluff WCM93-014 phonotephrite cinder cone gms-pt NM-34 (5186) fp 6 77 2672 ± 67

WCM93-010 phonolite dome lava gms-ph NM-34 (5237) fp 4 76 2768 ± 109 WCM93-037 phonolite lava at bluff base gms-ph NM-34 (5198) fp 5 100 2823 ± 242 WCM93-053 trachyte clast in hyalotuff gms-tr NM-34 (5199) fp 7 100 2722 ± 86 WCM93-069 trachyte clast in hyalotuff gms-tr NM-34 (5200) fp 7 100 3735 ± 967 WCM93-121 phonolite clast in hyalotuff gms-ph NM-34 (5201) fp 5 100 2759 ± 121 WCM93-250 trachyte clast in hyalotuff gms-tr NM-34(5186) fp 6 100 2684 ± 126

Mount Berlin–Brandenberger Bluff trachyte/phonolite Mean 5 2738 ± 63

Mount Berlin–distal englacial tephra at Mount Moulton WCM93-313 tephra layer # 2 anorth NM-32 (5002-12) fp 11 100.0 18 ± 13 5 cm thick NM-36 (5002) fp 7 51.8 14.2 ± 3.3 NM-32 (5002) lm 6 29 ± 30 Mean 2 14.5 ± 3.8 WCM93-315 tephra layer # 4 anorth NM-32 (5004-11) fp 8 92.3 32 ± 13 and BIT 152 12 cm thick NM-36 (5352) fp 11 100.0 28.0 ± 4.0 NM-32 (5004) lm 6 21 ± 22 anorth NM-78 (8417-01) fp 6 86.9 26.4 ± 2.2 NM-78 (8418) lm 29 29.9 ± 4.3 Mean 5 27.3 ± 2.3

BIT 156 tephra layer # 7 anorth NM-78 (8435-01) fp 5 96.5 118.9 ± 9.2 anorth NM-78 (8436) lm 7 99 ± 127

WCM93-316 tephra layer # 8 anorth NM-32 (5005) fp 10 100.0 90.7 ± 3.0 and BIT 157 3 cm thick NM-36 (5361) fp 12 100.0 92.2 ± 5.7 NM-36 (5355) lm 10 91.8 ± 7.6 anorth NM-78 (8420-01) fp 8 97.6 91.2 ± 4.9 anorth NM-78 (8420-02) fp 8 99.2 92.6 ± 4.5 anorth NM-78 (8419) lm 35 94.0 ± 2.3 Mean 6 92.5 ± 2.0

WCM93-317 tephra layer # 9 anorth NM-32 (5006) fp 10 100.0 106.5 ± 4.3 and BIT 158 2 cm thick NM-36 (5362) fp 13 100.0 108.1 ± 8.5 NM-36 (5356) lm 14 110 ± 11 anorth NM-78 (8422-01) fp 8 98.4 105.2 ± 5.4 anorth NM-78 (8422-02) fp 8 98.6 106.0 ± 3.2 anorth NM-78 (8425) lm 52 113.5 ± 2.2 Mean 5 106.3 ± 2.4

BIT 159 tephra layer # 10 anorth NM-78 (8429-01) fp 8 100.0 147 ± 15 WCM93-314 tephra layer # 11 anorth NM-36 (5359) fp 11 100.0 119.5 ± 6.0 and BIT 160 15 cm thick NM-36 (5354) lm 22 119 ± 10 anorth NM-78 (8424-01) fp 8 98.8 120.9 ± 4.2 anorth NM-78 (8423) lm 23 117.7 ± 2.5 Mean 5 118.7 ± 2.5

BIT 162 tephra layer # 13 anorth NM-78 (8426-01) fp 8 98.1 139.6 ± 4.2 anorth NM-78 (8425) lm 21 147.1 ± 7.0 Mean 2 141.6 ± 7.5 BIT 163 tephra layer # 14 anorth NM-78 (8437-11) fp 11 100.0 396 ± 26 anorth NM-78 (8437) lm 19 428 ± 18

the Mount Moulton tephra samples (BIT 156 from the sample (Fig. 6). A few samples ERUPTION RECORDS and BIT 163) contained crystals of several dif- (WCM93-25 and WCM93-23 from Mount ferent ages. The age discordance may result Berlin and WCM93-313 from Mount Moulton) Mount Berlin from xenocrystic contamination or excess 40Ar produced discordant, slightly saddle-shaped age (e.g., van den Bogaard, 1995; Esser et al., spectra. For these samples, the mean of the Mount Berlin, an actively steaming volcano 1997). For most laser samples, however, youngest ages in each spectrum was calculated in western Marie Byrd Land with a prominent xenocrystic and/or excess argon contamination as a maximum age for the sample. As with the summit crater and a subsidiary crater at Merrem is not evident. laser results, evidence for contamination by Peak, is the only volcano with documented ge- Furnace step-heating of multiple anortho- xenocrystic and/or excess argon is rare. We in- othermal activity in West Antarctica (LeMa- clase crystals (50–200 mg) produced repro- terpret laser and furnace results that are both re- surier and Wade, 1968) (Figs. 1, 2, and 7). ducible and flat age spectra, yielding plateau producible and consistent with stratigraphy as Mount Berlin is characterized by a south to ages that comprise most of the 39Ar released eruption ages. southeastward migration of volcanic activity

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TABLE 1. LATE QUATERNARY MARIE BYRD LAND VOLCANOES 40Ar/39Ar AGE SUMMARY (Continued) Sample Description Sample type* Irrad.# (L#)† Method§ n# %39Ar** Age†† ± 2 σ§§ (ka) BIT 166 tephra layer # 15 anorth NM-78 (8438-01) fp 11 100.0 601 ± 38 anorth NM-78 (8438) lm 15 492.4 ± 9.7

Mount Berlin–distal englacial tephra at Mount Moulton 2 14.5 ± 3.8 in stratigraphic order from young to old 5 27.3 ± 2.3 692.5± 2.0 5 106.3 ± 2.4 4 118.7 ± 2.5 2 141.6 ± 7.5 1 492.4 ± 9.7 Mount Siple summit crater WCM93-278 near summit, lava anorth NM-32 (5028) fp 11 100.0 168.9 ± 5.4 WCM93-277 crater wall, densely anorth NM-15 (1871) fp 6 97.3 230.0 ± 6.7 welded fall NM-32 (5025) fp 11 100.0 234 ± 17 NM-32 (5026) fp 11 100.0 222.1 ± 6.9 NM-15 (1300) lm 10 189 ± 60 Mean 3 227 ± 7.6

Mount Takahe summit crater W85009 Bucher Rim (south) pumice anorth NM-36 (5342) fp 10 100.0 8.2 ± 5.4 and obsidian bombs W85013 Bucher Rim (north) lava anorth NM-36 (5379) fp 11 100.0 93.3 ± 7.9 beneath welded fall W85015 Bucher Rim (north), anorth NM-36 (5372) fp 9 91.3 102.0 ± 7.4 welded fall obsidian MT85006 9300’ outcrop lava anorth NM-36 (5357) fp 11 100.0 154.1 ± 8.2 W85022 Bucher rim (south), lava anorth NM-36 (5343) fp 11 100.0 167 ± 14 W85011 Bucher Rim (south), 10 m anorth NM-36 (5369) fp 12 100.0 192.0 ± 6.3 lava over W85-09 Note: Samples from each site are listed in stratigraphic order from youngest to oldest. Unit eruption ages are listed in bold type. Ages not used in unit eruption ages are italicized. *Sample type: anorth—anorthoclase; gms—groundmass; bas—basanite; haw—hawaiite; ph—phonolite; pt—phontephrite; tr—trachyte. †NMGRL reference numbers: Irrad#— irradiation batch number, L#—lab analysis number. §Methods: fp— furnace step-heating plateau age (after Fleck et al., 1977), lm— laser fusion weighted mean age. #n—the number of apparent ages included in furnace or laser mean age or site mean age calculation. **%39Ar—percentage of total 39Ar that is included in mean or plateau age. ††mean sample ages and unit ages were calculated by weighting by the inverse of variance. §§Two-sigma uncertainties of mean sample and unit ages were calculated using method of Samson and Alexander (1987).

that is perpendicular to the regional westward Ma (LeMasurier and Rex, 1983), within 2 σ un- Merrem Peak and dated to 571 ± 8.9 ka. Sam- migration pattern of felsic volcanism reported certainty of the mean 40Ar/39Ar age. Branden- ples from these outcrops were previously dated by LeMasurier and Rex (1989). Growth of berger Bluff is a 300-m-thick section composed by the conventional K-Ar method to 620 ± 100 Mount Berlin occurred in three stages (Fig. 7): of steeply dipping (20º–30°) fine-grained tra- and 630 ± 60 ka (2 σ uncertainty; LeMasurier first, volcanism was centered at Brandenberger chytic-phonolitic lapilli tuff and tuff that over- and Rex, 1983). Three subsequent episodes of Bluff (~1650 m above sea level [a.s.l.]) at 2.7 lies a phonolitic lava. Lapilli tuff at the top of activity at Merrem Peak crater are recorded by Ma; second, eruptions from the 3000 m a.s.l. the bluff exhibits shallow-dipping planar beds proximal deposits west of the crater, including a Merrem Peak crater occurred from 571 to 141 and cross-beds and contains abundant armored trachytic lava dated to 443 ± 52 ka, an 18+ m ka; and third, volcanic activity shifted to the lapilli, features indicative of subaerial phreato- thick, densely to incipiently welded, trachytic 3478 m a.s.l. summit crater from 25.5 to 0 ka. magmatic base surge eruptions. This section is pumiceous pyroclastic breccia (Fig. 2C) dated The volcanic history is recorded in crater rim interpreted as an emergent phreatomagmatic to 183.6 ± 2.9 ka, and a 1-m-thick, phonolitic, and flank deposits on the volcano and in distal tuff cone, with subaqueously deposited flank nonwelded pumice lapilli layer dated to 141.4 tephra exposed in bare ice at the margin of the deposits and subaerially deposited bluff-top de- ± 5.7 ka. Both of the 183.6 ka and 141.4 ka fall summit ice cap of Mount Moulton, located 30 posits. The highest elevation outcrop (at ~2000 deposits mantle topography. km east of the Mount Berlin summit. The m a.s.l.) associated with the ancestral stage con- Flank eruptions also occurred during this in- chronology, geochemistry, and explosivity of sists of 2.67 ± 0.07 Ma phonotephritic cinder terval of volcanism at Merrem Peak crater. late Quaternary activity at Mount Berlin are cone deposits. Previous workers interpreted Groundmass samples from parasitic basanite summarized in Figure 8 and discussed in the Brandenberger Bluff as subglacially erupted to hawaiite cinder cone remnants on the north- following. hyaloclastite, a remnant of a table mountain west flank of Mount Berlin yield a mean age of Stage I. Ancestral Volcano. The oldest evi- (LeMasurier and Rex, 1983). 211 ± 18 ka. A pyroclastic vent breccia and a dence for volcanic activity at Mount Berlin is Stage II. Merrem Peak Crater. The second near-vent trachyte lava (231 ± 11 ka) located on 2.74 ± 0.06 Ma (n = 5) phonolitic-trachytic lava and apparently most voluminous phase of activ- the northeast flank and a welded trachytic ig- and pyroclastic rocks and 2.67 ± 0.07 Ma (n = 1) ity at Mount Berlin is characterized by growth nimbrite (228 ± 12 ka) on the southeast flank phonotephritic cinder cone deposits at and near of the volcano to Merrem Peak at 3000 m a.s.l. are geochemically identical, with a mean age of Brandenberger Bluff on the north side of Mount and by eruptions from the 2.5 × 1 km diameter 229.8 ± 8.3 ka (Fig. 3; Table 1). The ignimbrite Berlin (see footnote 1). Trachytic clasts from the Merrem Peak crater. The oldest available evi- contains ~30% fiamme, with typical aspect ra- Brandenberger Bluff section were previously dence for activity is dense foliated and clasto- tios of 1:10 (h/l). A benmoreite lava (see foot- dated by conventional K-Ar method to 2.2–2.7 genic trachyte lava located in the vicinity of note 1) overlies the ignimbrite and signifies a ca.

1570 Geological Society of America Bulletin, October 1999

TABLE 2. GLASS SHARD AND PUMICE MICROPROBE DATA WCM93 25 313 315 316 317 314 130 W-9 W-15 W-16 Volcano Berlin Moulton Moulton Moulton Moulton Moulton Moulton Takahe Takahe Takahe Rock* trach trach trach trach trach trach trach trach trach trach Age† 18 ka 15 ka 27 ka 93 ka 106 ka 119 ka 141 ka 8.2 ka 102 ka <8.5 ka Electron microprobe major element data (wt%) Number 6 5 5 8 6 6 5 6 6 6 SiO2 60.77 61.76 62.95 63.04 64.39 64.51 63.13 59.54 60.09 60.14 (0.85) (0.17) (0.44) (1.03) (0.33) (0.61) (1.62) (0.63) (0.44) (0.67) TiO2 0.45 0.42 0.51 0.45 0.43 0.40 0.40 0.85 0.55 0.57 (0.06) (0.02) (0.02) (0.15) (0.01) (0.02) (0.03) (0.03) (0.01) (0.02) AL2O3 14.73 14.22 14.04 15.12 13.75 14.30 14.15 15.56 14.78 14.37 (0.73) (0.14) (0.15) (1.34) (0.18) (0.32) (0.75) (0.24) (0.24) (0.10) FeO 8.64 9.10 8.99 7.35 7.97 7.61 8.57 8.15 8.51 9.28 (0.43) (0.15) (0.11) (2.44) (0.18) (0.36) (0.59) (0.33) (0.19) (0.30) MnO 0.28 0.31 0.31 0.26 0.25 0.21 0.28 0.30 0.32 0.37 (0.04) (0.02) (0.04) (0.12) (0.03) (0.03) (0.06) (0.05) (0.05) (0.03) MgO 0.04 0.01 0.01 0.02 0.00 0.00 0.01 0.41 0.14 0.03 (0.04) (0.01) (0.01) (0.01) (0.00) (0.00) (0.01) (0.07) (0.02) (0.02) CaO 1.10 0.94 1.07 1.18 0.97 0.81 0.86 1.87 1.28 1.40 (0.20) (0.06) (0.05) (0.41) (0.01) (0.04) (0.07) (0.46) (0.19) (0.02) Na2O 8.91 8.36 7.27 7.09 7.22 7.16 7.69 8.03 9.13 8.61 (1.33) (0.41) (0.32) (0.40) (0.25) (0.49) (2.62) (0.29) (0.43) (0.18) K2O 4.43 4.16 4.34 4.92 4.42 4.37 4.10 4.87 4.70 4.74 (0.20) (0.11) (0.11) (1.27) (0.11) (0.11) (0.47) (0.23) (0.13) (0.13) P2O 5 0.07 0.07 0.04 0.04 0.07 0.02 0.02 0.11 0.02 0.06 (0.07) (0.05) (0.05) (0.04) (0.04) (0.02) (0.03) (0.09) (0.03) (0.05) Cl 0.25 0.31 0.22 0.24 0.24 0.28 0.35 0.09 0.18 0.15 (0.03) (0.02) (0.02) (0.09) (0.02) (0.03) (0.01) (0.01) (0.02) (0.02) F 0.34 0.34 0.26 0.30 0.31 0.34 0.44 0.24 0.32 0.30 (0.04) (0.03) (0.01) (0.10) (0.02) (0.05) (0.05) (0.03) (0.04) (0.04)

P.I.§ 1.32 1.28 1.19 1.12 1.21 1.15 1.21 1.19 1.36 1.34 Ion microprobe trace element data (in ppm) Number 4 3 3 5 2 3 4 4 Li 39 41 57 41 44 22 47 31 (13) (13) (36) (10) (5) (3) (8) (4) B1720 151518 698 (2) (2) (1) (2) (1) (1) (1) (1) P 203 281 198 281 194 375 305 283 (12) (45) (12) (32) (6) (26) (52) (21) Rb 153 168 141 180 182 93 163 138 (12) (9) (14) (36) (4) (7) (28) (8) Sr 8 6 5 4 2 19 7 2 (1) (0) (0) (2) (0) (3) (1) (1) Y 124 142 110 122 143 60 97 98 (17) (9) (6) (7) (3) (5) (2) (4) Zr 1495 1631 1171 1282 1527 533 983 833 (145) (30) (38) (36) (24) (26) (15) (21) Nb 253 300 207 206 254 100 185 166 (17) (10) (3) (12) (23) (3) (10) (5) Ba 84 65 57 58 22 356 27 29 (29) (21) (10) (9) (11) (46) (2) (2) La 176 203 148 171 186 94 154 142 (28) (19) (5) (12) (16) (9) (18) (14) Ce 355 413 295 345 386 185 301 284 (39) (22) (9) (26) (18) (14) (19) (8) Nd 133 147 117 149 141 71 115 113 (20) (15) (3) (39) (11) (11) (6) (8) Eu 6 7 7 10 6 7 4 6 (2) (1) (0) (7) (2) (2) (2) (1) Th 25 29 18 25 28 9 17 15 (3) (2) (2) (10) (0) (2) (4) (3) U1010 7206 24 2 (1) (0) (1) (27) (2) (1) (0) (0) Notes: Geochemical quantities in weight % or ppm are data averaged from multiple analyses; quantities in parenthesis are 1 standard deviation uncertainties. *Rocks classified according to LeBas et al. (1986). †Ages are 40Ar/39Ar mean ages, except age of Mount Takahe sample W-16, which is inferred from field relationships. § P.I.—peralkaline index = molecular (K2O + Na2O)/ Al2O3.

230 ka or younger effusive eruption. Plagioclase than 400 m to 3478 m a.s.l. and a southeastward 150 m in thickness, are exposed in the eastern from the benmoreite yielded uninterpretable shift of the vent area to the 2-km-diameter sum- wall of the summit crater (Fig. 2A). These pyro- laser fusion age data. mit crater (Fig. 2A). The constructional slopes clastic fall deposits are composed of slightly Stage III. Summit Crater. The final and of the volcano above Merrem Peak crater are flattened pumiceous bombs and abundant cog- still-active phase of Mount Berlin volcanism typically 20º–25°. Two prominent welded tra- nate xenoliths (Fig 3B). We attribute the weld- was marked by growth of the volcano by more chytic pyroclastic fall units, totaling more than ing to agglutination with minor load pressure

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compaction. Recumbent folds seen in one unit 15 15 suggest that part of the pyroclastic deposit TAS B 10 flowed rheomorphically. The ages of the two lower welded units are analytically indistin- B 141 ka 5 2738 ka B guishable at 25.5 ± 2.0 ka. These are locally 571 ka 14 0 overlain by a >10-m-thick sequence of welded Phonolite 40 50 60 70 and nonwelded pyroclastic fall beds. The fall B T <8.2 ka S deposits include a 6-m-thick lithic- and ash-rich T B 25.5 ka explosion breccia containing bomb and lithic 13 B B B B BB T B

clasts to 50 cm in diameter and a 2-m-thick 2 454 184 ka B B S B ka B densely welded obsidian fall. Anorthoclase phe- BBB B B B nocrysts from the obsidian yielded a maximum T B age of 18.2 ± 5.8 ka. These crater-wall deposits 12 BBB B 450 ka BB B 230 ka at Mount Berlin were previously interpreted as >100 ka T 2 571 ka B T lava on the basis of reconnaissance investiga- K O + Na (wt%) tions (LeMasurier and Kawachi, 1990). B XRF data points Several fumarolic ice towers and steaming 11 10.3 ka Trachyte B = vents along the summit crater rim attest to on- Mount Berlin S = Mount Siple going geothermal activity. One ice tower opens T = Mount Takahe into an underlying ice cave system more than 70 ± 10 m long; a lava at the cave floor is dated to 10.3 58 60 62 64 66 5.3 ka. Surface temperatures of the cave floor lava are as high as 12 °C. SiO2 (wt%) Distal Mount Berlin Tephra. Glacial ice at the summit of Mount Moulton volcano, 30 km Figure 3. Total alkalies vs. silica (TAS) plot of whole-rock X-ray fluorescence data (XRF) from east of Mount Berlin (Fig. 1), contains at least 19 Mount Takahe (T), Mount Siple (S), and Mount Berlin (B) samples. Same-aged samples are cir- nonwelded pumice lapilli and ash layers. Five of cled. Phonolite and trachyte field boundaries are from LeBas et al. (1986). Mount Takahe data the layers with the coarsest pumice were sam- are from Palais et al. (1988). In calculating weight percents, major element data were normal- pled in 1993–1994 and the entire suite of coarse ized to 100% volatile free. Inset shows entire TAS plot after LeBas et al. (1986). pumice to fine ash layers was sampled in 1996–1997 (Figs. 2 and 9). The tephra layers are as thick as 15 and locally dip steeply, 40°–45°N, suggesting an upstream ice source at the summit of Mount Moulton and a downstream impedi- ment to ice flow at the 5.9 Ma Prahl Crags. Of 10

samples 40Ar/39Ar dated by furnace and laser ) 600

σ methods, seven yielded unimodal laser ages

and/or concordant furnace ages (Figs. 5, A–F, ± and 6, A–E). The mean ages of these seven tephra layers are consistent with stratigraphic order: 14.5 ± 3.8 ka, 27.3 ± 2.3 ka, 92.5 ± 2.0 ka, 400 106.3 ± 2.4 ka, 118.7 ± 2.5 ka, 141.6 ± 7.5 ka, 1:1 age and 492.4 ± 9.7 ka (Fig. 9; Table 1). Only four of the seven dated tephra layers correspondence record eruptions that correlate in age to dated Mount Berlin outcrops. The youngest tephra has 200 a trace element composition that is indistin- guishable from that of the youngest Mount Berlin crater wall obsidian, but that is very dif-

ferent from the youngest Mount Takahe sample Furnace mean ages (in ka (Fig. 10). We infer, on the basis of similar trace 0 element trends, that all seven of the dated Moul- 0 200 400 600 ton tephra layers were derived from Mount Laser mean ages (in ka +_ 2σ) Berlin. The uppermost tephra layers are possibly coeruptive with the 18.2 ± 5.8 ka (maximum age) and 25.5 ± 2.0 ka units exposed in the wall Figure 4. Comparison between replicate sample mean ages obtained by single-crystal laser- of the Mount Berlin crater (Table 1). Average fusion and bulk sample furnace step-heating methods. The center of the ellipse is the age inter- ages of these two youngest potentially coerup- cept, and the width and height correspond to two standard deviation uncertainties of furnace tive units from Mount Berlin and Mount Moul- and laser mean ages, respectively. Ages are weighted by the inverse of variance and uncertain- ton units are 15.6 ± 4.6 ka and 26.3 ± 2.4 ka. ties are calculated by the method of Samson and Alexander (1987).

1572 Geological Society of America Bulletin, October 1999

BIT 152 , same as WCM93-314 (L#8418) WCM93-314, same as BIT 160 (L#5003&5354) Mount Berlin Englacial tephra layer #4 Mount Berlin Englacial tephra layer #11

n = 30 n = 22 31.4 121.3 Mean age = 29.6 ± 2.0 ka Mean age = 119.5 ± 4.7 ka M.S.W.D. = 1.29 M.S.W.D. = 0.900

-20 0 20 40 60 80 100 0 50 100 150 200 250 300

BIT 157 , same as WCM93-316 (L#8419) BIT 162 (L#8425) Mount Berlin Englacial tephra layer #8 Mount Berlin Englacial tephra layer #13

n = 35 n = 21

94.3 143.6 Mean age = 147.1 ± 3.5 ka Mean age = 94.0 ± 1.1 ka M.S.W.D. = 0.311 M.S.W.D. = 0.981

50 100 150 200 250 60 100 140 180 220 260 300

BIT 158 , same as WCM93-317 (L#8421) BIT 166 (L#8438) Mount Berlin Englacial tephra layer #9 Mount Berlin Englacial tephra layer #15

n = 53 n = 15

113 488.3 Mean age = 492.4 ± 4.8 ka Mean age = 113.6 ± 1.1 ka M.S.W.D. = 0.487 M.S.W.D. = 0.462

50 100 150 200 250 350 400 450 500 550 600 650 Age ± 1 s.d. Age ± 1 s.d. (ka) (ka)

40 39 Figure 5. Six representative ideogram plots showing Ar/ Ar CO2 single-crystal laser-fusion age data of anorthoclase samples extracted from the englacial tephra layer sequence at Mount Moulton. The ideogram is a probability distribution diagram plot of apparent age vs. the summa- tion of the normal distribution of each individual analysis (Deino and Potts, 1990). The age listed above the curve corresponds to the peak value. The age and error (1σ) for each crystal is shown by the symbols and horizontal lines in the upper section of each plot. Solid symbols represent the analyses used in the weighted mean age calculation and the generation of the solid line on the ideogram. The open symbols are data omitted from the mean age calculation and the dashed line on the ideogram represents the probability distribution of all of the displayed data. Mean ages (±1σ) were calculated according to method of Samson and Alexander (1987). M.S.W.D.—mean squared weighted deviation, calculated according to Mahon (1996). The laser mean ages combined with furnace plateau ages in Figure 6 are consistent with stratigraphy and indicate that the en- closing summit ice cap at Mount Moulton dates to 492 ka.

The younger age is a maximum age for the de- The distribution and ages of deposits at Mount contain a complete record of explosive volcanism posit because all dated samples produced Berlin suggest that the volcano grew by more than at Mount Berlin. Off-axis winds or wind erosion slightly discordant and saddle-shaped age spec- 400 m during the 141–25 ka interval. We specu- may have removed some tephra time horizons tra. The 141.6 ± 7.5 ka englacial tephra appears late that the 119–93 ka tephra may be present at from the Mount Moulton sequence. to be correlative with the 141.4 ± 5.4 ka pumice Mount Berlin, buried beneath ice and younger deposit at Merrem crater, Mount Berlin. The rocks. This extensive englacial record of inferred Mount Siple Crater Rim 492.4 ± 9.7 ka englacial tephra is possibly cor- Mount Berlin eruptions illustrates the fragmen- relative with the 443 ± 52 ka trachyte lava at tary nature of the record at Mount Berlin volcano. Mount Siple is a 3110-m-high undissected is- Merrem crater, Mount Berlin. In turn, the englacial tephra sequence may not land volcano located along the relatively inac-

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A WCM93-022, Mount Berlin Summit Crater, laminated pumice D BIT 158, same as WCM93-317, Mt. Berlin, distal tephra layer #9 200 250

Plateau age = Plateau age = 200 ± 150 106.5 ± 4.3 ka 108.1 8.5 ka

150 100 Plateau age = 27.1 +_ 3.1 ka Age (ka) Age (ka) 100 50 50 Plateau age = 106.0 ± 3.2 ka Plateau age = 105.2 ± 5.4 ka 0 Plateau age = 0 25.2 +_ 3.1 ka

-50 0 50 100 0 50 100

B BIT 152, same as WCM93-315, Mount Berlin, distal tephra layer #4 E BIT 160, same as WCM93-314, Mount Berlin, distal tephra #11 400 300

250 Plateau age = Plateau age = 119.5 ± 6.0 ka 120.9 ± 4.2 ka 200 Plateau age = 200 26.4 +_ 2.2 ka 150 (ka) Age (ka) Age

0 100 Plateau age = 28.0 +_ 4.0 ka 50

-200 0 0 50 100 0 50 100

C BIT 157, same as WCM93-316, Mount Berlin, distal tephra layer #8 F W85-009, Mount Takahe, south Bucher Rim 250 100

200 Plateau age = 91.2 +_ 4.9 ka Plateau age = 150 90.7 +_ 3.0 ka Age (ka) 100 0 Age (ka) Plateau age = 50 8.2 +_ 5.4 ka Plateau age = Plateau age = 92.6 ± 4.5 ka ± 0 92.2 5.7 ka

-100 0 50 100 0 50 100 Cumulative %39Ar Cumulative %39Ar Figure 6. Representative 40Ar/39Ar furnace step-heating age spectra of six bulk anorthoclase samples. Plots show cumulative %39Ar released during furnace-heating experiment vs. apparent age (±2 s.d.). Samples WCM93-022 (A) and BIT 152 (B) are correlative pyroclastic trachyte units from the summit crater wall of Mount Berlin and englacial tephra sequence at Mount Moulton, respectively. Both units have geochemical similarities to the 20–30 ka Byrd Station ice-core tephra. (C–E) Samples BIT 157, BIT 158, and BIT 160 also date englacial tephra layers at Mount Moulton, with ages that are consistent with stratigraphy. (F) Sample W85-09 is from the youngest summit crater rim deposit on Mount Takahe and is correlated to the youngest 7.5 ka tephra in the Byrd Station ice core.

cessible Hobbs Coast (Fig. 1). Because of ice Mount Siple have been limited to coastal satel- ism is preserved in the shallow-marine environ- cover, outcrops at the 4–5-km-diameter summit lite centers (LeMasurier and Rex, 1990). In the ment adjacent to the volcano. caldera and upper slopes of Mount Siple are ex- first visit to the Mount Siple summit, we sam- tremely limited. The lack of dissection of the pled the 20+ m thick, moderately to densely Mount Takahe Crater Rim coastal volcano combined with the <0.1 Ma age welded, pyroclastic fall deposit forming the of a satellitic basalt cone near sea level has led highest point of the caldera rim. Anorthoclase Mount Takahe is an undissected shield volcano to speculation of recent activity of Mount Siple from the trachytic fall deposit yielded a mean (constructional side slopes 8º–12°) with an 8-km- volcano (LeMasurier and Rex, 1990). Satellite age 226.7 ± 7.6 ka, and a lava sample from a diameter ice-filled summit caldera located in east- images interpreted as active eruption plumes subsidiary vent below the summit crater yielded ern Marie Byrd Land, ~250 km northeast of the during 1988 were later discounted (Smithsonian an age of 168.9 ± 5.4 ka. It is probable that a planned WAISCORES drill site on the West Institution, 1988). Previous investigations at more complete record of Mount Siple volcan- Antarctic Ice Sheet ice divide (Fig. 1). Limited ex-

1574 Geological Society of America Bulletin, October 1999

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76°S 2000 m 1400 m Figure 7. Distribution and ages of outcrops II. Merrem Peak Crater on Mount Berlin. Three stages of activity are 140-–600 ka interpreted from these data, with a southeast- 1600 m c III. Berlin Summit Crater ward shift in the vent location. Base map is 0–26 ka from the Mount Berlin quadrangle, scale 3000 m 1:250 000. (U.S. Geological Survey, 1973). c 3478 m

Outcrop, 0–26 ka Outcrop,140–250 ka Outcrop, 400–600 ka Outcrop, 2.7 Ma Outcrop, not dated 2000 m 0 5 c Crater

200 m contour 136°W km

High E

Figure 8. Summary of volcanic activity at Explosivity

Low Mount Berlin, based on field observations, O 40 39 2 14 D Ar/ Ar dating, and geochemical analyses of +K samples from in situ outcrops at Mount Berlin O

2 and the englacial tephra sequence at Mount 10 = 6.2% Moulton. On the basis of age and rock lithol-

%Na C ogy, 12 eruptive units are identified. A proba- 2 64 bility distribution plot (A) shows 10 distinct 40 39 = 46.7% eruptive episodes, based on Ar/ Ar mean

%SiO 60 ages and uncertainties (B). Ages (see text and Table 1) from 68 laser and furnace analyses 10 B Mean unit ages ± 2 σ(ka) were used to calculate mean ages. Most of the 5 Laser and furnace analyses (n = 68) eruptive products were alkalic trachytes (C n events 0 and D), with SiO2 contents between 60% and 26.8 93.0 A 65% and alkali contents between 11% and 14.1 105.6 15%. Major oxides are based on X-ray fluorescence analyses of whole-rock powders. The 118.3 relative explosivity, shown in E, is based on the 183.1 nature of volcanic deposits. Trachytic lavas and mafic cinder cone deposits are classified

Relative 140.8

probability as effusive (below dashed line), although both 228.2 570.4 deposits may have had explosive phases. Tra- 493 chytic pyroclastic fall and flow deposits are classified as explosive; the Mount Moulton tephra layers are associated with the most explosive eruptions. 0 100 200 300 400 500 600 700 Eruption age (ka)

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Plan View Prahl Crags BIT-156 5.88 Ma BIT-157 BIT-158 BIT-159 BIT-160 BIT-161 BIT-154 BIT-153 BIT-152

BIT-166 BIT-151 134o42’36"W

BIT-150 BIT-163 WCM93-313 Figure 9. Map and cross section of englacial BIT-162 tephra layers in the summit ice cap of Mount Moulton, located 30 km east of source at GPS locations Mount Berlin. We infer that the tephra layers Estimated locations are exposed at the surface because ice flow Sample locations from the summit ice cap is deflected upward North as it approaches a late Miocene outcrop ob- 0 100 struction at Prahl Crags. Wind ablation has m resulted in interbedded ice and tephra ex- 76o04’15"S 76o04’05"S posed at the surface. The upper diagram is a global positioning system (GPS) map and the lower drawing is a cross-sectional view of the Schematic cross section ice-tephra stratigraphy. The thicknesses of Tephra 15 Tephra 11 Tephra 8 Tephra 2 tephra layers are exaggerated. (BIT-166) Tephra 13 (BIT-160) Tephra 9 (BIT-157) Tephra 4 (WCM-313) <1 cm thick (BIT-162) 15 cm thick (BIT-158) 3 cm thick (BIT-152) 5 cm thick 492 +/- 4 ka <1 cm thick 119 +/- 3 ka 2 cm thick 92 +/- 2 ka 12 cm thick 15 +/- 4 ka 142 +/- 8 ka 106 +/- 3 ka 27 +/- 2 ka

0 100 m

ice flow Prahl Blue Crags ice

South North

posures at and near the Mount Takahe caldera rim gested that much of the volcano construction oc- from mildly explosive Strombolian to highly include a 60 m section of welded and nonwelded curred after 40 ka, the age of the oldest Byrd ice- explosive Plinian, could produce the ubiquitous pyroclastic lapilli deposits, obsidian-bearing core tephra layers. welded and nonwelded proximal fall deposits bomb-and-block layers, hydrovolcanic tuffs, and seen at Marie Byrd Land volcanoes. Lavas are lavas (McIntosh et al., 1985). Anorthoclase from DISCUSSION associated with nonexplosive eruptive episodes, a sample of a nonwelded obsidian and pumice although they do not preclude the possibility of bomb-and-block layer near the top of this se- Explosivity of Marie Byrd Land Eruptions an associated explosive phase. quence has an 40Ar/39Ar age of 8.2 ± 5.4 ka (Fig. Although near-vent deposits provide only a 6F). This young unit, currently being rapidly The dispersive power of volcanic eruptions is qualitative measure of explosivity, the thickness eroded, overlies a sequence of lavas and densely largely controlled by the height of the eruption and coarse grain size of englacial tephra layers welded pyroclastic deposits with ages ranging column, which can be estimated from lateral on Mount Moulton at a distance of 30 km from from 93.3 ± 7.9 to 192.0 ± 6.3 ka. These older changes in deposit thickness and grain size their source demonstrate the highly explosive units indicate that Mount Takahe volcano reached (Carey and Sparks, 1986). Near-vent deposits nature of several Mount Berlin eruptions. its present elevation by 192 ka, contrary to a pre- provide only qualitative measures of eruption Pumice densities and dimensions were mea- vious interpretation (Palais et al., 1988) that sug- explosivity. A wide range of eruption styles, sured in four of the tephra layers (Table 3).

1576 Geological Society of America Bulletin, October 1999

1000

Figure 10. Mean ion micro- 100 probe trace element data (±1σ)

) of Mount Berlin sample,

σ WCM93-025; Mount Moulton englacial tephra, WCM93-313; ppm (±1 and Mount Takahe, W85-09. 10 Berlin obsidian (18 ka) Moulton tephra (15 ka) Takahe obsidian (8.2 ka)

1 Li B P Rb Sr Y Zr Nb Ba La Ce Nd Eu Th U Trace element

Mean diameters of the five largest clasts are logical matches between some of the Byrd Sta- Glacial Tephrochronology—Dating Ice 17–18 mm in the 27, 93, and 119 ka tephra lay- tion ice-core tephra and Mount Berlin tephra and Marine Cores ers; maximum anorthoclase crystal lengths in (Table 2, details discussed in the following sec- these coarser tephra layers are 10 mm. Such tion) are consistent with these model predictions. Ash layers in ice or marine cores can serve as limited data do not allow precise reconstruc- A further application of the 28-km-high precisely dated time-stratigraphic markers, if tions of eruption conditions, but can be used for model result predicts that the maximum particle they can be geochemically correlated with well- first-order estimates of eruption column height. size at a distance of 600 km from vent (distance dated source volcanic units. Tephrochronology Assuming a case of no-wind conditions, the from Mount Berlin to Byrd Station) would have correlation studies typically rely on chemical tephra dispersal model of Carey and Sparks been 5 µm. Byrd ice-core tephra reached sizes “fingerprinting” of individual tephra units. (1986) predicts an ~40-km-high eruption col- to 60 µm in diameter, which suggests that the Grain-discrete analyses of glass fragments, re- umn for the given density, size, and distance Mount Berlin eruption intensity or wind speed quiring microanalysis techniques, provide the from source of coarsest pumice at Mount Moul- is underestimated. The dispersive power of most reliable data. Our preliminary results ton. Assuming a maximum wind speed of 20 m/s Mount Berlin eruptions can be further evaluated based on multiple techniques indicate the po- (72 km/hr) and a westerly wind direction, the by geochemical and geochronological analyses tential for future correlation of ice-core tephra same model predicts an ~28-km-high eruption of englacial tephra layers at Mount Waesche, to source volcano tephra. column for the given clast parameters. In either which may be correlated to englacial tephra at In this study, Mount Takahe and Mount case, fine ash from such explosive eruptions Mount Moulton. In the 1996–1997 field season, Berlin are both recognized as sources for Byrd would have penetrated the stratosphere and several coarse ashes at Mount Waesche (with Station ice-core tephra. Palais (1985) favored been deposited over large portions of the West anorthoclase to 0.2 mm) were collected and only Mount Takahe as the source of the Byrd Antarctic Ice Sheet and the southern Pacific found to be petrographically similar to Mount core tephra, although she concluded that Mount Ocean. The close geochemical and geochrono- Berlin and Mount Moulton tephra. Berlin could not be ruled out as a source of the

TABLE 3. MOUNT MOULTON ENGLACIAL TEPHRA-PUMICE CLAST DATA WCM 93- Age Diameter n Density Vesicularity† (ka) (mm) (kg/m3) (%) max mean* mean ± 1σ range mean range 313 15 13 9 20 620 ± 150 440–890 76 66%–83% 315 27 25 18 30 780 ± 180 520–1140 70 56%–80% 316 93 28 17 30 770 ± 190 480–126 70 52%–82% 314 119 27 18 30 540 ± 100 400–910 79 65%–85% Notes: Density measurements made following technique by Gay and Smith (1996), a modification of method by Houghton and Wil- son (1989). *Average of long, intermediate, and short axes of five largest clasts. †Based on estimate of DRE density of 2610 kg/m3.

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0.5 Takahe EMP data(10 ka) 8.2 ka Takahe Berlin Crater EMP data (18 ka) Crater Wall tephra Berlin Crater XRF data (10, 26 ka) 0.4 Berlin Distal tephra (15, 27 ka) Byrd Core EMP data 7.5 ka Byrd Byrd Core EMP data (from Core tephra Palais et al., 1988) 0.3

20–30 ka

MgO 10–27 ka Byrd Core tephra 0.2 Berlin tephra

0.1

0.0 60 61 62 63 64 65 66

SiO 2

Figure 11. Representative plot of geochemical data from young Marie Byrd Land tephra samples. All data, except X-ray florescence (XRF) data, are based on electron microprobe analyses of glass shards. XRF data are based on analyses of whole-rock samples at the University of Keele, United Kingdom. EMP—electron microprobe.

ash layers. Subsequently, Palais et al. (1988), on the basis of additional analyses from Mount Takahe and data provided to the authors on two 350 samples from Mount Berlin, concluded that sig- 15 ka nificant differences between the chemical com- 300 position of the lavas from Mount Berlin and the Byrd core tephra support the identification of Mount Takahe as the most likely source for the 250 119 ka tephra. However, a comparison of new micro- Byrd Core 93 ka probe data from dated Mount Berlin and Mount 19–60 ka Mount Berlin Takahe samples and Byrd ice-core tephra layers Nb 200 supports the new correlation of 20–30 ka Byrd (ppm) 106 ka No age 27 ka core samples to Mount Berlin (Fig. 11). Figure 102 ka 11 shows that the 20–30 ka Byrd core tephra 150 Mount Takahe samples are geochemically similar to the 10–27 ka Mount Berlin volcano samples, whereas the 8.2 ka youngest dated Byrd core sample is geochemi- 100 Byrd Core cally similar to the youngest Mount Takahe 19–60 ka sample. The geochemical affinities between the ice-core tephras and source volcanoes are con- 50 sistent with the new 40Ar/39Ar ages (Fig. 12). 400 600 800 1000 1200 1400 1600 1800 Although the age of the base of the Byrd ice Zr core is controversial, the lack of tephra correla- (ppm) tive with the older eruptive units at Mount Berlin is consistent with a basal ice-core age of less than 93 ka. Figure 12. Plot showing niobium (Nb) vs. zirconium (Zr), based on individual ion microprobe The new correlation of Byrd core tephra to analyses of volcanic trachytic-phonolitic glass fragments from Mount Takahe, Mount Berlin the more distant Mount Berlin indicates the po- (Mount Moulton tephra layers), and Byrd core. The 27 ka point is from X-ray fluorescence data. tential of Mount Berlin tephrochronology for Byrd core data are from a composite sample of five Byrd core tephra layers with model ages be- dating deeper and older ice cores. Preliminary tween 19 and 60 ka. ion microprobe data indicate a unique trace element signature of each of the Mount Moulton

1578 Geological Society of America Bulletin, October 1999

Base of Byrd ice core core Takahe crater

Siple crater

Berlin tephra ? Berlin craters Potential tephra layers in WAISCORES

500 400 300 200 100 0 Eruption ages (ka)

Figure 13. 40Ar/ 39Ar volcanic time horizons. Time line summarizing tephrochronology in Marie Byrd Land. The black diamonds represent the 40Ar/39Ar eruption ages (±2 s.d.) from Mount Takahe, Mount Siple, and Mount Berlin. Most of the eruption ages are mean ages of different and/or replicate samples from each site (see Table 1). The vertical lines at top indicate model ages of coarse tephra layers in the Byrd ice core (Palais et al., 1988). The age of basal ice in the Byrd ice core is estimated as 74 ka, based on the electrical conductivity method (Hammer et al., 1994). Dou- ble-pointed arrows show correlations among Marie Byrd Land tephra and Byrd ice-core tephra layers. Potential Marie Byrd Land tephra time horizons in planned WAISCORES are shown at bottom.

tephra layers and Mount Takahe crater rim de- to Prahl Crags, including an ~55 m stratigraphic explosive, trachytic eruptions traces the late posits (Table 2; Fig. 13). The Moulton and thickness of ice between the oldest tephra layer Quaternary evolution of Mount Berlin, Mount Takahe trace element data resemble trace ele- (492 ka) and Prahl Crags (Fig. 9). This well- Siple, and Mount Takahe, three large alkaline ment signatures from glass shards in the com- dated section of glacial ice contains a potential composite volcanoes. This record provides a posite Byrd ice-core tephra sample (Fig. 13). If “horizontal ice core” record of paleoclimate that previously unrecognized potential independent these 40Ar/39Ar dated Marie Byrd Land ashes dates back to before 492 ka. Survival of such an- chronology for future WAISCORES and marine are identified by geochemical fingerprinting in cient ice in West Antarctica does not preclude core records. future ice and marine cores, then they will pro- West Antarctic Ice Sheet collapse during or 2. A detailed history of Mount Berlin records vide the cores with independently dated time since the ca. 125 ka interglacial or previous in- a three-stage growth and southeastward vent horizons. terglacial intervals, because temperatures at the migration. Many Mount Berlin eruptions have The small number of eruptions identified at the high-elevation tephra deposition site (3000 m been explosive, capable of depositing ash source volcanoes contrasts sharply with the large a.s.l.) would remain below freezing even with widely over the West Antarctic Ice Sheet. number of tephra layers in the Byrd ice core. This dramatic low-elevation temperature increases. 3. The youngest trachytic tephra layer in the discrepancy results in part from the poor expo- In theory, oxygen isotope data from such an- Byrd ice core, which was dated by ice-flow sure and burial by subsequent eruptions of source cient ice could test the West Antarctic Ice Sheet models to 7.5 ka, is correlated by age and geo- volcano rocks. In addition, the abundant chemi- collapse hypotheses (where ice-sheet collapse chemistry to an 8.2 ± 5.4 ka tephra layer at cally recognized fine ash layers in the Byrd ice and deglaciation is reflected in relatively higher Mount Takahe. Older (13–40 ka) Byrd core core might be overlooked at volcano outcrops δ18O values associated with decreased conti- tephra layers are not recognized in crater rim ex- and bare-ice areas, where only visual outcrop nentality and warmer local temperatures). The posures at Mount Takahe. sampling techniques have been applied. deglaciation hypothesis could be further tested 4. The 19–30 ka tephra layers in the Byrd ice by examining englacial tephra records from core are similar in age and geochemistry to the Oldest Ice in West Antarctica? Mount Waesche or other bare-ice areas on the 10–27 ka eruptive units at Mount Berlin. One- West Antarctic Ice Sheet for evidence of older to-one correlations between Byrd core tephra The Mount Moulton summit ice cap contains Mount Berlin tephra. and Mount Berlin deposits are not possible due the oldest dated ice in West Antarctica. The to the lack of available Byrd core material for englacial record of Mount Berlin eruptions at SUMMARY geochemical fingerprinting. These Byrd ice- Mount Moulton indicates that the interbedded core tephra units were previously correlated to glacial ice is older than the 125 ka interglacial The 40Ar/39Ar chronology of tephra layers Mount Takahe. temperature maximum and has survived more and source volcanoes in Marie Byrd Land of- 5. The Mount Moulton summit ice cap con- than 492 k.y. of climatic fluctuations. An ~320 fers a regional record of late Quaternary vol- tains the oldest dated ice in West Antarctica with m stratigraphic thickness of dipping ice and canic activity that includes the following im- a minimum age of 492 ka. On a horizontal patch tephra layers is exposed along the horizontal portant features. of bare ice at Mount Moulton, coarse-grained, section from the youngest tephra layer (15 ka) 1. The 8.2–571 ka chronology of 20 mostly englacial tephra layers derived from Plinian

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Devine, J. D., Sigurdsson, H., and Davis, A. N., 1984, Esti- Volcanic Province and its relation to the Cainozoic West eruptions of Mount Berlin delineate 12 time mates of sulfur and chlorine yield to the atmosphere from Antarctic rift system, in Tingey, R. J., ed., The geology of horizons from 492 to 15 ka within an ~320 m volcanic eruptions and potential climatic effects: Journal Antarctica: Oxford, Clarendon Press, p. 249–284. stratigraphic thickness of dipping ice and tephra of Geophysical Research, v. 89, p. 6309–6325. LeMasurier, W. E., and Wade, F.A., 1968, Fumarolic activity in Drewry, D. J., Jordan, S. R., and Jankowski, E., 1982, Mea- Marie Byrd Land: Science, v. 162, p. 352. layers. This tephra and ice stratigraphic se- sured properties of the Antarctic ice sheet: Surface con- Lo Bello, P., Feraud, G., Hall, C. M., York, D., Lavina, P., and quence offers the potential of an easily accessi- figuration, ice thickness, volume, and bedrock character- Bernat, M., 1987, 40Ar/39Ar step heating and laser fusion ble, stratigraphically coherent, horizontal ice istics: Annals of Glaciology, v. 3, p. 83–91. dating of a Quaternary volcanic from the Neschers, Mas- Drewry, J., 1983, Antarctica: Glaciological and geophysical fo- sif Central, France: The defeat of xenocrystic contamina- core with a local climate record that can be cor- lio: Cambridge, University of Cambridge Scott Polar Re- tion: Chemical Geology, v. 66, p. 61–71. related by stratigraphy and tephrochronology to search Institute. MacAyeal, D., 1992, Irregular oscillations of the West Antarc- Esser, R. P., Kyle, P. R., McIntosh, W. C., and Heizler, M. T., tic Ice Sheet: Nature, v. 27, p. 321–325. deep ice and marine core records. 1997, Excess argon in melt inclusions in zero-age Mahon, K. 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