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Surface Exposure Dating Using Cosmogenic

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Surface Exposure Dating Using Cosmogenic Surface exposure dating using cosmogenic isotopes: a field campaign in Marie Byrd Land and the Hudson Mountains Joanne S Johnson1, Karsten Gohl2, Terry L O’Donovan1, Mike J Bentley3,1 1British Antarctic Survey, High Cross, Madingley Road, Cambridge, CB3 OET, UK 2Alfred Wegener Institute, Postfach 120161, D-27515, Bremerhaven, Germany 3Durham University, South Road, Durham, DH1 3LE, UK INTRODUCTION SUMMARY 2. HUNT BLUFF Here we present preliminary findings from fieldwork undertaken in Marie • We collected 7 erratic and 2 bedrock surface samples, which are Byrd Land and western Ellsworth Land (Fig.1) in March 2006, during This site is a granite outcrop along currently being processed for 10Be, 26Al and 3He dating. We will visit which we obtained samples for surface exposure dating. This work was the western side of Bear the Hudson Mountains again in 2007/8 for further sampling and supported by helicopters from RV Polarstern, on an expedition to Pine Peninsula, adjacent to the Dotson collection of geomorphological data. Island Bay. Ice Shelf. Lopatin et al. (1974) report an age of 301 Ma for the • Samples from Turtle Rock, Mt Manthe and the un-named island are Surface exposure dating provides an estimate of the time when ice granite. granite/granitoids; from Hunt Bluff we collected a meta-sandstone and retreated from a rock surface, leaving it exposed. Nucleii in rocks are basalt, in addition to granite bedrock. split apart by neutrons produced when secondary cosmic rays enter the In half a day, we found only a few • Erratic boulders at Mt Manthe and Hunt Bluff are scarce; at Turtle atmosphere. The resulting cosmogenic isotopes (such as 10Be and 26Al) erratic samples (Fig. 4), and one Fig. 4. Meta-sandstone boulder Rock and the un-named island, they are relatively common. accumulate within the upper few cm of a rock surface over the duration example of a striated surface (Fig. lying on granite bedrock. of exposure, and can be measured to give an 'exposure age'. 5). The striation direction of N-S • Striated surfaces (N-S orientation) were found only at Hunt Bluff. gives an indication of ice flow Surface exposure dates from the samples we collected will provide a direction over this site. • Elevation of samples 470m • Ages from Mt Manthe will tell us when the surface of Pine Island record of elevation change for the ice sheet surface in this region, and Glacier receded below 500m, and from Turtle Rock and Hunt Bluff • Erratics are small (largest 30cm) thus will help to constrain thinning of the ice sheet since the Last Glacial about the thinning of ice to the west of Pine Island Bay, close to Smith Maximum. A second field campaign is planned in this area for 2007/8. • Lithology of erratics: meta- Glacier. sandstones and an alkali basalt • An age for the sample from the un-named island may represent the • Striations in bedrock granite date that the island emerged above sea level during post-deglaciation isostatic rebound. Fig. 5. N-S trending striations • Changes in the extent and thickness of the ice sheet in the Fig. 1. Map of Quaternary will be used to help constrain ice sheet models, thus field area, aiding efforts to predict future behaviour of the West Antarctic Ice showing Sheet. sampling 3. MOUNT MANTHE 4 sites. The nunatak has one exposed hyaloclastite breccia/capping lava lava 2 4. UN-NAMED ISLAND Fig. 8. Granitoid 3 sequence on the northern side (Fig. hcb This island, one of the erratics on the 6). Le Masurier & Rex (1982) report island. Inset: 1 a K-Ar date of 5.5±1.9 Ma for the Edwards Islands, lies west of colony of elephant capping lavas. Pine Island Glacier Canisteo Peninsula (Fig. 1). It seals (PIG) lies immediately to the SW of is low-lying but has no ice the Hudson Mountains; dates from Fig. 6. Hyaloclastite (hcb) cover, unlike adjacent these samples will tell us when the overlain by lavas islands. It is home to seals surface of PIG receded below 500m. and penguins, and is almost 1. TURTLE ROCK • Elevation of samples: 500m entirely covered by a brownish sediment-like This site lies adjacent to the • Erratic boulders are sparse: we found <5; all are sub-rounded material, presumably guano. Pope Glacier and a few km • Elevation of sample: 8m west of Mt Murphy. Two granites (Fig. 7) • Elephant seal colony present (Fig. superimposed hyaloclastite •The granite boulders have a 8); evidence for penguin colony as breccia sequences are similar lithology to some of those well overlain by vesicular capping at Turtle Rock. lavas (Fig. 2). It was Fig. 7. Granite boulder (inset) lying • Thick deposit of ?guano previously visited in 1990 and on basaltic capping lavas. an eruption age for the lavas Fig. 2. Sample sites on Turtle Rock. • Numerous granitoid erratic boulders obtained (4.7±0.15 Ma, (Fig. 9) are both embedded in, and lie Smellie, pers. comm. 2006). Fig. 9. Sampling an erratic on top of, guano (Fig. 8). • Elevation of samples: 629-700m boulder • Several fairly small (less than 1m diameter) granite and granitoid erratics (Fig. 3) • No striated surfaces found, but boulders are occasionally facetted Fig. 3. Granitoid erratics amongst hyaloclastite.
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