Surface exposure geochronology using cosmogenic nuclides : applications in Antarctic glacial geology

Abstract

Cosmogenic 3 He, 2 6 A1, and 10 Be have been measured in a variety of Antarctic glacial deposits in the McMurdo Sound-Dry Valleys region. The goals of this project were to provide age constraints for Antarctic glacial events, to investigate production mechanisms of 3 He, 10Be, and 2 6 A1 in terrestrial rocks, to constrain the importance of loss of 3 He from quartz due to diffusion, and to refine methods of exposure-age dating.Moraines deposited in Arena Valley by the Taylor Glacier, an outlet glacier of the East Antarctic Ice Sheet, have exposure ages from -120 kyr to 2 myr. 1 0 Be and 3 He ages of 122 + 29 and 134 ± 54 kyr, respectively, for the Taylor II moraine are consistent with deposition during isotope stage 5e (-120 kyr) and with aerial expansions of the East Antarctic Ice Sheet during interglacial periods. Mean 1 0 Be exposure ages for older moraines in the valley are 362 + 26 kyr (Taylor III), 1.1 ± 0.1 myr (Taylor IVa) and 1.9 ± 0.1 myr (Taylor IVb). Because these older moraines were deposited at most -200 m above the Taylor II limit their ages suggest that major ice sheet advances during the last 2 myr have been broadly similar in magnitude to changes during the last glacial-interglacial cycle. 10 Be ages for stratigraphically older drift deposited by the Taylor Glacier allow extension of this conclusion to -3 myr.1 0 Be measurements in high altitude, pre-Pleistocene glacial deposits in the Dry Valleys preclude rapid uplift of the Transantarctic Mountains (400-1000 m/myr) suggested by controversial biostratigraphic studies of Sirius Group tills. Comparison of measured 10Be concentrations in Sirius Group deposits with those predicted with a model of the effects of uplift on 1 0 Be production suggests minimal uplift of the Transantarctic Mountains over the last 3 myr.3 He, 1 0 Be and 2 6 A1 ages for the "late Wisconsin" Ross Sea Drift, a glacial drift deposited on the coast of McMurdo Sound by the Ross Sea Ice Sheet, range from 8-106 kyr. The age range suggests that this deposit does not, as previous studies suggested, represent a single ice advance in response to lowered sea level at the last glacial maximum. The age range may reflect several ice sheet advances during the last glacial period.Paired measurements of 3 He and 1 0 Be in a number of quartz sandstones from the Dry Valleys region show that cosmogenic 3 He is not completely retained on time scales of greater than 200 kyr. The 3 He and 1 0 Be data, combined with measurements of 3 He concentrations in quartz as a function of grain size, suggest effective 3 He diffusion coefficients in quartz from ~10 -19 to 10-17 cm 2 s -1. These values are one to three orders of magnitude greater than previous experimental determinations extrapolated to low temperature. The data also suggest that 3 He diffusion rates in quartz do not follow simple volume diffusion trends and are probably sample dependent.Crushing quartz grains in vacuo releases variable concentrations of 3 He and 4 He and releases helium with surprisingly high isotopic compositions, u...

Antarctic glacial chronology : new constraints from surface exposure dating

“…For example, the paired 'He and 21Ne data in this study indicate only 35 percent retention. Revised age estimates from the paired 'He and lOBe data presented by Brook (1993), using the present, best estimates for production rates, indicate greater than 25 percent 'He loss from samples -100 ka. Thus, although some of the 'He ages fall within stage 6, when diffusive loss is considered, all ten samples indicate an age older than stage 6.…”

“…Meyer soil development fell between that of Taylor II (stage 5) and Taylor III (stage 7) which were dated by correlation with U/Th dated drifts farther down Taylor Valley . Subsequent surface exposure dating has confirmed the stage 5 age of the Taylor II moraine and indicates that the Taylor III moraine is >350 kyr (Brook, 1993). The degree of soil development on Meyer drift is significantly greater than that of Taylor II and only slightly less than that of Taylor III.…”

“…Brook (1993) and Brook et 01., (1993) ages of 122 ± 29 kyr (114 ± 25 with new scaling and production rate) on the youngest (lowest elevation) moraine are consistent with ThiU ages on associated lake carbonates down valley (Hendy et al, 1979). The lOBe exposure ages of the oldest moraines (-2 Ma) are also consistent with 4OAr/39Ar and K-Ar ages of volcanics interbedded with correlated moraines down valley .…”

“…Diffusion rates of 'He in clinopyroxene (cpx) and olivine, are very low compared to the time periods of interest (Trull et 01.,1991)_ Experimentally determined diffusion rates in quartz suggested that quartz would also retain helium over time scales useful for exposure dating (Trull et 01.,1991). However, 'He loss due to diffusion has been shown to occur in quartz Staudacher and Allegre, 1991) and is strongly dependent on grain size )_ On the basis ofthe concordance of paired 'He and lOBe ages up to -100 kyr, Brook (1993) and Brook et 01. (1995) tentatively concluded that diffusion in quartz is slow enough in Antarctica to have minimal effect on young «100 kyr) samples.…”

“…Exposure ages are calculated using the scaling of and a lOBe production rate of 5 atoms/g/yr (Clark et al, 1995). lOBe data is from Brook (1993).…”