Susan Ivy-Ochs

Cosmogenic noble gas studies in the oldest landscape on earth: surface exposure ages of the Dry Valleys, Antarctica

Extraordinarily high surface exposure ages have been determined for Sirius Group tillites of Mt. Fleming and Mt. Feather as well as at localities in the Inner Dry Valleys using cosmogenic helium and neon. Ages of 10 Ma at Mt. Fleming, 5.3 Ma at Mt. Feather and 6.5 Ma at Insel Mountain are among the highest nominal exposure ages published so far. These values are minimal ages as they are based on the assumption of zero erosion and uplift. The Mt. Feather sample independently confirms the pre-Pliocene age of the Sirius Group sediments in the Dry Valleys as previously determined at Mt. Fleming. The Insel Mountain samples provide evidence for a landscape formation of the Inner Dry Valleys not later than Late Miocene time. Assuming conservatively low values of 2.5 cm Ma−1 for erosion rate and 50 m Ma−1 for uplift rate we infer that the Sirius Group tillites at Mt. Fleming were deposited earlier than 20 Ma ago. This indicates that the overriding of the Dry Valleys block of the Transantarctic Mountains by the East Antarctic Ice Sheet occurred not later than the Early Miocene. Maximum long-term erosion rates in the Inner Dry Valleys must be <15 cm Ma−1 down to altitudes <1000 m. Since such low erosion rates require permanently cold and hyperarid conditions, the response of Antarctica to the Pliocene warm climatic episode must have been small. Cosmogenic nuclide data from both the Inner Dry Valleys and the Sirius Group sediment localities support the hypothesis of a stable East Antarctic Ice Sheet since at least Late Miocene time, implying that the climate of Antarctica was decoupled from that of lower southern latitudes. We present also new elemental 21Ne production rates of P21(Mg) = 196 atoms g−1 yr−1 and P21(Al) = 55 atoms g−1 yr−1 at sea level and high geomagnetic latitude. These figures are consistent with a 3He production rate of P3 = 110 atoms g−1 yr−1, similar to previously published values. This consistency provides evidence that pyroxene is retentive for both helium and neon over at least 10 Ma. Cosmogenic Ne in quartz and pyroxene has a (22Ne/21Ne)cos ratio of 1.266 ± 0.040 and 1.159 ± 0.040, respectively.

Low slip rates and long-term preservation of geomorphic features in Central Asia.

In order to understand the dynamics of the India–Asia collision zone, it is important to know the strain distribution in Central Asia, whose determination relies on the slip rates for active faults. Many previous slip-rate estimates of faults in Central Asia were based on the assumption that offset landforms are younger than the Last Glacial Maximum (∼20u2009kyr ago). In contrast, here we present surface exposure ages of 40 to 170u2009kyr, obtained using cosmogenic nuclide dating, for a series of terraces near a thrust at the northern margin of the Tibetan Plateau. Combined with the tectonic offset, the ages imply a long-term slip rate of only about 0.35u2009mmu2009yr-1 for the active thrust, an order of magnitude lower than rates obtained from the assumption that the terraces formed after the Last Glacial Maximum. Our data demonstrate that the preservation potential of geomorphic features in Central Asia is higher than commonly assumed.

Moraine Exposure Dates Imply Synchronous Younger Dryas Glacier Advances in the European Alps and in the Southern Alps of New Zealand

Samples taken from the top surfaces of boulders on the Lake Misery moraines at Arthurs Pass, in the Southern Alps of New Zealand, were analysed for 10 Be by accelerator mass spectrometry. Exposure ages calculated with the currently accepted production rate, along with scaling corrections for sample latitude and elevation (42°50S, 960 m), are: 9300 ± 990, 1 1,000 ± 1360, 11,410 ± 1030, 12,050 ± 960, and 12,410 ± 1180 years. We consider the date of 9300 years to be an outlier, not included in our mean exposure age of 11,720±320 years for the Lake Misery moraines. Based on exposure ages and geomorphologic similarities, we compare the Lake Misery moraines with an Egesen moraine complex at Julier Pass in the Swiss Alps (46°30N, 2200 m). Based on the 10 Be, 26 Al, and 36 Cl exposure ages of three boulders, we calculate a mean exposure age of 11,750 ± 140 years for the outer Egesen moraine at Julier Pass. Based solely on 10 Be measurements, we obtain a mean exposure age of 11,860 ± 210 years for this outer moraine. Egesen moraines in the Swiss Alps represent glacier readvance during the Younger Dryas cold reversal, based on regional correlations and on basal radiocarbon dates from bogs located up-valley of Egesen moraines. The exposure dates from Arthurs Pass and Julier Pass show synchronous glacier advances both in the Southern Alps and in the European Alps during the European Younger Dryas chronozone of Mangerud et al.

The oldest ice on Earth in Beacon Valley, Antarctica: new evidence from surface exposure dating

Abstract Beacon Valley, Antarctica, contains unique remnants of glacier ice underneath a till layer covering the valley floor. To constrain the age and evolution of this important indicator of Antarctic paleoclimate, we analyzed two dolerite erratics from the till surface and one from within the ice for cosmogenic helium and neon. A conservative minimum exposure age of the older surface sample is 2.3 Ma, but taking into account erosion, the true exposure age of this boulder is likely to be considerably higher. The buried sample contains more than 20 times less cosmogenic noble gases than the old surface sample, although its current shielding would imply only a three times lower production rate. This indicates that the ice level has slowly been lowered by sublimation at the rate of a few m/Ma. The high exposure age of the surface sample as well as the very low sublimation rate of the relict ice both support the conclusion that the remnant ice in Beacon Valley was deposited many million years ago [Sugden et al., Nature 376 (1995) 412–414] and has never been thinner than at present. In addition, we found that cosmogenic helium and neon are released quantitatively from pyroxene at temperatures of 1000°C, respectively.

Late Pleistocene/Holocene slip rate of the Zhangye thrust (Qilian Shan, China) and implications for the active growth of the northeastern Tibetan Plateau

luminescence dating, and 10 Be exposure dating. The seismically active Zhangye thrust transects late Pleistocene alluvial fan deposits and forms a prominent north facing scarp. The fault consists of two segments that differ in orientation, scarp height, and age. A series of loess-covered terraces records the uplift history of the western thrust segment. Loess accumulation on all terraces started at 8.5 ± 1.5 kyr and postdates terrace formation. Gravels from the

Holocene glacier culminations in the Western Alps and their hemispheric relevance

The natural variability of Holocene climate defi nes the baseline to assess ongoing climate change. Greenland ice-core records indicate warming superimposed by abrupt climate oscillations in the early Holocene, followed by a general cooling trend throughout the middle and late Holocene that culminated during the Little Ice Age (LIA). Tropical precipitation changes correlate with these patterns throughout the Holocene. Here we use mountain glaciers in the European Alps to reconstruct the regional Holocene climate evolution and to test for a link between mid-latitude, North Atlantic, and tropical climate. Our precise 10 Be chronology from Tsidjiore Nouve Glacier, western Swiss Alps, indicates a glacier culmination during the earliest Holocene ~11.4 k.y. ago, likely related to the Preboreal Oscillation. Based on our data, no Holocene glacier advance of similar amplitude occurred until ~3.8 k.y. ago, when the glacier reached LIA limits. The 10 Be ages between 500 and 170 yr correspond to the LIA, while the youngest 10 Be ages overlap with the historically recorded post-LIA glacier positions. Integrating our data with existing records, we propose a hemispheric climate link between the Alps, North Atlantic temperature, and tropical precipitation patterns for the Holocene, supporting the concept of a pervasive climate driver. These fi ndings from northern mid-latitudes are consistent with the hypothesis formulated for the tropics that the Earth’s thermal equator, responding to North Atlantic temperature changes, might have migrated southward throughout the Holocene, reaching the southern turning point toward the end of the LIA.

10Be Analyses with a Compact AMS Facility—Are Bef2 Samples the Solution?

The injection of (super 10) BeF (super -) instead of (super 10) BeO (super -) into a compact accelerator mass spectrometry system with a terminal voltage of 0.58 MV was investigated, because BF (super -) molecules are unstable and isobaric interference of (super 10) B with (super 10) Be can thus be significantly reduced. We describe the method we developed to prepare BeF (sub 2) samples. (super 10) Be was measured in a segmented gas ionization detector. Separation of (super 10) Be from (super 10) B could be achieved both for ions in the 1+ charge state with an energy of 0.8 MeV and in the 2+ charge state with an energy of 1.4 MeV. The 2+ ions are better separated, whereas the 1+ charge state has a higher transmission. (super 10) Be/ (super 9) Be ratios ( approximately 10 (super -12) ) in a suite of rock samples were successfully determined for exposure dating in either charge state and compared with measurements made on the 6MV tandem.

Cosmogenic Nuclide Dating of Swiss Deckenschotter

The goal of our project is to date selected sites of the Swiss Deckenschotter using depth-profile and isochron-burial dating with the cosmogenic nuclides 10Be, 26Al and 36Cl in order to reconstruct the timing of Early and Middle Pleistocene glaciations in the Alps. Obtaining ages for these glaciofluvial units will provide fundamental information about the onset of glaciation in the Alps and the northern Hemisphere as a whole. Furthermore it will become possible to quantify the magnitude of incision in the foreland. In the first year of the project, 54 samples were collected from three different sites for depth-profile and isochron-burial dating. First results from the depth-profile for the Lower Deckenschotter at Pratteln yields a model age of around 270 ka. Similarly, the Higher Deckenschotter at Stadlerberg indicates a depth-profile age of around 1.7 Ma. Samples from the Higher Deckenschotter at Irchel are still in progress. Meanwhile, we are currently working on several fronts to improve and optimize the isochron-burial dating methodology, especially in measuring very low cosmogenic nuclide concentrations with low uncertainties. s o u r c e : h t t p s : / / d o i . o r g / 1 0 . 7 8 9 2 / b o r i s . 8 4 6 5 9 | d o w n l o a d e d : 2 7 . 1 . 2 0 2 0