John O. Stone

Air pressure and cosmogenic isotope production

The cosmic ray flux increases at higher altitude as air pressure and the shielding effect of the atmosphere decrease. Altitude-dependent scaling factors are required to compensate for this effect in calculating cosmic ray exposure ages. Scaling factors in current use assume a uniform relationship between altitude and atmospheric pressure over the Earths surface. This masks regional differences in mean annual pressure and spatial variation in cosmogenic isotope production rates. Outside Antarctica, air pressures over land depart from the standard atmosphere by ±4.4 hPa (1σ) near sea level, corresponding to offsets of ±3–4% in isotope production rates. Greater offsets occur in regions of persistent high and low pressure such as Siberia and Iceland, where conventional scaling factors predict production rates in error by ±10%. The largest deviations occur over Antarctica where ground level pressures are 20–40 hPa lower than the standard atmosphere at all altitudes. Isotope production rates in Antarctica are therefore 25–30% higher than values calculated by scaling Northern Hemisphere production rates with conventional scaling factors. Exposure ages of old Antarctic surfaces, especially those based on cosmogenic radionuclides at levels close to saturation, may be millions of years younger than published estimates.

Cosmogenic chlorine-36 from calcium spallation

Calcium is a major target element for cosmogenic 36Cl production. Consequently 36Cl rapidly reaches detectable levels in minerals such as calcite and calcium feldspar exposed at the Earths surface. Spallation of calcium isotopes typically accounts for 80–90% of 36Cl production in these minerals, with subsidiary contributions from negative muon capture by 40Ca and thermal neutron capture by 35Cl. To provide a basis for surface exposure dating, we have calibrated cosmogenic 36Cl production in calcium feldspar from the 17,300 year old Tabernacle Hill basalt. At an altitude of 1445 m and an effective geomagnetic latitude of 40.9 ° the calcium spallation rate is 152 ± 11 atoms (g Ca)−1 a−1. The corresponding rate at sea level and high latitude is estimated at 48.8 ± 3.4 atoms (g Ca)−1 a−1. The muon capture rate used to derive these values is 8.8 ± 2.2 atoms (g Ca)−1 a−1 at the Tabernacle Hill site, scaled from a value of 4.8 ± 1.2 atoms (g Ca)−1 a−1 at sea level and high latitude. The calcium spallation rate determined in this study is in excellent agreement with previous whole-rock calibration measurements at Tabernacle Hill, when these are recalculated with respect to the absolute timescale. The calibration of 36C1 production from calcium underpins development of an exposure dating technique for calcite. Due to its high calcium content, the 36Cl production rate in calcite is higher than in any other common rock-forming mineral. Measurement of 36Cl in calcite, with an accelerator mass spectrometric detection limit of ~5 × 103 atoms per gram, allows dating of limestone surfaces exposed for periods ranging from ~102–106 years. Alternatively, erosion rates from less than 1 to greater than 1000 μm per year can be determined in the case of eroding karst surfaces. Though the 36Cl production rate is lower in calcium feldspar than in calcite, measurements on this mineral will provide a useful means of dating young basalt lavas.

Cosmogenic Chlorine-36 Production in Calcite by Muons

At depths below a few metres, 36Cl production in calcite is initiated almost entirely by cosmic ray muons. The principal reactions are (1) direct negative muon capture by Ca; 40Ca(μ−,α)36Cl, and (2) capture by 35Cl of secondary neutrons produced in muon capture and muon-induced photodisintegration reactions. We have determined rates for 36Cl and neutron production due to muon capture in calcite from a 20 m (5360 g cm−2) depth profile in limestone. The 36Cl yield from muon capture by Ca in pure calcite is 0.012 ± 0.002 atom per stopped negative muon. The surface production rate of 36Cl by muon capture on Ca in calcite is, therefore, 2.1 ± 0.4 atom g−1a−1 at sea level and high latitude, approximately 11% of the production rate by Ca spallation. If it is assumed that 34% of the negative muons are captured by the Ca atom in calcite, the α-yield from 40Ca following muon capture is 0.043 ± 0.008, somewhat lower than the result of a recent muon irradiation experiment (0.062 ± 0.020), but well within the extremes of existing theoretical predictions (0.0033–0.15). The average neutron yield following muon capture in pure calcite is 0.44 ± 0.15 secondary neutrons per stopped negative muon, in good agreement with existing theoretical predictions. Cosmogenic isotope production by muons must be taken into account when dating young geomorphic surfaces, especially those created by excavation of only a few metres of overlying rock. Attention to isotope production by muons is also crucial to determining surface erosion rates accurately. Due to the deep penetration of muons compared to cosmic ray hadrons, the accumulation of muon-produced 36Cl is less sensitive to erosion than that of spallogenic 36Cl. Although production by muons at the surface is only a small fraction of production by spallation, the fraction of muon-produced 36Cl in rapidly eroding limestone surfaces can approach 50%. In such cases, erosion rates estimated using conventional models which attribute production solely to spallation will be in error by up to 40%. The difference in sensitivity to erosion of spallogenic and muon-produced 36Cl suggests methods for dating deeply eroded surfaces, checking the assumption of steady-state when calculating erosion rates, and unravelling multi-stage exposure and erosion histories.

THE LAST ICE SHEET IN NORTH-WEST SCOTLAND: RECONSTRUCTION AND IMPLICATIONS

Recent models of the last Scottish ice sheet suggest that nunataks remained above the ice surface in areas peripheral to the main centres of accumulation. This proposition has been investigated on 140 mountains over an area of 10,000 km2 in NW Scotland. Outside the limits of the later Loch Lomond Readvance in this area there is evidence for a single high-level weathering limit that separates glacially eroded terrain from higher areas of in situ frost debris. This limit occurs at altitudes ranging from 425 to 450 m in the Outer Hebrides to >950 m on the mainland, and is best developed on lithologies that resisted breakdown after ice-sheet downwastage. Interpretation of this weathering limit as a periglacial trimline cut by the last ice sheet at its maximum thickness is supported by: (1) joint-depth and Schmidt hammer measurements that indicate significantly more advanced rock breakdown above the weathering limit; (2) a much greater representation of gibbsite (a pre-Late Devensian weathering product) in the clay fraction of soils above the limit; (3) cosmogenic isotope dating of the exposure ages of rock outcrops above and below the limit; (4) the sharpness of the limit at some sites and its regular decline along former ice flowlines; and (5) shear stress calculations based on the inferred altitude and gradient of the former ice surface. Reconstruction of the ice surface based on trimline evidence indicates that the mainland ice shed lay near or slightly east of the present watershed and descended northwards from >900 m to ca. 550 m at the north coast. Independent dispersion centres fed broad ice streams that occupied major troughs. On Skye an ice dome >800 m deflected the northwestwards movement of mainland ice, but the mountains of Rum were over-ridden by mainland ice up to an altitude of ca. 700 m. The Outer Hebrides supported an independent ice cap that was confluent with mainland ice in the Minches. Extrapolation of the trimline evidence indicates that most reconstructions of ice extent are too conservative, and suggests that low-gradient ice streams extended across the Hebridean Shelf offshore. Wider implications of this research are: (1) that blockfields and other periglacial weathering covers are not all of the same age or significance, depending on the resistance of different lithologies to frost weathering; (2) that the contrasting degree of glacial modification in the Western and Eastern Highlands of Scotland may reflect a former cover of predominantly warm-based ice in the former and predominantly cold-based ice in the latter; and (3) that the approach and techniques developed in this study have potential application for constraining ice-sheet models, not only in areas peripheral to the main centres of ice accumulation in Britain and Ireland, but also in other mountain areas where nunataks protruded through warm-based Late Pleistocene ice masses.

Coupling of rock uplift and river incision in the Namche Barwa-Gyala Peri massif, Tibet

Geodynamic modeling demonstrates the strong potential for erosion to influence the pattern and style of deformation in active mountain belts, but field studies yield conflicting views on the importance of erosion in influencing orogenesis. Here we compare patterns in river power, inferred excess fluvial-transport capacity, topographic relief, precipitation, and mineral-cooling ages to assess the coupling between surface erosion and rock uplift within the vicinity of the Namche Barwa–Gyala Peri massif, an active antiformal structure within the eastern Himalayan syntaxis. Our rich and dense data set reveals a tight spatial correspondence of fluvial incision potential, high relief, and young cooling ages. The spatial coincidence is most easily explained by a sustained balance between rock uplift and denudation driven by river incision over at least the last ∼1 m.y. The Yarlung Tsangpo–Brahmaputra River is the largest and most powerful river in the Himalaya, and two lines of evidence point to its active role in the dynamic interaction of local erosion, rock uplift, thermal weakening of the lithosphere, and deformation: (1) Whereas along the rest of the Himalayan front, high relief and high rock uplift rates are essentially continuous, the high relief and rapid exhumation in the syntaxis is restricted to a “bulls-eye” pattern exactly where the largest river in the Himalaya, the Yarlung Tsangpo–Brahmaputra, has the most energy per unit area available to erode its channel and transport sediment. (2) The location of rapid incision on the Yarlung Tsangpo–Brahmaputra has been pinned for at least 1 m.y., and without compensatory uplift of the Namche Barwa–Gyala Peri massif during this time the river would have eroded headward rapidly, incising deeply into Tibet.

Exposure dating and validation of periglacial weathering limits, northwest Scotland

Periglacial weathering limits on two mountains in northwest Scotland separate zones of contrasting exposure history. Exposure dating of bedrock below the weathering limits gives ages consistent with late Devensian deglaciation, but six out of seven samples from above the weathering limits give minimum exposure ages older than late Devensian ice expansion. These results suggest that mountain summits stood as nunataks above the last ice-sheet surface and rule out formation of weathering limits by periglacial rock breakdown since deglaciation, trimming of frost debris during an ice-sheet readvance, or enhanced weathering prior to climatic warming during ice-sheet downwastage. The dating results do not preclude the possibility that weathering limits mark a former englacial boundary between passive cold-based ice on mountain summits and erosive, warm-based ice at lower elevations, although 26 Al/ 10 Be ratios for high-level bedrock surfaces provide no evidence of prolonged static ice cover.

Denudation rates for the southern Drakensberg escarpment, SE Africa, derived from in-situ-produced cosmogenic 36C1: initial results

Cosmogenic 36C1 concentrations in basalt samples from the Drakensberg escarpment on the SE African passive margin give quantitative estimates of denudation and scarp retreat rates. Over the 104–106 year timespan addressed by these data, the calculated escarpment retreat rate has been 50–95 m Ma-1 and the average summit denudation rate 6 m Ma-1. The scarp retreat rate is an order of magnitude less than previously suggested. The rate of summit lowering is sufficient to prevent the long-term intact survival of erosion cycle surfaces formed in the Mesozoic that were previously inferred for this region.

Rapid soil production and weathering in the Southern Alps, New Zealand

Weathering Heights The production of soil is the result of chemical weathering of rocks and minerals. In regions where tectonic uplift brings fresh material to Earths surface, erosion and weathering can accelerate. Using chemical tracers, Larsen et al. (p. 637, published online 16 January; see the Perspective by Heimsath) measured soil production rates of over 2 millimeters per year in New Zealands Southern Alps, which are some of the fastest uplifting mountains in the world. Because chemical weathering consumes CO2, these rapid rates may over time influence global climate. Fast weathering rates in the New Zealand Alps point to a strong influence of tectonic processes on global climate. [Also see Perspective by Heimsath] Evaluating conflicting theories about the influence of mountains on carbon dioxide cycling and climate requires understanding weathering fluxes from tectonically uplifting landscapes. The lack of soil production and weathering rate measurements in Earth’s most rapidly uplifting mountains has made it difficult to determine whether weathering rates increase or decline in response to rapid erosion. Beryllium-10 concentrations in soils from the western Southern Alps, New Zealand, demonstrate that soil is produced from bedrock more rapidly than previously recognized, at rates up to 2.5 millimeters per year. Weathering intensity data further indicate that soil chemical denudation rates increase proportionally with erosion rates. These high weathering rates support the view that mountains play a key role in global-scale chemical weathering and thus have potentially important implications for the global carbon cycle.

The Beinn Alligin rock avalanche, NW Scotland: cosmogenic 10Be dating, interpretation and significance

A tongue of very coarse rockslide debris that extends 1.25km downvalley below Beinn Alligin in NW Scotland has been variously interpreted as a glacier-cored rock glacier, landslide debris redistributed by glacier ice or an excess-runout landslide. Exposure dating with cosmogenic 10Be demonstrates that the the debris mass was emplaced at 3950±320 yr BP, and therefore was not associated with glacier ice. Calculations based on frictional considerations imply that the feature is an excess-runout rock avalanche (sturzstrom) deposit. The morphological characteristics of the deposit appear consistent with movement by grainflow or fragmental flow. Failure is inferred to reflect time-dependent paraglacial stress release and consequent propagation of an internal joint network, but may have been triggered by seismic activity. The late-Holocene age of failure implies persistence of the eŒects of paraglacial stress release over a time-scale of several millennia.

Cosmogenic Cl-36 dating of postglacial landsliding at The Storr, Isle of Skye, Scotland

Major postglacial rock slope failures are a common feature of the Scottish Highlands and other mountainous areas that were deglaciated at the end of the Pleistocene, but evaluation of the causes and triggers of failure has been hindered by a lack of reliable dating evidence. We report the result of a pilot study designed to establish the absolute age of a large postglacial rotational rockslide at The Storr on the Isle of Skye, Scotland, using 36Cl surface exposure dating. Exposure ages of 6.3 ± 0.7 cal. ka BP and 6.6 ± 0.8 cal. ka BP were obtained for rock samples from two separate landslide blocks, giving an overall age estimate of 6.5 ± 0.5 cal. ka BP for rock slope failure at this site. This date is consistent with AMS radiocarbon dating of windblown sand derived from the failure scarp, and with previous inferences (based on relative dating evidence) concerning an early-Holocene age for most rock slope failures in the Scottish Highlands. The long time lag (. 7 ka) between deglaciation and failure suggests that progressive joint extension and shearing of rock bridges and asperities were of critical importance in conditioning failure, though a seismic trigger cannot be ruled out. The methodology of surface exposure dating in this context is described and its future potential assessed.