Exposure Ages Range Research Articles

Pleistocene glacial advances and exposure age scatter in the Olympus Range, Antarctica: A study of cosmogenic 36Cl/3He in dolerites and 10Be in sandstones

In three cirques in the western Olympus Range of the McMurdo Dry Valleys, Antarctica, previous advances of cirque glaciers are recorded by a sequence of three drifts in each of the cirques. We dated drift limits and the deposits on modern glaciers in two of these cirques, Dean and Dipboye, via cosmogenic 3He in pyroxene from 41 dolerite boulders, 36Cl in pyroxene from 12 of those dolerites, and 10Be in quartz from 11 sandstone boulders. Exposure age scatter is high on all deposits. The 3He exposure ages across all deposits range from ∼35 to ∼2300 ka and 10Be exposure ages range from ∼7 to ∼435 ka. Coupled 36Cl/3He from dolerites support constant exposure with erosion for nine of the 12 samples, while the other three might have experienced complex exposure-burial histories. Due to the mesa-butte topography and slow bedrock erosion rates, nuclide inheritance is the primary cause of age scatter in dolerites, accounting for >1 Myr of exposure age error. Mean exposure ages from sandstones are 2–7 times younger than those from dolerites for the same deposits, indicating that inheritance is less common in sandstones in this region. Weathering analyses of sandstone boulders show an increase in average siliceous crust thickness and rock strength with deposit age, an example of case hardening. Based on both relative and exposure age dating, drift age increases with distance from the modern glaciers in both Dean and Dipboye cirques, with three advances during the past <700 ka. However, due to high exposure age scatter, it cannot be determined if the three drifts are temporally correlated across the two cirques and therefore the drifts might record different glacial advances in Dean Cirque vs. Dipboye Cirque despite the apparent stratigraphic correlation of the drifts. This study has implications for drift depositional processes of cold-based glaciers and the importance of source-bedrock lithology and geomorphology on nuclide inheritance in Antarctica.

10Be exposure age data reveals last Glacial ice sheet histories in the Larsemann Hills of East Antarctica

The Larsemann Hills, near Prydz Bay is a key region to understand past glacial dynamics in East Antarctica. However, multi-proxy dating constraints for this area indicate an uncertain chronology for deglaciation history during the Last Glacial Period (LGP). In this paper, we report two 26Al exposure ages and eighteen 10Be exposure ages for bedrock samples to better understand past glacial histories. The minimum exposure ages range from 82.84 ± 4.67 ka to 15.1 ± 1.95 ka corresponding to Marine Isotope Stages (MIS) 5–2, and fourteen of these samples date from before the global Last Glacial Maximum (c. 19–26.5 ka). Analysis of 10Be and 26Al concentrations implies a complex deglaciation history, and surface lowering of bedrock may have caused systematic age underestimation. Further comparisons between two adjacent regions in the Mirror Peninsula reveal two major deglaciations involving the Lambert Glacier / Amery Ice Shelf System dated at 61.78 ka to 82.84 ka and 21.98 ka to 49.5 ka during the LGP.

Light noble gases in 11 achondrites: Cosmic ray exposure ages, gas retention ages, and preatmospheric sizes

AbstractWe report light noble gas (He, Ne, and Ar) concentrations and isotopic ratios in 11 achondrites, Tafassasset (unclassified primitive achondrite), Northwest Africa (NWA) 12934 (angrite), NWA 12573 (brachinite), Jiddat al Harasis (JaH) 809 (ureilite), NWA 11562 (ungrouped achondrite), four lodranites (NWA 11901, NWA 7474, NWA 6685, and NWA 6484), NWA 2871 (acapulcoite), and Sahara 02029 (winonaite), most of which have not been previously studied for noble gases. We discuss their noble gas isotopic composition, determine their cosmogenic nuclide content, and systematically calculate their cosmic ray exposure (CRE) and gas retention ages. In addition, we estimate their preatmospheric radii and preatmospheric masses based on the shielding parameter (22Ne/21Ne)cos. None of the studied meteorites shows evidence of contribution from solar cosmic rays (SCRs). JaH 809 and NWA 12934 show evidence of 3He diffusive losses of &gt;90% and 40%, respectively. The winonaite Sahara 02029 has lost most of its noble gases, either during or before analysis. The average CRE age of Tafassasset of ~49 Ma is lower than that reported by Patzer et al. (2003), but is consistent with it within the uncertainties; this confirms that Tafassasset and CR chondrites are not source paired, CR chondrites having CRE ages from 1 to 25 Ma (Herzog &amp; Caffee, 2014). The ureilite JaH 809 has a CRE age of ~5.4 Ma, which falls into the typical range of exposure ages for ureilites; the angrite NWA 12934 has a CRE age of ~49 Ma, which is within the main range of exposure ages reported for angrites (0.2–56 Ma). We calculate a CRE age of ~2.4 Ma for the brachinite NWA 12573, which falls into a possible “cluster” in the brachinite CRE age histogram around ~3 Ma. Three lodranites (NWA 11901, NWA 7474, and NWA 6685) have CRE ages higher than the average CRE ages of lodranites measured so far, NWA 11901 and NWA 6685 having CRE ages far higher than the CRE age already reported by Li et al. (2019) on NWA 8118. The measured 40K‐40Ar gas retention ages fit well into established systematics. The gas retention age of Tafassasset is consistent, within respective uncertainties, with that previously calculated by Patzer et al. (2003). Our study indicates that Tafassasset originates from a meteoroid with a preatmospheric radius of ~20 cm, however discordant with the radius of ~85 cm inferred in a previous study (Patzer et al., 2003).

Chronology of Glacial Advances and Deglaciation in the Encierro River Valley (29° Lat. S), Southern Atacama Desert, Based on Geomorphological Mapping and Cosmogenic 10Be Exposure Ages

The high mountain segments of the valleys of the southernmost Atacama Desert of Chile present Late Quaternary glacial landforms that developed in already incised valleys. Glacier advances and deglaciation have left a geomorphic imprint in the southernmost Atacama Desert. In this work, the glacial landforms of the Encierro River Valley (29.1°S–69.9°W) have been revisited and new detailed geomorphological mapping is provided. This work also includes new 10Be exposure ages from moraine boulders and one age from an ice-molded bedrock surface. The former glacier of the El Encierro valley extended 16 km down the valley during the last local glacial maximum recorded by a terminal moraine (ENC 1a) with an exposure age of ∼40 ka. Four inboard moraine arcs were deposited upstream in telescopic patterns (ENC 1b–d), whose exposure ages range between ∼25 and ∼33 ka (ENC 1d). Exposure ages between ∼17–24 ka on lateral moraines (ENC 1L) developed during the later ice recession of the ENC 1 drift. Thus, the ice mostly disappeared in the main valley before ∼18 ka, as is also supported by the exposure age obtained from an ice-molded bedrock surface. Seven kilometers up the valley from the ENC 1, the ENC 2a–d moraine arcs correspond to a small ice advance by ∼17–20 ka. The last glacial advance (ENC 2), which occurred after deglaciation of the last local glacial maximum (ENC 1), coincides with the start of the Heinrich Stadial Event 1 (HS1; 18–14.5 ka), which is thought to play a direct role in the last glacial termination in the Andes.

Light noble gas records and cosmic ray exposure histories of recent ordinary chondrite falls

AbstractWe measured noble gas concentrations and isotopic ratios (He, Ne, and Ar isotopes) in six recent ordinary chondrite falls: Mangui (L6), Viñales (L6), Ozerki (L6), Tamdakht (H5), Kheneg Ljouâd (LL5/6), and Katol (L6). Among them, the three L6 chondrites Mangui, Viñales, and Ozerki fell in only a few months' interval; their apparent similar petrographic and mineralogic characteristics might indicate source crater pairing. To test this hypothesis, we have investigated those meteorites for their cosmic ray exposure (CRE) histories, using the cosmogenic noble gases 3He, 21Ne, and 38Ar. We systematically (re)calculated the CRE ages as well as the gas retention ages of these meteorites. The CRE age of the Mangui is, based on noble gases, &lt;1 Ma, which is unusually short for an L chondrite. Indeed, the range of exposure ages for L chondrites is generally distributed between ˜1 and ˜60 Ma, with major peaks occurring around ˜5, ˜30, and ˜40 Ma. In addition, the cosmogenic 3Hecos data of two Mangui duplicates are consistent with a remarkably high loss of helium by diffusion due to heating by solar radiation. Such a short parent body‐Earth transfer time (&lt;1 Ma) can be explained by a delivery from an Earth‐crossing object. Regarding the other L6 chondrites, Viñales has a nominal CRE age of ˜9.4 Ma, whereas the Ozerki meteorite has a nominal CRE age of ˜1.2 Ma, which is consistent with Korochantseva et al. (2019). Based on their CRE ages as well as on their gas retention ages, it appears that none of these three recent L6 chondrite falls are source crater paired, and therefore, all three originate from different meteoroids. The nominal exposure ages of Tamdakht, Kheneg Ljouâd, and Katol are ˜3.2, ˜11, and ˜30 Ma, respectively, and are consistent with identified age peaks on the exposure age histogram of H, LL, and L chondrites, respectively. The nominal CRE age of Tamdakht is consistent with previous observations for H chondrites and implies that they are dominated by small impact events occurring in several parent bodies.

Timing of exotic, far-traveled boulder emplacement and paleo-outburst flooding in the central Himalayas

Abstract. Large boulders, ca. 10 m in diameter or more, commonly linger in Himalayan river channels. In many cases, their lithology is consistent with source areas located more than 10 km upstream, suggesting long transport distances. The mechanisms and timing of “exotic” boulder emplacement are poorly constrained, but their presence hints at processes that are relevant for landscape evolution and geohazard assessments in mountainous regions. We surveyed river reaches of the Trishuli and Sunkoshi, two trans-Himalayan rivers in central Nepal, to improve our understanding of the processes responsible for exotic boulder transport and the timing of emplacement. Boulder size and channel hydraulic geometry were used to constrain paleo-flood discharge assuming turbulent, Newtonian fluid flow conditions, and boulder exposure ages were determined using cosmogenic nuclide exposure dating. Modeled discharges required for boulder transport of ca. 103 to 105 m3 s−1 exceed typical monsoonal floods in these river reaches. Exposure ages range between ca. 1.5 and 13.5 ka with a clustering of ages around 4.5 and 5.5 ka in both studied valleys. This later period is coeval with a broader weakening of the Indian summer monsoon and glacial retreat after the Early Holocene Climatic Optimum (EHCO), suggesting glacial lake outburst floods (GLOFs) as a possible cause for boulder transport. We, therefore, propose that exceptional outburst events in the central Himalayan range could be modulated by climate and occur in the wake of transitions to drier climates leading to glacier retreat rather than during wetter periods. Furthermore, the old ages and prolonged preservation of these large boulders in or near the active channels shows that these infrequent events have long-lasting consequences on valley bottoms and channel morphology. Overall, this study sheds light on the possible coupling between large and infrequent events and bedrock incision patterns in Himalayan rivers with broader implications for landscape evolution.

The age of Wolfe Creek meteorite crater (Kandimalal), Western Australia

AbstractWolfe Creek crater lies in northwestern Australia at the edge of the Great Sandy Desert. Together with Meteor Crater, it is one of the two largest craters on Earth from which meteorite fragments have been recovered. The age of the impact is poorly constrained and unpublished data places the event at about 300,000 years ago. In comparison, Meteor Crater is well constrained by exposure dating. In this paper, we present new ages for Wolfe Creek Crater from exposure dating using the cosmogenic nuclides 10Be and 26Al, together with optically stimulated luminescence ages (OSL) on sand from a site created by the impact. We also present a new topographic survey of the crater using photogrammetry. The exposure ages range from ~86 to 128 ka. The OSL ages indicate that the age of the impact is most likely to be ~120 ka with a maximum age of 137 ka. Considering the geomorphic setting, the most likely age of the crater is 120 ± 9 ka. Last, we review the age of Meteor Crater in Arizona. Changes in production rates and scaling factors since the original dating work revise the impact age to 61.1 ± 4.8 ka, or ~20% older than previously reported.

Weathering in the Glacial Foreland of Southern and Western Greenland

Glaciers physically grind up underlying rock and create fine-grained sediment that has a high potential for chemical weathering. Subsequent weathering of these sediments produces solutes, including nutrients and radiogenic isotopes, which are transported by streams to the world’s oceans where they can impact primary productivity and record past ice sheet activity. Previous research on the geochemistry of bedrock, bedload sediment and stream waters demonstrated that the extent of chemical weathering varies across a transect in western Greenland due to variations in either exposure age or precipitation (Scribner et al., 2015), and that variations in the extent of weathering associated with retreat of the Greenland Ice Sheet (GrIS) may account for observed trends in solutes during the last deglacial period. This project adds a new study area in southern Greenland that has higher precipitation, more vegetation, a different lithology, and a range of exposure ages, to address the leading factors contributing to the extent of weathering. Based on the relative proportions of cations and percent change between Na+K concentrations of bedload to waters, the southern region is undergoing less extensive weathering than the western transect, which suggests that lithology may be an important driver of weathering, and that exposure age and precipitation are less important factors than expected. The results also suggest there are additional factors that contribute to weathering that were not accounted for in this study, such as dissolved organic matter and microbial activity.

How significant is inheritance when dating rockslide boulders with terrestrial cosmogenic nuclide dating?—a case study of an historic event

Terrestrial cosmogenic nuclide (TCN) exposure dating of boulders is frequently used for rockslide chronology. A well-recognized source of error that cannot be readily quantified is related to inheritance of TCN produced in the rock prior to failure. The effect of inheritance is not constant and will be greatest in the instance of a very recent shallow failure on a high-altitude surface with low event frequencies. We illustrate the effect by measuring 10Be concentrations in six boulders exposed for only 9 years before sampling, on a rock avalanche in Puerto Aysen, Chile. Their apparent exposure ages range from 216 ± 76 to 1755 ± 436 years. The mean apparent exposure age of a statistical cluster of three samples exceeds the real exposure time by 345 ± 36 years (3800%), implying that all sampled rock surfaces experienced pre-failure TCN production. A reconstructed pre-failure topography enables the analysis of possible pre-failure boulder positions and an estimate of the range of possible inherited concentrations along a 2D transect. Despite a maximum failure-mass thickness of 110 m, the boulders seem to have originated from depths shallower than 14 m. Because of the likelihood that large boulders, prioritized for TCN sampling, originate from relatively shallow pre-failure depths owing to surface-near transport with minor turbation, it is necessary to consider potentially inherited TCN concentrations and their effect on the age determination, especially in cases of young rockslides, where the commonly adjusted effects of boulder erosion and snow, ash, or vegetation shielding are negligible in comparison.

Lavini di Marco (Trentino, Italy): 36Cl exposure dating of a polyphase rock avalanche

The Lavini di Marco rock avalanche deposit (“Marocca di Marco”) is located along the left side of the middle Adige Valley, south of the town of Rovereto (NE Italy). The deposit is estimated to have a volume of ∼2 × 108 m3 and cover an area of ∼6.8 km2. It comprises Jurassic Calcari Grigi limestones that detached from the western slope of Mt. Zugna Torta. The Lavini di Marco is composed of at least two different rock avalanche bodies, the main deposit known as Lavini di Marco (the principal) and the much smaller Costa Stenda deposit. Costa Stenda deposits overlie Lavini di Marco deposits. Samples for 36Cl exposure dating were collected from boulders within the deposits, from sliding plane bedrock and from the bedrock wall at the head scarp. Exposure ages range from 800 ± 210 to 21310 ± 1000 years. The latter age stands as a notable outlier suggesting that that Costa Stenda boulder was exposed for a considerable amount of time in the pre-slide bedrock. Lavini di Marco and Costa Stenda boulder ages are 2600 ± 200, 2700 ± 200, 3100 ± 300, 3300 ± 300, 3400 ± 300, 4400 ± 290, 5300 ± 300, and 5400 ± 300 years. The latter three are Costa Stenda boulders which we also interpret to contain inherited nuclide concentrations. The five remaining boulder ages cluster around 3000 years. We calculate a mean age for the Lavini di Marco and Costa Stenda rockslides of 3000 ± 400 years. Within the uncertainties of our data the two slides were simultaneous. For the bedrock sliding plane we obtained significantly younger ages, 1600 ± 100 and 1400 ± 100 years, and for the head scarp 800 ± 200 years. The sliding plane ages record small-scale reactivation which seems to overlap in time with a catastrophic flood event of the Adige River in Verona, as reported in the Fulda Annales, in 883 AD. Only the single age of 800 ± 210 years suggests activity at Lavini di Marco coincident with the well-known Verona earthquake (1117 AD).

Cosmic ray exposure ages of Rumuruti chondrites from North Africa

We analyzed noble gases and determined 3He, 21Ne, and 38Ar cosmic ray exposure ages (CREAs) of Rumuruti chondrites from North West Africa (NWA) to rule on potential pairings and/or source pairings of North Africa R chondrite samples. The 21Ne exposure ages range between 10 and 74 Ma, with NWA 2897 and 1668 having the highest known exposure ages among R chondrites. We also include other R chondrites from North Africa ( Schultz et al., 2005) and, based on their noble gas characteristics and their 21Ne CREAs, propose pairings of the following samples: NWA 2198, 5069, 755, 4615, 845, 851, 978, 1471, and possibly DaG 013 belonging to one fall with a CREA of ∼10 Ma, and NWA 753, 4360, 4419, 5606, 1472, 1476, 1477, 1478, and 1566 representing one fall with a CREA of ∼14 Ma. NWA 2821, 2503, 2289, 3364, 3146, 4619, 4392, 3098, and 2446 seem to belong to one single fall with a CREA of ∼20 Ma, and NWA 2897 and 1668 seem to be paired and show a common CREA of ∼66 Ma. Overall, all R chondrite samples from North Africa analyzed for noble gases so far represent ∼16 individual falls. Comparing falls from North Africa to literature CREAs of R chondrites worldwide, it seems possible that a significant number of all R chondrite falls studied for noble gases were ejected from the R chondrite parent body during one large collisional event between 15 and 25 Ma ago. However, the database is still too small to draw definitive conclusions. The large portion of brecciated R chondrites in collections suggests severe impact brecciation of the R chondrite parent body.

Cosmogenic radionuclide chronology of pre-last glacial cycle moraines in the Western Arthur range, Southwest Tasmania

Cosmogenic 10Be and 26Al exposure ages of 12 boulders from two moraine complexes in the Western Arthur Range of southwest Tasmania, which previously were considered to have been deposited during the Last Glacial Maximum, predate the Last Glacial Cycle. Zero-erosion minimum exposure ages range from 95 ka to 232 ka based on weighted mean 10Be and 26Al ages per boulder. For a reasonable choice of erosion rates, 10Be boulder ages range from 105 to 326 ka, respectively. Although a direct association of moraine construction to a specific marine isotope stage (MIS) glaciation is not definitive, erosion-corrected exposure ages indicate glacial advances commensurate with MIS-6 and -10, although the latter advance may be attributable to MIS-8 if the erosion rate correction is an over-estimate. Given the relatively close proximity of the dated moraines to their source cirques, this new cosmogenic glacial chronology implies a very limited extent for any younger glacial advances that occurred in the Western Arthur Range during the Last Glacial Cycle (post MIS-5) including the global LGM.

Late Quaternary ice sheet extents in northeastern Germany inferred from surface exposure dating

We determined in situ cosmogenic 10Be ages for 11 boulders on the Hoher Fläming ice marginal belt and five boulders on the Gerswalder moraine, a recessional moraine of the Pomeranian stage. Previous time estimations for the deposition of these moraines along the southern margin of the Scandinavian Ice Sheet (SIS) in northeastern Germany are mostly based on geomorphology and stratigraphy, and on few radiocarbon dates and recently published surface exposure ages in the case of the Pomeranian moraine. Our new exposure ages range from 21.7±1.2 10Beka to 172.6±6.0 10Beka for the Hoher Fläming ice marginal belt. The wide range of exposure ages may reflect episodic ice marginal belt modification due to intensified erosion by meltwater channeled in the Baruther Urstromtal, (ice marginal valley) accompanied by long-lasting erosional processes like solifluction and aeolian deflation.For the Gerswalder moraine, our new exposure ages range from 12.3±0.6 10Beka to 16.6±1.0 10Beka. Excluding the youngest sample (BER-97-03), we calculated error-weighted mean ages ranging from 15.2±0.5 10Beka to 16.0±0.5 10Beka (n=4), depending on choice of scaling methods, surface erosion and snow cover. These results firmly establish the position of the southern margin of the SIS in northeastern Germany during the Gerswalder substage a recessional phase of the Pomeranian stage.

Cosmic-Ray Exposure Ages of Large Presolar SiC Grains

AbstractAmong presolar SiC grains found in the Murchison carbonaceous meteorites (average size less than 0.5 μm) are very large grains, ranging in size up to 50 μm. We interpret 6Li excesses measured in eight of these grains as being the result of spallation reactions by Galactic cosmic rays during the time the grains spent in the interstellar medium before their incorporation into the meteorite. Derived interstellar exposure ages range from 40 My to 1 Gy, the highest values being consistent with theoretical expectations of interstellar grain lifetimes. Although six grains have almost identical C and Si isotopic compositions, their exposure ages are very different. This observation, combined with low trace element contents, and unusual grain sizes, raises fundamental questions about their stellar sources.

A laser probe 40Ar/ 39Ar and INAA investigation of four Apollo granulitic breccias

Infrared laser probe 40Ar/ 39Ar geochronology, instrumental neutron activation analysis (INAA) and analytical electron microscopy have been performed on four 0.5 × 1.0 × 0.3 cm polished rock tiles of Apollo 16 and 17 granulitic breccias (60035, 77017, 78155, and 79215). Pyroxene thermometry indicates that these samples were re-equilibrated and underwent peak metamorphic sub-solidus recrystallization at 1000–1100 °C, which resulted in homogeneous mineral compositions and granoblastic textures. 40Ar/ 39Ar data from this study reveal that three samples (60035, 77017, and 78155) have peak metamorphic ages of ∼4.1 Ga. Sample 79215 has a peak metamorphic age of 3.9 Ga, which may be related to Serenitatis basin formation. All four samples contain moderately high concentrations of meteoritic siderophiles. Enhanced siderophile contents in three of the samples provide evidence for projectile contamination of their target lithologies occurring prior to peak metamorphism. Post-peak metamorphism, low-temperature (<300 °C) events caused the partial resetting of argon in the two finer-grained granulites (60035 and 77017). These later events did not alter the mineralogy or texture of the rocks, but caused minor brecciation and the partial release of argon from plagioclase. Interpretation of the low-temperature data indicates partial resetting of the argon systematics to as young as 3.2 Ga for 60035 and 2.3 Ga for 77017. Cosmic ray exposure ages range from 6.4 to ∼339 Ma. Our results increase the amount of high-precision data available for the granulitic breccias and lunar highlands crustal samples. The results demonstrate the survival of pre-Nectarian material on the lunar surface and document the effects of contact metamorphic and impact processes during the pre-Nectarian Epoch, as well as the low-temperature partial resetting of ages by smaller impact events after 3.9 Ga. The mineralogy and chemical composition of these rocks, as well as exhumation constraints, indicate that the source of heat for metamorphism was within kilometers of the surface via burial beneath impact-melt sheets or hot ejecta blankets.

Late Glacial ice advances in southeast Tibet

The sensitivity of Tibetan glacial systems to North Atlantic climate forcing is a major issue in palaeoclimatology. In this study, we present surface exposure ages of erratic boulders from a valley system in the Hengduan Mountains, southeastern Tibet, showing evidence of an ice advance during Heinrich event 1. Cosmogenic nuclide analyses ( 10Be and 21Ne) revealed consistent exposure ages, indicating no major periods of burial or pre-exposure. Erosion-corrected (3 mm/ka) 10Be exposure ages range from 13.4 to 16.3 ka. This is in agreement with recalculated exposure ages from the same valley system by [Tschudi, S., Schäfer, J.M., Zhao, Z., Wu, X., Ivy-Ochs, S., Kubik, P.W., Schlüchter, C., 2003. Glacial advances in Tibet during the Younger Dryas? Evidence from cosmogenic 10Be, 26Al, and 21Ne. Journal of Asian Earth Sciences 22, 301–306.]. Thus this indicates that local glaciers advanced in the investigated area as a response to Heinrich event 1 cooling and that periglacial surface adjustments during the Younger Dryas overprinted the glacial morphology, leading to deceptively young exposure ages of certain erratic boulders.

Surface exposure dating of the Flims landslide, Graubünden, Switzerland

Sixteen boulder and bedrock surfaces related to the Flims landslide (volume 8–12 km 3) were dated with 36Cl and 10Be. Exposure ages range from 4900 ± 250 yr to 15,440 ± 1480 yr, including corrections made due to snow cover and karst erosion. Ages of 11,410 ± 590 yr and 13,340 ± 1090 yr were obtained on bedrock surfaces outside of the landslide zone. These are minima for deglaciation of Segnes valley and provide constraints on possible amounts of inherited nuclides. Based on seven boulder ages, which range from 8200 ± 260 to 9520 ± 990 yr, a mean of 8900 ± 700 years is calculated for the Flims landslide. We exclude three outliers (one significantly older and two significantly younger than the others) and the ages from the Cassons bedrock site from the mean calculation. The latter is excluded as shielding due to snow and vegetation cover is difficult to constrain there. The oldest boulder on the landslide yielded an age (15440 ± 1480 yr) more than 5000 years older than any other boulder age, suggesting that it included the surface of the pre-slide bedrock. The exposure ages are consistent with a single failure event. The erratics and patches of till lying on the landslide debris must have been carried piggy-back on top of the landslide rather than having been deposited by the late Pleistocene Vorderrhein Glacier. The Flims rockslide is about the same age as the Köfels (9800 yr) and Kandertal (9600 yr) rockslides. All three occurred during the marked transition to warmer, wetter conditions during the early Holocene.

Biogeochemical stoichiometry of Antarctic Dry Valley ecosystems

Among aquatic and terrestrial landscapes of the McMurdo Dry Valleys, Antarctica, ecosystem stoichiometry ranges from values near the Redfield ratios for C:N:P to nutrient concentrations in proportions far above or below ratios necessary to support balanced microbial growth. This polar desert provides an opportunity to evaluate stoichiometric approaches to understand nutrient cycling in an ecosystem where biological diversity and activity are low, and controls over the movement and mass balances of nutrients operate over 10–106 years. The simple organisms (microbial and metazoan) comprising dry valley foodwebs adhere to strict biochemical requirements in the composition of their biomass, and when activated by availability of liquid water, they influence the chemical composition of their environment according to these ratios. Nitrogen and phosphorus varied significantly in terrestrial and aquatic ecosystems occurring on landscape surfaces across a wide range of exposure ages, indicating strong influences of landscape development and geochemistry on nutrient availability. Biota control the elemental ratio of stream waters, while geochemical stoichiometry (e.g., weathering, atmospheric deposition) evidently limits the distribution of soil invertebrates. We present a conceptual model describing transformations across dry valley landscapes facilitated by exchanges of liquid water and biotic processing of dissolved nutrients. We conclude that contemporary ecosystem stoichiometry of Antarctic Dry Valley soils, glaciers, streams, and lakes results from a combination of extant biological processes superimposed on a legacy of landscape processes and previous climates.

Erosion in northwest Tibet from in‐situ‐produced cosmogenic 10Be and 26Al in bedrock

AbstractConcentrations of in‐situ‐produced cosmogenic nuclides 10Be and 26Al in quartz were measured by accelerator mass spectrometry for bedrock basalts and sandstones located in northwest Tibet. The effective exposure ages range between 23 and 134 ka (10Be) and erosion rates between 4·0 and 24 mm ka−1. The erosion rates are significantly higher than those in similarly arid Antarctica and Australia, ranging between 0·1 and 1 mm ka−1, suggesting that precipitation is not the major control of erosion of landforms. Comparison of erosion rates in arid regions with contrasting tectonic activities suggests that tectonic activity plays a more important role in controlling long‐term erosion rates. The obtained erosion rates are, however, significantly lower than the denudation rate of 3000–6000 mm ka−1 beginning at c. 5‐3 Ma in the nearby Godwin Austen (K2) determined by apatite fission‐track thermochronology. It appears that the difference in erosion rates within different time intervals is indicative of increased tectonic activity at c. 5–3 Ma in northwest Tibet. We explain the low erosion rates determined in this study as reflecting reduced tectonic activity in the last million years. A model of localized thinning of the mantle beneath northwest Tibet may account for the sudden increased tectonic activity at c. 5–3 Ma and the later decrease. Copyright © 2006 John Wiley &amp; Sons, Ltd.

Cosmogenic 3He exposure ages of Pleistocene debris flows and desert pavements in Capitol Reef National Park, Utah

The Quaternary history of the Capitol Reef area, Utah, is closely linked to the basaltic-andesite boulder deposits that cover much of the landscape. Understanding the age and mode of emplacement of these deposits is crucial to deciphering the Quaternary evolution of this part of the Colorado Plateau. Using cosmogenic 3He exposure age dating, we obtained apparent exposure ages for several key deposits in the Capitol Reef area. Coarse boulder diamicts capping the Johnson Mesa and Carcass Creek Terraces are not associated with the Bull Lake glaciation as previously thought, but were deposited 180±15 to 205±17 ka (minimum age) and are the result of debris flow deposition. Desert pavements on the Johnson Mesa surface give exposure ranging from 97±8 to 159±14 ka and are 34–96 kyears younger than the boulder exposure ages. The offset between the boulder and pavement exposure ages appears to be related to a delay in pavement formation until the penultimate glacial/interglacial transition or periodic burial and exposure of pavement clasts since debris flow deposition. Incision rates for the Capitol Reef reach of the Fremont River calculated from the boulder exposure ages range from 0.40 to 0.43 m kyear −1 (maximum rates) and are some of the highest on the Colorado Plateau.