Late Pleistocene and Holocene glaciation of the Fish Lake valley,northeastern Alaska Range,Alaska

We reconstructed a chronology of glaciation spanning from the Late Pleistocene through the late Holocene for Fish Lake valley in the north‐eastern Alaska Range using ¹⁰Be surface exposure dating and lichenometry. After it attained its maximum late Wisconsin extent, the Fish Lake valley glacier began to retreat ca. 16.5 ka, and then experienced a readvance or standstill at 11.6 ± 0.3 ka. Evidence of the earliest Holocene glacial activity in the valley is a moraine immediately in front of Little Ice Age (LIA) moraines and is dated to 3.3–3.0 ka. A subsequent advance culminated at ca. AD 610–900 and several LIA moraine crests date to AD 1290, 1640, 1860 and 1910. Our results indicate that ¹⁰Be dating from high‐elevation sites can be used to help constrain late Holocene glacial histories in Alaska, even when other dating techniques are unavailable. Close agreement between ¹⁰Be and lichenometric ages reveal that ¹⁰Be ages on late Holocene moraines may be as accurate as other dating methods. Copyright © 2009 John Wiley & Sons, Ltd.     

The Donegal ice dome,northwest Ireland: dimensions and chronology

Geomorphological evidence indicates that Donegal was formerly occupied by an ice dome that extended offshore to the west, northwest and north and was confluent with adjacent ice masses to the east and south. Erosive warm‐based ice over‐rode almost all the highest mountains, implying an ice‐divide altitude greater than 700 m. Only six peripheral summits escaped glacial modification, implying either that they remained above the ice surface as nunataks or supported a thin cover of protective cold‐based ice. Gibbsite, a pre‐last glacial weathering product, is preferentially represented on summits that escaped glacial modification. Cosmogenic ¹⁰Be exposure ages of 18.6 ± 1.4 to 15.9 ± 1.0 k yr for coastal sites confirm that Donegal ice extended offshore at the last glacial maximum. Reconstruction of the form of the Donegal ice dome suggests a former minimum ice thickness of ～500 m close to the present coastline in the west and northwest, and ～400 m near the coast of the Inishowen Peninsula in the north, with the ice extending at least 20 km across the adjacent shelf to the west and northwest. Copyright © 2007 John Wiley & Sons, Ltd.     

原地生成宇宙成因核素测年技术思考(二):持续抬升区单样品直接计算抬升速率方法

原地生成宇宙成因核素测年技术在计算暴露年龄和侵蚀速率时,一直默认样品所在位置不随时间发生高程变化,从而核素生成速率不因高程变化而改变。在构造稳定区,这样的假设是合理的。在构造活动区,往往会因构造运动,地表样品高程发生改变。大冰盖地区（如南极）的冰川消融或加厚也会引起地壳均衡反弹,从而导致样品高程变化,而核素的生成速率将随高程变化而改变。本文对连续抬升情况下,地表样品中宇宙成因核素浓度与生成速率加速率间的计算关系进行了三种方法的数学推导,并提出如何利用单块样品中两种宇宙成因核素（以10Be和26Al为例）浓度计算地表抬升速率,最后指出已往利用宇宙成因核素方法对构造活动区的侵蚀速率的计算存在高估。本文首次提出利用地表在接近稳态侵蚀状态下的单块样品的两种宇宙成因核素直接计算地表持续抬升速率的方法,从而将原地生成宇宙成因核素方法和构造运动学研究直接联系起来。     

A Karst Connection model for Grand Canyon, Arizona, USA

Along the Asturian coast of northern Spain an uplifted wave-cut platform extends for  100 km east–west. The steep cliff which bounds the gently seaward-dipping platform to the north increases in height from 30 m in the west to 100 m in the east and reflects the overall eastward increase in platform elevation. The southern edge of the 2–4 km-wide platform runs along the foothills of the Cantabrian Mountains, as constrained by a high-resolution digital elevation model. The marine platform, which was carved into deformed Paleozoic bedrock with abundant quartzite beds, is largely covered by weathered marine and continental sediments. Quartzite samples from flat bedrock outcrops which are currently not covered by sediment or soil yield cosmogenic nuclide concentrations (²¹Ne, ¹⁰Be and ²⁶Al) that demonstrate a long and complex exposure history, including periods of burial with partial or complete shielding from cosmic rays. The combination of multiple cosmogenic nuclides yields a minimum age of 1–2 Ma for the platform. Taking into account (i) the horizontal and vertical extent of the platform, (ii) the high resistance to erosion of the quartzitic bedrock, and (iii) published data on the magnitude of past sea level fluctuations, we suggest that the wave-cut platform formed in the Pliocene. Subvertical faults cutting the platform at high angles to the coastline offset the southern edge of the platform by 20 to 40 m and reactivate the pre-existing anisotropy in the Paleozoic bedrock. Uplift and crustal deformation of the coastal region have occurred after platform formation in the Pliocene and may still be active. The slow deformation of the northern edge of the Iberian plate including the Cantabrian Mountains may result from the ongoing slow convergence at an incipient subduction zone extending along the coast of northern Spain.     

10Be‐derived assessment of accelerated erosion in a glacially conditioned inner gorge,Entlebuch, Central Alps of Switzerland

Inner gorges often result from the propagation of erosional waves related to glacial/interglacial climate shifts. However, only few studies have quantified the modern erosional response to this glacial conditioning. Here, we report in situ ¹⁰Be data from the 64 km² Entlen catchment (Swiss Alps). This basin hosts a 7 km long central inner gorge with two tributaries that are >100 m‐deeply incised into thick glacial till and bedrock. The ¹⁰Be concentrations measured at the downstream end of the gorge yield a catchment‐wide erosion rate of 0.42 ± 0.04 mm yr^‐1, while erosion rates are consistently lower upstream of the inner gorge, ranging from 0.14 ± 0.01 mm yr^‐1 to 0.23 ± 0.02 mm yr^‐1. However, ¹⁰Be‐based sediment budget calculations yield rates of ~1.3 mm yr^‐1 for the inner gorge of the trunk stream. Likewise, in the two incised tributary reaches, erosion rates are ~2.0 mm yr^‐1 and ~1.9 mm yr^‐1. Moreover, at the erosional front of the gorge, we measured bedrock incision rates ranging from ~2.5 mm yr^‐1 to ~3.8 mm yr^‐1. These rates, however, are too low to infer a post‐glacial age (15–20 ka) for the gorge initiation. This would require erosion rates that are between 2 and 6 times higher than present‐day estimates. However, the downcutting into unconsolidated glacial till favored high erosion rates through knickzone propagation immediately after the retreat of the LGM glaciers, and subsequent hillslope relaxation led to a progressive decrease in erosion rates. This hypothesis of a two‐ to sixfold decrease in erosion rates does not conflict with the ¹⁰Be‐based erosion rate budgets, because the modern erosional time scale recorded by ¹⁰Be cover the past 2–3 ka only. These results point to the acceleration of Holocene erosion in response to the glacial overprint of the landscape. Copyright © 2012 John Wiley & Sons, Ltd.     

10Be dating of boulders on moraines from the last glacial period in the Nyainqentanglha mountains,Tibet

Chronologies of glacial advances during the last glacial period in the Nyainqentanglha mountain range may provide constraints on the past climate in a transition zone of the Asian monsoon.We present 15 new10Be exposure ages from two moraines in the Payuwang valley,on the north slope of the range.The inner moraine has exposure ages ranging from 18.0±1.7 to 30.6±2.8 ka(n=10),with a mean age of 23.8±4.0 ka,corresponding to the global Last Glacial Maximum(LGM).The outer moraine yields exposure ages ranging from 18.0±1.6 to 39.9±3.7 ka(n=5).Evidence for weathering leads us to view the oldest age as a minimum age,placing moraine formation during MIS3.Chronologies from the last glacial period from south slope of the Nyainqentanglha support this interpretation.Thus,there appears to have been a local LGM(LLGM)during MIS3 and a more limited glacial advance during the global LGM.Glacial advances during MIS3 in the Nyainqentanglha may correlate with millennial-scale climate change(Heinrich events).     

An approach for optimizing in situ cosmogenic 10Be sample preparation

Optimizing sample preparation for the isotopic measurement of ¹⁰Be extracted from quartz mineral separates has a direct positive effect on the accuracy and precision of isotopic analysis. Here, we demonstrate the value of tracing Be throughout the extraction process (both after dissolution and after processing), producing pure Be (by optimizing ion exchange chromatography methods and quantifying quartz mineral separate and final Be fraction purity), and minimizing backgrounds (through reducing both laboratory process blanks and ¹⁰B isobaric interference). These optimization strategies increase the amount of ¹⁰Be available for analysis during accelerator mass spectrometry (AMS), while simultaneously decreasing interference and contamination, and ensuring that sample performance matches standard performance during analysis. After optimization of our laboratory's extraction methodology, ⁹Be³⁺ ion beam currents measured during AMS analysis, a metric for sample purity and Be yield through the extraction process, matched the ⁹Be³⁺ beam currents of AMS standards analyzed at the same time considering nearly 800 samples. Optimization of laboratory procedures leads to purer samples that perform better, more consistently, and more similarly to standards during AMS analysis, allowing for improved precision and accuracy of measurements used for dating and quantification of Earth surface processes.     

REVIEW ON IN-SITU COSMOGENIC 14C DATING AND POTENTIAL APPLICATION IN PALEOEARTHQUAKE

Over the last two decades, in-situ cosmogenic ¹⁴C dating has become an import tool in Quaternary geology and is recognized to geoscientists because of its potential to provide information on exposure age and process rate estimates for geomorphic studies. The in-situ cosmogenic radiocarbon has a relatively short half-life(5730 yr)and is substantially more sensitive than all the other cosmogenic nuclides used so far. It is therefore particularly useful to determine surface-exposure ages of Holocene landforms and quantify erosion rates in rapidly denuding landscapes during the past few tens of thousands of years. Moreover, in situ ¹⁴C is produced in quartz which is both highly resistant to weathering and common in nature, so it can be used in combination with other in-situ cosmogenic nuclides such as ³He, ¹⁰Be,²¹Ne,²⁶Al, and ³⁶Cl to constrain complex exposure histories involving burial and/or erosion occurring over the past 25ka. The age and slip rate of Holocene normal fault have been undoubtedly a challenge for seismologists to be faced with as result from lack of appropriate late Quaternary sediment. Recently, the cosmogenic nuclides such as ³⁶Cl of preserved, seismically exhumed normal fault scarps were used to identify the last few major earthquakes and recover their ages and displacements through the modeling of the content of ³⁶Cl in the scarp rocks. This paper mainly summarizes the development of in-situ ¹⁴C dating, including its research history, production rate estimate, production mechanism, chemical behavior and experimental method. The potential application of in-situ ¹⁴C dating to recovering past earthquakes, their timing, and the regularity of their recurrence for preserved, seismically exhumed normal fault scarps is also introduced.     

原地宇宙成因核素测年中石英样品分离提纯的几组实验对比

原地宇宙成因核素测年方法在地质及地理研究领域应用广泛,测年对象众多。石英因其致密的晶体结构不易被大气成因10Be污染,以及较低的27Al含量的特点而成为宇宙成因核素测年方法中一种理想的测年物质。在分离石英样品10Be和26Al的实验流程中,石英的提纯是其中的一个关键环节。原始的HCl/H2O2和HF/HNO3蚀刻分离提纯石英方法应用广泛、安全可靠,但处理样品周期长、效率较低。因此,在原始流程的基础上,设计了3组实验流程与原始流程进行对比。结果显示,在样品石英纯度较高的基础上,可以通过减少超声波使用时间、增加蚀刻溶液浓度及每步样品处理量的方法,提高样品的处理效率,同时可以用加热磁力搅拌器替代超声波清洗器进行石英样品的HF/HNO3蚀刻提纯     

Long-term denudation rates of actively uplifting hillcrests in the Boso Peninsula, Japan, estimated from depth profiling of in situ-produced cosmogenic Be and Al

Depth profiling of in situ-produced cosmogenic ¹⁰Be and ²⁶Al was applied to estimate the rate of long-term denudation on actively uplifting hillcrests made of weakly consolidated sandstone in the Boso Peninsula, Japan. The nuclide concentration in the topmost meter of the hillcrest has reached an equilibrium value owing to surface denudation at 90 mm/kyr. This rate is much smaller than the regional uplift rate, and is consistent with the current isolated residual topography of the hillcrest. By considering the epoch of emergence of the sandstone hillcrests, we estimate the summit denudation rate of the adjacent mudstone hill formation to be 720 ± 110 mm/kyr.