Quantifying hillslope erosion rates and processes for a coastal California landscape over varying timescales

The Earth's surface erodes by processes that occur over different spatial and temporal scales. Both continuous, low‐magnitude processes as well as infrequent, high‐magnitude events drive erosion of hilly soil‐mantled landscapes. To determine the potential variability of erosion rates we applied three independent, field‐based methods to a well‐studied catchment in the Marin Headlands of northern California. We present short‐term, basin‐wide erosion rates determined by measuring pond sediment volume (40 years) and measured activities of the fallout nuclides ¹³⁷Cs and ²¹⁰Pb (40–50 years) for comparison with long‐term (>10 ka) rates previously determined from in situ‐produced cosmogenic ¹⁰Be and ²⁶Al analyses. In addition to determining basin‐averaged rates, ¹³⁷Cs and ²¹⁰Pb enable us to calculate point‐specific erosion rates and use these rates to infer dominant erosion processes across the landscape. When examined in the context of established geomorphic transport laws, the correlations between point rates of soil loss from ¹³⁷Cs and ²¹⁰Pb inventories and landscape morphometry (i.e. topographic curvature and upslope drainage area) demonstrate that slope‐driven processes dominate on convex areas while overland flow processes dominate in concave hollows and channels. We show a good agreement in erosion rates determined by three independent methods: equivalent denudation rates of 143 ± 41 m Ma^?1 from pond sediment volume, 136 ± 36 m Ma^?1 from the combination of ¹³⁷Cs and ²¹⁰Pb, and 102 ± 25 m Ma^?1 from ¹⁰Be and ²⁶Al. Such agreement suggests that erosion of this landscape is not dominated by extreme events; rather, the rates and processes observed today are indicative of those operating for at least the past 10 000 years. Copyright © 2006 John Wiley & Sons, Ltd.     

Production of selected cosmogenic radionuclides by muons 1. Fast muons

To investigate muon-induced nuclear reactions leading to the production of radionuclides, targets made of C₉H₁₂, SiO₂, Al₂O₃, Al, S, CaCO₃, Fe, Ni, Cu, Gd, Yb and Tl were irradiated with 100 and 190 GeV muons in the NA54 experimental setup at CERN. The radionuclide concentrations were measured with accelerator mass spectrometry and γ-spectroscopy. Results are presented for the corresponding partial formation cross-sections. Several of the long-lived and short-lived radionuclides studied are also produced by fast cosmic ray muons in the atmosphere and at depths underground. Because of their importance to earth sciences investigations, calculations of the depth dependence of production rates by fast cosmic ray muons have been made.     

Landscape preservation under Fennoscandian ice sheets determined from in situ produced Be and Al

Some areas within ice sheet boundaries retain pre-existing landforms and thus either remained as ice free islands (nunataks) during glaciation, or were preserved under ice. Differentiating between these alternatives has significant implications for paleoenvironment, ice sheet surface elevation, and ice volume reconstructions. In the northern Swedish mountains, in situ cosmogenic ¹⁰Be and ²⁶Al concentrations from glacial erratics on relict surfaces as well as glacially eroded bedrock adjacent to these surfaces, provide consistent last deglaciation exposure ages (∼8-13 kyr), confirming ice sheet overriding as opposed to ice free conditions. However, these ages contrast with exposure ages of 34-61 kyr on bedrock surfaces in these same relict areas, demonstrating that relict areas were preserved with little erosion through multiple glacial cycles. Based on the difference in radioactive decay between ²⁶Al and ¹⁰Be, the measured nuclide concentration in one of these bedrock surfaces suggests that it remained largely unmodified for a minimum period of 845₋₄₁₈⁺⁴⁶¹ kyr. These results indicate that relict areas need to be accounted for as frozen bed patches in basal boundary conditions for ice sheet models, and in landscape development models. Subglacial preservation also implies that source areas for glacial sediments in ocean cores are considerably smaller than the total area covered by ice sheets. These relict areas also have significance as potential long-term subglacial biologic refugia.     

Isotopic analysis of early solar system objects by an ion microprobe: Parametric studies and initial results

Magnesium, potassium and calcium isotope compositions in terrestrial samples and refractory phases from primitive meteorites are determined using an ion microprobe. A thorough investigation of the different instrument parameters is carried out to ensure that conditions necessary for high mass resolution and high precision isotopic studies are adequately satisfied. The instrument can be tuned to achieve mass resolution (M/ΔM) of up to 10,000 (M≤60); it has a very good dynamic stability (ΔB/B≤10 ppm over durations of ≤40 minutes) and the counting system has an effective dead-time of ≤25 nsec and a dynamic background of ≤0·01 c/s. Reproducibility and precision of isotopic measurements are checked by analyzing magnesium and titanium isotopic compositions in terrestrial standards and isotopically doped silicate glasses. A precision of 2‰ (2σ _m ) was achieved during magnesium isotopic analysis in samples with low Mg content (200 ppm). Results from studies of magnesium and potassium isotopic compositions in several Ca−Al-rich refractory inclusions (CAIs) from the primitive meteorites Efremovka and Grosnaja, representing some of the early solar system objects, are presented. The well-behaved Mg−Al isotopic systematics confirm the pristine nature of the Efremovka CAIs inferred earlier from petrographic and trace element studies. The Grosnaja CAIs that have experienced secondary alterations show disturbed magnesium isotopic systematics. Observation of excess²⁶Mg in several of the analyzed CAIs confirms the presence of the now extinct²⁶Al (t _1/2=7×10⁵ years) in the solar nebula at the time of CAI formation. Our data also suggest a relatively uniform distribution of²⁶Al in the solar nebula. Several Efremovka CAIs with excess²⁶Mg also have excess⁴¹K resulting from the decay of⁴¹Ca (t _1/2≃10⁵ years). This observation constrains the time interval between cessation of nucleosynthetic input to the solar nebula and the formation of some of the first solar system solids (CAIs) to less than a million years.     

Short-lived nuclides in the early solar system

Isotopic records in meteorites provide evidence for the presence of several short-lived nuclides in the early solar system with half-lives varying from 10⁵ to ∼8x10⁷ years. Most of the nuclides with longer half-life (> 10⁷ years) are considered to be products of stellar nucleosynthesis taking place over long time scales in our galaxy. However, for the relatively shorter-lived nuclides, two possibilities exist; they could be products of energetic particle interactions taking place in a presolar or early solar environment, or, they could have been produced in a stellar source and injected into the protosolar molecular cloud just prior to its collapse. The presently available data appear to support the latter case and put a stringent constraint of less than a million years for the time scale for the collapse of the protosolar molecular cloud to form the Sun and some of the first solar system solids. This short time scale also suggests the possibility of a triggered origin for the solar system with the very process of injection of the short-lived nuclides acting as the trigger for the collapse of the protosolar molecular cloud. Fossil records of the short-lived nuclides in meteorites also provide very useful chronological information on the early solar system processes like the time scale for nebular processing, the time scales for differentiation and for metal/silicate fractionation within planetesimals. The currently available data suggest a time scale of a few million years for nebular processing and a relatively short time scale of about ten million years within which differentiation, melting and recrystallization in some of the planetesimals took place.     

10Be和26Al在地表形成和演化研究中的应用

地球表面丰富多样的地形、地貌是地球内外动力相互作用的结果。宇宙射线辐射地表岩石、沉积物等产生的¹⁰Be和²⁶Al放射性核素（半衰期分别为1．5Ma和0．71Ma）,它们的浓度主要取决于由地理位置和地形、地貌条件所决定的宇宙射线辐射的通量和时间,从而能够记录地表的形成和演化历史。因此,宇宙成因核素是研究地表形成历史和作用过程的有力工具。     

Marine Oxygen Isotope Stage (MIS)-6 Glacial Advances on the Tibetan Plateau More Extensive than during MIS-2 due to More Abundant Precipitation

Numerous studies dated glacial deposits within the Himalayan-Tibetan orogen. While most focus on young deposits, i.e., younger than the Last Glacial Maximum (LGM or Marine oxygen Isotope Stage (MIS)-2, ~20 ka), older moraines such as those from MIS-6 (~130–191 ka) are much harder to date and interpret due to the less well-preserved nature of their surfaces and boulders, as well as their scattered and continuous age distribution due to long-lived erosion since deposition. Here, we dated with 10Be, two imbricated moraines near Yadong in southern Tibet, as MIS-2 and 6, showing that the most extensive, smooth surfaces were abandoned during MIS-6. Compiling published data from 54 MIS-6 moraines on the Tibetan Plateau reveals that they exist in most regions, dry or humid. They are particularly well-preserved (sharp crests) in eastern and northern Tibet, while in southern and central Tibet, their crests are rounded to sub-rounded. Because both MIS-2 and 6 were equally cold, and because MIS-6 moraines are much more extensive than those from the LGM, we conclude that MIS-6 glacial advances were controlled by more abundant precipitation than during MIS-2. This would be consistent with the peak in Asian monsoon during MIS-6, revealed by sediments from the South China Sea.     

宇生核素深度剖面暴露测年的原理与应用

宇生核素暴露测年技术是目前第四纪地貌年代测定最常用的方法之一,由于难以定量化继承性核素以及侵蚀对样品暴露年代的影响（通常假设继承性和侵蚀的影响为0）,其测年结果难以反映地貌的真实暴露年代。通过测定地貌剖面不同深度样品的宇生核素浓度,建立"宇生核素浓度-深度"剖面曲线,可同时定量化研究地貌体的暴露年龄、继承性宇生核素浓度以及侵蚀速率,有效地弥补了宇生核素暴露测年中继承性核素以及侵蚀速率不确定性的缺陷。基于蒙特卡洛方法的宇生核素深度剖面暴露测年模型（简称蒙特卡洛深度剖面模型）是最常用的计算模式之一,然而,国内关于该方法的原理及其应用研究相对较少,大大限制了该方法的广泛使用。本文详细阐述了蒙特卡洛深度剖面模型的原理及其在冰川地貌、河流阶地、冲洪积扇等地貌测年中的应用。此外,介绍了深度剖面模型的复现、非稳态模型、线性反演模型等其他几种计算模式并进一步探讨了蒙特卡洛深度剖面模型暴露测年技术的野外采样方法,以使其能够广泛应用于第四纪地貌学和年代学研究中。     

青藏高原东南部他念他翁山全新世早中期冰进事件研究

他念他翁山位于横断山脉西部,是海洋性冰川向亚大陆性冰川的过渡区,维持冰川发育的降水补给主要由西南季风带来,该区第四纪冰川进退对西南季风波动有较为直接的反映。正是这一地理位置的特殊性,使得该区第四纪冰川研究具有重要意义。采用野外地貌调查与宇宙成因核素测年技术相结合的方法,试图查明他念他翁山青古隆槽谷全新世期间冰川地貌学特征,并确定其发生的具体时间,探讨其响应机制。结果显示：他念他翁山全新世早中期冰川波动的时限介于（6.13±0.37）~（8.83±0.50） ka,可能是当时西南季风强盛,降水增加的结果。研究可为探究冰川作用与西南季风气候变化及全球气候变化之间的动力学联系提供新依据。     

10Be Exposure Ages Obtained From Quaternary Glacial Landforms on the Tibetan Plateau and in the Surrounding Area

In situ terrestrial cosmogenic nuclide (TCN) exposure dating using ¹⁰Be is one of the most successful techniques used to determine the ages of Quaternary deposits and yields data that enable the reconstruction of the Quaternary glacial history of the Tibetan Plateau and the surrounding mountain ranges. Statistical analysis of TCN ¹⁰Be exposure ages, helps to reconstruct the history of glacial fluctuations and past climate changes on the Tibetan Plateau, differences in the timing of glacier advances among different regions. However, different versions of the Cosmic‐Ray‐prOduced NUclide Systematics on Earth (CRONUS‐Earth) online calculator, which calculates and corrects the TCN ages of Quaternary glacial landforms, yield different results. For convenience in establishing contrasts among regions, in this paper, we recalculate 1848 ¹⁰Be exposure ages from the Tibetan Plateau that were published from 1999 to 2017 using version 2.3 of the CRONUS‐Earth calculator. We also compare the results obtained for 1594 ¹⁰Be exposure ages using different versions (2.2, 2.3 and 3.0) of the CRONUS‐Earth calculator. The results are as follows. (1) Approximately 97% of the exposure ages are less than 200 ka. A probability density curve of the exposure ages suggests that greater numbers of oscillations emerge during the Holocene, and the peaks correspond to the Little Ice Age, the 8.2 ka and 9.3 ka cold events; the main peak covers the period between 12 and 18 ka. (2) In most areas, the newer versions of the calculator produce older ¹⁰Be exposure ages. When different versions of the CRONUS‐Earth calculator are used, approximately 29% of the ¹⁰Be exposure ages display maximum differences greater than 10 ka, and the maximum age difference for a single sample is 181.1 ka.