The sensitivity of hillslope bedrock erosion to precipitation

Decoupling the impacts of climate and tectonics on hillslope erosion rates is a challenging problem. Hillslope erosion rates are well known to respond to changes in hillslope boundary conditions (e.g. channel incision rates) through their dependence on soil thickness, and precipitation is an important control on soil formation. Surprisingly though, compilations of hillslope denudation rates suggest little precipitation sensitivity. To isolate the effects of precipitation and boundary condition, we measured rates of soil production from bedrock and described soils on hillslopes along a semi‐arid to hyperarid precipitation gradient in northern Chile. In each climate zone, hillslopes with contrasting boundary conditions (actively incising channels versus non‐eroding landforms) were studied. Channel incision rates, which ultimately drive hillslope erosion, varied with precipitation rather than tectonic setting throughout the study area. These precipitation‐dependent incision rates are mirrored on the hillslopes, where erosion shifts from relatively fast and biologically‐driven to extremely slow and salt‐driven as precipitation decreases. Contrary to studies in humid regions, bedrock erosion rates increase with precipitation following a power law, from ～1 m Ma^?1 in the hyperarid region to ～40 m Ma^?1 in the semi‐arid region. The effect of boundary condition on soil thickness was observed in all climate zones (thicker soils on hillslopes with stable boundaries compared to hillslopes bounded by active channels), but the difference in bedrock erosion rates between the hillslopes within a climate region (slower erosion rates on hillslopes with stable boundaries) decreased as precipitation decreased. The biotic‐abiotic threshold also marks the precipitation rate below which bedrock erosion rates are no longer a function of soil thickness. Our work shows that hillslope processes become sensitive to precipitation as life disappears and the ability of the landscape to respond to tectonics decreases. Copyright © 2010 John Wiley & Sons, Ltd.     

A noble gas and cosmogenic radionuclide analysis of two ordinary chondrites from Almahata Sitta

Abstract– We present the results of a noble gas (He, Ne, Ar) and cosmogenic radionuclide (¹⁰Be, ²⁶Al, ³⁶Cl) analysis of two chondritic fragments (#A100, L4 and #25, H5) found in the Almahata Sitta strewn field in Sudan. We confirm their earlier attribution to the same fall as the ureilites dominating the strewn field, based on the following findings: (1) both chondrite samples indicate a preatmospheric radius of approximately 300 g cm^?2, consistent with the preatmospheric size of asteroid 2008 TC₃ that produced the Almahata Sitta strewn field; (2) both have, within error, a ²¹Ne/²⁶Al‐based cosmic ray exposure age of approximately 20 Ma, identical to the reported ages of Almahata Sitta ureilites; (3) both exhibit hints of ureilitic Ar in the trapped component. We discuss a possible earlier irradiation phase for the two fragments of approximately 10–20 Ma, visible only in cosmogenic ³⁸Ar. We also discuss the approximately 3.8 Ga (⁴He) and approximately 4.6 Ga (⁴⁰Ar) gas retention ages, measured in both chondritic fragments. These imply that the two chondrite fragments were incorporated into the ureilite host early in solar system evolution, and that the parent asteroid from which 2008 TC₃ is derived has not experienced a large break‐up event in the last 3.8 Ga.     

High resolution absorption cross section measurements of SO2 at 213 K in the wavelength region 172–240 nm

Laboratory measurements at high resolution of the absorption cross section of SO₂ at the temperature 213 K have been performed in the wavelength region 172–240 nm with a 6.65 m scanning spectrometer/spectrograph operated at an instrumental width of 0.002 nm. The measured cross sections are presented graphically in representative wavelength regions and are available throughout the region 172–240 nm at wavenumber intervals of 0.4–0.1 cm^?1 as a numerical tabulation stored on magnetic tape from the National Space Science Data Center, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, U.S.A. The measured cross sections, which are relevant to the photochemistry of planetary atmospheres, possess significantly more spectroscopic structure, and are more accurate, than previous measurements made at lower resolution.     

Detection of Aseismic Slip on an Inland Fault by Crustal Movement and Groundwater Observations: A Case Study on the Yamasaki Fault, Japan

To understand the detailed process of fault activity, aseismic slip may play a crucial role. Aseismic slip of inland faults in Japan is not well known, except for that related to the Atotsugawa fault. To know whether aseismic slip does not occur, or is merely not detected, is an important question. The National Institute of Advanced Industrial Science and Technology constructed an observation site near Yasutomi fault, a part of the Yamasaki fault system, and has collected data on the crustal strain field, groundwater pressures, and crustal movement using GPS. In a departure from the long-term trend, a transient change of the crustal strain field lasting a few months was recorded. It indicated the possibility of an aseismic slip event. Furthermore, analyses of data from the extensometers at Yasutomi and Osawa observation vaults of Kyoto University, as well as GPS data from the Geographical Survey Institute (GEONET), revealed unsteady crustal strain changes. All data could be explained by local, left-lateral, aseismic slip of the order of 1 mm in the shallow part of the Yasutomi fault.     

Climatology of yellow sand (Asian sand,Asian dust or Kosa) in East Asia

In order to study climatology of yellow sand (Asian sand, Asian dust or Kosa) in East Asia, secular fluctuation in China, Korea and Japan in the recent 30 years was presented. The number of days with sand-dust storm at five stations in China—Hotan, Zhangye, Minqin, Jurh and Beijing, decreases a lot at the former three stations, but changed little at the latter two stations. Suggesting that the recent global warming is more evident in Xinjiang and Gansu, where the frequency of cold air invasions from the higher latitudes is decreasing. But, the eastern parts of Mongolia, inner Mongolia, and North China encounter stronger cyclones in early spring as a result of global warming. These cyclones bring cold air from higher latitudes, causing severe dust storms. Secular variation in the annual days with sand-dust storms in China and Kosa days in Korea and Japan show a parallel change with higher frequency from 1975 to 1985. This may be related to the higher frequency of La Nina years. However, different tendency was shown in the period from 1986 to 1996. Since 1996 or 1997, a sharp increase is clear, which may be caused by the developed cyclones in East Asia as well as human activities, and stronger land degradation under La Nina conditions. Anomalies of the total number of stations with Kosa days were discussed in accordance with some synoptic meteorological conditions such as the differences between Siberian anticyclone and Aleutian cyclone center at 500 hPa level during the previous winter.     

Winter and summer monsoons and the navigation in East Asia in historical age

In order to make clear a problem in the human impact on the East Asian monsoons, navigation in the period from the 7th century to the 19th century was studied from the standpoint of historical climatology. 1. are described the periods of winter and summer monsoons over East Asia in relation to the division of the natural season. 2. based on the characteristics of the wind direction and velocity during the monsoons, it is shown that the winter monsoon is extremely unfavorable for navigation in contrast to the favorable summer monsoon. 3. statistical results on the possible frequency of navigation over the East China Sea by small boats are dealt with according to the meteorological records. 4. it is indicated that the navigations of the missions between Japan and China, Korea or Po Hai in the 7th – 9th centuries showed clear relations to the monsoon seasons. Lastly, the navigation mainly in 17th, 18th and 19th centuries around Ryukyu Islands, on the Chinese coast and the Japanese Islands are mentioned in detail. Even in this period the winter monsoon affected strongly on the navigation, that is, sailing and entries kept away the winter monsoon season almost completely.