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801.
Seasonal variation in albedo and radiation exchange between a burned and unburned forested peatland: implications for peatland evaporation 下载免费PDF全文
Forested boreal peatlands represent a precipitation‐dependent ecosystem that is prone to wildfire disturbance. Solar radiation exchange in forested peatlands is modified by the growth of a heterogeneous, open‐crown tree canopy, as well as by likely disturbance from wildfire. Radiation exchange at the peat surface is important in peatlands, as evaporation from the peat surface is the dominant pathway of water loss in peatlands of continental western North America. We examined shortwave and longwave radiation exchange in two forested ombrotrophic peatlands of central Alberta, Canada: one with (>75 years since wildfire; unburned) and another without a living spruce canopy (1–4 years since wildfire; burned) between the autumn of 2007 and 2010. Above‐canopy winter albedo was nearly two times greater in the recently burned peatland than the unburned peatland. Incoming shortwave radiation at the peat surface was much higher at the burned peatland, which increases the amount of energy available for evaporation. This is especially true for hollow microforms that are generally shaded by the tree canopy in unburned peatlands. Snow‐free albedo was similar between peatlands, although an increase in longwave losses at the burned site resulted in slightly greater net radiation at the unburned site. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
802.
The effect of increasing gravel cover on forest roads for reduced sediment delivery to stream crossings 下载免费PDF全文
Kristopher R. Brown Kevin J. McGuire W. Michael Aust W. Cully Hession C. Andrew Dolloff 《水文研究》2015,29(6):1129-1140
Direct sediment inputs from forest roads at stream crossings are a major concern for water quality and aquatic habitat. Legacy road–stream crossing approaches, or the section of road leading to the stream, may have poor water and grade control upon reopening, thus increasing the potential for negative impacts to water quality. Rainfall simulation experiments were conducted on the entire running surface area associated with six reopened stream crossing approaches in the south‐western Virginia Piedmont physiographic region, USA. Event‐based surface run‐off and associated total suspended solid (TSS) concentrations were compared among a succession of gravel surfacing treatments that represented increasing intensities of best management practice (BMP) implementation. The three treatments were no gravel (10–19% cover), low gravel (34–60% cover), and high gravel (50–99% cover). Increased field hydraulic conductivity was associated with maximized surface cover and ranged from 7.2 to 41.6, 11.9 to 46.3, and 16.0 to 58.6 mm h−1 respectively for the no gravel, low gravel, and high gravel treatments. Median TSS concentration of surface run‐off for the no gravel treatment (2.84 g l−1) was greater than low gravel (1.10 g l−1) and high gravel (0.82 g l−1) by factors of 2.6 and 3.5 respectively. Stream crossing approaches with 90–99% surface cover had TSS concentrations below 1 g l−1. Reducing the length of road segments that drain directly to the stream can reduce the costs associated with gravel surfacing. This research demonstrates that judicious and low‐cost BMPs can ameliorate poor water control and soil erosion associated with reopening legacy roads. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
803.
Fluvial organic carbon composition and concentration variability within a peatland catchment—Implications for carbon cycling and water treatment 下载免费PDF全文
Fluvial organic carbon (OC) transformations are an important component of carbon cycling and greenhouse gas production in inland waters resulting in considerable recent interest in the fate of fluvial OC exported from carbon rich soils such as peatlands. Additionally, peatland catchments are important drinking water collection areas, where high OC concentrations in runoff have water treatment implications. This analysis presents the results from a year‐round intensive study within a water treatment catchment draining an area of peatland, considering carbon transformations along a continuum from headwater river, through a storage reservoir and pipe, to a water treatment works. The study uses a unique combination of methods (colourmetric, ultrafiltration, and 14C radiocarbon dating) to assess catchment wide changes in fluvial carbon composition (colour, size, and age) alongside concentration measures. The results indicate clear patterns of carbon transformations in the river and reservoir and dissolved low molecular weight coloured carbon to be most subject to change, with both loss and replacement within the catchment residence time. Although the evidence suggests dissolved OC (DOC) gains are from particulate OC breakdown, the mechanisms of DOC loss are less certain and may represent greenhouse gas losses or conversions to particulate OC. The transformations presented here appear to have minimal impact on the amount of harder to treat (<10 kDa) dissolved carbon, although they do have implications for total DOC loading to water treatment works. This paper shows that peatland fluvial systems are not passive receptors of particulate and dissolved organic carbon but locations where carbon is actively cycled, with implications for the understanding of carbon cycling and water treatment in peatland catchments. 相似文献
804.
Low‐temperature thermochronology (LTT) is commonly used to investigate onshore records of continental rifting and geomorphic evolution of passive continental margins. The SE Australian passive margin, like many others, has an elevated plateau separated from the coastal plain by an erosional escarpment, presumed to originate through Cretaceous rifting prior to Tasman Sea seafloor spreading. Previous LTT studies have focused on reconciling thermal histories with development of the present‐day topography. New apatite LTT data along an escarpment‐to‐coast transect define a classic “boomerang” (mean track length vs. fission‐track age), indicating variable overprinting of late‐Palaeozoic cooling ages by a younger, mid‐Cretaceous cooling event. Regionally, however, the boomerang trend diverges NNW away from the coast and crosses the escarpment, implying the underlying thermal history pre‐dates escarpment formation and is largely independent from post‐breakup landscape evolution. We suggest that Cretaceous cooling might relate to erosion of Permo‐Triassic sedimentary cover from a formerly more extensive Sydney Basin. 相似文献
805.
Shuangwen Yi Jan‐Pieter Buylaert Andrew S. Murray Huayu Lu Christine Thiel Lin Zeng 《Boreas: An International Journal of Quaternary Research》2016,45(4):644-657
In this study, we report standard quartz SAR OSL and post‐IR infrared (IR) stimulated luminescence (post‐IR IRSL; pIRIR290) measurements made on sand‐sized quartz and K‐feldspar extracts from the loess‐palaeosol sequence at Niuyangzigou in northeastern China. The quartz OSL characteristics are satisfactory. Extensive pIRIR50,290 dose recovery tests were performed by adding doses on top of the natural dose. We found that dose recovery ratios improve significantly when the test dose ranges between ~15 and ~80% of the total dose, and good dose recovery (within ±5% of unity) can be obtained up to ~800 Gy. Otherwise, the dose recovery ratio deviates from unity. The De values also depend on the test dose size and so we conclude that the effect of test dose size should be routinely considered in pIRIR dating. First IR stimulation plateau pIRIR290 results are compared with multiple elevated temperature ‐pIRIR (MET‐pIRIR) data. It appears that the low temperature MET‐pIRIR data are strongly affected by poor dose recovery, but this is not the case for the pIRIR290 results. Natural signal measurements at the highest (first IR) stimulation temperature on a sample expected to be in field saturation suggest that ~10% signal loss occurs in pIRIR signals. Long‐term laboratory bleaching experiments (>80 days) show that a constant (or very difficult to bleach) residual pIRIR290 signal is reached after ~300 h, corresponding to a dose of 6.2±0.7 Gy. Quartz OSL and feldspar pIRIR50,290 ages are in good agreement at least back to c. 70 ka. Beyond this the quartz ages begin to underestimate but the feldspar ages are in agreement with the expected Last Interglacial age palaeosol. 相似文献
806.
807.
808.
石榴石Lu-Hf、原位独居石U-Pb定年对多期变形的时代制约——以北祁连托勒牧场为例 总被引:2,自引:1,他引:1
多期变质变形事件的精确年代限定是造山构造年代学研究的热点问题之一。本文尝试运用面理弯切轴测量技术,结合石榴石Lu-Hf和原位独居石U-Pb定年,厘定北祁连托勒牧场地区石榴石和斜长石斑晶记录的两期构造变形事件:石榴石斑晶生长记录的早期构造变形事件年代为512.3±2.7Ma;斜长石斑晶生长记录的晚期构造变形事件年代不早于481.0±2.3Ma,并推断该期构造变形水平挤压主应力方向为北东-南西。斜长石斑晶内未发现独居石,用于年代学测试的独居石颗粒均位于斜长石斑晶外基质中。显微构造分析认为,独居石生长不早于斜长石斑晶。481.0±2.3Ma的独居石U-Pb年龄,应为斜长石斑晶所记录构造变形的时代下限。结合前人锆石U-Pb定年和Hf同位素研究结果分析认为,获得的512.3±2.7Ma石榴石-全岩Lu-Hf等时线年龄,代表了祁连洋俯冲过程中石榴石的生长时间,后期变质变形作用未对石榴石的Lu-Hf同位素体系产生明显影响。结合显微构造分析,石榴石Lu-Hf定年可为早期构造变形提供有效年代学制约。 相似文献
809.
810.
Andrew Morton Colin Waters Mark Fanning Ian Chisholm Matt Brettle 《Geological Journal》2015,50(5):553-574
A study of detrital zircon age populations in Namurian–Westphalian (Carboniferous) sandstones in the southern Central Pennine Basin of the UK has revealed considerable complexity in their provenance history. The Pendleian–Marsdenian Morridge Formation, which is known to have been derived from the Wales‐Brabant Massif to the south on the basis of palaeocurrent and petrographic information, is dominated by zircons ultimately derived from the Caledonian belt to the north. These zircons were recycled from sandstones of northern origin that had been previously deposited over the massif during Middle to Late Devonian times. The Morridge Formation also includes Late Neoproterozoic zircons of local Wales‐Brabant Massif origin. The south lobe of the Yeadonian Rough Rock has been previously interpreted as having a complex provenance including sediment of northern origin interbedded with sediment ascribed to a Wales‐Brabant Massif source. However, the zircon spectrum lacks a Late Neoproterozoic component that would have been diagnostic of input from the Wales‐Brabant Massif, and the provenance history of the Rough Rock south lobe therefore remains enigmatic. The Langsettian Ludgbridge Conglomerate is dominated by Late Neoproterozoic zircons of Wales‐Brabant Massif origin, but even in this evidently proximal deposit, the provenance is complex since the main zircon group (ca. 640 Ma) cannot be matched with known local Neoproterozoic basement sources. The data either indicate the presence of hitherto‐unknown magmatic rocks of this age adjacent to the South Staffordshire coalfield or indicate that the zircons were recycled from sediment with a more distal origin. Finally, the Duckmantian Top Hard Rock contains zircons that can be reconciled with a source in the Irish Caledonides, consistent with the palaeocurrent evidence, supplemented by zircons derived from the Wales‐Brabant Massif, possibly including the Monian Composite Terrane of Anglesey. The study reinforces the important message that failure to recognize the presence of recycled zircon could lead to erroneous reconstructions of sediment provenance and transport history. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献