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231.
A jökulhlaup burst from the head of Grasshopper Glacier in Wyoming's Wind River Mountains during early September 2003. Five reaches with distinct sedimentation patterns were delineated along the Dinwoody Creek drainage. This paper focuses on a portion of the jökulhlaup route where erosion of the forested banks created 16 large logjams spaced at longitudinal intervals of tens to hundreds of meters. Aggradation within the main channel upstream from each logjam created local sediment wedges, and the jams facilitated overbank deposition during the jökulhlaup. Field surveys during 2004 and 2006 documented logjam characteristics and associated erosional and depositional features, as well as initial modification of the logjams and flood deposits within the normal seasonal high-flow channel. Overbank deposits have not been altered by flows occurring since 2003. Field measurements supported three hypotheses that (i) logjams present along the forested portions of the jökulhlaup route are larger and more closely spaced than those along adjacent, otherwise comparable stream channels that have not recently experienced a jökulhlaup; (ii) logjams are not randomly located along the jökulhlaup route, but instead reflect specific conditions of channel and valley geometry and flood hydraulics; and (iii) the presence of logjams facilitated significant erosional and depositional effects. This paper documents a sequence of events in which outburst floodwaters enhance bank erosion and recruitment of wood into the channel, and thus the formation of large logjams. These logjams sufficiently deflect flow to create substantial overbank deposition in areas of the valley bottom not commonly accessed by normal snowmelt peak discharges, and through this process promote valley-bottom aggradation and sediment storage. Changes in the occurrence of glacier outburst floods thus have the potential to alter the rate and magnitude of valley-bottom dynamics in these environments, which is particularly relevant given predictions of worldwide global warming and glacial retreat. Processes observed at this field site likely occur in other forested catchments with headwater glaciers. 相似文献
232.
Haidi J. L. Hancock Gerald R. Dickens Ellen Thomas Kevin L. Blake 《International Journal of Earth Sciences》2007,96(5):925-946
Bulk carbonate content, planktic and benthic foraminiferal assemblages, stable isotope compositions of bulk carbonate and
Nuttallides truempyi (benthic foraminifera), and non-carbonate mineralogy were examined across ∼30 m of carbonate-rich Paleogene sediment at Deep
Sea Drilling Project (DSDP) Site 259, on Perth Abyssal Plain off Western Australia. Carbonate content, mostly reflecting nannofossil
abundance, ranges from 3 to 80% and generally exceeds 50% between 35 and 57 mbsf. A clay-rich horizon with a carbonate content
of about 37% occurs between 55.17 and 55.37 mbsf. The carbonate-rich interval spans planktic foraminiferal zones P4c to P6b
(∼57–52 Ma), with the clay-rich horizon near the base of our Zone P5 (upper)—P6b. Throughout the studied interval, benthic
species dominate foraminiferal assemblages, with scarce planktic foraminifera usually of poor preservation and limited species
diversity. A prominent Benthic Foraminiferal Extinction Event (BFEE) occurs across the clay-rich horizon, with an influx of
large Acarinina immediately above. The δ13C records of bulk carbonate and N. truempyi exhibit trends similar to those observed in upper Paleocene–lower Eocene (∼57–52 Ma) sediment from other locations. Two successive
decreases in bulk carbonate and N. truempyi δ13C of 0.5 and 1.0‰ characterize the interval at and immediately above the BFEE. Despite major changes in carbonate content,
foraminiferal assemblages and carbon isotopes, the mineralogy of the non-carbonate fraction consistently comprises expanding
clay, heulandite (zeolite), quartz, feldspar (sodic or calcic), minor mica, and pyrolusite (MnO2). The uniformity of this mineral assemblage suggests that Site 259 received similar non-carbonate sediment before, during
and after pelagic carbonate deposition. The carbonate plug at Site 259 probably represents a drop in the CCD from ∼57 to 52–51 Ma,
as also recognized at other locations. 相似文献
233.
Evaluating channel response to an extreme sedimentation event in the context of historical range of variability: Upper Colorado River,USA 总被引:1,自引:0,他引:1
In May 2003, a breach in a large irrigation ditch within Rocky Mountain National Park (RMNP) initiated a debris flow that entered Lulu Creek and the Colorado River, where 36 000 m3 of sediment substantially altered channel forms and processes. We present a proof of concept to understand whether the 2003 disturbance is within the historical range of variability (HRV), and whether the recovery potential of the system is sufficient to adapt to the disturbance. Flow and sediment regimes, and channel morphology and stability were monitored on Lulu Creek and the Colorado River from 2004 to 2011. Dominant channel response following the debris flow within Lulu Creek included step development, bed armoring, and channel widening. Step height‐to‐length ratios (H/L) for three reaches on Lulu Creek are outside the HRV of reference channels, with one reach approaching reference conditions. Erosion of approximately 23% of the debris fan volume occurred as a result of the long duration 2011 peak flow. Sediment within the Lulu Creek fan will persist for ~30–190 years, assuming current maximum and mean removal rates. Planform changes on the Colorado River since the debris flow include an increase in single‐thread geometries, with braided reaches where bar deposition occurred. Bedload transport and grain‐size analysis of bedload indicate translational spreading of a sand wave front with a dispersive component in steeper reaches. Lulu Creek is returning to a condition of natural variability, but the Colorado River is outside the HRV expected for steep‐gradient, pool‐riffle channels. Applying HRV to a situation where management questions require a longer term perspective, and pre‐disturbance baseline data are limited, is a useful approach. The HRV analysis facilitates a better understanding of site variability and delineates the range of possibilities of channel form and process to achieve management goals. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
234.
Beth Ellen Clark Richard P. Binzel Ellen S. Howell Edward A. Cloutis Maureen Ockert-Bell Phil Christensen Maria Antonietta Barucci Francesca DeMeo Dante S. Lauretta Harold Connolly Jr. Alicia Soderberg Carl Hergenrother Lucy Lim Josh Emery Michael Mueller 《Icarus》2011,216(2):462-475
We present reflectance spectra from 0.4 to 2.4 μm of Asteroid (101955) 1999 RQ36, the target of the OSIRIS-REx spacecraft mission. The visible spectral data were obtained at the McDonald Observatory 2.1-m telescope with the ES2 spectrograph. The infrared spectral data were obtained at the NASA Infrared Telescope Facility using the SpeX instrument. The average visible spectrum is combined with the average near-infrared wavelength spectrum to form a composite spectrum. We use three methods to constrain the compositional information in the composite spectrum of Asteroid (101955) 1999 RQ36 (hereafter RQ36). First, we perform a least-squares search for meteorite spectral analogs using 15,000 spectra from the RELAB database. Three most likely meteorite analogs are proposed based on the least-squares search. Next, six spectral parameters are measured for RQ36 and their values are compared with the ranges in parameter values of the carbonaceous chondrite meteorite classes. A most likely meteorite analog group is proposed based on the depth of overlap in parameter values. The results of the least-squares search and the parametric comparisons point to CIs and/or CMs as the most likely meteorite analogs for RQ36, and COs and CHs as the least likely. RQ36 has a spectrally “blue” continuum slope that is also observed in carbonaceous chondrites containing magnetite. We speculate that RQ36 is composed of a “CM1”-like material. Finally, we compare RQ36 to other B-type asteroids measured by Clark et al. (Clark, B.E. et al. [2010]. J. Geophys. Res. 115, E06005). The results of this comparison are inconclusive. RQ36 is comparable to Themis spectral properties in terms of its albedo, visible spectrum, and near-infrared spectrum from 1.1 to 1.45 μm. However, RQ36 is more similar to Pallas in terms of its near-infrared spectrum from 1.6 to 2.3 μm. Thus it is possible that B-type asteroids form a spectral continuum and that RQ36 is a transitional object, spectrally intermediate between the two end-members. This is particularly interesting because Asteroid 24 Themis was recently discovered to have H2O ice on the surface (Rivkin, A., Emery, J. [2010]. Nature 464, 1322–1323; Campins, H. et al. [2010a]. Nature 464, 1320–1321). 相似文献
235.
Michael K. Shepard Alan W. Harris Beth Ellen Clark Michael C. Nolan Christopher Magri Lance A.M. Benner 《Icarus》2011,215(2):547-551
We report new radar observations of E-class Asteroid 64 Angelina and M-class Asteroid 69 Hesperia obtained with the Arecibo Observatory S-band radar (2480 MHz, 12.6 cm). Our measurements of Angelina’s radar bandwidth are consistent with reported diameters and poles. We find Angelina’s circular polarization ratio to be 0.8 ± 0.1, tied with 434 Hungaria for the highest value observed for main-belt asteroids and consistent with the high values observed for all E-class asteroids (Benner, L.A.M., Ostro, S.J., Magri, C., Nolan, M.C., Howell, E.S., Giorgini, J.D., Jurgens, R.F., Margot, J.L., Taylor, P.A., Busch, M.W., Shepard, M.K. [2008]. Icarus 198, 294-304; Shepard, M.K., Kressler, K.M., Clark, B.E., Ockert-Bell, M.E., Nolan, M.C., Howell, E.S., Magri, C., Giorgini, J.D., Benner, L.A.M., Ostro, S.J. [2008b]. Icarus 195, 220-225). Our radar observations of 69 Hesperia, combined with lightcurve-based shape models, lead to a diameter estimate, Deff = 110 ± 15 km, approximately 20% smaller than the reported IRAS value. We estimate Hesperia to have a radar albedo of , consistent with a high-metal content. We therefore add 69 Hesperia to the Mm-class (high metal M) (Shepard, M.K., Clark, B.E., Ockert-Bell, M., Nolan, M.C., Howell, E.S., Magri, C., Giorgini, J.D., Benner, L.A.M., Ostro, S.J., Harris, A.W., Warner, B.D., Stephens, R.D., Mueller, M. [2010]. Icarus 208, 221-237), bringing the total number of Mm-class objects to eight; this is 40% of all M-class asteroids observed by radar to date. 相似文献
236.
Fossilization and degradation of intact polar lipids in deep subsurface sediments: A theoretical approach 总被引:1,自引:0,他引:1
Stefan Schouten Jack J. Middelburg Ellen C. Hopmans 《Geochimica et cosmochimica acta》2010,74(13):3806-3814
Intact polar membrane lipids (IPLs) are frequently used as markers for living microbial cells in sedimentary environments. The assumption with these studies is that IPLs are rapidly degraded upon cell lysis and therefore IPLs present in sediments are derived from in situ microbial production. We used a theoretical approach to assess whether IPLs in surface sediments can potentially represent fossilized IPLs derived from the upper part of the water column and whether IPLs can be preserved during sediment burial. Previous studies which examined the degradation kinetics of IPLs show that phospholipids, i.e. ester-linked lipids with a phosphor-containing head group, degrade more rapidly than glycosidic ether lipids, i.e. ether-linked lipids with a glycosidically bound sugar moiety. Based on these studies, we calculate that only a minor fraction of phospholipids but a major fraction of glycosidic ether lipids biosynthesized in the upper part of the water column can potentially reach deep-sea surface sediments. Using a simple model and power law kinetic degradation parameters reported in the literature, we also evaluated the degradation of IPLs during sediment burial. Our model predicts a log-log relationship between IPL concentrations and depth, consistent with what has been observed in studies of IPLs in subsurface sediments. Although our results do not exclude production of IPLs in subsurface sediment, they do suggest that IPLs present in the deep biosphere may contain a substantial fossil component potentially masking in situ IPL production. 相似文献
237.
For the semi-arid environment of southern California, a reach-scale classification system and conceptual model were created
to synthesize the observed floodplain forms into three basic floodplain continuums: armored, non-armored, and active-regional
alluvial fan. These continuums are comprised of three to five alluvial floodplain forms (cascade, step-pool, plane-coarse-bed,
plane-mixed-bed, plane-fine-bed, pool-riffle, braided, and dune-ripple). For the non-armored floodplain continuum, a floodplain
state diagram has been generated to quantitatively describe the natural downstream progression of floodplain forms and geometry,
using specific stream power and the width-to-depth ratio as the state and shape metrics, respectively. Based on field data
for 91 cross sections along 14 watercourses, this floodplain state diagram provided the basis to define geomorphic thresholds
for floodplain instability and braiding using logistic regression analyses. This method for defining floodplain instability
thresholds provides a unique management tool by providing both a systematic and a quantitative means to assess the stability
state of watercourses and, thereby, provide a means for assessing the potential for the hazards associated with watercourse
instability. 相似文献
238.
239.
Jacob Mulder Graham Hagen-Peter Teresa Ubide Rasmus Andreasen Ellen Kooijman Melanie Kielman-Schmitt Yue-Xing Feng Bence Paul Andreas Karlsson Christian Tegner Charles Lesher Fidel Costa 《Geostandards and Geoanalytical Research》2023,47(2):311-336
Laser ablation multi-collector mass spectrometry (LA-MC-ICP-MS) has emerged as the technique of choice for in situ measurements of Sr isotopes in geological minerals. However, the method poses analytical challenges and there is no widely adopted standardised approach to collecting these data or correcting the numerous potential isobaric inferences. Here, we outline practical analytical procedures and data reduction strategies to help establish a consistent framework for collecting and correcting Sr isotope measurements in geological materials by LA-MC-ICP-MS. We characterise a new set of plagioclase reference materials, which are available for distribution to the community, and present a new data reduction scheme for the Iolite software package to correct isobaric interferences for different materials and analytical conditions. Our tests show that a combination of Kr-baseline subtraction, Rb-peak-stripping using βRb derived from a bracketing glass reference material, and a CaCa or CaAr correction for plagioclase and CaCa or CaAr + REE2+ correction for rock glasses, yields the most accurate and precise 87Sr/86Sr measurements for these materials. Using the analytical and correction procedures outlined herein, spot analyses using a beam diameter of 100 μm or rastering with a 50–65 μm diameter beam can readily achieve < 100 ppm 2SE repeatability ("internal") precision for 87Sr/86Sr measurements for materials with < 1000 μg g-1 Sr. 相似文献