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1.
Stable oxygen isotope analysis and measurement of several dissolved cations and anions of bulk meltwater samples have provided information about the hydrochemical environment of the glacial hydrological system at Imersuaq Glacier, an outlet tongue from the Greenland ice‐sheet, West Greenland. The samples were collected at frequent intervals during the period 20–28 July 2000 in a small (<20 L s?1) englacial meltwater outlet at the glacier margin. The results document the following findings: (i) a marked diurnal variation of δ18O is related to the composition of oxygen isotope provenances, mainly near‐marginal local superimposed ice and basal up‐sheared ice further up‐glacier; (ii) a relationship is seen between all base cations (Na+, K+, Ca2+, Mg2+), SO42? and δ18O, indicating that solute acquisition is provided by solid–solution contact with the up‐sheared ice—as the relationship with Cl? is weak the influence of seasalt‐derived solutes is small in the area; (iii) when the melt rate is high, two diurnal maxima of δ18O values and solute concentrations are measured, and it is suggested that a snow meltwater component is responsible for the second maximum of δ18O—a short residence time leads to a delayed decrease in ion concentrations. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
2.
The ionospheric eclipse factor method (IEFM) and its application to determining the ionospheric delay for GPS 总被引:4,自引:1,他引:3
A new method for modeling the ionospheric delay using global positioning system (GPS) data is proposed, called the ionospheric
eclipse factor method (IEFM). It is based on establishing a concept referred to as the ionospheric eclipse factor (IEF) λ
of the ionospheric pierce point (IPP) and the IEF’s influence factor (IFF) . The IEF can be used to make a relatively precise distinction between ionospheric daytime and nighttime, whereas the IFF
is advantageous for describing the IEF’s variations with day, month, season and year, associated with seasonal variations
of total electron content (TEC) of the ionosphere. By combining λ and with the local time t of IPP, the IEFM has the ability to precisely distinguish between ionospheric daytime and nighttime, as well as efficiently
combine them during different seasons or months over a year at the IPP. The IEFM-based ionospheric delay estimates are validated
by combining an absolute positioning mode with several ionospheric delay correction models or algorithms, using GPS data at
an international Global Navigation Satellite System (GNSS) service (IGS) station (WTZR). Our results indicate that the IEFM
may further improve ionospheric delay modeling using GPS data. 相似文献
3.
4.
Hui Jiang Mads F. Knudsen Marit‐Solveig Seidenkrantz Meixun Zhao Longbin Sha Lihua Ran 《Boreas: An International Journal of Quaternary Research》2014,43(1):208-219
We present a new reconstruction of summer sea‐surface salinity (SSS) over the past 15 000 years based on a diatom record from piston core 17940, located on the northern slope of the South China Sea (SCS). The reconstructed diatom‐based summer SSS values for the modern period are in accord with instrumental observations of summer SSS in the area. Here, the modern summer SSS is primarily controlled by river runoff, in particular from the Pearl River. The reconstruction presented in this study shows that the summer SSS varied between 33.3 and 34.2 psu over the past 15 000 years. The long‐term summer SSS trend closely followed the trend of the orbitally controlled solar insolation at 20°N, suggesting that orbital forcing was the dominant driver of changes in summer SSS in this area. Comparisons to speleothem δ18O data and studies of surface hydrography in the region suggest that changes in solar insolation affected the summer SSS through changes in the East Asian Monsoon and sea‐level changes associated with the last deglaciation. Univariate spectral analyses indicate that centennial‐scale oscillatory variations in summer SSS were superimposed on the long‐term trend. During the deglacial period (c. 12 000–9000 cal. a BP), the dominant periodicity was centred around 230–250 years, whereas a ~350‐year oscillation dominated in the period 2200–4500 cal. a BP. The balance of evidence suggests that these centennial‐scale changes in summer SSS may have been driven by solar‐induced changes in the East Asian Monsoon, but further evidence is needed to firmly establish this relationship. 相似文献
5.
R.I. Waller A.J. Russell T.A.G.P. van Dijk & Ó. Knudsen 《Geografiska Annaler: Series A, Physical Geography》2001,83(1-2):29-38
During the initial stages of the November 1996 jökulhlaup at Skeiðarárjökull, Iceland, floodwaters burst onto the glacier surface via a series of fractures. This supraglacial drainage led to the formation of a number of distinct ice surface depressions, one of which is investigated in detail. The morphology and structural characteristics of this feature are described, as well as the sedimentology of an associated assemblage of debris-filled fractures. This work suggests that debris-charged subglacial floodwaters travelled up to the glacier surface, where supraglacial flow occurred initially via an extensive network of fractures, orientated parallel to the glacier margin. Supraglacial discharge became progressively more focused into a series of discrete outlets, leading to the mechanical erosion of a number of depressions on the glacier surface. The associated transfer of subglacially derived floodwaters to high levels within the glacier resulted in the rapid entrainment of large volumes of sediment which may influence the patterns, processes and products of ice-marginal sedimentation in the future. 相似文献
6.
7.
Noah Planavsky Andrey Bekker Balz Kamber Andrew Knudsen 《Geochimica et cosmochimica acta》2010,74(22):6387-6405
The ocean and atmosphere were largely anoxic in the early Precambrian, resulting in an Fe cycle that was dramatically different than today’s. Extremely Fe-rich sedimentary deposits—i.e., Fe formations—are the most conspicuous manifestation of this distinct Fe cycle. Rare Earth Element (REE) systematics have long been used as a tool to understand the origin of Fe formations and the corresponding chemistry of the ancient ocean. However, many earlier REE studies of Fe formations have drawn ambiguous conclusions, partially due to analytical limitations and sampling from severely altered units. Here, we present new chemical analyses of Fe formation samples from 18 units, ranging in age from ca. 3.0 to 1.8 billion years old (Ga), which allow a reevaluation of the depositional mechanisms and significance of Precambrian Fe formations. There are several temporal trends in our REE and Y dataset that reflect shifts in marine redox conditions. In general, Archean Fe formations do not display significant shale-normalized negative Ce anomalies, and only Fe formations younger than 1.9 Ga display prominent positive Ce anomalies. Low Y/Ho ratios and high shale-normalized light to heavy REE (LREE/HREE) ratios are also present in ca. 1.9 Ga and younger Fe formations but are essentially absent in their Archean counterparts. These marked differences in Paleoproterozoic versus Archean REE + Y patterns can be explained in terms of varying REE cycling in the water column.Similar to modern redox-stratified basins, the REE + Y patterns in late Paleoproterozoic Fe formations record evidence of a shuttle of metal and Ce oxides across the redoxcline from oxic shallow seawater to deeper anoxic waters. Oxide dissolution—mainly of Mn oxides—in an anoxic water column lowers the dissolved Y/Ho ratio, raises the light to heavy REE ratio, and increases the concentration of Ce relative to the neighboring REE (La and Pr). Fe oxides precipitating at or near the chemocline will capture these REE anomalies and thus evidence for this oxide shuttle. In contrast, Archean Fe formations do not display REE + Y patterns indicative of an oxide shuttle, which implies an absence of a distinct Mn redoxcline prior to the rise of atmospheric oxygen in the early Paleoproterozoic. As further evidence for reducing conditions in shallow-water environments of the Archean ocean, REE data for carbonates deposited on shallow-water Archean carbonate platforms that stratigraphically underlie Fe formations also lack negative Ce anomalies. These results question classical models for deposition of Archean Fe formations that invoke oxidation by free oxygen at or above a redoxcline. In contrast, we add to growing evidence that metabolic Fe oxidation is a more likely oxidative mechanism for these Fe formations, implying that the Fe distribution in Archean oceans could have been controlled by microbial Fe uptake rather than the oxidative potential of shallow-marine environments. 相似文献
8.
Using a quasi-two-dimensional model of the Venus ionosphere, we calculated the ion number densities and horizontal ion bulk velocities expected for a range of solar zenith angles near the terminator (80 to 100°), and compared them with data obtained from the Pioneer Venus Orbiter retarding potential analyzer. The calculated ion bulk velocity arises entirely from the solar EUV-induced plasma pressure gradient and has a magnitude consistent with observations; ionization by suprathermal electrons is neglected in those computations. We find that while photoionization is the dominant source of ionospheric plasma for solar zenith angles less than 92°, plasma transport from the dayside is the dominant plasma source for solar zenith angles greater than 95°. We also show that the main nightside plasma peak at approximately 140 km altitude is of the F2 type (i.e., is diffusion controlled). Its altitude and shape are thus quite insensitive to the altitude of the ion source. 相似文献
9.
10.
Theoretical studies of glacial outburst floods (jökulhlaups) assume that: (i) intraglacial floodwater is transported efficiently in isolated conduits; (ii) intraglacial conduit enlargement operates proportionally to increasing discharge; (iii) floodwater exits glaciers through pre‐existing ice‐marginal outlets; and (iv) the morphology and positioning of outlets remains fixed during flooding. Direct field observations, together with historical jökulhlaup accounts, confirm that these theoretical assumptions are not always correct. This paper presents new evidence for spatial and temporal changes in intraglacial floodwater routing during jökulhlaups; secondly, it identifies and explains the mechanisms controlling the position and morphology of supraglacial jökulhlaup outlets; and finally, it presents a conceptual model of the controls on supraglacial outbursts. Field observations are presented from two Icelandic glaciers, Skeiðarárjökull and Sólheimajökull. Video footage and aerial photographs, taken before, during and after the Skeiðarárjökull jökulhlaup and immediately after the Sólheimajökull jökulhlaup, reveal changes in floodwater routing and the positioning and morphology of outlets. Field observations confirm that glaciers cannot transmit floodwater as efficiently as previously assumed. Rapid increases in jökulhlaup discharge generate basal hydraulic pressures in excess of ice overburden. Under these circumstances, floodwater can be forced through the surface of glaciers, leading to the development of a range of supraglacial outlets. The rate of increase in hydraulic pressure strongly influences the type of supraglacial outlet that can develop. Steady increases in basal hydraulic pressure can retro‐feed pre‐existing englacial drainage, whereas transient increases in pressure can generate hydraulic fracturing. The position and morphology of supraglacial outlets provide important controls on the spatial and temporal impact of flooding. The development of supraglacial jökulhlaup outlets provides a new mechanism for rapid englacial debris entrainment. Copyright © 2000 John Wiley & Sons, Ltd. 相似文献