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211.
A first-order leveling survey across the northeast part of the Yellowstone caldera in September 1998 showed that the central
caldera floor near Le Hardy Rapids rose 24±5 mm relative to the caldera rim at Lake Butte since the previous survey in September
1995. Annual surveys along the same traverse from 1985 to 1995 tracked progressive subsidence near Le Hardy Rapids at an average
rate of –19±1 mm/year. Earlier, less frequent surveys measured net uplift in the same area during 1923–1976 (14±1 mm/year)
and 1976–1984 (22±1 mm/year). The resumption of uplift following a decade of subsidence was first detected by satellite synthetic
aperture radar interferometry, which revealed approximately 15 mm of uplift in the vicinity of Le Hardy Rapids from July 1995
to June 1997. Radar interferograms show that the center of subsidence shifted from the Sour Creek resurgent dome in the northeast
part of the caldera during August 1992 to June 1993 to the Mallard Lake resurgent dome in the southwest part during June 1993
to August 1995. Uplift began at the Sour Creek dome during August 1995 to September 1996 and spread to the Mallard Lake dome
by June 1997. The rapidity of these changes and the spatial pattern of surface deformation suggest that ground movements are
caused at least in part by accumulation and migration of fluids in two sill-like bodies at 5–10 km depth, near the interface
between Yellowstone's magmatic and deep hydrothermal systems.
Received: 30 November 1998 / Accepted: 16 April 1999 相似文献
212.
Characteristics of Danish estuaries 总被引:2,自引:0,他引:2
Daniel J. Conley Hanne Kaas Flemming Møhlenberg Bjarke Rasmussen Jørgen Windolf 《Estuaries and Coasts》2000,23(6):820-837
We review various aspects of the structure and functioning of Danish estuaries from data collected by the National Monitoring Program and from information in published sources. We present data on the physical, chemical, and biological characteristics of estuaries in Denmark, we evaluate the functioning of these systems as filters and transformers of nutrients and we evaluate the outlook for Danish estuaries in the future. Danish estuarine systems are for the most part shallow (<3 m deep), have short residence times, and tend to be heavily loaded with nutrients primarily from agricultural sources. Total average loads from land per unit watershed area are 112 kg P km?2 yr?1 and 2,400 kg N km?2 yr?1 during the period 1989–1995. The total phosphorus (TP) load in estuaries has been significantly reduced over the last decade, following implementation of the 1987 Action Plan for the Aquatic Environment (Vandmiljøplan in Danish) that prescribed that nitrogen loads to the total aquatic environment should be reduced by 50% and phosphorus loads by 80%. Reductions in the total nitrogen (TN) load have been more modest. Nutrient loading is one of the primary determinants of estuarine nutrient concentration with 70% of the annual variation in TN concentration and 55% of the annual variation in TP concentration explained by variation in the load. Many Danish estuaries have rich communities of macrophytes and benthic filter feeders, such asMytlis edulis andCiona intestinalis, that can control water column chlorophyll concentrations by their filter feeding activities. Many of the estuaries experience hypoxia and anoxia, especially during warm and calm summer months. Further reductions in nutrient loading are expected following implementation of the Action Plan for the Aquatic Environment II, with predicted improvements in oxygen concentrations and in the functioning of these shallow, dynamic estuarine systems. 相似文献
213.
William W. ChadwickJr Sigurjon Jónsson Dennis J. Geist Michael Poland Daniel J. Johnson Spencer Batt Karen S. Harpp Andres Ruiz 《Bulletin of Volcanology》2011,73(6):679-697
The May 2005 eruption of Fernandina volcano, Galápagos, occurred along circumferential fissures parallel to the caldera rim
and fed lava flows down the steep southwestern slope of the volcano for several weeks. This was the first circumferential
dike intrusion ever observed by both InSAR and GPS measurements and thus provides an opportunity to determine the subsurface
geometry of these enigmatic structures that are common on Galápagos volcanoes but are rare elsewhere. Pre- and post- eruption
ground deformation between 2002 and 2006 can be modeled by the inflation of two separate magma reservoirs beneath the caldera:
a shallow sill at ~1 km depth and a deeper point-source at ~5 km depth, and we infer that this system also existed at the
time of the 2005 eruption. The co-eruption deformation is dominated by uplift near the 2005 eruptive fissures, superimposed
on a broad subsidence centered on the caldera. Modeling of the co-eruption deformation was performed by including various
combinations of planar dislocations to simulate the 2005 circumferential dike intrusion. We found that a single planar dike
could not match both the InSAR and GPS data. Our best-fit model includes three planar dikes connected along hinge lines to
simulate a curved concave shell that is steeply dipping (~45–60°) toward the caldera at the surface and more gently dipping
(~12–14°) at depth where it connects to the horizontal sub-caldera sill. The shallow sill is underlain by the deep point source.
The geometry of this modeled magmatic system is consistent with the petrology of Fernandina lavas, which suggest that circumferential
eruptions tap the shallowest parts of the system, whereas radial eruptions are fed from deeper levels. The recent history
of eruptions at Fernandina is also consistent with the idea that circumferential and radial intrusions are sometimes in a
stress-feedback relationship and alternate in time with one another. 相似文献
214.
Christopher Packham Keith L. Thompson Almudena Zurita Johan H. Knapen Ian Smail Robert Greimel Daniel F. M. Folha Chris Benn rew Humphrey Rene Rutten David Ciardi Matthieu Bec Richard Bingham Simon Craig Kevin Dee Derek Ives Paul Jolley Peter Moore Marti Pi i Puig Simon Rees Gordon Talbot Sue Worswick 《Monthly notices of the Royal Astronomical Society》2003,345(2):395-405
215.
Abstract— We report new 39Ar‐40Ar measurements on 15 plagioclase, pyroxene, and/or whole rock samples of 8 Martian shergottites. All age spectra suggest ages older than the meteorite formation ages, as defined by Sm‐Nd and Rb‐Sr isochrons. Employing isochron plots, only Los Angeles plagioclase and possibly Northwest Africa (NWA) 3171 plagioclase give ages in agreement with their formation ages. Isochrons for all shergottite samples reveal the presence of trapped Martian 40Ar (40Arxs), which exists in variable amounts in different lattice locations. Some 40Arxs is uniformly distributed throughout the lattice, resulting in a positive isochron intercept, and other 40Arxs occurs in association with K‐bearing minerals and increases the isochron slope. These samples demonstrate situations where linear Ar isochrons give false ages that are too old. After subtracting 40Ar*that would accumulate by 40K decay since meteorite formation and small amounts of terrestrial 40Ar, all young age samples give similar 40Arxs concentrations of ?1–2 × 10?6cm3/g, but a variation in K content by a factor of ?80. Previously reported NASA Johnson Space Center data for Zagami, Shergotty, Yamato (Y‐) 000097, Y‐793605, and Queen Alexandra Range (QUE) 94201 shergottites show similar concentrations of 40Arxs to the new meteorite data reported here. Similar 40Arxs in different minerals and meteorites cannot be explained as arising from Martian atmosphere carried in strongly shocked phases such as melt veins. We invoke the explanation given by Bogard and Park (2008) for Zagami, that this 40Arxs in shergottites was acquired from the magma. Similarity in 40Arxs among shergottites may reveal common magma sources and/or similar magma generation and emplacement processes. 相似文献
216.
Miquel Poyatos‐Mor George D. Jones Rufus L. Brunt Daniel E. Tek David M. Hodgson Stephen S. Flint 《Basin Research》2019,31(5):920-947
Exhumed basin margin‐scale clinothems provide important archives for understanding process interactions and reconstructing the physiography of sedimentary basins. However, studies of coeval shelf through slope to basin‐floor deposits are rarely documented, mainly due to outcrop or subsurface dataset limitations. Unit G from the Laingsburg depocentre (Karoo Basin, South Africa) is a rare example of a complete basin margin scale clinothem (>60 km long, 200 m‐high), with >10 km of depositional strike control, which allows a quasi‐3D study of a preserved shelf‐slope‐basin floor transition over a ca. 1,200 km2 area. Sand‐prone, wave‐influenced topset deposits close to the shelf‐edge rollover zone can be physically mapped down dip for ca. 10 km as they thicken and transition into heterolithic foreset/slope deposits. These deposits progressively fine and thin over tens of km farther down dip into sand‐starved bottomset/basin‐floor deposits. Only a few km along strike, the coeval foreset/slope deposits are bypass‐dominated with incisional features interpreted as minor slope conduits/gullies. The margin here is steeper, more channelized and records a stepped profile with evidence of sand‐filled intraslope topography, a preserved base‐of‐slope transition zone and sand‐rich bottomset/basin‐floor deposits. Unit G is interpreted as part of a composite depositional sequence that records a change in basin margin style from an underlying incised slope with large sand‐rich basin‐floor fans to an overlying accretion‐dominated shelf with limited sand supply to the slope and basin floor. The change in margin style is accompanied with decreased clinoform height/slope and increased shelf width. This is interpreted to reflect a transition in subsidence style from regional sag, driven by dynamic topography/inherited basement configuration, to early foreland basin flexural loading. Results of this study caution against reconstructing basin margin successions from partial datasets without accounting for temporal and spatial physiographic changes, with potential implications on predictive basin evolution models. 相似文献
217.
Jonathan A. Warrick David M. Rubin Peter Ruggiero Jodi N. Harney Amy E. Draut Daniel Buscombe 《地球表面变化过程与地形》2009,34(13):1811-1821
A new application of the autocorrelation grain size analysis technique for mixed to coarse sediment settings has been investigated. Photographs of sand‐ to boulder‐sized sediment along the Elwha River delta beach were taken from approximately 1·2 m above the ground surface, and detailed grain size measurements were made from 32 of these sites for calibration and validation. Digital photographs were found to provide accurate estimates of the long and intermediate axes of the surface sediment (r2 > 0·98), but poor estimates of the short axes (r2 = 0·68), suggesting that these short axes were naturally oriented in the vertical dimension. The autocorrelation method was successfully applied resulting in total irreducible error of 14% over a range of mean grain sizes of 1 to 200 mm. Compared with reported edge and object‐detection results, it is noted that the autocorrelation method presented here has lower error and can be applied to a much broader range of mean grain sizes without altering the physical set‐up of the camera (~200‐fold versus ~6‐fold). The approach is considerably less sensitive to lighting conditions than object‐detection methods, although autocorrelation estimates do improve when measures are taken to shade sediments from direct sunlight. The effects of wet and dry conditions are also evaluated and discussed. The technique provides an estimate of grain size sorting from the easily calculated autocorrelation standard error, which is correlated with the graphical standard deviation at an r2 of 0·69. The technique is transferable to other sites when calibrated with linear corrections based on photo‐based measurements, as shown by excellent grain‐size analysis results (r2 = 0·97, irreducible error = 16%) from samples from the mixed grain size beaches of Kachemak Bay, Alaska. Thus, a method has been developed to measure mean grain size and sorting properties of coarse sediments. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
218.
Darrell S. Kaufman Yarrow Axford R. Scott Anderson Scott F. Lamoureux Daniel E. Schindler Ian R. Walker Al Werner 《Journal of Paleolimnology》2012,48(1):9-26
Sediment cores from Lone Spruce Pond (60.007°N, 159.143°W), southwestern Alaska, record paleoenvironmental changes during the global Last Glacial Maximum (LGM), and during the last 14,500 calendar years BP (14.5?cal?ka). We analyzed the abundance of organic matter, biogenic silica, carbon, and nitrogen, and the isotope ratios of C and N, magnetic susceptibility, and grain-size distribution of bulk sediment, abundance of alder shrub (Alnus) pollen, and midge (Chironomidae and Chaoboridae) assemblages in a 4.7-m-long sediment sequence from the depocenter at 22?m water depth. The basal unit contains macrofossils dating to 25?C21?cal?ka (the global LGM), and is interpreted as glacial-lacustrine sediment. The open water requires that the outlet of the Ahklun Mountain ice cap had retreated to within 6?km of the range crest. In addition to cladocerans and diatoms, the glacial-lacustrine mud contains chironomids consistent with deep, oligotrophic conditions; several taxa associated with relatively warm conditions are present, suggestive of relative warmth during the global LGM. The glacial-lacustrine unit is separated from the overlying non-glacial lake sediment by a possible disconformity, which might record a readvance of glacier ice. Non-glacial sediment began accumulating around 14.5?cal?ka, with high flux of mineral matter and fluctuating physical and biological properties through the global deglacial period, including a reversal in biogenic-silica (BSi) content during the Younger Dryas (YD). During the global deglacial interval, the ??13C values of lake sediment were higher relative to other periods, consistent with low C:N ratios (8), and suggesting a dominant atmospheric CO2 source of C for phytoplankton. Concentrations of aquatic faunal remains (chironomids and Cladocera) were low throughout the deglacial interval, diversity was low and warm-indicator taxa were absent. Higher production and air temperatures are inferred following the YD, when bulk organic-matter (OM) content (LOI 550?°C) increased substantially and permanently, from 10 to 30?%, a trend paralleled by an increase in C and N abundance, an increase in C:N ratio (to about 12), and a decrease in ??13C of sediment. Post-YD warming is marked by a rapid shift in the midge assemblage. Between 8.9 and 8.5?cal?ka, Alnus pollen tripled (25?C75?%), followed by the near-tripling of BSi (7?C19?%) by 8.2?cal?ka, and ??15N began a steady rise, reflecting the buildup of N and an increase in denitrification in soils. Several chironomid taxa indicative of relatively warm conditions were present throughout the Holocene. Quantitative chironomid-based temperature inferences are complicated by the expansion of Alnus and resulting changes in lake nutrient status and production; these changes were associated with an abrupt increase in cladoceran abundance and persistent shift in the chironomid assemblage. During the last 2,000?years, chironomid-assemblage changes suggest cooler temperatures, and BSi and OM values were generally lower than their maximum Holocene values, with minima during the seventh and eighth centuries, and again during the eighteenth century. 相似文献
219.
To reconstruct oceanographic variations in the subtropical South Pacific, 271-year long subseasonal time series of Sr/Ca and δ18O were generated from a coral growing at Rarotonga (21.5°S, 159.5°W). In this case, coral Sr/Ca appears to be an excellent proxy for sea surface temperature (SST) and coral δ18O is a function of both SST and seawater δ18O composition (δ18Osw). Here, we focus on extracting the δ18Osw signal from these proxy records. A method is presented assuming that coral Sr/Ca is solely a function of SST and that coral δ18O is a function of both SST and δ18Osw. This method separates the effects of δ18Osw from SST by breaking the instantaneous changes of coral δ18O into separate contributions by instantaneous SST and δ18Osw changes, respectively. The results show that on average δ18Osw at Rarotonga explains ∼39% of the variance in δ18O and that variations in SST explains the remaining ∼61% of δ18O variance. Reconstructed δ18Osw shows systematic increases in summer months (December-February) consistent with the regional pattern of variations in precipitation and evaporation. The δ18Osw also shows a positive linear correlation with satellite-derived estimated salinity for the period 1980 to 1997 (r = 0.72). This linear correlation between reconstructed δ18Osw and salinity makes it possible to use the reconstructed δ18Osw to estimate the past interannual and decadal salinity changes in this region. Comparisons of coral δ18O and δ18Osw at Rarotonga with the Pacific decadal oscillation index suggest that the decadal and interdecadal salinity and SST variability at Rarotonga appears to be related to basin-scale decadal variability in the Pacific. 相似文献
220.
Various network model creation algorithms have been introduced to demonstrate a better approximation of the actual walking pattern and to provide a better wayfinding guide. However, it is under‐investigated which algorithm creates the most appropriate indoor navigation network model in the context of wayfinding applications. Due to the lack of discussion, some studies unconsciously extended an algorithm designed for creating an outdoor navigation network model to indoor space applications. This is problematic because indoor space has different spatial contexts from outdoor space, such as non‐linear space and no‐designated walking space. Our solution is to select five well‐known algorithms that have been introduced, to reproduce the algorithm for the automated construction of the indoor navigation network model, and to evaluate the applicability of algorithms for indoor wayfinding applications. This article compares the quality of wayfinding results from the output of the indoor navigation network model against two criteria: route efficiency (i.e., length) and route simplicity (i.e., number of directions). Our statistical analysis illustrates that the visibility graph algorithm is the most appropriate for indoor wayfinding applications. 相似文献