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101.
When a sediment laden river reaches a flat basin area the coarse fraction of their sediment load is deposited in a cone shaped structure called an alluvial fan. In this article we used the State Space Soil Production and Assessment Model (SSSPAM) coupled landform–soilscape evolution model to simulate the development of alluvial fans in two- and three-dimensional landforms. In SSSPAM the physical processes of erosion and armouring, soil weathering and sediment deposition were modelled using state-space matrices, in both two and three dimensions. The results of the two-dimensional fan showed that the fan grew vertically and laterally keeping a concave up long profile. It also showed a downstream fining of the sediments along the fan profile. Both of these observations are in agreement with available literature concerning natural and experimental fan formations. Simulations with the three-dimensional landform produced a fan with a semicircular shape with concave up long profiles and concave down cross profiles which is typical for fans found in nature and ones developed in laboratory conditions. During the simulation the main channel which brings sediment to the fan structure changed its position constantly leading to the semicircular shape of the fan. This behaviour is similar to the autogenic process of ‘fanhead trenching’ which is the major mechanism of sediment redistribution while the fan is developing. The three-dimensional fan simulation also exhibited the downstream fining of sediments from the fan apex to the peripheries. Further, the simulated fan also developed complex internal sediment stratification which is modelled by SSSPAM. Currently such complex sediment stratification is thought to be a result of allogenic processes. However, this simulation shows that, such complex internal sediment structures can develop through autogenic processes as well. © 2020 John Wiley & Sons, Ltd. 相似文献
102.
Greg Brown Jonathan Rhodes Daniel Lunney Ross Goldingay Kelly Fielding Nicole Garofano Scott Hetherington Marama Hopkins Jo Green Skye McNamara Angie Brace Lorraine Vass Linda Swankie Clive McAlpine 《Transactions in GIS》2019,23(6):1184-1203
Geographic citizen science has much potential to assist in wildlife research and conservation, but the quality of observation data is a key concern. We examined the effects of sampling design on the quality of spatial data collected for a koala citizen science project in Australia. Data were collected from three samples—volunteers (n = 454), an Internet panel (n = 103), and landowners (n = 35)—to assess spatial data quality, a dimension of citizen science projects rarely considered. The locational accuracy of koala observations among the samples was similar when benchmarked against authoritative data (i.e., an expert‐derived koala distribution model), but there were differences in the quantity of data generated. Fewer koala location data were generated per participant by the Internet panel sample than the volunteer or landowner samples. Spatial preferences for land uses affecting koala conservation were also mapped, with landowners more likely to map locations for residential and tourism development and volunteers less likely. These spatial preferences have the potential to influence the social acceptability of future koala conservation proposals. With careful sampling design, both citizen observations and land use preferences can be included within the same project to augment scientific assessments and identify conservation opportunities and constraints. 相似文献
103.
Greg Egan 《Celestial Mechanics and Dynamical Astronomy》2018,130(1):5
Oks proposes the existence of a new class of stable planetary orbits around binary stars, in the shape of a helix on a conical surface whose axis of symmetry coincides with the interstellar axis, and rotates with the same orbital frequency as the binary pair. We show that this claim relies on the inappropriate use of an effective potential that is only applicable when the stars are held motionless. We also present numerical evidence that the only planetary orbits whose planes are initially orthogonal to the interstellar axis that remain stable on the time scale of the stellar orbit are ordinary polar orbits around one of the stars, and that the perturbations due to the binary companion do not rotate the plane of the orbit to maintain a fixed relationship with the axis. 相似文献
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105.
Pacific water exits the Chukchi Sea shelf through Barrow Canyon in the east and Herald Canyon in the west, forming an eastward-directed shelfbreak boundary current that flows into the Beaufort Sea. Here we summarize the transformation that the Pacific water undergoes in the two canyons, and describe the characteristics and variability of the resulting shelfbreak jet, using recently collected summertime hydrographic data and a year-long mooting data set. In both canyons the northward-flowing Pacific winter water switches from the western to the eastern flank of the canyon, interacting with the northward-flowing summer water. In Barrow canyon the vorticity structure of the current is altered, while in Herald canyon a new water mass mode is created. In both instances hydraulic effects are believed to be partly responsible for the observed changes. The shelfl)reak jet that forms from the canyon outflows has distinct seasonal configurations, from a bottom-intensified flow carrying cold, dense Pacific water in spring, to a surface-intensified current advecting warm, buoyant water in summer. The current also varies significantly on short timescales, from less than a day to a week. In fall and winter much of this mesoscale variability is driven by storm events, whose easterly winds reverse the current and cause upwelling. Different types of eddies are spawned from the current, which are characterized here using hydrographic and satellite data. 相似文献
106.
New osmium (Os) isotope and platinum group element (PGE) concentration data are used in conjunction with published 3He and Th isotope data to determine the relative proportions of lithogenic, extraterrestrial and hydrogenous iridium (Ir) in a Pacific pelagic carbonate sequence from the Ocean Drilling Program (ODP) Site 806 on the Ontong Java Plateau (OJP). These calculations demonstrate that lithogenic and extraterrestrial contributions to sedimentary Ir budget are minor, while hydrogenous Ir accounts for roughly 85% of the total Ir. Application of analogous partitioning calculations to previously reported data from a North Pacific red clay sequence (LL44-GPC3) yields very similar results. Total Ir burial fluxes at Site 806 and LL44-GPC3 are also similar, 45 and 30 pg cm−2 kyr−1, respectively. Average Ir/3He and Ir/xs230Thinitial ratios calculated from the entire Site 806 data set are similar to those reported earlier for Pacific sites. In general, down-core profiles of Ir, 3He and xs230Thinitial, are not well correlated with one another. However, all three data sets show similar variance and yield sediment mass accumulation rate estimates that agree within a factor of two. While these results indicate that Ir concentration has potential as a point-paleoflux tracer in pelagic carbonates, Ir-based paleoflux estimates are likely subject to uncertainties that are similar to those associated with Co-based paleoflux estimates. Consequently, local calibration of Ir flux in space and time will be required to fully assess the potential of Ir as a point paleoflux tracer. Measured 187Os/188Os of the OJP sediments are systematically lower than the inferred 187Os/188Os of contemporaneous seawater and a clear glacial-interglacial 187Os/188Os variation is lacking. Mixing calculations suggest Os contributions from lithogenic sources are insufficient to explain the observed 187Os/188Os variations. The difference between the 187Os/188Os of bulk sediment and that of seawater is interpreted in terms of subtle contributions of unradiogenic Os carried by particulate extraterrestrial material. Down-core variations of 187Os/188Os with Pt/Ir and Os/Ir also point to contributions from extraterrestrial particles. Mixing calculations for each set of several triplicate analyses suggest that the unradiogenic Os end member cannot be characterized by primary extraterrestrial particles of chondritic composition. It is noteworthy that in efforts aimed at determining the effect of extraterrestrial contributions, 187Os/188Os of pelagic carbonates has greater potential compared to abundances of PGE. An attempt has been made for the first time to estimate sediment mass accumulation rates based on amount of extraterrestrial Os in the OJP samples and previously reported extraterrestrial Os flux. Throughout most of the OJP record, Os isotope-based paleoflux estimates are within a factor of two of those derived using other constant flux tracers. Meaningful flux estimates cannot be made during glacial maxima because the OJP sediments do not record the low 187Os/188Os reported previously. We speculate that this discrepancy may be related to focusing of extraterrestrial particles at the OJP, as has been suggested to explain down-core 3He variations. 相似文献
107.
Greg BIGNALL 《Resource Geology》2000,50(2):131-135
Abstract: Exploration drilling at Orakeikorako provides information on the hydrologic and thermal regime of the geothermal system, which is presently as high as 265C. The presence of almandine in Drillhole OK–1, at 1312. 5m drilled depth, is the only known occurrence of hydrothermal garnet in an active geothermal system from New Zealand. The formation temperature of the almandine, is not consistent with measured (bore) temperatures, but does coincide with fluid inclusion data and temperatures inferred from other secondary minerals, which suggest it formed at >240C, and possibly as much as 310C. Almandine is an important mineral geothermometer in active hydrothermal systems, and implications for its occurrence should not be discounted in fossil (mineralised) epithermal-type hydrothermal systems. 相似文献
108.
109.