排序方式： 共有26条查询结果，搜索用时 15 毫秒
21.
Mark S. Fonseca W. Judson Kenworthy Emily Griffith Margaret O. Hall Mark Finkbeiner Susan S. Bell 《Estuarine, Coastal and Shelf Science》2008
The scale of landscape pattern formation of an ecological community may provide clues as to the processes influencing its spatial and temporal dynamics. We conducted an examination of the spatial organization of an annual seagrass (Halophila decipiens) in an open ocean setting at two spatial scales and growing seasons to identify the relative influence of external (hurricanes) versus internal (clonal growth) factors. Visual surveys of seagrass cover were conducted over 2 years within three replicate 1 km2 study areas each separated by ∼25 km in an inshore–offshore transect along the southwest coast of Florida at depths between ∼10 and 30 m. A towed video sled allowed observations of seagrass cover of 1 m2 areas approximately every 6 m over thousands of meters of evenly spaced transects within the study areas (coarse scale). The towed video revealed that 17.5% of the seafloor was disturbed irrespective of location or sample time. Randomly selected 10 × 10 m quadrats within the larger, 1 km2 study areas were completely surveyed for seagrass cover by divers at 0.625 m2 resolution (fine scale). The coarse-scale observations were tested using both conventional geostatistics and an application of a time-series technique (Runs test) for scale of seagrass cover contiguity. Fine-scale observations were examined using conventional geostatistics and a least squares approach (cumulative logistic). 相似文献
22.
We have applied techniques used in rock magnetism to the study of possible temporal changes in provenance and firing conditions of a collection of Etruscan bucchero pottery fragments representing the interval between 800 B.C. to 400 B.C. To our knowledge, this is the first time that such procedures have been applied to ancient ceramic wares. Our preliminary magnetic measurements demonstrate that this approach can provide useful archaeological information, as exemplified by the following results: (1) Bucchero ceramic styles sottile, spesso, and possibly pesante, contained magnetic particles of similar grain sizes, which suggested a common provenance and similar firing conditions for these pot typologies. (2) Ceramic styles buccheroid impasto and grey bucchero each had magnetic characteristics different from those in (1). However, too few samples of these were available for any definite conclusions. (3) The occurrence of magnetite, and not hematite, in all samples indicated that firing conditions were effectively reducing. (4) Variations in the amount of magnetite in different samples indicated that slight inhomogeneities must have existed in the iron oxide/hydroxide content of the local clay source. Most magnetic techniques are fast, easy, and generally nondestructive. Some magnetic properties, such as initial susceptibility, saturation remanent magnetization, and coercivity of remanence, can even be measured in the field. At the least, magnetic measurements can be used initially to discriminate among different groups of potsherds. Selected samples can then be analyzed with more elaborate methods of archaeometry. 相似文献
23.
24.
The effective elastic thickness of the lithosphere in the Williston Basin region has been determined for several time intervals by fitting an elastic flexure equation to the shape of the basin over time. The elastic thickness increases from about 40 km just after the basin began subsiding (450 m.y. B.P.) to about 80 km at the present.These results agree well with the increasing elastic thickness with age predicted for a cooling, 250 km-thick plate (taking elastic thickness to be depth to the 450°C isotherm) if we assume a thermal age of the lithosphere of about 500 m.y. Radiometric dates of basement in the area, however, generally yield ages of 1.7 b.y. (Churchill Province) and 2.5 b.y. (Superior Province).Because basement ages are generally much greater than the inferred lithospheric thermal age in the Williston Basin region, we suggest that lithosphere in this area was thermally rejuvenated, about 500 m.y.B.P., by the same process(es) responsible for the thermal subsidence of the basin. Temperatures were not high enough in the upper crust to reset most radiometric clocks. Additional support for a thermal event 500 m.y.B.P. comes from fission-track ages and a single Rb-Sr age of basement rocks of about 500 m.y.Lithospheric rejuvenation has been modelled using a one-dimensional finite-difference thermal model. The base of a 250 km-thick plate is heated from 1333°C to 1850°C (approximately the solidus of basalt at that depth) for approximately 100 m.y. With conduction only, the depth to the 450°C isotherm slowly decreases, then slowly increases after heating ceases. Rapid thinning and slow thickening of the lithosphere can be achieved only if convective thinning is simulated in the model. The model with convection yields results consistent with the observed increase in lithospheric thickness as well as the observed subsidence of the basin over time. 相似文献
25.
Sediment macropores (with effective diameters larger than 100 μm) comprise 11% of the bulk sediment volume in a tidal freshwater wetland vegetated withPeltandra virginica. In order to determine effects of macroporous sediment structure on solute transport, we conducted a solute tracer experiment in the sediment. The effective transport volume (θeff, the volume of sediment through which solute was transported normalized to sediment bulk volume) was 0.15 cm3 cm?3, which is considerably smaller than the total pore space that is potentially available for transport (porosity of sediment is 0.63 cm3 cm?3). A mean transport time of 13 d was required to flush preferential flow paths inPeltandra hummocks; hydrologic turnover of the volumetrically dominant matrix pores (0.53 cm3 cm?3) was apparently much slower. Based on porewater sampler design and hydrological principles, we suggest that N2-purged tension solution samplers and diffusion equilibrators preferentially sample porewater from macropore and matrix domains, respectively. Dissolved ammonium and orthophosphate concentrations were three-fold higher in matrix pores compared to macropores, which is consistent with our finding that more rapid hydrological flushing occurred in macropores compared to matrix pores. Further evaluation of porewater sampler designs in macroporous sediment is needed to improve studies of hydrologic transport and biogeochemical cycling in wetlands. 相似文献
26.
Seagrasses are typically light limited in many turbid estuarine systems. Light attenuation is due to water and three optically active constituents (OACs): nonalgal particulates, phytoplankton, and colored dissolved organic matter (CDOM). Using radiative transfer modeling, the inherent optical properties (IOPs) of these three OACs were linked to the light attenuation coefficient, K PAR, which was measured in North River, North Carolina, by profiles of photosynthetically active radiation (PAR). Seagrasses in the southern portion of Albemarle-Pamlico Estuarine System (APES), the second largest estuary in the USA, were found to be light limited at depths ranging from 0.87 to 2 m. This corresponds to a range of K PAR from 0.54 to 2.76 m?1 measured during a 24-month monitoring program. Turbidity ranged from 2.20 to 35.55 NTU, chlorophyll a from 1.56 to 15.35 mg m?3, and CDOM absorption at 440 nm from 0.319 to 3.554 m?1. The IOP and water quality data were used to calibrate an existing bio-optical model, which predicted a maximum depth for seagrasses of 1.7 m using annual mean water quality values and a minimum light requirement of 22% surface PAR. The utility of this modeling approach for the management of seagrasses in the APES lies in the identification of which water quality component is most important in driving light attenuation and limiting seagrass depth distribution. The calibrated bio-optical model now enables researchers and managers alike to set water quality targets to achieve desired water column light requirement goals that can be used to set criteria for seagrass habitat protection in North Carolina. 相似文献