Macrobenthic assemblages in Calcasieu Lake estuary (Louisiana) were sampled at 11 sites from October 1983 through November 1985. The sites were numerically dominated by subsurface-deposit feeders, consisting mostly of polychaetes. Greater densities of macrofauna were collected at the northern (upper) stations of the lake than were collected in West Cove or the southern stations. Abundances of polychaetes, oligochaetes, and amphipods at the upper lake stations accounted for most of the differences among stations. The numerical dominance by detritivores (97% of the fauna) and lack of strong sediment or salinity gradients across the estuary, resulted in an absence of temporal pattern in trophic structure of the macrofauna. 相似文献
Sheet structure, or large-scale exfoliation, is the division of a rock mass into lenses, plates or “sheets” approximately parallel to the earth's surface. Sheet fractures, which separate the plates, are characterized by surface markings resembling those formed during the brittle fracture of metals, glass and ceramics and those on joints in rock. Lineations, or hackle marks, parallel to the direction of fracture propagation, are common. Branching occurs during propagation so that the fractures appear as many echelon segments in profile. The opposing surfaces of sheet fractures observed in quarry walls and natural exposures are typically in contact. Damage, if any, to rock adjacent to sheet fractures is generally limited to a zone less than one centimeter wide. Sheets tend to parallel preferred orientations of microcracks in many rock masses. Most sheets that are exposed today are of prehistoric origin, but in some rock masses, such as Chelmsford granite in Massachusetts, new sheets continue to form.
Sheet structure forms in environments of high differential stress, dominated by large-scale compression parallel to an exposed rock surface. In parts of the Chelmsford granite the magnitude of this compression is greater than 30 MPa in the zone of sheeting. High differential stresses near the ground surface can result from several natural agents, including contemporary tectonic forces, vertical unloading of a rock mass that formed at depth under high triaxial compression, and suppression of expansion that would otherwise result from temperature increases or chemical alteration of the rock. Exfoliation of boulders during forest fires and spheroidal weathering of boulders appears to occur under similar states of stress, although the agents responsible for the stresses differ. 相似文献
Experiments (P=6.9 kb; T=900–1000°C) on four crustal xenoliths from Kilbourne Hole demonstrate the varying melting behavior of relatively dry crustal lithologies in the region. Granodioritic gneisses (samples KH-8 and KH-11) yield little melt (<5–25%) by 925°C, but undergo extensive (30–50%) melting between 950 and 1000°C. A dioritic charnockite (KH-9) begins to melt, with the consumption of all modal K-feldspar, by 900°C. It is as fertile a melt source as the granodiorites at lower temperatures, but is outstripped in melt production by the granodiorite gneisses at high temperature, yielding only 26% melt by 1000°C. A pelitic granulite (KH-12) proved to be refractory (confirming earlier predictions based on geochemistry) and did not yield significant melt even at 1000°C. All melts have the composition of metaluminous to slightly peraluminous granites and are unlikely to be individually recognizable as magma contaminants on the basis of major element chemistry. However, the relative stability of K-feldspar during partial melting will produce recognizable signatures in Ba, Eu, K/Ba, and Ba/Rb. Melts of KH-11, which retains substantial K-feldspar throughout the melting interval, are generally low in Ba (<500–800 ppm), have high K/Ba and low Ba/Rb (est.) (62–124 and 1–3, respectively). Melts of KH-9, in which all K-feldspar disappears with the onset of melting, are Ba-rich [2000–2600 ppm, K/Ba=16–22; Ba/Rb (est.) =25–47]. Melts of KH-8 have variable Ba contents; <500 ppm Ba at low temperature but >900 ppm Ba in high-temperature melts coexisting with a K-feldspar-free restite. Although REE were not measured in either feldspar or melt, the high Kspar/melt Kds for Eu suggests that the melts coexisting with K-feldspar will have strong negative Eu anomalies. Isotopic and trace element models for magma contamination need to take into account the melting behavior of isotopic reservoirs. For example, the most radiogenic (and incompatible element-rich) sample examined here (the pelitic granulite,87Sr/86Sr=0.757) is refractory, while samples with far less radiogenic Sr (87Sr/86Sr=0.708-0.732) produced substantial melt. This suggests that, in this area, the isotopic signature of contamination may be more subtle than expected. The experimental results can be used to model the petrogenesis of Oligocene volcanic rocks exposed 150 km to the NW of Kilbourne Hole, in the Black Range in the Mogollon-Datil volcanic field. The experimental results suggest that a crustal melting origin for the Kneeling Nun and Caballo Blanco Tuffs is unlikely, even though such an interpretation is permitted by Sr isotopes. Curstal contamination of a mantle-derived magma best explains the chemical and isotopic characteristics of these tuffs. Both experimental and geochemical data suggest that the rhyolites of Moccasin John Canyon and Diamond Creek could represent direct melts of granodiorite basement similar, but not identical, to the Kilbourne Hole granodiorites, perhaps slightly modified by crystal fractionation. The absence of volcanic rocks having87Sr/86Sr>0.74 in the region is consistent with the refractory character of the pelitic granulite. 相似文献
Nitrate concentrations have increased twofold in the Mississippi River during the past three decades. The increased nitrogen loading to the Louisiana shelf has been postulated as a factor leading to eutrophication and the subsequent development of hypoxia west of the Mississippi River delta. While ratios of nitrogen:phosphorus and nitrogen:silica are relatively high in surface waters on the western Louisiana shelf, nitrogen has been posed as the ‘limiting’ nutrient in this region. Bioassays were performed with nutrient additions to surface waters collected from the Louisiana shelf to examine the potential for specific nutrient limitation. Experiments were conducted in March and September 1991, and May 1992. The growth responses of natural and cultured phytoplankton populations were determined by measuring the time course of in vivo and 3-(3,4 dichlorophenyl)-1, 1-dimethylurea (DCMU)-induced fluorescence, as well as initial and final chlorophylla concentrations. The results suggest that phosphate and silicate potentially limit phytoplankton growth during the winter-spring, particularly at low salinities. In late summer, in contrast, nitrogen limitation may be prominent at higher salinities. 相似文献