Momentum and energy processes during jackhammer operation     

S. S. Pang  W. Goldsmith 《Rock Mechanics and Rock Engineering》1989,22(3):205-229

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HUMAN GEOGRAPHY OF THE AMERICAN TROPICS: A FORTY-YEAR REVIEW     

Kent Mathewson  Ph.D. 《Singapore journal of tropical geography》1994,14(2):123-156

For a decade or so after World War II, human geographers working in the American tropics found socio-political conditions and resultant research topics little changed from those before the war. This was in contrast to the Old World tropics where decolonization processes and the demands of economic development and new nation building produced divergent research currents, For the first half of the period under review, the American tropics continued to be the province largely of geographers with culturalhistorical questions grounded in natural historical bases. The legacy is Humboldt's; the practitioners most notably students of Carl Sauer or German counterparts such as Carl Troll. French and British regionalist approaches, strong in the region before World War II, survived less successfully. By the 1980s, however, broad pan-tropical currents of geographic discourse and debate had become established. New practices and theories were formulated and tested as North Atlantic geographers borrowed from antipodean innovators and others working in the Asian and African tropics. Since the 1980s, there have been greater efforts at dialogue and collaboration with host country colleagues. As might be expected in this era of ‘globalization’, national research styles and agendas have become less evident. This paper offers a highly selective map of research nodes within tropical Americanist geography since the early 1950s. The selection of examples includes qualitative criteria, but more importantly, research that signify stasis or intensification as well as turning points and departures in the overall development of this literature. The contours of this history suggest some highly evolved, even idiosyncratic enterprises, but in the main, it is an unfolding that suggests broad congruencies with human geographic work elsewhere in the tropics.  相似文献   

Organic matter fluxes and marsh stability in a rapidly submerging estuarine marsh     

J. A. Nyman  R. D. DeLaune  S. R. Pezeshki  W. H. Patrick 《Estuaries and Coasts》1995,18(1):207-218

We studied organic matter cycling in two Gulf Coast tidal, nonsaline marsh sites where subsidence causes marine intrusion and rapid submergence, which mimics increased sea-level rise. The sites experienced equally rapid submergence but different degrees of marine intrusion. Vegetation was hummocked and much of the marsh lacked rooted vegetation. Aboveground standing crop and production, as measured by sequential harvesting, were low relative to other Gulf CoastSpartina patens marshes. Soil bulk density was lower than reported for healthyS. alterniflora growth but that may be unimportant at the current, moderate sulfate levels. Belowground production, as measured by sequential harvesting, was extremely fast within hummocks, but much of the marsh received little or no belowground inputs. Aboveground production was slower at the more saline site (681 g m^?2 yr^?1) than at the less saline site (1,252 g m^?2 yr^?1). Belowground production over the entire marsh surface averaged 1,401 g m^?2 yr^?1 at the less saline site and 585 g m^?2 yr^?1 at the more saline site. Respiration, as measured by CO₂ emissions in the field and corrected for CH₄ emissions, was slower at the less saline site (956 g m^?2 yr^?1) than at the more saline site (1,438 g m^?2 yr^?1), reflecting greater contributions byS. alterniflora at the more saline site which is known to decompose more rapidly thanS. patens. Burial of organic matter was faster at the less saline site (796 g m^?2 yr^?1) than at the more saline site (434 g m^?2, yr^?1), likely in response to faster production and slower decomposition at the less saline site. Thus vertical accretion was faster at the less saline site (1.3 cm yr^?1) than at the more saline site (0.85 cm yr^?1); slower vertical accretion increased flooding at the more saline site. More organic matter was available for export at the less saline site (1,377 g m^?2 yr^?1) than at the more saline site (98 g m^?2 yr^?1). These data indicated that organic matter production decreased and burial increased in response to greenhouse-like conditions brought on by subsidence. *** DIRECT SUPPORT *** A01BY069 00016  相似文献   

High precision measurements of sediment elevation in shallow coastal areas using a sedimentation-erosion table   总被引：1，自引：0，他引：1  

Roelof M. J. Boumans  John W. Day 《Estuaries and Coasts》1993,16(2):375-380

The use of a sedimentation-erosion table (SET) for the measurement of small changes in sediment surface elevation in intertidal and shallow subtidal areas is described. The SET provides a constant reference plane in space from which the distance to the sediment surface can be measured by lowering pins to the surface. The precision of the method was determined by repeated measurements in coastal marshes, mudflats, and subtidal areas in Louisiana and Georgia. The confidence interval of the SET is about ±1.5 mm. The SET is more accurate if pins are lowered manually and the sediment surface is determined visually than if pins are lowered by gravity and the sediment surface is determined by soil resistance.  相似文献   

Geochemistry of meteoric calcite cements in some Pleistocene limestones     

ARTHUR H. SALLER  CLYDE H. MOORE  JR 《Sedimentology》1991,38(4):601-621

In this study, the stable isotope and trace element geochemistries of meteoric cements in Pleistocene limestones from Enewetak Atoll (western Pacific Ocean), Cat Island (Bahamas), and Yucatan were characterized to help interpret similar cements in ancient rocks. Meteoric calcite cements have a narrow range of δ¹⁸O values and a broad range of δ¹³C values in each geographical province. These Pleistocene cements were precipitated from water with stable oxygen isotopic compositions similar to modern rainwater in each location. Enewetak calcite cements have a mean δ¹⁸O composition of ?6.5%₀ (PDB) and δ¹³C values ranging from ?9.6 to +0.4%0 (PDB). Sparry calcite cements from Cat Island have a mean δ¹⁸O composition of ?4.1%₀ and δ¹³C values ranging from ?6.3 to + 1.1%₀. Sparry cements from Yucatan have a mean δ¹⁸O composition of ?5.7%₀ and δ¹³C values of ?8.0 to ?2.7%₀. The mean δ¹⁸O values of these Pleistocene meteoric calcite cements vary by 2.4%₀ due to climatic variations not related directly to latitude. The δ¹³C compositions of meteoric cements are distinctly lower than those of the depositional sediments. Variations in δ¹³C are not simply a function of distance below an exposure surface. Meteoric phreatic cements often have δ¹³C compositions of less than —4.0%₀, which suggests that soil-derived CO₂ and organic material were washed into the water table penecontemporaneous with precipitation of phreatic cements. Concentrations of strontium and magnesium are quite variable within and between the three geographical provinces. Mean strontium concentrations for sparry calcite cements are, for Enewetak Atoll, 620 ppm (σ= 510 ppm); for Cat Island, 1200 ppm (σ= 980 ppm); and for Yucatan, 700 ppm (σ= 390 ppm). Equant cements, intraskeletal cements, and Bahamian cements have higher mean strontium concentrations than other cements. Equant and intraskeletal cements probably precipitated in more closed or stagnant aqueous environments. Bahamian depositional sediments had higher strontium concentrations which probably caused high strontium concentrations in their cements. Magnesium concentrations in Pleistocene meteoric cements are similar in samples from Enewetak Atoll (mean =1.00 mol% MgCO₃; σ= 0.60 mol% MgCO₃) and Cat Island (mean = 0.84 mol% MgCO₃; σ= 0.52mol% MgCO₃) but Yucatan samples have higher magnesium concentrations (mean = 2.20 mol% MgCO₃: σ= 0.84mol% MgCO₃). Higher magnesium concentrations in some Yucatan cements probably reflect precipitation in environments where sea water mixed with fresh water.  相似文献   

Tide gauge records,water level rise,and subsidence in the Northern Gulf of Mexico     

R. Eugene Turner 《Estuaries and Coasts》1991,14(2):139-147

Long-term water level changes in the northern Gulf of Mexico were examined using tide gauge records for this century. Strong coherence exists between the annual mean water changes at Galveston, Texas, and (1) the relatively geologically-stable west coast of Florida, (2) global mean sea level, and 93) the subsiding Louisiana coast. Water levels at the Galveston gauge, one of the longest records (81 yr), have risen steadily, but not accelerated over the long-term. The apparent acceleration of water rise in the recent two decades is within the historical pattern, and is probably driven by regional or global, but not local climatic factors. Because eustatic sea level has risen steadily this century, the analysis supports the conclusion that regional geologic subsidence has not varied significantly over the tide gauge record (1909–1988). Variations in the estimates of subsidence in the surface layers are generally consistent with the generally accepted understanding of the geology of deltaic processes on this coast.  相似文献   

A univariate model for long-term streamflow forecasting   总被引：1，自引：0，他引：1  

P. F. Krstanovic  V. P. Singh 《Stochastic Environmental Research and Risk Assessment (SERRA)》1991,5(3):173-188

This paper, the first in a series of two, employs the principle of maximum entropy (POME) via maximum entropy spectral analysis (MESA) to develop a univariate model for long-term streamflow forecasting. Three cases of streamflow forecasting are investigated: forward forecasting, backward forecasting (or reconstruction) and intermittent forecasting (or filling in missing records). Application of the model is discussed in the second paper.  相似文献   

Acid Neutralization by Gulf Coast Sediments     

Linn E. Fink  Winton G. Aubert  William J. Bernard 《Ground water》1991,29(4):489-494

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Factors influencing the salinity regime of Lake Pontchartrain,Lousiana, a shallow coastal lagoon: Analysis of a long-term data set     

Walter B. Sikora  Björn Kjerfve 《Estuaries and Coasts》1985,8(2):170-180

Lake Pontchartrain., Louisiana is a 1,630 km² shallow brackish lagoon with a mean salinity from 1.2‰ in the west to 5.4‰ in the east., The construction of a 120 km long deep-water connection to the Gulf of Mexico in 1963, was expected to cause a 5‰ increase in lagoon salinity. However, the actual increase was everywhere in the lagoon less than 2‰ Analysis of 31 years of daily salinity and discharge records indicates that discharge is the most important factor controlling salinity variations in Lake Pontchartrain., Seventy-four percent of low-frequency salinity variations are explained by freshwater discharge and the completion of the deep-water canal. Lake Pont.-chartrain experiences annual variations of salinity as high as 8‰ This salinity signal has remained constant since the beginning of the salinity records in 1946. It appears that the deep-water canal is not responsible for observed die-back of freshwater swamps and retreat of lagoon shorelines.  相似文献   

Seasonality of materials transport through a coastal freshwater marsh: Riverine versus tidal forcing     

Marlene K. Stern  John W. Day  Kenneth G. Teague 《Estuaries and Coasts》1986,9(4):301-308

Transports of nitrate and suspended solids were measured six times from January 1984 until January 1985 in a small freshwater tidal bayou in south-central Louisiana. The bayou and adjacent marshes are influenced by Atchafalaya River discharges, tides, and coastal weather patterns. Large net ebb-directed water transports occurred in winter, spring, and summer, coincident with high river discharges, indicating riverine dominance. A very small net flood-directed water transport occurred in fall, indicating tidally dominated hydrology. Nitrate and suspended solids transports were net ebb-directed in all seasons, but were two orders of magnitude higher during high river flow. Exports changed as hydrology switched from river dominated to tidally dominated, and as concentrations of materials changed. Comparison of suspended solids and nitrate concentrations in the river and bayou shows that these materials were usually lower in the bayou, indicating retention by the marsh/aquatic system.  相似文献