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91.
Nikolaus Gussone Gerald Langer Markus Geisen Blair A. Steel Ulf Riebesell 《Earth and Planetary Science Letters》2007,260(3-4):505-515
Four species of marine calcifying algae, the coccolithophores Calcidiscus leptoporus, Helicosphaera carteri, Syracosphaera pulchra and Umbilicosphaera foliosa were grown in laboratory cultures under temperatures varying between 14 and 23 °C, and one species, C. leptoporus, under varying [CO32−], ranging from 105 to 219 μmol/kg. Calcium isotope compositions of the coccoliths resemble in both absolute fractionation and temperature sensitivity previous calibrations of marine calcifying species e.g. Emiliania huxleyi (coccolithophores) and Orbulina universa (planktonic foraminifera) as well as inorganically precipitated CaCO3, but also reveal small species specific differences. In contrast to inorganically precipitated calcite, but similar to E. huxleyi and O. universa, the carbonate ion concentration of the medium has no statistically significant influence on the Ca isotope fractionation of C. leptoporus coccoliths; however, combined data of E. huxleyi and C. leptoporus indicate that the observed trends might be related to changes of the calcite saturation state of the medium. Since coccoliths constitute a significant portion of the global oceanic CaCO3 export production, the Ca isotope fractionation in these biogenic structures is important for defining the isotopic composition of the Ca sink of the ocean, one of the key parameters for modelling changes to the marine Ca budget over time. For the present ocean our results are in general agreement with the previously postulated and applied mean value of the oceanic Ca sink (Δsed) of about − 1.3‰, but the observed inter- and intra-species differences point to possible changes in Δsed through earth history, due to changing physico-chemical conditions of the ocean and shifts in floral and faunal assemblages. 相似文献
92.
Yongping YUAN Ronald BINGNER Randall WILLIAMS Richard LOWRANCE David BOSCH Joe SHERIDAN 《国际泥沙研究》2007,22(1):60-69
The United States Department of Agriculture (USDA) Annualized Agricultural Non-Point Source Pollution model (AnnAGNPS) is used to help evaluate a watershed response to agricultural management practices to control water quality. However, AnnAGNPS version 3.5 does not contain features to estimate the effect of a riparian buffer (RB) system on water quality. The Riparian Ecosystem Management Model (REMM) is used to simulate the impact of riparian buffer systems on water quality. However, frequently the lack of measured upland loadings that are required by REMM simulation limits the application of REMM. To address this data gap, a study was conducted to integrate AnnAGNPS with REMM for RB system simulation. AnnAGNPS was used to simulate water and sediment loadings from an upland field into a three-zone RB system at the Gibbs Farm located in the Georgia coastal plain. These AnnAGNPS outputs were used as the inputs to REMM. REMM was used to simulate water and sediment movement along the riparian buffers. The AnnAGNPS simulated amount of annual runoff at the edge of the field was close to observed amounts (Nash-Sutcliffe efficiency of 0.92). It is believed that a substantial portion of sand was removed from the runoff one meter into the grass buffer where the samplers were located; therefore, sand was excluded from the AnnAGNPS simulation for comparison with observed sediment. Excluding sand, the AnnAGNPS predicted amount of annual sediment matches the observed amount fairly well (Nash-Sutcliffe efficiency of 0.46). In addition, based on evaluating the percent reduction of sediment at each zonal interface, the AnnAGNPS/REMM model well simulated the function of the RB system to reduce sediment. 相似文献
93.
An intensive approach to Barrow, Alaska’s adaptations to climate change and variability during recent decades suggests reconsideration
of the interconnected roles of science, policy, and decision-making structures. First, profound uncertainties are inherent
in unique interactions among the many natural and human factors affecting Barrow’s vulnerability. Science cannot significantly
reduce these uncertainties through extensive approaches, but intensive approaches can reconstruct and update local trends,
clarify the underlying dynamics, and harvest experience for policy purposes. Second, sound policies to reduce Barrow’s vulnerability
to coastal erosion and flooding must incorporate these profound uncertainties and the multiple values of the community. Minimizing
vulnerability to climate change is only one of the community’s interests, and must compete with other interests for limited
time, attention, funds and other resources. Third, the community itself is in the best position to understand its own context,
to decide on sound policies, and to take responsibility for those decisions. In short, local context matters in science, policy,
and decision-making structures for adaptation to climate change and variability. Overall, cognitive constraints may be the
most important human dimension of climate change. Factoring the global problem into more tractable local problems would make
the most of our cognitive capacity. 相似文献
94.
Teh-Lung Ku Shangde Luo Tim K. Lowenstein Jianren Li Ronald J. Spencer 《Quaternary Research》1998,50(3):261-275
Uranium-series dating on a 186-m core (DV93-1) drilled from Badwater Basin in Death Valley, California, and on calcareous tufas from nearby strandlines shows that Lake Manly, the lake that periodically flooded Death Valley during the late Pleistocene, experienced large fluctuations in depth and chemistry over the last 200,000 yr. Death Valley has been occupied by a long-standing deep lake, perennial shallow saline lakes, and a desiccated salt pan similar to the modern valley floor. The average sedimentation rate of about 1 mm/yr for core DV93-1 was punctuated by episodes of more-rapid accumulation of halite. Arid conditions similar to the modern conditions prevailed during the entire Holocene and between 120,000 and 60,000 yr B.P. From 35,000 yr B.P. to the beginning of the Holocene, a perennial saline lake existed, over 70 m at its deepest. A much deeper and longer lasting perennial Lake Manly existed from about 185,000 to 128,000 yr B.P., with water depths reaching about 175 m, if not 330 m. This lake had two significant “dry” excursions of 102–103yr duration about 166,000 and 146,000 yr B.P., and it began to shrink to the point of halite precipitation between 128,000 and 120,000 yr B.P. The two perennial lake periods correspond to marine oxygen isotopic stages (OIS) 2 and 6. Based on the shoreline tufa ages, we do not rule out the possible existence 200,000 yr ago of yet a third perennial lake comparable in size to the OIS 6 lake. The234U/238U data suggest that U in tufa owes its origin mainly to Ca-rich springs fed by groundwater that emanated along lake shorelines in southern Death Valley, and that an increase of this spring-water input relative to the river-water input apparently occurred during OIS 6. 相似文献
95.
Simon J. Woodings rew J. Williams Ralph Martin Ronald R. Burman & David G. Blair 《Monthly notices of the Royal Astronomical Society》1998,301(1):L5-L9
Following the proposal by Damineli that the central object of Eta Carinae may be an early-type binary, we perform numerical simulations of the X-ray emission from colliding stellar winds. A synthetic light curve has been generated which qualitatively agrees with the recent X-ray variability, and provides further support for the binary model. In particular, the model predicts a rise in the observed X-ray emission towards periastron, followed by a sharp drop and subsequent recovery. This is indeed what is seen in the RXTE light curve, although some problems concerning the X-ray spectrum at periastron still need to be explained. The simulations suggest that the width of the periastron dip will provide strong constraints on the binary and stellar wind properties of the components of Eta Car. 相似文献
96.
The emission of gas from the earth's crust is a complex process influenced by meteorological and seasonal processes which must be understood for effective application of gas emission to geochemical exploration. Free mercury vapor emission and radon emanation are being measured in a shallow instrument vault at a single nonmineralized site in order to evaluate these influences on gas emission.Mercury concentrations in the instrument vault average 9.5 ng/m3 and range from < 1 ng/m3 to 53 ng/m3 with a strong seasonal effect. Mercury has a direct relationship to vault temperature, air temperature, soil temperature, barometric pressure, water table, and the frozen or thawed state of the soil. Air and soil temperature, barometric pressure, and relative humidity are most important in influencing mercury emission while soil moisture is also important in radon emanation. Diurnal cycles are common but do not occur on all days. A heavy precipitation event on a dry soil seals the soil resulting in a rise in mercury concentration. Precipitation on a soil that is already wet does not increase mercury emission because of the compensation caused by lowering of the soil temperature by the precipitation event. Freezing of the soil changes the physical state of the vault-soil-soil gas-atmosphere system and emits the lowest concentrations of mercury. Phase lag effects are likely important. Stepwise multiple regression of mercury as dependent variable with meteorological and seasonal parameters as independent variables gives a cumulative R value of 0.563 and R2 of 0.317. The short-term noise coupled with phase lags are an important factor.The radon measurements integrated over weekly intervals smooth out much of the short-term noise. Stepwise multiple regression of radon as dependent variable with meteorological and seasonal parameters as independent variables gives a cumulative R value of 0.967 and R2 of 0.934. In this portion of the study the variation in the radon emanation is adequately predicted by meteorological and seasonal parameters. 相似文献
97.
Ronald T. Merrill 《Surveys in Geophysics》1975,2(3):277-311
Experimental evidence and theory indicate that chemical changes occur in many igneous rocks at sufficiently low temperatures to significantly affect the remanent magnetization. Some chemical changes lead to self-reversals of magnetization that are not reproducible in laboratory experiments. Such self-reversals appear to be very rare in subaerially-erupted basalts, but they probably are much more common in some other rock types, such as granites and diorites. The stability of the natural remanent magnetization in igneous rocks can be decreased, left unaltered, or increased by chemical changes. In addition, chemical changes will usually affect the intensity of magnetization in a rock; the intensity can increase, decrease, or (rarely) be left unaltered by a chemical change. Such changes are important to consider in the development of improved techniques for obtaining reliable estimates of the intensity of the Earth's magnetic field in the past and in correctly interpreting marine magnetic anomalies. Finally, experiments and theory are presented which suggest that many of the chemical changes in igneous rocks will only occasionally produce significant changes in the direction of the magnetization. 相似文献
98.
99.
Transmission of tidal energy over a plateau 总被引:1,自引:0,他引:1
Ronald Allan Heath 《Ocean Dynamics》1979,32(6):289-296
100.
James I. Drever James R. Lawrence Ronald C. Antweiler 《Earth and Planetary Science Letters》1979,42(1):98-102
Gypsum and halite crystals, together with saponite and phillipsite, were found in a vein in a basalt sill 625 m below the sea floor at DSDP Site 395A, located 190 km west of the crest of the Mid-Atlantic Ridge. The δ34S value of the gypsum (+19.4‰) indicates a seawater source for the sulfate. The δ18O values of the saponite (+19.9‰) and phillipsite (+18.1‰) indicate either formation from normal seawater at about 55°C or formation from18O-depleted seawater at a lower temperature.The gypsum (which could be secondary after anhydrite) was formed by reaction between Ca2+ released from basalt and SO42? in circulating seawater. The halite could have formed when water was consumed by hydration of basalt under conditions of extremely restricted circulation. A more probable mechanism is that the gypsum was originally precipitated as anhydrite at temperatures above 60°C. As the temperature dropped the anhydrite converted to gypsum. The conversion would consume water, which could cause halite precipitation, and would cause an increase in the volume of solids, which would plug the vein and prevent subsequent dissolution of the halite. 相似文献