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Abstract. Four sites were sampled in kelp (Macrocysiis pyrifera) forests occupying rocky bottom habitats along a wave exposure gradient in central California. Consistent betwecn-site differences were found in the three major structural elements - the surface canopy, the undcrstory assemblage, and the ground cover/turf assemblage - of kelp forest communities. Macrocysiis pyrifera was found at all four sites. Nereucyslis tuelkeana only at the most exposed site. The understory kelps Laminaria setchellii and Pterygopltora californica were also characteristic of exposed sites. Articulated coralline algae were more abundant at exposed sites than protected, while fleshy red algae showed the opposite pattern. All four study sites were located along 8.5 km of coastline, and thus were assumed to have available to them the same species pool for colonization. The substrate composition was the same and the amount of unconsolidated substrate was similar at all four sites. We suggest that exposure to wave-generated water motion, through its influence on the surface canopy and therefore on the amount of light reaching the bottom, is responsible for these between-site differences. 相似文献
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Brooks B. Ellwood Joo Zilho Francis B. Harrold William Balsam Burke Burkart Gary J. Long Andr Debnath Abdeljalil Bouzouggar 《Geoarchaeology》1998,13(1):55-71
We have sampled Upper and Middle Paleolithic sediments in Caldeirão Cave, Portugal, for paleoclimatic analysis. This work involved measuring magnetic susceptibility (MS) on continuous vertical profiles of a series of ∼8 cc sediment samples, and using the MS data as a paleoclimate proxy. Previous work has shown that caves can be ideal recorders of paleoclimatic variations because they are protected environments. Pedogenesis outside the cave during time of cool climate produces sediments with low MS magnitudes, while warmer climates yield higher MS magnitudes. Eroded soils collect in caves in sediment sequences where futher pedogenesis and biological disturbance is minimal. Continuously sampled profiles of those sediments found in an archaeological context then allow paleoclimatic estimates for all archaeological levels. Results of our work here include identification of the last glacial maximum, at ca. 24,000–22,000 B.P. (calibrated), defined by very low MS values found in basal Solutrean levels in the cave. Distinctive ca. 2500 year Neo-glacial cycles, defined by the MS data within the Early Upper Paleolithic and Solutrean, are consistent with published 14C duration estimates for the Upper Paleolithic in Caldeirão Cave. © 1998 John Wiley & Sons, Inc. 相似文献
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R. Srikanthan T. I. Harrold A. Sharma T. A. McMahon 《Stochastic Environmental Research and Risk Assessment (SERRA)》2005,19(3):215-226
There has been extensive research on the problem of stochastically generating daily rainfall sequences for use in water management applications. Srikanthan and McMahon [Australia Water Resources Council, Canberra, 1985] proposed a transition probability matrix (TPM) model that performs better for Australian rainfall than many alternative models, particularly where long records (say 100 years) are available. Boughton [Report 99/9, CRC for Catchment Hydrology, Monash University, Melbourne, 21pp, 1999] incorporated an empirical adjustment into the TPM model that allows the model to reproduce the observed variability in the annual rainfall. More recently, Harrold et al. [Water Resour Res 39(10, 12):1300, 1343, 2003a,b] proposed nonparametric models for the generation of daily rainfall occurrences and rainfall amounts on wet days. By conditioning on short, medium and long-term characteristics, this approach is also able to preserve the variability in annual rainfall. In this study, the above two approaches were used to generate daily rainfall data for Sydney and Melbourne, and the results evaluated. Both approaches preserved most of the daily, monthly and annual characteristics that were compared, with the nonparametric approach providing marginally better performance at the cost of greater model complexity. The nonparametric approach was also able to preserve the variability and persistence in the annual number of wet days. 相似文献
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The cassiterite-sulfide deposits of western Tasmania are spatially and temporally related to Devonian granitoids. The stable isotope and fluid inclusion data of these deposits are best explained by a genetic model in which Sn is leached from the granites by reduced, non-magmatic fluids mixed with late-stage magmatic fluids. These fluids mineralize the overlying dolomite horizons and subjacent faults by a combination of wallrock and boiling induced reactions. The model requires co-incidental release of volatiles and ingress of groundwaters to sustain the leach mechanism. This implies some specific structural/deformational controls on the process and an intimate association between cassiterite-sulfide deposits and volatile-rich granites.The sulfur isotope data show a wide range in values, from about −2 to 20 per mil, and are explained in terms of a near-zero magmatic component and three heavier country-rock sources: the Precambrian Oonah Formation, the Crimson Creek Formation and the Mount Read Volcanics. Following this interpretation, the heavy sulfur isotope values from the Heemskirk Granite (Hajitaheri, 1985) and the Federal-Bassett Fault at Renison (Kitto, 1994) indicate ingress of groundwater into the granites. Salinities of fluid inclusions from the deposits show a restricted range (around 5 to 15 weight percent NaCl equivalent) compared with the range shown by inclusions from alteration zones within the granites (around 5 to 40 weight percent NaCl equivalent). It is suggested that the wide range of salinities in the granites is generated in the critical region of the NaCl---H2O system at temperatures around 400 to 500°C and pressures around 400 to 500 bars. The limited range of salinities in the deposits reflects cooling to sub-critical conditions. Cooling through the critical region promotes homogenization of groundwater and magmatic vapor and brine. Condensation of magmatic volatiles within this zone of mixing maintains acidity and promotes fluid-rock reaction.The geochemistry of the granites underlying the Renison Bell area (Bajwah et al., 1995) is interpreted in terms of two granites. The geological relationships indicate the more mafic phase, the Renison Granite, was intruded by the more fractionated Pine Hill Granite. Fluids from the Pine Hill Granite sericitised and tourmalinized both the Renison Granite and Pine Hill Granite along their mutual contact, generating a broad north-east trending zone of alteration. This contact may have been offset by the Federal-Bassett Fault before the Renison deposit was formed. It is suggested that groundwater flow was essentially constrained within this NW-SE trending structure and subsidiary structures. Magmatic volatiles, emanating from deep within the Pine Hill Granite, acidified the reduced groundwater and promoted leaching of the granite. The siting of the Renison deposit on the margin of the Pine Hill Granite is consistent with a strong up-flow zone on the margin of a cooling pluton. The very constant S-isotope signature in the deposit is consistent with a large-scale homogenization process operating over a sustained period. The very large size of the Renison deposit is accounted for by a long-lived groundwater system maintained by the high heat flow from a fractionated granite. 相似文献
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T. I. Harrold A. Sharma S. Sheather 《Stochastic Environmental Research and Risk Assessment (SERRA)》2001,15(4):310-324
Mutual information is a generalised measure of dependence between any two variables. It can be used to quantify non-linear
as well as linear dependence between any two variables. This makes mutual information an attractive alternative to the use
of the correlation coefficient, which can only quantify the linear dependence pattern. Mutual information is especially suited
for application to hydrological problems, because the dependence between any two hydrologic variables is seldom linear in
nature. Calculation of the mutual information score involves estimation of the marginal and joint probability density functions
of the two variables. This paper uses nonparametric kernel density estimation methods to estimate the probability density
functions. Accurate estimation of the mutual information score using kernel methods requires selection of appropriate smoothing
parameters (bandwidths) for use with the kernels. The aim of this paper is to obtain a practical method for bandwidth selection
for calculation of the mutual information score. In this paper, the lag-one dependence structures of several autocorrelated
time series are analysed using mutual information (note that this produces the lag-one auto-MI score, the analog of the lag-one
autocorrelation). Empirical trials are used to select appropriate bandwidths for a range of underlying autoregressive and
autoregressive-moving average models with normal or near-normal parent distributions. Expressions for reasonable bandwidth
choices under these conditions are proposed. 相似文献
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