Changing Holocene snail faunas and environments along the Eastern Caprock Escarpment of Texas

Detailed study of gastropods recovered from the Lake Theo Site, Briscoe County, Texas, has revealed a progressive decline in species diversity from early Holocene to the present. Knowledge of the microhabitats preferred by the recovered species allows a partial reconstruction of environments required to produce such communities. Since the early Holocene, the Lake Theo area has experienced a progressive loss of deciduous riparian gallery woodland habitat. Alteration in plant communities was caused by a dramatic decrease in effective precipitation. Analysis of the sequence of local extirpation of the various snail species indicates that the decrease in precipitation and increase in temperature probably were not concurrent.     

On the metal content and metal ion uptake of botanically specific peats and the derived humic acids

With respect to ten metals (Al, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga) which are continguous in the Periodic Table the natural metal content and the ability to take up metal ions has been determined for three peats of defined botanical origin, each peat being sampled at three different levels. A correlation is found between the natural metal content and botanical origin. Apart from aluminium, the concentration of individual metals increased in the series Sphagnum < fen sedge < reed, the increase being statistically significant (P = 0.001) for V, Mn, Co, Ni, and Ga and for the sum of the metal ions measured.No clear correlation emerged between botanical origin and metal uptake. The metal uptake by the humic acid derived from each peat sample was also determined, and correlated with the metal uptake of the source peat itself.In all the humic acid and peat metal-uptake experiments the selectivities were very similar. V, Cr, Cu, Ga were readily taken up while Mn, Co and Zn were consistently less preferred.     

Deccan flood basalts at the Cretaceous/Tertiary boundary?

Joint consideration of new paleomagnetic, paleontological and geochronological data from the Deccan continental flood basalts in India and critical discussion of earlier results lead us to suggest that volcanic activity may have lasted less than 1 Ma, thus possibly ranking as one of the largest volcanic catastrophes in the last 200 Ma. Available data are best satisfied if volcanism spanned the Cretaceous/Tertiary boundary, followed shortly afterwards by rifting of the Arabian Sea. These results point out the need for further work which may help in choosing between “external” and “internal” models of the Cretaceous/Tertiary boundary events.     

An indicator kriging model for investigation of seismic hazard

Time domain probabilistic techniques most often are used for assessment of seismic hazard. Such techniques are based on the historic frequency of ground motion. Hazard is expressed as a probability of experiencing a particular level of seismic activity over a given length of time. One of these techniques utilizes frequency of extreme values for assessment of hazard. The major disadvantage of this technique, however, becomes evident when maximum seismic activity for two consecutive years occurs only a few weeks or months apart. In this case, the extreme value approach overestimates seismic hazard. A new approach for hazard assessment is founded on principles of indicator kriging. This technique evaluates seismic hazard as a simple frequency record, which is more realistic for regions of little to moderate seismicity.     

Growth of juvenile spot,Leiostomus xanthurus Lacépède,in the nursery region of the James River,Virginia

Thein situ growth of juvenile spot,Leiostomus xanthurus Lacépède, was investigated in a nursery area of the James River, Virginia. Juvenile growth was fairly rapid (11.34 mm standard length per month) during their first spring and summer in the nursery grounds, and leveled off in the autumn. The mean size of spot after the first year was calculated to be 148 mm standard length. The weight-length relationship was log₁₀W = ?5.018+3.246 (log₁₀L), where W = wet weight in grams and L = standard length in millimeters. The weight-length relationship of juvenile spot of the James River estuary differed statistically from that reported from South Carolina, Mississippi, Louisiana, and Texas. Spot of the James River, Virginia, were found to be heavier per unit length than those of other regions, which suggests that the Chesapeake Bay system provides an optimum environment for growth of 0-year class fish.     

Determination of hydrogen ion concentration in softwater lakes using carbon dioxide equilibria

A strategy for determining the hydrogen ion content of fresh waters is proposed that involves total dissolved inorganic carbon (DIC or σCO₂) and CO₂ partial pressure (P_CO2) measurements rather than pH electrode measurements. This recommendation derives from discrepancies between pH and carbon dioxide equilibria measurements made on several softwater lakes at the Experimental Lakes Area, northwestern Ontario. The pH calculated from DIC, P_CO2, and the first dissociation constant of carbonic acid (K₁) data was consistently higher than that directly measured with a pH electrode. Similarly, calculation of P_CO2 of surface waters from pH, DIC, and K₁ data gave values up to twice that of atmospheric saturation despite repeated equilibrations with atmospheric P_CO2. Laboratory experiments demonstrated that the high dissolved organic carbon content of these waters appears to alter the electrode response yielding pH values lower than the true values. Furthermore, the uptake of protons by weak organic acid anions appear to be the cause of the measured difference between total (Gran) and carbonate (DIC — dissolved CO₂) alkalinity. Therefore bicarbonate ion concentration must be calculated from the difference between the total dissolved inorganic carbon content and uncharged dissolved CO₂ content. These procedures should provide more accurate and consistent results in the pH trend in surface waters and hence yield a solid baseline against which the effects of acid precipitation can be assessed.     

Distribution and abundance of grapsid crabs (Grapsidae) in a mangrove estuary: Effects of sediment characteristics,salinity tolerances,and osmoregulatory ability

Crabs (Grapsidae,Sesarma) are the dominant macrofaunal group of mangrove forest soils in northern Australia. Little is known about the ecology of these crabs or the factors that influence their distribution in mangrove forests. Pitfall traps were used to sample grapsid crabs in the Murray River estuary in north Queensland. Sampling was conducted at five sites along a salinity gradient from <1‰ at upstream sites to >35‰ at the river mouth. At each site, trapping was done in both low and high intertidal forests. We characterized the sediments at each site by measuring percent sand, silt, clay and organic matter, Eh, pH, and soil pore-water salinity. Four species of grapsids dominated the crab fauna along the Murray River (Sesarma semperi-longicristatum, S. messa, S. brevicristatum, andS. brevipes). Distinct zonation patterns were found along the salinity gradient and between high and low intertidal forests.S. messa was dominant in high intertidal, downstream forests, high and low intertidal forests in the middle to downstream portion of the river, and in low intertidal forests in the central reach of the river.S. brevipes was dominant in both low and high intertidal zone forests at low salinity upstream sites.S. brevicristatum was most abundant in the central reaches of the river and only in the high intertidal zone.S. semperi-longicristatum was found only in the low intertidal zone, downstream forest. Subsequently, tests of salinity tolerances of these crabs were carried out in the laboratory. These indicated very wide tolerances over salinities from completely fresh to hypersaline (60‰). The osmoregulatory abilities of the crabs were also found to vary. However, neither their salinity tolerance nor osmoregulatory ability adequately explain the zonation patterns were measured in the field. For example,S. brevicristatum had the most restricted distribution, but it had the second broadest salinity tolerance and osmoregulatory ability. Sediment characteristics explained a significant amount of the variation in abundance for two of the crab species. Pore-water salinity provided no explanatory power for any of the species. Individual species abundances are probably influenced by additional factors such as interspecific competition and predation.     

A distribution-free alternative to least-squares regression and its application to Rb/Sr isochron calculations

A distribution-free estimator of the slope of a regression line is introduced. This estimator is designated S_m and is given by the median of the set of n(n – 1)/2 slope estimators, which may be calculated by inserting pairs of points (X _i, Y_i)and (X _j, Y_j)into the slope formula S _i = (Y _i – Y_j)/(X _i – X_j),1 i < j n Once S _m is determined, outliers may be detected by calculating the residuals given by R_i = Y_i – S_mX_i where 1 i n, and chosing the median R_m. Outliers are defined as points for which |R_i – R_m| > k (median {|R _i – R_m|}). If no outliers are found, the Y-intercept is given by R_m. Confidence limits on R_m and S_m can be found from the sets of R_i and S_i, respectively. The distribution-free estimators are compared with the least-squares estimators now in use by utilizing published data. Differences between the least-squares and distribution-free estimates are discussed, as are the drawbacks of the distribution-free techniques.