Rates of cave and landform development in the Yorkshire Dales from speleothem age data

Uranium-series ages have been obtained for 87 speleothems collected from nine major cave systems in the Craven district of northern England. Large systems such as Ease Gill Caverns, the West Kingsdale caves, and Gaping Gill-Ingleborough Cave, which contain relict high-level tunnels, are found to be older than 350,000 years (the limit of the ²³⁰Th/²³⁴U dating method). There is little evidence to indicate a significant enlargement of these passages since this time. Estimates of the age of Victoria Cave from ²³⁴U/²³⁸U isotopic ratios suggest that the cave has been fully relict for more than 500,000 years. Ages of in situ speleothems immediately adjacent to local base level cave streams show that mean maximum downcutting rates in limestone channels are about 2 to 5 cm 1000 yr. These rates are significantly lower than those obtained from direct measurements on limestone bedrock in stream channels but are comparable to areal denudation rates based on solute budgets. Using the present elevation of caves with respect to adjacent valley floor levels, mean maximum valley entrenchment rates are found to range between 5 cm/ky and < 20cm/ky, which corresponds to 6 m to < 24 m of lowering per glacial/interglacial cycle. These rates suggest that upper beds of the limestone were incised to form the Yorkshire Dales between 1 and 2 million years ago. The results indicate that the erosional effects of individual glaciations are not as severe as previously proposed in the literature.     

Quantifying anthropogenic influence on recent near-surface temperature change

We assess the extent to which observed large-scale changes in near-surface temperatures over the latter half of the twentieth century can be attributed to anthropogenic climate change as simulated by a range of climate models. The hypothesis that observed changes are entirely due to internal climate variability is rejected at a high confidence level independent of the climate model used to simulate either the anthropogenic signal or the internal variability. Where the relevant simulations are available, we also consider the alternative hypothesis that observed changes are due entirely to natural external influences, including solar variability and explosive volcanic activity. We allow for the possibility that feedback processes, other than those simulated by the models considered, may be amplifying the observed response to these natural influences by an unknown amount. Even allowing for this possibility, the hypothesis of no anthropogenic influence can be rejected at the 5% level in almost all cases. The influence of anthropogenic greenhouse gases emerges as a substantial contributor to recent observed climate change, with the estimated trend attributable to greenhouse forcing similar in magnitude to the total observed warming over the 20th century. Much greater uncertainty remains in the response to other external influences on climate, particularly the response to anthropogenic sulphate aerosols and to solar and volcanic forcing. Our results remain dependent on model-simulated signal patterns and internal variability, and would benefit considerably from a wider range of simulations, particularly of the responses to natural external forcing.     

Determination of HBCD, PBDEs and MeO-BDEs in California sea lions (Zalophus californianus) stranded between 1993 and 2003

Blubber samples from male California sea lions (Zalphophus californianus) stranded between 1993 and 2003 were analyzed for 27 polybrominated diphenyl ether (PBDE) congeners, three isomers of hexabromocyclododecane (HBCD) and 14 methoxylated polybrominated diphenyl ether (MeO-BDE) congeners. Total PBDEs ranged from 450 ng/g to 4740 ng/g wet mass and total HBCD ranged from < 0.3 ng/g to 12 ng/g wet mass. The concentration of HBCD increased from 0.7 ng/g to12.0 ng/g wet mass in sea lion blubber between 1993 and 2003. However, no significant temporal trend was observed for any of the other brominated compounds over this 10 year period. Only one of the 14 MeO-BDE congeners was detected in the blubber samples, 6-methoxy-2,2',4,4'-tetrabromodiphenyl ether (6-MeO-BDE 47), and concentrations ranged from < 0.2 ng/g to 12 ng/g wet mass. A bromo-, chloro-heterocyclic compound, 1,1'-dimethyl-tetrabromo-dichloro-2,2'-bipyrrole (DBP-Br4Cl2), previously reported in marine species along the Pacific coast, was also identified in the sea lion blubber. DBP-Br4Cl2 ranged from 44 ng/g wet mass to 660 ng/g wet mass and was present at concentrations rivaling the dominant PBDE congener, BDE 47 (2,2',4,4'-tetrabromodiphenyl ether). Concentrations of DBP-Br4Cl2 were positively correlated with 6-MeO-BDE 47 (r = 0.7; p < 0.05). Both of these compounds have been identified in marine algae and sponges, and studies suggest they are both produced from natural sources. This study demonstrates that brominated compounds from both anthropogenic and biogenic sources can accumulate to similar levels in marine mammals. In addition, HBCD concentrations appear to be increasing in California sea lion populations, whereas PBDE concentrations, between 1993 and 2003, were highly variable.     

Ice sheets and the CO2 problem

In this review, the carbon dioxide problem is discussed, with special reference to the possible effects of a global warming on the ice sheets of Greenland and Antarctica. Instead of detailed projections of future climate and the consequences, the basic mechanisms are explained and illustrated with results described in the literature.It is concluded that a doubling of the atmospheric CO₂ content (most likely to occur somewhere in the second half of the next century) will result in a globally-averaged warming of 2–4°C, and an intensification of the hydrological cycle. In the polar regions, this warming will be a few degrees larger and as a consequence the Greenland Ice Sheet will decrease in size. Antarctica, on the other hand, is expected to grow because of the increased snowfall. The instability of the West Antarctic Ice Sheet is also discussed and, although no conclusive prediction to its long-term response can be made, it is argued that on a short time scale (less than about 100 y) nothing dramatically wil happen to this part of Antarctica.     

Evidence of the impacting body of the Ries crater — the discovery of Fe-Cr-Ni veinlets below the crater bottom

Fe-Cr-Ni particles and veinlets have been discovered in the top 15 m of the compressed zone with abundant shatter cones below the bottom of the Ries crater. The metallic particles are less than a few microns across. They occur in various minerals along healed intergranular and locally in intragranular microfractures in quartz diorite, amphibolite and chloritized granite of the basement crystalline rocks.The particles consist of major Fe, Cr, and Ni with minor Si and Ca. Origin due to contamination is absolutely ruled out. We believe that these Fe-Cr-Ni particles are probably condensed from the vaporized impacting body which produced the Ries crater. These particles were injected with high velocity into microfractures near the top of the compressed zone, implanted in and across various minerals before these microfractures were resealed. The presence of Si and Ca as well as the fact that the Cr content is nearly twice that of Ni, led us to conclude that the Ries impacting body is very likely not an iron meteorite but a stony meteorite.     

Compositional variation in three Cascade stratovolcanoes: Jefferson,Rainier, and Shasta

Detailed major and trace element studies of volcanic rocks from Jefferson, Rainier, and Shasta stratovolcanoes in the Cascade Range indicate that each volcano has distinct geochemical distribution patterns. Silica variation diagrams are not smooth nor, in general, continuous for any volcano. Portions of stratigraphic sections within the volcanoes exhibit compositional coherency and are interpreted as eruptive groups which were extruded over time intervals which are short compared to the lifetimes of the volcanoes. The results of this investigation indicate the leasibility of geochemically mapping eruptive groups within stratovolcanoes. Systematic compositional trends are not observed within thick (500–1000 m) eruptive groups but may occur over thicknesses of <200 m. Compositional variations within eruptive groups are commonly non-systematic and show ranges similar to the ranges observed in individual flows. Correlations between the amounts or kinds of phenocryst phases present and intra-group compositional variation is not observed. Inter-group compositional differences are sometimes accompanied by mineralogical differences. Late andesites and dacites at Rainier and Shasta are characterized by decreases in K and Rb while at Jefferson increases in these elements and other compositional changes occur in the late eruptives. Progressive fractional crystallization models do not seem capable of explaining the element distributions observed in the three volcanoes. Existing data are consistent with a model involving varying degrees of melting of some combination of amphibolite, eclogite or peridotite in or above a subduction zone with varying water contents. Segregation and sequential eruption of small batches of magma may produce the eruptive groups characterizing the volcanoes. Late mafic magmas erupted at satellite vents appear to be produced in different (deeper?) mantle source areas.     

A method of searching for radial velocity anomalies in the lower mantle using interfering signals

Radial velocity anomalies in the lower mantle that give rise to triplications in the travel-time curve for short-periodP waves will produce arrivals havingdT/d values that differe by roughly 0.2–0.5 s/deg. The first two arrivals associated with such triplications will be separated by less than one second over a distance range of 4°–10° they may not, therefore, be separable visually on single seismograms, so that their presence can only be inferred from some measurable property that depends on their mutual interference. If there are lateral variations in the regions of anomalous velocity gradients, the interfering signals will also have different azimuths of arrival. Using two synthetic wavelets we have investigated the effect of interference on bothdT/d and azimuth measurements at the Yellowknife Array. We found that if the interfering pulses have a dominant frequencyv, there is a range of time separations (0.30/v0.55/v) over which the measureddT/d and azimuth values may fluctuate by much more than the differences indT/d and azimuth between the interfering signals. We have evaluated the following empirically defined functions for three different primary signals, and for three different relative amplitudes of the interfering signals:f (t), the drift function, which expresses how the measured slownesses,p, and azimuths, , differ from the slownesses and azimuths of the primary wavelets; f(), the range function, which describes the behaviour of the upper and lower bounds ofp and as a function of the difference in arrival times of the signals, andf , studied the properties of these functions, and have outlined how these properties provide criteria based on the numerical and statistical characteristics of the arrival vectors, and on the waveform of the signal that will enable small radial velocity anomalies to be more clearly delineated.Contribution No. 863 from the Earth Physics Branch.     

Predation Efficacy of Larvivorus Fishes on Cyclops,Transmitting Dracunculiasis

The predatory potentiality of G. affinis and P. reticulata fishes on cyclops was tested by ‘no-choice’ and ‘choice’ experiments. In ‘no-choice’ experiments predation increased significantly with the number of cyclops exposed (1, 3, 5, 7 and 10 preys) and decreased with the increase in the water volume (1, 2 and 3 litres) in case of both the fishes. A similar trend for cyclops predation was observed in ‘choice’ experiments. The relative rate of predation of G. affinis and P. reticulata was more in ‘no-choice’ as compared to in ‘choice’ experiments. However, the predatory efficacy of G. affinis was relatively higher than that of P. reticulata in all the experiments. The comparative rate of predation on three zooplanktons by G. affinis and P. reticulata showed that Daphnia spec. was most susceptible to predation by both the fishes. It was followed by calanoids and the least preferred prey being cyclops.