Tropical Deforestation and Climate Change: What Does the Record Reveal?

Tropical deforestation is widely believed to directly influence the climate at a number of scales. Yet while much has been written about the tropical forest-climate relationship, there is little empirical evidence showing if and how local and regional climates are modified by deforestation. This study presents the results of an analysis of deforestation and climate change in a rain forest in southern Mexico. Records from 18 climate stations in the Selva Lacandona of Chiapas, Mexico were examined and related to an analysis of deforestation based on Landsat images. The area surrounding some stations has been deforested since the stations were established, while the area surroundings others has remained forested. Strong climatic trends were generally evident at the deforested stations, including decreases in the average daily maximum temperature and temperature range. No precipitation changes were observed. A comparison of the results with microclimatic experiments and modeling studies suggests that the climatic impacts of deforestation are overgeneralized at the local scale. Landscape heterogeneity appears to influence the biophysical mechanisms linking tropical forests and climate, and should be explicitly represented in modeling studies.     

Paleolimnological significance of spinose and non-spinose morphs of Pyrgophorus hibbardi (Leonard and Franzen, 1944)

The basal portion of the Ogallala Formation (=Laverne Formation) (Lower Pliocene) Beaver County, Oklahoma, contains an interesting assemblage of non-marine fossil molluscs that include both spinose and non-spinose forms of the aquatic gastropod species Pyrgophorus hibbardi. The origin and paleolimnological significance of the spinose morph has been a source of much conjecture that has influenced environmental reconstructions of this assemblage. In one hypothesis the spinose forms of P. hibbardi are assumed to be associated with brackish water conditions by analogy with some populations of a related hydrobiid Potamopyrgus jenkinsi. To test the hypothesis that the spinose forms lived under different water conditions than the non-spinose morphs, we analyzed 10 specimens each of the two varieties for stable oxygen and carbon isotope ratios in the shell aragonite.The mean isotope ratios for the smooth and spinose morphs show no significant difference (oxygen: t = 0.28, df = 18, P (T t) 0.78 n.s.; carbon: t = 0.96, df = 18, P (T t) 0.35 n.s). We conclude that the lack of a statistically significant difference between the means of the oxygen and carbon isotope values for the smooth and spinose morphs suggests that the two forms lived in waters having similar isotope signatures. The considerable range in oxygen isotope values recorded by both morphs of P. hibbardi, including values as high as 5–6, suggest that both morphs were associated with waters which were periodically evaporatively enriched in ¹⁸O.     

Holocene environmental changes in the Whitefish Dunes Area, Door Peninsula, Northern Lake Michigan Basin, USA

A two meter sediment core taken from an inter-dune pond in Whitefish Dunes State Park, Door Peninsula, Wisconsin, provides a record of paleoenvironmental changes in the area from approximately 3600 RCYBP to 5100 RCYBP. The hydrology of the Whitefish Dunes pond is now apparently dependent on ground water recharge from local precipitation that infiltrates into the dunes and from Clark Lane, making it very sensitive to low magnitude, short term climate fluctuations.Changes in lithology, molluscan species diversity, oxygen isotopes from gastropod shells, and sediment organic content permit division of the core into several intervals representing different environmental regimes. The basal sediments are older than 5130 RCYBP and resemble those of the Liberty Grove Member of the Pleistocene Horicon Formation. The oldest Holocene sediments recorded in the core are an organic rich layer dated at 5130 RCYBP, which accumulated in a depression on the surface of a sand bar. Continued rise in the local water table during the Nipissing Transgression produced paludal conditions in the area, which were followed by the onset of lacustrine conditions. During this interval, the area supported a diverse molluscan fauna. Minor, short term climate fluctuations are reflected in sediment and oxygen isotope changes which are absent from the record in deep water cores from Lake Michigan. This phase in the history of the area ended with the eutrophication of the pond and the development of dunes. Information from Whitefish Dunes pond and other onshore sites within the Lake Michigan basin are an important source of data for reconstructing the history of water levels and climate changes for this area of the Great Lakes drainage.     

The emergence of a Galápagos shield volcano, Roca Redonda

Roca Redonda volcano is a mostly submarine shield volcano that rises nearly 3 km from the adjacent seafloor. Over twenty lava flows and palagonite tuff are exposed in a 60 meter high oblong outcrop above sea level, and several other flows are exposed in the shallow water surrounding the islet. Thick, slightly alkaline picritic flows form the base of the section. Thinner picrites interbedded with sparsely porphyritic alkali-olivine basaltic pahoehoe toes characterize the upper section. The subaerial section probably records the filling of a palagonite tuff cone with younger lavas. Numerous fumaroles that may have a magmatic component are present in the shallow (<30 m) submarine zone and indicate that the volcano is probably still active. Three lava types are exposed: the basal picrites with 19% > MgO > 14%, high-Mg basalts with MgO of about 9%, and low-Mg basalts with MgO of about 6%. The Sr and Nd isotopic ratios of the three lava types are within analytical uncertainty. Olivine compositions indicate that the picrites are basaltic liquids that have accumulated olivine whose composition is in equilibrium with the host basaltic liquid. Apparently, basaltic magmas percolated through dunite and troctolite that had crystallized from slightly older Roca Redonda basaltic magma. Lavas from Roca Redonda have enriched trace element contents and isotopic ratios relative to nearby Wolf volcano, but they are quite similar to lavas from Cerro Azul and Ecuador volcanoes. The common characteristic of these volcanoes is that they lie on the periphery of the archipelago and are in a stage of subaerial growth. This suggests that Galápagos volcanoes may go through a juvenile alkaline stage before a mature tholeiitic stage, analogous to the Loihi stage of Hawaiian volcanism. A low ³He/⁴He ratio in olivine from one of the picrites indicates a small contribution by the Galápagos mantle plume. Received: 15 December 1997 / Accepted: 6 May 1998     

Are Q-stars a serious threat for stellar-mass black hole candidates?

We examine the status of the threat posed to stellar-mass black hole candidates by the possible existence of Q-stars (compact objects with an exotic equation of state that might have masses well above the normally accepted maximum for standard neutron stars). We point out that Q-stars could be extremely compact (with radii less than 1.5 times the corresponding Schwarzschild radius) making it difficult to determine observationally that a given object is a black hole rather than a Q-star, unless there is direct evidence for the absence of a material surface. On the other hand, in order for a Q-star to have a mass as high as that inferred for the widely favoured black hole candidate V404 Cygni, it would be necessary for the Q-matter equation of state to apply already at densities an order of magnitude below that of nuclear matter and this might well be considered implausible on physical grounds. We also describe how rotation affects the situation and discuss the prospects for determining observationally that black hole candidates are not Q-stars.     

The UVX quasar optical luminosity function and its evolution

The recently finished Edinburgh UVX quasar survey at B <18 is used together with other complete samples to estimate the shape and evolution of the optical luminosity function in the redshift range 0.3< z <2.2. There is a significantly higher space density of quasars at high luminosity and low redshift than previously found in the PG sample of Schmidt & Green, with the result that the shape of the luminosity function at low redshifts ( z <1) is seen to be consistent with a single power law. At higher redshifts the slope of the power law at high luminosities appears to steepen significantly. There does not appear to be any consistent break feature which could be used as a tracer of luminosity evolution in the population.     

Oscillations in radiation-driven outflows

In this paper we investigate the dynamical behaviour of radiation-driven winds, specifically winds that arise when Compton scattering transfers momentum from the radiation field to the gas flow. Such winds occur during strong X-ray bursts from slowly accreting neutron stars, and also may be driven from the inner regions of a black hole or neutron star accretion disc when the mass transfer rate is very high. By linearizing the radiation hydrodynamic equations around steady spherical outflow, we evaluate the time-dependent response of these winds to perturbations introduced at their inner boundaries. We find that although radiation-driven winds are generally stable, they act as mechanical filters that should produce quasi-periodic oscillations or peaked noise in their radiation output when perturbations force them stochastically. This behaviour may underlie the photospheric oscillations observed in some strong Type I X-ray bursts.     

Origin of crystalline,cold desert salts in the McMurdo region,Antarctica

The distribution of crystalline salt minerals in deposits in the McMurdo region of Antarctica has been examined to study the origin of these salts. Sulphate, chloride, sodium and calcium salts are most frequently encountered. Salts containing chloride and sodium ions become less common away from the coast. Sulphate salts are more regularly distributed but tend to be related isotopically to sea water sulphate. Salts containing magnesium ion tend to exist mainly on substrates composed of basic igneous rocks, whereas calcium and carbonate salts are present on all rock types in the region.These distributions show that salts of marine origin are regionally and quantitatively most important but that chemical weathering of mafic materials in rocks and soils is also significant. However, biological, volcanic and hydrothermal processes are or have been active contributing to salts in local areas, that include penguin rookeries and eastern Taylor Valley, the summit area of Erebus Volcano, and subsurface rocks, respectively.     

Paläozoische und frühmesozoische Orogenesen am pazifischen Rand Gondwanas

Zusammenfassung In Südamerika, in der Antarktis und in Australien läßt sich gleichermaßen vom Kambrium bis in die Trias ein pazifikwärtiges Wandern der orogenen Aktivität beobachten. Aus der relativen Lage dieser Orogene ergibt sich, daß vor dem Jura die Antarktische Halbinsel westlich der Südspitze von Südamerika gelegen hat.

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In South America, Antarctica and Australia, from the Cambrian to the Triassic Pacificward shifting of the orogenic activity is observed. From the relative position of these orogenes is deduced that before the Jurassic the Antarctic Peninsula was situated west of the southern tip of South America.

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Resumen Tanto en Sudamérica como en la Antártida y en Australia desde el Cámbrico hasta el Triásico se observa una transposición continua de la actividad orogénica hacia el Pacífico. De la posición relativa de estos orógenos se concluye que antes del Jurásico la Península Antártica fue situada al Oeste de la punta Sur de Sudamérica.

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Age and intensity of thermal events by fission track analysis: The Ries impact crater

A thermal event reduces the number of previously registered fission tracks in a mineral dependent upon the track retention properties of the individual mineral. Apatite, sphene and zircon have retention properties over a wide range of temperatures (from 100° to 550°C); apatite data reveal information at lowest temperatures while sphene and zircon data are useful for higher temperatures.Thermal events within this temperature range of 100°C to about 550°C are suitable for study with this technique. The age of the event is determined from samples in which the fission tracks are completely erased, while minerals containing partially removed (erased) tracks provide information on the temperatures occurring during the thermal event.As a test case, the analysis of the temperatures developed by the meteorite impact which produced the Ries crater at 14.7 m.y. ago is presented.