Hasil untuk "Geology"

L. Royden, B. Burchfiel, Robert D. van der Hilst

R. Fell, J. Corominas, C. Bonnard et al.

J. Ogg, G. Ogg, F. Gradstein

D. Burdige

A. Favre, M. Päckert, S. Pauls et al.

Biodiversity is unevenly distributed on Earth and hotspots of biodiversity are often associated with areas that have undergone orogenic activity during recent geological history (i.e. tens of millions of years). Understanding the underlying processes that have driven the accumulation of species in some areas and not in others may help guide prioritization in conservation and may facilitate forecasts on ecosystem services under future climate conditions. Consequently, the study of the origin and evolution of biodiversity in mountain systems has motivated growing scientific interest. Despite an increasing number of studies, the origin and evolution of diversity hotspots associated with the Qinghai‐Tibetan Plateau (QTP) remains poorly understood. We review literature related to the diversification of organisms linked to the uplift of the QTP. To promote hypothesis‐based research, we provide a geological and palaeoclimatic scenario for the region of the QTP and argue that further studies would benefit from providing a complete set of complementary analyses (molecular dating, biogeographic, and diversification rates analyses) to test for a link between organismic diversification and past geological and climatic changes in this region. In general, we found that the contribution of biological interchange between the QTP and other hotspots of biodiversity has not been sufficiently studied to date. Finally, we suggest that the biological consequences of the uplift of the QTP would be best understood using a meta‐analysis approach, encompassing studies on a variety of organisms (plants and animals) from diverse habitats (forests, meadows, rivers), and thermal belts (montane, subalpine, alpine, nival). Since the species diversity in the QTP region is better documented for some organismic groups than for others, we suggest that baseline taxonomic work should be promoted.

N. Shapiro, M. Campillo, L. Stehly et al.

P. Falkowski, R. Barber, V. Smetacek

J. Bibring, Y. Langevin, J. Mustard et al.

K. E. Peters, J. Moldowan

D. Benn, D. Evans

S. Strebelle

B. Berkowitz

R. Riding

R. Ketcham, W. Carlson

D. Turcotte, G. Schubert

E. Bonnet, O. Bour, N. Odling et al.

Scaling in fracture systems has become an active field of research in the last 25 years motivated by practical applications in hazardous waste disposal, hydrocarbon reservoir management, and earthquake hazard assessment. Relevant publications are therefore spread widely through the literature. Although it is recognized that some fracture systems are best described by scale‐limited laws (lognormal, exponential), it is now recognized that power laws and fractal geometry provide widely applicable descriptive tools for fracture system characterization. A key argument for power law and fractal scaling is the absence of characteristic length scales in the fracture growth process. All power law and fractal characteristics in nature must have upper and lower bounds. This topic has been largely neglected, but recent studies emphasize the importance of layering on all scales in limiting the scaling characteristics of natural fracture systems. The determination of power law exponents and fractal dimensions from observations, although outwardly simple, is problematic, and uncritical use of analysis techniques has resulted in inaccurate and even meaningless exponents. We review these techniques and suggest guidelines for the accurate and objective estimation of exponents and fractal dimensions. Syntheses of length, displacement, aperture power law exponents, and fractal dimensions are found, after critical appraisal of published studies, to show a wide variation, frequently spanning the theoretically possible range. Extrapolations from one dimension to two and from two dimensions to three are found to be nontrivial, and simple laws must be used with caution. Directions for future research include improved techniques for gathering data sets over great scale ranges and more rigorous application of existing analysis methods. More data are needed on joints and veins to illuminate the differences between different fracture modes. The physical causes of power law scaling and variation in exponents and fractal dimensions are still poorly understood.

R. Goldfarb, D. Groves, S. Gardoll

S. Calvert, T. Pedersen

V. Zgonnik

Abstract Using an interdisciplinary approach, this paper reviews current knowledge in the field of natural hydrogen. For the first time, it combines perspectives on hydrogen from the literature of the former Eastern bloc with that of the West, including rare hardcopies and recent studies. Data are summarized and classified in three main sections: hydrogen as a free gas in different environments, as inclusions in various rock types, and as dissolved gas in ground water. This review conclusively demonstrates that molecular hydrogen is much more widespread in nature than was previously thought. Hydrogen has been detected at high concentrations, often as the major gas, in all types of geologic environment. A critical evaluation of all the proposed mechanisms regarding the origin of natural hydrogen shows that a deep-seated origin is potentially the most likely explanation for its abundance in nature. By combining available data, an estimate of 23 Tg/year for the total annual flow of hydrogen from geologic sources is proposed. This value is an order of magnitude greater than previous estimate but most likely still not large enough to account for recently discovered worldwide diffusive seepages. Hydrogen could play a critical role in mechanisms taking place in both the shallow and deep geospheres and it can influence a very wide range of natural phenomena. Hydrogen is an essential energy source for many microorganisms. Sampling for hydrogen can be a useful tool in studying natural environments, geologic mapping, monitoring of earthquakes, plotting fault traces and resource exploration. Hydrogen of geologic origin has the potential to become the renewable energy source of the future, with exploratory projects ongoing at the present time. The topic of natural hydrogen is therefore relevant from many different perspectives.

D. Wilhelms, N. Trask