Capture of planetesimals by the giant planets

We investigate the possibility of gravitational capture of planetesimals as temporary or permanent satellites of Uranus and Neptune during the process of planetary growth. The capture mechanism is based in the enhancement of the Hill's sphere of action not only due to the mass acquired by the planet, but also by the variation of the planet-Sun distance as a consequence of the scattering of planetesimals by the planets of the outer solar system. Our calculations indicate that satellite capture was very important, specially during the first stages of the accretion process, contributing in a significant way to the planetary growth.     

Modelling rainfall/watertable relations using linear response functions

Linear system theory can be used to model and predict watertable responses to precipitation inputs in an artificially drained field. The response function is mathematically equivalent to the Unit Hydrograph concept familiar to hydrologists. This paper shows that it is possible to derive such response functions, and comments on the problems encountered in their derivation. Response functions for two contrasted sites are presented, and the possibilities for their use discussed.     

The climate of Europe 6000 years ago

Palaeoclimates across Europe for 6000 y BP were estimated from pollen data using the modern pollen analogue technique constrained with lake-level data. The constraint consists of restricting the set of modern pollen samples considered as analogues of the fossil samples to those locations where the implied change in annual precipitation minus evapotranspiration (P–E) is consistent with the regional change in moisture balance as indicated by lakes. An artificial neural network was used for the spatial interpolation of lake-level changes to the pollen sites, and for mapping palaeoclimate anomalies. The climate variables reconstructed were mean temperature of the coldest month (T _c ), growing degree days above 5  °C (GDD), moisture availability expressed as the ratio of actual to equilibrium evapotranspiration (α), and P–E. The constraint improved the spatial coherency of the reconstructed palaeoclimate anomalies, especially for P–E. The reconstructions indicate clear spatial and seasonal patterns of Holocene climate change, which can provide a quantitative benchmark for the evaluation of palaeoclimate model simulations. Winter temperatures (T _c ) were 1–3 K greater than present in the far N and NE of Europe, but 2–4 K less than present in the Mediterranean region. Summer warmth (GDD) was greater than present in NW Europe (by 400–800 K day at the highest elevations) and in the Alps, but >400 K day less than present at lower elevations in S Europe. P–E was 50–250 mm less than present in NW Europe and the Alps, but α was 10–15% greater than present in S Europe and P–E was 50–200 mm greater than present in S and E Europe. Received: 3 January 1996 / Accepted: 15 July 1996     

Bias in estimating fractal dimension with the rescaled-range (R/S) technique

Fractal geostatistics are being applied to subsurface geological data as a way of predicting the spatial distribution of hydrocarbon reservoir properties. The fractal dimension is the controlling parameter in stochastic methods to produce random fields of porosity and permeability. Rescaled range (R/S)analysis has become a popular way of estimating the fractal dimension, via determination of the Hurst exponent (H). A systematic investigation has been undertaken of the bias to be expected due to a range of factors commonly inherent in borehole data, particularly downhole wireline logs. The results are integrated with a review of previous work in this area. Small datasets. overlapping samples, drift and nonstationariry of means can produce a very large bias, and convergence of estimates of H around 0.85–0.90 regardless of original fractal dimension. Nonstationarity can also account for H>1, which has been reported in the literature but which is theoretically impossible for fractal time series. These results call into question the validity of fractal stochastic models built using fractal dimensions estimated with the R/Smethod.     

Occurrence and possible significance of rare Ti oxides (Magnéli phases) in carbonaceous chondrite matrices

Abstract— Rare, ultrafine-grained Ti oxides (Ti₃O₅ and the Magnéli phases, Ti₅O₉ and Ti₈O₁₅) have been identified by transmission electron microscopy in the CM2 carbonaceous chondrite, Bells and a carbonaceous chondrite matrix clast from the Nilpena polymict ureilite. In both meteorites the Ti oxides occur in the matrix as isolated grains and clusters of two or more grains. They are euhedral in shape and have grain sizes of 0.05–0.3 μm. Magnéli phases have been recently shown to be a common component in some interplanetary dust particles, but this is the first reported occurrence in a meteorite. The morphological properties and grain size of the Ti oxides are consistent with formation by vapor phase condensation either within the solar nebula or possibly in a presolar environment.     

Dynamical model of prominence formation and oscillation,I

We investigate a dynamical model of prominence formation in a current sheet at the boundary between two regions of opposite magnetic polarity. Coupled nonlinear equations describing the temporal compression and condensation of plasma in the current sheet are set up as a natural extension of the usual equations for current sheet collapse (Imshennik and Syrovatskii, 1967). It is shown that under certain conditions the current sheet undergoes a nonlinear oscillation during the compression. The thermal instability with cooling is driven by a density enhancement produced during the current sheet formation stage.     

Seismic stratigraphy, buried beach ridges and contourite drifts: the Late Quaternary history of the closed Lago Cardiel basin, Argentina (49°S)

The results of a seismic stratigraphic analysis of a closed lake basin, Lago Cardiel, in southernmost South America are reported. Very few high-resolution, continental records spanning the Late Quaternary have been obtained from this region. Seismic sequence stratigraphic analysis allows a reconstruction of lake level variations. Two major hiatuses of unknown age occurred during the early evolution of the basin with the deposition of an alluvial fan in a restricted area in the intervening time period. Following the development of a relatively shallow lake during the late Pleistocene and a short desiccation pulse around 11 220 ¹⁴C yr BP, a transgression of over 135 m occurred at the beginning of the Holocene. The transgression was associated with the formation of beach ridges preserved in the lake stratigraphy on the floor of the modern Lago Cardiel at four different elevations. The preservation of largely unreworked beach ridges indicates a stepwise rise in the lake level. There is no seismic evidence of a major lowering of the lake below modern level during the entire Holocene. Deposition since the mid-Holocene is marked by strong lateral differences in sediment accumulation with a depocentre slightly to the north of the basin midpoint and a pronounced mounded distribution. Seismic reflection geometries, as well as sedimentological characteristics indicate a lacustrine contourite drift covering an area of 80–100 km². As Lago Cardiel is under the influence of westerly winds, these most likely drove lake circulation. The identification of drowned beach ridges and of contourite drifts illustrates that high-resolution seismic stratigraphy is not only a powerful tool in reconstructing past lake level elevations for closed lake basins, but it can also provide information about the rate of lake level changes and the presence and strength of lake currents.     

Clay mineral records of East Asian monsoon evolution during late Quaternary in the southern South China Sea

High-resolution clay mineral records combined with oxygen isotopic stratigraphy over the past 190 ka during late Quaternary from core MD01-2393 off the Mekong River in the southern South China Sea are reported to reconstruct a history of East Asian monsoon evolution. The dominating clay mineral components indicate a strong glacial-interglacial cyclicity, with high glacial illite, chlorite, and kaolinite contents and high interglacial smectites content. The provenance analysis indicates the direct input of clay minerals via the Mekong River drainage basin. Illite and chlorite derived mainly from the upper reach of the Mekong River, where physical erosion of meta-sedimentary rocks is dominant. Kaolinite derived mainly from active erosion of inhered clays from reworked sediments in the middle reaches. Smectites originated mainly through bisiallitic soils in the middle to lower reaches of the Mekong River. The smectites/(illite+chlorite) and smectites/kaolinite ratios are determined as mineralogical indicato