Conductivity anomalies: lower crust or asthenosphere?

The indication from surface measurements of a zone of relatively high conductivity (resistivity<200 ohm-m) at depths between 20 and 50 km has become so general over the Earth that regions without this zone can be considered anomalous. However, the depths indicated often span the lower crust and the uppermost mantle, so that before any effect can be definitely attributed to one or the other, the depth resolution in the electromagnetic measurements must be carefully considered. This paper applies the eigenvector decomposition of generalized linear inverse theory to soundings by Schlumberger resistivity, by magnetotellurics, by man-made electromagnetic fields formed by controlled current flow in grounded electric transmission lines, and by natural magnetic field variation studies to improve the bounds on depth, thickness and conductivity of a conductive layer. It is shown that many of the methods are capable of giving the depth to the top of a conductor with remarkable accuracy. Joint inversion of two or more of them offers an advantage in the separation of thickness and conductivity of both conductive and resistive layers. Natural geomagnetic field transfer functions, while accurately mapping the position of the edge of a conductor, do not provide the resolution of the other techniques, largely because the frequencies that can be practically measured at present are much too low.     

Enhancement of the accuracy of single-epoch GPS positioning for long baselines by local ionospheric modelling

Single-epoch relative GPS positioning has many advantages, especially for monitoring dynamic targets. In this technique, errors occurring in previous epochs cannot affect the position accuracy at the current epoch, but careful processing is required, and resolving carrier phase ambiguities is essential. Statistical ambiguity resolution functions have been used to determine the best values of these ambiguities. The function inputs include as a minimum the known base station position, the approximate roving antenna “seed” position, and the dual-frequency carrier phase measurements from both receivers. We investigate different solutions to find the ambiguity function inputs that achieve the highest ambiguity resolution success rate. First, we address the rover seed position. A regionally filtered undifferenced pseudorange coordinate solution proves better than a double-differenced one. Multipath errors approximately repeat themselves every sidereal day in the case of static or quasi-static antennas; applying a sidereal filter to the pseudorange-derived positions mitigates their effects. Second, we address the relative carrier phase measurements, which for medium to long baselines are significantly affected by ionospheric propagation errors imperfectly removed during differencing. In addition to the International GNSS Service ionospheric model, we generate a local pseudorange-based ionospheric correction. Applying this correction improves the quality of the phase measurements, leading to more successful ambiguity resolution. Temporally smoothing the correction by means of a Kalman filter further improves the phase measurements. For baselines in the range 60–120 km, the mean absolute deviation of single-epoch coordinates improves to 10–20 cm, from 30–50 cm in the default case.     

Weathering processes in baked sediments and their effects on archaeomagnetic field-intensity measurements

Thellier-type measurements of ancient field intensity on specimens of clay and other sediments, which were apparently well-baked and oxidised in ancient times, often fail to give consistent results over part or even most of the blocking-temperature spectrum. It is suggested that post-baking chemical alteration, or weathering, leading to the presence of hydrated Fe minerals, is a major cause of non-ideal behaviour in such material. The behaviour of hypothetical specimens containing either goethite or lepidocrocite can be predicted using simple models, and actual examples are given from two sites in southern Australia which show some similarities with the predicted model behaviour. The results of the Thellier measurements, after interpretations, agree closely with previously published results from the northern hemisphere for the period 4700-4200 yr. B.P. The presence of hydrated minerals may not be readily detected by methods other than the Thellier technique and, if so, would result in estimates of the ancient geomagnetic field strength that are systematically too low.     

LONG TERM HISTORY (DEXTRAL TO SINISTRAL AND DUCTILE TO BRITTLE) OF THE DAMXUNG—JIALI SHEAR ZONE IN SOUTHERN TIBET

LONG TERM HISTORY (DEXTRAL TO SINISTRAL AND DUCTILE TO BRITTLE) OF THE DAMXUNG—JIALI SHEAR ZONE IN SOUTHERN TIBET     

Rescue palaeontology

A technique for replicating large rock outcrops has been developed so that casts of these surfaces can be archived in their present stage of preservation. Researchers can then study these casts in the laboratory under optimum lighting conditions, sometimes revealing new fossil evidence, while students and members of the public can be shown geological features that might otherwise be inaccessible to them.     

Interhemispheric anti-phasing of rainfall during the last glacial period

We have obtained a high-resolution oxygen isotopic record of cave calcite from Caverna Botuverá (27°13′S, 49°09′W), southern Brazil, which covers most of the last 36 thousand years (ka), with an average resolution of a few to several decades. The chronology was determined with 46 U/Th ages from two stalagmites. Tests for equilibrium conditions show that oxygen isotopic variations are primarily caused by climate change. We interpret our record in terms of meteoric precipitation changes, hence the variability of South American Monsoon (SAM) intensity. The oxygen isotopic profile broadly follows local insolation changes and shows clear millennial-scale variations during the last glacial period with amplitudes as large as 3‰ but with smaller centennial-scale shifts (<1‰) during the Holocene. The overall record is strikingly similar to, but strongly anti-correlated with, a number of records from the Northern Hemisphere.We compared our record to other precisely dated contemporaneous records from Hulu Cave eastern China. Minima in δ¹⁸O (wet periods, intense SAM) at our site are synchronous with maxima in δ¹⁸O (dry periods, weak East Asian Monsoon, EAM) in eastern China (within precise dating errors) and vice versa. This anti-phased precipitation relationship between two low-latitude locations may be interhemispheric in extent, based on comparison with records from other sites. Precipitation anti-phasing may be related to north–south shifts in the mean position of the intertropical convergence zone (ITCZ) and asymmetry in Hadley circulation in two hemispheres, associated not with seasonal changes as observed today, but with millennial-scale climate shifts. The millennial-scale atmospheric see-saw patterns that we observe could have important controls and feedbacks on climate within hemispheres because of water vapor's greenhouse properties.