Efficient two-dimensional magnetotellurics modelling using implicitly restarted Lanczos method

This paper presents an efficient algorithm, FDA2DMT (Free Decay Analysis for 2D Magnetotellurics (MT)), based on eigenmode approach to solve the relevant partial differential equation, for forward computation of two-dimensional (2D) responses. The main advantage of this approach lies in the fact that only a small subset of eigenvalues and corresponding eigenvectors are required for satisfactory results. This small subset (pre-specified number) of eigenmodes are obtained using shift and invert implementation of Implicitly Restarted Lanczos Method (IRLM). It has been established by experimentation that only 15–20% smallest eigenvalue and corresponding eigenvectors are sufficient to secure the acceptable accuracy. Once the single frequency response is computed using eigenmode approach, the responses for subsequent frequencies can be obtained in negligible time. Experiment design results for validation of FDA2DMT are presented by considering two synthetic models from COMMEMI report, Brewitt-Taylor and Weaver (1976) model and a field data based model from Garhwal Himalaya.     

Improvements in medium range weather forecasting system of India

Medium range weather forecasts are being generated in real time using Global Data Assimilation Forecasting System (GDAFS) at NCMRWF since 1994. The system has been continuously upgraded in terms of data usage, assimilation and forecasting system. Recently this system was upgraded to a horizontal resolution of T574 (about 22 km) with 64 levels in vertical. The assimilation scheme of this upgraded system is based on the latest Grid Statistical Interpolation (GSI) scheme and it has the provision to use most of available meteorological and oceanographic satellite datasets besides conventional meteorological observations. The new system has an improved procedure for relocating tropical cyclone to its observed position with the correct intensity. All these modifications have resulted in improvement of skill of medium range forecasts by about 1 day.     

Two layers of Australasian impact ejecta in the Indian Ocean?

Abstract— Only 2 Australasian tektites have been found in the Indian Ocean, and both are associated with surficial sediments. We collected cores from both locations where the tektites have been reported. The microtektites in these cores (and both the tektites, as reported earlier) have chemical compositions within the compositional range previously reported for Australasian tektites and microtektites. In both locations, while the tektites are occurring at the sediment/water interface, the microtektites are found buried in older horizons beneath the seafloor at stratigraphic levels, conforming to the radiometric age of the strewn field. Thus, at first glance, there appear to be 2 layers of Australasian impact ejecta in the Indian Ocean. However, the manganese nodules are associated with the tektites which, although millions of years old, are invariably resting on recent sediments. Therefore, the mechanism that retains nodules at the seafloor also seems to be operative on the tektites, thus leading to this apparent “age paradox” of tektite/microtektite distribution in the Indian Ocean, although they both belong to the same impact event.     

Influence of rigid boundary on the propagation of torsional surface wave in an inhomogeneous layer

Journal of Earth System Science - The present work illustrates a theoretical study on the effect of rigid boundary for the propagation of torsional surface wave in an inhomogeneous crustal layer...     

Tectonics or climate: What drove the Miocene global expansion of C4 grasslands?

Investigations into the most plausible causes which triggered the Late Miocene global expansion of C₄ grasslands have reached no consensus and are still being debated. The global decline in CO₂ levels in the Late Miocene has been thought to be the most common driver. Although C₄ grassland expansion was largely confined to the Late Miocene, their first appearance varied significantly in different parts of the Old and New world and therefore emphasizes role of local and/or regional factors may or may not be in combination with one or more global factors. The dynamic Himalayan orogen constitutes a significant global tectonic event that is believed to have affected global climate. Hence, a study of the Himalayan foreland basin sediments could help in gleaning out possible causes behind this major paleoecologic event, which could perhaps be directly or indirectly related. Stable carbon isotope analyses of a total of 141 pedogenic carbonates in the Mio-Pleistocene Siwalik paleosols of the Ramnagar sub-basin are coherent, providing a better insight into paleovegetational changes across the sub-basin on temporal and spatial scales. Furthermore, paleovegetational history during Oligocene is reconstructed from Dagshai paleosols exposed in the Subathu sub-basin. The isotopic results show dominance of C₃ vegetation pre-7 Ma and dominance of C₄ vegetation post-5 Ma. Percentage abundance of C₄ vegetation was less than 20 % pre-7 Ma but increased to more than 40 % post-5 Ma, reaching up to 100 % in the youngest analyzed sediments. There is exclusive dominance of C₃ vegetation during Oligocene. These results conform to the pattern of change in vegetation documented in other parts of the Himalayan belt. The global expansion of C₄ grasslands largely during Late Miocene have long been linked with then climate changes, particularly brought by declining atmospheric CO₂ level, large-scale fires, intensification of monsoon, seasonality, and aridity as demonstrated by various researchers. These major hypotheses explaining expansion of C₄ grasslands during Late Miocene are not very convincing due to significant shortcomings associated with them, for example, expansion of C₄ grasslands in non-monsoonal regions put serious constraints on the monsoonal intensification hypothesis. Likewise, aridity as being the primary driver of C₄ grassland expansion seems hard to support since there is no substantial evidence that could suggest global onset of arid conditions at about same geologic time, and some most arid periods, like the Triassic, show no evidence of C₄ plants. We suggest that the initial lowering of CO₂ below 450 ppmV created an environment for the beginning of C₄ vegetation, but the persistence of this threshold value for a considerable time during Late Miocene appears to be the probable cause of the Late Miocene global expansion of C₄ grasslands irrespective of their time of first appearance. It has been deduced through the correlation of Himalayan tectonic events with atmospheric CO₂ levels and paleovegetational changes since Upper Miocene times that it was indeed the Late Miocene continuous, intense tectonic instability of the Himalayas that significantly decreased atmospheric CO₂ levels and which perhaps played a key role in changing the nature of photosynthetic pathways.