High-resolution profile of stable isotopes and iridium across a K/T boundary section from Koshak Hill, Mangyshlak, Kazakhstan

Oxygen and carbon isotopes were measured to a high depth resolution in a nannoplankton carbonate sequence spanning the Cretaceous-Tertiary boundary at Koshak Hill, Mangyshlak Peninsula, Kazakhstan. The boundary is characterized by the presence of a ∼ 1 cm thick clay layer having a sharp peak in iridium concentration with a maximum value of 3.7 ng g^-1. The δ¹⁸O data reveal rapid (∼10³ years) excursion of sea-surface temperature at the boundary where an initial cold pulse is followed by a persistent warm period. The δ¹³C data, in contrast, indicate only a gradual change in productivity across the boundary. The observations suggest a biogeochemical scenario for the boundary event pertaining to shallow epicontinental seas.     

Highly heterogeneous Precambrian basement under the central Deccan Traps, India: Direct evidence from xenoliths in dykes

Crustal or mantle xenoliths are not common in evolved, tholeiitic flood basalts that cover huge areas of the Precambrian shields. Yet, the occasional occurrences provide the most direct and unequivocal evidence on basement composition. Few xenolith occurrences are known from the Deccan Traps, India, and inferences about the Deccan basement have necessarily depended on geophysical studies and geochemistry of Deccan lavas and intrusions. Here, we report two basalt dykes (Rajmane and Talwade dykes) from the central Deccan Traps that are extremely rich in crustal xenoliths of great lithological variety (gneisses, quartzites, granite mylonite, felsic granulite, carbonate rock, tuff). Because the dykes are parallel and only 4 km apart, and only a few kilometres long, the xenoliths provide clear evidence for high small-scale lithological heterogeneity and strong tectonic deformation in the Precambrian Indian crust beneath. Measured ⁸⁷Sr/⁸⁶Sr ratios in the xenoliths range from 0.70935 (carbonate) to 0.78479 (granite mylonite). The Rajmane dyke sampled away from any of the xenoliths shows a present-day ⁸⁷Sr/⁸⁶Sr ratio of 0.70465 and initial (at 66 Ma) ratio of 0.70445. The dyke is subalkalic and fairly evolved (Mg No. = 44.1) and broadly similar in its Sr-isotopic and elemental composition to some of the lavas of the Mahabaleshwar Formation. The xenoliths are comparable lithologically and geochemically to basement rocks from the Archaean Dharwar craton forming much of southern India. As several lines of evidence suggest, the Dharwar craton may extend at least 350–400 km north under the Deccan lava cover. This is significant for Precambrian crustal evolution of India besides continental reconstructions.     

Relationships of surface ozone with its precursors,particulate matter and meteorology over Delhi

Journal of Atmospheric Chemistry - The paper presents the temporal variations of surface ozone (O3) and its precursors (oxides of nitrogen (NOX), carbon monoxide (CO), methane (CH4) and non-methane...     

<Emphasis Type="Italic">Holigarna</Emphasis> (Anacardiaceae) from the early Eocene of Western India and its palaeogeographical and palaeoclimatological significance

Thick lignite deposits on the western margin of India were preserved on account of thick vegetation which got buried underneath due to tectonic activities that occurred in northwestern part of India during the early Palaeogene. Study of the fossil flora preserved in these beds is important to deduce the palaeoclimate of the region. Therefore, a field work was undertaken to collect plant fossils from the Tarkeshwar lignite mine, Surat district, Gujarat (western India) which is still unexplored as far as the plant mega-remains are concerned. A fossil wood is systematically described from the Cambay Shale Formation of the mine considered to be of early Eocene in age. It shows close affinities with Holigarna (Anacardiaceae), particularly with H. beddomei and H. grahamii. This genus is evergreen in nature and grows in the Indo-Malayan region, while both the modern analogues of the fossil are endemic to semi-evergreen to evergreen forests ofWestern Ghats, south India, located at the equator. This not only confirms the palaeolatitudinal position of the Indian subcontinent at the equator during the early Palaeogene, but also indicates the presence of equatorial rain forest in the Indian subcontinent during the period. Based on the timetree, Sapindales started to diverge in the early Palaeocene (62–57 Ma) as its oldest fossil is from the K-Pg boundary. Discovery of the present anacardiaceous fossil (belonging to Sapindales) from the early Eocene strengthens the above phylogenetic view.     

Discovery of <Emphasis Type="Italic">Elephas</Emphasis> cf. <Emphasis Type="Italic">namadicus</Emphasis> from the late Pleistocene strata of Marginal Ganga Plain

We describe an elephant skull recovered from a cliff section of Dhasan river of Marginal Ganga Plain. The dental morphology and cranial features of the skull have been compared with the known species of Elephas from the Indian subcontinent. Although it shows very near resemblance to Elephas namadicus, but being an isolated specimen its specific identity cannot be proclaimed with certainty. As such, the specimen is provisionally referred as E. cf. namadicus. The Optically Stimulated Luminescence ages place this find at ~56 ka BP. This is the first chronologically well constrained report of E. cf. namadicus from the Ganga Plain.     

Influence of basin-scale and mesoscale physical processes on biological productivity in the Bay of Bengal during the summer monsoon

Physical forcing plays a major role in determining biological processes in the ocean across the full spectrum of spatial and temporal scales. Variability of biological production in the Bay of Bengal (BoB) based on basin-scale and mesoscale physical processes is presented using hydrographic data collected during the peak summer monsoon in July–August, 2003. Three different and spatially varying physical processes were identified in the upper 300 m: (I) anticyclonic warm gyre offshore in the southern Bay; (II) a cyclonic eddy in the northern Bay; and (III) an upwelling region adjacent to the southern coast. In the warm gyre (>28.8 °C), the low salinity (33.5) surface waters contained low concentrations of nutrients. These warm surface waters extended below the euphotic zone, which resulted in an oligotrophic environment with low surface chlorophyll a (0.12 mg m⁻³), low surface primary production (2.55 mg C m⁻³ day⁻¹) and low zooplankton biovolume (0.14 ml m⁻³). In the cyclonic eddy, the elevated isopycnals raised the nutricline upto the surface (NO₃–N > 8.2 μM, PO₄–P > 0.8 μM, SiO₄–Si > 3.5 μM). Despite the system being highly eutrophic, response in the biological activity was low. In the upwelling zone, although the nutrient concentrations were lower compared to the cyclonic eddy, the surface phytoplankton biomass and production were high (Chl a – 0.25 mg m⁻³, PP – 9.23 mg C m⁻³ day⁻¹), and mesozooplankton biovolume (1.12 ml m⁻³) was rich. Normally in oligotrophic, open ocean ecosystems, primary production is based on ‘regenerated’ nutrients, but during episodic events like eddies the ‘production’ switches over to ‘new production’. The switching over from ‘regenerated production’ to ‘new production’ in the open ocean (cyclonic eddy) and establishment of a new phytoplankton community will take longer than in the coastal system (upwelling). Despite the functioning of a cyclonic eddy and upwelling being divergent (transporting of nutrients from deeper waters to surface), the utilization of nutrients leading to enhanced biological production and its transfer to upper trophic levels in the upwelling region imply that the energy transfer from primary production to secondary production (mesozooplankton) is more efficient than in the cyclonic eddy of the open ocean. The results suggest that basin-scale and mesoscale processes influence the abundance and spatial heterogeneity of plankton populations across a wide spatial scale in the BoB. The multifaceted effects of these physical processes on primary productivity thus play a prominent role in structuring of zooplankton communities and could consecutively affect the recruitment of pelagic fisheries.     

Analytical expressions for determining the stability of cohesionless soil slope under generalized seismic conditions

In recent major earthquakes, the researchers have found the need for consideration of vertical seismic acceleration for the stability analysis of the man-made and natural slopes. However, in most past studies, the performance of slopes has been assessed by accounting only the horizontal seismic component of the ground motion, without giving due weightage to the effect of vertical component. In the present study, analytical expressions are derived to determine the factor of safety, yield seismic coefficient and consequently the seismic displacement of cohesionless soil slope under combined horizontal and vertical components of the ground motion. The derivation uses the Newmark’s sliding block approach, in which the soil slope with a planar failure surface within the framework of conventional pseudo-static analysis is assumed to follow the Mohr-Coulomb failure criterion. The effects of vertical seismic coefficient on the stability of cohesionless slope have been studied through a set of graphical presentations for a specific range of soil parameters. It is observed that overlooking the effect of the vertical component of the ground motion on factor of safety and the displacement while designing the slope may be detrimental, resulting in the slope failure. The general expressions presented in this paper may be highly useful in the field of earthquake geotechnical engineering practice for designing the cohesionless soil slopes under combined horizontal and vertical seismic loads.