Santosh K. Rai

Temporal variation in Sr and 87Sr/86Sr of the Brahmaputra: Implications for annual fluxes and tracking flash floods through chemical and isotope composition

Temporal variations in dissolved Sr and its 87Sr/86Sr in the Brahmaputra River were determined for the first time by analyzing biweekly samples collected over a period of about a year at Guwahati. Sr in the Brahmaputra range from 0.604 to 1.392 μM with 87Sr/86Sr of 0.71594 to 0.71802. Sr concentration decreases in monsoon (except for the 15 June 2000 sample, which had anomalously high concentrations of Sr and major ions) compared to the nonmonsoon period. The decrease in elemental abundances, a factor of ∼2, is not proportional to the increase in discharge, an order of magnitude, indicating enhanced weathering during monsoon. This can be a cumulative effect of an increase in drainage area and in physical weathering during monsoon. 87Sr/86Sr and other proxies of silicate weathering show a relatively lower contribution from silicate weathering to the major ion and Sr budget during monsoon with a concomitant increase in carbonate weathering contribution. Shorter interaction time between water and minerals during monsoon coupled with the slower weathering kinetics of silicate compared to carbonate can be contributing factors to the seasonal changes in their relative contributions to the major ion budget. This study shows that the annual fluxes of various ions, ΣCatsil, and Srsil calculated on the basis of measured biweekly concentrations and monthly water discharge data are roughly within ±35% of those calculated using either the lowest or the highest concentrations and annual water discharge. This implies that a tropical region such as in India, where river discharge is governed mostly by monsoon rains, the annual elemental fluxes measured during monsoon would be a good approximation for their discharge weighted annual fluxes; however, an additional sampling during a drier period would be required to understand the weathering processes. The geochemistry of the 15 June 2000 sample is anomalous. The chemistry of this sample is similar to those from the Tibetan region, an inference consistent with reports of a flash flood in the Brahmaputra due to a natural dam burst in the Yigong River of Tibet. These results highlight the use of the chemical and isotopic composition of water samples to track and quantify the source and discharge of a flash flood.

Stable (δ 13 C and δ 15 N) Isotopes and Magnetic Susceptibility Record of Late Holocene Climate Change from a Lake Profile of the Northeast Himalaya

This study aims at reconstructing paleovegetation history in the northeast Himalaya during the past 2700 yrs using a 100 cm long sedimentary section selected near Anini village, Dibang valley, Arunachal Pradesh (India). Stable carbon and nitrogen isotopic composition (δ13C and δ15N); and total organic carbon (TOC) and total nitrogen (TN) content in organic matter as well as magnetic susceptibility were determined. Results obtained show a strong correlation between TOC-TN indicating similar source for organic matter. δ13C and TOC/TN ratio throughout the stratigraphic unit seems to be poorly correlated which suggests that the primary carbon isotope signatures are preserved in the sedimentary environment. Overall, δ13C values of the organic matter in the section vary in the range from–23.9 to–21.2 ‰ which is typical of mixed C3-C4 plants.. A distinguishable increasing trend in δ13C during 2700 to 1300 yrs indicates change in vegetation pattern from C3 to C4 plants due to significant climate change from wetter to drier conditions. After 1300 yrs δ13C values decrease rapidly by 1.3‰ and suggest increasing abundance of C3 plants in wetter climatic condition. The relatively small variation in δ13C values in the upper part of the section during 1200 yrs to present suggest a stable climatic condition.

Traumatic thrombosis of internal carotid artery

Abstract 27 year old soldier sustained closed head injury in a road accident. He was unconscious for about 6hr. When seen at our hospital his GCS was 15/15; normal pupil but he had left hemiplegia. Head CT scan showed a low attenuation lesion in right fronto-parietal region suggestive of infarct. A cerebral angiogram revealed right internal carotid occlusion. He was treated with unsuccessful thrombectomy and STA-MCA by pass, anti-platelet agents and improved partially. Problems of traumatic ICA thrombosis in the setting of head injury are discussed.

A quantitative analysis of the ramganga drainage basin and structural control on drainage pattern in the fault zones, Uttarakhand

The present study incorporates quantitative analysis of drainage of the Ramganga basin, Garhwal Himalaya, Uttarakhand. The drainage parameters (linear, areal and relief) of the Ramganga basin from Dudhatolidhar peak its point of origin to Kalagarh from where the river Ramganga enter into the plain, are covered. The impact of the morphological characters on the terrain is reflected by the drainage basin of the area. The calculated values of all the morphometric parameter of 38 Vth order intrabasins for different lithotectonic units were computed under four sectors viz., NE of NAT, between NAT and SAT, between SAT and MBT and downstream of MBT. In the lower reaches of Ramganga basin the various morphometric parameters shows existence of proportionality between stream length and catchment area while such proportionality is non-existence in the crystalline zone. Structural control on the drainage patterns of the Ramganga river and its tributaries are observed in the form of straight course, wide valleys with thick fluvial deposits and relatively gentle stream gradient along the transverse faults to incised valleys; less fluvial deposits in the banks and steeper stream gradient with knick points in the hanging wall of thrusts.

Fluid Inclusion Study of the Higher Himalayan Quartzitic Pelites, Garhwal Himalaya, India: Implications for Recrystallization History of Metasediments

Quartzitic pelites forms a part of Higher Himalayan Crystalline of higher geotectonic zone in Garhwal Himalaya. Quartzitic pelites (locally known as Pandukeshwar Quartzite) in Garhwal Himalaya is sandwiched between high grade metamorphic rocks of Central Crystallines and Badrinath Formation. Fluid inclusion studies are carried out on the detrital, and recrystallized quartz grains of quartzitic pelites to know about the fluid phases present during recrystallization processes at the time of maximum depth of burial. The quartzitic pelite (Pandukeshwar Quartzite) essentially consists of recrystallised quartz with accessory minerals like mica and feldspar. Fluid microthermometry study reveals the presence of three types of fluids: (i) high-salinity brine, (ii) CO2-H2O and (iii) H2O-NaCl. These fluids were trapped during the development of grain and recrystallization processes. The high saline brine inclusions and CO2-H2O fluid with the density of 0.90 to 0.97 gm/cm3 are remnants of provenance area. CO2 density in detrital quartz grains characterise the protolith of the sandstone as granite or metamorphic rock. The H2O-NaCl fluids involved in the recrystallization processes at temperature-pressure of 430-350°C; 4.8 to 0.5 Kbars as constrained by fluid isochores of CO2-H2O and H2O-NaCl inclusions and bulging and subgrain development during recrystallization processes. The re-equilibration of the primary fluid due to elevated internal and confining pressure is evident from features like ‘C’ shaped cavities, stretching of the inclusions, their migration and decrepitation clusters. The observed inclusion morphology revealed that the rocks were exhumed along an isothermal decompression path.