Sunil K Singh
Articles written in Journal of Earth System Science
Volume 107 Issue 4 December 1998 pp 283-291
The role of silicate and carbonate weathering in contributing to the major cation and Sr isotope geochemistry of the headwaters of the Ganga-Ghaghara-Indus system is investigated from the available data. The contributions from silicate weathering are determined from the composition of granites/ gneisses, soil profiles developed from them and from the chemistry of rivers flowing predominantly through silicate terrains. The chemistry of Precambrian carbonate outcrops of the Lesser Himalaya provided the data base to assess the supply from carbonate weathering. Mass balance calculations indicate that on an average ∼ 77% (Na + K) and ∼ 17% (Ca + Mg) in these rivers is of silicate origin. The silicate Sr component in these waters average ∼40% and in most cases it exceeds the carbonate Sr. The observations that (i) the87Sr/86Sr and Sr/Ca in the granites/gneisses bracket the values measured in the head waters; (ii) there is a strong positive correlation between87Sr/86Sr of the rivers and the silicate derived cations in them, suggest that silicate weathering is a major source for the highly radiogenic Sr isotope composition of these source waters. The generally low87Sr/86Sr (< 0.720) and Sr/Ca (∼ 0.2 nM/ μM) in the Precambrian carbonate outcrops rules them out as a major source of Sr and87Sr/86Sr in the headwaters on a basin-wide scale, however, the high87Sr/86Sr (∼ 0.85) in a few of these carbonates suggests that they can be important for particular streams. The analysis of87Sr/86Sr and Ca/Sr data of the source waters show that they diverge from a low87Sr/86Sr and low Ca/Sr end member. The high Ca/Sr of the Precambrian carbonates precludes them from being this end member, other possible candidates being Tethyan carbonates and Sr rich evaporite phases such as gypsum and celestite. The results of this study should find application in estimating the present-day silicate and carbonate weathering rates in the Himalaya and associated CO2 consumption rates and their global significance.
Volume 108 Issue 1 March 1999 pp 1-14
The cause for prolific seismicity in the Koyna region is a geological enigma. Attempts have been made to link occurrence of these earthquakes with tectonic strain as well as the nearby reservoirs. With a view to providing reliable seismological database for studying the earth structure and the earthquake process in the Koyna region, a state of the art digital seismic network was deployed for twenty months during 1996–97. We present preliminary results from this experiment covering an area of 60 × 80 km2 with twenty seismic stations. Hypocentral locations of more than 400 earthquakes confined to 11×25 km2 reveal fragmentation in the seismicity pattern — a NE — SW segment has a dip towards NW at approximately 45°, whilst the other two segments show a near vertical trend. These seismic segments have a close linkage with the Western Ghat escarpment and the Warna fault. Ninety per cent of the seismicity is confined within the depth range of 3–10 km. The depth distribution of earthquakes delimits the seismogenic zone with its base at 10 km indicating a transition from an unstable to stable frictional sliding regime. The lack of shallow seismicity between 0 and 3 km indicates a mature fault system with well-developed gouge zones, which inhibit shallow earthquake nucleation. Local earthquake travel time inversion for P- and S-waves show ≈ 2% higher velocity in the seismogenic crust (0–10 km) beneath the epicentral tract relative to a lower velocity (2–3%) in the adjoining region. The high P- and S-wave velocity in the seismogenic crust argues against the presence of high pressure fluid zones and suggests its possible linkage with denser lithology. The zone of high velocity has been traced to deeper depths (≈ 70 km) through teleseismic tomography. The results reveal segmented and matured seismogenic fault systems in the Koyna region where seismicity is possibly controlled by strain build up due to competent lithology in the seismic zone with a deep crustal root.
Volume 108 Issue 3 September 1999 pp 179-187
The applications of the187Re-187Os isotope pair as a petrogenetic and geologic tracer are increasing in recent years due to several advances in the chemical extraction and purification of Re and Os, occurring at ppb levels in environmental samples, and in the precise determination of the Os isotope composition. We have established in our laboratory; based on available methods, chemical procedures and Negative Thermal Ionisation Mass Spectrometric techniques for the measurement of Re-Os concentrations in environmental samples and the Os isotope composition in them. Using these techniques, we are able to determine187Os/186Os ratios with a precision of ∼ 1% (±2σμ; twice the standard error of the mean) in several tens of picogram of Os. Preliminary analysis of black shales from the Lower Tal section of the Maldeota phosphorite mine yields a mean187Re-187Os model age of 597 ± 30 Ma. The187Os/186Os and Os concentration in black shales of the Lesser Himalaya range from 8 to 96 and 0.02 to 13 ng g-1 respectively. The mean187Os/186Os in these samples is ∼ 25, significantly higher than the crustal value of ∼ 10.5, suggesting that these black shales could be an important source of radiogenic Os to the rivers draining the Himalaya and to the steady increase in187Os/186Os of the oceans through the Cenozoic.