A new occurrence of thorianite from syenitic pegmatite near Bhaluchuan,Odisha

We report a new occurrence of thorianite from syenitic pegmatite near Bhaluchuan, Sambalpur district, Odisha. The thorianite is brown to deep-brown with round grains of 2 to 10 mm size. The chemical analysis of the investigated thorianite reveals 64.8% ThO₂, 25% U₃O₈, 3.81% PbO and 1.7% Fe₂O₃. Calculated structural formula of the thorianite is (Th⁺⁴ _0.61U⁺⁴ _0.14U⁺⁶ _0.08ΣREE⁺³ _0.017Pb⁺² _0.04Ca⁺² _0.01Mn⁺² _0.001Fe⁺³ _0.05Al⁺³ _0.003Sc⁺² _0.002K⁺¹ _0.005Na⁺¹ _0.008 Si⁺⁴ _0.04Ti⁺⁴ _0.02)O_2.08. Chondrite-normalised rare-earth element (REE) plot of the thorianite reveals enrichment of light REE (LREE) over heavy REE (HREE) with pronounced negative Eu-anomaly (Eu/Eu* = 0.35). The (ΣLREE/ΣHREE)_N ratio is perceptibly high (2.76). The (La/Lu)_N (42.31), (La/Yb)_N (27.49) and (Ce/Yb)_N (21.58) ratios are also very high. X-ray diffraction (XRD) pattern of the investigated thorianite displays sharply-defined reflections. Corresponding interplanar spacings (d-spacings) of all the reflections are in very close agreement with those published for thorianite standard in International Centre for Diffraction Data (ICDD) Card No. 4-556. However, I/Io of two reflections (1.9694Å and 1.6787Å) are lower than those published for thorianite standard. The unit cell parameter (a_o) of the investigated thorianite (a_o 5.5750Å) is also less than a_o of thorianite standard (a_o 5.6000Å and V 175.62ų), which is because of extensive substitution of Th by U.     

Thermohaline structure of an inverse estuary--The Gulf of Kachchh: measurements and model simulations

The Gulf of Kachchh (GoK) is situated in the northeastern Arabian Sea. The presence of several industries along its coastal belt makes GoK a highly sensitive coastal ecosystem. In the present study, an attempt is made for the first time to study GoK thermohaline structure and its variability, based on field measurements and model simulations. Though GoK is considered as a well-mixed system, the study reveals that only the central Gulf is well mixed. Vertical gradients in temperature and salinity fields are noticed in the eastern Gulf, where a cold and high saline tongue is observed in the subsurface layers. Salinity indicates the characteristic feature of an inverse estuary with low values (37.20 psu) near the mouth and high values (>40.0 psu) near the head of the Gulf. The model simulated temperature and salinity fields exhibit semidiurnal oscillations similar to that of field observations. Model results show cold, high saline waters advecting from the east during ebb forming a transition zone, which oscillates with tides. A high salinity tongue is seen in the bottom layer, indicating a westward flowing bottom current. The transient zone acts as an dynamic barrier, and plays a vital role in the pollutant transport.     

Correlation of the oldest Toba Tuff to sediments in the central Indian Ocean Basin

We have identified an ash layer in association with Australasian microtektites of ∼0.77 Ma old in two sediment cores which are ∼450 km apart in the central Indian Ocean Basin (CIOB). Morphology and chemical composition of glass shards and associated microtektites have been used to trace their provenance. In ODP site 758 from Ninetyeast Ridge, ash layer-D (13 cm thick, 0.73–0.75 Ma) and layer-E (5 cm thick, 0.77–0.78 Ma) were previously correlated to the oldest Toba Tuff (OTT) eruptions of the Toba caldera, Sumatra. In this investigation, we found tephra ∼3100 km to the southwest of Toba caldera that is chemically identical to layer D of ODP site 758 and ash in the South China Sea correlated to the OTT. Layer E is not present in the CIOB or other ocean basins. The occurrence of tephra correlating to layer D suggests a widespread distribution of OTT tephra (∼3.6 × 10⁷ km²), an ash volume of at least ∼1800 km³, a total OTT volume of 2300 km³, and classification of the OTT eruption as a super-eruption.     

Shift in detrital sedimentation in the eastern Bay of Bengal during the late Quaternary

Down-core variations of granulometric, geochemical and mineral magnetism of a 70-cm long sediment core retrieved from the eastern Bay of Bengal abyssal region were studied to understand sedimentation pattern and sediment provenance during the last ～12 kyr BP. Based on down-core physical and elemental variations, three units were identified: unit 3 (70–43 cm) is a ～30 cm thick clayey silt organic carbon-rich (0.5–0.92%) turbidite probably delivered by the Brahmaputra River during the late Quaternary period. Units 2 (43–24 cm) and 1 (24–0 cm) represent enhanced and reduced supply of coarse-grained detrital sediments from the Ganges River during early and late Holocene period, respectively. Increased terrigenous supply dilutes calcium carbonate (CaCO₃) and biogenic elements (P, Ba and Cu) in units 3 and 2. On the contrary, a reduction in detrital input enhances CaCO₃ and biogenic elements in unit 1. Lithogenic elements (Ti, Al, K and Rb) and shale-normalized REE patterns in all three units suggest terrigenous source. The shift in provenance from the Brahmaputra to the Ganges derived sediments is evident by a sharp increase in sediment grain size, increased concentration and grain size assemblages of magnetic minerals, lithogenic elements concentration and La_n/Yb_n ratio. This study highlights terrigenous dilution on biogenic sedimentation in the eastern Bay of Bengal sediments.