Organic carbon in sediments of the Southwestern margin of India: influence of productivity and monsoon variability during the late Quaternary

The texture, organic carbon (OC), CaC03 and Rock-Eval parameters of the sediments from two gravity cores collected at depths below the oxygen minimum zone (OMZ) of the southwestern margin of India are presented and compared the results with those within the OMZ. Clayey silt/ silty clays are the characteristic sediments. The OC in the core top sediments between Cape Comorin and Mangalore is higher below the OMZ than those from the OMZ. However, it is higher within the OMZ than those below the OMZ in the sediments between Mangalore and Goa. The down-core variations of OC are identical in these cores. In both the cores, relatively high OC content and low sedimentation rates correspond to the intervals of late Holocene and Last Glacial Maximum (LGM) and, low OC and high sedimentation rates to the early Holocene sediments. The CaC03 follows sand content in a core off Cape Comorin, with low values at the core top, increase marginally in the early Holocene and LGM and then decrease in the late Pleistocene sediments. The CaC03 values in a core off Mangalore are higher in the intervals of the late Holocene and early deglaciation than m early Holocene and LGM intervals. Rock-Eval parameters distinguish the sources of organic matter only at high OC concentrations. The high OC during the LGM may be related to the productivity, associated with convective mixing occurring during the NE monsoon. The low OC/CaC03 and high clay content during the early Holocene may be the consequences of the intensified SW monsoon that results in stronger near-surface stratification leading to low productivity High OC and low CaC03 during the late Holocene suggest increased productivity and early diagenesis in the near surface sediments. We suggest that the variations in productivity and downslope transport of sediment controlled the OC enrichment

Dune associated calcretes, rhizoliths and paleosols from the Western continental shelf of India

The calcareous deposits on the western continental shelf of India, off Bhatkal (water depths 50 to 58 m) occur as crusts, sheets, cylinders and reddish brown mudstones. The outer layers of the crusts are micrite-dominated and beneath this layer detrital/carbonate sands with thin heavy mineral laminations are found. Textural studies indicate that the detrital sands are derived from dunes. Drusy calcite and neomorphic calcite cements are associated with the sands. Sheet deposits contain coarse calcareous component-dominated layer within micrite layers and abundant micrite cements. These are similar to dune associated calcretes. Cylinders are rhizoliths and show different stages of root calcification. Circular bodies, about 0.15 mm to 0.70 mm diameter, interpreted as vascular cylinders of the root tissues are typical. Radial fibrous calcite and spar calcite with inclusions indicate neomorphic cements. Reddish brown mudstones contain calcified root-hair sheaths, micrite glaebules and reworked pollen suggesting that these are indurated soils. X-ray diffraction studies indicate the presence of ferroan calcite, quartz, pyriie and dolomite in rhizoliths and ferroan calcite, goethite and quartz in mudstones. Mudstones and some rhizoliths are Fe-rich and some other rhizoliths are Fe-poor but enriched with Mg and Mn. The particulate matter in the calcareous deposits were initially at the proximity of the coast and cemented by metastable calcites during the ultimate Pleistocene intcrglacial sea-level stands on the shelf. Pedogenic cementation processes overprinted and developed them into eolianites and paleosols during the subsequent Late Pleistocene sea-level regression. The compositional differences of these deposits were apparently controlled by type of sediments and associated sedimentary environments

Wind-driven estuarine turbidity maxima in Mandovi Estuary, central West coast of India

Systematic studies on the suspended particulate matter (SPM) measured on a seasonal cycle in the Mandovi Estuary, Goa indicate that the average concentrations of SPM at the regular station are ~20mg/l, 5mg/l, 19mg/l and 5mg/l for June–September, October–January, February–April and May, respectively. SPM exhibits low-to-moderate correlation with rainfall indicating that SPM is also influenced by other processes. Transect stations reveal that the SPM at sea-end stations of the estuary are at least two orders of magnitude greater than those at the river-end during the monsoon. Estuarine turbidity maximum (ETM) of nearly similar magnitude occurs at the same location in two periods, interrupted by a period with very low SPM concentrations. The ETM occurring in June–September is associated with low salinities; its formation is attributed to the interactions between strong southwesterly winds (5.1–5.6ms-1) and wind-induced waves and tidal currents and, dominant easterly river flow at the mouth of the estuary. The ETM occurring in February–April is associated with high salinity and is conspicuous. The strong NW and SW winds (3.2–3.7ms-1) and wind-driven waves and currents seem to have acted effectively at the mouth of the estuary in developing turbidity maximum. The impact of sea breeze appears nearly same as that of trade winds and cannot be underestimated in sediment resuspension and deposition

Origin of Cretaceous phosphorites from the onshore of Tamil Nadu, India

Cretaceous phosphorites from the onshore of Tamil Nadu have been investigated for their origin and compared with those in the offshore. Cretaceous phosphorites occur as light brown to yellowish brown or white nodules in Karai Shale of the Uttatur Group in the onshore Cauvery basin. Nodules exhibit phosphatic nucleus encrusted by a chalky shell of carbonate. The nucleus of the nodules consists of light and dark coloured laminae, phosphate peloids/coated grains and detrital particles interspersed between the laminae. Scanning electron microscope (SEM) studies reveal trapping and binding activity of microbial filaments. A mat structure with linearly arranged microbial filaments and hollow, cell-based coccoid cyanobacterial mat are present. Nodules contain abundant carbonate fluorapatite, followed by minor calcite, quartz and feldspar. The P2O5 content of the phosphorites ranges from 18 to 26%. The CaO/P2O5, Sr and F contents are higher than that of pure carbonate fluorapatite. Concentrations of Si, Al, K, Fe, and Ti are low. We suggest that the nuclei of the nodules represent phosphate clasts related to phosphate stromatolites formed at intertidal conditions. At high energy levels the microbial mats were disintegrated into phosphate clasts, coated with carbonate and then reworked into Karai Shale. On the other hand, Quaternary phosphorites occur as irregular to rounded, grey coloured phosphate clasts at water depths between 180 and 320m on the continental shelf of Tamil Nadu. They exhibit grain-supported texture. Despite Quaternary in age, they also resemble phosphate stromatolites of intertidal origin and reworked as phosphate clasts onto the shelf margin depressions. Benthic microbial mats probably supplied high phosphorus to the sediments. Availability of excess phosphorus seems to be a pre-requisite for the formation of phosphate stromatolites

Fluctuations in productivity and denitrification in the Southeastern Arabian Sea during the Late Quaternary

Sedimentological and stable isotopic characteristics of sediments have been studied in a core from the southeastern Arabian Sea containing records of the past 70 ka. Palaeoproductivity proxies such as organic carbon (Corg), total nitrogen (TN) and calcium carbonate (CaCO3) contents, show high values at the core top and during the Last Glacial Maximum (LGM) and marine isotope stage (MIS) 4, suggesting high productivity, whereas low Corg and CaCO3 contents are associated with the MIS ½ and mid-MIS 3, indicating reduced productivity. The δ18O values in planktonic foraminifera range between - 2.7% and - 0.1%, with a large glacial-interglacial amplitude Δδ18O of ∼2.6%, suggesting changes related to monsoonal precipitation/ runoff. The δ15N values fluctuate between 5.4% and 7.3%, signifying variation in denitrification intensity. The δ15N indicates an overall increase in denitrification intensity during MIS 1 and MIS 3 and, reduced intensity during MIS ½, LGM and mid-MIS 3. Higher primary productivity and reduced denitrification intensity during LGM and MIS 4 might be due to convective winter mixing and more oxygenated subsurface waters. Reduced primary productivity during MIS ½ and mid-MIS 3 might be the effect of enhanced precipitation associated with the intensified southwest monsoon fortifying near-surface stratification

Suspended sediment dynamics on a seasonal scale in the Mandovi and Zuari estuaries, Central West coast of India

Suspended particulate matter (SPM) collected at regular stations from the Mandovi and Zuari estuaries indicates that the peaks of high SPM coincide with peaks of high rainfall and low salinity and also with peaks of moderate/low rainfall coupled with high salinity during the monsoon. The estuarine turbidity maximum (ETM) is a characteristic feature, it occurs in the channel accompanying spring tide during the monsoon and pre-monsoon, and shifts to the bay on neap tide during post-monsoon. ETM remains at the same position in the Mandovi River, both during the monsoon and pre-monsoon, whereas in Zuari it stretched upstream during monsoon and migrates seaward of the channel during pre-monsoon. The ETM coincides with the freshwater-seawater interface during the monsoon and is formed by the interaction between tidal currents and river flows. The ETM during pre-monsoon is associated with high salinities and is generated by tidal and wind-induced currents. The turbidity maximum on neap tide during post-monsoon may be due to the erosion and resuspension of sediments from the emergent tidal flats and transport of these turbid waters into the bay. Funneling effect of the narrowing bay in the Zuari estuary and associated physical processes effectively enhance the magnitude of the currents and transports sediments to the channel. SPM retention percentage indicates that the estuarine channel is prone to siltation

Efficient conversion of aldoximes into nitriles and ketoximes into amides using bis-morpholinophosphorylchloride

Bis-morpholinophosphorylchloride(bmpc) has been identified as a new reagent to efficiently convert aldoxime into nitriles through dehydrogenation and ketoximes into amides through Beckmann rearrangement. When compared to other chlorophosphate reagents used earlier, which are liquids and irritating, bmpc is a non-irritating stable solid. In all the reactions, products are obtained in high yields and purity

A study of temperature profiles and trends as revealed by COSMIC RO technique and balloon –borne radiosonde instrument

This research presents atmospheric temperature profiles and trends retrieved using COSMIC RO technique and balloon-borne radiosonde instrument in 2007 and a few cases during 2017. By effectively using ‘wet’ temperature product available at COSMIC Data Analysis and Archive Center (CDAAC) website, an analysis has been made to present temperature profiles and trends at various regions including, Indian, Taiwan and Japan. A one-to-one correspondence is, clearly, seen between temperature profiles retrieved with COSMIC RO and radiosonde instrument. But, few and dominant differences in temperature profiles are found below at an altitude of ~5 km and above around tropopause (~16-17 km). The dominant differences found at below ~5km could be due to the inhomogeneous distribution of humidity present, generally, at the tropical regions, whereas above the tropopause altitudes, differences might be due to the ionospheric residual correction as reported by other researchers. Further, temperature monthly trends at various regions show distinct characteristics including, a sharp temperature inversion up to tropopause altitude. In addition, it is also observed maximum temperatures (peaks) during the northern summer seasons (May, June, July, and August) and minimum temperatures (troughs) during the northern winter seasons (November, December, January, and February) near to the surface of the Earth. Interestingly, although it is generally observed that the tropopause altitude is located at ~ 16-17 km at various regions, a keen observation reveals that distinct seasonal and latitudinal variations can be witnessed. With this case study, it may be concluded that the COSMIC RO technique is able to provide very accurate measurement, which reiterates its importance as a powerful tool to explore the Earth’s atmosphere on the local and global scale

Anomalous wind circulation over Taipei, Taiwan during the northern winter seasons of 2004 and 2005- A case study

This research reports, for the first time, an anomalous wind circulation over Taipei (Latitude 25.030N, Longitude 121.510E), Taiwan during the northern hemisphere winter season (December, January, and February) of years 2004 and 2005. The anomalous wind circulation of meridional winds, which showed southward directions during the winter seasons of 2004 and 2005 instead of northward winds, is noticed from one kilometer altitude range (lower troposphere) and that trend continued till around 20 km altitude range (lower stratosphere). To ascertain whether such a disturbed nature of wind pattern existed over nearby locations to Taipei, we have analyzed radiosonde-measured meridional and zonal winds over four nearby stations station to Taipei including, Roig, Xiamen, Minami and Fuzhou. Surprisingly, no anomalous wind behavior is seen except over Taipei during the northern winter seasons of 2004 and 2005. On the other hand, the European Centre for Medium-Range Weather Forecasts (ECMWF) model-predicted winds do not show any anomalous wind patterns over Taipei and other nearby stations, possibly due to the large averaging of internal variabilities of reanalysis databases. The plausible physical mechanisms of these disturbed meridional wind patterns are not understood at this juncture, but it is believed that local winds and atmospheric pollutants might have created an amicable environment as to provide such a disturbed meridional wind pattern over Taipei, Taiwan in the winter season of 2004 and 2005