G. J. Chakrapani

Erosion of Deccan Traps determined by river geochemistry: impact on the global climate and the 87Sr/86Sr ratio of seawater

Abstract The impact of the Deccan Traps on chemical weathering and atmospheric CO2 consumption on Earth is evaluated based on the study of major elements, strontium and 87Sr/86Sr isotopic ratios of the main rivers flowing through the traps, using a numerical model which describes the coupled evolution of the chemical cycles of carbon, alkalinity and strontium and allows one to compute the variations in atmospheric pCO2, mean global temperature and the 87Sr/86Sr isotopic ratio of seawater, in response to Deccan trap emplacement. The results suggest that the rate of chemical weathering of Deccan Traps (21–63 t/km2/yr) and associated atmospheric CO2 consumption (0.58–2.54×106 mol C/km2/yr) are relatively high compared to those linked to other basaltic regions. Our results on the Deccan and available data from other basaltic regions show that runoff and temperature are the two main parameters which control the rate of CO2 consumption during weathering of basalts, according to the relationship: f=R f ×C 0 exp −Ea R 1 T − 1 298 where f is the specific CO2 consumption rate (mol/km2/yr), Rf is runoff (mm/yr), C0 is a constant (=1764 μmol/l), Ea represents an apparent activation energy for basalt weathering (with a value of 42.3 kJ/mol determined in the present study), R is the gas constant and T is the absolute temperature (°K). Modelling results show that emplacement and weathering of Deccan Traps basalts played an important role in the geochemical cycles of carbon and strontium. In particular, the traps led to a change in weathering rate of both carbonates and silicates, in carbonate deposition on seafloor, in Sr isotopic composition of the riverine flux and hence a change in marine Sr isotopic composition. As a result, Deccan Traps emplacement was responsible for a strong increase of atmospheric pCO2 by 1050 ppmv followed by a new steady-state pCO2 lower than that in pre-Deccan times by 57 ppmv, implying that pre-industrial atmospheric pCO2 would have been 20% higher in the absence of Deccan basalts. pCO2 evolution was accompanied by a rapid warming of 4°C, followed after 1 Myr by a global cooling of 0.55°C. During the warming phase, continental silicate weathering is increased globally. Since weathering of continental silicate rocks provides radiogenic Sr to the ocean, the model predicts a peak in the 87Sr/86Sr ratio of seawater following the Deccan Traps emplacement. The amplitude and duration of this spike in the Sr isotopic signal are comparable to those observed at the Cretaceous–Tertiary boundary. The results of this study demonstrate the important control exerted by the emplacement and weathering of large basaltic provinces on the geochemical and climatic changes on Earth.

Himalayan uplift and osmium isotopes in oceans and rivers

Previous studies have shown that ^(187)Os/^(188)Os in seawater has become increasingly radiogenic over the last 40 Ma in a manner analogous to strontium. This rapid rise in the marine ^(187)Os/^(188)Os over the last 17 Ma has been attributed to an increase in the bulk silicate weathering rates resulting from the rise of the Himalayas and/or selective weathering and erosion of highly radiogenic organic rich ancient sediments. The key test of this hypothesis is the ^(187)Os/^(188)Os and the total osmium concentration of the Himalayan rivers. We report the concentration and isotopic composition of osmium in the Ganges, the Brahmaputra, and the Indus rivers. The ^(187)Os/^(188)Os of the Ganges close to its source (at Kaudiyal, 30°05′N, 78°50′E) is 2.65 and [Os] = 45 fM/kg. A second sample of the lower reaches of the Ganges at Patna (25°30′N, 85°10′E) gives ^(187)Os/^(188)Os =1.59 and [Os] = 171 fM/kg. The ^(187)Os/^(188)Os of the Brahmaputra at Guwahati (26°10′N, 91°58′E) is 1.07 and [Os] = 52 fM/kg. A sample of the Indus (Besham, 34°55′N, 72°51′E) has a ^(187)Os/^(188)Os of 1.2 and [Os] = 59 fM/kg. We infer that the Himalayas do not provide either a high flow of osmium or a highly radiogenic osmium component to the oceans. The overall trend for osmium and strontium could be explained by a regularly increasing input of global continental weathering sources but the Himalayas themselves appear not to be the dominant source.

Preliminary studies on the geochemistry of the Mahanadi River basin, India

Water, and suspended and bed sediments were collected during 1986/1987 for the two seasons and analysed to study the geochemistry of the Mahanadi River basin, which is one of the eight major drainage systems in India. The water chemistry reflects the control exercised on weathering by the most abundant lithology, namely the carbonates in the basin. Water composition is in equilibrium with dolomite, kaolinite and chlorite. XRD studies in suspended and bed sediments show the presence of dolomite, kaolinite, chlorite and minor quantities of montmorillonite. The Mahanadi annually delivers 15.74·106t of sediments to the Bay of Bengal, which is high compared to its basin size. The dissolved load dominates over physical load by a factor of 4, with 95% of the transport taking place in the monsoon season. The basin is characterised by an erosion rate of 138.24 t km−2 yr.−1, with a lowering rate of 0.31 mm yr.−1. Textural studies indicate that the bed sediments are relatively coarser grained than the suspended sediments. Due to reworking, the sediments are finer downstream. Fe, Mn, Cu, Zn and Pb concentrations in the sediments are higher compared to the other peninsular Indian rivers. Eventhough, Mahanadi transports annually to the Bay of Bengal < 10% of the total load by other Indian rivers, and its weathering in a complex geological terrain with rich mineral deposits controls to a larger extent the chemistry of the sediments in the Bay of Bengal.

Heavy metals distribution and fractionation in sediments of the Mahanadi River basin, India

Heavy metals distribution in core sediments, different size fractions of bed sediments (>212 urn, 90-212 jam, 63–90 urn, 53–63 urn, < 53 urn), and suspended sediments (>30 urn, 20–30 μm, 10–20 urn, 2–10 urn, <2 μm) have been discussed. Pb, Zn, and Cr have been accumulating in recent years in the sediments. Si, Al, Fe, Ca, and Mg dominate the bed and suspended sediment composition. Metals show increasing concentrations in finer sediments. Applying multivariate analysis to sediment composition, metals have been grouped into different factors depending upon their source of origin. Chemical fractionation studies on suspended and bed sediments show Fe, Zn, Cu, and Pb are associated with the residual fraction and Mn with the exchangeable fraction.

Rates of erosion and sedimentation in the Mahanadi river basin, India

Abstract The Mahanadi river basin extends over an area of 1.42 × 105 km2 and discharges about 15.74 × 106 t of sediment to the Bay of Bengal. The geology of the basin is mostly characterised by Precambrian rocks of the Eastern Ghats, rocks associated with the Gondwana supercontinent and recent littoral deposits. Sampling was done to determine chemical erosion, physical erosion and sedimentation rate in the basin. Carbonate rock weathering is the dominant control on the chemical composition of the river water. There is a spatial and temporal variation in water and sediment flow. The chemical erosion rate for individual elements at different sites has been established. Sedimentation rate determined by the 210Pb isotope method in core samples gives rates of 5.08 to 20.39 mm year−1. This high sedimentation rate is attributed to local anthropogenic activities carried out in the basin.

Size characteristics and mineralogy of suspended sediments of the Ganges river, India

With a mean annual flow of 5.9×1011m3yr−1 and sediment load of 1600x1012gyr−1 the Ganges river ranks second and third, respectively, in terms of water flow and sediment load among the worlds rivers. Considering the enormous sediment transport by Ganges to the Bay of Bengal, a study was conducted on the size distribution and mineral characteristics of the suspended sediments of the Ganges river and is reported here. Most of the sediment load has a size range between <4–5.75 φ). The sediments are mostly medium to coarse silt and are poorly sorted. Mica dominates among the clay minerals, followed by chlorite, vermiculite, kaolinite, and smectite. Due to differences in geology, smectite becomes a major clay mineral in downstream rivers. At Calcutta, the clay mineral transport in millions of tons per year is 18,464, 8000, and 2147, for mica, smectite, and chlorite, respectively.

Assessment of trace metal toxicity in soils of Raniganj Coalfield, India

Soil, rock and water samples were collected from India’s oldest coalfield Raniganj to investigate trace metal contamination from mining activity. Our data reveal that trace metal concentration in soil samples lies above the average world soil composition; especially, Cr, Cu, Ni and Zn concentrations exceed the maximum allowable concentration proposed by the European Commission for agricultural soils. In particular, Cr, Cu and Ni exceed the ecotoxicological limit, and Ni exceeds the typical value for cultivated soils. Mineral dissolution from overburden material and high adsorption capacity of laterite soil are responsible for the elevated concentrations. This is evident from enrichment factor (Ef), geoaccumulation index (Igeo) and metal pollution index values. Sediment quality guideline index indicates toxicity to local biota although enrichment index suggests no threat from consuming crops cultivated in the contaminated soil.

Fractionation of heavy metals and phosphorus in suspended sediments of the Yamuna river, India.

The Yamuna river is the largest tributary of the Ganges river system. It originates in the Himalayas and flows through a varied geological terrain encompassing a large basin area. Metals Fe, Mn, Pb, Zn, Cu in different chemical fractions of suspended sediments such as exchangeable, carbonates, Fe−Mn oxides, organics and residual fractions were studied. Phosphorus associated with different chemical forms are discussed. The metals are mostly associated with residual fractions in the sediments followed by organics, Fe−Mn oxides, exhangeable and carbonates. Intensive use of chemical fertilizers and pesticides in agriculture in the basin affects the high inorganic phosphorus content in sediments.

Factors controlling sediment discharge in the Mahanadi River Basin, India

Annual, seasonal, monthly and daily variation in the water and sediment discharge in the Mahanadi River and its controlling factors are discussed in detail. The water discharge varies from 9.61 to 1809.71 m3s−1 and the total suspended matter varies from 130 to 806 mg l−1. The smaller tributaries upstream carry higher sediment concentration. No yearly cyclic pattern in the water flow in the basin is noticed. More than 95% of the sediment discharge takes place in the monsoon, of which the discharge during July, August and September is 90% of the annual total. On certain days of the year, the sediment discharge accounts for 10–15% of the total annual load. Water discharge, rainfall, geology of the basin and the smaller tributaries upstream seem to control the bulk of the sediment discharge. The Mahanadi annually delivers 15.74 million tonnes of sediment to the Bay of Bengal and more than 80% of the sediment load is carried in the coarse silt fraction.

An environmental record of changes in sedimentary organic matter from Lake Sattal in Kumaun Himalayas, India

Sattal a small mountainous lake in the Kumaun Himalayas has been impacted by various cultural activities in recent years. We explored the effects of human-induced changes in this lake by using various geochemical proxies. Shifts in TOC and N flux, C/N ratio, stable isotopes (delta13C and delta(15)N), n-alkane, and pigment concentrations in sediments indicate a steady increase in primary productivity over the last few decades. The trophic status of the lake has changed from mesotrophic to eutrophic condition. The C/N, CPI, and TAR based ratios in sediments indicate accumulation of algal matter derived primarily from in situ production, with limited input of terrestrial organic matter from the watershed. The low (between 0.1 and 1 per thousand) delta15N values imply N2-fixation by cyanobacteria, and the decrease in delta13C values up-core represent the effect of sewage input and land based runoff, or possible contribution from microbial biomass. The pigments change from non-N2 fixing cyanobacterial species to the N2-fixing community, and are consistent with the proxy-based productivity shifts inferred in the lake. The deeper sediments are affected by post-diagenetic changes causing an increase in delta13C (and possibly delta15N) due to mineralization of organic C and N.