silicate weathering rates under the colder climatic condition. The silicate weathering is the only long term sink of CO 2 and is crucial in

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1 1.1 Introduction: Continental weathering and subsequent transport of solutes and sediments by rivers to the oceans is a major geological process which is responsible for landscape evolution and geochemical cycling of elements (Louvat and Allegre, 1997; Dessert et al., 2001). The chemical and mechanical weathering processes enhance each other and their intensity is controlled by several parameters, such as, climate, lithology, topography, physico-chemical and biological aspects (Mortatti and Probst, 2003). Deriving functional relationships between chemical weathering rates and their controlling parameters in the present-day environment is important to develop and interpret proxies for paleo-climate studies. During chemical weathering the primary silicate minerals, formed at elevated pressure and temperature conditions within the earth are transformed into more stable secondary minerals and solute species through interaction with water close to the earth's surface (Shand et al., 2007). Weathering of carbonate minerals is rapid in comparison to silicates, and therefore provides an unequal input of Ca (and Mg) into natural waters. Weathering reactions between rainwater and soil-derived carbonic acid with mineral phases provide plant nutrients (base cations and anions) and neutralize acidity (Drever, 1997). During chemical weathering, for each mole of calcite break down one mole of - atmospheric CO 2 is consumed and two mole of HCO 3 produced. But re-precipitation of calcite from sea water releases same amount of CO 2 to the atmosphere in about years time scale. Whereas, atmospheric CO 2 consumed during silicate weathering is subsequently precipitated as carbonates (Berner et al., 1983; Moon et al., 2007; Wu et al., 2008). Mantle outgassing via mid-oceanic ridges, hot spots and volcanoes on active margins and metamorphic break down of carbonates in active orogenes in arc system and collision zones are the primary source of atmospheric CO 2 (Huh, 2003). Different models have been proposed to understand the functional relationship between atmospheric CO 2 level and silicate weathering rate. While modeling the evolution of Phanerozoic atmospheric CO 2 Walker et al. (1981) suggested that silicate weathering rate is positively correlated with CO 2 abundance of the atmosphere. Huh (2003) also proposed a relationship between chemical weathering and climate. In this model, increase in atmospheric CO 2 level is balanced by higher silicate weathering rate under greenhouse conditions and lower atmospheric CO 2 level is balanced by decreased

2 silicate weathering rates under the colder climatic condition. The silicate weathering is the only long term sink of CO 2 and is crucial in determining the surface environmental conditions of the earth (Huh, 2003). Hence, study of silicate weathering and determining its rates have attracted much interest as regulators of atmospheric CO 2 levels over geological timescales (Walker et al., 1981; Berner, 1991; Edmond, 1992; Raymo and Ruddiman, 1992; Moon et al. 2007). The soil zone, where weathering is intense, is traditionally considered as the dominant source for solutes present in the surface water. However, less intensely weathered, large volumes of rocks that occur below the soil zone could also contribute to appreciable amount of solutes (Shand et al., 2007). Hence, geochemical studies of surface and sub-surface water are important to understand the cycling of elements such as, C, Si and Sr between earth, ocean and atmosphere. Strontium isotope ratios have been effectively used as a tracer of weathering and solute transport in catchment studies (Gaillardet et al., 1999; Krishnaswami et al., 1999; Bickle et al., 2005; Shand et al., 2007). Minerals with which surface water interacts have a wide and predictable range of 87 Sr/ 86 Sr ratios (Cartwright, 2007). 87 Sr is produced by the decay of 87 Rb with a half-life of 48.8 Ga (Faure, 1986). Rb readily substitutes for K and to a lesser extent Na, while Sr substitutes for Ca in minerals. Thus, the 87 Sr/ 86 Sr ratio of a mineral is governed by its initial 87 Sr/ 86 Sr ratio, its Rb/Sr ratio, and its age (Faure, 1986, McNutt, 2000). Weathering of ancient minerals and rocks with high Rb/Sr ratio will release Sr having high 87 Sr/ 86 Sr ratio, whereas weathering of rocks and minerals having low 87 Sr/ 86 Sr ratios such as, plagioclase and calcite will yield low 87 Sr/ 86 Sr ratios in water. The 87 Sr/ 86 Sr ratio in the rivers varies from in young basaltic terrains (Wadleigh et al., 1985) to in granitic terrains of Proterozoic age (Åberg and Wickman, 1987; Åberg et al., 1989). Elemental ratio such as Ca/Mg, Ca/Na and Mg/Na could be used to estimate the weathering rate and relative contribution of major ions to the river from different rock types. However, these elemental ratios may be affected by preferential removal of one of the elements involved. Unlike C, O, H, or S isotopes, Sr isotope ratios are not fractionated significantly in the river water by any of the processes, such as, precipitation or absorption. Thus, using Sr isotope abundances of the water, it may be possible to quantify the relative contribution from rocks having distinct 87 Sr/ 86 Sr ratios. Sr is an alkaline earth element like Ca, and has similar hydrated ionic radius (r Ca = nm, r Sr = nm) and ionic charge (Marcus and Kertes, 1968). Given a range in 2

3 87 Sr/ 86 Sr ratios among Ca sources, Ca/Sr ratios can be used in combination with 87 Sr/ 86 Sr ratios to identify the sources of these cations (Pett-Ridge, 2009). Wickman and Jacks (1992) emphasized that, in order to improve the application of Sr isotopes to weathering studies, knowledge of Sr isotope signatures is required for different parts of the drainage basin. Globally, the large rivers have been studied for understanding the chemical weathering and its controlling factors (Edmond, 1992; Negrel et al., 1993; Probst et al., 1994 and 2000; Bluth and Kump, 1994; Gaillardet et al., 1999; Viers et al., 2000; Wu et al., 2005). Rivers draining the Himalaya have been studied extensively because of the high chemical weathering rates, delivery of radiogenic Sr to the ocean, and temporal association between the rise of the Himalayas and the Cenozoic global cooling (Raymo and Ruddiman, 1992; Singh et al., 1998; Galy et al., 1999; Krishnaswami et al., 1999; Bickle et al., 2003). Ganga-Brahmaputra and Indus river systems draining Himalayas (Sarin et al., 1984; Pande et al., 1994, Trivedi et al., 1995; Singh et al., 1998, b; Krishnaswami et al., 1999; Bickle et al., 2003) and Peninsular rivers such as, Krishna and Narmada (Das et al., 2005; Dessert et al., 2001) have been studied in detail. Whereas, a few studies have been attempted on other Peninsular rivers (Pattanaik et al., 2007, Jha et al., 2009). However, the conclusion drawn for above river systems could not be generalized because of different controlling parameters. Except for Krishna and Godavari, all other Peninsular rivers are non-perennial and fed mainly during monsoon rains. During non-monsoon period the flow is lean or non-existent. However these rivers drain diverse Precambrian terrains and studying abundance of various ions in them will help to better understand weathering of various rock types in the catchment area. Kaveri, Ponnaiyar and Palar river systems drain through Precambrian rocks of the Dharwar craton, which includes Archean granitoid gneisses and intrusives, meta-volcanic rocks and meta-sediments occurring as greenstone belts, granulites, Cretaceous sedimentary formations and Recent alluvium. Gunnel (1998a) has suggested that rate of weathering of granulites are lower compared to the Peninsular gneisses and hence granulites occur as topographic highs. Whereas, Valdiya (1998) considered that neotectonics was responsible for certain topographic features observed along the west coast and Mysore plateaus. This work is focused on understanding the chemical weathering of southern granulites, Peninsular gneisses and other rock types that are exposed in the drainage basins of Kaveri, Palar and Ponnaiyar rivers. 3

4 During NE monsoon of the year 2005, eastward flowing Kaveri, Palar and Ponnaiyar river basins received much higher rainfall (more than 1200 mm) compared to previous 50 years. River Palar which is usually ephemeral (flows only for about 15 days in a year) was flooded during the November and December of This has provided an unique opportunity to sample these rivers to study solute and Sr isotopic composition when there was natural flow. Due to damming of these rivers there is little natural flow during most part of the year. During May-June 2006 pre-monsoon water samples were collected only from river Kaveri and its tributaries to compare the abundance of solutes and 87 Sr/ 86 Sr ratio with the monsoon river water, and to estimate annual flux of different major ions to the ocean and weathering rate. The onset of monsoonal climate regimes over the Indian sub-continent and Asia has considerably accelerated the rate of deposition of sediments. Long lived cosmogenic radionuclide with short residence time in atmosphere can be used to estimate rate of deposition of sediments. Several studies on Pacific and Atlantic Ocean sediments show the variation in 10 Be content over geological timescales (Raisbeck et al., 1980; Chase et al., 2002; Robinson et al., 1995). 10 Be is produced in the upper atmosphere via spallation reactions: 14 N (n, x) 10 Be and 16 O (n, x) 10 Be (Tuniz et al., 1998) Be decays by emission to stable 10 B with a decay constant ( ) of y -1 (half life 1.51 Ma). Concentration of 10 Be is very small in natural sample and generally it varies from atoms / g in polar ice to about atoms/g in Mn-nodules or younger marine sediments (other concentration are atoms / g in in situ quartz and in continental sediments and ranges between 10 7 to 10 8 atoms / g). To detect this isotope with the conventional beta counting method is not possible with small sample size. Hence, accelerator mass spectrometry (AMS) is the only method with the required sensitivity to measure 10 Be in geological samples (Tuniz et al., 1998). AMS facility was developed at Inter University Accelerator Center (IUAC), New Delhi (formerly Nuclear Science Center), based on a 15UD Pelletron accelerator, during the course of this study for the measurement of 10 Be. The residence time of 10 Be in the atmosphere is about 1.5 year (Lal, 1992), but quickly removed from the atmosphere by wet fall or aerosol (Gu et al., 1997; Graham et al., 2003) and incorporated to sediment. In the continent, it is generally transported via river or as direct fallout from atmosphere into lake or other inland water body. The concentration of 10 Be in the lake sediment depends primarily on atmospheric production, 4

5 but also controlled by climate, topography, lithology and sedimentation rate (Ning et al., 1994; Beer and Sturm, 1995) The production rate of 10 Be in atmosphere increases from atoms g -1 sec -1 at 0º latitude to atoms g -1 sec -1 at 90º latitude. The production rate is constant with latitude in troposphere, with 10 5 atoms g -1 sec -1 (Lal and Peters, 1967). Fluctuation in the 10 Be concentration in sediment is due to global climatic changes and variation in cosmic ray flux which depends on the paleomagnetic intensity (Eisenhauer et al., 1994; Robinson et al., 1995; Frank et al., 1995 and 1997; Chase et al., 2002). Several studies on younger marine sediments, Mn-nodules, lake sediments and soil profile using 10 Be have been carried out for understanding the climate change and sedimentation rate (Somayajulu, 1967; Brown et al., 1981; Sharma and Somayajulu, 1982; You et al., 1988; Eisenhauer et al., 1994; Frank et al., 1995 and 1997; Chase et al. 2002; St-Onge et al., 2003). The half-life of 10 Be (1.51 Ma) and its production rate is well suited for the investigation of the geomorphology of the Quaternary and late Tertiary. The longest continental dating record of sediment of fluvial and lacustrine sequence, using 10 Be is up to ~ 5.4 Ma, whereas, marine sediments can be dated upto ~12 Ma (Ku et al., 1985; Bourles et al., 1989) as these have higher 10 Be concentration. Kaluveli lake is located in the east coast of Tamilnadu, 15 km away from Pondicherry town towards Chennai. This is a swampy wetland of about 70 sq. km and lies between to N and to E. A number of gullies and small streams are connected to this lake from the adjacent area. East of the lake, the Recent alluvium that separates it from the Bay of Bengal, is consists of barrier dunes (medium to fine grained grey white sand); tidal flat (mud, slit and sand) and strand plain (medium to coarse sand). Western and northern parts of the lagoon mainly consists of low lying charnockitic rocks made up of quartz, plagioclase (Na Ca), hypersthenes and hornblende, which is covered by admixture of gravely lateritic-cum-calcritic reddish brown sandy soil. Some places rocks of Cuddalore Fm., considered as Mio-Pliocene age, are also exposed as patches. Rocks of upper Cretaceous and Paleocene age and Cuddalore Fm. occur south of the lake (District Resource Map, GSI, 2001). In this study, sediment core samples collected from Kaluveli lake were analyzed for 10 Be concentration using AMS to understand the paleoclimatic fluctuations and sedimentation rate during the Holocene. 5

6 1.2 Objectives: The goals of these studies are to: determine major ion composition and Sr isotope abundance of the Kaveri, Palar and Ponnaiyar rivers and to identify their sources and processes contributing them, derive chemical weathering rates and associated CO 2 consumption for these river basins, determine different ion flux to the ocean and compare the present results with those available for other Peninsular and Himalayan river, develop chemical procedure for separation of Be from the geological samples and participate in establishing the AMS facility at IUAC, New Delhi for 10 Be measurement determine the 10 Be abundance in the Kaluveli lake sediments and estimate the sedimentation rate. 6