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1.1 Introduction
Indian monsoon, an important component of the climate system of the Tropical region, is characterized by seasonal reversal of wind. During summer moist ocean air moves toward the continent and causes rain. On the other hand, during winter, wind direction reverses and wind blow from continent. It is believed that the Indian monsoon got initiated during late Miocene due to uplift of the Himalaya/Tibetan plateau up to a critical height (Kutzbach and Guetter, 1986; Ruddiman and Kutzbach, 1989; Harrison et al., 1992; Prell et al., 1992). The strong solar forcing coupled with critical height of the Himalaya produces large scale atmospheric pressure gradient (known as the monsoon pressure index) between the Tibetan plateau and Indian Ocean and drives seasonally reverse wind pattern (monsoonal wind; Fig.1.1). Subsequent variations in monsoon are mainly explained by the Glacial-Interglacial (G-I) cycles i.e. ice volume in both Hemispheres where Northern Hemisphere glacial build up possibly demands
Figure 1.1: Schematic diagram of atmospheric circulation over the Himalayan Mountain and Indian Ocean during summer and winter. (a) During Summer heating of air over the Tibetan plateau creates low pressure zone and attracts air from Indian Ocean. (b) During winter due to low solar forcing wind direction reverses and wind blow from cool Tibetan plateau.
major share (Maslin et al., 1998). Any increase or decrease in monsoon pressure index produced by solar radiation and G-I condition causes higher/lower rainfall over southern Asia than the normal rainfall (Prell and Kutzbach, 1987). Recent studies demonstrated that ice cover over the Himalaya and Tibetan plateau also played a major
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role in regulating the monsoonal condition (Owen et al., 2002; Schaefer et al., 2008).
Large quantity of ice cover over the Himalaya and Tibaten plataeu could have enhanced land albedo and prevented the sensible heating necessary to drive the Indian monsoon circulation.
The variations in the Indian monsoon are well recorded from biogeochemical and lithological changes of ocean core sediments from the Indian Ocean (Prell, 1984;
Leuschner and Sirocko, 2003) (Fig. 1.2).
Figure 1.2: Time series faunal records of monsoon related upwelling with respect to Marine Isotopic Stages (MIS; labeled in-between the records) with the gray bars are marking warm stages. MIS and paleoclimatic records shows weak monsoon conditions during MIS 2, MIS 4 and MIS 6 and strong monsoon conditions during MIS 1, MIS 3 and MIS 5 (after Prell et al., 1984).
During summer monsoon, wind flows from ocean which drags surface water and develops intense centers of upwelling. Upwelling brings cold, nutrient-rich waters from several hundred meters depth to the surface and trigger high productivity in the photic zone. It is reflected by the geochemical and biological changes in the ocean which has a direct link to the structure and intensity of the monsoonal winds. But the
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winds which blow from ocean to continent causes maximum rain during summer in Indian subcontinent and the amount of rainfall depends on the moisture content and transportation paths of the monsoon wind rather than speed (Clemens et al., 1996;
Sarkar et al., 2000). The variations in monsoon in term of rainfall amount are poorly recorded. Relative variations in monsoon rainfall intensity during late Miocene were reconstructed from the older Himalayan foreland basin sediments (Siwalik sediments) (Quade et al. 1989, 1995; Sanyal et al. 2004, 2005). Few paleoclimatic reconstructions from late Quaternary time are also available from different parts of Indian subcontinent (Rajasthan, Himalayas, Nilgiri hills, and Ganga basin) (Wasson et al., 1983; Tandon et al., 1997, 2006 ; Juyal et al., 2000, 2006, 2009; Jain and Tandon, 2003; Sharma et al., 2004; Gibling et al., 2005, 2008; Sinha et al., 2006a; Williams et al., 2006), but these records do not provide continuous spectrum and are qualitative in nature. Sediments of
Figure 1.3: Map showing wind pattern and depression track during Indian summer monsoon. Dates of onset of summer monsoon are presented by thick dashed lines whereas the thick solid lines indicate the withdrawal of summer monsoon in different parts of India. The thin dashed lines with small arrow indicate the direction of surface wind. Thick grey arrows indicate possible transport of Arabian Sea vapour through west coast and Bay of Bengal vapour through east coast which mix over the Ganga plain.
(After Sengupta and Sarkar, 2006).
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the Ganga plain provide opportunity to reconstruct the past rainfall variations as it falls in track of the southwest Indian monsoon (Fig. 1.3). The sediments in the Ganga plain are deposited by perennial and monsoon fed rivers. During pause in sedimentation, by the isotopic exchange authigenic mineral like soil carbonate attained the isotopic signature of rainwater which can be used to reconstruct the past rainfall.
It would be interesting to see the effect of monsoonal rainfall variations on vegetation. Photosynthetic pathways of plants have evolved with time. Early vegetation was characterized by Calvin cycle (C3 plants) where fixation of CO2 during photosynthesis is done by Ribulose 1, 5 Bisphosphate Carboxylase/Oxygenase (Rubisco) (Jordan et al., 1984; Sharkey, 1988; Pagani et al., 2005). During late Miocene, a new photosynthetic pathway i.e. Hatch-Slack cycle (C4 plants) evolved in low-latitude areas across the globe (Taiz and Zeiger 1998; Hatch and Osmond, 1976;
Sage, 2001). Compared to the C3 plants, the C4 community is more favored in a low pCO2 environment. Therefore, it is suggested that C4 plants, being efficient CO2 user, evolved in response to the lowering of atmospheric CO2 during the late Miocene (Ehleringer et al., 1991; Cerling et al., 1997). However, reconstructions of pCO2
estimation using different proxies like stomatal index (ratio of stomata to epidermis cell in leaf) of leaf, boron isotope ratio of foraminifera and carbon isotope ratio of alkenone are not consistent for the late Miocene period (Fig.1.4; Spivack et al., 1993; Van Der Burgh et al., 1993; Pagani et al., 1999). The stomatal index variation of fossil Quercus petraea leaves suggested 280 to 370 ppmV CO2 concentration during the last 10 Myr (Van Der Burgh et al., 1993; Fig. 1.4a). The boron isotopic composition of foraminifera indicates 4.5 times higher CO2 concentration at 21 Ma than the present day concentration, and at around 7.5 Ma CO2 reached the present day concentration (Spivack et al., 1993; Fig. 1.4b). Further, on the basis of alkenone-based atmospheric CO2 estimation Pagani et al. (1999) showed gradual increasing pCO2 trend from 15 to 9 Ma and atmospheric CO2 attained pre-industrial value at around 9 Ma (Fig. 1.4c).
This observation led Pagani et al. (1999) to suggest that seasonal patterns of rainfall and changes in growing season condition were responsible for appearance and expansion of C4 plants during late Miocene rather than lowering of pCO2 in the atmosphere. This finding is also supported by Sanyal et al. (2004, 2005, 2010) which
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shows strong influence of seasonality on the emergence and proliferation of C4 plants.
It is apparent from the foregoing discussion that factors responsible for the abundance of C3 and C4 plants are still a subject of raging debate. The effect of temperature, rainfall, and pCO2 of the atmosphere in relative abundance of C3-C4 vegetation can be tested for relatively younger period for which pCO2 is well constrained (Fig. 1.4d) (Barnola et al., 1987).
Figure 1.4: Diagram showing palaeo CO2 concentration variation obtained from (a) stomatal index variation of leaf which show CO2 concentration has fluctuated between 370 ppmV to 280 ppmV during last 10 Myr, (b) Boron isotope of foraminifera show at around 7.5 Ma ago the CO2 concentration was at present day level and (c) alkenone based CO2 concentration estimation show CO2 concentration was lowest around 15 Ma ago followed by increasing CO2 concentration at about 9 Ma ago reached the present day value. (d) Atmospheric CO2 estimation from trapped air collected from Vostok ice core for late Quaternary.
Thus, in this study carbon isotope ratios of soil carbonate and organic matter trapped within soil carbonate nodules (NOM) have been used as a proxy for vegetation change over the landscape. Also, with the help of change in rainfall amount it is possible to find out the effect of monsoonal rainfall on ambient vegetation. However, limited occurrence of carbonate nodules is a potential obstacle for such reconstruction.
Thus, carbon isotope ratio of residual organic matter dispersed in paleosols (SOM) has been widely used to reconstruct past vegetation. The assumption in such reconstruction is based on the fact that SOM preserves the isotopic signature of vegetation which thrived on the landscape. However, various studies on SOM degradation portrayed contrasting opinions varying from enrichment to depletion by several per mil in 13C
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compared to the original SOM 13C value (Dzurec et al., 1985; Benner et al., 1987;
Nadelhoffer and Fry, 1988; Boutton et al., 1993; Wedin et al., 1995; Huang et al., 1996;
Van Bergen et al. 1997, 1998; Bird et al., 2001; Krull et al., 2002). Moreover, C4 plant derived SOM is thought to be less resistant compared to C3 counterpart (Wynn and Bird, 2007) thus resulting in underestimation of C4 plants abundance and estimation of paleo-soil CO2 concentration. It is generally agreed that during degradation of SOM, less resistant organic fractions like carbohydrates, proteins degrade early by microorganisms compared to more resistant fractions such as lignin and lipids (Benner et al., 1987). Removal of these less resistant fractions gradually lowers 13C values of SOM as carbohydrates and proteins are less depleted in 13C than lignin and lipids (Goni et al., 1997; Huang et al., 1999). In addition, microbial biomass also contributes to SOM pool during degradation processes (Parton et al., 1987; Coleman and Jenkinson, 1996; Trumbore, 1997). Therefore, molecular proxies such as distribution and isotopic composition of more resistant lipids (compound specific isotopic analysis) in residual organic matter have been used to identify the sources of SOM and paleoclimatic change (Huang et al., 1999; Freeman and Colarusso, 2001; Hughen et al., 2004).
Comparative compound specific isotopic study of organic matter from Siwalik and Bay of Bengal sediments spanning ~12 myr, by Freeman and Colarusso, (2001), showed that preservation of organic matter in marine sediments is better than the continental and encouraged the use of compound specific isotopic study in marine record for shorter time scale. Galy et al. (2008a) showed that marine organic matter preserves climatic signal even down to glacial-integlacial cycle such as higher abundance of C4
plants in Ganga plain during Last Glacial Maxima (LGM). However, organic matter delivered by river into ocean reflect a complex mixture of different sources, i.e., organic matter derived from vascular plant, soil microbial production, in-river autotrophy and heterotrophy, and petrogenic carbon derived from rock erosion (Blair et al., 2004; Komada et al., 2004; Leithold et al., 2006; Galy et al., 2007). It suggests that the export of organic matter by rivers from its site of formation to the deposition can alter the composition of organic matter, thereby limiting its use as a good paleoclimatic proxy. Thus, to understand the terrestrial paleoenvironmental changes in spatial and temporal manner it is crucial to understand the complex interplay of processes
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occurring during transient time in the continent. Therefore in this study, paleosols of the Ganga plain have been used to understand the intermediate exchange/transformation of SOM during this transient time. Also, it will be helpful to reconstruct the paleovegetation and climate history over last 80 ka from the continental part as the continental archive responds faster to the climatic variations than the ocean.
It is also important to understand the fluvial response to environmental changes.
The Himalaya, a young orogenic belt, transfers large amounts of particulate and dissolved materials to foreland basin and to the Bay of Bengal via Himalayan river (Fig.
1.5; Galy and France-Lanord, 2001; Singh et al., 2008). A significant amount of sediment is also supplied by peninsular Indian rivers like Chambal, Betwa and Son (Fig.
1.5), which drain through strikingly different lithologies. These source terrains have contrasting climatic regime and both regions are tectonically active as one is part of the orogenic belt and the other one belongs to the forebulge (Fig. 1.5).
Figure 1.5: Younger Himalayan Foreland Basin of south Asia, showing the main drainage elements (After Tandon et al., 2006). The foreland basin is bordered by Himalaya in north and peninsular India in south. KP - Kalpi, BP - Bhognipur, FP - Firozpur
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Study suggests that exhumation of the central Himalayan rock during the Neogene played a major role in controlling the 87Sr/86Sr ratio in soil carbonate (Quade et al., 1997). Paired Sr and Os isotopic analysis of the Himalayan paleosols indicated similar source terrain (central Himalaya) for Sr and Os during the Neogene and showed potential role of Himalayan weathering for evolution of the Sr and Os in the seawater (Chesley et al., 2000). However, such study emphasizes only multimillion year change in isotopic ratio of the Himalayan rivers. But weathering processes might have varied in much shorter time scales. Therefore, comprehensive geochemical study of the low land basin area is necessary to characterize change in weathering rate in the source area.
In the current study fluvial sediments from the younger Himalayan foreland basin (Ganga plain) have been used to investigate weathering of the Himalayan rocks, and impact of climate and tectonics on weathering in millennial time scale. As peninsular Indian rivers also contribute substantial amount of sediment into the Ganga plain, it is important to estimate the contribution of these river into the sediment budget. It has been shown that soil carbonate developed in the floodplain soil represents the contemporaneous 87Sr/86Sr ratio of river water and can be linked to source rock (Quade et al., 1997). Since the majority of the sediments are generated from monsoon dominated and tectonically active Himalaya and peninsular India, the 87Sr/86Sr ratio in soil carbonate thus can be used as a surrogate for ascertaining the spatio-temporal changes (millennial scale) in sediment provenance under the influence of climate and tectonic.
In this study, four cores which represent varying geographical and depositional environment have been raised from the Ganga plain (Fig.1). These are the Firozpur core (FP) collected from valley fill deposits of the Ganga river, IITK from interfluve area of Ganga-Yamuna river, Bhognipur (BP) from the southern interfluve margin area of Ganga-Yamuna river (northern bank of Yamuna river) and Kalpi (KP) from the Yamuna-Betwa interfluve region (southern bank of Yamuna river; Fig. 1.5) (Sinha et al., 2007; 2009). 18O, 13C values, concentration of Sr and 87Sr/86Sr ratios are measured in soil carbonate collected from these cores. In addition to this, concentration and 13C values in SOM, NOM, fatty acid and n-alkanes (selected samples) have been measured. Also, selected silicate samples of FP and KP cores have been analyzed for Sr
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and Nd isotope ratio. 18O values of soil carbonate have been used to reconstruct monsoon rainfall variations whereas 13C values of soil carbonate, SOM, NOM, fatty acid and n-alkanes have been used to reconstruct vegetation history for last 100 ka.
Further, by comparing the rainfall-vegetation data with available pCO2 records, attempts have been made to ascertain the forcing factors (rainfall and pCO2) in the abundance of C4 plants during the late Quaternary. Finally, available information on rainfall ( 18O values of soil carbonate) along with Sr and Nd data and sediment composition are used to assess (i) relative contribution of sediments from the Himalaya and peninsular India and (ii) processes regulating the sediment supply.
1.2 Objectives
1.2.1 Reconstruction of Indian summer monsoonal rainfall
As mentioned, monsoon reconstruction from oceans in terms of wind speed variation is well recorded. However, rainfall which is manifestation of monsoon in continent is poorly understood. The best way to investigate paleomonsoon is to study rainfall proxies from monsoon sensitive regions. The sediments of the Ganga plain provide an excellent opportunity to reconstruct past rainfall variations as it falls on the track of the Indian summer monsoon. Oxygen isotope composition in soil carbonates from the Ganga plain sediments have been used to reconstruct the temporal change in rainfall.
1.2.2 Reconstruction of paleovegetation
Carbon isotope ratio of plants using C3 and C4 photosynthetic pathway have mean 13C value around −27 and −12.5 ‰, respectively and calculation shows −12.5 and +2 ‰
13C value for soil carbonate under pure C3 and C4 vegetation, respectively. Therefore, any change in the 13C value of soil carbonate will reflect the change in vegetation type.
In addition, during formation of soil carbonate, organic matter gets trapped inside the nodule which also provides information regarding vegetation change. But, soil carbonate does not precipitate if the annual rainfall amount is more than 75 cm (Cerling, 1984). In such situation vegetational reconstruction can be done by measuring the δ13C values of SOM.
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1.2.3 Assessment of preservation of carbon isotope ratio in SOM
Various studies related to preservation of organic matter show degradation of organic matter with time in soil profile. During such degradation carbon isotope ratio of SOM may get enriched or depleted in 13C depending on the environmental condition in the soil profile. In this process algae and microorganisms also contribute organic matter into soil. Therefore, molecular proxies such as distribution and carbon isotope ratio of alkanes and fatty acid provide opportunity to evaluate the relative importance of such influences in residual organic matter. In addition, during early diagenesis calcium carbonates precipitates and form discrete nodules. Organic matter in carbonate nodule (NOM) as well as in loose sediments (SOM) from same paleosol bed can be used to understand role of early diagenesis in preservation of pristine carbon isotopic ratio in soil.
1.2.4 Role of monsoon and atmospheric CO2 on the relative abundance of C3 and C4 plants
The causes of appearance and expansion of C4 plants during the late Miocene is debatable between the influence of lowering of atmospheric CO2 concentration and establishment of seasonality. Although, during the Quaternary period C4 plants were already firmly established in terrestrial ecosystems, but it would be interesting to see the effect of variations of monsoon(seasonality) and atmospheric CO2 on relative abundance of C3 and C4 plants.
1.2.5 Spatio-temporal changes in sedimentation over southern central Ganga plain The sediments of the Ganga plains consist of two major provenances namely the Himalaya and peninsular India. These source terrains have contrasting climatic regime and both regions are tectonically active as one is part of orogenic belt and the other one belongs to forebulge. Under the influence of climate and tectonics both terrain produce variable amount of sediments which are deposited by various Himalayan and peninsular Indian rivers into Ganga plain. In this study, for the first time, Sr isotope ratio in soil carbonate and Sr and Nd isotope compositions of the silicate have been used to investigate spatio-temporal variations in the provenance of sediments and their causative factors.
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1.3 Structure of thesis
This thesis is divided into seven chapters. The content of these seven chapters are as follows.
Chapter 1
Outlines the thesis topics addressed in this study. This chapter presents the current state of knowledge on these topics through a brief description of earlier studies.
Chapter 2
First part of this chapter deals with the general geographical position and geology of the Ganga plain whereas the latter part gives a brief description of sampling of soil carbonate and paleosols and the various analytical methods used for their chemical and isotope analyses.
Chapter 3
This chapter presents evidence concerning the late Quaternary (100 ka) climate changes at southern central part of the Ganga plain, India based on the oxygen isotope ratio of soil carbonates.
Chapter 4
The chapter deals with the vegetation change over the southern central part of the Ganga plain during last 100 ka using carbon isotope ratio of soil carbonate, NOM and SOM as a proxy.
Chapter 5
The chapter present results of concentration and carbon isotope ratio of fatty acid and n-alkane in the SOM to evaluate relative contribution of organic matter from different source like algae, microorganism and vascular plants. Last part of the chapter deals with the factor controlling relative abundance of C3 and C4 plants during last 100 ka.
Chapter 6
The chapter presents spatial and temporal variations in strontium and neodymium data in the Ganga plain. These measurements have been used to determine spatio-temporal change in sedimentation over the Ganga plain.
Chapter 7
The Chapter 7 deals with conclusions and future scope of study.
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1.4 Contribution of the thesis
This study focuses on local and regional climates changes over the Ganga plain during last 100 ka. It particularly emphasizes the change in rainfall amount evidenced by 18O values in soil carbonates, shifts in paleovegetation evidenced by 13C values in soil carbonates, NOM, SOM and fatty acids, and change in sediments provenance supported by 87Sr/86Sr ratios in soil carbonate and Sr/86Sr and 143Nd/144Nd ratios in silicates.. Finally the effect of climate changes on the vegetation and river in the time interval of 100 to 18 ka have been inferred from the paleoclimate proxy data. Overall, it has been found that monsoon has intensified at three different phases during last 100 ka.
Vegetational reconstruction showed that abundance of C3 and C4 plants has varied during this period, and relative abundance of C3 and C4 plants mainly controlled by rainfall variation. Isotope study of bulk organic matter implies preservation problem of pristine organic matter signature and in such situation molecular level isotopic data can be used to reconstruct vegetation. This study demonstrate that the role of climate and tectonics on the sediments source of fluvial architecture of the Ganga plain.
