Tropical rainforest vegetation, climate and sea level during the Pleistocene in Kerala, India

Tropical rainforest vegetation, climate and sea level during the Pleistocene in Kerala, India

Quaternary International 213 (2010) 2–11 Contents lists available at ScienceDirect Quaternary International journal homepage: www.elsevier.com/locat...

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Quaternary International 213 (2010) 2–11

Contents lists available at ScienceDirect

Quaternary International journal homepage: www.elsevier.com/locate/quaint

Tropical rainforest vegetation, climate and sea level during the Pleistocene in Kerala, India Anjum Farooqui a, *, J.G. Ray b, S.A. Farooqui c, R.K. Tiwari d, Z.A. Khan c a

Birbal Sahni Institute of Palaeobotany, 53, University Road, Lucknow, Uttar Pradesh 226 001, India Department of Botany, St. Berchman’s College, Chaganacherry, Kerala, India c Directorate of Geology and Mining, Lucknow, India d Geological Survey of India, Northern Wing, Lucknow, India b

a r t i c l e i n f o

a b s t r a c t

Article history: Available online 7 October 2009

The southwestern Ghats region of the Indian Peninsula is unique for its extant endemic rainforest flora supported by high rainfall throughout the year. The record of tropical rainforest corresponding to the dynamic series of Pleistocene interglacial/glacial cycles is poorly known from peninsular India. This communication discusses the palynological study of organic matter (OM) deposits (>40 ka BP) in two well sections (Chaganachery, Kerala) from the Indian Peninsula (west coast). A rich archive of tropical rainforest pollen/spores and marine dinoflagellate cysts indicates anoxic fluvio-marine/estuarine depositional environments during warmer climates with an intensified Asian monsoon. The geochemical fingerprinting of glass shards indicates the presence of Youngest Toba ash of w74 ka from northern Sumatra, and therefore establishes a time-controlled stratigraphy. Thus, the depositional time period of the OM is related to the sea level highstand of Marine Isotopic Stage 5.1 (w80 ka) which was the host to the YTT shards. The Late Quaternary pollen/spores diversity suggests that the modern climatic conditions in the southwestern Ghats have facilitated the conservation of moist evergreen rainforest and dry/moist deciduous forest. The pollen grains show its lineage with the extant flora and some of the fossil pollen recorded during the mesic Tertiary period from the Indian peninsula. Thus, it appears that the tropical rainforest survived here as ‘Plant Refugia’ in xeric (glacial) Quaternary periods, perhaps as riparian vegetation, and was rejuvenated during the Holocene as modern extant flora. Ó 2009 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction The tropical Rainforest ecosystem has had a long geological history (Corlett and Primack, 2006) since its emergence. The diversification of rainforests occurred in the Late Cretaceous and Early Palaeogene during the movement of the Indian Plate from southern to mid- latitudes, coupled with changes in the global climate pattern (Givinish, 1999). The Quaternary period witnessed the most dynamic climatic conditions (Morley, 2000), leading to a situation in which extinction of plants was more important than speciation. The stability of a tropical rainforest largely depends on low seasonality, with few dry months (Barboni et al., 2003). The extant tree diversity in the Amazon forest is the legacy of the Palaeogene and Neogene rather than the evolutionary product of the Quaternary (Hooghiemstra and Van der Hammen, 1998). Similarly, the southwestern Ghats in India is home to many

* Corresponding author. Tel.: þ91 0522 2740008; fax: þ91 0522 2740485. E-mail address: [email protected] (A. Farooqui). 1040-6182/$ – see front matter Ó 2009 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2009.09.024

endemic evergreen to semi-evergreen rainforest plants, due to the high precipitation and few dry months annually (Pascal, 1982). Palynological signatures of Tropical Montane Forests from Middle Holocene lake sediments in the Annamalai and Nilgiri Hills in the southwestern Ghats have been documented (Vishnu- Mittre and Gupta, 1971; Blasco and Thanikaimoni, 1974; Vasanthy, 1988; Gupta and Bera, 1996; Rajagopalan et al., 1997). Similar palynological studies in lake sediments from the Palni Hills have revealed the presence of Tropical rainforest w7 ka BP (Bera et al., 1997; Bera and Farooqui, 2000). These palyno-chronological studies show that evergreen to semi-evergreen rainforest dominated in the warm and humid Holocene epoch, but show its absence during the LGM, marked by savanna grassland. However, information on the palaeovegetation response to Pleistocene climate is still meagre (Narayanan et al., 2002) from the western Ghats. The present communication aims to unravel the linkages among Pleistocene vegetation, climate and sea level, providing a clue to its lineage with the extant flora of the western Ghats and its resemblance to the Neogene rainforest flora.

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2. Regional setting Vazhapally in Chaganchery lies in Kottayam district (8 350 N; 76 300 E, Fig. 1), 25 km inland from the present Alleppey shoreline and southeast of Vembanad Lake. Highlands (>650 m asl), midlands (300–650 m asl) and lowlands (300 m asl to sub-mean sea level) are differentiated. The basin of the Meenachil, Manimala, and Muvattupuzha Rivers and their tributaries is fed by several streams crossing the southwestern Ghats montane forest (914–1097 m asl) in the Idukki district, and extends to Vembanad Lake in the west. The vegetation is mainly tropical evergreen and moist deciduous, adapted to moderate climate and low seasonal temperature variation (20 –35  C). It receives an average of 3150 mm of rainfall during the southwest monsoon, with high levels of humidity. During the retreat of the monsoon in winters, it falls on the leeward side, receiving little rain. The topography is undulating with laterite hills, interconnected plains and low-lying marshy lands in the south. Two laterite ridges separated by a valley dominate the Vazhapally area, which has an average elevation of 5 m asl. Heavy rainfall and high temperature prevalent in the state are conducive to the process of laterisation. The soil is reddish brown to yellowish red, mostly gravelly loam to gravelly clay loam. The present vegetation, floristic types and monsoon seasonality gradient have been well documented from the southwestern Ghats (Pascal, 1988; Barboni et al., 2003). Champion and Seth (1968) classify these forests as Southern Indian Moist Deciduous Forest. 3. Materials and methods Two well sections measuring 39 m (CHY-I) and 16 m (CHY-II) depth from Vazhapally, Chaganachery (Fig. 1) were studied. CHY-I is located on the laterite ridge lying parallel to the MC Road. CHY-II

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is located slightly to the north on the slope of the ridge. In both wells, a single layer of organic matter (OM) varies between 3 m and 0.6 m in thickness, and is present below the thick laterite cover (Table 1). The OM layer contains ash, carbonized seeds, plant twigs and abundant resinous matter with a strong odour of sulphur. Two 14C dates of the organic carbon, one from each well (Figs. 3 and 4) were obtained from Birbal Sahni (BS) Institute of Palaeobotany, India, calibrated following Stuiver et al. (1998). Five samples from the OM layer in CHY-I and three samples from CHY-II were taken for palynological study (pollen, spores and dinoflagellate cysts). For this, 10 g samples were treated with warm 10% potassium hydroxide and later sieved through 150mesh (105 mm). The filtrate was settled overnight and the supernatant drained. The residue was treated with 40% Hydrofluoric acid. The sample was then acetolysed, following Faegri and Iverson (1989). The acetolysed samples were caught in 650 mesh size (10 mm) and the residue was mounted on glass slides in glycerine jelly for palynological study under a high power light microscope (Olympus BX-52). The pollen/spore spectra prepared is the percentage of total palynological counts in the air dried 10 g sample. The thick laterite above the OM is low in pollen count (2–6/10 g sample) or barren, and therefore it is not included in the pollen diagram. Pollen atlases of Guinet (1962) and Tissot et al. (1994) and reference slides from the Herbarium of Birbal Sahni Institute of Palaeobotany were consulted for identification of the palynomorphs. Determination of the percentage of organic matter and sulphur was carried out following Arinushkin (1978). Trace elements in the OM were analysed using a Metal Analyser ‘InnovX-systems’ (a-4000S), USA. The surface morphology of the carbonized seeds and glassy shards present along with the OM were studied under a Scanning Electron Microscope (SEM- LEO, 430). The shards were separated by gravity using heavy liquid

Fig. 1. Location map of the study area and past/present vegetation cover in southern India.

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Fig. 2. 1: Carbonized fruit (inside dark black impression of seed) of Terminalia catappa; 2: Carbonized seed of Mallotus; 3: Macroscopic resinous fragments with adhering greyish ash; 4: Light microscopic photomicrographs of resin showing numerous gas bubbles indicating intense heat; 5: Scanning Electron Microscopic Photomicrographs of surface of the carbonized seed test showing layer of parenchyma tissue and secondary mineralisation; FP, Framboidal pyrite. A: Charred vesicular burst epidermal cell on the surface (detailed in 8); B: unburst pore space; C: Unidentified Secondary mineral UM; 6: Light microscopic photomicrographs of bromoform separated glass shards showing small bubbles either scattered or arranged in linear fashion. 7: V-shaped notched glass shards stained with yellowish resinous substance. 8: Detail of SEM photograph shown in 5; 9: cuspate to angular glass shards showing conchoidal fractures; 10: broken vesicular junction with braided surface of glass shards; 11: Highly angular shard showing elongated vesicles; 12: Y-shaped bubble junctions in glass shards.

Bromoform. Pure glass shards were fused with sodium carbonate at 900–1000  C, and wet digestion was carried out following IBM (1979) for the analysis of major element oxides. The geochemical fingerprinting of trace and Rare Earth Elements (REE) in glass shards was estimated using a ICP-Mass Spectrometer (Varian 820-MS), Geological Survey of India, Northern Wing, Lucknow. Thin sections of the desiccated hard layer above the organic matter were studied under a petrological microscope.

4. Results 4.1. Lithology The thickness of the soil cover varies from 1.5 to 3.5 m (Table 1) reddish loam and contains meagre pollen/spores (5–6 pollen per 10 g sample). Beneath the soil is 4.5–10 m of lateritic sediment which is palynologically barren (Figs. 5 and 6). This is followed by

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Table 1 Lithology and Radiocarbon dates (k yrs BP) in Well Sections CHY-I and CHY-II. CHY-I

CHY-II

Depth (meters)

Sediment texture

Depth (meters)

Sediment texture

0–2.1 2.1–11 11–11.25 11.25–14.25 13–13.5 ¼ >40 k yr. BP  2420 (BS- 1871) 14.25–14. 55 14.55–39 Bedrock

Red loams Laterite Ferruginous sandstone Unconsolidated blackish organic matter, resin fragments, carbonized plant twigs, seeds, fruits etc. and Ash content Ferruginous sandstone Yellowish solidified fine clay Granite

0–3 3–10.5 10.5–10.7 10.7–11.3 10.7–11.3 ¼ >41 k yr. BP  2364 (BS-2195) 11.3–11.45 11.45–12.35 12.35–15.85 15.85 and below

Red loams Laterite Ferruginous sandstone Organic Matter, resinous fragments, less ash content

Table 2 Major Element oxides (in % including loss on ignition) of glass shards from CHY-I and CHY-II compared with earlier records. Element

CHY-I

CHY-II

Song et al., 2000 South China Sea

Pattan et al., 2001 Indian Ocean

BeddoeStephens et al., (1983)

Raj, 2008

SiO2 CaO MgO Na2O K2O FeO Al2O3 MnO TiO2

70.39 0.67 0.36 2.18 6.24 4.20 13.95 0.01 0.09

72.15 0.79 0.39 2.42 4.45 3.63 11.91 0.02 0.13

78.12 0.62 0.15 2.68 5.20 1.02 12.04 0.07 0.04

76.94 0.83 0.05 3.40 5.04 0.88 12.45 0.09 0.12

73.12 0.69 – 3.13 4.75 0.84 12.42 – –

71.15 1.54 1.4 2.53 3.52 3.47 13.23 0.09 0.48

Total

98.09

95.89

99.95

99.8

94.95

97.41

a thin desiccated reddish brown hardened layer, below which is a black to gray unconsolidated OM layer containing carbonized plant parts and ash, with a strong odour of sulphur. The thickness of the OM varies between 1.0 m and 3.0 m in the entire Chaganachery area, and it is 3 m and 0.6 m thick in CHY-I and CHY-II, respectively. The entire Chaganachery area shows a similar OM layer below the laterites at depths from 8 to 11 m from the surface (Fig. 1). It sharply decreases within a distance of 60 m towards the east. The organic matter in CHY-I includes more ash along with macroscopic carbonized plant parts, and abundant resin fragments ranging in size from microscopic to 1 cm (Fig. 2.3). Below the OM is palynologically barren yellowish sandy-clay 3.0–6.0 m thick, overlying granitic rocks in CHY-I and the water table in CHY-II.

Fig. 3. Logarithmic graph of elemental oxides in pure glass shards from CHY-I and CHY-II in comparison with earlier records.

Ferruginous sandstone Very hard, dark reddish-yellow baked type of clay Solidified pale yellow clay Water Table

Table 3 Trace element and REE composition (ppm) of glass shards in CHY-I, CHY-II, Bori place (India), YTT of Toba Caldera and Tejpur (India). Elements

CHY-I

CHY-II

Bori place (Kukadi) Westgate et al., 1998

YTT, Toba Caldera (Song et al., 2000)

Madhumati River, Tejpur, (Raj, 2008)

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Be Ge Mo Sn Hf Ta W Th U

34.41 63.14 7.04 27.96 4.96 1.07 3.49 0.545 2.73 0.462 1.436 0.2086 1.382 0.2146 0.5872 1.3172 6.3445 5.9077 6.7143 1.3084 1.9215 11.005 5.2911

34.72 54.27 5.45 19.36 2.67 0.36 1.96 0.26 1.14 0.20 0.65 0.10 0.65 0.11 0.19 1.59 6.04 12.58 8.89 0.50 0.30 15.43 2.0590

24.24 46 5.25 18.3 3.95 0.39 4.06 0.72 4.64 0.96 3.06 0.51 3.52 0.59 – – – – – – – 26.2 4.84

20.35 40 4.92 18.2 4.21 0.03 4.92 0.92 6.25 1.35 3.90 0.77 6.25 4.88 – – – – 3.42 – – 30.3 6.04

32.05 61.65 6.44 24.63 5.57 0.95 4.48 0.84 5.36 1.17 3.69 0.57 3.76 0.66 – – – – – – – 27.79 4.30

4.2. Carbonized plant parts The carbonized seeds/fruits of Terminalia catappa (indigenous Almond; Fig. 2.1), seeds of Mallotus (Fig. 2.2), and ash laden twigs of plants were identified visually. SEM study of the seed coat shows blistered parenchymatous cells (Fig. 2.1 and 2.8) with numerous

Fig. 4. Chondrite normalized (Evensen et al., 1978) REE of pure glass shards from Chaganachery showing strong Europium (Eu) anomaly.

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Fig. 5. Palynological spectrum in CHY-I, Chaganachery, Kerala.

vesicular structures, perhaps formed during intense heat. The glass shards are strongly adhered to its surface (Fig. 2.5). The abundant framboidal pyrite observed on the seed coat shows biologically mediated oxidation of the organic matter. The presence of framboidal pyrite in the marine environment suggests either diagenetic sulphate reduction or euxinic bottom waters (Masuzawa et al., 1992). The glass shards adhering to the organic matter probably settled through the water column in an anoxic estuarine depositional environment. Estuarine organic matter associated with abundant framboidal pyrite coupled with toxic trace metals

have been reported previously from the east coast of India (Farooqui and Bajpai, 2003). The microscopic resinous fragments derived from terrestrial plants show numerous small to large vesicles (Fig. 2.4) indicating its subjection to intense heat, perhaps due to fire in the forested provenance, prior to its burial in an estuarine ecosystem. The abundance of resinous matter in the studied sediment could be due to the abundance of common gum-resin yielding plants (Giang et al., 2006) such as Bombax/Ceiba, Garcinia, Terminalia, Combretaceae, Canarium, Melia, Garuga, and Gluta, which is evident from the palynological record.

Fig. 6. Palynological spectrum of Well Section CHY-II, Chaganachery.

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4.3. Geochemical analysis of OM and geochemical fingerprinting of glass shards The OM layer shows a high percentage (w36%) of total organic matter and high sulphate (w14%) content. The pH of the aqueous soil solution of the sediment ranges from 1 to 2. Metal analysis of the homogenized organic matter and ash content from both well sections shows very high concentrations of Fe (2,25,000– 1,95,000 ppm) followed by Ti (9,0400–1,1400 ppm), Co (1230– 1410 ppm), Mn (660–492 ppm), As (94.7–81.5 ppm), Mo (51.0– 11.7 ppm), Pb (33.7–27.2 ppm), and Rb (6.2–5.5 ppm). The major elemental oxides of pure glass shards range from 95 to 98% and total LOI (loss on ignition) is 2–4.8 wt % (Table 2), which indicates that the rocks containing volcanic glass have been hydrated. A comparative data set of major elemental oxides shown in Fig. 3 reveals high percentages of silica, aluminum and iron oxides, showing close similarity with values recorded by BeddoeStephens et al. (1983), Chesner (1998; Indonesia) and Raj (2008; India), and with the trend in variability (4–6%) recorded by Shane et al. (1995), Song et al. (2000), and Pattan et al., 2001. Nearly identical concentrations of trace elements and REE of these glass shards were recorded (Table 3) compared to the YTT shards reported elsewhere (Fig. 4) by Song et al. (2000), Pattan et al. (2001) and Raj (2008) (see Fig. 6). The present analysis provides a new data of 7 other trace elements including Beryllium(Be), Germanium (Ge), Molybdenum (Mo), Tin (Sn), Hafnium (Hf), Tantalum (Ta), and Tungsten (W) in glassy shards of Toba tuff for the first time, except for previous reports of Hafnium from some records. 4.4. Morphology of glass shards Light microscopic study of pure glass shards show very minute bubble inclusions, either scattered or arranged in a linear fashion (Fig. 2.6). These also show V-shaped notches and sharp angularity. Some shards are stained with a yellowish resinous substance (Fig. 2.7). This indicates either fire or intense heat that induced melting of resins in the forested provenance. The study of larger glass shards under a low power reflected light microscope showed conchoidal fractures (Fig. 2.9), and pumice surfaces (Fig. 2.10). The SEM study of microscopic glass shards showed elongated vesicular structures (Fig. 2.11) and bubble wall junctions (Fig. 2.12). These show sharp angularity and are block-like, flat to cuspate, and triangular with multi bubble wall junctions. Braided, broken elongated vesicles and fine pits are present on the surface. This is attributed to the explosive nature of magmatic activity: magmatic volatiles are largely dissipated when tephra is explosively elevated into the atmosphere, causing rapid chilling of the bubble surfaces (Karmalkar et al., 1998). 4.5. Thin section study The petrological study of the thin section of the desiccated hard layer present above the OM reveals medium to fine grained sands, poorly to moderately sorted. The framework constituents are quartz, feldspars, and rock fragments cemented by ferruginous material. The quartz grains are predominantly monocrystalline and exhibit undulose extinction. These are subangular to subrounded and tend to show elongation. The few grains of feldspars, mainly orthoclase, are subangular in nature. The sandstone shows a high order of compositional maturity, and therefore is termed ferruginous sandstone. 4.6. Palynology The soil cover from 1.5 to 3.5 m is represented by meagre pollen/ spores (5–6 number of pollen per 10 g sample). Pollen includes dry

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and moist deciduous vegetation such as Hibiscus, Annona, and Casuarina, and members of Meliaceae, Sapotaceae, and Palmae. Beneath the upper soil, the thick lateritic sediment is palynologically barren. The palynological study of the OM from both CHY-I and CHY-II shows excellent preservation of pollen/spores and dinoflagellate cysts, dating to >40 ka BP (BS- 1871; CHY-I) and 41 ka BP (BS- 2195; CHY-II). OM from CHY-I and CHY-II reveal similar palynofloral assemblages (Figs. 5 and 6). Most of the pollen spores show pyrite inclusions, suggesting burial under an anoxic depositional environment. The pollen grains of mangrove and associated vegetation in CHY-I constitute a high percentage (10.8%) of true mangroves belonging to Rhizophoraceae (3.9%), out of which Rhizophora species dominate, followed by Bruguiera (Fig. 5). The other mangrove assemblage taxa are comprised of Aegialitis, Aegiceras, Avicennia, Brownlowia, Excoecaria, Herietera, Xylocarpus, Nypa, and Sonneratia, constituting about 6.9% of the total pollen count. The non-arboreals constitute 14.7%, with Liliaceae (2.7%), Eriocaulon (1.7%) and Euphorbiaceae (1.4%) pollen. Myriophyllum, Eriocaulon, Cyanotis, Citrullus and Potamogeton are adapted to ephemeral ponds/puddles or nearby areas, suggesting a wetter climate. Other species present are creepers or small herbaceous taxa, indicating a low percentage of ground-cover vegetation. Arboreal pollen constitutes w78% of the total count (Fig. 5). These are dominated by the moist evergreen rainforest community, including about 26 families. About 13.2% is Bombacaceae, such as Bombax/Ceiba and Cullenia species, followed by Sapotaceae (7.3%), Combretaceae (6.4%), Dipterocarpaceae (4.9%), Meliaceae (4.7%), Clusiaceae (4.4%), Myrtaceae (3.1%), and Celastraceae (2.9%). Pollen grains of Aglaea, Canarium, Chrysophyllum, Eugenia, Euonymous, Garcinia, Garuga, Eugenia, Euonymous, Garcinia, Hopea, Knema, Ligustrum, Madhuca, Mallotus, Myristica, Melia, Palaquim, Poeciloneuron, Schleichera, Shorea, Symplocos, Syzygium, Tabernaemontana, Terminalia, Tilia, Toddalia and Podocarpus (Fig. 7) indicate a thick evergreen to semi-evergreen rainforest community in the vicinity, constituting 55–60% of the total arboreal count. The absence of poaceae/cyperaceae pollen indicates a dense forest cover restricting its growth. The percentage of trilete pteridophytic spores is 1.7%, monolete spores is 1.7%, fungal spores is 2.7%, Botryococcus algae/algal cyst is 0.6% and Dinoflagellate cysts is 4.3%. Among these, the highest percentage is of Spiniferites (2.9%) followed by Operculodinium (0.7%), Lingulodinium (0.2%), and Bitectodinium (0.2%), indicating a marginal marine to estuarine depositional environment. As compared to CHY-I, the palynotaxa assemblages recovered from CHY-II are similar, although with fewer pollen and spores. Mangroves and estuarine to marginal marine dinoflagellate cysts were more represented in CHY-I than in CHY-II. The CHY-II spectrum comprises about 5.8% mangroves (Fig. 6), including Rhizophora, Excoecaria, Bruguiera, Avicennia, and Xylocarpus. The non-arboreals comprise 8.8% of the total count. Boerhavia, Citrullus, Cyanotis, Derris, members of Euphorbiaceae, Lamiaceae, Liliaceae, Menispermaceae and Tinospora constitute the herbaceous/creeper community. Arboreal palynotaxa, 76.6%, belong to 23 families. Members of Sapotaceae and Bombacaceae dominate (w12.0%), followed by Combretaceae, Meliaceae, Dipterocarpaceae, and Myrtaceae (Fig. 6). Evergreen to semi-evergreen pollen taxa constitutes 50–60% of the total count, dominated by Cullenia, Garcinia, Mesua, Diospyros, Memecylon, Podocarpus, Palaquium, Dipterocarpaceae, Mallotus, Aglaia, Eugenia, Myristica, Ligustrum, Sterculia and Knema. Pteridophytic trilete spores constitute 1.7%, monolete spores 0.9%, fungal spores 4.0%, and dinoflagellate cysts 1.0%. The Poaceae and Cyperaceae pollen percentages were very low (0.001%), indicating a larger area covered by closed canopy dense forest in the provenance.

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Fig. 7. Light Microscopic Photographs 1–6- Rhizophoraceae; 7- Sonneratia; 8- Nypa; 9- Aegiceras; 10- Bruguiera; 11- Excoecaria; 12- Bruguiera; 13- Xylocarpus; - 14- Aegiceras; 15Avicennia; 16- Eriocaulon ; 17- Symplocos; 18- Sapotaceae having pyrite crystal inclusion; 19- Palmae; 20- Potamogeton; 21- Brownlowia; 22- Excoecaria species 23- Aegiceras; 24- Herietera; 25- Bombacaceae; 26- Terminalia; 27- Combretaceae; 28- Unidentified; 29- Meliaceae; 30- Madhuca; 31- Sapotaceae; 32- Derris; 33- Citrullus; 34- Euphorbiaceae; 35- Syzygium; 36- Helicteres; 37- Eugenia; 38- Tabernaemontana; 39- Celastraceae; 40- Hybanthes type; 41- Shorea; 42- Hopea; 43- Anacardiaceae; 44- Blepharistema; 45- Dipterocarpaceae; 46- Bignoniaceae; 47- Bombax; 48- Myristica; 49- Durio; 50- Ligustrum; 51- Meliaceae; 52- Menispermaceae; 53- Sterculiaceae; 54- Hopea species; 55- Scolopia;

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5. Discussion Important floristic sorting periods occurred during the shift from the wet Neogene to the unusually dry Quaternary (Axelrod, 1979), initiating a wave of emerging new plant species, presumably in response to the new climate. The Quaternary vegetation record in the Indian subcontinent, particularly prior to Last Glacial Maximum, is quite meagre. The few conclusions resulting from the various isotopic and palynological studies show significant changes in vegetation due to variations in climate (Prabhu et al., 2004). The evergreen and moist/dry deciduous forest recorded in the present study generally was confined to low elevation (0–650 m) where the coldest months remain >23  C (Pascal, 1988), and 3–4 dry months occur during the year (Barboni et al., 2003). The important determinant of distribution of wet, evergreen forest types in the present day western Ghats is the shorter dry season, rather than high total precipitation (Pascal, 1982). Most of the palynotaxa recorded in the present study are representatives of the medium to high precipitation zone which receives annual rainfall in excess of 2500 mm (Cullenia, Garcinia, Poeciloneuron, Mesua, Diospyros, Memecylon, Podocarpus, Palaquium, Dipterocarpaceae, Mallotus, Aglaia, Eugenia, Myristica, Ligustrum, Sterculia, Knema, Terminalia, Meliaceae, Sapotaceae etc.). However, evidence of pollen grains of deciduous taxa such as Bombax, Terminalia, Diospyros, Lagerstroemia, Schleichera, Sterculia and Artocarpus indicates <2500 mm annual precipitation. This indicates variability in the rainfall distribution pattern in the southern part of the Indian peninsula during the Pleistocene, depending on local variations in altitude and moisture, similar to the present day precipitation pattern ranging from 800 to 5000 mm of rainfall and differing number of dry months in a year, recorded through palynological study of surface sediment (Barboni and Bonnefille, 2001). Many Tertiary plant species that were adapted to very moist conditions did not go extinct despite the development of a progressively drier climate (Herrera, 1992) and repeated Quaternary glacial aridification which strongly stimulated the emergence of new taxa (Axelrod, 1979). At present, there is a gap in the knowledge of historical ecological processes that have eliminated species from communities or allowed them to persist (Wiens and Donoghue, 2004). The tropical rainforest palynofloral diversity in India is known to exist since the Palaeogene in coal and lignite deposits of western and north-eastern coastal and adjoining lowland areas (Venkatachala and Kar, 1969; Ramanujam, 1987; Hait and Banerjee, 1994; Tripathi et al., 2009). Apparently, except for the smaller size of the pollen and minor variability in the ornamentation, the morphology of Quaternary pollen grains such as Garcinia, Cullenia, Xanthophyllum, Bombax, Dipterocarpus, Hopea, Shorea, Memecylon, Podocarpus, Aglaia, Meliaceae, Toona/Walsura, Myrtaceae, Eugenia, Gomphandra, Lagerstroemia, and Sapotaceae show resemblances to the morphology of the pollen grains recorded from India during the Palaeocene/Eocene. This suggests that the modern climate conditions in the southwestern Ghats have facilitated the conservation of many plant species which existed during the Palaeogene. Some of the morphogenera, e.g., Bombacacidites, Lakiapollis and Tricolporocolumellites, have affinities with extant Bombacaceae (Thanikaimoni et al., 1984; Venkatachala

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et al., 1989; Mandal, 2005). A diversity and abundance of Durio/ Cullenia (Lakiapollis) flourished during the Eocene in a warm equatorial climate and declined with climate change after the Eocene. The taxon was common in the Miocene of south India, e.g., Kerala, Neyveli (see Mandal, 2005). The abundance of Cullenia and Bombax in the present study and their existence as important extant flora in the southwestern Ghats provides a clue to its continuous existence in the Indian subcontinent since the Palaeogene. Bombax is found to be abundant throughout the warmer forest regions and tropical Eastern Himalaya (Bose et al., 1998) and has wide ranges of temperature and rainfall tolerance. The geochemical fingerprinting of the glass shards show major elemental oxides, trace elements and REE (Rare Earth Elements) comparable to the YTT glass shards reported elsewhere (Chesner, 1998; Pattan et al., 2001). The highly silicic nature of the ash is very typical of the rhyolite and closely resembles the Toba ash of Indonesia. The Al2O3 value of 13.23% is very close to that of Toba ash (Ninkovich et al., 1978). The Indian tephra beds are chemically identical to deep sea and terrestrial proximal correlatives of the YTT (Acharyya and Basu, 1993; Pattan et al., 2001; Raj, 2008). This catastrophe may have blasted so much ash and sulphur dioxide into the stratosphere that it blocked out the Sun, causing Earth’s temperature to plummet and possibly triggering the glacial period (Chesner et al., 1991; Dehn et al., 1991). The presence of high sulphur in the studied OM substantiates the signatures of YTT shards in Chaganachery. Characteristic V-shaped notches are generally formed during aeolian transport, and are also reported from the Northern Indian Ocean and the Bay of Bengal (Rose and Chesner, 1987; Pattan et al., 2003). The abnormal presence of reduced Eu2þ (Europium) in the processes of fractionation of the REE group is thought to be the reason causing Eu maxima and minima in the spectra of REE distribution in various minerals and magmatic rocks. One of the unique features of melt flow and melt– rock reaction is the strong fractionation of Eu from other REE, because Eu2þ is more incompatible and expected to diffuse faster than the other REE (Morgan et al., 2003). The glass/chondrite normalized trace elements show virtually identical values to those recorded elsewhere in YTT shards, suggesting similar origins. Results are consistent with LREE, MREE and HREE patterns. Therefore, the time period of estuarine organic matter deposition is related to the MIS 5.1 (5a) sea level highstand, which was the host to YTT and high terrigenous organic sediment from the surrounding highly diverse rainforest. The distal tephra is constrained by an isothermal plateau fissiontrack age on glass in Malaysia of 68000  7000 a (Chesner et al., 1991) and by the O-isotope chronology at the stage 5.1/4 boundary (74,000  7000 a) in deep sea cores (Ninkovich et al., 1978; Rampino and Self, 1993). In the Indian subcontinent, MIS 5 shows three interstadials (5.1, 5.3 and 5.5) associated with relatively larger terrigenous sediment deposits (TSD), suggesting humid conditions and intense precipitation (Pattan et al., 2005) that led to high OM deposition during this period. Organic rich sediment accretion in an anoxic estuarine ecosystem triggered build-up of the trace elements recorded in the present analysis. High values of titanium, rubidium, arsenic, cobalt and lead were associated with the high percentage of OM. Titanium and rubidium along with other heavy

56- Bignoniaceae; 57- Myristica type; 58- Boerhavia; 59- Liliaceae; 60- Madhuca; 61- Lagerstroemia; 62- Podocarpus; 63- Symplocos; 64- Palaquim; 65- Chrysophyllum; 66- Aegialitis; 67Lamiaceae; 68- unidentified; 69- Mesua; 70- Casearia; 71- Eriocaulon; 72- Tinospora; 73–74- Garcinia; 75- Cullenia; 76- Unidentified; 77- Mallotus; 78- Tilia; 79- Citrullus; 80- Cynotis; 81- Gomphandra; 82- Canarium; 83- Myrtaceae; 84- Knema; 85- Anacardiaceae; 86- Meliaceae; 87- Schleichera; 88- Garuga; 89- Euonymous; 90- Toddalia; 91- Cullenia exarillata; 92Walsura; 93- Myriophyllum; 94- Moraceae; 95- Corchorus; 96- Euphorbiaceae; 97–98- Palmae; 99- Xanthophyllum; 100- Myristica; 101- Sesuvium; 102- Anacardiaceae; 103- Striga angustifolia; 104- Shorea; 105; Meliaceae; 106- Corchorus; 107 and 108-Palmae; 109-Ceiba/Bombax; 110-Asteraceae; 111- Scolopia; 112- Dipterocarpaceae; 113- Poeciloneuron; 114Dipterocarpaceae; 115,117- Liliaceae; 116- Unidentified; 118–119- Harpullia; 120- Crepis; 121- Homalium; 122- Aglaea; 123- Unidentified; 124- Sapotaceae; 125- Liliaceae; 126Botryococcus algae; 127 to 129- Trilete pteridophytic spores; 130–134- Monolete spores; 135–139- Fungal spores; 140- algal cyst; 142 and 150- Dinoflagellate cysts of Operculodinium spp.; 151- Lingulodinium; 141,143–149- Spiniferitis species. 152–162- Estuarine to marginal marine dinoflagellate cysts; 163- Homotryblium type; All scales ¼ 10 mm, or as mentioned.

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metals such as Co, Pb, As, and Mn are generally associated with biologically mediated organic framboidal pyrite that indicates an anoxic estuarine depositional environment (Whitehead et al., 1993; Farooqui and Bajpai, 2003). Several records of high arsenic concentration associated with framboidal pyrite and organic matter deposited under anoxic conditions have induced serious health problems in coastal areas of India (West Bengal) and Bangladesh (Acharyya et al., 2000). Therefore, high concentrations of heavy metals probably bound to abundant framboidal pyrite concretions are likely to contaminate the groundwater aquifers and pose health problems through groundwater drawn from the wells around Chaganachery or other similar contemporary areas. The distribution and chemistry of the OM layer thus requires a thorough investigation. Radiocarbon dates of the organic matter are >40 ka BP in CHY-I and >41 ka BP in CHY-II, and the associated glass shards confirm the YTT origin. Therefore, the estuarine sediment recorded here in Chaganchery is related to the sea level highstand during MIS-5.1 around 80 ka BP (Fairbanks and Matthews, 1978; Toscano and Lundberg, 1999 and Shackleton, 2000). The presence of the hard cemented ferruginous framework of quartz, feldspars and rock fragments above the estuarine sediment could be due to sub-aerial exposure of marine deposits in the subsequent sea level lowstand during the glacial period triggered by YTT aerosols in the atmosphere. Such Pleistocene dessicated clays have been reported earlier in similar conditions by Yim and Tovey (1995). Ice core data show that the weather was cooler by 3–5  C for several centuries after the Toba eruption, possibly due to the large amount of stratospheric sulphur aerosols ejected by the volcano, and increased albedo (Zielinski et al., 1996; Rampino, 2002). In India during the LGM, most of the peninsular areas were vegetated by savanna forest, indicated by the dominance of C4 vegetation (Sukumar et al., 1993; Bera et al., 1997; Meher-Homji and Gupta, 1999) from the southwestern Ghats. The extant tropical rainforest in the region spread in the southwestern Ghats between 7 and 4 kyr. BP (Gupta, 1973; Bera and Farooqui, 2000). It has been documented (Hooghiemstra and Van der Hammen, 1998, 2004) that the Quaternary unfavourable stadial climate cycles forced the rainforest flora into isolation in pockets as ‘plant refugia’ which existed as riparian vegetation due to soil moisture availability. Such ‘plant refugia’ evidence has been reported from Africa (Plana et al., 2004). Considerable fragmentation of the rainforest in southeast Asia occurred during the LGM (Hardy et al., 2002). During this period, rainforest refugia were probably present in northern and eastern Borneo, northern and western Sumatra, and the Mentawai Islands. The evergreen and semi-evergreen species that generally grow on the sides of rivers/streams as moist tropical riparian forest, including Terminalia arjuna, Hopea parviflora, Mangifera indica, Drypetes roxburghii, Garcinia gummi-gutta, Mallotus stenanthus, Calophyllum calaba, Syzygium cumini, and Schefflera racemosa are common inhabitants of the extant flora in the region, and are recorded in good percentages in the studied Pleistocene sediment. The vegetation in southern peninsular India probably took refuge in the riparian zone or around large water bodies within the forests. Despite the wave of plant extinctions that occurred with increasing aridity from Neogene to Quaternary, the fossil record shows that large taxonomic components of the ancient flora remain in contemporary communities of Mediterranean regions and tropical non-Mediterranean regions around the world (ValienteBanuet et al., 2006) and include some of the most abundant modern genera. The ecological characteristics of ancient taxa appear to be highly conserved over geological time scales. Thus, the tropical rainforest during the Palaeogene and Neogene in India and southeast Asia shows a lineage with the palynological archive of

Pleistocene documented from Chaganachery. During the LGM, these took refuge as riparian vegetation and rejuvenated during the warm and humid Holocene. The study bridges the gap in the knowledge of the sustenance and lineage of extant flora to Pleistocene and older periods.

6. Conclusions Palynological study has revealed a fluvio-marine to estuarine depositional environment in Chaganachery, Kerala, which was host to aeolian YTT glass shards (w74 ka BP). Terrigenous organic sediment is dominated by pollen grains and macro-remains of coastal moist forest, southwestern Ghats moist deciduous forest, dry deciduous forest and southwestern Ghats montane forest. The rich palynofloral archive (>80 pollen taxa) of the last interglacial period associated with a sea level highstand (MIS-5.1) is a useful analogue to interpret Palaeogene, Neogene, and modern vegetational lineages. Long-term persistence of moist climates in the past and present (high precipitation, humidity and fewer dry months) in southern India have facilitated the continuity of many ancient rainforest flora and other coastal species, as well as low-lying riparian zone flora which took refuge during the last glacial period. The study substantiates the hypothesis of ‘rainforest refugia’ during arid/glacial periods in the Indian subcontinent.

Acknowledgments Thanks are due to the Directors, BSIP, GSI, DGM and principal, SBC for providing necessary facilities and encouragement in accomplishing this research work. The authors wish to thank the Radiocarbon laboratory, BSIP for providing 14C dates of the organic matter. We are grateful for the constructive comments by anonymous referees and the Editor-in-Chief which helped us in improving the manuscript.

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