Hydroclimatic variability and corresponding vegetation response in the Darjeeling Himalaya, India over the past ~2400 years

Catena 170 (2018) 84–99

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Hydroclimatic variability and corresponding vegetation response in the Darjeeling Himalaya, India over the past ~2400 years

T

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Ruby Ghosha, , Oindrila Biswasb, Dipak Kumar Paruyab, Shailesh Agrawala, Anupam Sharmaa, ⁎⁎ C.M. Nautiyala, Meghma Berab, Subir Berab, a b

Birbal Sahni Institute of Palaeosciences, 53, University Road, Lucknow 226007, India Centre of Advanced Study, Palaeobotany-Palynology Laboratory, Department of Botany, University of Calcutta, 35, Ballygunge Circular Road, Kolkata 700019, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Hydroclimate Vegetation response MWP LIA Late Holocene Darjeeling Himalaya

To understand the present day climate change impacts on ecosystem, knowledge of the rapid climatic events occurred within the last few thousand years is crucial. Indian summer monsoon (ISM) dominated eastern Himalayan vegetation is sensitive to even a minor change in climate parameters, hence suitable for studying climate-plant interactions. We reconstruct a ~2400 years climatic history of the Darjeeling area, eastern Himalaya combining pollen, phytoliths, non-pollen palynomorphs (NPPs), δ13C signatures, sediment texture and total organic carbon (TOC) records from a lacustrine deposit to explore ecosystem response to climate change and to understand the possible forcing mechanisms behind it. This study is centred on two northern hemispheric late Holocene climatic events namely Medieval Warm Period (MWP) and Little Ice Age (LIA). Although considerable variations exist globally for these warm (moist) and cool (dry) periods with respect to their timing, duration, and hydroclimatic dynamics, our results identify a humid climatic phase at the beginning of the last millennium, a pre-MWP less humid phase, while MWP was wetter than the former phase and a wet LIA in the Darjeeling Himalaya. Our results indicate that this climatic variability also induced changes in the regional vegetation. During 364 BCE to 131 CE, the region was humid harbouring a dense broad-leaved evergreen forest; a comparatively drier condition prevailed between 131 CE and 624 might be the reason behind the thinning in the forest cover. A wet phase is observed during 1118 CE. A further increase in monsoonal strength is apparent between 1367 CE and 1802. Considering the available records from the eastern Himalaya and peninsular India it is inferred that centennial scale variations in frequencies of “active dominated” and “break-dominated” periods govern the internal dynamics of the ISM, and considered to be the key forcing mechanism behind the differential behaviour of the ISM over these regions.

1. Introduction

well as regional variations both in time span and in magnitude. Signatures of ‘MWP’ and ‘LIA’ are most prominent in the Northern Hemisphere where these phases are manifested by corresponding warming and cooling trends respectively. Although MWP displayed a warming trend that matches or sometimes exceeds that of the past decade in some regions, but are comparatively lower than the recent global levels (Mann, 2002b). However, during LIA, a cooling over the extratropical Northern Hemisphere continents is noticed (Mann et al., 2009). These phases are less reliable in the Southern Hemisphere, especially in the extratropics. In the monsoon dominated tropical regions, these temperature anomalies also influenced monsoon as expressed by regional hydroclimatic variations, though heterogeneity exists in timing and extent of these RCCs (Mann et al., 2009; Wanner et al., 2008). A good number of proxy- records have reconstructed the nature and

The nature and causes of rapid climatic changes (RCCs) during the late Holocene remain controversial due to disparity in their timing, duration, magnitude and dynamics among different regions of the globe. These inconsistencies may be attributed to sensitivity of the proxies used, time lag factor among different proxy parameters, regional climatic differences and their forcing factors and sometimes chronological constrains (Mayewski et al., 2004). Two significant late Holocene climate events spanning the last millennium a cool-dry phase known as ‘Little Ice Age’ (LIA), a period approximately ranged between ca. 1400 to 1850 CE (Mann, 2002a) preceded by a contrasting warmmoist phase known as Medieval Warm Period (MWP) spanning between ca. 900 to 1300 CE (Mann, 2002b) have shown considerable global as

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Correspondence to: R. Ghosh, Birbal Sahni Institute of Palaeosciences, 53, University Road, Lucknow 226007, India. Correspondence to: S. Bera, Centre of Advanced Study, Department of Botany, University of Calcutta, 35, Ballygunge Circular Road, Kolkata 700019, West Bengal, India. E-mail addresses: [email protected] (R. Ghosh), [email protected] (S. Bera).

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https://doi.org/10.1016/j.catena.2018.05.043 Received 9 December 2017; Received in revised form 21 May 2018; Accepted 29 May 2018 0341-8162/ © 2018 Elsevier B.V. All rights reserved.

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phytoliths recovered from ancient sediments represent a highly localized, ín situ deposition. Due to their durable nature and distinctive shapes phytoliths can identify plants up to family, generic or sometime even specific level also, and thus help in reconstruction of past vegetation (Piperno, 1988). Moreover, phytoliths also can distinguish grasslands from woodlands, C3 from C4 grasses in an assemblage and among the C4 grasses dominance of mesophytic from xerophytic grasses (Pearsall, 2000). However, a certain amount of phytoliths in assemblage may also be contributed due to their long-distance transport (Piperno, 1988). The combination of dry season winds and arid vegetation with bare ground cover are the two factors that favour longdistance phytolith transport, which is not the case for the forested Darjeeling Himalaya. Hence, phytoliths recovered in ancient sediments will reflect mostly the local vegetation scenario. Non-pollen palynomorphs (algal, fungal and zoological remains) may also provide useful information on local ecological characteristics of a site due to their definitive ecological preferences. They are widely used in reconstructing vegetation dynamics, land-use and hydrological changes (Barthelmes et al., 2012; Cugny et al., 2010; Dietre et al., 2012; Feeser and O'Connell, 2010; Ghosh et al., 2017; Wünnemann et al., 2010). Stable carbon isotopic signature preserved in organic matter associated with soil/sediments (SOM) is another tool frequently used in palaeovegetation as well as palaeoclimate reconstructions (Meyers, 1994). It is a reliable proxy to discriminate past distribution of C3 and C4 plants. Environmental factors like the temperature and precipitation influence carbon isotopic fractionation in both C3 and C4 plants and alter the δ13C values of these plants. C3 plants adapted to humid climatic conditions show a large range in δ13C values (−20‰ to −34‰, VPDB), while C4 plants adapted to less humid conditions show a range between −9‰ to −19‰ (O'Leary, 1988; O'Leary, 1981; Sage, 1999; Agrawal et al., 2012; Basu et al., 2015). A minor variation in temperature–rainfall gradients significantly alter δ13C values of C3 and C4 plants, which is reflected in δ13C value of SOM (Deines, 1980; O'Leary, 1981; Farquhar et al., 1989). Hence, together all these proxies may provide insights into long term changes in climate, vegetation and land-use.

magnitude of the late Holocene climate variations in centennial- to millennial-scale (and some even up to decadal scale) from different parts of the geographically and climatically varied Indian subcontinent (for example, Agnihotri et al., 2002; Berkelhammer et al., 2010, 2012; Bhattacharyya et al., 2007; Chauhan and Quamar, 2010; Demske et al., 2016; Dixit and Bera, 2013; Gadgil et al., 2005; Kotlia et al., 2012, 2014, 2016; Kotlia and Joshi, 2013; Liang et al., 2015; Menzel et al., 2014; Mishra et al., 2015; Phadtare, 2000; Prasad et al., 2014; Quamar and Chauhan, 2012, 2014; Rühland et al., 2006; Shankar et al., 2006; Sinha et al., 2007, 2011a, 2011b, 2015 and Tripathi et al., 2014). In most of these records, variations in the Indian summer monsoon (ISM) was emphasized as it supplies nearly 80% of the annual precipitation over the Indian subcontinent as the JJAS (June–July-August-September) seasonal precipitation vital for the agrarian-based Indian socioeconomy. Records exhibit conspicuous climatic variations in the Indian subcontinent in the last two millennia. Identification of the forcing factor(s) behind this regional inconsistency is an urgent need to understand the pattern of variability and to enhance the future climate predictability. Earlier asynchronous behaviour of the ISM is also observed during the MWP and LIA in India. For example MWP in general is observed as an interval of strong monsoonal period (Dixit and Tandon, 2016 and references therein), but high resolution speleothem oxygen isotopic (δ18O) records from Dandak Cave, central-eastern India (Sinha et al., 2007) and Dharamjali Cave, Central Kumaun Himalaya (Sanwal et al., 2013) inferred that droughts occurred frequently on either side of MWP. Furthermore, records from the Indian core monsoon zone (CMZ) suggest that the entire span of the LIA was punctuated by episodic and widespread reoccurrences of monsoon megadroughts (Sinha et al., 2011b), while from a general cool-dry trend, departures are observed in the central Himalaya and in the northeast India, where signatures of a wet LIA was evident (Sinha et al., 2011a; Sanwal et al., 2013; Dixit and Tandon, 2016 and references there in). In their multi-proxy study and model simulations, Polanski et al. (2014) have showed that heterogeneity exists among different regions within the Indian subcontinent regarding the summer and winter moisture anomalies, where western and central Himalaya are influenced by variations in intensity of extratropical Westerlies during winter. Variations in thermal gradient between the Bay of Bengal and the Indian subcontinent as well as the zonal band of strengthened ISM–EASM (East Asian Monsoon) link influence the eastern Himalayan region while, the summer moisture anomalies of the Central India are affected by the SST (sea surface temperature) pattern in northern Arabian Sea. More studies from comparatively underexplored regions of this vast subcontinent may improve our understanding of these late Holocene RCCs and identify the nature and possible forcing factors of these multi-century climatic variations. The ‘eastern Himalaya’ may serve as an excellent archive for studying past monsoonal impacts on vegetation, as this highly precipitated region is exclusively influenced by the Bay of Bengal branch of the ISM with a rich and diverse flora. Moreover, from this geographically and floristically vast region, very little is known on the late Holocene monsoon variability and its ecosystem impacts. Available records inferred asynchronous monsoon trends in the eastern Himalaya in the last few thousand years (Chauhan and Sharma, 1996; Sharma and Chauhan, 2001; Bhattacharyya et al., 2007; Agrawal et al., 2015). One of the reasons may be the age uncertainties of these records attributable to poor age resolution. Besides, possible forcings behind this asynchronous behaviour of the ISM are not even considered in these studies. Our aim behind this multiproxy study is to identify the trends of past changes in vegetation distribution with reference to climatic forcing over the last ca. 2400 years in the Darjeeling area, eastern Himalaya and to discuss the possible underlying mechanisms. We have used pollen grains as one of the proxies as they depict a picture of both the local (herbs and azonal elements) and regional vegetation (due to their long distance wind transport). While, large proportions of the

2. Study area, climate and vegetation The Darjeeling Himalaya bounded by Sikkim in the north, Nepal in the west and Bhutan in the east is a part of the eastern Himalayan mountain ranges. Samples for the present study were collected from a 70 cm deep profile dug out at the bed of a small, dried lake located at an elevation of 1943 m a.s.l. (above sea level) (27°01′45.24″N; 88° 19′18.71″E) of the Darjeeling Himalaya. Today the surface area of the dried lake is only about 35 × 30 m and situated about 15 km southeast from Darjeeling Township at the Sixth Mile area of the Darjeeling district (Fig.1). Presently this dried lake is located at the temperate vegetation belt of the Darjeeling Himalaya, surrounded by an open wood land. The region is influenced by the Bay of Bengal branch of ISM. Between 1500 and 3000 m a.s.l. a warm-temperate humid climate prevails, where a warm summer and a dry winter characterize the region (as per Köppen-Geiger climate classification, Peel et al., 2007). Annual mean temperature of the area is about 14.7 °C. Mean temperature of the summer months reaches to 21.3 °C, while mean temperature for the winter months goes down to 4.5 °C. Though snowfall is not a common feature in Darjeeling town, but sometimes may takes place once or twice in a year between January and February. However, high elevated sites of the Darjeeling Himalaya get plenty of snow between end of December and February. From the gridded climate data, the mean annual rainfall of the area is estimated to be about 2716 mm (Hijmans et al., 2005). Maximum extent of rainfall takes place between the months of June and September. The dominant forest type of the study area falls under east Himalayan wet temperate forest. Higher stretches of the temperate vegetation belts are occupied by oaks (Quercus spp.) and chestnuts 85

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Fig. 1. Map of India showing location of the SMP (red box) in the Darjeeling Himalaya (area marked in red) and other sites discussed in this work (blue dots). Black arrows indicate the direction of the Arabian Sea and Bay of Bengal branches of the ISM. ITCZ lies in its northern position during the summer monsoon months (double red lines) and moisture laden winds (black arrows) cross India from the South-West to North-East. [Data points other than present study are 1. Mirik Lake, Darjeeling (Sharma and Chauhan, 1994), 2. Jore-Pokhari, Darjeeling (Chauhan and Sharma, 1996), 3. Kupup Lake, Sikkim (Sharma and Chauhan, 2001), 4. Paradise Lake, Arunachal Pradesh (Bhattacharyya et al., 2007), 5.Dandak cave (CMZ) (Sinha et al., 2007), 6. Jumar cave, central India (Sinha et al., 2011b), 7. Wah Sikhar cave, Northeast India (Sinha et al., 2011b), 8. NGHP-16A core, Bay of Bengal (Ponton et al., 2012), 9. Dharamjali cave, Central Kumaum Himalaya (Sanwal et al., 2013), 10. Panigarh cave, Central Himalaya (Liang et al., 2015), 11. Dibang valley, Arunachal Pradesh (Agrawal et al., 2015)]. Colour legends for elevation indicate land elevation, and ocean is demarcated as blue. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Palaearctic distribution also have characterized this zone. Major plant species between 1800 and 2100 m a.s.l. around the study area as per their frequency in the regional vegetation are, Machilus edulis (Lauraceae, 12%), Magnolia cathcartii (Magnoliaceae, 8%), Beilschmiedia spp. (Lauraceae, 5%), Cinnamomum bejolghota (Lauraceae, 2%), Litsea spp. (Lauraceae, 1.5%), Machilus sp. (Lauraceae, 1.5%), Magnolia campbellii (Magnoliaceae, 2%), Engelhardtia spicata (7%), Schima wallichii (Theaceae, 5%), Castanopsis tribuloides (Fagaceae, 3%), Lithocarpus spicatus

(Castanea spp.). Rhododendron spp. are extended down from the subalpine forests above. In the lower stretches, Lauraceae members are equally abundant and predominate over the oaks while, Engelhardia and Schima are found extended fairly up from the lower forests (Champion and Seth, 1968). Magnoliaceae are present throughout the temperate vegetation belt but do not generally contribute greatly to the canopy. Among the conifers Pinus roxburghii is abundant at the slopes of Rangit valley of Darjeeling. Acer, Prunus, Ulmus and other deciduous genera of

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Palaeosciences (BSIP), Lucknow, India.

(2%), Prunus nepalensis (3%), Mallotus nepalensis (Euphorbiaceae, 2.5%), Betula alnoides (Betulaceae, 2.5%), Acer laevigatum (Sapnidaceae, 1%), A. campbellii (Sapnidaceae, 1%) Eurya acuminata (Pentaphylacaceae), Symplocos theaefolia (Symplocaceae), Araliaceae, Ilex spp. (Aquifoliaceae), Turpinia cochinchinensis (Staphyleaceae) and Mahonia spp. (Berberidaceae) (Champion and Seth, 1968).

3.3. Analyses of sediment texture and loss on ignition (LOI) To perform sediment textural analysis (grain size), 10 g of each sample was air dried and subjected to treatment with different chemicals to remove excess salts and carbonate, organic carbon and iron manganese coatings following the procedure of Jackson (1956) and Knuze (1965). To remove sand fraction, samples were then sieved by 53 μ sieve. Silt and clay fractions were determined using pipetting technique (Day, 1965; Tanner and Jackson, 1947). For determination of loss on ignition (LOI), about 5 g (− 200 mesh) sample powder was taken and heated in quartz crucibles and loss of weight was measured at 110 °C, 550 °C and 950 °C to determine the moisture, organic carbon and carbonate carbon (see supplementary table S2) present respectively in the samples (Bengtsson and Enell, 1986; Dean Jr., 1974). This technique is widely used as this is rapid and provides fairly consistent results compared to that obtained using carbon analyzer and therefore widely used.

3. Methodology 3.1. Pollen, phytoliths and NPPs: extraction and study Standard technique i.e. “boiling the representative sample (10 gm) in 10% KOH solution followed by HF treatment and acetolysis using acetic anhydride and conc. H2SO4 mixture in 9:1 ratio was followed for extraction of palynomorphs” (Erdtman, 1954; Faegri and Iversen, 1989). Detail method for the palynological study is mentioned in supplementary information (S3.1). Herbaceous pollen taxa belonging to the same family were grouped together, as high pollen diversity with negligible occurrence may create noise and dilute the indicative potential of the datasets. From the sediments, phytoliths were extracted following Fredlund and Tieszen (1994) and Kelly (1990) with slight modifications which involved organic matter oxidation using H2O2 followed by a densimetric separation with heavy liquid CdI2 + KI solution maintaining a specific gravity of 2.3. For detail method of phytolith analysis adopted in this study see supplementary information (S 3.1.1). While interpreting palaeoclimate from fossil phytolith assemblages, we used four phytolith indices such as Ic, EhIc, Iph and Fs. Details of which are mentioned in supplementary information (S 3.1.1). For the extraction of the NPPs, 10 g of each of the samples were processed following the similar protocol followed for palynomorph extraction. NPPs were counted independent of palynomorphs (including fern spores) and their frequencies were expressed as percentages of the total sum of the NPPs. In this study total counts of the NPPs varied between 164 and 325 depending on yield of the samples. Identification of the NPPs was made with the help of some already published literatures (for example Cugny et al., 2010; Demske et al., 2013; Feeser and O'Connell, 2010; Gelorini et al., 2011; Hooghiemstra and van Geel, 1998; López-Vila et al., 2014; Montoya et al., 2010; van Geel and Aptroot, 2006).

3.4. Chronology Three bulk sediment samples rich in organic matters were selected for C14 radiocarbon dating (Table 1). All the C14 dates were calibrated in OxCal v.4.2 software package (Bronk Ramsey, 2001) using calibration curve IntCal13 (Reimer et al., 2013). Poisson process deposition model (Bronk Ramsey, 2008) was used to establish the age-depth relationship of the Sixth Mile profile (SMP) using calibrated dates and depths of the corresponding samples (Fig. 2) and considering the lithological boundaries. The model used here assumes a non-uniform sedimentation rate throughout the analyzed profile. A critical value of 60% and 95% respectively for the agreement index and the convergence index was selected in the model. The agreement index gives the relative likelihood of the posterior from the model to that of a ‘null’ model (considering no constraints or interrelationships between the events) and the threshold for acceptable agreement index is 60% which has been selected by empirical comparison to χ2 tests (Bronk Ramsey, 2008). To build up a representative sample of possible solutions OxCal uses a Markov Chain Monte-Carlo (MCMC) and the convergence index measures the extent to which it is able to do so. A value above 95% indicates a good convergence (Bronk Ramsey, 2016).

3.2. Stable carbon isotopic analysis of sedimentary organic matter (SOM) 3.5. Numerical analysis

For carbon isotopic analysis of SMP sediments, ~1 g of each sample was finely powdered and treated with 5% HCl to remove carbonate. Further, acid and soluble salts were removed from the sediments by centrifugation (~3000 rpm) process. The de‑carbonated samples were dried in an oven at about 50 °C and were subsequently crushed to fine powder. All the acid treated powdered samples were individually packed into tin capsules and introduced into the pre-filled and conditioned reactor of Elemental Analyzer (Model no. Flash EA 2000 HT) through an auto sampler attached with the instrument. The CO2 gas produced through the combustion was introduced into the Continuous Flow Isotope Ratio Mass Spectrometer (CFIRMS, MAT 253) coupled with ConFlow IV interface. Signals corresponding to masses 44, 45 and 46 were measured for each sample and a reference gas and carbon isotopic composition was calculated. Three pulses of reference CO2 were allowed to enter into CFIRMS through reference open split, followed by single pulse of sample CO2. Reproducibility of samples was also checked by repeated measurements that ranged between 0.1 and 0.3‰ for δ13C (see supplementary table S1). The tank CO2 was calibrated using NBS CH3. The carbon isotopic data is reported against VPDB. Internal standards (Sulfanilamide and Marine Sediment Standard) were run to check the accuracy for the CO2 measurements with an external precision of ± 0.1‰ (1σ). All samples were analyzed in the Stable Isotope Laboratory, Birbal Sahni Institute of

In order to avoid bias related to low pollen counts (< 200 grains) in some of the SMP samples, and to provide more confidence in our pollen dataset, 0.95 confidence intervals for pollen proportions of the major taxa were calculated following the equations proposed by Mosimann (1965). This will also help us to understand if the observed variations in the pollen assemblages are statistically significant. Excluding the azonal marshy and aquatic elements from the dataset, palynotaxa for the present analysis were selected following the method proposed by Prentice (1980) i.e., presence in at least 3 samples with a maximum ≥3% in at least one sample. Further all the non-arboreal elements were grouped together (as Sum_NAP), and finally the dataset was reduced to Table 1 Lithological description of the Sixth Mile profile. Profile depth (cm)

Nature of the sediment

0–10

Loose blackish carbonaceous clay with silt with signature of anthropogenic disturbance Compact blackish carbonaceous clay with fine silt and sand Greyish clayey silt with much sand Compact blackish carbonaceous clay with fine silt and sand

10–30 30–60 60–70

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detrending by linear segments, and down-weighting of rare species using statistical program CANOCO version 4.5 (ter Braak, 1986, 1988; ter Braak and Smilauer, 2002). DCA produces a low-dimensional space scaled in standard deviations (SD) that conserves ecological distances and original relationships between species and samples (Gauch, 1982). With the palaeoecological datasets, this ordination method defines an ecological space delimited by the variability of taxa and environmental conditions through time. 4. Results Lithological description of the Sixth Mile Profile (SMP) is provided in Table 1. The profile was comprised of alternative layers of compact blackish carbonaceous clay with fine silt and sand, and greyish clayey silt with profuse sand fraction. The upper 0–10 cm part of the profile was anthropogenically disturbed and thus not included in this study. Fig. 2 and Table 2 exhibit well-constrained ages and relatively narrow uncertainty ranges for the SMP. The Poisson process model confirmed compatibility of the three dates (as agreement index was higher than the critical value of 60%) with the overall age-depth relationship (Fig. 2). Variations in pollen, phytoliths, NPPs, sediment texture, TOC, δ 13C values and sedimentation rate across the SMP are represented in Figs. 3–6. As mentioned earlier, we have calculated 0.95 confidence limits of proportion for the twenty major pollen taxa in the SMP assemblage following Mosimann (1965) (Fig. 4). In case of low pollen sum, pollen percentages may vary widely. In this study the confidence intervals quite accurately represent that the variations seen within most individual taxa are not equally significant (Fig.4). The main signal in the pollen assemblage appears to be driven by changes in Pinus roxburghii, Quercus spp., Mallotus, and the combined non-arboreal pollen data. It further shows that temporal variations in the combined arboreal and non-arboreal taxa are most significant in interpreting changes in the past vegetation. Based on the observed changes in the SMP sediment record and constrained cluster analysis using CONISS three zones have been identified (Figs. 3–7). 4.1. Zone I: between 70 and 60 cm depths (ca. 364 BCE–131 CE)

Fig. 2. Age-depth model reconstructed for the SMP using OxCal v4.2.4 (Bronk Ramsey and Lee, 2013). The outer, lighter shade indicates 95% probability distribution range and the inner, darker shade represents 68% probability distribution range. Dark circles represent modeled age medians for the original radiocarbon dates, while open circles represent the modeled age medians for every depth derived through age-depth model. See the details in chronology section.

This zone is characterized by high frequency of arboreal pollen (AP) taxa (~81%) over the non-arboreals (NAP) (~19%) (Fig. 3a). High percentages of aquatics with moderate occurrence of marshy elements have characterized this zone (Fig. 3b).Grass phytolith assemblage displays high percentages of bilobates, flat top rondels, two horned rondels and medium/plateau saddles. Flat tower types and cuneiform bulliform cells are also recovered in significant amounts. Among the non-grass types, globular rugose (globular granulate), globular echinate and blocky cells are frequently recovered in this zone with negligible occurrence of Cyperaceae types (Fig. 5a). Phytolith indices like Ic (65.35%), EhIc (68.75%), Iph (20%) and Fs (0.10%) also show significant variability (Fig. 5a). Fern spores are abundantly observed in NPP assemblage. Ustulina deusta, Arnium, Agaricus, and Glomus dominate the fungal spore assemblage followed by a low percentage of Sordaria. Of the zoological remains Hydrozetes and Arthropod hairs are

20 taxa. We have performed detrended correspondence analysis (DCA) on the frequency datasets of pollen (arboreal and non-arboreals only), phytolith, and NPPs (other than phytoliths) separately to understand the drivers that might explain the variations in the datasets. During the analysis, percentage data for the pollen, phytoliths and NPPs were square root transformed to stabilize the variance and to optimize the signal-to-noise ratio. DCA was performed selecting the options such as

Table 2 14 C Radiocarbon dating results obtained on bulk sediments from the Sixth Mile profile. Calibration was done using IntCal13 (Reimer et al., 2013) calibration curve and calibrated dates are expressed as 95% probability ranges and median point estimates (following, Scott et al., 2007; Rull et al., 2015; Demske et al., 2016). Laboratory code

Sample depth (cm)

Radiocarbon date (14C yr BP)

Calibrated age, 95% range (yr AD/BC)

Calibrated age, median (yr AD/BC)

Calibrated age, median (yr BP)

BS-3676 BS-3747 BS-3748

30 40 70

620 ± 90 860 ± 60 2330 ± 100

1248–1610 CE 1022–1246 CE 746–57 BCE

1367 CE 1118 CE 364 BCE

583 832 2313

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Fig. 3. a. Frequency (%) spectra of the non-arboreal and arboreal pollen taxa along the SMP. All the percentages are calculated in respect to pollen sum which excludes aquatic and marshy taxa. b. Percentage diagram of the aquatic and marshy pollen taxa only. All the percentages are calculated in respect to total pollen count. In both the diagrams grey shaded margins indicate a 5 fold exaggeration.

armigera) have started appearing in the later part of this zone i.e. between depth 40 and 30 cm. Organic carbon content shows a sudden drop in this zone, but ca.1.5 to 2.4‰ increase in stable carbon isotopic values is also noticed (Fig. 6). Sediment textural analysis also reveals increase in percentages of sand and decrease in clay (Fig. 6). Sedimentation rate has declined than the earlier zone (0.02 cm yr−1) and found to be more or less constant throughout this zone.

found in high frequencies. Spirogyra zygospores are also found in significant amount. Fragments of dark brown coloured unidentified biostructured phytoclasts and tracheidal fragments cf. conifer wood are also recovered in considerable percentages (Fig. 5b). Rate of sedimentation in the lake is observed to be 0.03 cm yr−1 in this zone. Textural analysis of sediments shows 49.35%, 27.30% and 23.36% of sand, silt and clay respectively. The average content of organic carbon in this zone is around 9.32% and the stable carbon isotopic value of SOM is about ~ −26.3‰ (Fig. 6).

4.3. Zone III: between 30 and 10 cm depths (ca. 1118 CE–1802 CE and onwards)

4.2. Zone II: between 60 and 30 cm depths (ca. 131 CE–1118 CE) A rise in percentages of APs over the NAPs is observed at a depth between 30 and 20 cm (Fig. 3a). Of the NAPs, Saxifraga marks its first appearance in this zone along with Sambucus and Smilax (Fig. 3a). However at depth above 20 cm, disappearance or decline of certain AP and NAP elements is noticed. Rest of the species in the assemblage shows remarkable similarity in frequency and diversity with that observed between depths 70 and 60 cm (Fig. 3a). Of the aquatics and marshy species, except Trapa, all the species show conspicuous presence. However, except Cyperaceae, all other taxa are found absent above 20 cm depth (Fig. 3b). Although bilobates have decreased but ‘Stipa type’ bilobates enhanced with corresponding increase of crosses and rondels. All the nongrass morphotypes show a rise in this zone. Polygonal cones, globular granulates and blocky types also exhibit increasing trend above 20 cm (Fig. 5a). Both the climatic index (Ic) and the eastern Himalayan climatic index (EhIc) show considerable rise than the earlier phase, but amplification of the EhIc is more prominent than the latter (Fig. 5a). Again no clear trend is observed for Iph and Fs (Fig. 5a). Within NPPs, fern allies show considerable increment than in the earlier zone, and continued upward. Tracheidal fragments cf. conifer woods and other biostructured phytoclasts also have shown a rise

A considerable overall decline in AP (~50–64%) percentages and rise in NAPs (37–49%) are observed in this zone, though arboreal species still dominate the assemblages. Aquatics and marshy taxa show overall decline in this zone, except Typha, which shows considerable presence (Fig. 3a, b). In case of grass phytolith assemblages, a minor drop in medium/ plateau saddle percentages and significant rise in tower, long saddle and collapsed saddle types mark this zone. However, no such significant variation is observed for bilobates and crosses. All the non-grass morphs such as polygonal cones (found in Cyperaceae), blocky types, globular granulates and globular echinates show considerable drop in frequency than earlier zone. Both the Ic and EhIc also show decline, but no clear trend is noticed for Iph and Fs (Fig.5a). Of the NPPs, considerable fall is observed for the dark brown coloured biostructured unidentified phytoclasts and tracheidal fragments cf. conifers, with enhancement of both the grass and non-grass cuticular fragments. A marked increase of Agaricus and Glomus as well as coprophilous fungal species (such as Cercophora, Podospora, Sordaria and Sporormiella) is noticed in this zone. Spirogyra has shown a slight declining trend (Fig. 5b). Neorhabdocoela oocytes (cf. Microdalyellia 89

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Fig. 4. Showing estimates of the 0.95 confidence interval (CI) of the true proportions of pollen taxa for counts inside the sum for selected taxa from the SMP following Mosimann (1965). Dotted coloured lines indicate 95% CI of the true proportions of the pollen grains in the spectra and red lines indicate their true proportions. ‘X’ axis of every abundance curve represents percentage frequency of the representative pollen taxa. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

35.8% variance in the datasets respectively (Table 3). In all the cases, the first eigenvalue is fairly high, suggesting that first axis represents a strong gradient. The first axis in DCA bi-plot for the pollen data shows high scores for the moist loving plants adapted to comparatively low elevation subtropical to temperate zones such as, Alnus, Araceae, Araliaceae, Betula, Calamus, Caryophyllaceae, Elaeocarpus, Eurya, Magnoliaceae, Malvaceae, Premna and Quercus. Plants adapted to comparatively less moist environments (such as Abies, Acer, Pinus wallichiana, Rosaceae, Saxifraga) in the high altitudes show low scores in axis 1 (Fig. 7a). Here the first axis appears to represent a moisture gradient. In the phytolith DCA bi-plot, moist environment indicators such as three lobates, rondels, medium or plateau saddles, trapeziforms and globular rugose types mostly occur in C3 grasses and woody dicots exhibit high scores in axis 1. While, towers, bilobates and short saddles, comparatively dry indicator elements show low scores in axis 1, and further indicate a moisture gradient (Fig. 7b). Similarly, among the NPPs, Hydrozetes, Neorhabdocoela oocytes, Ustulina, Nigrospora, Spirogyra and fern allies show high scores in axis 1. All the coprophilous fungal spores and Glomus show very low scores in axis 1 (Fig. 7c) indicating a moisture gradient. Whereas, the NPPs like Hydrozetes, Neorhabdocoela oocytes, Ustulina, Nigrospora, Spirogyra and fern allies are adapted to high available soil moisture conditions and coprophilous fungal spores indicate open forest environment with comparatively low available soil moisture.

together with non-grass cuticular fragments. Substantial decrease in coprophilous fungal spores and Glomus is noteworthy in this zone. However, fungal spores like Rosellinia, Ustulina deusta and Diporotheca show considerable rise. Neorhabdocoela oocytes show a sharp rise than in the earlier zone and decline upwards. Loricas cf. Callidina and mite eggs also found substantially present (Fig. 5b). Sediment textural records show a declining trend in sand with increasing silt and clay fractions (Fig.6) between 30 and 20 cm, but, above 20 cm depth, proportion of sand increases (Fig.6). Rate of sedimentation also rises between 30 and 20 cm (0.02 to 0.04 cm yr−1) than earlier and increased further upwards (0.07 cm yr−1) (Fig.6). δ 13C values range between −26.2 to −26.8‰ between 30 and 20 cm and further decreased to −27.6‰ above 20 cm depth (Fig.6). Results of the DCA performed separately on the pollen, phytolith and NPP datasets are presented in Table 3 and Fig. 7. Environmental gradient lengths for pollen, phytoliths and NPPs calculated in DCA for axis 1 are 1.27 SD, 0.80 SD and 1.42 SD respectively. A < 2 SD gradient length always indicates a small environmental gradient and suggests to employ a linear ordination method (Birks, 1995). However, we have used DCA here as it corrects the ‘arch effect’ caused due to a functional dependence of the second axis on the first and further compresses intra-sample distances towards the ends of the axes (Hill and Gauch Jr., 1980; Gauch, 1982). Distances anywhere in the ordination are thought to represent ecological distances. The first two DCA axes for pollen, phytoliths and NPPs accounted for 46.9%, 32.6% and

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Fig. 5. a. Variations of a. grass and non-grass phytoliths and phytolith indices and b. non-pollen palynomorphs (NPPs) along the SMP. Percentages of the NPPs are calculated in respect to total NPP count which is independent of the total pollen count.

Fig. 6. Variations in δ 13C records, organic carbon contents, loss on ignition values (LOI) after losing the organic matter, inorganic carbon and volatile contents, sandsilt-clay percentages and rate of sedimentation along the SMP. 91

Fig. 7. Detrended correspondence analysis results showing the first two axes scores for a. pollen, b. phytolith and c. NPP data. Here square symbols indicate samples and hollow circles represent different types of pollen, NPP and phytoliths recovered from SMP. (Pollen taxa codes used in the analysis are as follows: Api = Apiaceae, Ama = Amaranthaceae, Ara = Araceae, Ast = Asteraceae, Camp = Campanulaceae, Cary = Caryopteris, Caryophyl = Caryophyllaceae, Ran = Ranunculus, Lili = Liliaceae, Poa = Poaceae, Pot = Potentilla, Poly = Polygonaceae, Rubi = Rubiaceae, Saxi = Saxifraga, Abi = Abies, Pi rox = Pinus roxburghii, Pi wal = Pinus wallichiana, Tsu = Tsuga, CT = Cupressaceae/Taxaceae, Acer = Acer, Alnus = Alnus, Aral = Araliaceae, Ber = Berberidaceae, Betula = Betula, Cala = Calamus, Car = Carpinus, Eurya = Eurya, Frax = Fraxinus, Ind = Indigofera, Jug = Juglans, Fag = Fagaceae, Loni = Lonicera, Mag = Magnoliaceae, Mal = Mallotus, Pre = Premna, Quer = Quercus spp., Rhod = Rhododendron, Sal = Salix, Schi = Schima, Elaeo = Elaeocarpaceae, Rosa_AP = Rosaceae_AP, Symplo = Symplocos, Ulm = Ulmus, Ado = Adoxaceae, Smi = Smilax).

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oaks at lower elevation (Champion and Seth, 1968). However, absence of Lauraceae members in the present pollen record may not be due to their absence in the past flora but can be attributed to their low pollen production or poor preservation potential (Behling and Negrelle, 2006; Macphail, 1980). Correspondence between fossil palynoassemblage and the modern vegetation around Darjeeling indicates that a similar climatic condition as prevailing now might have supported analogous vegetation type in the past. Janssen (1966, 1973, 1984) categorized pollen grains in the assemblages in terms of their source area i.e., local, extra-local, regional and extra-regional types. Local pollen types are those that are contributed by plants growing within a radius of 20 to 30 m in a closed forest (Andersen, 1970; Bradshaw, 1981; Jacobson and Bradshaw, 1981). Hence, local pollen types are expected to be high near the source plants. Aquatic and marshy taxa and herbaceous and entomophilous plants are regarded as the ‘local types’ and are expected to be higher near the site of deposition. Hence, successional changes within these categories may provide insight into the changes in the aquatic communities and sometimes replacement of the canopy trees within a closed forest (Andersen, 1984). A low frequency of NAPs during this span suggests a dense forest around the lake. Comparatively high percentages of the aquatic species like Nuphar, Typha and Trapa indicate a deep water condition. High percentage of fern spores and moderate presence of Cyperaceae are also suggestive of a wet-land around the lake and a humid climate. Among the NPPs, higher recovery of brown coloured opaque phytoclasts and biostructured phytoclasts cf. tracheidal fragments of conifers indicates a humid climate as abundance of these particles are linked to warm and humid climatic conditions (Medeanic and Silva, 2010). Low percentage of Spirogyra, the only recovered alga is suggestive of comparatively higher lake level, as this taxon is commonly found in shallow and stagnant water and their higher recovery is linked to a lowering of the water table (Medeanic and Silva, 2010; van Geel et al., 1989). Dominance of Ustulina deusta, and comparatively low recovery of Glomus, Agaricus and coprophilous taxa also suggest a dense woodland around the lake. Ustulina deusta (presently known as Kretzschmaria deusta) is a pathogenic parasite causing soft-rot of wood to a variety of broad-leaved trees like Quercus, Tilia and Acer etc. (Hepting, 1971; Wilkins, 1934), so their high recovery is suggestive of high percentages of these trees around the lake. Another noteworthy NPP found in significant percentage is the Hydrozetes, an aquatic mite. All the developmental stages of these modern mites live in the lake margins or up to several meters below the water line (Seniczak, 2011). Water temperature, depth, and availability of nutrients in an aquatic ecosystem may increase their numbers, while their absence or low abundance indicates a cold climate, with reduced nutrient cycling or dry conditions (Erickson and Platt Jr., 2007 and references therein). So, frequent recovery of these aquatic mites also suggests high water level at the lake and a warm condition. A mesophytic condition with dominance of C3 grasses such as Pooideae, Arundinoideae and Bambusoideae in the grass cover is apparent in the phytolith data. Considerable recovery of medium/plateau saddles in the GSSC (grass silica short cell) assemblages indicates presence of Phragmites sp. around the lake, as this marshy grass species are known to produce medium/plateau saddles abundantly in the Darjeeling Himalaya (Biswas et al., 2016). High value of Ic and EhIc, and low Iph and Fs also confirm a humid climate with high monsoonal strength during the deposition. It is noteworthy here that both the Ic and EhIc show high values, but EhIc is considerably higher than Ic, due to inclusion of medium/plateau saddles into the calculation. This further proves that in the eastern Himalayan regions, where mesophytic C3 grasses dominate, EhIc is more suitable in climate interpretations than Ic (Biswas et al., 2016). This phase of richness of C3 plants (also proved from stable carbon isotopic signatures) is characterized by a comparatively high proportion of organic carbon that might have generated from the degraded biomass accumulated on the forest floor. Similarly low carbonate carbon (see supplementary table S2) content indicates shrinkage of erosion prone land surface due to a dense forest around. In the entire SMP profile sand

Table 3 Summary results of detrended correspondence analysis (DCA) on the pollen, phytolith and NPP data of the SMP (SD = standard deviation units). DCA Axis 1

Axis 2

Axis 3

Axis 4

0.208 1.271 46.9

0.05 0.695 58.1

0.013 0.557 61.1

0.002 0.773 61.5

0.071 0.803 32.6

0.042 0.661 51.8

0.009 0.655 56

0 0.485 56

0.171 1.422 35.8

0.043 0.959 44.8

0.002 0.818 45.2

0 0.818 45.2

Pollen data Eigenvalue Gradient length (SD) Cumulative percentage variance of pollen data (%) Phytolith data Eigenvalue Gradient length (SD) Cumulative percentage variance of phytolith data (%) NPP data Eigenvalue Gradient length (SD) Cumulative percentage variance of NPP data (%)

5. Discussion In order to understand the history of changes in the forest cover of the Darjeeling Himalaya in the last ~2400 years, and to explore the forcing factors behind such changes, we should have knowledge of the modern vegetation and climate of the study area which is discussed in detail in section 2. 5.1. Development and changes in the vegetation succession over the ca. 2400 years 5.1.1. Span ca. 364 BCE and 131 CE The multiproxy record reveals regional forest history of the Darjeeling Himalaya since ca. 364 BCE. It is observed from the pollen record that between ca. 364 BCE and 131 CE, forest around Darjeeling was dominated by Quercus, Pinus roxburghii, Betula, Alnus and diverse evergreen species like Magnoliaceae, Castanopsis, Symplocos, Lithocarpus, Elaeocarpus, Schima, Prunus and Sloanea, just as in the present day forest. Mallotus, Premna and Eurya were some other noteworthy members present in the forest. SMP pollen spectra also show some conifers namely Abies, Tsuga and Cupressaceae/Taxaceae though in very low frequencies that are not the components of modern vegetation around the study area but are growing at comparatively higher elevation sites. Occurrence of these conifers between ca. 364 BCE and 131 CE in very low percentages may reflect their down slope transport through katabatic winds which carry very little pollen from upper elevation to the lowlands (Markgraf, 1980; Sjögren et al., 2008; Li et al., 2013). In general the major abundances of the wind-pollinated pollen grains within an area extending from a 200 m radius up to an outer limit of approximately 200 km may also be represented in the pollen spectra in substantial and consistent percentages (Jacobson and Bradshaw, 1981; Janssen, 1984; Prentice, 1985). However, the katabatic winds which flow in the evening through mountain valleys when begin to roll down the slopes, most of the uphill pollen grains and spores primarily get deposited by that time on the forest floor, so only very little amount of pollen grains are left for downhill transport (Markgraf, 1980; Sjögren et al., 2008; Li et al., 2013). Overall coexistence of warm-temperate and sub-tropical taxa indicates a warm temperate and moist climate during this time span with strong ISM. Modern vegetation of the temperate zone of the Darjeeling Himalaya also consists of the above-mentioned species, where oaks (Quercus spp.) and chestnuts (Castanea spp.) dominate the forests at higher elevation and the laurels (Lauraceae) are abundant and predominating over the 93

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further decrease in the stable carbon isotopic values indicating further expansion of C3 plants might be due to strengthening of the ISM. As discussed earlier, our data do not capture the span that distinguishes MWP from LIA. Ic and EhIc show an accelerating trend with overall low values of Iph and Fs and suggest dominance of C3 grasses and a humid climate. However, considerable presence of cool loving taxa such as Fraxinus, Lonicera, Spiraea, Rhododendron, Chrysosplenium, Saxifraga, Arenaria, Anemone and Potentilla between 1367 CE and 1802 indicate a cooler climate than the earlier. Extra-local conifers showing a further rise (Tsuga and Cupressaceae/Taxaceae), especially considerable presence of Pinus wallichiana confirm that cooling was a regional phenomenon in the Darjeeling Himalaya during the LIA span when ISM was also strong. A complementary trend is noticed for silt and clay fraction with comparatively low proportion of sand particles than earlier suggesting that the soils were tightly held back by the growing vegetation and only finer fractions were supplied to the lake. High rainfall might have induced higher rate of surface runoff and a faster sedimentation rate. Higher rate of sediment input might have made the lake shallower (as shown in percentages of aquatics and marshy species that declined considerably in spite of a high rainfall regime). Due to considerable expansion of tree species forest became more intense than earlier and that probably restricted the space for herbivory, as reflected by overall drop in the percentages of Glomus and the coprophilous taxa. Rise in Neorhabdocoela oocytes cf. Microdalyellia armigera during this time further suggests an increasing eutrophication due to high nutrient input caused by high rainfall. Species like Microdalyellia armigera are eurythermic and prefer lake environments with phytosociologically poor situations and high nutrient input (Haas, 1996). Hence, high surface run-off due to strong monsoon infiltrated excessive nutrients in the form of leaf litter in the lake and probably made the lake shallower. However, after ca. 1802 CE, in the last ca. 200 years, the lake became shallower with input of more leaf litter from adjacent forest as reflected in the absence of aquatics and marshy elements and rise in organic carbon content. A further acceleration in the sedimentation rate than the preceding phase with input of more sand also corroborates the fact. Trees still found dominating over NAPs, but increase in Alnus suggest a soil disturbance, possibly reflecting increased fluvial activities at riverside habitats. Increments of Quercus, Magnoliaceae, P. roxburghii, Tsuga, Betula and Ulmus together with Symplocos, Elaeocarpus, Castanopsis and Calamus indicate return of a warm-temperate climate again. A further depletion in stable carbon isotopic value and high Ic and EhIc values also confirm the fact. Significant rise in coprophilous fungi suggests that the bed of the shrinking lake might be the place of grazing within the forest. In general high grazing activity is related to high pastoral activity in the region. Rearing cattle is one of the principle sources of income of the inhabitants of the Darjeeling Himalaya. Hence, a sudden rise in coprophilous fungi may also indicate an increase in pastoral as well as anthropogenic activities in the Darjeeling Himalaya in the last ca. 200 years. The results of DCA analyses portray significant influence of available soil moisture on the pollen, phytolith and NPP distribution, maintained by precipitation and evapo-transpiration balance. In all the cases moist loving elements and dry loving elements found widely separated from each other in the ordination diagrams and mostly in opposite directions (Fig. 7a–c) indicating strong influence of rainfall.

fraction is found dominating, but in this phase a comparatively high proportion of clay further supports the earlier inference of a high rainfall regime under a strong ISM (Fig. 6). 5.1.2. Span ca. 131 CE and 624 CE By the ca. 131 CE and 624, a prominent change in forest cover is noticed. Although arboreals were still dominant but forest became open during this time as observed by considerable rise in the NAPs. Sudden increase in percentages of dry loving Amaranthaceae, Asteraceae and Poaceae and enrichments noticed in the stable carbon values is also indicative of a drier condition than the preceding phase. Considerable decline or disappearance of certain moisture loving broad leaved taxa such as Quercus, Magnoliaceae, Alnus, Betula, Castanopsis, Elaeocarpus, Lithocarpus, Schima, Ulmus along with Pinus roxburghii, as well as declining trend in both the Ic and EhIc suggest a weakening of ISM. Though ISM became weak in this phase but the region still received sufficient rainfall to support the forest. Similarly, grass cover was still dominated by C3 species. Marshy taxon Phragmites might have been decreased around the lake as percentages of medium/plateau saddles declined considerably. Percentages of the bilobates have been improved than the earlier phase. While, short saddles chiefly occurring in the C4 Chloridoideae grasses do not show any significant increase. In an earlier study from the eastern Himalaya, it was observed that certain C4 chloridoid grasses also produce considerable amounts of rondels that are generally found in C3 pooid grasses, and some C3 grasses also produce bilobates (Biswas et al., 2016), so multiplicity and redundancy within the grass phytoliths should also be considered here. Moreover it has also been observed that grass phytolith indices such as Iph and Fs are not suitable for climatic reconstructions in the eastern Himalaya (except the foothills) where due to high rainfall the grass assemblage is dominated by C3 grasses (Biswas et al., 2016). Weakening of the ISM during ca. 131 CE and 624 probably was a regional phenomenon as indicated by and appearance of Pinus wallichiana adapted to cooler or drier environments at higher elevation. A decrease in aquatic species, enhancement of Cyperaceae and increase in sand percentages suggests that the lake became shallow during this time and sedimentation was slow due to low surface runoff. Decrease in frequencies of fern allies and biostructured phytoclasts further indicates a weakening of ISM. Considerable increase in Glomus and grass cuticles, high proportion of carbonate carbon, decrease in organic carbon percentages advocate for a higher rate of soil erosion due to shrinkage in the forest cover. Thinning in the forest cover probably created more space for herbivore grazing around the lake as coprophilous fungal spores like Cercophora, Podospora, Sordaria and Sporormiella has increased rapidly. After ca. 624 CE and until 1118 CE, a mild depletion in the stable carbon isotopic value, increments in Ic and EhIc values, decrease in Asteraceae, Amaranthaceae and Poaceae, and rise in Quercus have been noticed. All these suggest that ISM regained its strength during this time span corresponding to MWP, when monsoon was strong in the Himalaya as well as in the peninsular India (Bhattacharyya et al., 2007; Sinha et al., 2007, 2011b; Sanwal et al., 2013). In general, the MWP is thought to have range between 900 CE and 1300 (Mann, 2002b), however due to comparatively coarser resolution of our data we fail to capture the earlier part of the MWP, instead our data capture the middle part of the MWP, which was wetter than the earlier phase. Slight decrease in the frequencies of the coprophilous fungal species and Glomus is indicative of reduced availability of the open space for herbivore grazing as well as less soil erosion due to comparatively higher rainfall during this phase.

5.2. ISM variability in the Darjeeling Himalaya during the last 2400 years: synchronous or asynchronous? SMP multiproxy record provides a detailed reconstruction of the hydroclimatic changes in the Darjeeling Himalaya for the last ~2400 years. Successional changes in the vegetation and ecological conditions as inferred from the proxy data portray multiple short term changes in the ISM strength (Fig.6). In brief, it is suggested that the span between ~2400 and 1819 yr BP (~364 BCE and 131 CE) was a warm and humid phase when ISM was similar to that of the present in

5.1.3. Span ca.1118 CE and 1802 CE During the interval between ca. 1118 CE and 1802, frequent presence of Quercus and Betula, and rise in Magnoliaceae, Alnus, Juglans and Araliaceae suggests that the forest became denser than the preceding phase indicating a strengthening of the ISM. This time span covering the partial MWP and the entire LIA phase shows 1.4 to 2.8‰ 94

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Fig. 8. Comparison of stable carbon isotopic data, AP, NAP percentages and the Eastern Himalayan Ic (EhIc) data of SMP with proxy records for the ISM variations during the past ~2500 years.

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dominated northeast India, central Himalaya and core monsoon zone (CMZ) of India (Fig. 8). A stalagmite oxygen isotope record from the Dandak cave, CMZ with near annual resolution has captured multi-decadal period of weakened monsoon between 1300 CE and 1450, just at the onset of the LIA (Sinha et al., 2007). Multi-decadal scale weakening in ISM was also evident on either side of the MWP span (Fig. 8). In an another study on speleothem oxygen isotope records from Jhumar cave, CMZ and Wah Shikar cave, northeast India (Sinha et al., 2011b), it has been noticed that between ~1400 CE and 1700, centennial scale episodes of negative (positive) precipitation anomalies existed over central India (northeast India) (Fig. 8). In contrast, from 17th century onwards, an opposing trend in monsoon precipitation was found over central India (northeast India), when central India experienced a high monsoon and a weakening is observed over northeast India. Our data also shows a pre-MWP less humid phase with weak ISM, MWP with strong ISM and a humid LIA when ISM was strong and corroborating with the record from, northeast India (Sinha et al., 2011b; Fig. 8). Monsoon proxy records from the marine core off the mouth of Godavari River (CMZ) influenced by the Bay of Bengal branch of ISM (Ponton et al., 2012), the northern Bay of Bengal, the Andaman Sea and the eastern Arabian Sea also inferred higher salinities in the late Holocene (Govil and Naidu, 2010; Kudrass et al., 2001; Rashid et al., 2007) especially between ca.1700 and 1300 yr BP. This phase coincides well with the Holocene monsoon minimum as revealed in the wind proxy reconstruction from the Arabian Sea (Anderson et al., 2010). A similar weak ISM phase is evident in the Darjeeling Himalaya between ca. 1819 and 1326 yr BP. Both the onset and end of the MWP is known to be characterized by the weakening in the ISM strengths as observed in records from peninsular India, the Bay of Bengal and the Arabian Sea (Gupta et al., 2003; Laskar et al., 2013; Sinha et al., 2011a, 2011b) but not apparent in the Lesser and Central Himalaya (Denniston et al., 2000; Kotlia et al., 2014; Liang et al., 2015; Sanwal et al., 2013; Sinha et al., 2015). The time interval between ca. 950 CE and 1250 is characterized by the strengthening in the ISM corresponding to MWP. We also observe a weakened phase of the ISM between ca. 1819 and 1326 yr BP (i.e., 131 CE till 624 CE) which may be attributed to the preMWP less moist phase. A restoration phase of ISM during the MWP in the Darjeeling Himalayan record suggests an overall similar trend of ISM variability during the late Holocene (Fig. 8). It is to be noted that due to coarse sampling resolution, our data fail to capture the decadal to centennial scale variations in the ISM strength. Further the magnitude of the ISM weakening may be different in the highly precipitated eastern Himalaya. For similar reason, the weakening phase of the ISM, differentiating the MWP and LIA is not captured in our record. A more fine resolution study may throw light on this phase. During the LIA, ISM shows a weakened phase over the peninsular India (Dixit and Tandon, 2016 and references therein). However, considerable variability exists, where the central Himalaya, northeast India and Andaman received high precipitation during the LIA indicating a strong ISM (Denniston et al., 2000; Sanwal et al., 2013; Kotlia et al., 2014; Sinha et al., 2011b). Our data also shows a strong phase of the ISM during the LIA. This opposing behaviour of ISM over the peninsular India and northeast India may be attributed to the local internal dynamics of the ISM. In general, ISM has oscillating periods with a persistently “active dominated” and a “break-dominated” regimes. Active periods used to bring heavy precipitation over central and western India and are characterized by a sequence of time-clustered convective systems. While, monsoon trough (a contiguous system of shallow thermal lows and moist convective lows across central India oriented in NW-SE direction) shifts northwards during the break period, when convection used to enhance over Equatorial Indian Ocean (EIO) which led to drier conditions over much of the Indian subcontinent, and wetter conditions over the foothills of the Himalayas and northeast India (Rajeevan et al., 2010; Sinha et al., 2011b). Hence, the opposing behaviour of ISM over central and northeast India is attributable to sub-

the region. The phase between ca. 1819 and 1326 yr BP (ca. 131 CE and 624) was drier than the preceding phase and ISM was comparatively weaker. ISM was observed to regain its strength during ca. 832 and 583 yr BP (ca.1118 CE and 1367), which falls within the MWP phase. A further strengthening of ISM with a cooler temperature between ca. 583 and 148 yr. BP (ca. 1367 CE and 1802) during the LIA was noticed. It is evident that from 1802 CE onwards, the ISM regained its modern state. We consider present data in the light of some available late Holocene records from the eastern Himalaya in order to understand the regional relevance and consistency of the events related to the ISM variability (Fig. 8). Despite the difference in chronological resolution between the data, there are general trends that can be observed in the last ca. 2500 years. For example, Sharma and Chauhan (1994) inferred ‘a gradual deterioration of climate’ and ‘degradation of forest’ in temperate zone of the Darjeeling Himalaya between 2000 and 500 yr BP when anthropogenic activity increased substantially. However, the data lacks any concrete inference about to what extent the climate deteriorated in the last 2000 years, and hence not comparable with the present observation (Fig. 8). In an another study from the Darjeeling Himalaya, Chauhan and Sharma (1996) observed a warm-humid and temperate phase between 2500 and 1600 yr BP, a cool and less humid phase between 1600 and 1000 yr BP and a, warm-humid phase between 1000 and 300 yr BP. Our data portrays a moist phase between ca. 2313 and 1819 yr BP and less humid phase with a weaker ISM between ca. 1819 and 1326 yr BP and a restoration of ISM after ca. 1326 yr BP, with more strengthening between ca.832 and 583 yr BP (Fig. 8). These discrepancies may be attributed to the poor age control of the earlier data which is based on the single C14 radiocarbon date for the entire span. In Sikkim, eastern Himalaya between ca. 2000 and 1800 yr BP, a humid climate was prevailing and the vegetation was similar to the present day (Sharma and Chauhan, 2001) which is also evident in our data. Though, between ca. 1800 to 1450 yr BP the region became less moist than the earlier and turned to be warm and humid between ca. 1450 and 450 yr BP (Sharma and Chauhan, 2001) (Fig. 8). Our data also shows a less moist phase between ca. 1819 and 1326 yr BP, probably a pre-MWP phase. Both data from the Sikkim and Darjeeling Himalaya show a humid MWP when ISM was strong, as revealed by restoration in the forest cover than the earlier phase. In contrast to the Darjeeling Himalaya, record from Sikkim showed a drier trend during the LIA (Sharma and Chauhan, 2001). This incongruity may be due to the imprecise age-depth model applied to Kupup profile (Sharma and Chauhan, 2001). A combined pollen and stable carbon isotopic study from the alpine zone of the Arunachal Himalaya by Bhattacharyya et al. (2007) also has identified a warm-humid climatic phase around 1780 yr BP and a further warming around 1100 yr BP which has been identified as the MWP. A dry LIA has been identified in this study and that too on the basis of a single C14 age based chronology (Fig. 8). Moreover, detail discussion on the variations of the stable carbon isotopic signature during the MWP and LIA is also missing in the record. Agrawal et al. (2015) also inferred a wet condition around ca. 2700 yr BP in the Arunachal Himalaya, which gradually changed to a less humid condition during ca. 1300 yr BP. This study also identified a short spell of humidity attributed to MWP and a stable climate afterwards (Fig. 8). It is apparent from all these records that a warm climatic condition similar to present day with strong ISM was prevailing over the eastern Himalaya since ca 2700 yrs. BP until the beginning of the last millennium which turned out to be a little drier with a weakening of ISM before the onset as well as the beginning of the MWP. Though in all the records MWP was humid, when ISM was stronger than the earlier, but variability observed in the duration and magnitude of this climatic event may be attributed to the poor age resolution, use of different age-depth models and resolution of the data. Surprisingly, significant variations exist regarding the LIA span, where in contrast to our data most of the records portrayed a weak ISM in the eastern Himalaya (Fig. 8). To resolve this ambiguity and understand the ISM trends during the LIA, we have selected some high resolution proxy records from the ISM 96

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seasonal fluctuations in the mean position of the Inter tropical convergence zone (ITCZ) over the core monsoon zone (Goswami et al., 2006; Lawrence and Webster, 2001; Sinha et al., 2011a, 2011b). In general, thermal gradients between the Bay of Bengal and the Indian subcontinent drive majority of the summer season rainfall over northeast India and Andaman (Polanski et al., 2014). Hence, the ‘monsoonbreak conditions’ probably explain the increase in precipitation during LIA over the northeast India and central Himalaya. During LIA, the monsoon trough located close to the foot of the Himalaya, might lead to a striking decrease in rainfall over most of India but lead to an increase along the Himalaya and parts of northeast India and in the extreme southeast of Peninsular India (Liang et al., 2015). This also possibly explains the opposing precipitation trends in the northeast India and central India, observed around the late 17th Century, and was caused by a change in the frequency characteristics of Active-Break periods. During this time the ISM dynamics probably shifted to a predominantly “active” state, generating precipitation anomalies of the opposite sign over central India and northeast India (Sinha et al., 2011b). However, our record does not show any significant change in vegetation in the Darjeeling Himalaya except a drying trend in the lake and a slight opening in the forest which further indicates that in such a high rainfall region a minor change in the rainfall does not induce any major change in the vegetation.

change. Quat. Res. 77, 159–170. Agrawal, S., Srivastava, P., Sonam, Meena, N.K., Rai, S.K., Bhushan, R., Misra, D.K., Gupta, A.K., 2015. Stable (δ13C and δ15N) isotopes and magnetic susceptibility record of Late Holocene climate change from a lake profile of the northeast Himalaya. J. Geol. Soc. India 86, 696–705. Andersen, S.T., 1970. The relative pollen productivity and pollen representation of North European trees, and correction factors for tree pollen spectra determined by surface pollen analyses from forests. Danm. Gcol. Unders.II Rk 96, 1–99. Andersen, S.T., 1984. Forests at Lavenholm, Djursland, Denmark, at present and in the past. Biol. Skr. Dans. Vidensk. Selsk. 24, 1–211. Anderson, D.M., Baulcomb, C.K., Duvivier, A.K., Gupta, A.K., 2010. Indian summer monsoon during the last two millennia. J. Quat. Sci. 25 (6), 911–917. Barthelmes, A., de Klerk, P., Prager, A., Theuerkauf, M., Unterseher, M., Joosten, H., 2012. Expanding NPP analysis to eutrophic and forested sites: significance of NPPs in a Holocene wood peat section (NE Germany). Rev. Palaeobot. Palynol. 186, 22–37. Basu, S., Agrawal, S., Sanyal, P., Mahato, P., Kumar, S., Sarkar, A., 2015. Carbon isotopic ratios of modern C3–C4 plants from the Gangetic plain, India and its implications to paleovegetational reconstruction. Palaeogeogr. Palaeoclimatol. Palaeoecol. 440, 22–32. Behling, H., Negrelle, R.R.B., 2006. Vegetation and pollen rain relationship from the tropical Atlantic rain forest in southern Brazil. Braz. Arch. Biol. Technol. 49 (4), 631–642. Bengtsson, L., Enell, M., 1986. Chemical analysis. In: Berglund, B.E. (Ed.), Handbook of Holocene Palaeoecology and Palaeohydrology. John Wiley and Sons Ltd., Chichester, pp. 423–451. Berkelhammer, M., Sinha, A., Mudelsee, M., Cheng, H., Edwards, R.L., Cannariato, K., 2010. Persistent multidecadal power of the Indian summer monsoon. Earth Planet. Sci. Lett. 290, 166–172. Berkelhammer, M., Sinha, A., Stott, L., Cheng, H., Pausata, F.S.R., Yoshimura, K., 2012. An abrupt shift in the Indian monsoon 4000 years ago. Geophys. Monogr. Ser. 198, 75–87. Bhattacharyya, A., Sharma, J., Shah, S.K., Chaudhury, V., 2007. Climatic changes during last 1800 yrs BP from Paradise Lake, Sela Pass, Arunachal Pradesh, Northeast Himalaya. Curr. Sci. 93 (7), 983–987. Birks, H.J.B., 1995. Quantitative palaeoenvironmental reconstructions. In: Maddy, D., Brew, J.S. (Eds.), Statistical Modelling of Quaternary Science Data. Technical Guide. vol. 5. Quaternary Research Association, Cambridge, pp. 161–254. Biswas, O., Ghosh, R., Paruya, D.K., Mukherjee, B., Thapa, K.K., Bera, S., 2016. Can grass phytoliths and indices be relied on during vegetation and climate interpretations in the eastern Himalayas? Studies from Darjeeling and Arunachal Pradesh, India. Quat. Sci. Rev. 134, 114–132. Bradshaw, R.H.W., 1981. Modern pollen-representation factors for woods in south-east England. J. Ecol. 69, 45–70. Bronk Ramsey, C., 2001. Development of the radiocarbon calibration program. Radiocarbon 43 (2A), 355–363. Bronk Ramsey, C., 2008. Deposition models for chronological records. Quat. Sci. Rev. 27, 42–60. Bronk Ramsey, C., 2016. OxCal 4.2 Manual. Electronic document. https://c14.arch.ox.ac. uk/oxcalhelp/hlp_contents.html>, Accessed date: 9 August 2016. Bronk Ramsey, C., Lee, S., 2013. Recent and Planned Developments of the Program OxCal. Radiocarbon 55 (2-3), 720–730. Champion, H.G., Seth, S.K., 1968. A Revised Survey of Forest Types of India. Govt. of India Press, New Delhi. Chauhan, M.S., Quamar, M.F., 2010. Vegetation and climate change in southeastern Madhya Pradesh during late Holocene, based on pollen evidence. J. Geol. Soc. India 76, 143–150. Chauhan, M.S., Sharma, C., 1996. Late Holocene vegetation of Darjeeling (Jore- Pokhari), eastern Himalaya. Palaeobotanist 45, 125–129. Cugny, C., Mazier, F., Galop, D., 2010. Modern and fossil nonpollenpalynomorphs from the Basque mountains (western Pyrenees, France): the use of coprophilous fungi to reconstruct pastoral activity. Veg. Hist. Archaeobotany 19, 391–408. Day, P.R., 1965. Particle fractionation and particle-size analysis. In: Black, C.A. (Ed.), Methods of Soil Analysis. Agronomy. vol. 9. American Society of Agronomy Inc., Madison, Wisconsin, USA, pp. 545–566. Dean Jr., W.E., 1974. Determination of carbonate and organic matter in calcareous sediments and sedimentary rocks by loss on ignition: comparison with other methods. J. Sediment. Petrol. 44, 242–248. Deines, P., 1980. The isotopic composition if reduced organic carbon. In: Fritz, P., Fontes, J.C. (Eds.), Handbook of Environmental Isotope Geochemistry The Terrestrial Environment. vol. 1. Elsevier, Dordrecht, pp. 339–345. Demske, D., Tarasov, P.E., Nakagawa, T., Suigetsu, 2006. Project Members, 2013. Atlas of pollen, spores and further non-pollen palynomorphs recorded in the glacial-interglacial late Quaternary sediments of Lake Suigetsu, central Japan. Quat. Int. 290–291, 164–238. Demske, D., Tarasov, P.E., Leipe, C., Kotlia, B.S., Joshi, L.M., Long, T., 2016. Record of vegetation, climate change, human impact and retting of hemp in Garhwal Himalaya (India) during the past 4600 years. The Holocene 26 (10), 1661–1675. Denniston, R.F., González, L.A., Asmerom, Y., Sharma, R.H., Reagan, M.K., 2000. Speleothem evidence for changes in Indian summer monsoon precipitation over the last ~2300 years. Quat. Res. 53, 196–202. Dietre, B., Gauthier, E., Gillet, F., 2012. Modern pollen rain and fungal spore assemblages from pasture woodlands around Lake Saint-Point (France). Rev. Palaeobot. Palynol. 186, 69–89. Dixit, S., Bera, S.K., 2013. Vegetation vis a vis climate change around Bhogdoi swamp in Lower Brahmaputra flood plain of Assam, Northeast India since late Holocene. The Palaeobotanist 62, 19–27.

6. Conclusion Multiproxy record from the Sixth Mile profile provides evidence of ISM dynamics over the last 2400 years in the Darjeeling Himalaya. Although spatial variability exists with respect to timing and magnitude of these events, but a general trend is also noticed. Inconsistencies observed in the eastern Himalaya are primarily attributable to the low resolution of dating and sampling. For the two most significant northern hemispheric RCCs, i.e. MWP and LIA, wet conditions are observed in the eastern Himalaya. However, LIA and post-LIA phases show considerable contrasting trends between peninsular India and eastern Himalaya as well as northeast India which may be due to the variations in frequency distribution of active-break periods. This further suggests that ISM response to slowly-evolving changes in the external boundary conditions is principally governed by internal dynamics. Present study paves the way for future high resolution study that may reveal minor fluctuations in ISM during the late Holocene in the eastern Himalaya. Acknowledgements RG, SA, AS and CMN thankfully acknowledge the Director, Birbal Sahni Institute of Palaeosciences for his encouragement, support and permission to publish this work (Permission No. RDCC 65/2016-17). OB and SB thankfully acknowledge financial assistance from SERB, Government of India in the form of a research project (Sanction No. SERB/SR/SO/PS/94/2010 dated 28.05.2012). SB acknowledges UGCCAS (Phase VII) for infrastructural facilities. Thanks are also due to Dr. E.L.H. Cammeraat, Chair of Editors-in-Chief, CATENA and the anonymous reviewer for their valuable constructive comments. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.catena.2018.05.043. References Agnihotri, R., Dutta, K., Bhushan, R., Somayajulu, B., 2002. Evidence for solar forcing on the Indian monsoon during the last millennium. Earth Planet. Sci. Lett. 198, 521–527. Agrawal, S., Sanyal, P., Sarkar, A., Jaiswal, M.K., Dutta, K., 2012. Variability of Indian summer monsoon over the past 100 ka and its implications to C3-C4 vegetational

97

Catena 170 (2018) 84–99

R. Ghosh et al.

Liang, F., Brook, G.A., Kotlia, B.S., Railsback, L.B., Hardt, B., Cheng, H., Edwards, R.L., Kandasamy, S., 2015. Panigarh cave stalagmite evidence of climate change in the Indian central Himalaya since AD 1256: monsoon breaks and winter southern jet depressions. Quat. Sci. Rev. 124, 145–161. López-Vila, J., Montoya, E., Cañellas-Boltà, N., Rull, R., 2014. Modern non-pollen palynomorphs sedimentation along an elevational gradient in the south-central Pyrenees (southwestern Europe) as a tool for Holocene paleoecological reconstruction. The Holocene 24 (3), 327–345. Macphail, M.K., 1980. Fossil and modern Beilschmiedia (Lauraceae) pollen in New Zealand. New Zeal. J. Bot. 18, 453–457. Mann, M.E., 2002a. Little Ice Age. In: MacCracken, M.C., Perry, J.S. (Eds.), Encyclopedia of Global Environmental Change. John Wiley, Chichester, U. K., pp. 504–509. Mann, M.E., 2002b. Medieval climatic optimum. In: MacCracken, M.C., Perry, J.S. (Eds.), Encyclopedia of Global Environmental Change. John Wiley, Chichester, U. K., pp. 514–516. Mann, M.E., Zhang, Z., Rutherford, S., Bradley, R.S., Hughes, M.K., Shindell, D., Ammann, C., Faluvegi, G., Ni, F., 2009. Global signatures and dynamical origins of the Little Ice Age and Medieval Climate Anomaly. Science 326, 1256–1260. Markgraf, V., 1980. Pollen dispersal in a mountain area. Grana 19, 127–146. Mayewski, P.A., Rohling, E.E., Stager, J.C., Karlén, W., Maasch, K.A., Meeker, L.D., Meyerson, E.A., Gasse, F., van Kreveld, S., Holmgren, K., Lee-Thorp, J., Rosqvist, G., Rack, F., Staubwasser, M., Schneider, R.R., Steig, E.J., 2004. Holocene climate variability. Quat. Res. 62 (3), 243–255. Medeanic, S., Silva, M.B., 2010. Indicative value of non-pollen palynomorphs (NPPs) and palynofacies for palaeoreconstructions: Holocene Peat, Brazil. Int. J. Coal Geol. 84, 248–257. Menzel, P., Gaye, B., Mishra, P.K., Anoop, A., Basavaiah, N., Marwan, N., Plessen, B., Prasad, S., Riedel, N., Stebich, M., Wiesner, M.G., 2014. Linking Holocene drying trends from Lonar Lake in monsoonal central India to North Atlantic cooling events. Palaeogeogr. Palaeoclimatol. Palaeoecol. 410, 164–178. Meyers, P.A., 1994. Preservation of elemental and isotopic source identification of sedimentary organic matter. Chem. Geol. 114, 289–302. Mishra, P.K., Anoop, A., Schettler, G., Prasad, S., Jehangir, A., Menzel, P., Naumann, R., Yousuf, A.R., Basavaiah, N., Deenadayalan, K., Wiesner, M.G., Gaye, B., 2015. Reconstructed late Quaternary hydrological changes from Lake Tso Moriri, NW Himalaya. Quat. Int. 371, 76–86. Montoya, E., Rull, V., van Geel, B., 2010. Non-pollen palynomorphs from surface sediments along an altitudinal transect of the Venezuelan Andes. Palaeogeogr. Palaeoclimatol. Palaeoecol. 297, 169–183. Mosimann, J.E., 1965. Statistical methods for the pollen analyst: multinomial and negative multinomial techniques. In: Kummel, B., Raup, D. (Eds.), Handbook of Paleontological Techniques. Freeman, San Francisco, Calif., pp. 636–673. O'Leary, M.H., 1981. Carbon isotope fractionation in plants. Phytochemistry 20, 553–567. O'Leary, M.H., 1988. Carbon isotopes in photosynthesis. Bioscience 38, 328–336. Pearsall, D.M., 2000. Palaeoethnobotany: A Handbook of Procedures. Academic Press, New York. Peel, M.C., Finlayson, B.L., Mcmahon, T.A., 2007. Updated world map of the Köppen–Geiger climate classification. Hydrol. Earth Syst. Sci. 11, 1633–1644. Phadtare, N.R., 2000. Sharp decrease in summer monsoon strength 4000–3500 cal yr B.P. In the central higher Himalaya of India based on pollen evidence from alpine peat. Quat. Res. 53, 122–129. Piperno, D.R., 1988. Phytolith Analysis: An Archaeological and Geological Perspective. Academic Press, San Diego, CA, pp. 44. Polanski, S., Fallah, B., Befort, D.J., Prasad, S., Cubasch, U., 2014. Regional moisture change over India during the past millennium: a comparison of multi-proxy reconstructions and climate model simulations. Glob. Planet. Chang. 122, 176–185. Ponton, C., Giosan, L., Eglinton, T.I., Fuller, D.Q., Johnson, J.E., Kumar, P., Collett, T.S., 2012. Holocene aridification of India. Geophys. Res. Lett. 39, L03704. Prasad, S., Anoop, A., Riedel, N., Sarkar, S., Menzel, P., Basavaiah, N., Krishnan, R., Fuller, D., Plessen, B., Gaye, B., Röhl, U., Wilkes, H., Sachse, D., Sawant, R., Wiesner, M.G., Stebich, M., 2014. Prolonged monsoon droughts and links to Indo-Pacific warm pool: a Holocene record from Lonar Lake, central India. Earth Planet. Sc. Lett. 391, 171–182. Prentice, I.C., 1980. Multidimensional scaling as a research tool in Quaternary palynology: a review of theory and methods. Rev. Palaeobot. Palynol. 31, 71–104. Prentice, I.C., 1985. Pollen representation, source area, and basin size: towards a unified theory of pollen analysis. Quat. Res. 23 (1), 76–86. Quamar, M.F., Chauhan, M.S., 2012. Late Quaternary vegetation, climate as well as lakelevel changes and human occupation from Nitaya area in Hoshangabad District, southwestern Madhya Pradesh (India), based on pollen evidence. Quat. Int. 263, 104–113. Quamar, M.F., Chauhan, M.S., 2014. Signals of Medieval Warm Period and Little Ice Age from southwestern Madhya Pradesh (India): a pollen-inferred Late Holocene vegetation and climate change. Quat. Int. 325, 74–82. Rajeevan, M., Gadgil, S., Bhate, J., 2010. Active and break spells of the Indian summer monsoon. J. Earth Syst. Sci. 119, 229–247. http://dx.doi.org/10.1007/s12040-0100019-4. Rashid, H., P, B., Flower, R.Z., Poore, T., Quinn, M., 2007. A similar to 25 ka Indian Ocean monsoon variability record from the Andaman Sea. Quat. Sci. Rev. 26 (19–21), 2586–2597. Reimer, P.J., Bard, E., Bayliss, A., et al., 2013. IntCal13 and MARINE13 radiocarbon age calibration curves 0–50,000 years cal BP. Radiocarbon 55 (4), 1869–1887. Rühland, K., Phadtare, N., Pant, R., Sangode, S., Smol, J., 2006. Accelerated melting of Himalayan snow and ice triggers pronounced changes in a valley peatland from northern India. Geophys. Res. Lett. 33, L15709. Rull, V., Cañellas-Boltà, N., Margalef, O., et al., 2015. Late-Holocene vegetation dynamics

Dixit, Y., Tandon, S.K., 2016. Hydroclimatic variability on the Indian subcontinent in the past millennium: review and assessment. Earth Sci. Rev. 161, 1–15. Erdtman, G., 1954. An Introduction to Pollen Analysis. Waltham Mass, USA, pp. 239. Erickson, J.M., Platt Jr., R.B., 2007. Oribatid mite studies. In: Elias, Scott (Ed.), Encyclopedia of Quaternary Science. Elsevier, London, pp. 1547–1566. Faegri, K., Iversen, J., 1989. Textbook of Pollen Analysis, 4th Edition. Wiley, Chichester. Farquhar, G.D., Ehleringer, J.R., Hubick, K.T., 1989. Carbon isotope discrimination and photosynthesis. Annu. Rev. Plant Physiol. Mol. Biol. 40, 503–537. Feeser, I., O'Connell, M., 2010. Late Holocene land-use and vegetation dynamics in an upland karst region based on pollen and coprophilous fungal spore analyses: an example from the Burren, western Ireland. Veg. Hist. Archaeobotany 19, 409–426. Fredlund, G.G., Tieszen, L.L., 1994. Modern phytolith assemblages from the North American Great Plains. J. Biogeogr. 21, 321–335. Gadgil, S., Rajeevan, M., Nanjundiah, R., 2005. Monsoon prediction — why yet another failure? Curr. Sci. 88, 1389–1400. Gauch, H.G., 1982. Multivariate Analysis in Community Ecology. Cambridge University Press, Cambridge, UK. Gelorini, V., Verbeken, A., van Geel, B., et al., 2011. Modern non-pollen palynomorphs from east African lake sediments. Rev. Palaeobot. Palynol. 164, 143–173. Ghosh, R., Paruya, D.K., Acharya, K., Ghorai, N., Bera, S., 2017. How reliable are nonpollen palynomorphs in tracing vegetation changes and grazing activities? Study from the Darjeeling Himalaya, India. Palaeogeogr. Palaeoclimatol. Palaeoecol. 475, 23–40. Goswami, B.N., Venugopal, V., Sengupta, D., Madhusoodanan, M.S., Xavier, P.K., 2006. Increasing trend of extreme rain events over India in a warming environment. Science 314, 1442–1445. Govil, P., Naidu, P.D., 2010. Evaporation-precipitation changes in the eastern Arabian Sea for the last 68 ka: implications on monsoon variability. Paleoceanography 25, PA1210. http://dx.doi.org/10.1029/2008PA001687. Gupta, A.K., Anderson, D.M., Overpeck, J.T., 2003. Abrupt changes in the Asian southwest monsoon during the Holocene and their links to the North Atlantic Ocean. Nature 421, 354–357. Haas, J.N., 1996. Neorhabdocoela oocytes- palaeoecological indicators found in pollen preparations from Holocene freshwater lake sediments. Rev. Palaeobot. Palynol. 91, 371–382. Hepting, G.H., 1971. Diseases of forest and shade trees of the United States. In: U.S. Dep. Agric. Forest Serv. Agric. Handbook. 386. pp. 658. Hijmans, R.J., Cameron, S.E., Parra, J.L., Jones, P.G., Jarvis, A., 2005. Very high resolution interpolated climate surfaces for global land areas. Int. J. Climatol. 25, 1965–1978. Hill, M.O., Gauch Jr., H.G., 1980. Detrended correspondence analysis: an improved ordination technique. Vegetatio 42, 47–58. Hooghiemstra, H., van Geel, B., 1998. World list of Quaternary pollen and spore atlases. Rev. Palaeobot. Palynol. 104, 157–182. Jackson, M.L., 1956. Soil Chemical Analysis: Advanced Course. Published by the. Author, Department of Soils, University of Wisconsin, Madison 6, Wisconsin, USA, pp. 925. Jacobson, G.L., Bradshaw, R.H.W., 1981. The selection of sites for paleovegitational studies. Quat. Res. 16, 80–96. Janssen, C.R., 1966. Recent pollen spectra from the deciduous and coniferous-deciduous forests of northwestern Minnesota: a study in pollen dispersal. Ecology 47, 804–825. Janssen, C.R., 1973. Local and regional pollen deposition. In: Birks, H.J.B., West, R.G. (Eds.), Quaternary Plant Ecology. 14th Symposium, British Ecological Society. Blackwell, Oxford. Janssen, C.R., 1984. Modern pollen assemblages and vegetation in the Myrtle Lake peatland, Minnesota. Ecol. Monogr. 54, 213–252. Kelly, E.F., 1990. Method for Extracting Opal Phytoliths from Soil and Plant Material. Internal Report. Department of Agronomy, Colorado State University, Fort Collins. Knuze, G.W., 1965. Pre-treatment for mineralogical analysis. In: Black, C.A. (Ed.), Methods of Soil Analysis. Agronomy. vol. 9. American Society of Agronomy, Madison, Wisconsin, USA, pp. 568–577. Kotlia, B., Joshi, L., 2013. Late Holocene climatic changes in Garhwal Himalaya. Curr. Sci. 104, 911. Kotlia, B.S., Ahmad, S.M., Zhao, J.-X., Raza, W., Collerson, K.D., Joshi, L.M., Sanwal, J., 2012. Climatic fluctuations during the LIA and post-LIA in the Kumaun Lesser Himalaya, India: evidence from a 400 y old stalagmite record. Quat. Int. 263, 129–138. Kotlia, B.S., Singh, A.K., Joshi, L.M., Dhaila, B.S., 2014. Precipitation variability in the Indian central Himalaya during last ca. 4000 years inferred from a speleothem record: impact of Indian summer monsoon (ISM) and westerlies. Quat. Int. 371, 244–253. Kotlia, B.S., Singh, A.K., Zhao, J.-X., Duan, W., Tan, M., Sharma, A.K., Raza, W., 2016. Stalagmite based high resolution precipitation variability for past four centuries in the Indian Central Himalaya: Chulerasim cave re-visited and data re-interpretation. Quat. Int. http://dx.doi.org/10.1016/j.quaint.2016.04.007. Kudrass, H.R., Hofmann, A., Doose, H., Emeis, K., Erlenkeuser, H., 2001. Modulation and amplification of climatic changes in the Northern Hemisphere by the Indian summer monsoon during the past 80 k.y. Geology 29 (1), 63–66. Laskar, A.H., Yadava, M., Ramesh, R., Polyak, V., Asmerom, Y., 2013. A 4 kyr stalagmite oxygen isotopic record of the past Indian summer monsoon in the Andaman Islands. Geochem. Geophys. Geosyst. 14, 3555–3566. Lawrence, D.M., Webster, P.J., 2001. Interannual variations of the intraseasonal oscillation in the south Asian summer monsoon region. J. Clim. 14, 2910–2922. Li, S.P., Hu, Y.Q., Ferguson, D.K., Yao, J.X., Li, C.S., 2013. Pollen dispersal in a mountainous area based on pollen analysis of four natural trap types from Lugu Lake, southwest China. J. Syst. Evol. 51 (4), 413–425.

98

Catena 170 (2018) 84–99

R. Ghosh et al.

Sjögren, P., van der Knaap, W., Kaplan, J.O., van Leeuwen, J.F.N., Ammann, B., 2008. A pilot study on pollen representation of mountain valley vegetation in the central Alps. Rev. Palaeobot. Palynol. 149, 208–218. Tanner, C.B., Jackson, M.L., 1947. Monographs of sedimentation times for soil particles under gravity or centrifugal acceleration. Proc. Soil Sc. Soc. Am. 12, 50–55. ter Braak, C.J.F., 1986. Canonical correspondance analysis: a new eigenvector technique for multivariate direct gradient analysis. Ecology 67, 1167–1179. ter Braak, C.J.F., 1988. Canoco-A FORTRAN Program for Canonical Community Ordination by (Partial) (Detrended) (Canonical) Correspondence Analysis, Principal Components Analysis and Redundancy Analysis (Version 2.1). Technical Rep. LWA88-02. GLW, Wageningen. pp. 95. ter Braak, C.J.F., Smilauer, P., 2002. CANOCO 4.5. Biometris. Wageningen University and Research Center, Wageningen (500 pp.). Tripathi, S., Basumatary, S.K., Singh, V.K., Bera, S.K., Nautiyal, C.M., Thakur, B., 2014. Palaeovegetation and climate oscillation of western Odisha, India: a pollen databased synthesis for the Mid-Late Holocene. Quat. Int. 325, 83–92. van Geel, B., Aptroot, A., 2006. Fossil ascomycetes in Quaternary deposits. Nova Hedwigia 82, 313–329. van Geel, B., Coope, G.R., Hammen, T., 1989. Palaeoecology and stratigraphy of the Late glacial section at Usselo (The Netherlands). Rev. Palaeobot. Palynol. 60, 25–129. Wanner, H., Beer, J., Buetikofer, J., Crowley, T.J., Cubasch, U., Flueckiger, J., Goosse, H., Grosjean, M., Joos, F., Kaplan, J.O., 2008. Mid-to Late Holocene climate change: an overview. Quat. Sci. Rev. 27, 1791–1828. Wilkins, W.H., 1934. Studies in the genus Ustulina with special reference to parasitism. I. Introduction, survey of previous literature and host index. Trans. Br. Mycol. Soc. 18, 320–346. Wünnemann, B., Demsk, D., Tarasov, P., Kotlia, B.S., Reinhardt, C., Bloemendal, J., Diekmann, B., Hartmann, K., Krois, J., Riedel, F., Arya, N., 2010. Hydrological evolution during the last 15 kyr in the Tso Kar lake basin (Ladakh, India), derived from geomorphological, sedimentological and palynological records. Quat. Sci. Rev. 29, 1138–1155.

and deforestation in Rano Aroi: implications for Easter Island's ecological and cultural history. Quat. Sci. Rev. 126, 219–226. Sage, R.F., 1999. Why C4 photosynthesis? In: Sage, R.F., Monson, R.K. (Eds.), C4 Plant Biology. Academic Press, San Diego, CA, pp. 3–16. Sanwal, J., Kotlia, B.S., Rajendran, C., Ahmad, S.M., Rajendran, K., Sandiford, M., 2013. Climatic variability in central Indian Himalaya during the last ~1800 years: evidence from a high resolution speleothem record. Quat. Int. 304, 183–192. Scott, E.M., Cook, G.T., Naysmith, P., 2007. Error and uncertainty in radiocarbon measurements. Radiocarbon 49, 427–440. Seniczak, A., 2011. Ecology of Hydrozetes Berlese, 1902 (Acari, Oribatida) at various water bodies near Bydgoszcz (northern Poland). Biol. Lett. 48 (2), 185–192. Shankar, R., Prabhu, C.N., Warrier, A.K., Vijaya Kumar, G.T., Sekar, B.A., 2006. Multidecadal rock magnetic record of monsoonal variations during the past 3,700 years from a tropical Indian tank. J. Geol. Soc. India 68, 447–459. Sharma, C., Chauhan, M.S., 1994. Vegetation and climate since Last Glacial Maximum in Darjeeling (Mirik Lake), Eastern Himalaya. In: Proceeding 29th International Geological Congress Part B, pp. 279–288. Sharma, C., Chauhan, M.S., 2001. Late Holocene vegetation and climate of Kupup (Sikkim), Eastern Himalaya, India. J. Palaeontol. Soc. India 46, 51–58. Sinha, A., Cannariato, K.G., Stott, L.D., Cheng, H., Edwards, R.L., Yadava, M.G., Ramesh, R., Singh, I.B., 2007. A 900-year (600 to 1500 AD) record of the Indian summer monsoon precipitation from the core monsoon zone of India. Geophys. Res. Lett. 34. Sinha, A., Berkelhammer, M., Stott, L., Mudelsee, M., Cheng, H., Biswas, J., 2011a. The leading mode of Indian summer monsoon precipitation variability during the last millennium. Geophys. Res. Lett. 38. Sinha, A., Stott, L., Berkelhammer, M., Cheng, H., Edwards, R.L., Buckley, B., Aldenderfer, M., Mudelsee, M., 2011b. A global context for megadroughts in monsoon Asia during the past millennium. Quat. Sci. Rev. 30, 47–62. Sinha, A., Kathayat, G., Cheng, H., Breitenbach, S.F.M., Berkelhammer, M., Mudelsee, M., Biswas, J., Edwards, R.L., 2015. Trends and oscillations in the Indian summer monsoon rainfall over the last two millennia. Nat. Commun. 6. http://dx.doi.org/10. 1038/ncomms7309.

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