j ourna l homepage: www.e lsev ie r .com/ locate /g lop lacha
Miocene to Pleistocene floras and climate of the Eastern HimalayanSiwaliks, and new palaeoelevation estimates for theNamling–Oiyug Basin, Tibet
Mahasin Ali Khan a, Robert A. Spicer b,c, Subir Bera a,⁎, Ruby Ghosh d, Jian Yang c, Teresa E.V. Spicer c,Shuang-xing Guo e, Tao Su f, Frédéric Jacques f, Paul J. Grote g
a Centre of Advanced Study, Department of Botany, University of Calcutta, 35, Ballygunge Circular Road, Kolkata 700019, Indiab Environment, Earth and Ecosystems, Centre for Earth, Planetary, Space and Astronomical Research, The Open University, Milton Keynes MK7 6AA, UKc Institute of Botany, The Chinese Academy of Sciences, 20 Nanxincun Xiangshan, Beijing 100093, Chinad Birbal Sahni Institute of Palaeobotany, 53, University Road, Lucknow 226007, U.P., Indiae Department of Palaeobotany and Palynology, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, Jiangsu, Chinaf Key Laboratory of Tropical Forest Ecology, Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences, Mengla 666303, Chinag School of Biology, Institute of Science, Suranaree University of Technology,111 University Avenue, Nakhon Ratchasima 30000, Thailand
Four fossil floras ranging in age from the mid Miocene to the early Pleistocene from the eastern Siwaliks nearDarjeeling and in Arunachal Pradesh (AP) were compared taxonomically and subjected to a CLAMP (ClimateLeaf AnalysisMultivariate Program) analysis using a new calibration dataset that includes sites from India, south-ern China and Thailand and high resolution gridded climate data. Two lower Siwalik mid Miocene floras yieldedalmost the same values suggesting mean annual temperatures (MATs) of 25.4 and 25.3 °C ± 2.8 °C (all uncer-tainties ±2 sigma) with warm month mean temperatures (WMMTs) of 28.4 and 27.8 ± 3.39 °C and coldmonth mean temperatures (CMMTs) of 17.9 and 21.3 ± 4 °C. Precipitation estimates have high uncertaintiesbut suggest a weak monsoon with growing season precipitations of 242 ± 92 cm for Darjeeling and174 ± 92 cm for AP. Leaves from the middle Siwalik (Pliocene) sediments of AP indicate a lowering of theMAT to 23.7 °C, a function of cooler winter months (CMMT 16.9 °C). The AP early Pleistocene temperaturesand rainfall were similar to those of the mid Miocene. Changes in the monsoon index suggest that in the AParea there has been little change in the intensity of the monsoon since mid Miocene time, while further westat Darjeeling there has been an intensification since the mid Miocene. Mid Miocene CLAMP-derived enthalpyestimates provide sea level (b200 a.m.s.l.) data for a re-evaluation of the palaeoelevation of a 15 Ma flora fromthe Namling–Oiyug Basin, southern Tibet. Enthalpy values from Darjeeling and AP were 354.1 and355.8 ± 10.3 kJ/kg respectively, while that derived from the Namling–Oiyug flora was 296.3 kJ/kg. This yieldsa palaeoelevation of 5888 m for the Namling site using the Darjeeling enthalpy estimate as a sea level datumand 6065 m using the AP assemblage. The combined uncertainty is ±728 m. Model corrected enthalpy trendsat sea level across palaeolatitude and longitude reduce the mean elevation to 5.54 km. These elevations arehigher than earlier estimates from the same site (but within uncertainty) and the corrected mean is ~1 kmhigher than the present day basin floor elevation in the region, suggesting some deflation since 15 Ma associatedwith east–west extension, particularly if a shift in the locus of deformation and uplift south to the Himalayas inpost middle Miocene times relieved N–S compressional stress on southern Tibet.
Understanding the evolution of the Asian monsoon system and itssensitivity to future climate change are major challenges for Earth sys-tem science, not the least because the monsoon supplies the waterneeds of approximately half the worlds' population. The elevation andextent of the Himalaya–Tibetan plateau edifice (HTE) have a major
5x297 (Office).
ghts reserved.
influence on the monsoon although the mechanism for that influenceremains controversial (Molnar et al., 2010). A key component of differ-entiating long term (millions of years) influences from those that takeplace over millennial or decadal timescales lies in quantifying the uplifthistory of the HTE over time in relation to measured changes in mon-soon intensity both on and off the plateau.
Being directly exposed to, and processors of, the atmosphere thearchitecture and composition of vegetation are powerful proxies forclimate. Unlike chemical proxies the palaeoclimatic signature containedwithin the architecture of leaf fossils is not subject to diagenetic
Fig. 1.Map showing the location of the areas yielding fossil floras in this work. MBT—MainBoundary Thrust, MCT—Main Central Thrust, STD—South Tibetan Detachment, IYTS—Indus-Yarlung–Tsangpo suture. Faults are shown in grey.Modified from Rowley and Currie, 2006.
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alteration and pre-depositional transport differences are usually notmore than a few hundred metres (Spicer, 1981; Ferguson, 1985;Spicer and Wolfe, 1987). Here we explore changes in the vegetationand climate from mid Miocene times to the Present at sea level alongthe southern margin of the Himalayas towards their eastern limit bydocumenting the composition and palaeoclimatic signature plant fossilassemblages preserved in Siwalik sediments.We then compare themidMiocene Siwalik palaeoflora with that of a similar age in the Namling–Oiyug Basin on the Tibetan Plateau. By this means we refine a midMiocene palaeoelevation estimate for the Namling–Oiyug Basin previ-ously obtained using climate model-derived data and characterise themonsoon influence in mid Miocene times on the southern margin ofthe plateau.
1.1. Regional setting
1.1.1. Siwalik sediments‘Siwalik’ sediments make up a thick (~7000 m) succession of
Neogene freshwater coarsely bedded sandstone, siltstone, clayand conglomeratic molassic deposits exposed along the length ofthe Himalayan foothills from the Potwar Plateau in the west to theBrahmaputra River in the east (Parkash et al., 1980; Bora and Shukla,2005). They accumulated close to sea level in a long but narrow foredeep
Table 1A generalised lithostratigraphy of Siwalik sediments in the Eastern Himalaya. In the DarjeelingPradesh is derived from Anand-Prakesh and Singh (2000), Argawal et al. (1991), Karunakaran1996), Kunte et al. (1983), Ranga Rao (1983), and Singh (2007).
Generalised Siwalik lithology Age
Upper Siwalik Loosely packed, friable very course-grainedgrey sandstones with high limonitisation atplaces and intercalated with claystones andshales. Frequent boulder beds with a sandymatrix also occur in this formation. Remainsof wood, leaves and fruits have also beenrecorded.
Late P
Middle Siwalik Generally weakly indurated, medium tocoarse-grained sandstones with salt andpepper texture. Calcareous concretions ofvarious shapes and sizes occur in thesandstones, occasionally associated withgrey shales with plant fossils.
Plioce
Lower Siwalik Well-indurated medium to fine-grainedgenerally well-sorted sandstones, subordinatemicaceous sandstones, bluish nodular silty shale,claystone, and small lenses of coal; plant fossilsoccur frequently.
Late MMiddl
to the south of the rising Himalaya. During the latest phase of the rise ofthe Himalaya, in Pleistocene to Recent times, these ‘Siwalik’ sedimentswere uplifted, folded and faulted to form a continuous mountain rangeof relatively low height ranging from 1000 to 1200 m a.m.s.l., some2400 km in length and 20–25 km in width. From west to east alongtheir length the Siwaliks have been divided into seven sectors: Jammu,Himachal Pradesh, Uttar Pradesh, Nepal, Darjeeling-Sikkim, Bhutan, andArunachal Pradesh (Karunakaran and Ranga Rao, 1976; Ranga Rao et al.,1979). The Darjeeling-Sikkim and Arunachal Pradesh sectors are thefocus of the work presented here.
Midlicott (1864) and Middlemiss (1890) both recognised a three-fold classification of the Siwalik succession, an approach also adoptedby later workers (Pilgrim, 1913; Colbert, 1934, 1942) and still in usetoday. Although the characteristics of the three subdivisions vary some-what along the length of the Siwaliks and formation names differbetween sectors, in both the Darjeeling-Sikkim and Arunachal Pradesh(AP) sectors relevant to this study (Fig. 1) the general lithological char-acteristics of the three subdivisions are those given in Table 1.
Greywackes indicating a marine influence have been reportedfrom the lower part of the succession (Sikka et al., 1961), but mostof the Siwaliks represent non-marine sediments with a compositionthat suggests weathering and deposition under a warm and humidclimate. Sediments in the upper part of the succession are generallycoarser and more immature than those they overlie, indicative of in-creasingly proximal rapid Himalayan uplift. The three subdivisionsalso show clear variations in heavy mineral composition (Dehadrai,1958; Sinha, 1970; Chaudhri, 1972). Staurolite dominates theheavy mineral components of the lower Siwaliks, whereas kyaniteand hornblende are the dominant heavy minerals in the sedimentsof the middle and upper Siwaliks respectively. The ages of the sedi-mentary succession in the different Siwalik sectors are also deter-mined by means of magnetostratigraphy (Sangode et al., 2003;Kumaravel et al., 2005).
1.1.2. The Namling–Oiyug Basin, Southern TibetLeaf impression fossils have also been recovered from mid Miocene
lacustrine sediments belonging to the Wulong Formation in theNamling–Oiyug Basin, south central Tibet (Spicer et al., 2003) (Fig. 1).Situated on the Lhasa Terrane, the Namling–Oiyug Basin of southernTibet comprises Neogene lava flows and shales with abundant plant-rich horizons interbedded with tuffaceous material unconformablyoverlying an Upper Cretaceous/Eocene volcanic basement. Two fossilassemblages with similar species compositions have been recovered
area this is based on Ganguly and Rao (1970) and Acharya (1994) while that in Arunachaland Ranga Rao (1979), Kumar (1997), Kumar et al. (1983), Kumar and Singh (1980, 1982,
Fig. 2. Simplified section showing the Namling fossil locality in relation to 40Ar/39Ar datedhorizons within the section (Spicer et al., 2003).
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from steeply dipping, ash-rich, lacustrine units. These fine-grained sed-iments contain several lignite-rich horizons and immature palaeosolssuggesting variations in lake sedimentation and depth. Both assem-blages represent the same horizon although now separated by a verticaldistance of 300 m due to tectonic tilting. The present day averageelevation of the two assemblages is 4450 m. The assemblages havebeen dated as 15 Ma on the basis of 40Ar/39Ar analysis of single crys-tal sanadine feldspars recovered from volcanic ashes immediatelybelow them, and from phlogopite micas in potassic lavas 55 mstratigraphically above (Fig. 2).
2. Methods
2.1. Modern floras
At elevations similar to those inferred for the source vegetation ofthe Siwalik fossil assemblages (b300 m) modern vegetation close tothe fossil localities in both Darjeeling and AP is characterised as moist
Table 2Relationships between CLAMPmorphotypes (Operational Taxonomic Units—OTUs), individual fof the presumed nearest living relatives for the lower Siwalik sediments of the Darjeeling Foot
deciduous-tropical or humid tropical (Champion and Seth, 1968; Kauland Haridasan, 1987; Antal and Awasthi, 1993; Hazra et al., 1996;Baishya et al., 2001). Lists of the dominant taxa are given in the supple-mentary data.
2.2. Fossil floras
Fossil leaf, seed and fruit assemblages were collected from the GishClay in the Darjeeling area, the Dafla, Subansiri and Kimin formationsin AP, and the Wulong Formation in the Namling–Oiyug Basin. Herewe focus on the leaf impressions and compressions.
In the case of the Siwalik floras the material was cleaned, curated,and photographed using low angle incident light before being com-pared with modern taxa in the collections of the Central NationalHerbarium, Sibpur, Howrah, West Bengal, as well as with taxa growingnear to the fossil sites. The fossil material is held in the repository of thePalaeobotany–Palynology Section, Department of Botany, University ofCalcutta.
The Siwalik floras were then compared to a mid Miocene (15 Ma)leaf flora from the Namling–Oiyug Basin, southern Tibet, that has previ-ously been used to derive a palaeoelevation estimate (Spicer et al.,2003). A detailed systematic description of this impression flora is inpreparation. The specimens are held at the Nanjing Institute of Geologyand Palaeontology, Chinese Academy of Sciences, Nanjing, China.
2.3. CLAMP analysis
Climate Leaf Analysis Multivariate Program (CLAMP), introduced byWolfe (1993) and developed to include methodological refinements(Kovach and Spicer, 1995; Spicer et al., 2009; Teodoridis et al., 2011;Yang et al., 2011) and expanded training sets (Gregory-Wodzicki,2000; Spicer et al., 2004; Steart et al., 2010; Jacques et al., 2011;Srivastava et al., 2012), exploits the intimate relationship that existsbetween the leaf physiognomy (architecture) of woody dicots andclimate. Through evolutionary adaptation, selection, phenotypic plastic-ity and migration woody dicotyledonous plants growing in natural ornaturalised vegetation possess a spectrum of features that optimisecompetitive success under the specific environmental conditions to
ossil specimens (designated by specimen collection numbers), fossil names and the nameshills, latitude 26.8833°N, longitude 88.4667°E, elevation 194 to 250 m.
NLR
olata Phoebe lanceolata Nees (Lauraceae)gstonei Garcinia livingstonei T. Anderson (Clusiaceae)nii Combretum decandrum Roxb. (Combretaceae)
Cratoxylum sp. (Clusiaceae)latum Glochidion oblatum J. D. Hooker (Euphorbiaceae)f. tomentosa Lagerstroemia tomentosa C. Presl (Lythraceae)ika Callicarpa arborea Roxb. (Lamiaceae)
Table 3Relationships betweenCLAMPmorphotypes, (Operational TaxonomicUnits—OTUs), individual fossil specimens (designated by specimen collection numbers), fossil names and the namesof the presumed nearest living relatives for the lower Siwalik sediments of Arunachal Pradesh, latitude 27.043056°N, longitude 92.605°E, elevation 250 m.
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which they are exposed. No single leaf character in isolation confers thisoptimisation or correlates with any single environmental/climatic vari-able (Lande and Arnold, 1983; Ackerly et al., 2000; Spicer, 2007). Thesecomplex interactions operate across a number of leaf architectural
Table 4Relationships betweenCLAMPmorphotypes, (Operational TaxonomicUnits—OTUs), individualof the presumed nearest living relatives for the middle Siwalik sediments of Arunachal Pradesh
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character array where the physiognomy (architecture) of leaves from atleast 20 species ofwoody dicots fromeach site (vegetation sample) is de-scribed numerically for 31 character states encompassing lobing, marginform, size, apex and base form, length to width ratio and shape. This isthe physiognomy or ‘Physg’ file. The second array contains climaticobservations for those same sites. Canonical Correspondence Anal-ysis (ter Braak, 1986) positions the extant samples in multidimen-sional physiognomic space based on their physiognomic scores,while the climate array is used to position and calibrate climatevectors that best summarise climate trends through this physiog-nomic space. Fossil sites, scored in the same manner as the extantcalibration samples, are positioned passively (i.e. without climatedata) within physiognomic space and their location along thecalibrated climate vectors (their vector score) delivers the predict-ed palaeoclimate by means of a second order polynomial regres-sion transfer function summarising the relationship between thevector scores of the modern sites and their observed climate. Fulldetails of the method are given on the CLAMP website (http://clamp.ibcas.ac.cn).
Where possible species identified on the basis of charactersshared by presumed nearest living relatives were adopted asmorphotypes for the CLAMP analysis. The equivalence betweenthe morphotype identifiers, the fossil taxon names and the pre-sumed nearest living relatives for the Siwalik samples are given inTables 2–5. CLAMP does not require the species to be identified orreferred to modern presumed relatives and the term ‘morphotype’is used in CLAMP to refer to groups of specimens likely to representleaves of the same ancient species. These morphotypes are referredto as operational taxonomic units (OTUs) and the numbering is selfcontained within each site, so OTU 1 in one assemblage is not neces-sarily the same taxon as OTU1 in another site. The identified speciesand morphotypes were scored according to the standard CLAMPprotocols given on the CLAMP website, which are distinct from
Table 5Relationships betweenCLAMPmorphotypes, (Operational TaxonomicUnits—OTUs), individualof the presumed nearest living relatives for the upper Siwalik sediments of Arunachal Pradesh
those used to describe leaves for taxonomic purposes (e.g. Elliset al., 2009). The scoresheets are available as supplementary data.
To calibrate our analysis we initially used the PhysgIndia1 dataset(Srivastava et al., 2012), with a matching climate array based on thehigh resolution New et al. (2002) 0.16° × 0.16° gridded data. Climateparameters derived from this dataset were adjusted for the exactlocation and elevation of the modern vegetation sites using themethod of Spicer et al. (2009). We found that some of the Siwalikfossil sites plotted close to the ends of the temperature and enthalpyvectors, thereby potentially introducing uncertainties that are diffi-cult to quantify. We therefore supplemented this dataset with addi-tional samples from the PhysgAsia1 dataset (Jacques et al., 2011)with mean annual temperatures of 20 °C or higher, together withunpublished sites from southern China and Thailand in order toextend the number of warm Asian sites in the calibration. High-resolution gridded data were also used for the additional sites.Both the physiognomic and high-resolution gridded climate cali-bration data are available as PhysgAsia2 and HiResGridMetAsia2files in the supplementary information. The physiognomic scoresfor the Namling–Oiyug flora are given in the ‘Fossil’ file, also partof the supplementary information.
Although a range of methods have been used to measure thestrength of the monsoon (Liu and Yin, 2002; van Dam, 2006; Liu et al.,2011) here we used the monsoon index (MSI) of Xing et al. (2012) be-cause their MSI can be derived from the climate variables returned byCLAMP using the following expression:
MSI ¼ 3WET−3DRYð Þ � 100=GSP:
The higher the MSI the greater the difference in precipitation be-tween the wet and dry seasons and the stronger the monsoon.
fossil specimens (designated by specimen collection numbers), fossil names and the names, latitude 27.06589°N, longitude 93.64561°E, elevation 530 m.
NLR
tifolia Actinodaphne angustifolia Nees (Lauraceae)FabaceaeLitsea salicifolia Roxb. (Lauraceae)Quercus lamellosa Sm. (Fagaceae)
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2.4. Palaeoelevation estimates
For estimations of palaeoelevation we used the enthalpy method(Forest et al., 1999; Spicer et al., 2003). Enthalpy (more strictly moistenthalpy) H is given by:
H ¼ cpT þ Lvq
where cp is the specific heat capacity of moist air at a constant pressure,T is temperature (in Kelvin), Lv is the latent heat of vaporisation, and q isthe specific humidity. Being a combination of temperature andmoistureenthalpy is reflected strongly in leaf architecture (Forest et al., 1999).
Moist static energy (h) is a property of a parcel of air that is con-served as that parcel changes in altitude, moist static energy beinggiven by:
h ¼ H þ gZ
where gZ is gravitational potential energy given by the acceleration dueto gravity (g) and elevation (Z). The difference in enthalpy at sea level(Z = 0) and an unknownheight is ameasure of the difference in poten-tial energy and thus the height difference.
Lacking nearby direct measurements of sea level enthalpy for 15 Mathe previous estimate of the elevation of the Namling flora had to relyon the use of a mid Miocene climate model constrained for enthalpyas estimated from coeval fossil floras in Japan (Spicer et al., 2003).Given generally poor model congruence with proxy data in pre-Quaternary simulations (Spicer et al., 2008, and references therein)arguably this is unsatisfactory. By deriving enthalpy estimates fortwo mid Miocene floras proximal to the Namling site, here givenby the Darjeeling and Arunachal Pradesh lower Siwalik assemblagesthat must have been laid down close to sea level in the Himalayanforedeep, we can derive more direct estimates for the height ofthe Namling–Oiyug Basin flora at 15 Ma.
3. Results
3.1. Composition of the fossil floras
The Siwalik fossil assemblages show that there is some compositionalchange since the mid Miocene at the genus level, presumably as a resultof evolutionary competition, but the general character of the vegetationdoes not alter in that the fossil leaf forms are similar to those typical ofmodernmoist tropical forests. The CLAMP analyses show thatwhat com-positional change there is does not indicate marked climate change.
3.1.1. Darjeeling Lower Siwalik (middle Miocene) Gish ClayThe fossil flora consists of 19woody dicot taxa attributable to the fol-
lowing genera: Phoebe, Garcinia, Combretum, cf. Cratoxylum, Glochidion,Lagerstroemia, Callicarpa, cf. Bridelia, Dipterocarpus, Persea, Medinilla,Mesua, cf. Woodfordia, Hopea, Nephelium, Uvaria, cf. Atalantia, cf. Derrisand Albizia, collectively indicative of a tropical to subtropical humidenvironment. In addition a further seven fossil forms remain unidenti-fied. Table 2 provides a more complete list of the fossil taxa, theirnearest living relatives, their equivalent morphotype identifiers (Oper-ational Taxonomic Units (OTUs)) and specimen numbers used in theCLAMP analysis.
3.1.2. Arunachal Pradesh Lower Siwalik (middle to upper Miocene) DaflaFormation
The lower Siwalik sediments of Arunachal Pradesh yielded the 32woody dicot taxa listed in Table 3. Although to some extent composi-tionally distinct from the lower Siwalik flora of Darjeeling again themodern equivalents of these taxa are typical of tropical humid forests.Genera common to both fossil and living floras include: Actinodaphne,Aglaia, Combretum, Dipterocarpus, Lagerstroemia, and Terminalia.
3.1.3. Arunachal Pradesh Middle Siwalik (Pliocene) Subansiri FormationThe 26 taxa comprising this fossil flora are listed in Table 4 together
with their nearest living relatives and the morphotypes and specimenidentifiers used in the CLAMP analysis. The following genera are com-mon to the fossil and living floras: Calophyllum, Gynocardia, Pimenta,Pongamia, Quercus, and Terminalia.
A total of 29 species of woody dicots were recovered from the upperSiwalik (Kimin Formation) sediments in Arunachal Pradesh. These andtheir nearest living relatives are listed in Table 5. Twenty three ofthese taxa were previously described by Khan et al. (2011) who alsoused univariatemethods to infer palaeoclimate. The upper Siwalik fossilflorahas a tropicalmoist aspect and includes Actinodaphne, Calophyllum,Combretum, Dipterocarpus, Knema, Litsea, Pongamia, and Quercus thatare found at low elevations in AP today.
The megafossil plants present in the coal-bearing strata of theMiocene Wulong Formation at Wangdui Village in Namling Countyat a present day altitude 4300–4600 m were first studied by Li andGuo (1976). Initially they thought that there were two stratigraphicallydistinct horizons (an upper and lower plant bed) but the apparent sepa-ration is an artefact of a steep local dip and the beds are now known tobe lateral equivalents. Compositionally the two assemblages are similar(Li and Guo, 1976; Guo, 1981; Guo and Wu, 1988) and comprise collec-tively Salix, Quercus, Thermopsis, Rhododendron, Phragmites, Cyperacites,Populus, Betula, Carpinus,Ulmus, Ribes and Crataegus. Specimens of Betulaand Carpinus are the most numerous.
A collection of 458 plant megafossil specimens representing theNamling–Oiyug flora, the vast majority of which are leaves, was origi-nally made and reported on by Spicer et al. (2003). This material wasused in this study. The specimens came from a single steeply dipping(70°) lacustrine claystone horizon near Wangdui Village (29.6964°N,89.5788°E). All the fossil forms are assignable to modern genera typicalof cool temperate deciduous woodlands. The megafossil woody dicotsin the analysed flora comprise 7 families, 10 genera and 22 species intotal. Acer, Alnus, Salix, Betula, and Corylus predominate together withtypical Himalayan taxa such as Rhododendron. Collectively they repre-sent a typical boreal temperate broad-leaved deciduous forest foundtoday at high elevations in the Himalayas.
Minimal mechanical and biological degradation suggests that theleaves were not transported far before rapid burial. Possible conifer re-mainswere extremely rare in the assemblage and fragmentary, suggest-ing a more distant source. A full taxonomic treatment of the flora,including formal descriptions of new species, is in preparation. TheCLAMP scoresheet is given as supplementary data.
3.2. Palaeoclimate
CLAMP climate retrodictions and associated uncertainties for all thefossil sites are given in Table 6.
3.2.1. Siwalik palaeoclimateCLAMP analyses suggest that all the Siwalik fossil assemblages attest
to warm (tropical to subtropical) humid climates with a distinctivemonsoon signature. The CLAMP retrodictions for the Darjeeling andAPmiddle Miocene lower Siwalik assemblages are, within error, identi-cal as regards temperature, humidity and enthalpy estimates. Bothassemblages reveal a moderately high rainfall of around 2 m perannum and an average annual humidity of 80%. In such wet regimesprecipitation uncertainties are high because leaf form is only weaklyconstrained by water availability. The ratios of the precipitation in thethree wettest months to the three driest months for all the AP samples
Fig. 3. CLAMP regression model (transfer function) for moist enthalpy showing the posi-tions of the fossil floras. Note that all the Siwalik floras plot high and together indicatelow palaeoelevations, while the Namling site plots low indicating a high elevation.
Table 6CLAMP results and associated uncertainties. Abbreviations: MAT—mean annual temperature, WMMT—warm month mean temperature, CMMT—cold month mean temperature, LGS—length of the growing season where growth is regarded to take place when the mean daily temperature N10 °C (note—values N 12 months reflect the uncertainty in the estimates),GSP—growing season precipitation, MMGSP meanmonthly growing season precipitation, 3WET—precipitation during the three consecutive wettest months, 3DRY—precipitation duringthe three consecutive driest months, RH—mean annual relative humidity, SH mean annual specific humidity, ENTHAL—mean annual moist enthalpy, SD—standard deviation.
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are N6:1which defines them asmonsoonal (Lau and Yang, 1997; Zhangand Wang, 2008), although more weakly so than for the present daywhere the ratio for that region is over 120:1 based on the same griddeddataset used to calibrate CLAMP. The MSI for the fossil floras ranges be-tween 51 (lower Siwalik), 43.2 (middle Siwalik) and 48.8 for the upperSiwalik, while the modern MSIs are 52.5, 54.7 and 52.8 respectively.These results suggest that the monsoon in the AP region has remainedessentially the same since mid Miocene times. The Darjeeling fossilwet/dry ratio is 3.8:1 suggesting a more even distribution of rainfallthroughout the year due, apparently, to awetter dry season than furthereast. The MSI for the fossil flora is 34.2 while that of themodern is 68.1.InmidMiocene times themonsoon in theDarjeeling area, to thewest ofAP, was weaker than now.
The middle Siwalik Pliocene sample from Arunachal Pradesh ap-pears to represent slightly cooler conditions, notably in winter, thanthose of the lower and upper Siwaliks but the difference is withinerror andmay not be real. All other climate variables are identical, with-in error, to all other Siwalik samples. The uniformity of climate betweenall the Arunachal Pradesh Siwalik samples is notable.
3.2.2. Namling palaeoclimateThe retrodicted climate for the Namling–Oiyug Basin is, by contrast,
markedly cooler and drier than those of the Siwalikswith a considerablyshorter growing season. The difference exceeds±2 standard deviationsfor all temperature variables. With high uncertainties the precipitationestimates should be regardedwith caution but the Namling GSP (Grow-ing Season Precipitation) is a third less than those of the Siwaliks, butwith twice the dry season rainfall than that of the AP sample of thesame age. The shortfall in rainfall appears to be a wet season phenome-non. If these estimates are in any way accurate they imply blocking ofwet air onto the plateau, but to amuch lesser degree than at the presentday. Measures of present day blocking based on gridded data should beusedwith caution, however. TheMSI for the fossil flora is 35.2 while themodern is 1988. This implausibly highmodern value is the result 1) thelow density of climate stations across Tibet providing a climate recordfor the 1961–1990 interval used by New et al. (2002) for the griddeddata, and 2) the use of the GSP in calculating the MSI. At altitudesabove 4000 m the growing season (0.1 months) is strongly tempera-ture dependent and it is only later in the summer (WMMT 8.8 °C)that mean daily temperatures exceed the 10 °C that is used in CLAMPto define the onset of woody dicot growth. Much of the monsoon-driven precipitation determining the 3-WET value (123 mm) arrivesin the rain shadow north of the Himalaya before the onset of growthresulting in a very low GSP (6 mm). The precipitation during the threeconsecutive driest months (3 mm) coincides with lowwinter tempera-tures (CMMT −9.2 °C) and does not provide a readily available springsoil water reserve.
3.3. Palaeoelevation
TheNamling site has a significantly lower enthalpy value than any ofthe Indian sites, irrespective of age (Fig. 3). Moreover there is little
change in the enthalpy values over time for all the Siwalik floras mean-ing that lack of dating precision is not critical to quantifying thepalaeoelevation of the Namling flora. The palaeoelevation for Namlingresolves at 5888 m using the Darjeeling enthalpy estimate and6065 m using the lower Siwalik assemblage from Arunachal Pradesh.The combined uncertainty of the Siwalik and Tibet altitude estimates(2 sigma) is ±728 m. These raw values need to be corrected for thedifferent relative positions Namling and the Siwalik sites at 15 Ma. SeeSection 4.3 for how this was done.
4. Discussion
4.1. Vegetation and climate of the Eastern Siwaliks since the mid Miocene
Through time there is an increasing commonality between thecomposition of Siwalik fossil assemblages and the vegetation grow-ing today immediately around the fossil sites. This progressive floralmodernization appears to take place despite no major changes intemperature regime. All fossil floras contain tropical elements andoverall the species compositions indicate a continuance throughtime of tropical humid vegetation. In all Siwalik fossil sites CLAMPWMMTs are remarkably consistent. The CMMTs show more varia-tion, with the Darjeeling lower Siwalik and AP middle Siwalik
8 M.A. Khan et al. / Global and Planetary Change 113 (2014) 1–10
assemblages indicating the coolest conditions but barely descendbelow 17 °C.
The lower Siwalik assemblage from Darjeeling yields the highestGSP and thewettest dry season, but all sites suggest distinct seasonalityin rainfall. Inmid-Miocene times themonsoon in theDarjeeling area ap-pears to have beenweaker than now,whereas in AP there has been littlechange.
4.2. Vegetation and climate of Namling
The present-day vegetation surrounding the fossil site (N4300 m) ispredominantly herbaceous semi-arid grassland grazed extensively bydomesticated animals (sheep and yak), or turned over to growing cerealcrops, particularly barley. Trees are capable of growing when suppliedwithwater and protected fromanimals. Populus and Salix are commonlyplanted in and around farms and villages and provide shade andfirewood.
In contrast the fossil flora is indicative of a moderately diverse cooltemperate broad-leaved deciduousmontanewoodland growing aroundthe margins of the palaeolake. The extent and depth of the lake areunknown so it is difficult to estimate any ameliorating effects the lakemight have had on local climate but it undoubtedly supplied water tothe surrounding vegetation. Despite this it represents vegetation grow-ing under a drier regime than contemporaneous floras preserved inSiwalik sediments. The main difference is that in the mid Miocene wetseason less moisture was delivered to the Namling–Oiyug Basin thanto the floras to the south, possibly suggesting marked blocking of wetair or ‘rain out’ before reaching the Namling location. This blockingwas less pronounced that at the present day, suggesting a lowerHimalayan barrier, and the dry season shows no such effect.
The temperature regime revealed by CLAMP ismarkedly colder thanfor coeval floras at sea level N400 km to the south. The mid MioceneNamling MAT is around 17 °C lower than experienced by the Siwalikfossil floras. The largest difference is in the winter temperatures; theWMMT is only around 4 °C cooler while the winter temperatures aremore than 20 °C colder and, even allowing for uncertainties, indicativeof significant freezing (CMMT −5.71 ± 4 °C). These differences areindicative of a significantly higher elevation for Namling palaeofloracompared to that from the Siwaliks.
4.3. The mid Miocene elevation of the Namling–Oiyug Basin
A previous quantification of the palaeoelevation of this site (Spiceret al., 2003) relied on inferred reference enthalpies derived from ageneral circulation climate model because no fossil floras were thenavailable from proximal sea level locations. The model was constrainedby oxygen isotope sea surface temperatures and enthalpy estimatesderived from mid Miocene Japanese floras. The advantage of themodel approach was that reference enthalpies could be inferred at thesame location as the Namling fossil site, but the disadvantage was thatthere was no constraining data local to that site. In the present studywe assume that the palaeoenthalpy data derived from the lower Siwaliklocalities represent conditions at, or very close to, sea level. This isjustified based on the present elevation of the foredeep surface asrepresented by the Gangetic Plain across northern India. This seldomexceeds 200 m in areas likely to be accumulating sediments andacross much of northeastern India is below 100 m. The previouslyused model-derived sea level enthalpy was 335.5 kJ/kg comparedto the 354.1 and 355.8 kJ/kg obtained from the Darjeeling andArunachal Pradesh deposits respectively. Moreover, because higherenthalpy values are associated with lower altitudes we are confidentthat the fossil sites represent near sea level enthalpies.
The palaeoelevation for Namling resolves at 5888 m using theDarjeeling enthalpy estimate and 6065 m using the lower Siwalikassemblage from Arunachal Pradesh. The combined uncertainty(2 sigma) is ±728 m. However, this takes no account for the
palaeospatial trends in enthalpy. Assuming a roughly 22°N, 88.5°Eposition at 15 Ma for the Darjeeling and 22°N, 93.5°E for the APlocality and a 27°N, 90°E position for Namling based on the mapsin Molnar and Stock (2009) and Song et al. (2010), and the use ofthe same climate model for the mid Miocene as in Spicer et al.(2003), the enthalpy at sea level at the Namling site is 6.196 kJ/kgless than that at Darjeeling and 5.1106 kJ/kg less than in AP.Subtracting these values from the CLAMP-derived enthalpy for theSiwalik fossil sites to estimate the 15 Ma enthalpy at sea levelbeneath Namling gives a palaeoelevation of 5.26 km using theDarjeeling site as a sea level datum and 5.54 km if the AP site isused. The mean is 5.4 km. These values are, within error, similar tothe 5200 + 1370/−606 m derived from ∂18O from soil carbonatesobtained from the Namling site when allowances are made for tem-perature and humidity uncertainties (Currie et al., 2005). The meanis, however, 700 m higher than that reported in Spicer et al. (2003)who used the modelled enthalpy. We regard the value reportedhere to be the more reliable. Our new elevation estimates, and thatof Currie et al. (2005), are ~1 km higher than the present average el-evation for the region and certainly higher than the modern valleyfloor at the Namling site (~4.3 km). The fossil site must representvegetation on the floor of the ancient basin because it was proximalto, and accumulated in, a lake without showing any signs of long-distance transport to that site.
4.4. Implications of a 1 km reduction in elevation of the Namling–OiyugBasin since 15 Ma
Putting aside uncertainties, a height reduction from c. 5.4 km at15 Ma to c. 4.3 km today could be due to two factors that are not mutu-ally exclusive. One is erosion and the other is extensional deflation.Erosion, however, cannot explain the observed elevation reduction be-cause the previously elevated basin floor still survives. While the south-ern Himalayan flank of the plateau appears to have undergone aminimum of 25 km of denudation since the Miocene, the relativelyflat topography of the Tibetan interior shows little sign of significanterosion since that time.
Fielding (1996) ascribes the longwavelength flatness of Tibet to lith-ospheric viscous flow. It is an inevitable consequence of uplift that weaklower crustal rocks are subject to lateral pressure gradients, and thusflow, even when isostatically compensated (Bird, 1991). The effect ofthis flow is to reduce topographic variation even over geologicallyshort timescales. Moreover this flow can be focussed by areas ofdenudation (Beaumont et al., 2001). Lower crustal flow has proved tobe a popular model to explain the palaeoelevation dynamics in easternTibet and Yunnan (Clark and Royden, 2000; Clark et al., 2005a, 2005b;Schoenbohm et al., 2006; Royden et al., 2008) even though quantitativemeasures of diachronous height change in the region have yet to beobtained (Clark et al., 2005b) and mid crustal flow is thought to play asignificant role in the development of High Himalaya (Beaumont et al.,2001; Searle et al., 2011). Although Searle et al. (2011) question theeastward extrusion of crust from beneath Tibet they do not rule outthat this might be a phenomenon that does not have direct surfaceexpression. Flow in the lower crustmight result in surfaceheight chang-es without the low viscosity material finding surface expression orevidenced by significant transverse faulting. They do, however, findabundant evidence for mid crustal channel flow to the south from be-neath southern Tibet in early to mid Miocene times (23–15 Ma). N–Strending normal faulting and shoshonitic dyke emplacement indicativeof E–W extension had begun by 18 Ma (e.g. Williams et al., 2001) andN–S trending faulting may have begun even earlier. It is not unreason-able to propose that flowof lowormiddle crustalmaterial frombeneathsouthern Tibet, whether south or east or both, may account for a degreeof surface deflation, particularly if southern Tibet was relieved of acertain amount of compression due to transfer of the main locus of de-formation and uplift southward to theHimalaya in postmiddleMiocene
9M.A. Khan et al. / Global and Planetary Change 113 (2014) 1–10
times. To test this hypothesis additional palaeoelevation estimates arerequired from other sites across southern Tibet.
Taking uncertainties into account the difference in the mid Miocenepredicted elevation and that of today may not be significant. Neverthe-less the analysis of the Namling flora presented here, from both compo-sitional and CLAMP perspectives, confirms a high elevation, and onethat is likely to have been higher than the present altitude.
Acknowledgements
This work was supported by a Visiting Professorship for Senior Inter-national Scientists awarded to R.A.S. by the Chinese Academy of Sciences(2009S1-20), by an International S & T Cooperation Project of China No.2009DFA32210 and by funding from the University of Kolkata. Theworkwas also supported by the “Strategic Priority Research Program B” of theChinese Academy of Sciences (Grant No. XDB03010103) and the Nation-al Natural Science Foundation of China grant (41030212, 41272007).We thank Taksin Artchawakom, Director of Sakaerat EnvironmentalResearch Station, Thailand, for permission to collect material for CLAMPcalibration, and to PitchanatNgerndee, for her help inmaking that collec-tion.M. K. and S. B acknowledge thefinancial assistance from theDepart-ment of Science and Technology, New Delhi. Thanks are due to theauthorities of Central National Herbarium, Sibpur, Howrah for permis-sion to consult theHerbarium. R.G thankfully acknowledges theDirector,BSIP, Lucknow for his encouragement and permission to publish thiswork.
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.gloplacha.2013.12.003.
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