High frequency abrupt shifts in the Indian summer monsoon since Younger Dryas in the Himalaya

In order to quantify the Indian summer monsoon (ISM) variability for a monsoon dominated agrarian based Indian socio-economy, we used combined high resolution δ13C, total organic carbon (TOC), sediment texture and environmental magnetic data of the samples from a ~3 m deep glacial outwash sedimentary profile from the Sikkim Himalaya. Our decadal to centennial scale records identified five positive and three negative excursions of the ISM since last ~13 ka. The most prominent abrupt negative ISM shift was observed during the termination of the Younger Dryas (YD) between ~11.7 and 11.4 ka. While, ISM was stable between ~11 and 6 ka, and declined prominently between 6 and 3 ka. Surprisingly, during both the Medieval Warm Period (MWP) and Little Ice age (LIA) spans, ISM was strong in this part of the Himalaya. These regional changes in ISM were coupled to southward shifting in mean position of the Intertropical Convergence Zone (ITCZ) and variations in East Asian monsoon (EAM). Our rainfall reconstructions are broadly in agreement with local, regional reconstructions and PMIP3, CSIRO-MK3L model simulations.


Supplementary (1) Stable carbon isotopes and palaeoclimate: an introduction
The  13 C value of C 3 plants is controlled by the fractionation associated with atmospheric carbon dioxide (CO 2 ) uptake and fixation () during photosynthesis and can be described by mathematical model using the formula ( = a + (b -a)(p i /p a ) 1 . Here, 'a' and 'b' are the constant values of isotopic fractionation during diffusion of CO 2 through stomata (4.4‰) and during carboxylation (~29‰), respectively. 'p i ' and 'p a ' are the intercellular and ambient partial pressures of CO 2 , respectively 1 . The p i value is influenced by climatic factors like amount of rainfall and light. Slight alteration in these climate governed parameters affects the stomatal conductance, resulting change in p i values that alter the  13 C values of C 3 plants. During low rainfall condition plants narrow their stomatal opening to prevent water loss and leads to reduction in p i value, resulting in an increase in  13 C values of C 3 plants and vice-versa. The  13 C of modern C 3 plants and SOM responds principally to rainfall variations 2,3 . Thus, it can be suggested that  13 C values of organic matter in sedimentary archives derived from C 3 vegetation will potentially reflect paleoprecipitation 4,5 . Based on this concept, the variability in the Indian Summer Monsoon (ISM) precipitation has been reconstructed for last 12.7 ka. Moreover, the variations in magnetic susceptibility are also found to reflect paleoclimatic changes, because lf depends on the concentration and mineralogy of magnetic grains. A comparison of lf with a variety of other geochemical and lithologic indicators (e.g.  13 C, TOC etc.) reveals several defined co-relatable features, which are traditionally used to understand climate change (e.g. high TOC and low magnetic susceptibility).

(2) Geomorphology, geology, vegetation and climate of the study area
The present study has been carried out on a proglacial peat/bog profile (27° 54' 16.28" N; 88° 31' 31.32" E) retrieved from the Chopta Valley, north Sikkim, India. The study site lies in the transition zone between the dry steppe of the Tibetan plateau in the north and the sub-humid Himalayan climate in the south (Fig. 1, 2). Sikkim, a small state with an area of approx. 7299 km 2 , is one of the places in the North East Indian Himalaya (NEIH) that has been considered as 'biodiversity hotspots' [6][7][8][9] . The region is drained by the Tista River that flows from almost north to south and Chopta Valley lies in its upper reaches. Strategically the state is very important as it shares international borders with countries like Bhutan, China and Nepal Himalaya 10,11 . The region constitutes of the rocks of Lesser, Central and the Tethys Himalaya 12 . The state exhibits a complex topography with sheer altitudinal gradient that influences the vegetation and local weather patterns. These complex characteristics have resulted into existence of microclimatic conditions at small distances and shelter distinctive vegetation and wildlife 13 . The vegetation of the state is represented by the sub-Himalayan wet mixed forests, sub-tropical pine forests, wet temperate forests, mixed coniferous forests, eastern oak-Hemlock forests, Oak-fir forests, moist alpine scrubs and dry alpine scrubs 14,15 . The study area is extensively covered by grasses and the local communities use these proglacial valleys as a pasture land for grazing their herds. This proglacial valley is formed due to the damming of melt water stream (Chopta Chu) by late Quaternary recessional moraines resulting in the formation of this flat outwash plain that is filled with finer sediments 11 .  Climatologically, the state has distinct and wide meteorological variations resulting from complex topography and steep altitudinal gradient. Towards the south (foot hills), the state experiences a subtropical weather while the northern sector (towards Trans Himalaya) experiences tundra type weather. There is a significant and progressive decline in the precipitation gradient from the low altitude southern parts to high altitude northern sector. Since, the meteorological data is short near to sampling site, therefore, we obtained meaningful climate data from the gridded temperature and precipitation datasets of nearest grid points ( 16

(3) Materials and Methods, Samples collection
Samples for isotopic ( 13 C) analysis were collected from a 3m deep sedimentary profile that has been excavated in the proglaical outwash plain (27° 54' 16.28" N: 88° 31' 31.32" E; ~4000 m asl) of the Chopta valley. Lithologically, the profile has two distinct parts i.e. the top brown to dark brown peat/bog and the bottom light grey fine sandy silty part (Fig. 2). Stratigraphically, from bottom, 10 cm of the profile consists of a silty layer which is overlain by a grey silty-sandy layer of around 85 cm thickness. Sedimentologically and texturally this zone represents a typical succession of glacial sediments deposits by a meltwater stream under favorable melting conditions. A 5 cm dark peaty layer overlies the silty horizon. Brown husky layer with partially decomposed organic material of ~10 cm thickness is present and is overlain by a grey silty layer of 10 cm thickness and a silty-sandy horizon of 20 cm thickness. The top 160 cm are mostly organic rich peat/bog sediments with brown to dark brown colour (Fig. 2). The upper half of this sedimentary succession is characterized by high organic rich sedimentation and represents a very wet environment with peat/bog formation. The organic material-rich peat is formed by the deposition of layers of sand, silt, clay and plant matter, which undergo microbiological and chemical transformations on a timescale of hundreds of years 19 . In the present study the primary controlling parameter of peat/bog formation could be the changing height of the water table which is directly related to the dynamic fluvial system. The organic rich layers of the profile can be distinguished into three different types on the basis of colour 20 . These are (i) light peat (husky brown layer in the profile) that shows a little decomposition and shows a lot of grass fibers; (ii) dark peat that shows intermediate decomposition and has a dark brown colour with greasy feel and (iii) black peat which is highly decomposed and has a rich dark colour 21 . The bottom most grey sandy/silty sediment layer is clearly a melt water lain sediment layer, highly minerogenous and formed in a redox environment. The growth of peat/bog has taken place once the water level has slightly fallen and environment has changed to oxidizing condition. The husky brown bog layer shows partial decomposition that may be attributed to water level which has created a redox condition. The topmost layer which shows humification in oxidizing conditions produce humic acid and hence the colouring these layers 22 .
A total of 150 samples (each representing 2cm of the sediment profile; ~200 gm) were collected from this profile and ~1 gm of the sediment sample was taken after coning and quartering.
Sample preparation was done following the procedure discussed in detail in Agrawal et al. 23 and Dubey et al. 11 . The sample were dried at room temperature and subsequently powdered to clay size and poured into 50 ml centrifuge tubes. 5% HCl solution (three times) was added to the sediments for the removal of carbonates and washed with milli-Q water using a centrifuge machine (~3000 rpm) for the removal of acid and soluble salts. The de-carbonated samples were then dried in a hot air oven with temperature control (< ~45ºC). The oven dried samples were again powdered with an agate mortar to lose any clumps for during the earlier processes. The  (Table S2). The sample at the depth of 220 cm was exceptionally low considering the stratigraphy, and hence not considered for the age-depth model. Before that all the ages were calibrated using intcal13 and the calibrated ages (2 ) are given in Table S2. Assuming the modern sample is of 2±1 a, the age-depth was modeled and this is given in Fig. S2. From the age-depth model, the ages were interpolated for every sample location. The calibration and the age-depth model were done in R software 25 using Bchron package 26

(4) Results
The TOC in the de-carbonated samples of profile is highly variable and range from 0.01 to 67.4% (Table S1). Lower part of the sediments profile (~300 to 202 cm depth), deposited during ~12.7 to11.8 ka, is mainly composed of the light grey fine sandy silt and is characterized by low TOC which range from 0.01 to 1.93. Following this, TOC content increases rapidly from 202 to 186 cm depth (from ~11.8 to 11.3 ka) and reaches up to ~ 47% in the brown husky layer with partially decomposed organic material. Subsequently, TOC content decreases abruptly and relatively low TOC observed in the grey silty layer at ~180 to 170 cm depth (~11.1 to 10.2 ka). in the black peat layer (Fig. 3). The  13 C values in lower half of the sediment profile are also highly variable. This section of the profile is deposited during ~12.7 and 10.6 ka (between depth ~300 and 166 cm) and is characterized by alternate layers of silt and sand punctuated with dark and black peat layers. Here, we have observed some abrupt changes in  13 C values ranging There is a prominent anti-correlative trend is observed between MS and  13 C values, although the MS values are significantly lower that is attributed to the peaty nature of the sediments.
However, our interpretation of MS is in good agreement with carbon isotopic data (Fig. 3).

Fig. S3.
Correlation of reconstructed rainfall with other regional proxies via. GISP2 ice core temperature data, Core SK 218/1 from the western way of Bengal, Hulu and Timta cave data.

(5)  13 C values of organic matter as palaeo-precipitation indicator
The carbon isotope ratio of C 3 plants and modern surface sediments (C 3 vegetation dominated) is inversely correlated with the rainfall amount 2,3,29-35 . Kohn 35 and Diefendorf et al. 34 36 . In historical period, during preindustrial the precipitation anomaly is positive over the Indian region. However, it decreased in-between and then again it reconstructed to the normal but the extreme precipitation anomaly (positive as well as negative) years are increased (Fig. 4a). The annual and JJAS (June, July, August, September) precipitation climatology is shown in figure 4b, to see ISM extent from Holocene to historical period. That clearly justifies the diverse precipitation from LGM to historical (Fig. 4b).