- Research Letter
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Holocene loess in the Himalayas piedmont of southeastern Nepal
Geoscience Letters volume 11, Article number: 26 (2024)
Abstract
Loess, loess-like, sandy loess, and sandy-silty fluvial–aeolian deposits are intercalated to fluvial sediments and lie on a flat surface under intense agricultural land use in Bardibas, Mahottari district, central Nepal. To identify the depositional processes and provenance, we carried out geochemical, mineralogical, SEM, and grain-size analyses. Optically stimulated luminescence (OSL) dating shows the aeolian deposits are from the late Holocene (1.0–4.8 ka). Our results demonstrate that the loess and loess-like deposits (i) were dominantly sourced locally from deflation Quaternary sediments of the Terai Plain; (ii) are related to the composition of Siwalik rocks; and (iii) were deposited during dry periods of weak activity of the Indian Summer Monsoon.
Introduction
For decades, Asian loess deposits in China and the Tibetan Plateau have been used as a main proxy for paleogeographic and paleoclimatic reconstructions (Heller and Liu 1984; Kukla 1987; Pye 1987; Porter 2001; Sun 2002a, b; Lu et al. 2004; Owen et al. 2006; Sun et al. 2007, Jones and Pal 2009 among many others). The most studied locations with loess in the southwest region of the Himalayas are in the Kashmir region and some from small intramountain areas in the Central Himalayas (India) (Pant 1993; Dodonov and Baiguzina 1995; Ahmad and Chandra 2013; Liu et al. 2017; Muhs 2018; Li et al. 2020; Mir et al. 2022). However, loess records from the southeastern flanks and foothills of the Himalayas are less known. To the best of our knowledge, only a couple of articles describe paleosoils and briefly relate them to loess as overlying glacial deposits in the high mountains of eastern Nepal (Guggenberger et al. 1998; Baumler 2001). We are not aware of other published records of Holocene loess from the Himalayan foothills on the Gangetic Plain in Nepal.
Here, we present previously undescribed loess deposits from the Himalayan foothills in central Nepal (Fig. 1). We determine their sedimentological, geochemical, and mineralogical characteristics and their chronology, discuss their provenance, and correlate them with aeolian and other paleoenvironmental proxies in India and Tibet.
a DEM, regional geology of the Himalayas (modified from Dhital 2015), Nepal. b Enlarged Lidar DEM with geomorphologic map of the area showing the Terai Plain and studied localities, loess and loess-like deposits sampling sites in the Bardibas region, central Nepal. Yellow areas of the Terai Plain (T2) are dominantly covered by loess and loess-like deposits
Regional setting
We surveyed the piedmont area around Bardibas city (Fig. 1). The geomorphology of the area is characterized by the Lesser Himalayas to the north, and the widespread alluvial Terai Plain to the south, in the Kosi Gandak Interfan area (Sinha et al. 2014), a sub-unit of the foreland Gangetic morpho-sedimentary plain. The height of the Lesser Himalayas in the study area varies between 700 and 1500 m asl., whereas the average elevation of the plain is ~50–100 m asl.
The southwest summer monsoon moves north-westwards and dominates the current climate of central Nepal, bringing summer rainfalls (June–September), with a mean annual rainfall at Jaleshwar and Tulsi meteorological stations of 1035 mm and 1609 mm, respectively (Khanal et al. 2007). The winter monsoon propagates south-eastwards from October to April, bringing winds from the NW and drier conditions than the summer monsoon.
The Lesser Himalayas are drained by small fluvial catchments that erode the continental clastic rocks of the Siwalik Group (Medlicott 1864; Gansser 1974; Delcaillau 1992; Dhital 2015). The aggradational Southern Terai Plain consists of Pleistocene to Holocene fluvial and alluvial fan deposits of coarse to fine-grained gravel, sandy and muddy deposits (Dhital 2015), and the aeolian deposits described here. The study area is cut across by the active Main Himalayan Frontal Thrust (Patu and Bardibas Thrusts) which tectonically uplifted Quaternary fluvial terraces along the rivers of the Himalayan piedmont zone (Bollinger et al. 2014, Almeida et al. 2018).
Materials and methods
This study involved fieldwork, geomorphologic mapping, sedimentological descriptions, and sampling, optically stimulated luminescence dating, grain size by laser diffraction, geochemical and mineralogical analyses by X-ray fluorescence (XRF) and X-ray diffraction (XRD), and SEM (scanning electron microscope). We present selected results from five locations named Laksmi1, Laksmi downstream, RT4, RT5, and WSOT (Fig. 1). Sixteen samples were collected for grain-size analysis (Appendix 1). Satellite images and Digital Elevation Model (DEM) from the Shuttle Radar Topographic Mission (SRTM) at 90-m resolution were processed for geomorphologic mapping in Global Mapper and QGIS. A detailed 0.5-m high-resolution DEM was surveyed with Airborne Lidar by the Earth Observatory of Singapore from Khayarmara in the west to Puspalpu, with a coverage of 304 km2. We processed both sources, LIDAR and SRTM in the Bardibas area, with an overlap coverage of 91.35 km2 covering the study area.
Optically stimulated luminescence dating
Five loess and loess-like samples were dated for optically stimulated luminescence in the Luminescence Dating Laboratory at the Korea Basic Science Institute, using automated Risø TL/OSL measurement (Table 1). Quartz grains of 90–250 µm were prepared by wet sieving and acid treatments (10% HCl, 10% H2O2, and conc. HF). Equivalent doses were estimated using protocol SAR (Single-Aliquot Regenerative-Dose; Murray and Wintle 2000). For sample RT1-A which has higher Infrared Stimulated Luminescence of Feldspar (IRSL)/Blue light of Quartz signal ratios (> 10%), a double-SAR procedure was carried out to determine their equivalent dose (De) values (Roberts 2007). All dose rates were modified using both present and saturated water contents and attenuation factors (Zimmerman 1971). Cosmic ray contributions were estimated after Prescott and Hutton (1994).
Grain-size analysis
The grain-size distributions of sixteen samples were measured using a Malvern Mastersizer 3000 Hydro EV laser grain-sizer (Fig. 3). The distributions of the different particle sizes were quantified with GRADISTAT (Blott and Pye 2001).
Geochemistry
All samples were heated in an oven at 90 °C for 6 h to remove moisture, and crushed with a swing mill for XRF and XRD analyses. For XRF analysis, powdered samples were mixed with licowax binder to create stable pressed pellets. Major element analyses were carried out using an Energy-dispersive X-ray fluorescence (EDXRF) spectrometer S2 PUMA with the SPECTRA.ELEMENTS software.
Mineralogy
The XRD analyses were conducted using a D2 PHASER X-ray diffractometer, using CuKα radiation, an accelerating voltage of 30 kV, and a beam current of 10 mA. The scan range (2Ѳ) was from 1 to 70° with slit of 0.2 mm for 4 h. Minerals were identified qualitatively using phase identification by comparing the measured pattern of each crystalline phase with the Powder Diffraction File (PDF-2 2019) database. Qualitative and semi-quantitative analyses were carried out with the DIFFRAC.EVA software.
Scanning electron microscopy (SEM)
Eight loess and loess-like sediments have been studied under SEM to observe microtexture (Table 2). Preparation for grain morphology analyses involved 30% hydrogen peroxide (H2O2) and 10% hydrochloric acid (HCl). Samples were then washed with deionized water until the decanted water was clear. Grains were randomly selected under the binocular microscope and were placed on the stubs to get carbon coated. The Leica sputter coater EM ACE600 was used to produce very thin carbon layers. BE and SE images were obtained using JEOL SM-7800F Schottky Field Emission Scanning Electron Microscope. Twenty-five to thirty grains are observed per sample for this study of microtextures following the grain microtexture classification of Vos et al. (2014). The feature of surface grain micro-texture is called abundant when it is present on > 75% of the grains, common (50–75%), sparse (50–5%) and rare (< 5%).
Results
While the deformation of the Himalayan Main Frontal Thrust (MFT) has been typically reconstructed by focusing on morphotectonics and the deformation of fluvial terraces, the presence of beds of Quaternary massive and friable yellowish silt-sized sediments in the Terai plain and fluvial terraces confirm the occurrence of loess, sandy loess, and sandy-silty fluvial–aeolian deposits. The Terai Plain is characterized by a flat silty surface and intense agricultural land use. We regionally identified four terrace levels in the pedemountain rivers, named from T1 (younger) to T4 (older), being the present river floodplains the level T0 (Figs. 1, 2). Previous studies included a fragmentary higher terrace in the MFT belt, bringing the total to five levels (Bollinger et al. 2014). The aeolian deposits, the focus of this article, were identified in river banks outcrops, correlated with Terraces 2 and 1, and on the regional surface of the Terai Plain. They are typical loess and sandy loess deposits, a clastic loose silty sediment with unstratified structure, porous, permeable, erodible, brown in color and with vertical banks.
a Stratigraphic sections of the river terraces with OSL ages. Photographs of Loess deposits covering fluvial deposits in b Laksmi downstream, and c Ratu 5 site
Loess and related aeolian deposits
The composed Laksmi section is located on the Lakshmi River’s bank and exposes terraces T1 and T2 (Figs. 1, 2). Sample Lak 1–1, a silty fine sand from T2, yields an age of 4.8 ± 0.3 ka. The youngest OSL dating sample of a loess (fine sandy-silt sediment) from Laksmi’s section, Lak 2–1 (1.0 ± 0.1 ka), was collected from T1. The total thickness of the aeolian deposits in Lakshmi section (Fig. 2) reach 1.6 m of very fine sandy coarse silt loess (Lak 2.2).The correlative Lakshmi downstream section is composed also of ~ 1.7 m of loess (samples Lak 1-D with 51% silt, Lak 2-D with 64% silt, Fig. 3 and Appendix A.1).
Grain-size distribution curves of loess and loess-like in Bardibas
In the Laksmi Downstream profile (T1), Sample Lak 1-D has a unimodal size of 67 µm with 61% of silt-sized particles. Sample Lak 2-D has a unimodal size of 94 µm with 51% of silt-sized particles. Both samples Lak 1-D and Lak 2-D are defined as coarse primary loess according to Vandenberghe (2013), Vandenberghe et al. (2018) (Fig. 3, Appendix A.1), whereas Lak 2–3 show bimodality of 1 µm and 24 µm. They have 75% and 72% silt-sized particles, respectively.
In Ratu tributary, section RT 5 is from T2, with OSL ages of 5.8 ± 0.6 (RT1) for loessic fine sandy silt, 4.0 ± 0.2 (RT1-A) for very fine sand, and 2.6 ± 0.3 ka (RT2) for a primary loess with 62% of silt, (RT5a2-SAS) from the lower, middle and upper parts of the section, respectively (Fig. 2). In Ratu RT5 profile (T3), samples RT5 1st, RT 5 MLS, and RT5 LUS show unimodal grain-size distributions, with a modal size of more than 134 µm. They are fine to very fine sands, with 12–29% of silt-sized particles. Sample RT5a-2 SAS from RT5 profile shows polymodal sizes of 7.5 µm, 67 µm, and 267 µm with 62% of silt-sized particles.
In RT4 profile (T2), samples RT4 Loess1 and RT4 SL3 show unimodal size distribution ranging from 119 µm and 106 µm, respectively, with less than 50% silt-size particles. Sample RT4u30 from RT4 profile indicates trimodal sizes of 1 µm, 7 µm, and 150 µm with 55% of silt-sized particles. These sediments fall into the coarsest loess sediment group.
Importantly, widespread loess is blanketing the T2 surface that regionally dominates the Terai Plain (Fig. 1). The sample WSOT from that surface shows a primary loess population with a unimodal size of 53 µm, and 72% of silt-sized particles, and a modal grain size in the medium- to coarse-grained silt fraction. It is relevant because demonstrates that loess is even purer on the plain and becomes sandier when deposited closer to the fluvial belts. It has to be considered that sandy loess are derived from some proximal fine sand source such as fluvial deposits, sand dunes, coastal areas, piedmont areas, glaci-fluvial deposits, among others, and were recorded worldwide (Pye 1995; Ding et al. 1999; Zárate 2003; Scull and Schaetzl 2011; Vandenberghe 2013; Li et al. 2017; Costantini et al. 2018; among others). The presence of sandy loess close to the fluvial belts of the Terui Plain is a consequence of fluvial aeolian interactions in the Himalayas piedmont where loess deposition is widespread, but the additional source of fine sands from the fluvial systems occurs
Similarly to typical loess from other regions of the planet, the Bardibas loess and loess-like sediments are rich in SiO2, which ranges from 78.37% to 93.84% (Appendix A.2). Al2O3 ranges from 5.09% to 12.3%, Fe2O3 ranges from 0.95 to 3.94%, K2O ranges from 0.75% to 1.95%, and MgO ranges from 0.34% to 1.67%. Na2O, CaO, TiO2, and P2O5 are present only in minor (< 1%) amounts.
The XRD analyses on bulk samples of loess indicate that quartz, mica (muscovite and illite), and potassium feldspar (microcline, orthoclase, and sanidine) are the dominant minerals, comprising more than 90% of the total (Appendix A.3.). The remaining minerals include kaolinite, plagioclase (albite), and carbonate minerals (calcite, dolomite, and aragonite). The average mineralogical composition is 84.7% quartz, 5.4% potassium feldspar, 7.4% mica, 0.37% kaolinite, 0.9% plagioclase, and 0.22% carbonate minerals.
Discussion
Grain-size analysis concerning provenance
Based on the grain-size analysis, the aeolian deposits consist of primary loess, sandy loess, and to a minor degree local secondary loess (sandwiched loess beds in between alluvial beds).
Loess sediment with a modal grain size in the fine-sand modal fraction is possibly derived from a local sandy source between several hundred meters to a few kilometers away (Enzel et al. 2010; Vandenberghe 2013). However, loess particles in the medium- to coarse-grained silt modal fraction could have been transported across tens of kilometers (Vandenberghe 2013). Thus, primary loess covers the Terai Plain where the silt fraction can reach up to 72%. Dust particles transported across larger distances are characterized by the fine-silt and clay fraction (between 4 μm and 22 μm) (Fig. 3). This finer loess modal fraction is recorded only as a sub-population in secondary loess (reworked or mixed, loess-derived sediment). These polymodal grain-size distributions indicate an imprint of long-distance deposition of loess affected by (secondary) post-depositional processes of local sandy supply (fluvial and aeolian) and reworking by rivers or surface runoff (Vandenberghe 2013; Vandenberghe et al. 2018). It suggests that sediment is likely to experience cycling episodic transportation and deposition prior to dominant entrainment and deflation by wind.
At least one billion tons of siliciclastic detritus are currently produced annually by erosion of the active Himalayan range from 380 × 106 ty−1 to 480 × 106 ty−1 (Lupker et al. 2012; Lupker et al. 2013; Borromeo et al. 2019). The area has been sourced and acted as a trapping zone of fluvial sediments since the Pleistocene when large megafans developed in the area, and the Kosi megafans are still active. The vast Kosi and Gandak megafans (Mohindra and Parkash 1994; Sinha et al. 2014), and particularly the vast alluvial interfans area of the Tepui plain are possibly the main proximal sources of the Holocene loess in our study area.
Furthermore, fine-grained sediments (clay and perhaps silts) deposited during the Holocene in the Ganga Basin along the Ganga river system to the Bay of Bengal are also a possible secondary source for the silt and clay fraction although further analyses are required to confirm it.
Mineralogy, major element content, and SEM, regarding weathering and provenance
Beyond local–regional sources, there are other potential sources to be considered for the loess in our study, from the furthest to closest: Chinese Loess Plateau, Kashmir Valley, and, as discussed above, the Himalayan foreland fluvial systems of the Ganges River Basin. To interpret provenance and its weathering effect, we carried out bulk mineralogy and chemical analyses.
For comparison, we plotted the concentrations of the major elements that represent clay, carbonates, feldspars, and micas from the Chinese Loess Plateau, Kashmir Valley, and this study loess deposits. Loess in the study area contains less abundant clay minerals (Fig. 4a) and lacks a significant amount of carbonate minerals (Fig. 4b), and fewer feldspars and micas than in the other potential source regions (Fig. 4c). The predominant quartz and SiO2 content in the Bardibas samples indicates quartz-rich source rocks (Fig. 4b; Appendix A.3). The loess and loess-like deposits of our study are clustered differently than other characteristic loessic areas, indicating specific characteristics and different regional sources.
Plots of major elements concentrations indicatives of a clay minerals, b carbonate minerals, and c feldspars and micas from loess deposits in Chinese Loess Plateau, Kashmir Valley and this study. Data sources for the Chinese Loess Plateau from Muhs (2018) and Kashmir Valley from Ahmad and Chandra (2013)
The lack of correlation with distant sources indicates that the most probable are regional source areas of the lesser Himalayas and foothills areas where the Siwalik rocks and derived Quaternary alluvial proximal piedmont deposits are widespread. Thus, we compared the mineralogy of the Siwaliks rocks with the bulk mineralogy composition of loess and loess-like sediments in Bardibas.
Petrographically, the Siwaliks sedimentary rocks are predominantly quartzolithic and include monocrystalline to fine-grained polycrystalline quartz, followed by potassium feldspar, plagioclase, and common to rare lithic fragments comprised gneiss, schist, limestone, dolomite, chert, shale, crystalline quartzite, and volcanic rocks (Ranjan and Banerjee 2009; Tamrakar and Syangbo 2014). Muscovite and biotite are scarce in the Lower Siwaliks but increase toward the Upper Siwaliks (Tokuoka et al. 1988; Tamrakar and Syangbo 2014). The Siwaliks mudstone is composed of illite, chlorite, smectite, sepiolite, and mixed clays (Chaudhri and Gill 1983; Raiverman and Suresh 1997; Suresh et al. 2004).
The ternary plot of bulk clay minerals suggests a low kaolinite/ilite ratio. Indicates dominant physical erosion and weak chemical weathering (Fig. 5a; e.g., Liu et al. 2007, Liu et al. 2012; Chen et al. 2017; Sang et al. 2018). Potassium feldspar is found as the main component in the Siwaliks rocks and the Bardibas loess and loess-like sediments. Muscovite is probably derived from the middle and upper parts of the Siwaliks rocks and/or from the physical erosion of metamorphic lithic fragments. Illite may have been formed by weathering of potassium feldspar under moderate hydrolysis conditions, degradation of mica, and/or physical erosion of metamorphic and granitic lithic fragments (Chamley 1989; Thiry 2000; Liu et al. 2007).
a Ternary diagram comparing the relative abundance of Quartz–Feldspar–Calcite (Q–F–C) between the loess and loess-like deposits from Tibet, Loess Plateau (from Sun et al. 2007), and this study, and b clay minerals of kaolinite–illite + chlorite–smectite (K–IC–S) in loess and loess-like deposits from Bardibas
Bulk geochemical analyses were carried out to examine chemical signatures of the source terrain of loess and loess-like deposits. The geochemical signature provides information on the complex geologic interaction of several variables such as source terrain composition, weathering environment, transport, and diagenesis (McLennan 1993).
Grain size correlates with the Al2O3 and SiO2, which Al2O3 increases in finer sediments (primary loess silt) and SiO2 with sand content (Fig. 6). SiO2 shows a negative correlation with Al2O3 (R2 = 0.98) and TiO2 (R2 = 0.92) suggesting that SiO2 is mainly present as quartz grains. A strong positive correlation of TiO2 with Al2O3 (R2 = 0.93), Fe2O3 (R2 = 0.92), and MnO (R2 = 0.6) indicates the occurrence of TiO2 in the clays. A positive correlation of CaO to TiO2 (R2 = 0.67) and Al2O3 (R2 = 0.67) suggests the presence of carbonate minerals. The Al2O3 correlates positively with K2O (R2 = 0.95), Na2O (R2 = 0.7), and MgO (R2 = 0.91) suggesting that the concentrations of the K-bearing minerals have a significant influence on aluminum distribution (McLennan et al. 1983; Jin et al. 2006; Babeesh et al. 2017). A lower ratio of K2O/Al2O3 indicates a predominance of clay minerals over K-bearing minerals (Cox et al. 1995; Ahmad and Chandra 2013). The calculated Index of compositional variation for all samples shows values < 1 (from 0.5 to 0.79, average 0.67) pointing to clay minerals such as kaolinite, illite, and muscovite (Cox et al. 1995; Moosavirad et al. 2011; Babeesh et al. 2017).
Scatter plots of Bardibas loess and loess-like deposits major oxides and Al2O3 percentage
Constraining at the regional al scale, we compared our results with the Fe/Si, Na/Si, and K/Si ratios and the Al/Si ratio from the river sediments covering the Ganga basin rocks (Fig. 7a–c; Lupker et al. 2012). All elements, except Na, show a systematic positive correlation with Al/Si. This positive correlation suggests the concentration of Fe-bearing (e.g., biotite, sheet silicate iron clay, and Fe-hydrox.) and K-bearing minerals (such as illite and muscovite). The K and Na ratio indicates the relative abundance of potassium feldspar to plagioclase.
a Fe/Si, b K/Si, and c Na/Si ratios of the main Ganga rivers system and this study. The chemical compositions of the Ganga River system are from Lupker et al. (2012)
The chemical compositions of the loess and like-loess sediments also were compared to the Neogene Siwalik rocks by plotting the molar proportions of Al2O3, CaO* + Na2O, K2O (A-CN-K) and Al2O3, CaO* + Na2O + K2O, FeO + MgO (A-CNK-FM), where CaO* represents Ca in silicate-bearing minerals only (Nesbitt and Young 1982). The bulk composition plots on the A-CN-K and A-CNK-FM diagrams show that all fall onto one cluster (Fig. 8). A tendency of aeolian samples distribution to the left from the Siwaliks rocks (Fig. 8) suggests progressive processes of weathering, as the bulk composition evolves toward the A-K line. CaO* + Na2O is removed in preference to K2O. It results in a trend that is subparallel to the left boundary, which suggests that Ca and Na are taken up by illite and muscovite (Fig. 8).
The geochemical results point out that the Siwaliks rocks are the most plausible parent rocks for Bardibas loess and loess-like deposits.
We additionally estimated the degree of weathering by applying the Chemical Index of Alteration (CIA; Nesbitt and Young 1982) and Chemical Index of Weathering (CIW; Harnois 1988) (Table 3). CIA values range from 73 to 84 (average 78; Appendix A.2) similar to the CIA value of the Neogene Siwalik mudstones (average 75; Sinha et al. 2007) and sandstones (average 79; Ranjan and Banerjee 2009). The CIW value ranges from 84.8 to 97.5 (average 92, Appendix A.2) and is comparable to the Siwalik mudstones (average 88; Sinha et al. 2007) and sandstones (average 90; Ranjan and Banerjee 2009). Insignificant changes in CIA and CIW values suggest recycling or re-sedimentation of the possible Siwalik source into the Terai Plain without notable chemical weathering. The presence of clay minerals such as illite, muscovite, and kaolinite is characterized by a low value of the index of compositional variation (ICV from 0.5 to 0.79, average 0.67; Cox et al. 1995; Moosavirad et al. 2011; Babeesh et al. 2017).
In conclusion, the mineralogical and geochemical of the loess and like loess deposits and the regional geochemical and mineralogical data from the Siwalik sediments (Chaudhri and Gill 1983; Tokuoka et al. 1988; Raiverman and Suresh 1997; Suresh et al. 2004; Ranjan and Banerjee 2009; Tamrakar and Syangbo 2014) points the Siwalik Group as the most plausible parental rocks.
SEM—scanning electron microscope
In terms of loess genesis, there is still considerable debate about the specific processes whereby silt particles are generated before they are entrained by the wind (Muhs 2018). To address this issue we identified microtextures on grain surfaces using SEM (Table 2). Grain surface microstructures and morphology provide an insight into the sedimentary history and cycle of clastic sediments in different depositional environments (Velbel 2007; Vos et al. 2014).
In accordance with the geochemical result, grains of the loess and loess-like deposits are dominated by quartz grains that are typically sub-angular to sub-rounded, indicating that the edges and corners had been eroded by particle collisions during long transport or multicycle processes. Collision pits, such as dish-like and crescent-like depressions are usually formed as a result of high-speed impact among well-rounded particles and are the most typical features of quartz grain in aeolian sediment (Fig. 9a, Chen et al. 1986; Wang et al. 2018, 2019). In addition, upturned plate cleavage was also observed (Fig. 9b). Chemical weathering is indicated by pits, grooves, and holes on the grain surfaces (Fig. 9d).
SEM images showing a Sample Lak2-2: crescentic percussion marks, triangular etching pattern, and adhering particles. B Sample RT5-a2: (b) upturned plate, flat cleavage surface, straight steps and meandering ridges. c And (d) sample Lak2D: conchoidal fracture, arcuate and straight steps, adhering particles and solution pits
Loess and loess-like sediments chronology
The OSL ages are evidence that the Bardibas primary loess began to deposit at ca. 4.8 ka, and continued accumulating up to ca. 1 ka (Table 1). The age range correlates with the last recorded phase of loess deposition in intra-mountains basins of the Central Himalayas in the India territory (Pant 1993), ~ 700 km to the ENE of our study area, and the record of near 40 cm of loess deposition at ca. 2.5 ka in southern Zanskar range, northwestern Himalaya, approximately 1200km far away from our study area (Lone et al. 2023). The chronology is also correlative with dune building in the Thar Desert between 4.5 ka and 3.1 ka (Thomas et al. 1999). Additionally, the chronology is supported by a sharp drop in temperature and rainfall recorded around 4 ka in the Central Himalayas (Phadtare 2000). It overlaps with the decrease in precipitation from ~ 4 ky–3 ka linked to the deterioration and collapse of the Harappan/ Indus civilization and Ghaggar culture in northern India and the decrease in precipitation recorded in speleothems records (Kotlia et al. 2015; Leipe et al. 2014; Tripathi et al. 2004). Regarding loess records in Nepal, a couple of meters to less than a meter of loess ca. 3.0–4.0 ka were described along the Imja Drangka valley, in the Khumbu Himal high mountains, with thickness decreasing with elevation (Baumler 2001).
Despite the long distances and the orographic barrier represented by the Himalayas, there are potential correlations between the proxy records from the northern flank of the Himalayas and the Tibetan Plateau. Drought events correlate with loess units at ca. 5.8–6.8 ka in eastern Tibet (Lehmkuhl et al. 2014), while fluvial–eolian loess deposition at ca. 2.6 ka is synchronous with several lake and peat records from southeastern Tibet, pointing to a shift to a cold-dry climate at ca 3 ka to 2.5 ka (Shen 2003; Shen et al. 2006; Zhou et al. 2010). In the northeastern Tibetan Plateau, sand dunes have also been reactivated between 3 ka and the present (Stauch et al. 2012). A drier climate in the late Holocene is also well documented in many other records in monsoonal China and central Asia, and it is interpreted as a result of a decrease in monsoon intensity (An et al. 2000; Herzschuh et al. 2006; Zhao et al. 2009; Zhao and Yu 2012; Zhang et al. 2015). A decreasing energy of solar insolation in the area has been proposed (20°N to 35°N) from 5 to 1 ka (Yuan et al. 2004; Bird et al. 2014; Ramisch et al. 2016; Wang et al. 2019; Huang et al. 2020; Banerji et al. 2020) which might be related to monsoon weakening. In addition, Rashid et al. (2007) also reported a weak Indian Summer Monsoon phase after 5.5 ka in the Andaman Sea, and a significant weakening of ISM was also reported in North and northeast India (28°N to 36°N) ~ 4 ka (Berkelhammer et al. 2012; Bhushan et al. 2018; Banerji et al. 2020). The records reviewed above suggest phases of weakened summer monsoon with a drier and dustier climate in Southern Asia and Tibet during the late Holocene.
Currently and locally, dust storms commonly take place in Nepal during winters, with the Himalayas acting as a barrier to winds, and deflecting wind direction. Thus, we caution against a simplistic correlation of loess or other paleoenvironmental climatic proxy records on both flanks of the Himalayas. For example, the Nepal loess from this study has significantly higher amounts of quartz and lower amounts of calcite compared to loess samples from other regions in Asia (Fig. 4), which points to different provenances of the Bardibas loess and the Tibet and the Chinese Loess Plateau.
We suggest that the winter monsoon played a major role in the deposition of loess between 4.8 ka and 1 ka. Currently, the wind system is best developed during the dry season, which is typically October through May, with localized and strong mountain–valley wind circulations (Ramage 1971; Ohata et al. 1981). In comparison, winds are quite weak, with gusts at their minimum strength, particularly in August during the summer monsoon (Tartari et al. 1998). Although the data on the present winds patterns in the Lesser Himalayas foothills are scarce, near-surface winds blow mainly from the northwest (N330°E) to the east and northeast (N30°E–N110°E) (https://earth.nullschool.net). The average wind speed is less than 4 m/s during winter high-wind system and secondary local circulation piedmont winds (Laudari et al. 2018; https://earth.nullschool.net).
Final remarks
This study reports the first record of loess and loess-like deposition in eastern Nepal and presents integrated results of sedimentology, geochemistry, geochronology, mineralogy, and SEM. The late Holocene (4.8–1 ka) loess and related deposits in the Bardibas region were dominantly sourced locally from deflation Quaternary sediments of the Terai Plain which spread on the Kosi Gandak interfan area. The mineralogy, geochemistry, and SEM data of loess and loess-like deposits from the Bardibas area of the Terai plains indicate that these were derived from moderately weathered regional sedimentary source regions, and were subjected to a very low degree of chemical weathering after deposition. The high tectonic instability in the source area and geomorphology with cycles of aggradation and degradation by the fluvial systems in the Himalayas foothills did not favor intense and long-term chemical weathering of piedmont deposits. The mineralogical and geochemical compositions of the loess and loess-like deposits are related to those of the Siwalik rocks and those re-transported materials from rivers draining the Lesser Himalayas. The late Holocene loess and loess-like sediments in the Himalayan piedmont of Nepal were deposited during periods of weak activity of the Indian Summer Monsoon. Once entrained, this material would have been regionally carried in easterly and north-easterly directions, with the coarser material being deposited closer to the source area while the very fine material was carried out of the system in a long range of suspension. Specifically at the study area, the Terai alluvial plain sediments, sourced by fluvial catchments eroding the Siwalik Group in the Lesser Himalayas, were reworked by surface winds, probably with a northeast- and eastwards direction, during the winter monsoon. The role of the summer monsoon and the limited remote sources of fine dust can also be considered, but this needs further analysis. The chronologies of the loess and aeolian-related deposits in the Bardibas area correlate with thin intermountain loess deposits from the Central Himalayas in India.
Availability of data and materials
Additional lab data are provided in a supplementary file.
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Acknowledgements
The authors want to thank R. Almeida, M. Hamahashi, and L. Thu for their collaboration during fieldwork surveys, J. Hubbard for helpful discussions, and P. Adamek for his assistance with the manuscript style. We acknowledge Jeong-Heon Choi and Hou Ran for their OSL and XRF contributions.
Funding
This study was supported by the National Research Foundation Singapore under its Singapore NRF Fellowship scheme (National Research Fellow Award No. NRF-NRFF2013-06), and by the EOS and the National Research Foundation Singapore and the Singapore Ministry of Education under the Research Centres of Excellence initiative.
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Edgardo M. Latrubesse (EML) conducted fieldwork in Nepal, discovered the loess deposits, collected the samples for sedimentology, mineralogy, and OSL, and analyzed and mapped the geomorphology and stratigraphy of the region. Abang Nugraha (AN) conducted Remote sensing processing of SRTM and LIDAR, the XRD-XRF, grain-size analyses, and SEM, and prepared the figures. Both authors participated in the preparation of the manuscript, discussions, and interpretation of the data.
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Supplementary Information
Supplementary Material 1: Appendix A.1.
Grain size statistics of loess and loess-like deposits in Bardibas area. Appendix A.2. Major oxides of loess and loess-like deposits in Bardibas area. Appendix A.3. Major mineralogy of loess and loess-like deposits in Bardibas area.
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