Monitoring hydroecology and climatic variability since ~4.6 ka from palynological,sedimentological and environmental perspectives in an Ox-bow lake,Central Ganga Plain,India

Abstract

The Ganga-Sai River Interfluve contains several ox-bow lakes in the fertile Central Ganga plains (CGP). A ~2.20-meter deep sedimentary profile obtained near the Chandra Shekhar Azad bird sanctuary (Nawabganj lake-NL) of the CGP was studied to understand the evolution of the ecosystem and climate using pollen/spores, diatoms, testate amoebae, environmental magnetic data,and carbon and nitrogen isotopes. This sedimentary profile is chronologically well-constrained by five radiocarbon (¹⁴C) dates. Between 4.6 and 4.4 ka, the sandy sediment and pollen evidence for riparian forest, the absence of aquatic pollen and sponge spicules suggest scant water in the vicinity through the river channel. A semi-closed fluvial ecosystem between 4.4 and 4.2 ka is indicated by testate amoebae, sponge spicules and arboreal pollen. At least two intermittent warm conditions prevailed between 4.6 and 4.2 ka. Between 4.2 and 2.8 ka, high aquatic pollen, diatoms and testate amoebae indicate a lake ecosystem. By ~2.8–0.9 ka the gammoscleres from sponges formed during dry seasons indicate recharging during monsoon as the river shifted. Thereafter, agricultural pollen (Brassica and Apiaceae) indicates a further shift in the lake boundary exposing land. The highly sandy texture, fluctuating δ¹³C, δ¹⁵N and magnetic mineral values indicate an unstable fluvio-lacustrine deposition inducing hydroecological changes influenced by intermittent about 5–6 humid and dry climatic conditions since ~4.6 ka to present. The calcrete layer in the bottom sediments shows high aridity in CGP between ~5 and 4.6 ka reaching the climax cold-dry event of ~4.2 ka recorded worldwide. The spectral analysis of palynological data from NL and the contemporary Barela Lake, reveals de Vries and Gleissberg cycles of low and high solar irradiance at centennial to multi-centennial scale during the Holocene. The impact on vegetation, sediment depositional dynamics, and shift in river channel was more rapid showing the dominance of ~200 years. periodicity post ~5 ka as compared to ~300 years of dominance prior to this. This centennial timescale is of great speculation for future climate predictions in CGP coupled with the anthropogenic forcings.

Keywords

Central Ganga Plain climate India Ox-bow Lake palynology sediment

Introduction

India occupies about 56.9% of the total geographical area of the fertile alluvial Indo-Ganges basin (Figure 1a),a much larger area compared to other neighbouring countries, for instance, Bangladesh, Bhutan, Nepal, Pakistan, Afghanistan and China. The Ganges and Brahmaputra river systems form the major part of the Indo-Ganges Basin in the north of the Indian sub-continent and the source of water is predominantly through Indian summer monsoon (ISM) or South West Monsoon (SWM) and snow-melt in the Himalayan Mountains (Lutz et al., 2014). The magnitude, intensity and duration of the ISM in India and adjoining countries that fall on the Indo-Ganges Basin largely governs its societal, agricultural and economic prosperity (Attri and Tyagi, 2010). Climatic modelling with respect to the ISM, demands reliable records of the long-term spatio-temporal climate variability and its periodicity, especially focusing on the evolution of the agricultural ecosystem conducive to a sustainable future (Mishra, 2015; Molnar et al., 2010). Although the ISM is a regional phenomenon, its link to global- climate variability has been well evaluated (Sinha et al., 2007; Trenberth et al., 2000). Seasonal shifts in the Intertropical Convergence Zone(ITCZ) and temperature variability of the equatorial region have been perceived as phenomenon influencing global climate (Bordoni and Schneider, 2008; Chao and Chen, 2001; Privé and Plumb, 2007). The ITCZ dynamics are associated with the spatio-temporal variability in precipitation over the Indian region which influences the ISM (Gadgil, 2018; Hari et al., 2020; Schneider et al., 2014).

Figure 1.

(a) The land cover showing the Indo-Ganges Basin, the location of Rivers Ganga, Sai, Gomti and the study site. (b) Ox-bow lakes in Sai interfluve. (c) Map showing vegetation and land use in the vicinity of the study site between Ganga-Sai River interfluves. (d) The present-day Nawabganj Lake, Anthropogenically developed Vegetation cover and the location for sampling a 2.20 m deep trench.

Numerous ox-bow lakes, ponds and fluvial channels were formed mostly during the Late Quaternary in the Ganga plain (Singh, 1996) and host very good sedimentary archives which can be used to understand regional trends in climate, landscape changes and paleochannels. Past climate variability and its various manifestations both in terms of magnitude and periodicity can provide clues for plausible future changes along with anthropogenic forcings. Until now several sedimentary archives have been studied from the Ganga Plain (Misra et al., 2020; Saxena and Trivedi, 2017; Thakur et al., 2018; Tripathi et al., 2017). However, there is still a paucity of climatic data for the Late-Holocene (Meghalayan Age- 4.2 ka to present) in the river interfluve region. These floodplains receive the SWM from July-August and the North East monsoon (NEM) from December-January (although with fluctuating intensity) which supports agriculture (Sinha and Sarkar, 2009). Annually, the rivers are highly influenced by the intensity of SWM bringing about large-scale floods every year inducing enormous changes in sediment deposition influencing landscape alterations in time and space. The landscape change is influenced by the course of the rivers and their tributaries playing a major role in inducing changes in the seasonal hydrology of both lentic and lotic water bodies providing variable records for the advent of agricultural activity in this fertile Ganga plain.

The palynological record from the Ganga-Sai River interfluves has been published recently using the sedimentary archives in Barela Tal (Lake) which is also an Ox-bow Lake of the Sai River formed around 10 ka (Misra et al., 2020). The biotic and abiotic proxies from Barela Lake have provided insights into the changes in ISM in the CGP between ~14 to ~5 ka. Being an agricultural site, the top 1.5 m sediments deposited after ~5 ka were disturbed by ploughing and therefore, discarded for the study. The signatures of pollen, algae, testate amoebae, sponge spicules, environmental magnetic data, and Carbon and Nitrogen stable isotope data were used to infer palaeohydrology and paleoclimate from Barela Lake. Here, a weakened ISM was recorded with active NEM prior to 10 ka. Although pollen was scant, an excellent assemblage of diatoms and other algae provided clues to past climate dynamics in the region with reference to both SWM and NEM. Several other sedimentary profiles have been explored through palynology and other multi-proxy data in the Gangetic Plain to understand shifts in past vegetation and the shrinking of lake boundaries in response to a weakened monsoon after ~5 ka, shown by the diversification in deciduous trees (Chauhan, 2015; Saxena et al., 2015; Tripathi et al., 2017; Trivedi et al., 2013). The strengthened SWM rainfall was associated with the dynamics of the Inter Tropical Convergence Zone (ITCZ) (Hari et al., 2020). Rapid intensification of the ISM between 12 and 8.3 ka and weakening around 8.2 and 4.5 ka with the onset of aridity has been documented from eastern central Himalayas (Kotlia et al., 2015) and northeastern Himalayas (Kathayat et al., 2018). Studies carried out in lake sediments from the Gangetic Plain show different time periods for Ox-bow lake formation in contemporary areas for example, 10 ka for Barela lake (Misra et al., 2020), ~13 ka for Karela Lake (Tripathi et al., 2017), between ~8 and 9 ka from Jalesar lake (Trivedi et al., 2013) and the biotic and abiotic multi- proxy records provide information of lake expansion or shrinking in relation to Holocene climate dynamics (Misra et al., 2020 and references therein). However, in terms of the high resolution study of the Late-Holocene (Meghalayan Age-4.2 ka onwards) the vegetation data is scant, particularly during the global arid event of 4.2 ka.

Both biotic and abiotic proxies provide reliable signatures of climate and the depositional environment. The relatively high presence of pollen grains of evergreen and moist deciduous trees in sediments indicates low seasonality whereas higher abundances of dry deciduous trees indicate high seasonality in the region. The relatively high abundance of aquatic pollen and algal remains indicate the lentic status of the ecosystem. Testate amoebae species are microscopic heterotrophs that are indicators of aquatic ecosystems and moist terrestrial habitats. Freshwater sponges are very sensitive to ecological fluctuations and their siliceous skeletons preserved in sediments help to understand past environmental conditions (Kürten et al., 2013; Parolin et al., 2008). Gammoscleres help in the identification of sponge species, these are siliceous skeletons of reproductive gammules formed generally during stressed (dry) conditions, otherwise, vegetative propagation is dominant in clear and slow-flowing water. Increases or decreases of gammoscleres in sediments provide clues to the scarcity or abundance of water in the ecosystem. There has been a single study from the Ganga plains using sponge spicules for Holocene climatic interpretations (Tripathi et al., 2017). Sponge spicules have been recovered in Pleistocene sediments from the Kashmir valley from the fluvio-lacustrine to the lacustrine ecosystem of Karewa Lake (Farooqui et al., 2021). Every biogenic material is characterized by its own dynamic stable isotope fingerprint (Wada, 2009). The fluctuating stable isotope values in sediment organic matter suggest that δ¹³C and δ¹⁵N reflect shifts in trophic states (Brenner et al., 1999). Environmental magnetic data relates to the magnetic properties of the mineral in sediment and its transport either in present conditions or in the geological past which varies due to climate or other environmental influences on minerals and therefore, are sensitive recorders of climate and sediment transport (Maher, 2007).

The periodicity of climate change in time and space, particularly at the centennial scale is of importance for future predictions and sustainable strategies for agriculture. One of the external forcings inducing climate variability during the Holocene is the precession of the Earth’s orbit around the sun (Maher and Hu, 2006). In recent years, millennial to multi-millennial and centennial monsoon variability is modulated by the incoming solar radiation, inter-hemispheric heat transport and high latitude glacial status (Bond et al., 2001; Clemens et al., 1991; Kutzbach et al., 1993; Sinha et al., 2005). Climate rythmicities include the solar 50–100 Gleissberg-Yoshimura cycle, 120 years cycle, the 200 years solar deVries-Suess cycle and the millennial eddy cycle of the 2000–2500 year cycles (Dima and Lohmann, 2010; Kanda, 1933; Konecky et al., 2013; Obrochta et al., 2012).

In this study, we aim to use the multi-proxy data obtained from Barela Lake between ~14 -5 ka (Misra et al., 2020) in addition to new data generated from the contemporary Nawabganj lake in the Ganga-Sai interfluve region to obtain climate periodicities. To achieve this we aim to infer a continuous record of the vegetation- climate relationship, climate cyclicity and the dynamics of the riverine system and Ox-bow lakes influencing biota in the region since ~14 ka to the present. The study provides useful clues to predict future climate periodicity and monsoonal patterns in the fertile land of Ganga plains coupled with anthropogenic forcings in the near future.

Study area

The Indo-Ganges Basin (Figure 1a) in the Himalayan foreland represents one of the largest fertile alluvial plains in the World. The sedimentary profile from the Ganga-Sai River interfluve (Figure 1b) is one of the oldest geomorphic surfaces in the CGP at an average altitude of ~125 m above sea level (masl) (Singh, 1996; Srivastava et al., 2003). The Sai River is also referred to as Aadi Ganga (oldest Ganges) and is presently serving as a tributary of the Gomti River in the state of Uttar Pradesh, India. The Sai River has a unique meandering channel system (Figure 1b) comprising several sinuous abandoned channels with big to small-scale meander cut-offs forming present-day closed lake ecosystems. These water bodies support the vegetation and agriculture in the region (Figure 1c). The origin and existence of these lakes differ in time periods with respect to shifts in the river channels (Singh, 1996; Srivastava et al., 2003; Tripathi et al., 2017). The distinct sediment depositional domains recognized are silt, clayey silt along with shells, variegated silty sand/clayey silt and calcretized silt. In general, the overall stratigraphic succession in several ponds/lakes reveals a sandy micaceous layer overlain by silt since 9–8 ka which is due to disruption of water channels, sediment depositional environment of rivers/streams and by the post-tectonic activities in the Ganga plain which is also observed in the study area (Singh, 1996, 2001; Srivastava et al., 2003).

The study site is in the peripheral dried part of the Chandra Shekhar Azad Bird Sanctuary which is an ox-bow lake commonly known as Nawabganj Lake (NL) in the Ganga-Sai River interfluve situated at latitude 26.62260 and longitude 80.67018 (Figure 1b and c). The climate is characterized by extreme changes between winters and summers with annual winter temperatures dipping to ~7°C and summer temperatures rising up to ~48°C. Most of the rainfall is during the South West Monsoon (July-August) followed by North East Monsoon (NEM) in winter (December–January) for a short duration. As a result, the study area falls under the high seasonality zone and the vegetation is dry deciduous to moist deciduous type. These are Barringtonia acutangula in swampy areas along with Caesalpinia, Morus alba, Azadirachta, Terminalia, Pongamia, etc. (Sinha and Shukla, 2021; Tripathi et al., 2022). The lake is a shallow, static marshy/swampy that harbours about 111 species belonging to 67 genera and 40 families of aquatic/terrestrial plant diversity (Garg and Joshi, 2015; Narain and Kumar, 2008) and is home to more than 250 migratory bird species that flock to this area from northern higher latitudes every year between November and March (Rahmani, 1992) and is conserved as a bird sanctuary (migratory) by the Forest Department, Government of India. The water depth in the NL varies from 50 cm in the peripheral area to more than a meter in the deepest part. In the vicinity of the lake, there are the vegetated highlands, some of which were made artificially for the nesting of migratory birds. The common aquatic plants present in the lake are Ceratophyllum, Chara, Eichhornia, Hydrilla, Lemna, Najus, Nelumbo, Nymphaea, Pistia, Polygonum, Potamogeton, Sagittaria, Spirodella, Vallisneria, Wolffia. Marginal species present are Typha, Brachiaria and Scirpus. The sediment profile beneath the lake and surrounding areas show calcium–rich duricrust (~50–100 cm thick layer) commonly known as calcrete (Calcium Carbonate) at 1–2 m depth from the ground surface. Calcrete is a hardened layer formed as it precipitates in and on the soil as a result of climatic fluctuations in arid and semi-arid regions. The calcrete was formed when the soil was exposed to evaporation for a certain period of time in response to the dry climatic phase in the studied site at the beginning of the Late Holocene (~5 ka). Subsequently, the monsoon-driven river channel became active and gradually an Ox-bow lake was formed, At present, the remains of this exist as Nawabganj Lake (NL) which is 500 m away from the study site.

Materials and method

A 2.20m deep trench was dug very near to the present-day NL surrounded by mixed deciduous forest cover and agricultural land (Figure 1d). The uppermost ~10 cm surface samples were discarded on account of anthropogenic disturbances. The percentage of sand, silt and clay in the sedimentary profile was obtained by the density method (USDA-NRCS, 1997). For the chronology, five bulk samples enriched in organic carbon content were used for radiocarbon (¹⁴C) analysis in the Birbal Sahni Institute of Palaeosciences Lucknow, India (Supplemental Table 1). The sediments were manually sieved and treated with hydrochloric acid for the removal of carbonates. Using standard procedures and catalysts, extracted carbon dioxide was collected and converted to acetylene and then finally to liquid benzene. A Liquid Scintillation Counter (Quantulus-1220) was used for counting the decay of extracted benzene. Calibrated ages (cal yr. BP) (2σ ranges) (Supplemental Table I) were computed using IntCal20 (Reimer et al., 2020). The calibrated ages versus the sediment depth were used to create the Age-Depth Model (Figure 2) using the ‘CLAM’ extension of R studio version clam 2.3.2 (Blaauw, 2010).

Figure 2.

Age-depth model of the sedimentary profile from Nawabganj Lake.

Palynology

Ten grams of air-dried sediments were dissolved in distilled water at every 5 cm interval of the sedimentary profile(Figure 3). About 5–10 pellets of potassium hydroxide (KOH) were added and boiled for 5 min. After cooling, it was sieved through 150 mesh size (105 µm pore size). The filtrate was retained to settle overnight in a cool place. The supernatant was discarded and the rest of the sediment aliquot was divided into two parts. One part of the aliquot was treated with 40% Hydrofluoric acid (HF) to remove sand particles (silica). Later, the aliquot was centrifuged to remove excess HF. The sediments were then treated in series with glacial acetic acid and a mixture of anhydrous acetic acid and sulphuric acid (9:1) following Erdtman (1943). The processed samples were then sieved through 650 mesh size (<10µm pore size). The residue was collected in 10 ml of distilled water and glycerin mixture (1:1) to prevent drying and coagulation of fine particles. A drop of sample from this homogenized aliquot was mounted on the glass slides in glycerin jelly medium and ~150–300 pollen/spores were counted (Figure 4). The other half of the aliquot after treatment with warm KOH was sieved through a 650 micron mesh. A drop of the residue was mounted on glass slides in 20% glycerin medium having a refractive index of 1.35 (Hoyt, 1934) in order to see diatoms, desmids, algae, calcified algal spores, testate amoebae (siliceous and organic tests/shells) and sponge spicules (Opal Silica) with a refractive index of 1.43–1.47, respectively (Lewin, 1962). Normally, the diatoms are observed under a microscope in naphrax medium (RI: 1.65–1.7) but as it is hazardous to health, we obtained good results in a low RI of 15%–20% glycerin medium in our recent published methodology (Misra et al., 2020). The identification and counting were done in temporary slides using Olympus BX51 light microscopes at Birbal Sahni Institute of Palaeosciences, Lucknow. The temporary slides were prepared in order to roll the specimens for better identification in different views. The numerical data obtained for each proxy such as pollen/spores, algae/diatoms, testate amoebae and sponge spicules (Figures 3 and 4) were used for their relative percentages and graphed in Tilia 2.0.41 software (Grimm, 1991). The cluster analysis (CONISS) through this software helped to demarcate five phases chronologically (I-V: from bottom to top of the trench sediments, respectively). The photographs illustrated (Figure 5) were taken by a DP-26 camera attached to Olympus- BX-51 Microscope. The pollen grains were grouped into arboreal (AP- evergreen and deciduous trees) and non-arboreal (NAP) and monolete/trilete spores representing terrestrial palynomorphs. The identification of pollen/spores was carried out by consulting standard literature (Erdtman, 1943; Nayar, 1990; Tissot et al., 1994). For the diatom classification and identification standard literature was consulted (Gandhi, 1959a and 1959b; Round et al., 2007; Taylor et al., 2007). The testate amoebae were identified following Leidy (1879), Ogden and Hedley (1980), and Kumar and Dalby (1998).

Figure 3.

Relative percentages of biotic proxies and net rate of sedimentation in Nawabganj Lake since ~4.6 ka.

Figure 4.

A cumulative record of all the studied proxies and their correlation with mid (India) and high (Greenland) latitude climatic changes since 4.6 ka.

Figure 5.

1. Tectona, 2. Syzygium, 3. Combretaceae, 4. Bombax, 5. Ficus, 6. Hibiscus, 7. Portulaca, 8. Cucurbitaceae, 9. Momordica, 10 Brassica, 11 Apiaceae, 12 Persicaria, 13 Lemna, 14 Ludwigia, 15 Trilete, 16. Rhopalodia gibba, 17. Gomphonema, 18. Navicula, 19. Cymbella, 20. Oscillatoria type, 21. Pediastrum, 22–25. Microscleres of freshwater sponge spicules (vegetative body) of Spongilla/Dosilia spp., 26–28. siliceous gammoscleres- spicules from sponge gammule of Ephydatia spp., 29. Gammoscleres of Trochospongilla spp., 30. biorotule of gammosclere, 31. Centropyxis aculeata, 32–33 C. laevigata, 34. Arcella sp, 35. A. artocrea, 36. A vulgaris, 37. A. discoides, 38–40 Centropyxis aerophila complex (C aerophila aerophila; C. sphaginicola; C. sylvatica), 41. Cyclopyxis kahli, 42, 43. Difflugia, 44. Lesquereusia modesta, 45. Euglypha spp., 46. Trinema lineare.

Estimation of mean annual temperature (MAT) and mean annual precipitation (MAP)

Pollen assemblages (Supplemental Table2 and Supplemental Figure 1) were used to reconstruct MAT and MAP (Figure 4) using the ‘coexistence approach’ (CA) (Mosbrugger and Utescher, 1997; Utescher et al., 2014). Here,the tolerance range of temperature and precipitation for taxa recovered from sedimentary profiles is calculated by comparing it with the range of their living modern taxa. The range of temperature and precipitation that is, MAT and MAP was determined from climatological normal data of 30 years taken from 235 climate stations distributed all over the country (Climatological Tables of Observatories in India, 1967). The modern distribution of the living plant taxa acclimatized to a limited range of precipitation and temperature which correspond to pollen identified from sediments was used following Champion and Seth (1968). Here, forest types have been classified into five major groups based on temperature and moisture contents which are further divided into 16 groups and ~200 sub-groups. Logically, the changes in climate during the geological time period would alter plant distribution, and therefore, climate-vegetation studies reveal certain climatic regimes (eg.,temperature and precipitation) that are associated with plant communities (Walter, 1985). Despite several limitations such as the coarse resolution of potential climate change and forests in India, an assessment has been done using climatic data to project the Regional climate model of the Hadley Centre (HadRM3) and dynamic global vegetation model- Integrated Biosphere Simulator Model (IBIS) 2.5 (Chaturvedi et al., 2011; Foley et al., 2005). Besides the soil parameters (e.g. soil texture, Nitrogen and Carbon sequestration) IBIS model requires monthly mean precipitation, monthly relative humidity, monthly minimum, maximum and mean temperature (%), wind speed (arid or moist), etc. (Kucharik et al., 2000). This model simulates current vegetation distribution (Chaturvedi et al., 2011) with observed vegetation type documented by Champion and Seth (1968). However, the uncertainties, model and data limitations have been discussed earlier (Cramer et al., 2001; McGuire et al., 2001) and are down-scaled at regional levels (Chaturvedi et al., 2011).

Mineral magnetic analysis

Forty samples were air-dried and packed in standard non-magnetic plastic bottles of 10 cm³. Magnetic susceptibility (χ) at low (0.47 kHz) frequency (χ_lf) was determined on a Bartington Susceptibility Meter (Model MS2) with a dual sensor (Figure 5; Supplemental Figure 2). Anhysteretic remanent magnetization (ARM) was induced in the samples using a Molspin AF demagnetizer (with an ARM attachment) in a constant biasing field of 0.1 mT superimposed on a decaying alternating field (a.f.) with a peak field of 100 mT at a decay rate of 0.001 mT per cycle. The susceptibility of the ARM (χ_ARM) was calculated by dividing the mass-specific ARM by the size of the biasing field (0.1mT = 79.6 A/m; Walden et al., 1999). Isothermal remanent magnetization (IRM) was induced in selected samples at different field strengths (50, 100, 300, 500, 700 and 800 mT) and backfields of up to 300 mT using an impulse magnetizer (ASC Scientific, USA). Remanence was measured using the JR6 (AGICO). S-ratio, SIRM/χ_lf, χ_lf/SIRM, and Soft and hard IRM were used in the analysis. Five inter- parametric ratios that were used in the analysis are S-ratio, SIRM/χlf, χARM/SIRM and hard and soft IRM’s. The S-ratio is the ratio between the IRM induced at − 300 mT to the SIRM induced at 700 mT (-IRM-300 mT/SIRM700 mT). The SIRM/χlf ratio gives a determination of grain size with a higher ratio suggesting a coarse grain size and vice versa for χARM/SIRM. For magnetic mineralogy measurements, the IRM acquisition was performed on all samples. The IRM acquisitions also show the contribution of magnetite which is the primary mineral contributing to the remanence in these sediments. You need to state in the intro the rationale for using this method

Spectral analysis

Using the percentages of tree pollen, herbs and shrubs, the ratio of Arboreal and non-Arboreal pollen, and aquatic pollen from Nawabganj Lake and Barela Lake (Supplemental Figure 3), the cyclicity of climate was plotted at 90-95% significance. Later,eight time-series palynological dataset obtained from both these lakes was subjected to spectral analysis (Figure 6) using the software REDFIT4.1 (Schulz and Mudelsee, 2002). In order to standardize, different statistical parameters were set to the rectangular window and Monte-Carlo simulation (n50 = 1, and N_sim = 1000) for the periodogram of the time series. The spectral resolution of the Nawabganj Lake ranged from 0.09 to 0.14 ka and Barela Lake from 0.05 to 0.18 ka. The analysis provides periodicity in climate change responding to external forcings (solar irradiance).

Figure 6.

Time series Periodogram for corrected spectrum in Nawabganj Lake and Barela Lake, Gangetic Plain showing significant multi-centennial periodicities governed by monsoonal climate variability. (a) Pollen Sum of Tree taxa. (b) Pollen Sum of Herbs/shrubs. (c)NAP/AP. (d) Aquatic Pollen assemblage (a–d: details in Supplemental Figure 8).

Results

Lithology and chronology

At 220 cm deep the calcrete layer was encountered and beyond 215 cm the hard clay or calcrete layer was difficult to dig out. From ~200–215 cm depth, the average percentage of sand was ~65% followed by silt (~20%) and clay (~15%) (Table 1). This also included small interspersed calcrete nodules which increased in number and depth. The sediment between 170 and 195 cm depth was silty sand. However, between 90 and 165 cm depth the sand content was reduced with a further increase in silt but it was low in clay percentage. Between 40 and 85 cm depth an increase in silt and clay was observed (Table 1; Supplemental Figure 2;). The topmost surface sediments are equally represented by sand and silt. The sedimentary profile in the Nawabganj area and the lake area shows a calcrete layer at 1–2 m depth which is as thick as 50–100 cm in the region and below this, the lithofacies is sandy or mixed with calcrete in the entire interfluve region (Srivastava et al., 2003).

Table 1.

Chronology, sediment depth and values for each proxy from Nawabganj Lake.

Parameters	Phases
Parameters	5	4	3	2	1
Age (cal Yr BP)	558–859	909–2617	2833–4166	4200–4371	4405–4575
Depth (cm)	05–35	40–85	90–165	170–195	200–215
Sand %	40–62	60–40	55–45	62–40	80–50
Silt%	25–40	40–25	45–40	45–25	30–10
Clay%	10–20	25–15	10–10	25–10	25–10
Xlf (10–6 SI)	7.3–9.3	10.3–7.8	8.4–6.2	6.6–5.6	5.9–5.1
xARM (10–8 m³kg⁻¹)	26.57–39.52	24.55–14.16	13.2–7.56	7.23–4.89	7.35–5.34
SIRM (10–5 Am²kg⁻¹)	43.04–66.92	39.9–31	29–18.12	19.33–14.29	20.37–15.24
Hard IRM (10–5 Am²kg⁻¹)	9.78–13.49	10.64–9.2	10.56–9.16	10.2–7.87	10.56–8.22
Soft IRM (10–5 Am²kg⁻¹)	29.85–41.7	30.27–25.47	25.89–17.62	16.98–14.22	19.45–14.87
S-ratio	0.75–0.8	0.76–0.69	0.68–0.48	0.57–0.43	0.55–0.42
xlf/SIRM	0.14–0.18	0.29–0.2	0.36–0.26	0.4–0.34	0.38–0.26
TC%	0.23–0.71	0.32–0.18	1.17–0.13	0.4–0.11	1.6–0.85
δ13C	−20.8 − −17.3	−17.4 to −20	−4 to −20.6	−5.4 to −18.9	−2.5 to −3.7
TN%	0.02–0.07	0.03–0.02	0.02–0.01	0.01–0.01	0.01–0.01
δ15N	2.67–3.28	4.38–2.67	4.67–2.98	3.44–2.67	1.7–1.5
C/N %	9–14	16–6	117–7	40–11	160–85

Five phases were demarcated on the basis of cluster analysis of the palynological data (Figure 4) where all the counts of biotic proxies were summed together and the relative percentages have been given (Figures 3 and 4).

Phase-wise variations in the multi-proxy record

Phase 1: 215-200-cms depth (~4.6 to 4.4ka)

The vegetation reconstruction reveals the dominance of arboreal pollen (AP) or tree taxa (60.7%) followed by Non-Arboreal Pollen (NAP) which is ~23% of the total sum (Figures 3 and 4). The ratio of AP and NAP is highest in this phase accounting to ~39%. Of the tree taxa ~56% are evergreen against 5% of deciduous taxa. The major tree pollen recovered were Mangifera indica, Artocarpus, Syzygium, Ficus, Morus, Terminalia, Ziziphus, Ceiba bombax, Aegle marmelos, Azadirachta and Madhuca indica which are mostly the constituents of riparian forest (Figure 3). No aquatic pollen or swampy taxa were recorded. About ~7% of the total sum, the testate amoeba was dominated by Trinema which are mostly found in moist soil beneath the tree canopy. A scant presence of Oscillatoria (Figure 5), a cyanobacterium, was observed. The range of MAT evaluated from the pollen model was between 12°C to 25°C and the average was ~18°C. The MAP evaluated was between ~900 and 1050 mm and the average was ~925 mm. The sand fraction ranged between 80 to 50%, whereas the silt and clay fraction ranged from 10 to 25%. The magnetic concentration χlf is 5.1–5.9⁻⁸m³kg⁻¹×10⁻⁸m³kg⁻¹. This zone constituting 200-year period records decreasing χ_lf, χ_ARM, SIRM, and S-ratios (0.4–0.5) values, sharply fluctuating sand, silt, and clay percentages with high sand%, and an increase in silt and clay percentages. The total carbon (TC) ranged between 1.14 and 1.6 wt% with an average of 1.3% ± 0.18. The δ¹³ Cvalues varied in a narrow range of −2.5 to −3.7‰. TN contents were low (~0.01 wt.%) with δ¹⁵N values ranging between 1.5 and 1.7 ‰ (Figure 4 and Supplemental Figure 2).

Phase 2: 195–170cm (4.4–4.2ka)

A decrease in the ratio of AP and NAP was observed (Figure 4). Herbs and shrubs were reduced in accordance with the reduction in AP taxa with an increase in deciduous taxa to 11%. During this phase the siliceous microscleres of freshwater sponge spicules for example, Trichospongilla were recorded. Aquatic testate amoebae such as Centropyxis aculeata and Arcella vulgaris (Figure 5) were observed that are absent in phase 1. Aquatic pollen constitutes 1.2% for example, Nelumbo, Cocculus and Ranunculus. Low percentages of diatoms such as Pinnularia, Cymbella and Cosmarium were observed. About 15% of the testate amoebae recorded were those that are generally found in water (C. aculeata, Difflugia and Lesquiresia),or terrestrial moist habitats and moss cushions (e.g. Centropyxis aerophila complex, C. laevigata, C. delicatula), Monolete/trilete spores account for 0.3%?of the pollen sum, they were not recorded in phase 1. The minimum temperature evaluated was 10°C and the maximum was 28°C and the MAT was ~19°C. The minimum precipitation evaluated was ~900 mm and the maximum was 1130 mm. The silt content increases with less clay and sand in this zone. The magnetic concentration χlfis 5.6–6.6×10 ⁻⁸m³kg⁻¹. The increasing trends of χ_lf, χ_ARM, SIRM, fluctuating but low S-ratios (0.4–0.5) are encountered in this zone. TC contents ranged between 0.11 to 0.85%; whereas δ¹³ Cvalues displayed a large range varying from −3.2 to -17.4‰. TN contents were found be slightly enhanced (compared to the preceding phase) as they varied between ~0.01 and 0.02 (wt.%). δ¹⁵N values ranged between (−)1.5 and 3.4‰.

Phase 3: 170–90 cms (4.2–2.8 ka)

During this phase, signatures of lake formation are evident due to a three-fold increase in testate amoebae. The testate amoebae community constituted Centropyxisaculeata, C. aerophila complex (C. aerophila, C. sphaginicola and C. sylvatica), C. delicatula and C. constricta, and Difflugia (Figure 5). The microscleres of sponge spicules, chlorophycean algae along with diatoms and aquatic pollen were also recorded (Figures 3 –5). The presence of Rhopalodia, Cosmarium and Gomphonema were recorded. An increase in calcified Phacotus algal cysts in the latter part of this phase was observed. However, evergreen pollen taxa decreased over deciduous pollen taxa and the ratio of AP and NAP reduced to ~0.7. The NAP constitutes ~44%. Asteraceae pollen increased in percentage along with members of Malvaceae and Apocynaceae. A consistent presence of Nelumbo and other swampy pollen taxa such as Justicia and Ludwigia was recorded. A sharp decrease in cryptogamic spores (monolete/triletes) was observed along with an increase in Asteraceae and terrestrial taxa. The minimum temperature evaluated was 10°C and maximum was 28°C and the MAT was ~19°C. The minimum precipitation evaluated was ~900 mm and the maximum was 1500 mm. The MAP analyzed was ~1200 mm. The silty sediment was dominant. The gradual increasing trend of χlf and S Ratio was recorded. The magnetic concentration χlf is 6.2–8.4⁻⁸m³kg⁻¹×10⁻⁸m³kg⁻¹. This longer phase shows increasing χ_lf, χ_ARM, SIRM. The S-ratios are still low (0.4–0.6) denoting haematite formation due to warmer climates (Figure 5 and Supplemental Figure 2). The TC contents for this phase ranged between 0.11 and 1.17 wt.% with δ¹³ C values varying between −4.0 and −20.6‰. TN contents varied between ~0.01 to 0.02% with δ¹⁵ N values ranging between 2.9 and 4.2‰ with an average value of 3.6‰ ± 0.5.

Phase 4 (90–40cms), 2.8–0.9 ka

Palynological results reveal an increase in the pollen of evergreen taxa and deciduous taxa (Figure 4). The ratio of AP and NAP also increased. Relative to earlier phases, Mangifera indica, Syzygium, ArtocarpusandMadhucaindica increased which are fruit-yielding trees showing anthropogenic activity. The absence of aquatic pollen was observed. Among diatoms, only Cymbella was recorded. Testate amoebae such as C. aerophila complex and C. delicatula were recorded. An increase in vegetation cover was probably favoured by moist soil around the lake or along the moist paleochannels in the vicinity. The absence of aquatic pollen, scant algal remains, and water-dwelling testate amoebae such as Centropyxis aculeata, Arcella discoides, A. vulgaris, and Difflugia indicate drier substrate due to the shrinking of the lake boundary (Figure 7). Although the shrinking of the lake indirectly suggests reduced precipitation, the pollen record of Asteraceae members, Azadirachta indica, Madhuca indica, Mangifera indica, Syzygium, etc (Figures 3 and 4) could be attributed to a regional hydrological regime facilitating moist terrestrial substrates near the water body probably due to NEM or a highly fluctuating monsoonal pattern. The MAT evaluated was between ~10°C and 28°C and the average estimated was ~19°C. The MAP evaluated was between ~840 mm and 1500 mm. The estimated average was ~1170 mm. χ_lf, χ_ARM, and SIRM values are similar to the preceding zone with χ_lf ranging between 6.2–8.4×10 ⁻⁸m³kg⁻¹. The TC content for this phase ranged between 0.16 and 0.31 wt.% with δ¹³ Cranging between (−)17.4 % and (−)20.0‰ with an average value of −18.2 ± 0.8‰. TN contents increased with values ~0.02 ± 0.01% with an average δ¹⁵ Nvalue of 3.6 ± 0.8‰.

Figure 7.

Schematic map of the study area showing shifting of river channel and evolution from fluvial to the lacustrine ecosystem and the transitional phases in the past since 4.6 ka.

Phase 5 (Agricultural evidence) 35–5cms, 0.9–0.5 ka

The shrubs and herbs increased to ~50% as compared to 8-15% in the older four phases. However, tree taxa were reduced and no evidence of algae/diatom or sponge spicules was recorded along with the absence of aquatic plant taxa. The MAT ranged between ~10°C and ~27°C and the average was ~19°C. The MAP ranged between ~900 mm and ~1240 mm. The average was ~1070 mm. The magnetic characters such as S ratio, SIRM, and χ_lf support a relatively warm interval from 1060 to 759 cal yr BP. An abrupt decline in sand contents was recorded that remained consistent until modern times. The magnetic parameters also show a drastic fluctuation with increasing values. This short period shows a drastic lowering of lake levels. The increasing trend in magnetite (S-ratio is greater than 0.6%) shows agricultural activities (Frankl et al., 2022). The TC content for this youngest phase ranged between 0.23 and 0.71 wt.% with δ¹³ Cranging between (−)17.3 to (−)20.3‰. TN contents, however, showed noticeably increased values ~0.03 ± 0.02% with an average δ¹⁵ N value of 3.0 ± 0.3‰.

Spectral analysis and periodicities

The palynological data from Barela Lake deposited between ~14 and 5 ka and from Nawabganj Lake deposited from ~4.6 ka to the present (Supplemental Figure 3) used for spectral analysis shows multi-centennial scale periodicity (Figure 6). The arboreal pollen assemblage in Barela Lake sediments spanning between ~5 and ~14 ka interval reveal ~430, 350 years of cyclicity. The assemblages with herbs and shrubs pollen the cyclicity of 820, 550, 360 and 310 years were obtained. The aquatic forms and NAP/AP ratio revealed similar cycles of 630, 370 and 310 years (Figure 6). Therefore, the dominance of ~300 years of periodicity is observed from the Barela Lake data set prior to ~5 ka. The palynological assemblage of arboreal pollen from Nawabganj Lake revealed a cyclicity of 680, 410 years. Cycles of 210 years were observed with herbs and shrubs abundance, where the aquatic forms revealed cycles of 230 ka. The NAP/AP ratios revealed a cyclicity of 680 years. The dominance of ~200 years the cycle is observed with a data set obtained from Nawabganj Lake. Overall results indicate a multi-centennial scale of periodicity during Holocene. The investigated pollen recurrence of plant group (arboreals, AP/NAP, herbs and shrubs, aquatic pollen) in the Holocene epoch indicates vegetation-based climate phases under the influence of climatic periodicity due to Earth’s orbital changes (inducing differential distribution of solar input on Earth) in the geological past. Hence, a periodicity of about 300 years was observed prior to ~5 ka whereas, it was about 200 years during Late-Holocene with more rapid change in vegetation in CGP since 5 ka. However, an overall multi-centennial periodicity of 200, 350, 430, 550, 630, 850 years were obtained. This variability in cyclicity (recorded with different plant groups) can be attributed to the impact of delayed (for arboreals) or instant (for herbs and aquatic pollen) effects of hydroecological changes (landscape changes) influenced by the intensity of monsoon system due to intermittent low and high solar irradiance on Earth.

Discussion

Based on several proxy records, studies reveal that the sedimentary depositional environment in the flood plain of the River Ganges was largely influenced by climatic fluctuations during the late Quaternary (Goodbred and Kuehl, 2000; Goswami and Mishra, 2013; Singh, 1996). Vegetational reconstruction from lacustrine sediments from the Gangetic Plain in India is available from different areas such as Basaha Jheel (Chauhan et al., 2004), Lahuradeva (Chauhan et al., 2009), Misa Tal (Wasson et al., 2013) Karela Lake (Chauhan, 2015; Tripathi et al., 2017) and Barela Tal (Misra et al., 2020). Other palynological and sedimentary records from Sanai Tal (Sharma et al., 2004), Jalesar Lake (Trivedi et al., 2013), Chaudhary-Ka-Tal (Saxena et al., 2015), Ganga Plain sediments (Singh et al., 2015) and lacustrine sediments of the eastern Ganga Plain (Saxena and Singh, 2017) provide evidence of time period for each Ox-bow lake formation and therein signatures of climatic changes during the late Quaternary Period (since ~21 ka). The study of all these lake sites has been summarized recently (Misra et al., 2019, 2020). Accordingly, a peak strength of monsoon is evident between 7-5 ka and an increase in aridity after 5.0 ka from all the above-studied sites in Ganga Plain as well as from all over India (Farooqui and Nautiyal, 2016; Kar and Quamar, 2019; Roy and Singhvi, 2016). The palynological evidence was in response to climate-related hydroecological conditions. Our study highlights the variability in the response of each proxy record to palaeohydrological and palaeoclimatic conditions of Nawabganj Lake(NL) since ~4.6 ka.

Phase 1: Fluvial ecosystem 4.6–4.4 ka (Riparian forest)

The arboreal pollen (AP)such as Mangifera indica, Artocarpus, Syzygium, Ficus, Morus, Terminalia, Ziziphus, Ceibabombax, Aeglemarmelos, Azadirachta and Madhuca indica along with understorey herbs and shrubs indicate the dominance of riparian forest in the moist clayey sand substrate suggesting a fluvial depositional environment. The ratio of AP/NAP was highest (39%) and the vegetation here is perhaps the remains of plant diversity that flourished during a relatively warmer and humid climate prior to 5 ka during the Middle Holocene Climatic Optimum (MHCO: 9–5 ka). Similar evidence of rich plant diversity during MHCO has been reported in sedimentary profiles from the Gangetic Plain (Saxena and Trivedi, 2017; Tripathi et al., 2017; Trivedi et al., 2013) Central India (Chauhan and Quamar, 2012; Chauhan et al., 2013) Western Himalaya (Quamar, 2019), trans-Himalaya (Demske et al., 2009) and coastal areas (Farooqui and Nautiyal, 2016; Farooqui et al., 2013). Records of strengthened monsoon between 7-5 ka and subsequent increase in aridity throughout the Indian sub-continent have been documented (Phartiyal et al., 2020). The changes in the vegetation post Holocene Climatic Optima (HCO) have been attributed to a steady increase in aridity resulting in loss of surface water, shrinking of water bodies and loss of regional biodiversity as rainfall is the primary determinant of plant growth rates, productivity and hydrogeological changes (Hoffmann et al., 2012; Sankaran et al., 2005).

The absence of freshwater sponge spicules and aquatic pollen indicates an unstable aquatic ecosystem with low water levels even during the monsoon season. The presence of scant Oscillatoria filaments indicates occurrences of cyanobacterium indicating nitrogen fixation (Gallon et al., 1991). It is known that values of δ¹⁵ N ranging from ~˗1 to ~11 ‰ are quite reliable indicators of N₂ fixation by cyanobacterial mats (Rejmánková et al., 2004; Wada, 2009). Our values range between −1 and ~3.5‰ suggesting fluctuating conditions. The high percentage of sand (70%) and high sedimentation rate also support the pre-fluvial ecosystem during this phase (Figure 7). The inferred MAT and MAP show relatively low temperatures and precipitation during this phase which also supports the growth of cyanobacteria. Oxygen isotope data (Figure 4) from India at mid-latitude shows a relatively cooler climate ~5–4.6 ka (Kathayat et al., 2016; 2017) and similar evidence of lowered temperature from high latitudes has been observed in ice cores from Greenland, Northern Hemisphere (Cuffey and Clow, 1997). Our study shows the range of temperature varying between 0.5°C and 1°C during this phase. However, the precipitation was low ~90 cm (Figure 4). Hence, cooler and drier conditions prevailed in CGP much before the 4.2 ka (i.e. ~5 ka onwards) event known worldwide, but evidence of pollen/spores shows moisture availability in soil along the river channels favouring riparian vegetation even during low precipitation during this period. The low elemental carbon values from 1.2% to 0.2% are attributed to low organic matter input. The delta carbon fractionation values (¹²C/¹³C) of ~−4 to −6‰ indicate a drier condition and a low S ratio (~0.5) denotes the presence of haematite suggesting the fluctuation in depositional environment induced by intermittent dry and rewetted conditions. The river action and the early stages of lake formation, enhanced sediment input in the basin/lake is evident during this phase. Overall results indicate a fluvial (lotic) ecosystem during weakened SWM and active NEM suggesting that the Sai River was flowing in the study site during this time period.

Phase 2: 4.4–4.2ka (semi-closed fluvial ecosystem)

The paleochannels in the interfluve region of Ganga and Sai River (Figure 1b) have been well studied earlier (Misra et al., 2020; Sharma et al., 2004). Palynological results indicate a reduction in vegetation suggesting hydrological and landscape changes perhaps due to the gradual shifting of the Sai River course away from the study site (Figure 7). A slight increase in sponge spicules and a high percentage of testate amoeba, suggests now a semi-closed aquatic ecosystem and relatively good monsoon rainfall that recharged the channel in the beginning (4.4 ka) but intermittent dry conditions prevailed. About 8% of gammoscleres recorded in sediments indicate high seasonality which induced sexual reproduction during dry periods. Gammoscleres are the spicules derived from sponge gammules (reproductive bodies) which are known to form during water scarcity (Holmquist, 1973; Manconi and Pronzato, 2016; Pronzato et al., 1993). The increase in silty sediment and decline in the sand fraction suggests a weakening of the fluvial system which was consistently recharged during seasonal precipitations throughout in this phase. The increasing trends of χ_lf, χ_ARM, SIRM, fluctuating but low S-ratios support a sediment depositional environment under intermittent mild warm and moist climatic spells. The elemental carbon and nitrogen values show fluctuation between −8 to −16‰ suggesting input of organic carbon from both aquatic and terrestrial sources (Agrawal et al., 2015; Dubey et al., 2020; Hamilton and Lewis, 1992). The biotic and abiotic proxy records here reveal that the hydrological variations were of high magnitude due to fluctuating monsoons during this period. The intermittent signals of warm and dry climatic conditions are evident through biotic proxy records and dispersed calcrete nodules within the sediments suggesting intermittent drier events. Results show that an intermittent warm/humid spell occurred between ~4.6 and ~4.4 ka. The drier climatic event ~4.2 ka (Meghalayan Age) observed in parts of the Indian subcontinent (Kathayat et al., 2018) and World over (Railsback et al., 2018) is evident here (NL) in the form of dispersed calcrete formation and the development of an ox-bow lake (Figure 7). The Holocene climate history of the Indo-Gangetic plain shows a fabric of calcrete formation in older soils thereby precipitation of carbonates is common in the region (Srivastava, 2001). The hard duricrust is mostly formed in arid to semi-arid drier climatic conditions (Netterberg, 1980). As the thick calcrete layer is recorded at the bottom prior to 4.6 ka it is inferred that intense aridity began much earlier in CGP with intermittent variations in monsoonal pattern between ~4.6 and 4.2 ka. Similar calcrete deposits have been recorded in Jalesar Lake sediments ~5 ka (Trivedi et al., 2013). The aridity was not continuous and perhaps it reached the climax at around 4.2 ka. However, the herbs that crop up during the winter season such as Clerodendron and Asteraceae are attributed to moist substrate recharged by river channels in the vicinity through NEM. Records of active NEM from contemporary Barela Lake prior to the Middle Holocene (Misra et al., 2020) indicate an intermittent strengthening of NEM and weakening of SWM. Such a condition is earlier reported using clay mineralogy from Arabian Sea sediments which indicates a century-scale change in monsoon pattern and an inverse coupling of summer and winter monsoon (Chauhan et al., 2010) Other proxy records such as sponge gammoscleres and low counts of testate amoebae along with silty sediment texture, fluctuating values of carbon/nitrogen isotope and magnetic parameters support intermittent dry and wet climatic spells which were also influenced by a shift in river channel accompanied by the formation of cut off Ox-bow Lake (Nawabganj). The shifting of the ganga river course and its tributaries in time and space has been computed earlier from different sectors forming Ox-bow lakes (Pati et al., 2008; Singh, 1996).

Phase 3 (4.2–2.8 lake ecosystem, 90–165 cm)

In India, changes have been observed in the intensity of ISM during the Late Holocene (Enzel et al., 1996; Laskar et al., 2011; Misra et al., 2020). This was mainly mediated by the latitudinal shifts in the Inter Tropical Convergence Zone (ITCZ) and ISM changes (Farooqui et al., 2014; Prasad et al., 2014). The ratio of AP and NAP decreased sharply as compared to phases 1 & 2 (Figure 4). The presence of microscleres and absence of gammoscleres along with relatively high counts of aquatic vegetation suggest a continuous water supply in a closed-water body supported by rainfall. An increase in pollen counts of arboreal and non-arboreals from Jalesar Lake in the nearby Unnao district shows evidence of strengthened SWM (Trivedi et al., 2013). The presence of Rhopalodia (an epiphytic/benthic diatom) in the early part of this phase and the decline in the latter phase suggests the shift in the lake boundary from the study site at the end of this phase. An increase in Phacotus algal cysts formed during unfavourable ecological conditions at the end of this phase together with an increase in the C. aerophila complex (Foissner and Korganova, 2000) and C. delicatula (Farooqui et al., 2020) indicate moist peripheral part of the lake at the end of this phase (Figure 7). The occurrence of calcified cysts of Phacotus in a high percentage in the present study is of great palaeohydrological and palaeoecological significance since the Miocene (Lagerheim, 1902). The calcite lorica of Phacotus (chlamydomophycean alga) is often found in calcium-rich ecosystems at cooler temperatures between ~15° and 25°C (Schlegel et al., 1998). The calcified shells and the temperature ranges in which Phacotus blooms indicate a relatively cooler and drier climate. These cysts were perhaps favoured by high calcium concentrations in water derived from calcrete nodules recovered below in phases 1 and 2. Although the lake water ecology was changing in the study site due to a shift in river channel as is evidenced by increased silt and clayey sediment texture along with the environmental magnetic data, the soil moisture retained in the vicinity did not have much effect on the arboreal taxa. This suggests the active NEM during this phase prior to 2.8 ka and after the arid event of 4.2 ka. The MAT and MAP do not show drastic changes but the lake hyroecological changes are evident through variability in biotic and abiotic data because of a shift in the river channel and later shrinking of the lake from the study site due to inverse coupling of weak SWM with active NEM. The presence of arboreal taxa and diversification of deciduous taxa during this is evident from nearby sites (Trivedi et al., 2013) and it is also evident from Nawabganj Lake. A sharp decrease in cryptogamic spores (monolete/triletes) suggests drier conditions (Löbs et al., 2020) along with an increase in Asteraceae and terrestrial taxa indicating an increase in open land area and moisture through NEM as most of Asteraceae and other herbs have very shallow root system crop up after winter rainfall when the surface soil is moist (Cuffia et al., 2022). The loss of soil moisture from the sub-surface due to climatic conditions in CGP with intense dry and cold conditions from November to January is rewetted through NEM that favours the herbaceous flora (spring season). Therefore, the relative increase in Asteraceae and other herbs down-core in the sediment suggests active NEM. The testate amoebae community constituted C. aerophila complex (C. aerophila, C. aerophila and C. sylvatica), C. delicatula, and C. constricta indicating retreat off the lake boundary as these are more often found in the moist substrate (Farooqui et al., 2020). Thus, the river channel and lake both appeared shifting apart exposing land in the studied site (Figure 7). High variability in delta carbon fractionation values between around −4 and −22‰ suggests its input from aquatic productivity along with the fluctuating seasonal input of terrestrial organic matter (O'leary, 1988). A large spatial and temporal variability is observed in the annual and seasonal rainfall trends over the Ganga Basin and its sub-basin areas. Hence, the sub-basins (e.g. Ganga-Sai interfluve) that are influenced by rainfall during monsoon season play a major role in the fluctuation of water flow in the river Ganga (Bera, 2017).

Phase 4: 40–85 (2.8–0.9cal yr BP) (Ox-bow lake formation)

This phase is also characterized by intermittent dry and wet conditions with a slight reduction in the AP and NAP. This increase in the latter part of the phase was perhaps influenced by short-term Medieval warm period (MWP). Both evergreen pollen for example, Syzygium, Mangifera and Artocarpus were present along with deciduous pollen for example, Azadirachta and Ceiba bombax (Figure 4) suggesting intermittent moist conditions in the vicinity influenced by nearby River channel recharged during the rainy season. The herbaceous pollen reduced from 60% (phase 3) to ~48% in this phase suggesting a reduction in subsoil moisture. During this period an increase in deciduous pollen was observed. The testate amoebae recorded in this phase are mostly of terrestrial habitats such as Centropyxis aerophila complex and Cyclopyxis kahli dwelling in the seasonal moist substrate. Algal remains such as bacillariophyceae reduce drastically in this phase except for few counts of Cymbella and Navicula which probably occurred in small pools or puddles in the study site during rainy seasons as the lake boundary was continuously shifting from the study site. The sponge spicules reduced drastically suggesting water scarcity. Overall reductions in aquatic pollen/algal remains along with testate amoebae indicate landscape changes leaving behind an Ox-bow Lake which eventually drifted away from the study site (Figure 7). Here the lake is totally cut off which is supported by the increase in %clay. The environmental magnetic data indicates a lack of sediment input through the river channel. The high S-ratios (0.7–0.9) show the presence of magnetite, perhaps due to open land used for agriculture (Frankl et al., 2022). We infer, that the process of Ox-bow Lake formation and it’s shifting away from the study site during this phase influenced relative percentages of algae, testate amoebae and sponge spicules along with the sediment texture in the water body due to hydrological changes. However, the presence of the lake ecosystem and the river channel throughout in the vicinity during this phase facilitated moist substrates for arboreals and non-arboreals.

Phase 5: 05–35 (0.9–0.5calyr BP) agricultural pollen (exposed land)

Most of the tree pollen (Syzygium, Ceiba) continued its occurrence in the beginning of this phase but declined sharply in the latter part with the occurrence of Brassica, Apiaceae, Cucurbitaceae, Momordica (bitter gourd) suggesting anthropogenic intervention on land for agriculture practice with nearby availability of water. The absence of aquatic pollen, diatoms and aquatic testate amoebae for example, Arcella species, Centropyxis aculeata indicates the absence of a water body. Testate amoebae that are common in terrestrial habitats such as Centropyxis aerophila and Cyclopyxis kahlii were recorded suggesting moist substrate facilitated by seasonal rainfall and the nearby NL (Figure 7). The tree pollen recorded until phase 4 was drastically reduced by the end of this phase perhaps due to agricultural activity and reduced soil moisture suggesting continued shifting of the river water channel and the Ox-Bow Lake from the study site. At present the vast agricultural land between the Ganga-Sai River interfluve is irrigated by the water supplied through these Rivers, man-made water canals and the ox-bow lakes. As the study site was the flood plain of the existing River channel the percentage of sand remains relatively more although the silt and clay increased gradually from the bottom to the top of the sedimentary profile as the lake was formed. The magnetic parameters also show a drastic fluctuation with increasing values (Figure 4, Supplemental Figure 2). The increasing trend in S-ratio shows agricultural activities, as well as the elemental nitrogen values also depict relatively higher value in comparison to earlier phases suggesting the use of organic fertilizers (e.g. cow dung manure) for cultivation on exposed land. A similar landscape still exists in the present-day with stray occurrences of tree taxa although with an increased number of fast-growing trees such as Prosopis juliflora, Caesalpinia, Eucalyptus, and several invasive shrubs/herbs for example, Lantana camara and Aegiratum conizoides. Agriculture fields are common where cultivation of Triticum (wheat), Brassica (mustard) and Oryza (rice) is done majorly.

The dynamics of ISM during the Holocene

Holocene climatic variability in response to ISM has been recently reviewed over the Gangetic Plains through studies from several lake sedimentary profiles (Misra et al., 2020; Quamar and Kar, 2020a). The rise in temperature began both at higher and lower altitudes after ~10 ka until the cold spell ~8 ka (Achyuthan et al., 2016; Misra et al., 2019; Quamar et al., 2021). Broadly, moist conditions between 7 -6 ka and dry conditions since ~5 ka reached the climatic climax ~4.2 ka inducing extreme arid conditions which are also recorded the world over (Farooqui et al., 2013; Kathayat et al., 2017; Roy et al., 2022). Our results show intermittent wet conditions between ~4.5 and 4.4 ka and then between ~3.0 and 2.8 ka. The movement of the ITCZ over the equatorial region governs the weakening and strengthening of the ISM (Ghosh et al., 1978). Specifically, the summer monsoon is initiated by the northward movement of the ITCZ due to the warming of the Asian continents during summer (Wright et al., 2008). Speleothem records from north-eastern India show annually-resolved changes in the Indian Summer Monsoon (ISM) between 4.4 and 3.78 ka (Kathayat et al., 2017). The diversified arboreal pollen between 4.7 and 3.2 ka in Jalesar Lake sediments, Unnao (NL is 240 km from Jalesar) suggest a warm and humid spell with calcrete formation prior to 5 ka (Trivedi et al., 2013). Our pollen results too support diversified arboreals during this period. However, higher IpH (phytolith index) values during this period indicate the shrinking of the Karela Lake (Lucknow, Gomti River Interfluve) boundary due to weakened SWM (Tripathi et al., 2017). Overall results indicate that the weakened SWM monsoon prevailed in CGP post 5 ka which reduced the size of the lakes with the shifting of the river course simultaneously. The vicinity of the river channel and the ox-bow lakes in the region facilitated arboreals and non-Arboreals depending on the variability in hydrological conditions in time and space. Several other records from India suggest a weakening of ISM ~4.2 ka (Berkelhammer et al., 2013; Dixit et al., 2014; Staubwasser et al., 2003). The period of last two millennium is known to have witnessed a series of short-spell warmer periods such as the Medieval Warm Period (MWP), the Roman Warm Period (RWP), and the Minoan Warm Period (MnWP) and known colder spells during the Little Ice Age (Achyuthan et al., 2016; Borzenkova et al., 2015; Quamar et al., 2021)). A negative coupling has been recorded between local soil moisture and precipitation indicating land-atmosphere relation over the Gangetic plains (Agrawal and Chakraborty, 2016). In winter, as the region is cold and dry, a huge amount of soil moisture due to irrigation has a strong effect on a large scale resulting in the intensification of ISM (Agrawal et al., 2019). It is well known that the dry and moist durations show correspondence with the low and high solar irradiance (Roy et al., 2022). Therefore, the present study records intermittent ~5–6 wet and dry conditions throughout the Late-Holocene perhaps at the centennial to multi-centennial scale. Therefore, both external and internal forcings along with the recent anthropogenic forcings influence biotic and abiotic proxies through time buried in sedimentary archives.

Climatic periodicity

One of the external forcings hypothesized is cyclic changes in solar irradiance which affects the climate (Bond et al., 2001; Usoskin et al., 2016; Wu et al., 2018). The internal forcings are Atlantic Meridional Overturning Circulation (AMOC) (Stuiver et al., 1998). and Arctic sea ice (Müller-Wodarg et al., 2012). Hence, decadal to centennial timescale climate variations observed is attributed to change in solar irradiance during Holocene, Pleistocene, and Pliocene palaeoclimate records ((Askjær et al., 2022; Czymzik et al., 2016; Gray et al., 2010; Khan et al., 2022; Prasad et al., 2014; Steinhilber et al., 2009). The model infers that a ~2500 year Bray cycle and ~1000 year Eddy cycle have different modes of action. The manifestation of the Bray cycle acts through ~208 years de Vries cycle. Both de Vries and Gleissberg cycles influence the intensity of SWM and regional precipitation patterns (Dykoski et al., 2005; Fleitman et al., 2007). Such climatic periodicities and variabilities in time and space are indicated through changes in proxy records obtained from sedimentary profiles (Li et al., 2021; Xu and Grumbine, 2014). In the past, the periods of solar minima and the periodic cyclicity has been observed through proxy records from Arabian Sea sediments that coincided with solar cycles of 200, 100 and 60 years (Agnihotri et al., 2002; Azharuddin et al., 2019; Latif et al., 2013; Willard et al., 2005). The present study reveals that aridity was not a continuous process during the Late Holocene but was punctuated by wet climatic spells, that is, intermittent wet climate between 4.6 and 4.4 ka. Later, a wet climatic spell was recorded between ~3.8 and 2.8 ka after the 4.2 ka global cold/arid event reported from different parts of the world and from India. However, during these periods inverse coupling of SWM and NEM was observed through the present multi-proxy data. Thereby, external forcings such as low and high solar irradiance induced dry/cold and warm/wet climatic periodicity, respectively at centennial to multi-centennial scale from CGP. Hence, three intermittent wet events were punctuated by dry/cold events between 4.6 and 2.8 ka. During the last two millennium, we have three historic records of warmer/wet climate that is, MWP, RWP and MnWP but except for MWP the others could not be traced well from the studied site as the lake shifted away with anthropogenic intervention.

The external and internal forcings involved in multi-decadal to multi-centennial climate periodicity (Marchitto et al., 2010; Stuiver et al., 1998; Wanner et al., 2008) is important to understand the regional climatic dynamics governing landscape changes, particularly in the Indo-Gangetic plains. Our records from the present sedimentary profiles (Barela Lake and Nawabganj Lake) deposited since ~14 ka in the central Gangetic plain (Ganga-Sai interfluve) shows multi-centennial climatic periodicity (200, 300, 400, 600 and 700 years.) at 90-95% significance level during Holocene (Figure 6) suggesting de Vries and Gleissberg cycles influencing the intensity of SWM and regional precipitation patterns in CGP which governed the intermittent cyclicity of warm/humid and cold/dry conditions during low and high solar irradiance. So how do these periodicities relate (or not) to the literature…… Hence solar irradiance has been identified as a primary driver of centennial and multi-centennial climate variability which drives the changes in landscape and biodiversity. An additional driving force likely to contribute to climate change in the future is anthropogenic forcings (CO₂ and temperature enhancement). The climate oscillations on centennial timescales are of great interest and speculation for future climate changes and their socio-economic impact in CGP.

Conclusions

The regional hydroecological and landscape evolution in time and space play an important role in sustaining biotic forms in and around the water body which are directly or indirectly influenced by climate changes. The signatures of these are buried in sedimentary archives. A ~2.20-meter sedimentary profile deposited since ~4.6 ka near Chandra Shekhar Azad bird sanctuary (Nawabganj lake) of CGP was studied using pollen/spores, diatoms, testate amoebae, environmental magnetic data, carbon and nitrogen stable isotopes. The bottom most sample beyond 2.15 m depth is the hardened calcrete layer formed when the river bed was exposed for most of the time indicating highly dry and arid climatic conditions which began much earlier in CGP (~5 ka) than the worldwide 4.2 dry-cold event. The clayey sand sediments, absence of lentic biota and the arboreal pollen assemblage coupled with fluctuating C/N stable isotope values between 4.6 and 4.4 ka shows the occurrence of riparian forest and instability in climate influencing fluvial system. Dispersed calcrete nodules during this period support intermittent wet and dry climates. An intermittent short spell of strengthened monsoon was observed at ~4.4 ka that led to rewetting and water stagnation which is evidenced by the meager account of aquatic pollen, sponge spicules and algal forms. The dry/arid climatic climax is observed through both biotic and abiotic proxies in NL, CGP at ~4.2 ka. The calcified resting cysts of Phacotus during this dry period indicate a cooler and dry climate. The period between ~3.8 ka was again punctuated by a wet climate. Later, until ~2.8 ka the lake ecosystem was observed showing more evidence of aquatic pollen, algae, testate amoebae and microscleres of sponge spicules. Gammoscleres of sponges show seasonality indicating recharged lake ecosystem during monsoons. The presence of Asteraceae and other herbaceous pollen suggests an active NEM. Thus, the aridity from ~5 ka to 4.2 ka was not continuous but punctuated by two wet conditions evident through spectral analysis of the palynological data set. However, the continuous shrinking/shifting of the lake boundary and river channel from the study site since ~2.8 ka is shown by the decrease in aquatic forms and the presence of agricultural pollen later in the last millennium indicating a continued weakening of SWM. Our records of spectral analysis from two sedimentary profiles (Barela Lake and Nawabganj Lake) deposited since ~14 ka to present in the Ganga-Sai interfluve shows multi-centennial climatic periodicity of 200, 300, 400, 600 and 700 years. depicting cyclicity in monsoonal variations with regard to down-core palynological variations during Holocene. Therefore, the intermittent and periodic climatic fluctuations in CGP were influenced by de Vries and Gleissberg cycles influencing the intensity of SWM and regional precipitation patterns during low and high solar irradiance and the hydroecological conditions influencing the river basin or sub-basins in CGP during the Holocene. In view of additional anthropogenic forcings in the recent decades, a centennial to multi-centennial periodic climate change is important to understand and predict the magnitude of future changes likely to affect agriculture-based economy in CGP.

Supplemental Material

sj-docx-1-hol-10.1177_09596836231183067 – Supplemental material for Monitoring hydroecology and climatic variability since ~4.6 ka from palynological, sedimentological and environmental perspectives in an Ox-bow lake, Central Ganga Plain, India

Supplemental material, sj-docx-1-hol-10.1177_09596836231183067 for Monitoring hydroecology and climatic variability since ~4.6 ka from palynological, sedimentological and environmental perspectives in an Ox-bow lake, Central Ganga Plain, India by Anjum Farooqui, Salman Khan, Rajesh Agnihotri, Binita Phartiyal and Sunil Shukla in The Holocene

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Supplemental material, sj-jpg-3-hol-10.1177_09596836231183067 for Monitoring hydroecology and climatic variability since ~4.6 ka from palynological, sedimentological and environmental perspectives in an Ox-bow lake, Central Ganga Plain, India by Anjum Farooqui, Salman Khan, Rajesh Agnihotri, Binita Phartiyal and Sunil Shukla in The Holocene

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Supplemental material, sj-jpg-4-hol-10.1177_09596836231183067 for Monitoring hydroecology and climatic variability since ~4.6 ka from palynological, sedimentological and environmental perspectives in an Ox-bow lake, Central Ganga Plain, India by Anjum Farooqui, Salman Khan, Rajesh Agnihotri, Binita Phartiyal and Sunil Shukla in The Holocene

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Supplemental material, sj-jpg-5-hol-10.1177_09596836231183067 for Monitoring hydroecology and climatic variability since ~4.6 ka from palynological, sedimentological and environmental perspectives in an Ox-bow lake, Central Ganga Plain, India by Anjum Farooqui, Salman Khan, Rajesh Agnihotri, Binita Phartiyal and Sunil Shukla in The Holocene

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Supplemental material, sj-xlsx-2-hol-10.1177_09596836231183067 for Monitoring hydroecology and climatic variability since ~4.6 ka from palynological, sedimentological and environmental perspectives in an Ox-bow lake, Central Ganga Plain, India by Anjum Farooqui, Salman Khan, Rajesh Agnihotri, Binita Phartiyal and Sunil Shukla in The Holocene

Footnotes

Acknowledgements

The authors thank the Director, Birbal Sahni Institute of Palaeosciences, Lucknow for providing the necessary facilities to accomplish this work. Our heartfelt condolences to one of the authors Dr. Rajesh Agnihotri who expired in January 2023 after a brief illness. We thank Prof. Viv Jones and anonymous reviewers for their valuable suggestions that helped us improve the manuscript for better readability.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The author(s) received financial support for the research from Birbal Sahni Institute of Palaeosciences, Lucknow, Department of Science and Technology, Delhi. There is no authorship, and funding for publication of this article.

ORCID iDs

Anjum Farooqui

Salman Khan

Binita Phartiyal

Supplemental material

Supplemental material for this article is available online.

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