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Volume 31 Issue 5
Oct.  2020
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M. C. Manoj, Jyoti Srivastava, Prem Raj Uddandam, Biswajeet Thakur. A 2000 Year Multi-Proxy Evidence of Natural/Anthropogenic Influence on Climate from the Southwest Coast of India. Journal of Earth Science, 2020, 31(5): 1029-1044. doi: 10.1007/s12583-020-1336-4
Citation: M. C. Manoj, Jyoti Srivastava, Prem Raj Uddandam, Biswajeet Thakur. A 2000 Year Multi-Proxy Evidence of Natural/Anthropogenic Influence on Climate from the Southwest Coast of India. Journal of Earth Science, 2020, 31(5): 1029-1044. doi: 10.1007/s12583-020-1336-4

A 2000 Year Multi-Proxy Evidence of Natural/Anthropogenic Influence on Climate from the Southwest Coast of India

doi: 10.1007/s12583-020-1336-4
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  • The last millennium climate reconstructions are complex and limit our understanding of the mechanisms behind environmental and climate variability. We present multi-proxy centennial-scale records from the Cherai, southwest India. The last 2000 cal yr AD record suggests a complex environmental condition that prevailed at the depositional site augmenting the role of natural as well as anthropogenic agents. Increased elemental variations and indices values indicate stronger weathering, presumably wetter conditions and intense precipitation. Provenance studies suggest diverse sources and the main composition fall close to the Charnockite and Gneissic composition. Multi-proxy data suggests that a shift towards wetter climatic conditions, which occurred from 910 to 1230 cal yr AD. The core also records a shift towards the drier conditions that started around 1230 cal yr AD with a loss in vegetation diversity. The pollution load index values suggest that the overall study area falls in moderate contamination levels, which are also substantiated with the diatom data indicating human influence in the natural habitat during the deposition time. The present study reveals that the enhanced Cd and As concentration is due to strong anthropogenic influence. We compared the multi-proxy record with other continental and marine palaeoclimatic records to explore global and/or regional trends in climate variability during the last 2000 years.
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A 2000 Year Multi-Proxy Evidence of Natural/Anthropogenic Influence on Climate from the Southwest Coast of India

doi: 10.1007/s12583-020-1336-4

Abstract: The last millennium climate reconstructions are complex and limit our understanding of the mechanisms behind environmental and climate variability. We present multi-proxy centennial-scale records from the Cherai, southwest India. The last 2000 cal yr AD record suggests a complex environmental condition that prevailed at the depositional site augmenting the role of natural as well as anthropogenic agents. Increased elemental variations and indices values indicate stronger weathering, presumably wetter conditions and intense precipitation. Provenance studies suggest diverse sources and the main composition fall close to the Charnockite and Gneissic composition. Multi-proxy data suggests that a shift towards wetter climatic conditions, which occurred from 910 to 1230 cal yr AD. The core also records a shift towards the drier conditions that started around 1230 cal yr AD with a loss in vegetation diversity. The pollution load index values suggest that the overall study area falls in moderate contamination levels, which are also substantiated with the diatom data indicating human influence in the natural habitat during the deposition time. The present study reveals that the enhanced Cd and As concentration is due to strong anthropogenic influence. We compared the multi-proxy record with other continental and marine palaeoclimatic records to explore global and/or regional trends in climate variability during the last 2000 years.

M. C. Manoj, Jyoti Srivastava, Prem Raj Uddandam, Biswajeet Thakur. A 2000 Year Multi-Proxy Evidence of Natural/Anthropogenic Influence on Climate from the Southwest Coast of India. Journal of Earth Science, 2020, 31(5): 1029-1044. doi: 10.1007/s12583-020-1336-4
Citation: M. C. Manoj, Jyoti Srivastava, Prem Raj Uddandam, Biswajeet Thakur. A 2000 Year Multi-Proxy Evidence of Natural/Anthropogenic Influence on Climate from the Southwest Coast of India. Journal of Earth Science, 2020, 31(5): 1029-1044. doi: 10.1007/s12583-020-1336-4
  • Many paleoclimatic studies have been carried out to understand the climate of the last millennium for predicting future climate changes and its impact on societies. The last millennium is a crucial period in understanding the Earth's climate response to external forcing and its internal variability on decadal-to-centennial timescales (Kamae et al., 2017; PAGES 2k Consortium, 2013). Global climate variability during the past 2000 years is generally characterized by multi-century episodes of distinct temperature and precipitation anomalies such as the Roman Warm Period (RWP; 0-500 cal yr AD), Dark Age Cold Period (DACP; 500-900 cal yr AD), Medieval Climate Anomaly (MCA, 900-1300 cal yr AD), Little Ice Age (LIA, 1300-1850 cal yr AD), and subsequent Current Warm Period (CWP) beginning in the 19th century (Graham et al., 2011; Mann et al., 2009; Keigwin 1996). The concept of Medieval Warm Period (MWP), centered at around 1100-1200 cal yr AD, was first reported by Lamb (1965). These studies also attracted much attention because it is the most recent example of long-term, naturally generated global climate variability (e.g., Shi et al., 2016; Liu et al., 2011).

    Paleoclimate reconstruction using lake and wetland sediment deposits would improve the current understanding of the climate system and human-environment interactions. Mainly, the coastal wetlands are dynamic landforms that act as a transition between the continental and marine realm. They consist of a complex of natural habitat situated between the freshwater coastal ecosystems and the subtidal regions, forming ecotones of remarkable productivity and biodiversity (Paterson et al., 2011). Nevertheless, these coastal areas have become densely populated since prehistoric times, with most of the population living in the coastal fringe and consuming its natural resources (Kent, 2000). Hence, several coastal wetlands have deteriorated or destroyed by variation in freshwater input and sediment influx from the catchments to the coastal zone (Kennish, 2002). Previous studies have so far not examined the short-term events and their influence on the present climate.

    Multi-proxy studies are a valuable tool for reconstructing long-term environmental changes especially when they combine both marine and terrestrial environmental information (Pędziszewska et al., 2015; Engstrom and Rose, 2013; Birks and Birks, 2006; Birks and Ammann, 2000; Lotter et al., 2000). Biotic and geochemical studies help us to cross-validate, compare the reaction of elements to climatic stress, understand the responses of both terrestrial and aquatic ecosystems to climate change, and develop the relationships between biotic and abiotic elements (Pędziszewska et al., 2015). Geochemical records of the sediments are good indicators of the weathering trends and sources of pollution (e.g., Nath et al., 2000; Fedo et al., 1996; Nesbitt et al., 1996; Förstner and Salomons, 1980). Significant adsorption capabilities of the fine-grained sediments are the primary repository for trace metal. Along with this, the studies of heavy metal contamination in estuarine sediments indicate the environmental health issue (Li et al., 2007) and relevant human activities in the surrounding areas (Delgado et al., 2012). Many indices such as enrichment factor (EF) (Selvaraj et al., 2004), geoaccumulation index (Igeo) (Müller, 1969), contamination factor (CF) (Loska et al., 1997), and pollution load index (PLI) (Tomlinson et al., 1980) assess the state of metal pollution and their relationships. Amongst the biotic proxies, pollen serves as a valuable tool for environmental reconstructions due to its excellent preservation, abundance in both terrestrial and marine sediments and its sensitivity to hydrological changes (van Soelen et al., 2010). Pollen abundances reflect regional vegetation development and also record variation in runoff rates in marine sediments along with coastal vegetation (Srivastava and Farooqui, 2017, 2013; Farooqui et al., 2014). The relative distributions of diatom frustules and organic-walled dinoflagellate cysts had been used to reconstruct palaeoenvironment, stratification and paleoproductivity (Uddandam et al., 2017; Thakur et al., 2015; Smol and Stoermer, 2010; van der Meer et al., 2008; Marret and Zonneveld, 2003; Stoermer and Smol, 1999; Battarbee and Kneen, 1982).

    Kerala State is popularly known as the "Gateway of summer monsoon" over India. In general, the annual rainfall is maximum during the southwest (SW) monsoon (June-September) followed by pre-monsoon (March-May), post-monsoon (October-November) and winter (December-February) rainfall (Krishnakumar et al., 2009). The coastal zone of SW India especially the southern region of Kerala, encompasses the spectrum of coastal landforms such as barrier beaches, lagoons, ridge-runnel systems, wetlands, and estuaries. The SW coast of India is located in a vital position in atmospheric-oceanic circulation. This is a monsoon-dominated coast and has the most prolonged spell of SW monsoon among all other parts of the Indian coastline and having the largest area of wetlands considered as a unique area for oceanographic and meteorological processes. Historical variations in the monsoon have essential links to environmental changes and the cultural development of human societies (Kathayat et al., 2017; Clift and Plumb, 2008; Pandey, 2005; Sukumar, 2000). Geologically, the study area is composed of the Archean crystallines (Khondalites and Charnockite groups of rocks) and Quaternary deposits (represented by coastal sands, muds and alluvium) (Soman, 2002). This humid tropical coastline has been subjected to environmental dynamics of the past several years. The paleoenvironmental history of the Kerala coast is also of significant interest due to its long human occupation and fast-growing population.

    Here, we present a multi-proxy record from the wetland along the Kerala coast, SW coast of India. This record provides the reconstruction of the landscape dynamics in the Kerala coast from 225-2009 cal yr AD and allows inferences of climate variability within the MCA-LIA time frame. Our study is aimed to reconstruct palaeoenvironmental condition during the last 2000 cal yr AD using sedimentological, geochemical, palynological, dinoflagellate and diatom proxies from Cherai, Vembanad wetland region. We also compare the results with other regional and global records to better understand the climatic and environmental implications.

  • A 37 cm long sediment core was collected from the Cherai region (10°8'25.90"N, 76°11'0.18"E), which is lying parallel to Vembanad Wetland System, Kerala coast, SW India at a water depth of 6 m (Fig. 1). Vembanad Wetland is connected with a river and opens through a small opening in its northern part (Azhikode). The fluvial supply result in high siltation and the sediment flux from the catchments is about 32×106 tonnes/yr (Thomson, 2002). Hydrodynamics condition in the Vembanad wetland is very complex and various anthropogenic activities influence it. The core location is situated in a relatively less disturbed region and does not have a direct impact on wave activity and slumping. This region is not directly connected with the main Vembanad wetland. The sediment core was retrieved by pressing PVC pipe into the sediment by divers and the top and bottom capped with special caps. It consisted of silty-sand sediments and was sub-sampled at a 1 cm interval. Sediment samples were stored in clean, labelled low-density polythene bags, taking adequate care to avoid external and cross-sample contamination. The bulk sediments were dried at 50 ℃ in the oven and then grounded using an agate mortar.

    Figure 1.  Study area with the location of the sediment core-Cherai, southwest coast of India.

    A known weight of the dry powder sediment was open digested in clean PTFE Teflonℝ beakers for 8-9 h or until complete digestion using 10 mL of HF/HNO3/HClO4 acid mixture of (7:3:1). The digested sample was made up to 50 mL volume with 2% HNO3. This solution was analyzed for metal and trace elements using a Thermo inductively coupled plasma mass spectrometer (ICP-MS Thermo X-series with CCT) at National Centre for Polar and Ocean Research (NCPOR, Goa, India). The National Institute of Standards and Technology (NIST) standard reference materials (SRMℝ 1646a and SRM 4354) were analyzed in the same manner as the samples. Analytical accuracy was found to be better than 10%-5%. The total organic carbon (TOC) concentration was measured using a total organic carbon analyzer (TOC-V series SSM-5000A from Shimadzu) at NCPOR. The analytical precision of the TOC measurements was ±5%. Grain size analysis of sediments was carried out on bulk samples using Laser Particle Size Analyser (Beckman Coulter LS™ 13 320) at Birbal Sahni Institute of Paleosciences (BSIP), India. After that, the percentages of sand, silt (< 63 mm- > 4 mm) and clay (< 4 mm) were calculated in each sample.

    For Pollen analysis, 10 g of dried sediment samples were treated with KOH followed by HF to dissolve the silica. After following the standard preparation method, the acetolysed residue obtained after passing the sample through 10 μm mesh was studied under a high power light microscope (Olympus BX-53). The qualitative and quantitative study of pollen/spores has been documented as percentage values of more than 200 counts. For dinoflagellate cysts analysis samples were processed according to standard laboratory procedures. About 10% HCl and 40% HF were used to remove carbonates and silicates, respectively, at room temperature and the macerals were sieved through 20 µm mesh for identification. Palynomarine (PMI) index and Heterotrophs/Autotroph dinocysts ratio (H/H+A) was also calculated for palaeoenvironmental interpretation. For the diatom study, standard procedure for maceration and identification was followed. The diatom frustules were counted between 300-1500 as they were highly diverse in the entire dataset. The diatom range chart was plotted in Tilia 1.7 software.

  • Chronological control was obtained by accelerator mass spectrometry (AMS) radiocarbon (14C) dating on the total organic carbon content of the selected intervals of the Cherai sediment core from the SW Kerala coast at DirectAMS, USA. The age model is based on linear interpolation between the three dated horizons. All radiocarbon ages were calibrated to calendar ages using the Oxcal 4.3 program (Bronk Ramsay, 2009), and all the dates cited in the text thus refer to calendar years. The depth vs. age plot of the sediments, shown in Fig. 2, indicates that the rates of sedimentation vary between 1.1 and 3.5 cm/100 yr and they have been extrapolated to decipher the environmental changes and coeval climatic events in the region in a specific time frame. However, we recognize that the relative timing of events discussed here requires an element of caution. Relatively high TOC values of the sediments indicate that it must have been derived from the biogenic material and the possibility of contamination by a reworked terrestrial component is relatively little.

    Figure 2.  The age-depth model for the Cherai sediment core samples.

    Textural parameters are considered as an essential index for understanding the transport history, sediment composition, origin and depositional environment. The average grain-size composition of the Cherai core is as follows (wt.%): 34.42% sand; 59.45% silt and 6.13% clay. The upper part of the core shows a more fine-grained composition (higher silt content) as compared to the lower part. The sediments are mainly composed of sand and silt contributing > 90% and Pejrup (1988) ternary diagram reveals that the sediments fall mostly within Ⅳ-section of the ternary diagram, which is confirming that sediments were deposited under violent hydrodynamic condition (Fig. 3a). The variation in lithology coincides with the colour change, which varies from light-gray to dark grey (Fig. 2).

    Figure 3.  (a) Ternary diagram showing deposition of sand, silt and clay under varying hydrodynamic energy conditions; (b) Mixing diagrams of Mg/Na vs. Ca/Na molar ratios of Cherai sediment core (green). Blue, red, orange and black labels represent bedrock compositions of Charnockite, Gneiss, Amphibolite and Khondalite respectively (Braun et al., 2009; Soman, 2002; Sharma and Rajamani, 2000).

  • The geochemical record of major and trace elements of Cherai core from the Kerala coast is presented in Fig. 4. The TOC content in the core is 2.05%-2.91%, which is also showing a similar pattern with the above elements. Apart from this, Ba and Pb show a similar pattern and As and Cr do not show any similarity in their pattern (Fig. 4). Elements such as Al, Fe, Ti, Mg, Mn, Cd, U, Co, Ni, Zn, Cu, Sc and TOC registered a decrease in value and remained low between 220 and 545 cal yr AD. These elements gradually increased upwards from 590 cal yr AD, and then remained high and stable until recent (Fig. 4). Most of the elements show high values at 1430 cal yr. The elements Cu and Cd show high values around 1920 cal yr AD. The value of the Pb also showed a similar pattern with the Ba and high values were observed during 1180 cal yr AD to recent (Fig. 4). The concentration of As shows a gradual increase and the highest values observed at around 1045 cal yr AD and then gradually decreased. The concentration of the Cr shows high values between 355-865 cal yr AD and remains low during the remaining intervals.

    Figure 4.  The metal concentration of the Cherai core sediment samples.

  • A total of 16 pollen taxa are identified in the present study, including both coastal and terrestrial plants. True mangroves identified in the samples comprise of Rhizophora and Ceriops sp. along with several terrestrial taxa such as Hiptage, Casuarina equisetifolia, Hibiscus sp., Malvaceae, Asteraceae, Solanum sp., Bombacaceae, Convolvulaceae, and Poaceae. Aquatic plants such as Nymphaea, Eichhornia crassipes and Typha latifolia and xerophytic plants like Acacia and Chenopodiaceae were also reported in the study. Scolecodont fragments, microforaminiferal linings, fungal and pteridophytic spores represented the non-pollen palynomorphs in the studied samples (Fig. 5). Pollen concentration ranges between 1720-160 grains/cm3 with an average of 194 grains/cm3 during 225-545 cal yr AD. The assemblage is dominated by Hibiscus sp. (30%) in association with Eichhornia crassipes (10.3%) throughout the zone, whereas Chenopodiaceae (3.9%) was recorded from 410-545 cal yr AD. Microthyriaceae fungi (27%) and pteridophytic spores (18%) were predominant in the entire zone. Around 455 cal yr AD presence of Scolecodont fragments (5.2%) and foraminiferal lining (2.8%) coincides well with the decline in terrestrial taxa. Palynomarine index (ratio between the total marine palynomorphs and total terrestrial palynomorphs) for this zone ranges from 0.51 to 0.68, with an average of 0.61. Palynomorph diversity ranges from 0.9 to 1.5, with an average of 1.23, while the evenness in data ranges between 0.99-0.84 with an average of 0.93.

    Figure 5.  Palynological diagram showing pollen and dinoflagellate cyst absolute abundance for the Cherai core.

    The period between 545-1000 cal yr AD shows an increase in the pollen concentration range from 1980-60 grains/cm3 with an average of 314 grains/cm3. The assemblage shows the presence of true mangrove Rhizophora (5.3%) and Ceriops decandra (0.8%) along with Eichhornia (10.4%), Bombacaceae (3.6%) and Poaceae (5.3%). Hibiscus sp. (2.8%) and Typha (0.83%) were found only at one horizon in the zone c. 910 cal yr AD while Acacia (1.1%) and Chenopodiaceae (1.4%) were present at c. 1000 and 680 cal yr AD, respectively. Pteridophytic spores (27.4%) and foraminiferal linings (19.11%) dominated the non-pollen palynomorph assemblage along with the presence of Scolecodont fragments (3.0%) and Chytrid fungi (3.3%) from c. 680 to 770 cal yr AD. The higher palynomarine index is observed during this period that ranges from 0.59 to 0.85. Higher pollen and spore diversity ranging from 2.0 to 1.0 and an evenness ranging from 0.70 to 0.96 was also recorded.

    The total pollen concentration between 1000-1230 cal yr AD ranges from 1040 to 60 grains/cm3. In the absence of mangroves, the pollen assemblage is dominated by terrestrial taxa such as Hibiscus (12.2%) and other Malvaceae members (3.2%), Asteraceae (2.6%), Poaceae (3.2%), Acacia (1.9%) and aquatic taxa Eichhornia (7.7%) and Typha (8.3%). The non-pollen palynomorph assemblage is dominated by pteridophytic spores (33.3%) and microthyriaceae fungi (14.1%), followed by pseudoschizaea (8.3%) and scolecodont fragments (5.1%). The palynomarine index, palynomorph diversity and evenness range in this zone range from 0.28 to 0.77, 1.4 to 1.9 and 0.8 to 0.9, respectively. From 1230-1840 cal yr AD pollen concentration ranges between 1120 to 80 grains/cm3 with an average of 217 grains/cm3. There was a dominance of terrestrial pollen like Hibiscus sp. (20.8%), Asteraceae (10.4%), Eichhornia (8.4%), Convolvulaceae (6.0%), Acacia (3.6%), Malvaceae (2.4%) and Poaceae (2.4%). Non-pollen palynomorphs were also dominated by pteridophytic spores (22.4%) and microthyriaceae fungi (14.8%) along with Scolecodont fragments (4.4%), Chytrid fungi (1.6%) and fungal spores (2.8%). Palynomarine index ranged from 1.00 to 0.26, with an average of 0.63. Lower pollen and spore diversity is observed, ranging from 1.80 to 0 with an average of 1.07 along with the evenness ranging from 0.95 to 0, with a mean value of 0.70. The period from 1840 cal yr AD onwards indicates a low pollen concentration phase ranging from 520 to 60 grains/cm3 (average=132 grains/cm3) at c. 1960 cal yr AD. The rest of the zone was devoid of any pollen and spore. Assemblage around 1960 cal yr AD is dominated by terrestrial pollen such as Hibiscus sp. (14.5%), Asteraceae (9.9%), Malvaceae (6.6%), Acacia (6.0%) and Poaceae (5.3%). Nymphaea (4.0%), Casuarina equisetifolia (4.0%), Hiptage sp. (3.3%) and Solanaceae (2.0%) pollen were also observed from the same level. Non-pollen palynomorph content also shows dominance of microthyriaceae fungi (17.1%), Scolecodont fragments (10.5%), pteridophytic spores (8.5%), pseudoschizea and fungal spores (3.3%) and Chytrid fungi (2.0%). Palynomarine index for this zone ranges from 1.00 to 0.35, with an average of 0.84. This period marks the lowest palynomorph diversity, with an average of 0.63 and average evenness of 0.20.

  • Dinoflagellate cysts assemblage consist of 15 species with autotrophic ones represented by Bitectatodinium spongium, Lingulodinium machaerophorum, Spiniferites spp. Operculodinium centrocarpum and Tuberculodinium vancompoae and Heterotrophic species represented by Brigantedinium spp. Selenopemphix quanta, S. nephroides, Protoperidinium sp, Votadinium calvum, Lejeunecysta oliva, Lejeunecysta sabrina, Echinidinium spp. Trinovantedinium applanatum and Polykrikos Kofoidii. Autotrophic taxa show dominance over heterotrophic taxa during the entire period with B. spongium forming the majority of the assemblage (up to 60 % of the assemblage).

    The period between 225-545 cal yr AD is characterized by a high concentration of dinoflagellate cysts between 500-1620 cyst/g (average 928 cyst/g). Shannon diversity, species evenness and H/A index ranges from 0.8-1.1, 0.3-0.5 and 0.07-0.12, respectively. High dinocyst concentration from 1480-4180 cyst/g marks the period between 545-1000 cal yr AD and the maximum concentrations of the cyst is observed at 910 cal yr AD. Autotrophic species such as T. vancompoae and O. centrocarpum are confined only in this period, whereas L. machaerophorum shows maximum concentration. The majority of the heterotrophic species also confined to this period with maximum abundance (Fig. 5) with the highest species diversity index, evenness and H/A index ranging from 0.8-1.54, 0.2-0.4, 0.1-0.3, respectively. B. spongium concentration shows the highest abundance; however, the relative abundance shows a decreased concentration. During 1000-1230 cal yr AD shows a reduction in dinoflagellate cysts concentration ranging from 60-120 cysts/g. Species diversity, evenness and H/A index ranged from 0.1 to 0.8, 0.3 to 0.94 and 0 to 0.16, respectively. A decreased dinoflagellates concentration recorded during 1230-1840 cal yr AD (40-1220 cyst/g) and autotrophic taxa show dominance over heterotrophic species. The period between 1840-2009 cal yr AD is characterized by the rare occurrence of dinoflagellate cysts, which ranges from 20-40 except the 1980-2009 cal yr AD. Cyst diversity is lowest with the only occurrence of autotrophic species of B. spongium and Spiniferites spp.

  • About 52 diatom genera were recorded with the dominance of pennate (benthic, 41 genera) over the centric (planktic, 11 genera) forms in the assemblage. During 230-550 cal yr AD, the brackish to marine diatoms Diploneis, Thalassiosira dominated along with varying proportions of freshwater forms Gyrosigma, Navicula and Nitzschia indicating freshwater input in the vicinity of the depositional site. The cosmopolitan form Cyclotella (Oliva et al. 2008) is also found in low abundance during this interval. Thalassiosira and Diploneis shows the lower frequency at 230 cal yr AD followed by higher abundance around 365 cal yr AD and then declining at 545 cal yr AD. While Navicula and Nitzschia show relatively stable concentration during this period. Low turnout of marine centric diatoms tends towards lowering of wave/tidal activities.

    The centric diatoms are dominated by high values of Thalassiosira, Cyclotella, Biddulphia, Actinocyclus, Actinoptychus during 545-1090 cal yr AD while the pennate diatoms show the abundance of Navicula, Nitzschia, Surirella, Cocconeis, Anomoeoneis and Gyrosigma (Fig. 6). The time between 550-1090 cal yr AD witnesses an episodic variability of diatoms from 680-775, 820-1000 and 1045-1090 cal yr AD. These three short periods show variability in centric and pennate diatoms wherein during the period of 680-775 cal yr AD, the planktic forms rose over benthic ones suggesting a short inundation of sea water with mixed hydrodynamic behaviour of fresh and marine waters. The second period from 820-1000 cal yr AD shows higher freshwater benthic diatoms over planktic forms with high variation in the cosmopolitan species of Cyclotella. However, the total counts of the diatoms increased during this period indicating a moderate rise in productivity, indicating nutrient richness. The third intervening period from 1050-1090 cal yr AD also record higher counts of both planktic and benthic diatoms with high freshwater forms.

    Figure 6.  Range chart of diatoms from Cherai core.

    The period from 1140-1390 cal yr AD shows a sharp fall in the overall frequency of planktic diatoms while the benthic diatoms show an increasing trend. During this period, Diploneis increased along with Navicula, Nitzschia, Achnanthes, Amphora, Nitzschia panduriformis, Stauroneis and Surirella, suggesting a strong influence of anthropogenic activity during the depositional time. During the period of 1440-1660 cal yr AD diatom assemblage and diversity reduced to a great extent. Total diatom frustules also declined with lower counts of some major forms represented by Diploneis, Navicula, Nitzschia, Achnanthes, Amphora, Nitzschia panduriformis, Stauroneis and Surirella. The period above 1710 cal yr AD shows high variability in diatom assemblage and diversity. Around 1710 cal yr AD, an increase in diatom frequency followed by sharp fall from 1770-1875 cal yr AD is observed. The frequency slightly increased in 1930 cal yr AD and declined at 1980 cal yr AD. The role of climate cannot be subdued as the weakening of precipitation during the time periods 1765, 1875 and 2009 cal yr AD also points towards low diatom frequency.

  • The period during 220-2009 cal yr AD showed subtle changes and divided into different time units as per the global events such as RWP, DACP, MCA, LIA and recent. The sediment horizons around 1850, 1200, 1000 and 600 cal yr AD marked the common boundaries of these elemental content changes, according to the sedimentological and geochemical characteristics, five zones have been recorded. While the biotic responses (pollen, dinoflagellates and diatoms) to climate change have been recorded around 1840, 1230, 1000 and 545 cal yr AD based on which the palaeovegetation and climate have been reconstructed. All the proxies were sensitive climate indicators and complementary in their ability to reconstruct climate and environment. However, the fast-reacting proxies such as diatoms and dinoflagellates show climate signals directly, whereas the long-lived proxy such as pollen show delayed reactions. The combination of these helps in recording the leads and lags in the reaction of different biotic and abiotic systems to climate change and, in turn, enhances the understanding of community ecology under changing climatic conditions.

  • Water-soluble elements are dissolved due to chemical weathering by terrestrial water and transported into estuary and seawater (Zhao and Zheng, 2014), whereas water-insoluble elements are physically transported by the terrestrial water, oceanic current or wind. The geochemical composition of detrital sediments is mainly controlled by the property of source rocks and the intensities of chemical weathering. Therefore, the elemental oxide ratios have been used to trace the intensity of chemical weathering.

    The main rock sources in the study area are weathered lateritic, Archean crystalline rocks and Tertiary sediments are present in the course of the Periyar and Muvattupuzha River (Soman 2002; Padmalal et al., 1997). The plot of Ca/Na and Mg/Na molar ratios of the estuary and bedrocks (Braun et al., 2009; Rajamani et al., 2009; Soman, 2002; Sharma and Rajamani, 2000) (Fig. 3b) shows that core sediment falls close to the silicate end-member defined by Gaillardet et al. (1999). This suggests that the main composition of the source rocks fall close to the Charnockitic and gneissic composition and must have been derived from local erosion, probably under fluctuating climatic conditions. Chemical index of weathering (CIW) is defined as CIW=molar[Al2O3/(Al2O3+CaO+Na2O)]×100 (Harnois, 1988). The CIW index is used in this study to monitor paleoweathering at the source area, which is not sensitive to post-depositional K enrichments. The lower CIW values range from 48.17 to 62.88, with an average value of 57.37 of the studied core. The index values are low in comparison to upper continental crust (UCC) (McLennan, 2001) (65.24) and Post-Achaean Australian shale (PAAS) (Taylor and McLennan, 1985) (88.32), hence indicating low to moderate source area weathering. Higher values of CIW indicate stronger weathering and presumably wetter conditions and a strong monsoon (Sun et al., 2010). The increase in CIW during early DACP/MCA reflects a decrease in (MgO+CaO+Na2)/Al2O3, Ca/∑[Ti, Al, Fe], CaO/Al2O3, CaO/MgO, MgO/Al2O3 and FeO/Al2O3. The lowest CIW value is recorded during the DACP, which is suggesting a cold/arid condition. The rest of the period shows almost constant medium values suggesting a more humid condition and the little difference reflects the same amount of chemical weathering. We assume that lower rainfall during late DACP reduces the delivery of detrital elements Ti, Fe and Al to the estuary, which gradually increased during the MCA. Higher Ca/∑(Ti, Fe, Al) ratios reflect dry conditions and lower ratios reflect wet conditions. The Ca/∑(Ti, Fe, Al) ratio in the core display low and very small changes in values other than the DACP period (Fig. 7) and it reflects elevated amounts of authigenic carbonates that indicate higher ion concentration in the water. The increased Ca/∑(Ti, Fe, Al) values during the early MCA period suggest a drying trend in the study area, which terminated at ∼775 cal yr AD (Fig. 7). Studies showed that the CaO/MgO ratio could be used as an indicator of the water budget during deposition. Higher CaO/MgO suggests more carbonate under warm and dry climate conditions (Wang, 1992). The CaO/MgO record is coincident with the other records from the Cherai core.

    Figure 7.  Elemental and oxide ratios depicting the weathering and precipitation changes during last 2000 cal yr AD.

    Rubidium and strontium are easily fractionated during common processes at the earth's surface, including weathering, due to different geochemical behaviour. High Rb/Sr values during the warming intervals like DACP and MCA, indicating a higher intensity chemical weathering in the Cherai region during these periods (Fig. 7). There is a gradual increase in detrital elements observed during DACP, which may indicate increased precipitation. The DACP event generally regarded as cold, while many studies linked with hydroclimatic changes (wet, dry or unspecified change), most often with wet or moist conditions. This could be the reason for the strong weathering during the dry climate. Also, the climatic conditions during RWP are warm, there were wet and humid conditions also existed (Banerji et al., 2019, 2015; Martin-Puertas et al., 2009). Correspondingly, there are relatively weak chemical weathering phases where Rb/Sr ratios are lower than the former during late MCA, LIA and RWP. Most of the metals show good correlation with fine-grained sediment and also associate with the organic matter in the core. Heavy metals are generally adsorbed onto silt and clay particles and thus, metal trends could reflect a grain size trend (Grant and Middleton, 1990). So, both the finer grain size and total organic carbon can be the dominant factors controlling the metal distribution in the study area. The similarities between redox-sensitive metals such as Fe, Mn and U indicate that either these elements have undergone similar early diagenetic remobilization and reprecipitation around redox boundaries or that all these elements are derived from the same source.

  • The overall dominance of Hibiscus, pteridophytic spores and aquatic taxa Eichhornia suggests a warm and moderately wet climatic condition during 225-455 cal yr AD. Diatom records ~230-455 cal yr AD also show high freshwater discharge from land with a high abundance of Thalassiora sp. and other planktic diatoms. Between 455 and 545 cal yr AD, an expansion of salt-tolerant plants like Avicennia and Chenopodiaceae is interpreted as the development of supratidal back mangroves along with herbaceous vegetation in response to the decline in rainfall and onset of aridification. This phase corresponds to the end of the RWP and the beginning of the DACP. Palynological data from the Pichavaram estuary southeast coast of India also suggests the existence of salt-tolerant mangroves during this period under weakened monsoon conditions (Srivastava and Farooqui, 2013). High diversity of freshwater benthic diatoms after 455 cal yr AD also attributes to a fall in relative sea level. The period also shows eutrophic environmental conditions reflected through high abundances of B. spongium dinoflagellate cysts. High concentrations and relative abundance of B. spongium along with the occurrence of peridiniodes during this period indicating a well-mixed water column with a fully marine conditions and eutrophic environment (Zonneveld et al., 2013). B. spongium dominates the assemblages up to 60% during this period. It occurs in a fully marine environment with sea surface salinity ranging from 28-35.5 PSU and sea surface temperature ˃10 ℃ (Uddandam et al., 2017; Zonneveld et al., 2013). Protoperidinioid species L. oliva, L. sabrina and round browns, including Brigantedinium have also been reported from eutrophic environments at upwelling or river plume regions along with the low oxygenated bottom water conditions favouring their preservation (Uddandam et al., 2017; Zonneveld et al., 2013).

  • A general trend from low tree pollen and core mangrove abundance before 545 cal yr AD, towards higher terrestrial pollen percentage and Rhizophora abundances between 545-1000 cal yr AD suggests a change in prevailing hydrology from dry to humid conditions. True mangroves such as Rhizophora and Ceriops, along with terrestrial arboreal taxa attributes to a well-developed estuarine ecosystem with an excellent freshwater discharge into the sea (Yao and Liu, 2017; Srivastava et al., 2012). The presence of foraminiferal linings and scolecodont fragments around 680-770 cal yr AD suggests greater marine influence with well-mixed water conditions in the estuary. This species preference of mangrove development is attributed to the freshwater influx from land and saltwater incursion in the mangrove swamp from the sea. High abundance and diversity of freshwater planktic and benthic diatoms during this period point towards a high freshwater influx leading to a well-mixed environmental setting in the estuary (Smol, 2008). The presence of marine elements dinoflagellate cysts, foraminiferal linings and scolecodont fragments revealed that there had been marine incursion during the period, which led to a well-developed estuary during the beginning of MWP. An increase in diatom Eunotia sp. indicates a clean water condition with a mild acidic environment. The phase shows an increase in total dinoflagellate cysts along with protoperidiniod cysts indicating maximum productivity and highest nutrient concentrations in the last 2000 years indicating marine influx along with the high runoff. Protoperidinioides mainly feed on solitary and colonial diatoms and other small autotrophs (Jacobson and Anderson, 1986). An increase in the protoperidinioides indicates the availability of diatoms as prey (Matsuoka, 1999). This reveals enhanced nutrient supply in the Cherai region during MWP due to wet conditions associated with the high runoff discharge and strong summer monsoon. Such an increase in the precipitation during the MWP or early MCA has also been observed from other Indian monsoon regimes (Suokhrie et al., 2018; Quamar and Chauhan, 2014).

  • The MCA has been globally characterized by a warm climate in terms of temperature. Nonetheless, there are divergent evidences in relation to precipitation. In some regions, it was a dry period (Chen et al., 2015), while at some it was characterized by humid conditions (Sosa-Najera, 2013), thus, in general, it is a partially understood episode of Late Holocene climate change (Graham et al., 2011). In the present study, there was an abundance of herbaceous open ground vegetation and a decline in mangroves and arboreal suggesting a less humid late MCA in comparison to early MCA or MWP. Overall dominance of terrestrial nonarboreal taxa such as Hibiscus, Asteraceae, Poaceae, Acacia, Eichhornia and Typha suggests that the environment throughout the MCA was relatively less humid to support an estuarine ecosystem. Chauhan et al. (2010) reported an arid event centered at 1000 cal yr AD in the medieval warming. This is also evidenced by the absence of mangroves and arboreal plants along with the presence of xerophytic Acacia sp. and a lower marine index after 1000 cal yr AD, at the onset of MCA suggesting less fluvial input into the sea. A sharp decrease in the abundance of dinoflagellate cysts also indicates a reduction in productivity and runoff during this period. During this period the brackish-marine diatoms Diploneis, Nitzschia panduriformis increased along with freshwater diatoms such as Navicula, Nitzschia, Achnanthes, Amphora, Stauroneis and Surirella suggesting a high influence of anthropogenic activity in the area (Rimet et al., 2015; Kelly et al., 2008; Smol, 2002).

  • A further decrease in the pollen and dinoflagellate cysts diversity between 1230-1840 cal yr AD and the dominance of xerophytic taxa and autotroph dinoflagellates indicates a reduction in runoff from land and low productivity, respectively. The abundance of Acacia, which typically tolerates drier climatic conditions, confirms the prevalence of water-stressed environment, most probably caused by reduced rainfall (or excessive evapotranspiration) during the LIA. The climate during this period indicates reduced precipitation, which is unfavorable for tree growth, evidenced by declining plant diversity. The low abundance of the centric (planktic) diatoms during this period may be due to reduced water level in the estuarine complex leading to a dominance of benthic forms. An increase in Navicula, Nitzschia, and Amphora directly suggests human interference in the estuarine complex. The presence of diatoms sensitive to organic pollution such as Amphora, Achnanthes, Synedra and Achnanthidium may be a result of stressful climatic conditions due to weakened monsoon during LIA. The dominance of autotrophic dinoflagellate cysts species B. spongium and Spiniferites with a negligible contribution of protoperidinioides reveals a drastic reduction of productivity during this period. It could be possible that the reduction in the dinocyst abundance, mainly protoperidinioides could be due to the preservation effect as they are sensitive to oxygen degradation. However, this does not seem to be the case in the studied core, as the dominant species B. spongium is only reported from low oxygenated conditions (Uddandam et al., 2017; Zonneveld et al., 2013). Thus, the decreased diversity and abundance of dinocyst indicate climate deterioration during LIA, resulting in low nutrients and dry periods. This weak monsoon phase is inconsistent with the southern shift of the Inter-Tropical Convergence Zone (Yan et al., 2015; Gupta et al., 2003; Haug et al., 2001). The productivity decline shows consistency with the solar minima during the LIA (Kurian et al., 2009). This zone covers the global LIA that comprises of three low precipitation phases between 1450-1510, 1640-1720 and 1790-1820 cal yr AD (An, 2014) along with temperature decline between 1580-1850 cal yr AD (Osborn and Briffa, 2006).

  • Between 1840-1960 cal yr AD (post-LIA) absence of pollen and dinoflagellate cysts in the sediment is interpreted as the impact of dry climatic conditions during LIA, which had led to the deterioration of vegetation and loss of nutrients during post-LIA. The subsequent absence of vegetation post-LIA (1850-1960 cal yr AD) suggests climate deterioration due to a prolonged decline in precipitation (Kotlia et al., 2010), which was further enhanced by various anthropogenic activities. After 1960 cal yr AD, herbaceous plants recolonized in the area in response to the recent warming, which is enhanced by anthropogenic influence. The non-arboreal taxa recolonized the region, which may be due to the rejuvenation of climate post-1915 cal yr AD with excess summer and winter monsoon (Patnaik et al., 2012; Rao, 1999). However, a loss in plant diversity along with a decline in dinocysts count indicates a comparatively less humid Current warm period (CWP) than MWP or RWP. Dinoflagellate cysts show sensitivity to the stressed environment caused due to the high nutrient uploading due to cultural eutrophication (Dale, 2001). Two anthropogenic dinocysts signals, Oslofjord and heterotrophic signal has been reported. The Oslofjord signal shows an approximate doubling of the total cyst concentration with a sharp increase in the L. machaerophorum (cf. Dale, 2001). The heterotrophic signal shows the total decrease in the cyst concentrations but increase in the protoperidinioides abundance due to the increase in the prey availability (Matsuoka, 1999; Thorsen and Dale, 1997). The decrease in dinocyst abundance in the present study is gradual and started much before the human activities in the studied region. This indicates that dinocyst assemblage in the studied core predominantly reflects climate change rather than anthropogenic impacts. During this period, the diversity of diatoms also reduced significantly with the low abundance of Navicula, Nitzschia, Diploneis, Amphora. Increase in Biddulphia, Campylodiscus may be attributed to high salinity in the area suggesting wave/tide dominance in the Cherai region (Santhanam et al., 2018).

  • As one of the developing regions in SW India, the Cochin city, where the core site is located, also witnessed considerable economic growth. This rapid development can lead to the release of large amounts of metals into the environment, which results in the enrichment of sedimentary heavy metals (Morelli et al., 2012; Ip et al., 2005). This might be the major cause of the relative enrichment of heavy metals in the study area.

    In order to identify the possible contamination of the sediments during the past 2000 yrs at Cherai, we calculated the enrichment factor, which allowed us to identify the sources of natural elements and the quantitative assessment of the degree of pollution for each element (Ying et al., 2008). EF for each element was calculated using the formula: EF=(M/Al)sample/ (M/Al)shale (Abubakr, 2008). Where (M/Al)sample is the ratio to aluminium concentration of the metal in the sample, (M/Al)shale is the world shale average of the metal (Turekian and Wedepohl, 1961). The result shows that the metals (Mn, Fe, Co, Zn, Sc, and Mg) show no enrichment to minimal enrichment in Cherai region (Fig. 8a). This low to negligible enrichment value suggests its origin from lithogenous material derived entirely from the crustal materials or natural weathering and absence of contamination by these metals during the last 2000 yrs. While Ni and Cu show minimal to moderate enrichment during 590-1000 and around 1915 cal yr AD, respectively. Cr, Pb, Cd and As show moderate to significant enrichment values suggesting that it might be affected by anthropogenic inputs. Cr shows the highest EF value (410 cal yr AD) among the studied metals and shows moderate to significant enrichment during 319-910 cal yr AD periods and the remaining period shows minimal enrichment. High values of Cr concentration are mainly observed during RWP and the DACP/MCA transition. The Cr concentration in rocks varies from an average of 5 mg/kg (range of 2-60 mg/kg) in granitic rocks to an average 1800 mg/kg (range, 1 100-3400 mg/kg) in ultrabasic and serpentine rocks and higher concentrations mostly as a result of human activities, reflecting pollution from industrial activities and sewage discharges (WHO, 1988; US NAS, 1974; Perlmutter and Lieber, 1970). The high Cr values during 355-865 cal yr AD maybe suggests that it is present as Cr(Ⅳ), which is relatively mobile and after release in the pore waters, they migrate downward into the reducing zone and precipitations again as Cr(OH)2 (Jonathan et al., 2010; Kumar and Edward, 2009). The less variations of Cr in the above section indicate less mobility of this element. Pb shows moderate enrichment during 590-1100 cal yr AD and the remaining period shows significant enrichment. Cd in the studied core shows significant enrichment throughout the period, except minimal values during 410 and 545 cal yr AD. As also shows moderate to significant enrichment throughout the last 2000 cal yr AD. Previous studies from the Vembanad wetland surface sediments also point out the increasing contamination level in the recent sediments (Manoj et al., 2018; Salas et al., 2017; Balachandran et al., 2006). These results suggest that the study area is polluted by metals such as Cr, Cd, As, and Pb, which indicate the diverse source of these metals (e.g., anthropogenic) in the study area during the last 2000 cal yr AD. The anthropogenic sources may include the discharge of industrial effluents and increased use of fertilizers, pesticides, insecticides, petrol refining, metal processing and urban activities through rivers and small channels (Manoj et al., 2018).

    Figure 8.  (a) Enrichment factor, (b) geochemical index (Igeo), (c) modified degree of contamination and (d) pollution load index of the Cherai sediment.

    Geoaccumulation index was used to calculate the metal contamination levels for the sites from the Cherai region (Fig. 8b). The Igeo rank was assessed using the seven classes for increasing Igeo values proposed by Müller (1969). The present study reveals uncontaminated to moderate contamination of sediments by Mn, and moderate to strongly contamination by Fe, Co, Zn, Cu, Sc, and Mg. As shown in the EF values Cr shows strongly to extremely contaminated Igeo values 320-910 cal yr AD period (Fig. 8b). Ni and Pb show moderate to strongly contaminated values, especially after 590 cal yr AD. High Igeo values (strongly to extremely contaminated) of Cd and As confirm that these metals cause pollution to the study area. These results are supported by the enrichment factor values of the present study suggested that Cr, Cd, As and Pb were affected by anthropogenic inputs. The high Igeo values in most of the elements are started increasing after 590 cal yr AD.

    Metal records of the core sediment were also assessed for modified degree of contamination. The revised Hakanson equation was used to calculate the modified degree of contamination of metals. The mCd values for the individual sites lie in the range of 0.04-1.97 (Fig. 8c). Since 500 cal yr AD, the period shows values greater than 1.5, which suggests the overall contamination was nil to low before 500 cal yr AD and started increasing after that, probably due to the anthropogenic influence at the study area. The severity of pollution and its variation during the past were determined using PLI (Adebowale et al., 2008). The PLI values of the Cherai core ranged from 0.66-1.37 (Fig. 8d) and showed a similar pattern with the mCd profile. Till 590 cal yr AD, the study shows low values (< 1) and the remaining period shows values greater than 1. Values higher than 1 suggest moderate contamination levels in these areas. However, the values of the heavy metal pollution index were indicating moderate contamination, the heavy metals values were less than or close to the permissible limit of the U.S. Environmental Protection Agency and World Health Organization. This suggests that these elements can be considered as a low threat to the environment.

  • The occurrence of the warm (MWP/MCA) and cold (LIA) periods differs from region to region in terms of timing, duration and magnitude of the temperature anomalies (Dixit and Tandon, 2016; PAGES 2K consortium, 2013). Previous studies report centennial- to millennial-scale changes in the ISM during the Holocene from the Arabian Sea (e.g., Gupta et al., 2003; Staubwasser et al., 2003), the Arabian Peninsula (e.g., Fleitmann et al., 2007; Neff et al., 2001), and the Indian subcontinent (e.g., Dixit et al., 2015, 2014; Dutt et al., 2015; Dixit, 2013; Berkelhammer et al., 2012), while records from Indian subcontinent especially from south India is sparse. Dixit and Tandon (2016) complied with the past millennium hydroclimate variability in India from regions affected by the Indian Summer Monsoon (ISM) and the Westerlies. Their studies suggest that there were no globally synchronous warm or cold intervals that define an MCA or LIA on the Indian subcontinent, while a pattern of generally coherent regional precipitation variation during MCA and LIA period is observed. We compared the multi-proxy record of Cherai sediments with other continental and marine palaeoclimatic records to explore global and regional trends of climate variability during the last 2000 years (Fig. 9). Several centennial-scale warm/cool phases in the present study synchronize well with the SST record from Southern Okinawa Trough (Wu et al., 2012), G. Bulloides% record from Arabian Sea (Anderson, 2002) Northern Hemispheric (Arctic) temperature record (Mckay and Kaufman, 2014) (Fig. 9). This includes warm periods such as RWP, STWP, MWP and CWP (Patterson et al., 2010; Cronin et al., 2003) and cold periods such as DACP and LIA (Nyberg et al., 2002; Keigwin 1996).

    Figure 9.  Centennial-scale climate fluctuations and warm/cool events during the last 2000 cal yr AD and its relation with the SST record from Southern Okinawa Trough (Wu et el al., 2012), G. Bulloides% record from Arabian Sea (Anderson et al., 2002), total solar irradiance (TSI) data (Steinhilber et al., 2012), Northern Hemispheric (Arctic) temperature record (McKay and Kaufman, 2014). Warm periods (RWP, STWP, MWP and CWP) and cold periods (DACP and LIA) are also shown.

    A close link between monsoon, climate and silicate weathering intensity in the Cherai region can be observed from the multiple proxies (Fig. 9). The normal annual rainfall over Kerala from 1871 to 2005 is 2817 mm and the dependable seasonal rainfall at for pre-monsoon, southwest monsoon, post-monsoon and winter season is 269.3, 1624.2, 341.0 and 30.3 mm, respectively (Krishnakumar et al., 2009). Cherai region receives rainfall during most of the season, but the SW predominates above other season. The total solar irradiance (TSI) data (Steinhilber et al., 2012) and different records from Cherai core compared and show the relationship between sunspot activity during the last 2000 cal yr AD (Fig. 9). This close relationship is already established from Karnataka, southern India (Warrier et al., 2017), which suggests that solar activity has a profound influence on the SW monsoon. The influence of total solar irradiance on the Indian monsoon is well documented in several studies (e.g., Sandeep et al., 2015; Tiwari and Ramesh, 2007; Tiwari et al., 2005). Within the chronological error, the variation in the present record is in agreement with the sunspot minima of TSI during the Dalton, Maunder, Spörer, Wolf and Oort Minimum. Sandeep et al. (2015) also documented the rainfall variations in Pookot Lake (SW coast of India) and its connection to TSI data. However, some of the intervals are non-synchronous and/or leading with the present records could be due to the uncertainties in the age-depth model.

    A positive response of silicate weathering in the studied region catchment to monsoon climate is apparently observed according to the variations of these proxies. The present records show an increased weathering/higher energy and monsoon fluctuation at the beginning of MCA followed by strengthening of the monsoon. The higher CIW values, i.e., stronger silicate weathering intensity and higher energy condition during the MWP in this study point to fluctuating summer monsoon/humid climate, while the CIW values started decreasing during MCA and almost stable throughout LIA and showing a gradual increase in the present CWP. The results of this study are in accordance with recent researches. A small increase in the rainfall is also recorded during the LIA, around 1650 cal yr AD, which is also recorded from the other parts of India like, e.g., lake sediments (Sandeep et al., 2015; Shankar et al., 2006), Akalagavi speleothem (Yadava et al., 2004) western India tree ring (Bhattacharyya and Yadav, 1999). A weak monsoon during the LIA and a strong monsoon during the MWP documented in the lake sediment in Kerala (Sandeep et al., 2015) and marine core from the eastern Arabian Sea (Tiwari et al., 2005). Geochemical records from the present study also show low rainfall during LIA and high rainfall during MWP, which is also recorded from speleothem record (Sinha et al., 2007). The total loss of vegetation during the later part of this zone marks this lowering of temperature along with a low precipitation phase during the end of LIA (Patnaik et al., 2012).

    The transition from the MCA to the LIA occurred around 1310 cal yr AD based on a distinct shift to more positive δ18O values of speleothem record and recorded low rainfall during 1310 and 1660 cal yr AD (Sandeep et al., 2015; Fleitmann et al., 2004). Relatively stable weathering and drying phases are recorded during the LIA period and wet rain spells are not prominent during this phase. The events recorded in the Cherai proxies appear to anticipate most of the warming and cooling events, which must be due to the inadequate chronological assessment. When comparing different climatic proxy records from different environments, it shows responses to a particular climatic forcing, which is well documented in the schematic model (Fig. 10). Zonneveld et al. (1997) suggested a rapid teleconnection between the climates of high latitude and the SW monsoon, and the present study shows such relationships in the Cherai core. The stronger summer monsoon brings more precipitation and major runoff, are subjected to more intense chemical weathering, which is well documented by the elemental record and CIW values. These observations on the decoupling between silicate weathering and monsoon climate are confirmed to the prospection that chemical weathering is enhanced by warmer and wetter climate conditions (Filippelli, 1997). However, the present decline in vegetation at the Cherai region (Thakur et al., 2015) could be due to a distinct weakening of monsoon from 1960 to 2000 cal yr AD (Zhang et al., 2008) indicating the CWP. Considering the importance of the position of the SW coast of India, high-resolution sediment core/borehole sampling and multi-proxy analytical study from this coastal stretch are required to decode the past signatures behind the short term global climatic events, monsoon changes, marine-terrestrial teleconnection and its implication on the human settlement.

    Figure 10.  A schematic model showing the possible variation in the weathering, pollution, vegetation, and productivity during the global trends of climate variability during the past 2000 years.

  • Multi-proxy records reveal distinct environment shifts and monsoonal variation during the past 2000 years in the core sediments of Cherai, SW India. The past 2000 years showed subtle changes that were divided into different time units as per the global events such as RWP, Dark period, MCA, LIA and current warming. The abundance of terrestrial/freshwater taxa with high values of CIW and elemental record indicates stronger weathering and presumably wetter conditions/strong monsoon. The proxies suggest mainly warm humid and moderate wet to wetter climatic conditions and changes in precipitation. The MCA period shows marine and riverine influence with well-mixed water conditions, as evidenced by the presence of true mangroves along with terrestrial vegetation. The decreased shift in CIW during early dark ages reflects an increase in oxide ratios, suggesting more arid conditions. While during the transition between dark ages and MCA, the precipitation increased and the little difference after this period reflects more humid condition also suggests the same amount of chemical weathering. The record also suggests diverse sources and the main composition of the source rocks fall close to the Charnockite and Gneissic composition. The values of Cd and As could be due to strong anthropogenic influences and the pollution load index values suggest that the overall study area falls in moderate contamination levels, but a low threat to the environment. The heavy metal contents were controlled by grain size and organic matter. However, the diatom record suggests more influence of human activity on the natural habitat during the deposition time. The present study was compared with other continental and marine palaeoclimatic records to explore global and regional trends of climate variability. This points out that the present findings are also consistent with the different episodes of warm/cool events on centennial-scale such as RWP, DACP, STWP, MCA, LIA and current warming and support the hypothesis of marine-terrestrial teleconnection.

  • The authors are grateful to the Director, BSIP, for providing the funds and facilities to carry out the present study. We are thankful to Dr. Thamban Meloth B. L. Redkar and Ashish Painginkar, NCPOR, India, for the timely help to carry out the geochemical analysis work at their ICP-MS and TOC facility. This is BSIP contribution No. 68/2017-18. The final publication is available at Springer via https://doi.org/10.1007/s12583-020-1336-4.

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