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Holocene environmental changes as recorded in sediments of a paleodelta, southwest coast of India
Quaternary International xxx (2017) 1e9
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Holocene environmental changes as recorded in sediments of a paleodelta, southwest coast of India A.C. Narayana a, *, Vinu Prakash b, c, P.K. Gautam a, Swati Tripathi d a
Centre for Earth & Space Sciences, University of Hyderabad, Hyderabad, 500046, India Department of Marine Geology and Geophysics, Cochin University of Science & Technology, Cochin, 682016, India c Mar Athanasius College of Engineering, Perambavur, Kerala, India d Birbal Sahni Institute of Paleobotany, University Road, Lucknow, 226001, India b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 June 2016 Received in revised form 11 April 2017 Accepted 12 April 2017 Available online xxx
Sedimentary record of a 40 m deep borehole drilled in a paleodelta region of southwest coast of India has been studied for sedimentary proxies - texture, clay minerals, geotechnical parameters, and pollens. The sedimentary record, spanning from 12 ka to Recent in age, exhibits two different sedimentary environments of deposition. Sediments record high values of moisture content, organic carbon and plasticity index. Sediment texture and geotechnical properties indicate a distinct change in depositional environment from marine to fluvial during the sea level fall i.e., after ~7 ka. It also suggests that major rise of sea level from ~11 to ~7 ka and regression from ~7 ka to ~5 ka contributed to the changes in the environment of deposition. The downcore increase of illite and decrease of kaolinite at 12 m depth (~6 ka), and an upward increase of smectite and kaolinite, and decrease of illite concentration support the major fall in sea level in the region that accounted for the change of depositional environment. The pollen records reveal the abundant occurrence of semi-evergreen type of mangroves during early-to midHolocene. Thus, the multi proxy record provides an evidence of change in the environment of deposition from marine to fluvial, which might have been influenced by neotectonics, sea level variations, and monsoonal intensity in the region. © 2017 Elsevier Ltd and INQUA. All rights reserved.
Keywords: Paleoclimate Monsoon Depositional environment Holocene Paleodelta Southwest coast of India
1. Introduction Sedimentary archives from ocean and land have been studied extensively to infer paleo monsoonal intensity, climate change and environment of deposition during the Quaternary period (Singh, 1998; Govil et al., 2011; Das et al., 2013; Gupta et al., 2013; Narayana et al., 2014; Tiwari et al., 2015). Sedimentary records of coastal areas are used to infer past changes in sea levels and environment of deposition (Compton, 2001; Singh et al., 2001; Caldas et al., 2006; Carr et al., 2010; Wang et al., 2010). Sediment texture, physical and geotechnical properties, clay minerals, and pollen records were used as independent proxies in understanding the environment of deposition and sedimentation history of coastal environments and climate variations (Francus et al., 2002; Boulay et al., 2003; Kumaran et al., 2008; Dixit and Bera, 2013). However, most of these studies are based on short sediment cores and
* Corresponding author. E-mail address: [email protected] (A.C. Narayana).
limited to a single sediment proxy. In the present study, we use multi proxy approach to understand the past monsoonal intensity and environment of deposition in the coastal region of southwest India. Sediment texture and the geotechnical properties help to infer the hydro-dynamics of sediment deposition. Similarly, clay minerals are useful indicators of paleoclimate as they provide record of overall climatic signals (Singer, 1984; Thamban et al., 2002; Das et al., 2013). Pollens, transported by fluvial and aeolian action, accumulated in the sediments become a part of the stratigraphic record (Traverse, 1994), and are considered a sound proxy to reconstruct the vegetation history i.e. paleoenvironmental conditions of the region (Bradley, 1999; Kumaran et al., 2008; Nautiyal and Chauhan, 2009; Padmalal et al., 2011; Dixit and Bera, 2013). Based on short sediment cores, paleoclimatic conditions of southwest coastal region of India are investigated by various researchers (e.g. Rajendran et al., 1989; Narayana, 2007; Kumaran et al., 2008; Narayana et al., 2009). However, such studies on thick sedimentary sequences from onshore regions, covering the entire Holocene, are scanty. Here, we present a unique onshore
http://dx.doi.org/10.1016/j.quaint.2017.04.016 1040-6182/© 2017 Elsevier Ltd and INQUA. All rights reserved.
Please cite this article in press as: Narayana, A.C., et al., Holocene environmental changes as recorded in sediments of a paleodelta, southwest coast of India, Quaternary International (2017), http://dx.doi.org/10.1016/j.quaint.2017.04.016
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A.C. Narayana et al. / Quaternary International xxx (2017) 1e9
record of texture, geotechnical, clay minerals and pollens in a thick sedimentary sequence of a borehole drilled in the paleodelta of southwest coast of India. Sedimentary record of a paleodelta can be effectively used for climate change on high resolution scale as it is a repository of thick sediment sequences. In this paper, we discuss the sedimentary environment of deposition and the role of monsoon in depositional processes during the Holocene based on the multi proxy sedimentary record. 2. Study area The southwest coast of India, ~560 km long and varying width from 5 to 30 km, is remarkably straight because of faulting during the late Pliocene. The central part of the coast is marked by landforms such as barrier islands, paleo strandlines, beach ridges, alluvial plains, sand dunes, flood plains and marshy lands (Narayana and Priju, 2006). Paleodelta, an important landform encompassing an area of ~50 km2, lay landward of the modern Periyar River mouth (Narayana et al., 2001), which suggests that the River Periyar was once a major river system carrying large volume of sediments. The northern limb of the Periyar River exhibits remnant deltaic morphology and suggests the past position of the shore line (Fig. 1). Subsequent recession of the sea or uplift of the coastal tract initiated the change in shoreline position and depositional sequences. Several factors viz., tectonic, eustatic, climate and rainfall variations have contributed to the evolution of the paleodelta (Narayana et al., 2001). The study area is covered with Precambrian crystalline rocks, Tertiary sediments, laterites and Quaternary sediments. The Tertiaries and Holocene sediments over-lay the Precambrian high grade crystalline complex of khondalites, leptynites, charnockites and gneisses. Laterites separate the Quaternary sediments from the Tertiary sediments (Soman, 2002). Coastal sediments of the region comprise alluvium, beach sands, lime shells, peat beds, and calcareous clays. Mud flats blanket some parts of the coastal region. Unconsolidated Quaternary sand deposits are the result of detritus materials supplied by the erosion of Western Ghats and distributed extensively along the coast. The southwest coast of India experiences tropical humid climate. Summer season prevails during February to May, followed by southwest monsoon season (June to September) and postmonsoon season (OctobereJanuary). Most of the rainfall occurs during the southwest monsoon season and the annual average rainfall of the study area varies from 250 to 300 cm. The temperature is maximum (35 C) during pre-monsoon period (March to May) and it gradually comes down (24 C) from June because of monsoon precipitation. The study area experiences dynamic climate conditions and the sea level changes that resulted in modification of the geomorphic features of the region (Hashimi et al., 1995; Narayanan and Anirudhan, 2003; Jayalakshmi et al., 2004; Narayana and Priju, 2006). 3. Samples and methods of study A borehole was drilled to a depth of 40 m in a paleodelta region of the northern distributary of Periyar River (Fig. 1). Sediment samples were collected at different depths, sub-sampled and stored in airtight polythene bags for further analyses. Samples were analyzed for 14C ages, textural and geotechnical properties, clay minerals, and pollens following the standard procedures. Bulk sediment samples of the borehole representing muddy sand at 6.0 m, silty sand at 15 m depth, mud at 23 m, inter-face of mud and silty clay at 24.50 m, and peat horizon with silty clay at 31.50 m and 39.75 m depths were chosen for radiocarbon dating and analysed for 14C dating at the Birbal Sahni Institute of Paleobotany, Lucknow,
India, following the procedure of Rajagopalan et al. (1978). Ages for the top section of the borehole could not be obtained as the less amount of sample was retrieved. 3.1. Textural analysis Sand, silt and clay ratios in all the sediment samples were estimated following the standard sieve and pipette techniques (Carver, 1971; Folk, 1980). The salts from the samples were removed by repeated washing with distilled water, and subsequently organic matter and carbonates were removed by treating with H2O2 and glacial acetic acid. Sand and mud fractions were separated by sieving through 63 mm sieve, and the sand fraction (>63 mm) was dried and weighed. The mud fraction (<63 mm size) collected in a 1000 mL glass cylinder was subjected to pipette analysis for estimation of silt and clay fractions as per the procedure suggested by Folk (1980). 3.2. Geotechnical analysis The physical and geotechnical parameters of sediment samples consisting of water content, wet bulk density, shear strength, and Atterberg limits were analyzed as per the procedures suggested by ASTM (2005) and Singh and Punmia (1970). The Atterberg limits include liquid limit, plastic limit, plasticity index, and liquidity index. 3.3. Clay mineral analysis Clay minerals were analyzed employing X-Ray Diffractometer (XRD, Philips 1840 Model) at National Institute of Oceanography, Goa, as per the standard procedures (Biscay, 1964; Gibbs, 1977). Clay solutions of equal volume (1 mL) were pipetted out on glass slides from the disaggregated and deflocculated clay water suspension. The glass slides were dried at room temperature. Slides were scanned on XRD from 2 to 30 2theta at 1 2theta/minute using Ni filtered Cu-Ka radiation. Glycolated slides were further scanned for the confirmation of montmorillonite. The clay solution slides were scanned from 24 to 26 at 0.50 2theta/minute to differentiate the kaolinite from chlorite peaks (Biscay, 1964). Percentages of smectite, kaolinite, illite, and chlorite were semiquantified and their relative proportions are computed. 3.4. Palynological studies A total of forty samples were chosen for palynological studies. About 20 g of sample was boiled with 10% potassium hydroxide (KOH) for 5 min and pollens were deflocculated from the sediment and the humic acid was dissolved. Subsequently the samples were washed three times with distilled water and alkalis were removed by decantation process. Then the samples were sieved through 150 mesh sieve for separation of the coarse particles. The fine fraction samples were further treated with 10% hydrochloric acid and carbonates were removed, and after that acid content in the samples was removed by washing with distilled water. Silicates were removed from the samples by treating with 40% hydrofluoric acid (HF). The samples were again washed with distilled water to make them free from silicates and HF. The samples were further treated with glacial acetic acid (GAA) and centrifugation carried out for dehydration. Subsequently, samples were treated with acetolysing mixture (9:1 ratio of acetic anhydride and concentrated sulphuric acid), centrifuged and decanted for removal of acid, and again treated with GAA and washed with distilled water. Thereafter, 50% glycerin and a few drops of phenol were added to the residue to arrest microbial decomposition of samples. Pollen slides were
Please cite this article in press as: Narayana, A.C., et al., Holocene environmental changes as recorded in sediments of a paleodelta, southwest coast of India, Quaternary International (2017), http://dx.doi.org/10.1016/j.quaint.2017.04.016
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Fig. 1. Map of the study area with borehole location. Borehole (PBH) was drilled to a depth of 40 m in the paleodelta region at north limb of Periyar River of Central Kerala coast, India. Paleodelta, lagoonal system and small islands are shown in the figure.
prepared, and identification and counting of pollens was done under Olympus BX 61 light microscope. All the slides were photographed with Olympus DP-25 camera. Identification of palynomorphs in sediments was done with the help of reference slides available in Birbal Sahni Institute of Palaeobotany (BSIP), Lucknow, and with the help of published literature as described by various researchers (e.g. Tissot, 1990; Bera et al., 1997; Quamar and Bera, 2014). Approximately 100e150 pollens were counted in each sample, and percentages of recovered palynomorphs were computed in terms of total plant pollen count. Poacea (grasses) are categorized in to non-cereal (with pollen < 60 mm) and cereal with pollen > 60 mm) (Basumatary and Bera, 2012).
The palyno assemblages were categorized as arboreals (trees and shrubs) and non-arboreals (marshy and terrestrial herbs). 4. Results The radiocarbon ages of borehole sediments range from Recent to 11.9 ka, and the calibrated ages of ~4.69 and ~9.55 ka represent the sediment depths of 6.20 and 24.50 m respectively (Fig. 2), while the calibrated age of ~11.69 ka corresponds to the depth interval of 31.50 m. Sediments at the bottom of borehole i.e., at 39.75 m depth reveal the 14C age as ~11.9 ka. Inversion of calibrated ages of ~12.12 ka and ~10.33 ka are encountered at 15 m and 23 m depth
Please cite this article in press as: Narayana, A.C., et al., Holocene environmental changes as recorded in sediments of a paleodelta, southwest coast of India, Quaternary International (2017), http://dx.doi.org/10.1016/j.quaint.2017.04.016
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Fig. 2. Depth versus age diagram of borehole sediments from north limb of Periyar River of Kerala, Central Kerala, India. The age inversion at 25 m depth profile, shown in shaded portion, may be due to the neotectonic activity in the region.
respectively. 4.1. Textural properties The average sand, silt, and clay ratios of sediments are 39%, 34%, and 27% respectively (Table 1). However, sand content varies between 70 and 90% up to 5 m depth i.e., during the Late Holocene period. Sand-mud intercalations are recorded from 5 to 10 m depth (i.e., up to ~6 Ka) and sandy mud from 10 to 14.5 m depth (up to ~7 ka) as shown in Fig. 3. The borehole exhibits silty-sand overlying a thin layer of sandy mud between 14.5 and 16 m depth (i.e., around 7.5 ka), whereas mud dominates from 18 to 24 m depth (~8e9.5 ka), and peat deposit along with silty clay intercalations occurs below this depth (i.e., between 24 and 40 m), which ranges in age from ~9.5 to 11.9 ka. The borehole shows downcore change in texture varying from intercalations of sandy mud, silt to mud and peat deposits at 18 m depth (Fig. 3) indicating a change in the environment of deposition from fluvial to marine at ~8 ka. The borehole exhibits two distinct lithofacies e (i) muddy sands from surface to 18 m depth, i.e., ~8 ka, with sediments coarsening upwards, and (ii) muddy sediments with peat from 18 to 40 m depth i.e., during early Holocene. 4.2. Geotechnical properties Geotechnical properties of borehole sediments reveal that the wet bulk density ranges from 1.39 to 1.92 g/cc with an average of 1.64 g/cc. The maximum and minimum wet bulk density values are observed at 17 m (~7.5 ka) and 6.50 m depth (~4.6 ka) respectively. Moisture content varies from 24 to 98% with an average of 55%; the highest content is recorded at 6.50 m (~4.6 ka) and lowest at 17 m depth (~7.5 ka). Organic carbon also shows a similar trend like moisture content and varies from 0.1% (2.0 m depth - Recent
period) to 7.30% (38.50 m depth i.e., during 11e12 ka) with an average of 3.89%. It is observed that the physical and geotechnical parameters exhibit a significant change at 18 m depth of the borehole i.e., at ~8 ka (Fig. 3). Liquid limit varies from 46% at 10 m depth (~6 ka) to 108% at 19.75 m depth (~8.2 ka) with an average of 77%. These values are higher when compared to moisture content values indicating the plastic state of sediments. The variation of liquid limit values are similar to the moisture content, silt and clay contents, plastic limit, plasticity index, and shear strength, and show a positive correlation; whereas wet bulk density and sand content exhibit negative correlation with liquid limit. Plastic limit values range from 17% at 6 m depth (~4.6 ka) to 53% at 23 m depth (~8.5 ka), with an average of 34%. Plasticity index of the borehole sediments ranges from 23% at 11 m depth (~5.5 ka) to 61% at 21 m depth (~8.3 ka) with an average of 43% and shows medium to high plasticity in nature. The geotechnical properties exhibit increasing trend from surface to ~18 m depth i.e., from Recent to ~8 ka period, and slightly decreasing trend from 18 m to 40 m depth i.e., from ~8 ka to 12 ka. The plasticity values plotted on the plasticity chart (Fig. 4a) follow the A-Line. It is observed that majority of the samples exhibit very high plasticity. Activity chart (Fig. 4b) indicates the active state of the samples as they range from 0.78 to 2.77 with an average value of 1.41 (Skempton, 1953). 4.3. Clay minerals Relative percentages of clay minerals - smectite, kaolinite, chlorite, and illite and their ratios are shown in Fig. 5. Maximum, minimum, and average percentages of clay minerals are given in Table 1. The dominance of smectite and kaolinite in the upper section and illite in the lower section of the borehole is recorded; whereas chlorite is present in low concentration. Unlike textural,
Table 1 Maximum, minimum, and average values of textural & geotechnical properties and clayminerals. PBH: Paravur bore hole, g: wet bulk density, w: water content, Corg: organic carbon, S:sand, Z:silt, C: clay, Su: undrained vane shear strength, LL: liquid limit, PL: plastic limit, PI: plasticity index, A activity, wP/wL:plastic limit/liquid limit, Sm: smectite, Ka: Kaolinite, Ch: Chlorite, Ill: Illite. PBH (geotechnical)
g (g/cc)
W (%)
Corg (%)
S (%)
Z (%)
C (%)
Su (Kpa)
LL (%)
PL (%)
PI (%)
A
wP/wL
Sm (%)
Ka (%)
Ch (%)
Ill (%)
Maximum Minimum Average
1.92 1.29 1.64
92 24 55
7.21 0.1 3.99
95 0 29
53 5 34
54 0 25
55 3 19
105 66 77
52 17 24
61 23 42
2.77 0.78 1.41
0.52 0.25 0.44
71 10 14
50 4 20
14 0 6
63 5 29
PBH: Paravur bore hole, g: wet bulk density, w: water content, Corg: organic carbon, S:sand, Z:silt, C: clay, Su: undrained vane shear strength, LL: liquid limit, PL: plastic limit, PI: plasticity index, A activity, wP/wL:plastic limit/ liquid limit, Sm: smectite, Ka: Kaolinite, Ch: Chlorite, Ill: Illite.
Please cite this article in press as: Narayana, A.C., et al., Holocene environmental changes as recorded in sediments of a paleodelta, southwest coast of India, Quaternary International (2017), http://dx.doi.org/10.1016/j.quaint.2017.04.016
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Fig. 3. Lithofacies observed in vertical cross section of Paravur borehole recovered from the paleodelta region of Central Kerala, India. Downcore variation of texture, physical, and geotechnical parameter variations are also plotted. Based on lithology, the borehole can be divided into six facies as: muddy sand, sand, sandy mud, silty sand, sandy silt, and mud.
Fig. 4. a Figure shows liquid limit versus plasticity index value graph as plasticity chart to understand the plasticity level of borehole sediments recovered from north limb of Periyar river in the paleodelta regime. It can be observed that the points are falling on and around A-line in the Plasticity chart. b The activity chart of borehole sediments. Skempton (1953) classification of clays exhibits as active, normal, and inactive clays, satisfying the active state of the sediments.
and geotechnical parameters, clay minerals show two major changes in their relative concentrations at 12 m and 18 m depth (during ~6e8 ka). Smectite is dominant from surface to 12 m depth (i.e., during Early to Mid- Holocene) of the borehole. Further, low smectite content with concomitant abundance of chlorite and kaolinite from 12 m to 18 m depth (~6e8 ka) is observed. Increased amount of illite is recorded below 18 m depth of the borehole i.e., from ~8 ka to 12 ka. 4.4. Palynological assemblages Based on distinct palyno-assemblages retrieved from the borehole samples spanning from early to late Holocene period, five
pollen zones (P-I to P-V; Fig. 6) are broadly demarcated. Palynozone P-I (36e40 m depth; Late Holocene - ~11.9 ka): This palyno-spectrum is mainly dominated by the presence of 20e45% of grasses (Poaceae), followed by trilete and monolete spores up to a maximum of 13% and 26% respectively. Among the tree taxa, fabaceae and arecaceae are found to a maximum of 6%, whereas, sapotaceae, meliaceae and elaeocarpus are limited (2e4%) in their abundance. The core mangrove taxa are represented by rhizophora (1e2%) along with a scanty appearance of sonneratia, ceriops, avicennia and schleichera. Palynozone P-II (23e36 m depth; Late Holocene: ~9.5e11.7 ka): This pollen spectrum is marked by the proliferation of rhizophora to a maximum of 9%, followed by a small increase in abundance of
Please cite this article in press as: Narayana, A.C., et al., Holocene environmental changes as recorded in sediments of a paleodelta, southwest coast of India, Quaternary International (2017), http://dx.doi.org/10.1016/j.quaint.2017.04.016
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Fig. 5. Relative percentage of clay minerals (smectite, illite, chlorite, and kaolinite) in the borehole sediments.
Fig. 6. Shows the pollen variations in the borehole samples. Five pollen zones (P-I to P-V) on the basis of distinct palynoassemblage retrieved from the borehole spanning from early to late Holocene.
Please cite this article in press as: Narayana, A.C., et al., Holocene environmental changes as recorded in sediments of a paleodelta, southwest coast of India, Quaternary International (2017), http://dx.doi.org/10.1016/j.quaint.2017.04.016
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ceriops and schleichera. Rest of the core mangrove taxa like sonneratia, avicennia and lumnitzera are recorded in trace amounts. Tree taxa like sapotaceae, meliaceae, anacardiaceae and combretaceae are found varying in abundance from 3 to 5%; while fabaceae and arecaceae are encountered in low to moderate concentrations of 2e6%. Presence of regional taxa like alnus was noticed in P-II palynozone. Poaceae (9e20%) was observed with increase of terrestrial herbs like asteraceae and rutaceae to about 2e3%. Ferns represented as trilete (15e30%) and monolete (10e20%) in relatively higher values, compared to preceding P-I palynozone. Palynozone P-III (15e23 m depth; ~6e9.5 ka): This pollen zone is marked by the invasion of rhizophora (7e11%) along with occurrence of ceriops and schleichera up to 3% in concentration. Sporadic appearance of sonneratia, avicennia and lumnitzera was also recorded in P-III palynozone. Among the tree taxa, sapotaceae, meliaceae, combretaceae and anacardiaceae are found up to 4e5%, whereas other arboreal taxa like aquifoliaceae, chrysophyllum, elaeocarpus, semecarpus, fabaceae and moraceae are found in low quantity (1e3%). Surprisingly, lagerstroemia is found to be absent in this palynozone. Grasses occur in relatively higher amount (15e30%) as compared to P-II palynozone. Among ferns, monolete spores are recorded in more or less same quantity, whereas, trilete spores are observed in exceptionally higher amounts (up to 30%) as compared to palynozone P-II. Palynozone P-IV (5e15 m depth; Mid-Holocene): This zone is dominated by the presence of trilete spores ranging from 20 to 27%, followed by grasses (15e27%) and monolete spores (15e19%). Among the core mangrove taxa, rhizophora show a decreasing trend, compared to P-III zone. Striking increase of sonneratia was noticed for the first time in P-IV palynozone along with increment in values of lumnitzera, ceriops and schleichera. However, avicennia is very scanty. Among the arboreal taxa, increase of aquifoliaceae, fabaceae and arecaceae, while decrease of key tropical deciduous arboreals like sapotaceae, meliaceae, combretaceae, anacardiaceae and meliaceae are recorded. However, lagerstroemia, elaeocarpus and semecarpus show slight increase as compared to P-III zone. Palynozone P-V (0e5 m depth; Late Holocene): This zone is marked by the increase of rhizophora (up to 10%) along with ceriops, sonneratia, lumnitzera and schleichera. Among arboreals, a maximum of 10% of fabaceae and arecaceae are found. Some of the tree taxa such as sapotaceae and elaeocarpus record an increase up to 5%. Grasses are dominant with a maximum of 25%. Monolete spores slightly increased in abundance with the decrease of trilete spores. 5. Discussion Radiocarbon chronology suggests the Holocene period for the sediment sequence recorded in the borehole of central coastal region of southwest India. The higher radiocarbon ages at 15 m (~12.2 ka) and 23 m (~10.3 ka) depths (Fig. 2) indicate inversion of the ages, which is probably due to the slumping of sediments caused by neotectonic activity. The role of neotectonics on the sea level rise along the southwest coast of India has been discussed earlier (e.g., Nair et al., 2006; Valdiya and Narayana, 2007). Sea level reached its maximum position during the mid-Holocene, and subsequently lowered and stabilized at the present level (Kale and Rajaguru, 1983). Walcott (1972) estimated the post glacial sea level rise for different regions and assigned 88 m rise since 18 ka for the southwest coast of India. Based on 14C ages of on-shore shell deposits from the coastal plains of Karnataka (Caratini and Rajagopalan, 1992) and peat deposits along the Goa coast (Mascarranhas and Chauhan, 1998), it is suggested that the sea level was about 3.0 ± 1.0 m below the present level during the midHolocene i.e., at ~6.4 ka. Fairbanks (1989) observed that sea level
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rose worldwide rapidly (~24 m) during ~12 ka, followed by a slow rate of rise during ~11 ka, and rose again at a higher rate at ~9.5 ka. The sea level rose at a very faster rate (~20 m/1kyr) from ~10 to 7 ka, and after ~7 ka, particularly during the last 4 ka, it remained more or less at the present level along the southwest coast of India (Hashimi et al., 1995). Rising sea level is the primary control on aggradation of floodplains as observed in the deposition of RhineMoeuse coastal prism during middle Holocene back-filling (Van Dijk et al., 1991; Cohen et al., 2005). During the sea level rise, accommodation exceeded sedimentation in many regions. Further, peat can form extensively in flood basins because of the low supply of sediment. Local tectonic controls can lead to anomalies in subsurface elevation, sedimentary facies and aggradation rates in a longitudinal direction. Tectonic effects in deltaic plain are recorded by the changing distribution of organics and clastics within the sedimentary sequence. The control of active tectonics on alluvial deposits has long been recognized (Schumm et al., 2000). Deltaic systems have low-gradient reaches, and are believed to be sensitive to subtle neotectonic deformation (Holbrook and Schumm, 1999). Hence, the neotectonics might have induced the subsidence, resulting in the inversion of radiocarbon ages of sediments in the present study area. The Holocene fluvial record in the study area reflects the neotectonic control of differential subsidence during floodbasin aggradation. The sedimentary architecture of the borehole reveal floodbasin sediment deposits (intercalated beds of sand, muddy sand, sandy mud, silty sand, sandy silt, clay and peat). Widespread peat with intercalated clay occurs during ~9.5 and ~11.9 ka i.e., at the base of the Holocene sequence. Its presence indicates the onset of aggradation. The sediment characteristics give insights into evolution history of sedimentary deposit and paleoenvironmental history. The upward coarsening of sediments in the upper portion of the borehole suggests the change of sediment deposition in rough hydrodynamic conditions during the marine regression (Manojkumar et al., 1998). Thus, the textural characteristics of borehole sediments reveal a change in depositional environment of deltaic facies, apparently from marine to fluvial environment during mid-Holocene marine regression (Fig. 3). The geotechnical properties of sediments also exhibit distinct variation in the upper part of borehole i.e., from surface to 18 m depth (Recent to ~8 ka) corroborating with textural variation. The coarse sediments in the upper portion of borehole, representing Late Holocene period, suggest rapid erosion in the hinterland because of intense southwest monsoonal rainfall and initiation of high fluvial energy conditions, which resulted in the supply of coarse sediments to the paleodelta region. The geotechnical properties revealing high moisture content, organic carbon, and plasticity index, and low bulk density associated with low shear stress are conducive for erosion (Vanoni, 1975; Thorn and Parsons, 1980) and high erosion rate (Mehta et al., 1982). Further, sand and silt mixture in the upper part of borehole suggests that fluvial environment was influenced by the variation in the intensity of monsoon. Wang et al. (2010) also made similar observations in the evolution of subaquous Yangtza delta of China during post glacial period, and they attributed this to the sea level changes and the influence of Asian monsoon. The wet bulk density exhibits opposite trend with moisture content and organic carbon, particularly up to 18 m depth (~8 ka period) (Fig. 3). Moisture content shows pronounced positive relation with organic carbon content, particularly in bottom portion of the borehole. Thus, the sediments represent an increased erosion rate which can be accounted for the intense monsoon. The clay mineral record of the borehole further suggests the variation in depositional environment influenced by the sea level changes and monsoon intensity. Clay minerals are used as a potential proxy to understand climate change, the intensity of
Please cite this article in press as: Narayana, A.C., et al., Holocene environmental changes as recorded in sediments of a paleodelta, southwest coast of India, Quaternary International (2017), http://dx.doi.org/10.1016/j.quaint.2017.04.016
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weathering, and the degree of hydrolysis (Chamley, 1989; Thamban et al., 2002; Das et al., 2013). The increase in illite and to some extent smectite content, and corresponding decrease of kaolinite suggest increase in relative sea level or aridity during that time (Ruffel et al., 2002). In general, the local sea level fall would reflect in increased kaolinite deposition. The relative abundances (Fig. 5) of clay minerals varied significantly in time, while the illite and smectite contents show opposite trends in the borehole. Smectite is abundant in upper horizon from surface to 11.6 m depth (~6 ka) and below this depth the illite is the dominant one towards down core. Kaolinite exhibits similar trend like that of smectite, while chlorite concentration is very low throughout the section. This gives an evidence of major sea level fall causing the change in the environment of deposition from marine to fluvial and also the aridity of the climate in the region. It is suggested that the clay minerals are derived from the hinterland of the coast (Rao and Rao, 1995; Thamban et al., 2002; Narayana et al., 2008), which experiences intense southwest monsoon processes. Singer (1984) suggests that relative high percentage of chlorite and illite can be related to dry periods, while strong humid conditions lead to intense weathering and consequently the dominant occurrence of kaolinite. Smectite is taken as climate indicator of contrasting seasons, particularly a strong dry season. In this study, relative percentages of clay minerals show distinct temporal variations representing short interval of climate changes at depths of 2.0 m (early Holocene; wet to dry), 11.60 m (~6 ka; dry to wet), 17.0 m (~7.5 ka; dry to wet), and 18.90 m (~8 ka; wet to dry) of the borehole. Sea level changes and the geomorphic evolution of the coast since the Neogene contributed to substantial changes in the mangrove habitats, particularly the mangrove cover during the Late Holocene have been found to be different along the southwest coast of India, which is attributed to local hydrodynamics and rainfall in the mangrove swamps (Limaye et al., 2014). The sea level changes along the coast of Saxony, Germany, were effectively inferred using pollens (Freund et al., 2004). The palynological record of the borehole sediment samples shows that overall Rhizophora is the dominant core mangrove taxa. Apart from this, Rhizophoraceae-sonneratia also occurs between 24 and 40 m (Late Holocene) and 5e14 m depth (~4.5 - ~6.5 ka). The Rhizophorasonneratia transition from Middle to Late Holocene is considered as a distinctive feature of mangrove response to climate change (Kumaran et al., 2013). This modification may be attributed to the response of hydrological regimes. The ecological shift from core mangrove to fresh water facies is well marked in the study area. Increase in Rhizopora and decrease in grasses is well marked in P-II palynozone (~9.5e~11.7 ka) with further stabilization of both in the P-III zone (~7e~9.5 ka). Similar relation was noticed in P-IV (during Mid- Holocene) and P-V palynozones (Late Holocene), where decline in Rhizopora and rise in Poaceae pollen was observed in PIV palynozone (Mid-Holocene) with further stabilization in P-V palynozone i.e., during Late Holocene. The sediment samples at 5e14 m depth interval (i.e., during ~4.5e~6.5 ka) reveal the occurrence of mangrove associated tree elements in fair amount namely, Meliaceae, Anacardiaceae and Sapotaceae, etc., along with trace occurrence of Rhizophora. The herbaceous elements were also observed in moderate values. Grasses are found in higher values in almost all the palynozones except a small decrease in P-II palynozone. However, other terrestrial herbaceous taxa came into existence during ~9.5e11.7 ka as observed in P-II palynozone. Among ferns, trilete and monolete spores are dominant in almost all the samples, signifying moist and wet conditions in the study area. Monsoon variation and sea level changes could be the reason for increase and decrease of core mangrove taxa as well as change in frequencies of Poaceae and Arecaceae pollen in this entire Holocene sequence.
6. Conclusion The textural, physical and geotechnical parameters, clay minerals, and pollen records in sediments of a 40 m depth borehole from a paleodelta on the southwest coast of India throw insights on climate change and environment of deposition during the Holocene. The textural parameters reveal a distinct change in depositional environment from marine to fluvial coinciding with sea level fall. The concentrations of clay minerals - smectite, kaolinite and illite - also support the major sea level fall and the aridity in the region giving evidence to the change of environment of deposition. The variation in the fluvial facies of borehole section suggests the active monsoon and major sea level fall. Coarse grained sediment texture indicates the change in depositional conditions in the upper part of borehole sequence i.e., during Late Holocene, coinciding with strong fluvial action and marine regression. Geotechnical properties also suggest the change in the sedimentary facies because of varied hydrodynamic conditions. Pollen records of sediments indicate semi-evergreen type of mangrove plants from early-to mid- Holocene period, whereas the pollens that represent Mid-to Late Holocene suggest the change in hydrological regimes, which are reflected in the textural and geotechnical characteristics of sediments. The study reveals many episodes of change in the environment of deposition along the southwest coast of India during the Holocene, and the factors such as neotectonics, sea level changes, and varied intensity of southwest monsoon might have contributed, which in turn caused the change in mangrove vegetation along the southwest coast of India. Acknowledgement ACN thanks the All India Council for Technical Education for funding through a research project (No. 8019/RDII/TAP/CIV (232)/ 2000-01) granted when the author was with the Cochin University of Science and Technology, Cochin. C M Nautiyal is thanked for his help in radiocarbon dating. Girish Prabhu of National Institute of Oceanography (NIO) is acknowledged for his help during XRD analysis for clay mineral analysis. Authors thank S.K. Bera, for his help in pollen analysis, and R. Ramesh, PRL, for his valuable suggestions. References ASTM, 2005. Standard Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method. Annual book of American Society for Testing and Material standards, West Conshohocken, United States, p. E 691. Basumatary, S.K., Bera, S.K., 2012. Vegetation succession and climate change in Western Gharo hills, Meghalaya, India, since 11,643 year B.P: a palynological record. Int. J. Earth Sci. Eng. 5 (4), 748e758. Bera, S.K., Farooqui, A., Gupta, H.P., 1997. Late pleistocene/holocene vegetation and environment in and around Marian Shola, Palni hills, Tamil Nadu. Palaeobotanist 46, 191e195. Biscay, P.E., 1964. Distinction between chlorite and kaolinite in recent sediments by X-ray diffraction. Am. Mineral. 49, 1281e1289. Boulay, S., Colin, C., Trenteasaux, A., Pluquet, F., Bertaux, J., Blamart, D., Buehring, C., Wang, P., 2003. Mineralogy and sedimentology of Pleistocene sediment in the South China Sea (ODP site 1144): proceedings of the ocean drilling program. Sci. Results 184, 1e21. Bradley, R.S., 1999. Paleoclimatology: Reconstructing Climates of the Quaternary. Academic Press, San Diego. Caldas, L.H.O., Oliveira, J.G., Medeiros Jr., W.E., Stattegger, K., Vital, H., 2006. Geometry and evolution of the Holocene transgressive and regressive barriers on the semi-arid coast of NE Brazil. Geo-Marine Lett. 26, 249e263. Caratini, C., Rajagopalan, G., 1992. Holocene marine transgression marker on the Karnataka coast (India). Indian J. Mar. Sci. 2, 149e151. Carr, A.S., Bateman, M.D., Roberts, D.L., Murray-Wallace, C.V., Jacobs, Z., Holmes, P.J., 2010. The last interglacial sea-level high stand on the southern Cape coastline of South Africa. Quat. Res. 73, 351e363. Carver, R.E., 1971. Procedures in Sedimentary Petrology. Wiley-Interscience, NewYork.
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Holocene environmental changes as recorded in sediments of a paleodelta, southwest coast of India A.C. Narayana a, *, Vinu Prakash b, c, P.K. Gautam a, Swati Tripathi d a
Centre for Earth & Space Sciences, University of Hyderabad, Hyderabad, 500046, India Department of Marine Geology and Geophysics, Cochin University of Science & Technology, Cochin, 682016, India c Mar Athanasius College of Engineering, Perambavur, Kerala, India d Birbal Sahni Institute of Paleobotany, University Road, Lucknow, 226001, India b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 June 2016 Received in revised form 11 April 2017 Accepted 12 April 2017 Available online xxx
Sedimentary record of a 40 m deep borehole drilled in a paleodelta region of southwest coast of India has been studied for sedimentary proxies - texture, clay minerals, geotechnical parameters, and pollens. The sedimentary record, spanning from 12 ka to Recent in age, exhibits two different sedimentary environments of deposition. Sediments record high values of moisture content, organic carbon and plasticity index. Sediment texture and geotechnical properties indicate a distinct change in depositional environment from marine to fluvial during the sea level fall i.e., after ~7 ka. It also suggests that major rise of sea level from ~11 to ~7 ka and regression from ~7 ka to ~5 ka contributed to the changes in the environment of deposition. The downcore increase of illite and decrease of kaolinite at 12 m depth (~6 ka), and an upward increase of smectite and kaolinite, and decrease of illite concentration support the major fall in sea level in the region that accounted for the change of depositional environment. The pollen records reveal the abundant occurrence of semi-evergreen type of mangroves during early-to midHolocene. Thus, the multi proxy record provides an evidence of change in the environment of deposition from marine to fluvial, which might have been influenced by neotectonics, sea level variations, and monsoonal intensity in the region. © 2017 Elsevier Ltd and INQUA. All rights reserved.
Keywords: Paleoclimate Monsoon Depositional environment Holocene Paleodelta Southwest coast of India
1. Introduction Sedimentary archives from ocean and land have been studied extensively to infer paleo monsoonal intensity, climate change and environment of deposition during the Quaternary period (Singh, 1998; Govil et al., 2011; Das et al., 2013; Gupta et al., 2013; Narayana et al., 2014; Tiwari et al., 2015). Sedimentary records of coastal areas are used to infer past changes in sea levels and environment of deposition (Compton, 2001; Singh et al., 2001; Caldas et al., 2006; Carr et al., 2010; Wang et al., 2010). Sediment texture, physical and geotechnical properties, clay minerals, and pollen records were used as independent proxies in understanding the environment of deposition and sedimentation history of coastal environments and climate variations (Francus et al., 2002; Boulay et al., 2003; Kumaran et al., 2008; Dixit and Bera, 2013). However, most of these studies are based on short sediment cores and
* Corresponding author. E-mail address: [email protected] (A.C. Narayana).
limited to a single sediment proxy. In the present study, we use multi proxy approach to understand the past monsoonal intensity and environment of deposition in the coastal region of southwest India. Sediment texture and the geotechnical properties help to infer the hydro-dynamics of sediment deposition. Similarly, clay minerals are useful indicators of paleoclimate as they provide record of overall climatic signals (Singer, 1984; Thamban et al., 2002; Das et al., 2013). Pollens, transported by fluvial and aeolian action, accumulated in the sediments become a part of the stratigraphic record (Traverse, 1994), and are considered a sound proxy to reconstruct the vegetation history i.e. paleoenvironmental conditions of the region (Bradley, 1999; Kumaran et al., 2008; Nautiyal and Chauhan, 2009; Padmalal et al., 2011; Dixit and Bera, 2013). Based on short sediment cores, paleoclimatic conditions of southwest coastal region of India are investigated by various researchers (e.g. Rajendran et al., 1989; Narayana, 2007; Kumaran et al., 2008; Narayana et al., 2009). However, such studies on thick sedimentary sequences from onshore regions, covering the entire Holocene, are scanty. Here, we present a unique onshore
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Please cite this article in press as: Narayana, A.C., et al., Holocene environmental changes as recorded in sediments of a paleodelta, southwest coast of India, Quaternary International (2017), http://dx.doi.org/10.1016/j.quaint.2017.04.016
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A.C. Narayana et al. / Quaternary International xxx (2017) 1e9
record of texture, geotechnical, clay minerals and pollens in a thick sedimentary sequence of a borehole drilled in the paleodelta of southwest coast of India. Sedimentary record of a paleodelta can be effectively used for climate change on high resolution scale as it is a repository of thick sediment sequences. In this paper, we discuss the sedimentary environment of deposition and the role of monsoon in depositional processes during the Holocene based on the multi proxy sedimentary record. 2. Study area The southwest coast of India, ~560 km long and varying width from 5 to 30 km, is remarkably straight because of faulting during the late Pliocene. The central part of the coast is marked by landforms such as barrier islands, paleo strandlines, beach ridges, alluvial plains, sand dunes, flood plains and marshy lands (Narayana and Priju, 2006). Paleodelta, an important landform encompassing an area of ~50 km2, lay landward of the modern Periyar River mouth (Narayana et al., 2001), which suggests that the River Periyar was once a major river system carrying large volume of sediments. The northern limb of the Periyar River exhibits remnant deltaic morphology and suggests the past position of the shore line (Fig. 1). Subsequent recession of the sea or uplift of the coastal tract initiated the change in shoreline position and depositional sequences. Several factors viz., tectonic, eustatic, climate and rainfall variations have contributed to the evolution of the paleodelta (Narayana et al., 2001). The study area is covered with Precambrian crystalline rocks, Tertiary sediments, laterites and Quaternary sediments. The Tertiaries and Holocene sediments over-lay the Precambrian high grade crystalline complex of khondalites, leptynites, charnockites and gneisses. Laterites separate the Quaternary sediments from the Tertiary sediments (Soman, 2002). Coastal sediments of the region comprise alluvium, beach sands, lime shells, peat beds, and calcareous clays. Mud flats blanket some parts of the coastal region. Unconsolidated Quaternary sand deposits are the result of detritus materials supplied by the erosion of Western Ghats and distributed extensively along the coast. The southwest coast of India experiences tropical humid climate. Summer season prevails during February to May, followed by southwest monsoon season (June to September) and postmonsoon season (OctobereJanuary). Most of the rainfall occurs during the southwest monsoon season and the annual average rainfall of the study area varies from 250 to 300 cm. The temperature is maximum (35 C) during pre-monsoon period (March to May) and it gradually comes down (24 C) from June because of monsoon precipitation. The study area experiences dynamic climate conditions and the sea level changes that resulted in modification of the geomorphic features of the region (Hashimi et al., 1995; Narayanan and Anirudhan, 2003; Jayalakshmi et al., 2004; Narayana and Priju, 2006). 3. Samples and methods of study A borehole was drilled to a depth of 40 m in a paleodelta region of the northern distributary of Periyar River (Fig. 1). Sediment samples were collected at different depths, sub-sampled and stored in airtight polythene bags for further analyses. Samples were analyzed for 14C ages, textural and geotechnical properties, clay minerals, and pollens following the standard procedures. Bulk sediment samples of the borehole representing muddy sand at 6.0 m, silty sand at 15 m depth, mud at 23 m, inter-face of mud and silty clay at 24.50 m, and peat horizon with silty clay at 31.50 m and 39.75 m depths were chosen for radiocarbon dating and analysed for 14C dating at the Birbal Sahni Institute of Paleobotany, Lucknow,
India, following the procedure of Rajagopalan et al. (1978). Ages for the top section of the borehole could not be obtained as the less amount of sample was retrieved. 3.1. Textural analysis Sand, silt and clay ratios in all the sediment samples were estimated following the standard sieve and pipette techniques (Carver, 1971; Folk, 1980). The salts from the samples were removed by repeated washing with distilled water, and subsequently organic matter and carbonates were removed by treating with H2O2 and glacial acetic acid. Sand and mud fractions were separated by sieving through 63 mm sieve, and the sand fraction (>63 mm) was dried and weighed. The mud fraction (<63 mm size) collected in a 1000 mL glass cylinder was subjected to pipette analysis for estimation of silt and clay fractions as per the procedure suggested by Folk (1980). 3.2. Geotechnical analysis The physical and geotechnical parameters of sediment samples consisting of water content, wet bulk density, shear strength, and Atterberg limits were analyzed as per the procedures suggested by ASTM (2005) and Singh and Punmia (1970). The Atterberg limits include liquid limit, plastic limit, plasticity index, and liquidity index. 3.3. Clay mineral analysis Clay minerals were analyzed employing X-Ray Diffractometer (XRD, Philips 1840 Model) at National Institute of Oceanography, Goa, as per the standard procedures (Biscay, 1964; Gibbs, 1977). Clay solutions of equal volume (1 mL) were pipetted out on glass slides from the disaggregated and deflocculated clay water suspension. The glass slides were dried at room temperature. Slides were scanned on XRD from 2 to 30 2theta at 1 2theta/minute using Ni filtered Cu-Ka radiation. Glycolated slides were further scanned for the confirmation of montmorillonite. The clay solution slides were scanned from 24 to 26 at 0.50 2theta/minute to differentiate the kaolinite from chlorite peaks (Biscay, 1964). Percentages of smectite, kaolinite, illite, and chlorite were semiquantified and their relative proportions are computed. 3.4. Palynological studies A total of forty samples were chosen for palynological studies. About 20 g of sample was boiled with 10% potassium hydroxide (KOH) for 5 min and pollens were deflocculated from the sediment and the humic acid was dissolved. Subsequently the samples were washed three times with distilled water and alkalis were removed by decantation process. Then the samples were sieved through 150 mesh sieve for separation of the coarse particles. The fine fraction samples were further treated with 10% hydrochloric acid and carbonates were removed, and after that acid content in the samples was removed by washing with distilled water. Silicates were removed from the samples by treating with 40% hydrofluoric acid (HF). The samples were again washed with distilled water to make them free from silicates and HF. The samples were further treated with glacial acetic acid (GAA) and centrifugation carried out for dehydration. Subsequently, samples were treated with acetolysing mixture (9:1 ratio of acetic anhydride and concentrated sulphuric acid), centrifuged and decanted for removal of acid, and again treated with GAA and washed with distilled water. Thereafter, 50% glycerin and a few drops of phenol were added to the residue to arrest microbial decomposition of samples. Pollen slides were
Please cite this article in press as: Narayana, A.C., et al., Holocene environmental changes as recorded in sediments of a paleodelta, southwest coast of India, Quaternary International (2017), http://dx.doi.org/10.1016/j.quaint.2017.04.016
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Fig. 1. Map of the study area with borehole location. Borehole (PBH) was drilled to a depth of 40 m in the paleodelta region at north limb of Periyar River of Central Kerala coast, India. Paleodelta, lagoonal system and small islands are shown in the figure.
prepared, and identification and counting of pollens was done under Olympus BX 61 light microscope. All the slides were photographed with Olympus DP-25 camera. Identification of palynomorphs in sediments was done with the help of reference slides available in Birbal Sahni Institute of Palaeobotany (BSIP), Lucknow, and with the help of published literature as described by various researchers (e.g. Tissot, 1990; Bera et al., 1997; Quamar and Bera, 2014). Approximately 100e150 pollens were counted in each sample, and percentages of recovered palynomorphs were computed in terms of total plant pollen count. Poacea (grasses) are categorized in to non-cereal (with pollen < 60 mm) and cereal with pollen > 60 mm) (Basumatary and Bera, 2012).
The palyno assemblages were categorized as arboreals (trees and shrubs) and non-arboreals (marshy and terrestrial herbs). 4. Results The radiocarbon ages of borehole sediments range from Recent to 11.9 ka, and the calibrated ages of ~4.69 and ~9.55 ka represent the sediment depths of 6.20 and 24.50 m respectively (Fig. 2), while the calibrated age of ~11.69 ka corresponds to the depth interval of 31.50 m. Sediments at the bottom of borehole i.e., at 39.75 m depth reveal the 14C age as ~11.9 ka. Inversion of calibrated ages of ~12.12 ka and ~10.33 ka are encountered at 15 m and 23 m depth
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Fig. 2. Depth versus age diagram of borehole sediments from north limb of Periyar River of Kerala, Central Kerala, India. The age inversion at 25 m depth profile, shown in shaded portion, may be due to the neotectonic activity in the region.
respectively. 4.1. Textural properties The average sand, silt, and clay ratios of sediments are 39%, 34%, and 27% respectively (Table 1). However, sand content varies between 70 and 90% up to 5 m depth i.e., during the Late Holocene period. Sand-mud intercalations are recorded from 5 to 10 m depth (i.e., up to ~6 Ka) and sandy mud from 10 to 14.5 m depth (up to ~7 ka) as shown in Fig. 3. The borehole exhibits silty-sand overlying a thin layer of sandy mud between 14.5 and 16 m depth (i.e., around 7.5 ka), whereas mud dominates from 18 to 24 m depth (~8e9.5 ka), and peat deposit along with silty clay intercalations occurs below this depth (i.e., between 24 and 40 m), which ranges in age from ~9.5 to 11.9 ka. The borehole shows downcore change in texture varying from intercalations of sandy mud, silt to mud and peat deposits at 18 m depth (Fig. 3) indicating a change in the environment of deposition from fluvial to marine at ~8 ka. The borehole exhibits two distinct lithofacies e (i) muddy sands from surface to 18 m depth, i.e., ~8 ka, with sediments coarsening upwards, and (ii) muddy sediments with peat from 18 to 40 m depth i.e., during early Holocene. 4.2. Geotechnical properties Geotechnical properties of borehole sediments reveal that the wet bulk density ranges from 1.39 to 1.92 g/cc with an average of 1.64 g/cc. The maximum and minimum wet bulk density values are observed at 17 m (~7.5 ka) and 6.50 m depth (~4.6 ka) respectively. Moisture content varies from 24 to 98% with an average of 55%; the highest content is recorded at 6.50 m (~4.6 ka) and lowest at 17 m depth (~7.5 ka). Organic carbon also shows a similar trend like moisture content and varies from 0.1% (2.0 m depth - Recent
period) to 7.30% (38.50 m depth i.e., during 11e12 ka) with an average of 3.89%. It is observed that the physical and geotechnical parameters exhibit a significant change at 18 m depth of the borehole i.e., at ~8 ka (Fig. 3). Liquid limit varies from 46% at 10 m depth (~6 ka) to 108% at 19.75 m depth (~8.2 ka) with an average of 77%. These values are higher when compared to moisture content values indicating the plastic state of sediments. The variation of liquid limit values are similar to the moisture content, silt and clay contents, plastic limit, plasticity index, and shear strength, and show a positive correlation; whereas wet bulk density and sand content exhibit negative correlation with liquid limit. Plastic limit values range from 17% at 6 m depth (~4.6 ka) to 53% at 23 m depth (~8.5 ka), with an average of 34%. Plasticity index of the borehole sediments ranges from 23% at 11 m depth (~5.5 ka) to 61% at 21 m depth (~8.3 ka) with an average of 43% and shows medium to high plasticity in nature. The geotechnical properties exhibit increasing trend from surface to ~18 m depth i.e., from Recent to ~8 ka period, and slightly decreasing trend from 18 m to 40 m depth i.e., from ~8 ka to 12 ka. The plasticity values plotted on the plasticity chart (Fig. 4a) follow the A-Line. It is observed that majority of the samples exhibit very high plasticity. Activity chart (Fig. 4b) indicates the active state of the samples as they range from 0.78 to 2.77 with an average value of 1.41 (Skempton, 1953). 4.3. Clay minerals Relative percentages of clay minerals - smectite, kaolinite, chlorite, and illite and their ratios are shown in Fig. 5. Maximum, minimum, and average percentages of clay minerals are given in Table 1. The dominance of smectite and kaolinite in the upper section and illite in the lower section of the borehole is recorded; whereas chlorite is present in low concentration. Unlike textural,
Table 1 Maximum, minimum, and average values of textural & geotechnical properties and clayminerals. PBH: Paravur bore hole, g: wet bulk density, w: water content, Corg: organic carbon, S:sand, Z:silt, C: clay, Su: undrained vane shear strength, LL: liquid limit, PL: plastic limit, PI: plasticity index, A activity, wP/wL:plastic limit/liquid limit, Sm: smectite, Ka: Kaolinite, Ch: Chlorite, Ill: Illite. PBH (geotechnical)
g (g/cc)
W (%)
Corg (%)
S (%)
Z (%)
C (%)
Su (Kpa)
LL (%)
PL (%)
PI (%)
A
wP/wL
Sm (%)
Ka (%)
Ch (%)
Ill (%)
Maximum Minimum Average
1.92 1.29 1.64
92 24 55
7.21 0.1 3.99
95 0 29
53 5 34
54 0 25
55 3 19
105 66 77
52 17 24
61 23 42
2.77 0.78 1.41
0.52 0.25 0.44
71 10 14
50 4 20
14 0 6
63 5 29
PBH: Paravur bore hole, g: wet bulk density, w: water content, Corg: organic carbon, S:sand, Z:silt, C: clay, Su: undrained vane shear strength, LL: liquid limit, PL: plastic limit, PI: plasticity index, A activity, wP/wL:plastic limit/ liquid limit, Sm: smectite, Ka: Kaolinite, Ch: Chlorite, Ill: Illite.
Please cite this article in press as: Narayana, A.C., et al., Holocene environmental changes as recorded in sediments of a paleodelta, southwest coast of India, Quaternary International (2017), http://dx.doi.org/10.1016/j.quaint.2017.04.016
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Fig. 3. Lithofacies observed in vertical cross section of Paravur borehole recovered from the paleodelta region of Central Kerala, India. Downcore variation of texture, physical, and geotechnical parameter variations are also plotted. Based on lithology, the borehole can be divided into six facies as: muddy sand, sand, sandy mud, silty sand, sandy silt, and mud.
Fig. 4. a Figure shows liquid limit versus plasticity index value graph as plasticity chart to understand the plasticity level of borehole sediments recovered from north limb of Periyar river in the paleodelta regime. It can be observed that the points are falling on and around A-line in the Plasticity chart. b The activity chart of borehole sediments. Skempton (1953) classification of clays exhibits as active, normal, and inactive clays, satisfying the active state of the sediments.
and geotechnical parameters, clay minerals show two major changes in their relative concentrations at 12 m and 18 m depth (during ~6e8 ka). Smectite is dominant from surface to 12 m depth (i.e., during Early to Mid- Holocene) of the borehole. Further, low smectite content with concomitant abundance of chlorite and kaolinite from 12 m to 18 m depth (~6e8 ka) is observed. Increased amount of illite is recorded below 18 m depth of the borehole i.e., from ~8 ka to 12 ka. 4.4. Palynological assemblages Based on distinct palyno-assemblages retrieved from the borehole samples spanning from early to late Holocene period, five
pollen zones (P-I to P-V; Fig. 6) are broadly demarcated. Palynozone P-I (36e40 m depth; Late Holocene - ~11.9 ka): This palyno-spectrum is mainly dominated by the presence of 20e45% of grasses (Poaceae), followed by trilete and monolete spores up to a maximum of 13% and 26% respectively. Among the tree taxa, fabaceae and arecaceae are found to a maximum of 6%, whereas, sapotaceae, meliaceae and elaeocarpus are limited (2e4%) in their abundance. The core mangrove taxa are represented by rhizophora (1e2%) along with a scanty appearance of sonneratia, ceriops, avicennia and schleichera. Palynozone P-II (23e36 m depth; Late Holocene: ~9.5e11.7 ka): This pollen spectrum is marked by the proliferation of rhizophora to a maximum of 9%, followed by a small increase in abundance of
Please cite this article in press as: Narayana, A.C., et al., Holocene environmental changes as recorded in sediments of a paleodelta, southwest coast of India, Quaternary International (2017), http://dx.doi.org/10.1016/j.quaint.2017.04.016
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Fig. 5. Relative percentage of clay minerals (smectite, illite, chlorite, and kaolinite) in the borehole sediments.
Fig. 6. Shows the pollen variations in the borehole samples. Five pollen zones (P-I to P-V) on the basis of distinct palynoassemblage retrieved from the borehole spanning from early to late Holocene.
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ceriops and schleichera. Rest of the core mangrove taxa like sonneratia, avicennia and lumnitzera are recorded in trace amounts. Tree taxa like sapotaceae, meliaceae, anacardiaceae and combretaceae are found varying in abundance from 3 to 5%; while fabaceae and arecaceae are encountered in low to moderate concentrations of 2e6%. Presence of regional taxa like alnus was noticed in P-II palynozone. Poaceae (9e20%) was observed with increase of terrestrial herbs like asteraceae and rutaceae to about 2e3%. Ferns represented as trilete (15e30%) and monolete (10e20%) in relatively higher values, compared to preceding P-I palynozone. Palynozone P-III (15e23 m depth; ~6e9.5 ka): This pollen zone is marked by the invasion of rhizophora (7e11%) along with occurrence of ceriops and schleichera up to 3% in concentration. Sporadic appearance of sonneratia, avicennia and lumnitzera was also recorded in P-III palynozone. Among the tree taxa, sapotaceae, meliaceae, combretaceae and anacardiaceae are found up to 4e5%, whereas other arboreal taxa like aquifoliaceae, chrysophyllum, elaeocarpus, semecarpus, fabaceae and moraceae are found in low quantity (1e3%). Surprisingly, lagerstroemia is found to be absent in this palynozone. Grasses occur in relatively higher amount (15e30%) as compared to P-II palynozone. Among ferns, monolete spores are recorded in more or less same quantity, whereas, trilete spores are observed in exceptionally higher amounts (up to 30%) as compared to palynozone P-II. Palynozone P-IV (5e15 m depth; Mid-Holocene): This zone is dominated by the presence of trilete spores ranging from 20 to 27%, followed by grasses (15e27%) and monolete spores (15e19%). Among the core mangrove taxa, rhizophora show a decreasing trend, compared to P-III zone. Striking increase of sonneratia was noticed for the first time in P-IV palynozone along with increment in values of lumnitzera, ceriops and schleichera. However, avicennia is very scanty. Among the arboreal taxa, increase of aquifoliaceae, fabaceae and arecaceae, while decrease of key tropical deciduous arboreals like sapotaceae, meliaceae, combretaceae, anacardiaceae and meliaceae are recorded. However, lagerstroemia, elaeocarpus and semecarpus show slight increase as compared to P-III zone. Palynozone P-V (0e5 m depth; Late Holocene): This zone is marked by the increase of rhizophora (up to 10%) along with ceriops, sonneratia, lumnitzera and schleichera. Among arboreals, a maximum of 10% of fabaceae and arecaceae are found. Some of the tree taxa such as sapotaceae and elaeocarpus record an increase up to 5%. Grasses are dominant with a maximum of 25%. Monolete spores slightly increased in abundance with the decrease of trilete spores. 5. Discussion Radiocarbon chronology suggests the Holocene period for the sediment sequence recorded in the borehole of central coastal region of southwest India. The higher radiocarbon ages at 15 m (~12.2 ka) and 23 m (~10.3 ka) depths (Fig. 2) indicate inversion of the ages, which is probably due to the slumping of sediments caused by neotectonic activity. The role of neotectonics on the sea level rise along the southwest coast of India has been discussed earlier (e.g., Nair et al., 2006; Valdiya and Narayana, 2007). Sea level reached its maximum position during the mid-Holocene, and subsequently lowered and stabilized at the present level (Kale and Rajaguru, 1983). Walcott (1972) estimated the post glacial sea level rise for different regions and assigned 88 m rise since 18 ka for the southwest coast of India. Based on 14C ages of on-shore shell deposits from the coastal plains of Karnataka (Caratini and Rajagopalan, 1992) and peat deposits along the Goa coast (Mascarranhas and Chauhan, 1998), it is suggested that the sea level was about 3.0 ± 1.0 m below the present level during the midHolocene i.e., at ~6.4 ka. Fairbanks (1989) observed that sea level
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rose worldwide rapidly (~24 m) during ~12 ka, followed by a slow rate of rise during ~11 ka, and rose again at a higher rate at ~9.5 ka. The sea level rose at a very faster rate (~20 m/1kyr) from ~10 to 7 ka, and after ~7 ka, particularly during the last 4 ka, it remained more or less at the present level along the southwest coast of India (Hashimi et al., 1995). Rising sea level is the primary control on aggradation of floodplains as observed in the deposition of RhineMoeuse coastal prism during middle Holocene back-filling (Van Dijk et al., 1991; Cohen et al., 2005). During the sea level rise, accommodation exceeded sedimentation in many regions. Further, peat can form extensively in flood basins because of the low supply of sediment. Local tectonic controls can lead to anomalies in subsurface elevation, sedimentary facies and aggradation rates in a longitudinal direction. Tectonic effects in deltaic plain are recorded by the changing distribution of organics and clastics within the sedimentary sequence. The control of active tectonics on alluvial deposits has long been recognized (Schumm et al., 2000). Deltaic systems have low-gradient reaches, and are believed to be sensitive to subtle neotectonic deformation (Holbrook and Schumm, 1999). Hence, the neotectonics might have induced the subsidence, resulting in the inversion of radiocarbon ages of sediments in the present study area. The Holocene fluvial record in the study area reflects the neotectonic control of differential subsidence during floodbasin aggradation. The sedimentary architecture of the borehole reveal floodbasin sediment deposits (intercalated beds of sand, muddy sand, sandy mud, silty sand, sandy silt, clay and peat). Widespread peat with intercalated clay occurs during ~9.5 and ~11.9 ka i.e., at the base of the Holocene sequence. Its presence indicates the onset of aggradation. The sediment characteristics give insights into evolution history of sedimentary deposit and paleoenvironmental history. The upward coarsening of sediments in the upper portion of the borehole suggests the change of sediment deposition in rough hydrodynamic conditions during the marine regression (Manojkumar et al., 1998). Thus, the textural characteristics of borehole sediments reveal a change in depositional environment of deltaic facies, apparently from marine to fluvial environment during mid-Holocene marine regression (Fig. 3). The geotechnical properties of sediments also exhibit distinct variation in the upper part of borehole i.e., from surface to 18 m depth (Recent to ~8 ka) corroborating with textural variation. The coarse sediments in the upper portion of borehole, representing Late Holocene period, suggest rapid erosion in the hinterland because of intense southwest monsoonal rainfall and initiation of high fluvial energy conditions, which resulted in the supply of coarse sediments to the paleodelta region. The geotechnical properties revealing high moisture content, organic carbon, and plasticity index, and low bulk density associated with low shear stress are conducive for erosion (Vanoni, 1975; Thorn and Parsons, 1980) and high erosion rate (Mehta et al., 1982). Further, sand and silt mixture in the upper part of borehole suggests that fluvial environment was influenced by the variation in the intensity of monsoon. Wang et al. (2010) also made similar observations in the evolution of subaquous Yangtza delta of China during post glacial period, and they attributed this to the sea level changes and the influence of Asian monsoon. The wet bulk density exhibits opposite trend with moisture content and organic carbon, particularly up to 18 m depth (~8 ka period) (Fig. 3). Moisture content shows pronounced positive relation with organic carbon content, particularly in bottom portion of the borehole. Thus, the sediments represent an increased erosion rate which can be accounted for the intense monsoon. The clay mineral record of the borehole further suggests the variation in depositional environment influenced by the sea level changes and monsoon intensity. Clay minerals are used as a potential proxy to understand climate change, the intensity of
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weathering, and the degree of hydrolysis (Chamley, 1989; Thamban et al., 2002; Das et al., 2013). The increase in illite and to some extent smectite content, and corresponding decrease of kaolinite suggest increase in relative sea level or aridity during that time (Ruffel et al., 2002). In general, the local sea level fall would reflect in increased kaolinite deposition. The relative abundances (Fig. 5) of clay minerals varied significantly in time, while the illite and smectite contents show opposite trends in the borehole. Smectite is abundant in upper horizon from surface to 11.6 m depth (~6 ka) and below this depth the illite is the dominant one towards down core. Kaolinite exhibits similar trend like that of smectite, while chlorite concentration is very low throughout the section. This gives an evidence of major sea level fall causing the change in the environment of deposition from marine to fluvial and also the aridity of the climate in the region. It is suggested that the clay minerals are derived from the hinterland of the coast (Rao and Rao, 1995; Thamban et al., 2002; Narayana et al., 2008), which experiences intense southwest monsoon processes. Singer (1984) suggests that relative high percentage of chlorite and illite can be related to dry periods, while strong humid conditions lead to intense weathering and consequently the dominant occurrence of kaolinite. Smectite is taken as climate indicator of contrasting seasons, particularly a strong dry season. In this study, relative percentages of clay minerals show distinct temporal variations representing short interval of climate changes at depths of 2.0 m (early Holocene; wet to dry), 11.60 m (~6 ka; dry to wet), 17.0 m (~7.5 ka; dry to wet), and 18.90 m (~8 ka; wet to dry) of the borehole. Sea level changes and the geomorphic evolution of the coast since the Neogene contributed to substantial changes in the mangrove habitats, particularly the mangrove cover during the Late Holocene have been found to be different along the southwest coast of India, which is attributed to local hydrodynamics and rainfall in the mangrove swamps (Limaye et al., 2014). The sea level changes along the coast of Saxony, Germany, were effectively inferred using pollens (Freund et al., 2004). The palynological record of the borehole sediment samples shows that overall Rhizophora is the dominant core mangrove taxa. Apart from this, Rhizophoraceae-sonneratia also occurs between 24 and 40 m (Late Holocene) and 5e14 m depth (~4.5 - ~6.5 ka). The Rhizophorasonneratia transition from Middle to Late Holocene is considered as a distinctive feature of mangrove response to climate change (Kumaran et al., 2013). This modification may be attributed to the response of hydrological regimes. The ecological shift from core mangrove to fresh water facies is well marked in the study area. Increase in Rhizopora and decrease in grasses is well marked in P-II palynozone (~9.5e~11.7 ka) with further stabilization of both in the P-III zone (~7e~9.5 ka). Similar relation was noticed in P-IV (during Mid- Holocene) and P-V palynozones (Late Holocene), where decline in Rhizopora and rise in Poaceae pollen was observed in PIV palynozone (Mid-Holocene) with further stabilization in P-V palynozone i.e., during Late Holocene. The sediment samples at 5e14 m depth interval (i.e., during ~4.5e~6.5 ka) reveal the occurrence of mangrove associated tree elements in fair amount namely, Meliaceae, Anacardiaceae and Sapotaceae, etc., along with trace occurrence of Rhizophora. The herbaceous elements were also observed in moderate values. Grasses are found in higher values in almost all the palynozones except a small decrease in P-II palynozone. However, other terrestrial herbaceous taxa came into existence during ~9.5e11.7 ka as observed in P-II palynozone. Among ferns, trilete and monolete spores are dominant in almost all the samples, signifying moist and wet conditions in the study area. Monsoon variation and sea level changes could be the reason for increase and decrease of core mangrove taxa as well as change in frequencies of Poaceae and Arecaceae pollen in this entire Holocene sequence.
6. Conclusion The textural, physical and geotechnical parameters, clay minerals, and pollen records in sediments of a 40 m depth borehole from a paleodelta on the southwest coast of India throw insights on climate change and environment of deposition during the Holocene. The textural parameters reveal a distinct change in depositional environment from marine to fluvial coinciding with sea level fall. The concentrations of clay minerals - smectite, kaolinite and illite - also support the major sea level fall and the aridity in the region giving evidence to the change of environment of deposition. The variation in the fluvial facies of borehole section suggests the active monsoon and major sea level fall. Coarse grained sediment texture indicates the change in depositional conditions in the upper part of borehole sequence i.e., during Late Holocene, coinciding with strong fluvial action and marine regression. Geotechnical properties also suggest the change in the sedimentary facies because of varied hydrodynamic conditions. Pollen records of sediments indicate semi-evergreen type of mangrove plants from early-to mid- Holocene period, whereas the pollens that represent Mid-to Late Holocene suggest the change in hydrological regimes, which are reflected in the textural and geotechnical characteristics of sediments. The study reveals many episodes of change in the environment of deposition along the southwest coast of India during the Holocene, and the factors such as neotectonics, sea level changes, and varied intensity of southwest monsoon might have contributed, which in turn caused the change in mangrove vegetation along the southwest coast of India. Acknowledgement ACN thanks the All India Council for Technical Education for funding through a research project (No. 8019/RDII/TAP/CIV (232)/ 2000-01) granted when the author was with the Cochin University of Science and Technology, Cochin. C M Nautiyal is thanked for his help in radiocarbon dating. Girish Prabhu of National Institute of Oceanography (NIO) is acknowledged for his help during XRD analysis for clay mineral analysis. Authors thank S.K. Bera, for his help in pollen analysis, and R. Ramesh, PRL, for his valuable suggestions. References ASTM, 2005. Standard Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method. Annual book of American Society for Testing and Material standards, West Conshohocken, United States, p. E 691. Basumatary, S.K., Bera, S.K., 2012. Vegetation succession and climate change in Western Gharo hills, Meghalaya, India, since 11,643 year B.P: a palynological record. Int. J. Earth Sci. Eng. 5 (4), 748e758. Bera, S.K., Farooqui, A., Gupta, H.P., 1997. Late pleistocene/holocene vegetation and environment in and around Marian Shola, Palni hills, Tamil Nadu. Palaeobotanist 46, 191e195. Biscay, P.E., 1964. Distinction between chlorite and kaolinite in recent sediments by X-ray diffraction. Am. Mineral. 49, 1281e1289. Boulay, S., Colin, C., Trenteasaux, A., Pluquet, F., Bertaux, J., Blamart, D., Buehring, C., Wang, P., 2003. Mineralogy and sedimentology of Pleistocene sediment in the South China Sea (ODP site 1144): proceedings of the ocean drilling program. Sci. Results 184, 1e21. Bradley, R.S., 1999. Paleoclimatology: Reconstructing Climates of the Quaternary. Academic Press, San Diego. Caldas, L.H.O., Oliveira, J.G., Medeiros Jr., W.E., Stattegger, K., Vital, H., 2006. Geometry and evolution of the Holocene transgressive and regressive barriers on the semi-arid coast of NE Brazil. Geo-Marine Lett. 26, 249e263. Caratini, C., Rajagopalan, G., 1992. Holocene marine transgression marker on the Karnataka coast (India). Indian J. Mar. Sci. 2, 149e151. Carr, A.S., Bateman, M.D., Roberts, D.L., Murray-Wallace, C.V., Jacobs, Z., Holmes, P.J., 2010. The last interglacial sea-level high stand on the southern Cape coastline of South Africa. Quat. Res. 73, 351e363. Carver, R.E., 1971. Procedures in Sedimentary Petrology. Wiley-Interscience, NewYork.
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Please cite this article in press as: Narayana, A.C., et al., Holocene environmental changes as recorded in sediments of a paleodelta, southwest coast of India, Quaternary International (2017), http://dx.doi.org/10.1016/j.quaint.2017.04.016