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ISSN 0974-5904, Volume 07, No. 01
February 2014, P.P.289-296
Benthic Foraminifera in a Sedimentary Core from Kollam Coastal Plain, South Kerala, India R GAYATHRI 1, R NAGENDRA1, A N REDDY2, P SATHIYAMOORTHY 1 AND N SURESH1 1
2
Department of Geology, Anna University, Chennai – 600 0025, India 201, Majestic shore, 208, Choolaimedu High Road, Chennai-600094, India Email:
[email protected].
Abstract: A sediment core of 38m length collected from the Kollam coastal plain was analyzed for benthic foraminifera to understand paleoenvironmental evolution. Sedimentological analysis of the core recognized a sharp break in lithology at 13m depth which marked rapid upward decrease in foraminifera abundance and diversity. Total foraminifera abundance and species diversity, however, show a linear positive relationship with increased clay content and differing trend with increased sand content. The core analysis further revealed gradual upward transition in faunal composition and frequency variation of the most dominant taxa A.beccarii and N.scaphum suggests gradual environmental transition from estuarine complex to coastal plain complex in upcore direction. Keywords: Kollam coastal plain, Benthic foraminifera, Kerala coast. 1. Introduction: ◦
Kollam, South West part of Kerala (8 52'48.76"N: 76°36'00.14"E), has a coastline length of 41 km (Fig.I). It has geographical area of 2491 sq km. (Mini Chandran, 1998). Thickest Quaternary sediment. Sequence is exposed in the South Kerala Sedimentary basin which extends from Kollam to Kodungallur in a form of curvilinear area with a maximum width of approx. 25 km and a thickness of approx. 80 m. (Nair and Padmalal 2004). The basin is divided into central depression flanked by Southern Block and Northern Block. The first marine transgression took place around 42,000 years before present (42KyBP). The Holocene marine transgression was experienced by about 7kyBp. This was followed by a regression which left the present landscape of lagoons, wetlands and the ridge-runnel topography. Limaye et a1 (2009) studied Late Quaternary sediments from the boreholes of PanavalIy and Ayiramthengu of Kollam district and reported the occurrence of cyanobacteria. The present study focuses on sedimentological and foraminifera analysis to understand the environmental evolution through the deposition of sediment core. 2. Methodology: A land rig commonly used for ground water exploration was employed to drill a sediment core of a length of 38m and recovered 100%. The lithological study of sediment core reveals that it composed of 3m sand, 32 m lateritic clay. 13m sand with lateritic soil (0-13m), 10m clayey silt (13-23m) and 15m silty clay (23-38m).
About 10 gm dry sediment sample soaked in water for a while and washed over a 63µm sieve with distilled water and dried in an oven at 60°C. Then foraminifera were picked using a stereomicroscope (NOVEX AR 200M) and mounted on faunal slides and total faunal tests were counted. Subsequently the temporal distribution, absolute and relative abundances of benthic foraminifera is summarized in Table 2. Q-mode cluster analysis was carried out on the relative abundance of 29 foraminifera. The foraminifera were taxonomically identified by referring to Loeblich and Tappan (1988), Murray (1971), Boltovskoy et al. (1980) and World modern foraminifer database. 3. Results: 3.1 Sediments: The table I clearly indicates a sharp change in lithology at 13m depth, wherein sand/silt/clay ratios exhibit a turnaround in its percentage occurrence. The core can be divided into 3 litho units based on ratios of sand, silt and clay. The lower litho unit (L-1) between 38-23m contains predominance of clay (63.1-72%). The silt and sand represents 27.1-32.8% and 0.24-0.8% respectively. The litho unit 2 between 23m and 13m consists of clay (55.3-64.3%), silt (25.1-32.9) and sand (2.97-12.65%). This litho unit is transitional between L-1 and L-3, wherein increasing trend of % silt and sand is observed. The upper litho unit (L-3) shows predominance of sand (lateritic) (63.9-99.99%), silt (0-20.5%) and clay (018.9%). 3.2Foraminifera:
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Benthic Foraminifera in a Sedimentary Core from Kollam Coastal Plain, South Kerala, India
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The table II shows the vertical distribution of 30 benthic foraminifera species recorded from the sediment core at Kollam coastal plain. It is evident from the data that Ammonia beccarii (36.5-82.5%) is the most dominant almost at all the study depths followed by Nonion scaphum (7.4-54.2%). The remaining 28 species occur sporadically. Elphidium crispum contains 11% of total relative abundance at 3m depth, while Bolivina spathulata contains 25% and N.boueanum 27% relative abundance at 35m and 7m depths respectively. A. beccarii and N. scaphum occur throughout the core while other species show erratic presence and sparse abundance. The figure II illustrates the hierarchical dendrogram which explains biotopes recognized using Q-mode cluster analysis. The single linkage distance method recognized 8 biotopes at Eucledean distance of 52. The biotope-1 which covers the core depth mainly between 16m and 23m, shows very close similarity in faunal composition at a linkage distance of 10. This interval falls in litho unit 2 (L-2) and consists of mainly clayey silt lithology with minor sand content (2.9712.65%). The increased relative abundance of N.scaphum (31.24%) and reduced abundance of A.beccarii (56.1%) with respect to bottom L-1 unit characterizes this biotope. Biotopes 2,5,7 and 8 envelops the upper part of the core (14-0 m depth). However these biotopes grouped separately because of the contribution by other taxa in addition to two dominant taxa A.beccarii and N.scaphum. Biotope-2 has a linkage distance of 30 and characterized by Elphidium crispum (11%), Fursenkoina texturata (5.2%), Lobatula lobatula (3.4%) and Nonion boueanum (2.7%). Biotope5 is distinct by the presence of E. crispum (1.4%), F. texturata (1.1%) and E. hispidulum (0.4%), whereas Biotope-7 is distinguished by N.boueanum (25%) and Biotope-8 is diagnostic by the presence of E.crispum (5.7%), Bolivina spathulata (4.5%), F.texturata (1.1%) and Rectobolivina ?virgula (0.7%). Well preserved foraminifera specis are demonstrated in Plate I.
species and occurs predominantly in all the coastal water bodies either in polluted or natural environments revealing its high tolerance and adaptability to changing environmental variables. Ammonia beccarii is ubiquitous and dominant in all the coastal water bodies like lagoons and estuaries along the TamilNadu and Kerala coasts (Ramnathan, 1970; Reddy and Reddy, 1982; Jayaraju and Reddy, 1996; Kumar et al. 1996; Gandhi et al. 2002; Nagendra et al.2011). This taxa reported to occur abundantly in lower estuarine zone of Ashtamudi estuary of southern Kerala coast (Nagendra et al.2011), which is in close proximity to the Kollam coastal plain. The water depth in the lower estuary of Ashtamudi is about 4m and the predominant lithology represented by sand. N.scaphum was associated with A.beccarii in the middle estuarine zone (central and western kayals) of Ashtamudi estuary where water depth ranges from 2-4m and lithology mainly represented by clay and silt. In the central and western kayals of Ashtamudi abundance of A.beccarii and N.scaphum account to about 60% and 25-30% respectively.
4. Discussion:
The table I & figure III shows the vertical distribution of total foraminifer abundance (TNF) and species diversity (S). It is observed that a gradual increase of total abundance and diversity in litho unit 1 from 38m to 23m,where clay (63.1-72%) is the dominant lithology while sand represents
