Abstract

Twelve surface sediment samples have been studied on micropalaeontological and sedimentological aspects in the Rani-Garbhanga Reserve Forest, Assam to establish the ecological and morphological influence on the biological entities. Twelve surface samples were collected from Forest, Margin area, and Open land areas from two widely separated regions, namely Nalapur and Sukurbaria. The results from this study of total diatom count, the relative abundance of individual diatom species, sand-silt-clay percentage, and quartz grain microtexture shows that the population of diatom is low and the sand percentage is maximum within the forest area, whereas in margin area, diatom is increased and the sand percentage is decreased compared to forest area, and in open land area diatom is maximum and the sand percentage is low. Quartz grain microtexture also exhibits the more aeolian signature of transportation in forest areas and a more aqueous signature in open land areas. The study infers that biological and sedimentological parameters vary with the different settings within the forest. This study can be useful to decipher shifts in forest setup in past from the core collected around the study areas.

1. Introduction

Understanding the regional climate and its effect as a part of global changes depends on the discernment of the proper interactions of palaeoclimatic variation, human activity and environmental response. ^{1, 2} Dynamic process of changing climate is the most important factor, which governs the vegetational shifts periodically in any region in any course of geological time. There have been a number of repetitive climatic shifts recorded all over the world during the Quaternary period. The state of Assam, Northeast India, remains an ecologically sensitive area and highly responsive to climatic changes of every degree.

The present study has aim to provide the effect of climatic influence on its biotic and abiotic components from the tropical area of the Indian subcontinent. Though rainfall has a major control on monsoon variability this area was poorly understood so far with respect to climatic changes. This region has its uniqueness for its biodiversity controlled by its natural settings and hydro-geomorphic condition. Assam is considered an important biodiversity hotspot with a combination of deciduous forests, mixed deciduous forests, tropical evergreen, wetland ecosystems, grasslands, and bamboo plantations. However, among them, very few are designated as reserved forests and national parks. The valley around the mighty Brahmaputra river, expanding a total area of 3000 km² is called the most important river valley in northeast India as well as in the country. Due to drastic shifts in vegetational conditions influenced by climatic changes in this relatively high precipitation area, this area remains centre of palaeoclimatic studies with different proxies.

Single-celled photosynthetic eukaryotic algae diatoms belong to the class Bacillariophyceae. Diatom is one of the most prominent microorganisms which can be found in almost all aqueous environments. In freshwater environment diatoms play an important role as primary producers. More than 250 genera of diatoms with over 100,000 species have been recorded so far. ^{3, 4} These microalgae possess highly ornamented cell walls composed of silica (SiO₂) called frustules that provide a variety of shapes from micro to nano-scale structures. Diatoms can occur both in solitary and colony forms. Being a cosmopolitan organism, diatoms can be divided into planktonic (free-floating) and benthic (attached to a substrate) forms based on their habitat. ⁵ Benthic diatoms have very diverse habitats, which can thrive either as free-floating (phytoplanktons) or in the sediments (epipelon), or attached to the pebbles (epilithon), sand grains (epipsammon), and any other aquatic plants (epiphyton). Diatoms that live on soft sediment can also move up and down within the sediment column. On the other hand, other diatoms can be attached to the substrate with the help of mucilaginous stalks. Diatoms can also attach to one another to form communities of filamentous algal mats in lakes and rivers. ⁶

Variations in diatom assemblages have been controlled exclusively by the changes in water quality, such as temperature, salinity, light, oxygen content, conductivity, pH, moisture, current velocity, and inorganic and organic nutrients, which are sometimes influenced by human interference. ⁷ The purpose of bio-assessment based on diatom is to examine its distributional pattern in surface sediment within the pristine wetlands in Assam. Based on diatoms recovered from sediments of two wetlands of Assam namely Nalapur and Sukurbaria, a preliminary attempt has been made to understand the interrelation of diatoms with the environmental conditions of these wetlands.

2. Study Area

The Rani-Garbhanga Reserve Forest situated south of the Brahmaputra river falls under the Assam plain alluvial Sal forest in the Kamrup district. This area serves as a natural elephant corridor linking 70 km from the Meghalaya plateau to the Deepor Beel area. This Reserve Forest is divided into two major forest ranges; the Garbhanga range (Long: 91° 36’ to 91°47’E; Lat: 26°05’ to 25°54’N, 18861 ha) and the Rani range (Long: 91°35’ to 91°42’E; Lat: 26°05’ to 26°01’N, 4370 ha). Locations of the swamps of the current study area, namely Nalapur (25°59’45.6” N; 91°32’08.0” E) and Sukurbaria (25°58’35.7” N; 91°32’33.3” E) fall within the Nalapur forest division of Rani Reserve Forest (Figure 1).

To study the micropaleontological and sedimentological response to the environmental parameters twelve surface samples were collected from the respected areas (six samples from each area) from the centre of the forest (namely NP1, SKR1), the margin of the forest (namely NP2, NP3, SKR2, SKR3) and open land area (namely NP4, NP5, NP6, SKR4, SKR5, SKR6).

3. Methodology

Twelve surface samples were collected from Forest, Margin, and Open land areas from Nalapur and Sukurbaria, Rani-Garbhanga Reserve Forest. Each sample was treated differently for studies of micropalaeontological (diatom) and sedimentological aspects

The methodology for processing the samples for diatoms and making the diatom slides followed the procedure as described by Battarbee & Kneen ⁸ and Battarbee ⁹. Diatoms were cataloged with a minimum of 300 valves counted. The concentration of diatom per gm was calculated using the total number of valves counted in a specific area of the slide of the sample of known weight (modified after Battarbee ¹⁰).

Under the microscope, the diatoms were identified up to species level using the classical and advanced literature for identification ^{11, 12, 13, 14}, and count the abundance against the total of at least 300 complete valves (>90% intact) of the diatom. The relative abundance of each species was plotted against the sample number of both the study areas to understand the special change in the abundance of species. The relative abundance of all species was analysed by Constrained Incremental Sum of Squares (CONISS) Cluster Analysis with the help of Tilia Software Program Version 1.7.16 ¹⁵ to understand the affinity of the species presented in the samples of the study areas.

The grain size was measured on the pre-treated carbonate and organic-free sediments using laser scattering through Cilas 1190 Particle Size Analyzer. The grain-size classification was done following the Wentworth scale. ¹⁶ Grain-size data were analysed by Gradistat software ¹⁷ to assess different grain-size parameters. This software provides the measurement by Folk & Ward ¹⁸ as well as moments methods along with graphical representations of grain-size variation. In this present study, logarithmic Folk and Ward formulae (based on a log-normal distribution with size) have been used to interpret different parameters of grain size; such as the average size of the sediment (mean grain size), the dispersion of the distribution around the mean and the sorting of the sediment (standard deviation); the symmetry of distribution around the mean (skewness) and the peakedness of the distribution (kurtosis). This software also produces the frequency distribution curve based on the weight percentage of different fractions of sediments. The mean grain size is described using the Udden-Wentworth grade scale; while standard deviation, skewness, and kurtosis are illustrated using the scheme proposed by Folk & Ward ¹⁸. The maximum, minimum, and average values of these parameters from the samples of two locations are calculated. Bivariate scatter plots of mean grain size versus standard deviation, skewness, and kurtosis separately were derived from the statistical parameters using Microsoft Excel. These plots were used to understand the geological significance of grain size with regards to transporting medium, the energy of depositing medium, and the condition of deposition. [10-18].

Around 5 g of sediment samples from collection spots were processed for quartz grain analysis following the the procedure illustrated by Krinsley & Doornkamp ²¹. Fifteen to twenty grains of quartz of >63 μm size from each sample (total ~220 grains) from Nalapur and Sukurbaria were separated manually using a binocular stereo zoom microscope to observe the grain morphology and surface microtextures under SEM (JEOL model no. JSM7610F). Quartz grain morphology along with surface microtextures has been observed and the relative abundance of each microtexture has been tabulated. The relative abundance of all microtextures was analysed using Constrained Incremental Sum of Squares (CONISS) Cluster Analysis with the help of Tilia Software Program Version 1.7.16 ¹⁵ to infer both the mechanism of transportation and depositional condition of the sediments using the comprehensive method described by Krinsley & Doornkamp ²¹, Mahaney ²², Helland & Holmes ²³, Strand et al ²⁴ and Vos et al ²⁵.

4. Result

A total of 44 diatom species under 20 genera were found abundant in the Nalapur areas (Figure 2). The diatom species were identified from Nalapur as Achnanthidium exiguum (Grunow) Czarnecki 1994, Amphora sp., Caloneis bacillum (Grunow) Cleve 1894, C. silicula (Ehrenberg) Cleve 1894, Craticula cuspidata (Kutzing) D.G.Mann 1990, Cymbopleura sublanceolata Krammer 2003, Diploneis smithii (Brébisson) Cleve 1894, Encyonema minutiforme Krammer 1997, Eunotia arcus Ehrenberg 1837, E. camelus Ehrenberg 1843, E. circumborealis Lange-Bertalot & Nörpel 1993, E. minor (Kützing) Grunow 1881, E. pectinalis (Kützing) Rabenhorst 1864, E. valida Hustedt 1930, Frustulia crassinervia (Brébisson ex W.Smith) Lange-Bertalot & Krammer 1996, F. indica H.P.Gandhi 1959, Gomphonema affine Kützing 1844, G. augur Ehrenberg 1841, G. gracile Ehrenberg 1838, Luticola mutica (Kützing) D.G.Mann 1990, Navicula cryptotenella Lange-Bertalot 1985, Neidium gracile Hustedt 1938, Nitzschia frustulum (Kützing) Grunow 1880, Nitzschia cf. N. hantzschiana Rabenhorst 1860, N. umbonata (Ehrenberg) Lange-Bertalot 1978, Pinnularia acrosphaeria W.Smith 1853, P. amabilis Krammer 2000, P. conica H.P.Gandhi 1957, P. crucifera A.Cleve 1934, P. divergens W.Smith 1853, P. joculata (Manguin) Krammer 2000, P. latarea Krammer 2000, P. nodosa (Ehrenberg) W.Smith 1856, P. viridis (Nitzsch) Ehrenberg 1843, Pinnularia sp., Placoneis anglophila (Lange-Bertalot) Lange-Bertalot 2005, Rhopalodia musculus (Kützing) O.Müller 1900, Sellaphora bacillum (Ehrenberg) D.G.Mann 2018, Stauroneis phoenicenteron (Nitzsch) Ehrenberg 1843, S. smithii Grunow 1860, Surirella capronii Brébisson & Kitton 1869, S. capronioides H.P.Gandhi 1959, S. ceylonica Skvortsov 1930 and S. leyana Bramburger & P.B.Hamilton 2006. All diatom species are recognized as pinnate (benthic) form. The number of total diatoms in 1 gm of samples from Nalapur varies from 800 (sample no. NP1) to 5896100 (sample no. NP7) (Figure 3A).

On the other hand, a total of 60 diatom species under 23 genera were found abundant in the Sukurbaria areas (Figure 2). The diatom species were identified from Sukurbaria as Achnanthidium exiguum (Grunow) Czarnecki 1994, Amphora copulata (Kützing) Schoeman & R.E.M.Archibald 1986, Caloneis bacillum (Grunow) Cleve 1894, C. silicula (Ehrenberg) Cleve 1894, Craticula ambigua (Ehrenberg) D.G.Mann 1990, C. cuspidata (Kutzing) D.G.Mann 1990, Cymbopleura sublanceolata Krammer 2003, Diploneis smithii (Brébisson) Cleve 1894, Encyonema hustedtii Krammer 1997, E. javanicum (Hustedt) D.G.Mann 1990, E. minutiforme Krammer 1997, E. pergracile Krammer 1997, Encyonema sp., Eunotia minor (Kützing) Grunow 1881, E. valida Hustedt 1930, Eunotia sp., Fallacia pygmaea (Kützing) Stickle & D.G.Mann 1990, Frustulia crassinervia (Brébisson ex W. Smith) Lange-Bertalot & Krammer 1996, Gomphonema augur Ehrenberg 1841, G. gracile Ehrenberg 1838, G. parvulum (Kützing) Kützing 1849, G. pseudoaugur Lange-Bertalot 1979, Luticola goeppertiana (Bleisch) D.G.Mann ex J.Rarick, S.Wu, S.S.Lee & Edlund 2017, L. mutica (Kützing) D.G.Mann 1990, Navicula sp., Neidium gracile Hustedt 1938, Nitzschia compressa (Bailey) C.S.Boyer 1916, N. desertorum Hustedt 1949, N. linearis W.Smith 1853, N. obtusa W.Smith 1853, N. obtusa var. kurzii (Rabenhorst) Grunow 1880, N. palea (Kützing) W.Smith 1856, N. umbonata (Ehrenberg) Lange-Bertalot 1978, Nitzschia sp., Pinnularia acrosphaeria W.Smith 1853, P. amabilis Krammer 2000, P. complexa Krammer 2000, P. conica H.P.Gandhi 1957, P. crucifera A.Cleve 1934, P. divergens W.Smith 1853, P. latarea Krammer 2000, P. pisciculus Ehrenberg 1843, Pinnularia aff. P. saprophila Lange-Bertalot, Kobayasi & Krammer 2000, P. viridis (Nitzsch) Ehrenberg 1843, Pinnularia sp. A, Pinnularia sp. B, Pinnularia sp. C, Placoneis anglophila (Lange-Bertalot) Lange-Bertalot 2005, Planothidium biporomum (M.H.Hohn & Hellerman) Lange-Bertalot 1999, Rhopalodia musculus (Kützing) O.Müller 1900, Sellaphora bacillum (Ehrenberg) D.G.Mann 2018, S. capitata D.G.Mann & S.M.McDonald 2004, S. pupula (Kützing) Mereschkovsky 1902, Stauroneis amphicephala Kützing 1844, Stauroneis cf. S. kriegeri R.M.Patrick 1945, S. phoenicenteron (Nitzsch) Ehrenberg 1843, S. smithii Grunow 1860, Surirella capronioides H.P.Gandhi 1959, S. ceylonica Skvortsov 1930 and Tabularia fasciculata (C.Agardh) D.M.Williams & Round 1986. All diatom species are recognized as pinnate (benthic) form. The number of total diatom in 1 gm of samples from Sukurbaria varies from 61000 (sample no. SKR2) to 408000 (sample no. SKR6) (Figure 3B).

The relative percentages of all the species were plotted to show the distribution of each species over the areas. The CONISS Cluster Analysis of Nalapur based on the relative percentage of the species (considered as variables) divided the core into three parts (at the total sum of squares 0.5) (Figure 4); Zone I comprise of single sample NP1; Zone II constitutes of two samples, namely NP2 and NP3; whereas Zone III comprises of three samples, namely NP4, NP5, and NP6.

Cluster Analysis of Sukurbaria based on the relative percentage of the species (considered as variables) divided the core into three parts (at the total sum of squares 0.5) (Figure 5); Zone I comprise of single sample SKR1; Zone II constitutes of two samples, namely SKR2 and SKR3; whereas Zone III comprises of three samples, namely SKR4, SKR5 and SKR6.

Nalapur samples are mostly coarse silty medium sand, except NP3, which is categorized coarse silty fine sand (Figure 3A). On the other hand, Sukurbaria samples show a varied range of grain sizes (Figure 3B). SKR1 is categorized as very coarse silty fine sand, SKR2 is as fine sandy coarse silt and SKR3 is as fine sandy coarse silt; whereas SKR4 is categorized as medium sandy medium silt, SKR5 is as very coarse silty fine sand and SKR6 as fine sandy medium silt. All samples are poorly sorted Figure 6).

In Nalapur, the sand percentage varies from 57.0 (NP3) to 73.7 (NP1); whereas the silt percentage varies from 22.8 (NP1) to 35.87 (NP4) and the clay percentage varies from 3.11 (NP5) to 7.98 (NP3). The mean size ranges from 2.661Φ (NP1) to 4.033Φ (NP3) and sorting ranges from 2.356Φ (NP5) to 2.706Φ (NP6); whereas skewness varies between 0.418 (NP4) and 0.607 (NP1) and kurtosis varies between 0.816 (NP4) and 0.994 (NP3) (Figure 7). According to mean size, most of the samples are of sand size, except NP3 which is of very coarse silt size. Mean size versus standard deviation exhibits that all samples are very poorly sorted sediments. Mean size versus skewness exhibits that all samples are strongly fine-skewed irrespective of their mean size. Mean size and kurtosis shows that two samples (NP1 and NP3) are mesokurtic, while other samples are platykurtic (Figure 8).

In Sukurbaria, the sand percentage varies from 33.6 (SKR5) to 74.2 (SKR3); whereas the silt percentage varies from 22.2 (SKR3) to 57.3 (SKR5), and the clay percentage varies from 3.6 (SKR3) to 9.1 (SKR5). The mean size ranges from 3.171Φ (SKR1) to 5.304Φ (SKR6) and sorting ranges from 2.247Φ (SKR5) to 2.918Φ (SKR4); whereas skewness varies between 0.011 (SKR4) and 0.476 (SKR5) and kurtosis varies between 0.722 (SKR4) and 1.34 (SKR5) (Figure 7). According to mean size, two samples (SKR1 and SKR5) are of sand size, three samples (SKR2, SKR3, and SKR4) are of very coarse silt size and one sample (SKR6) is of coarse silt. Mean size versus standard deviation exhibits that all samples are very poorly sorted sediments. Mean size versus skewness exhibits that two samples (SKR1 and SKR5) are very fine skewed, one sample (SKR2) is fine skewed, and three samples (SKR3, SKR4, and SKR6) are near symmetrical. Mean size and kurtosis shows that one sample (SKR5) is leptokurtic, three samples (SKR1, SKR2, and SKR3) are mesokurtic and two samples (SKR4 and SKR6) are platykurtic (Figure 8).

The quartz grains from Nalapur and Sukurbaria studied under SEM exhibit textures that originated from both mechanical actions and chemical origin (Figure 9). The majority of quartz grains show high to moderate roundness and exhibit microtextures with low to moderate relief. Each grain shows characteristic microtextures of mechanical origin, such as featureless smooth grain surface, small pits, medium pits, large pits, small conchoidal fractures, medium conchoidal fractures, straight steps, arcuate steps, imbricated grinding features, meandering ridges, straight striations & scratches, curved scratches, random scratches and grooves, V-shaped patterns and Fracture plates/planes. Some textures of chemical origin, viz. adhering particles, solution pits, solution crevasses or channels, silica globules, silica pellicles, crystalline overgrowth (secondary dendritic structures), silica precipitation, silica tubes, and etched surface are also present.

The average percentage of the overall grain morphology and microtexture for 20 grains of each sample from Nalapur and Sukurbaria was tabulated and hence cluster analysis was performed separately for these two locations. For Nalapur samples, three major zones are recognized below 0.2 value of total sum of squares (Figure 10). Zone I represent two samples (namely NP1 and NP2), Zone II consists of two samples (namely NP3 and NP4), while Zone III comprises of two samples (namely NP5 and NP6). The predominant morphology and microtexture of Zone I are rounded to subrounded grain with bulbous edges, small to large pits, conchoidal fractures, meandering ridges, random scratches and grooves, and featureless smooth grain surface, which are formed mostly by aeolian mode of deposition. On the other hand, Zone III shows characters of an aqueous mode of deposition, such as rounded to subrounded grain with meandering ridges to subangular grain, straight striations & scratches, curved scratches, V-shaped patterns, adhering particles, solution pits, solution crevasses and channels, secondary crystalline dendritic overgrowth, silica precipitation, silica tubes, and etched surface. Zone II shows the mixed characteristics of these two zones, suggesting the effect of both aeolian and aqueous modes of deposition.

For Sukurbaria samples, three major zones are recognized below 0.2 value of the total sum of squares (Figure 11). Zone I represent the sole sample SKR1, Zone II consists of three samples (namely SKR2, SKR3, and SKR4), while Zone III comprises of two samples (namely SKR5 and SKR6). On the basis of quartz grain morphology and microtexture, Zone I show mostly an aeolian mode of deposition, while Zone III shows the dominance of the aqueous mode of deposition, and Zone II exhibits the mixture of both aeolian and aqueous mode of deposition.

The surface samples collected from two different areas, Nalapur and Sukurbaria, from Rani-Garbhanga Reserve Forest, were divided into three major environmental setups, namely forest area, marginal area, and open land area. The characteristics of each part on the basis of the biotic and abiotic proxies are given below.

(A) Forest area

(1) Nalapur

Only one sample (NP1) represents the forest area in Nalapur. In the case of diatom studies, NP1 has a very low count of diatom (800 per gm) and is dominated by Placoneis anglophila, Achnanthidium exiguum, Caloneis silicula, Diploneis smithii and Luticola mutica with the subordinate occurrence of Eunotia valida, Pinnularia latarea, and Rhopalodia musculus. Sedimentologically NP1 is designated as very poorly sorted coarse silty medium sand with the comparatively highest sand percentage (73.7%) and low clay percentage (3.5%). Grain size shows bimodal distribution with the major mode at 517.8µm and secondary mode at 31.54 µm. The mean value (158.1 µm) of NP1 falls within the fine sand range of grain size. This location exhibits very fine skewed (0.607) and mesokurtic (0.992) in nature. In the case of cluster analysis based on Quartz grain microtexture, this sample shows close affinity with sample NP2 (from the marginal area), together constitute Zone I. NP1 is dominated by rounded to subrounded grain (sometimes featureless grains) with bulbous edges with small to medium conchoidal fractures, small to medium pits, imbricated grinding features, meandering ridges, arcuate steps, random scratches, and grooves. These features are mostly associated with the aeolian mode of deposition.

(2) Sukurbaria

For Sukurbaria, sample SKR1 represents the sample collected from the forest area. In the case of the diatom study, this sample constitutes Zone I with the dominance of Stauroneis phoenicenteron, Rhopalodia musculus, and Encyonema hustedtii with a minor presence of Achnanthidium exiguum, Cymbopleura sublanceolata, Diploneis smithii, and Stauroneis cf. S. kriegeri. It has a low count of diatom (65000 per gm). Sedimentologically SKR1 is designated as very poorly sorted very coarse silty fine sand with a high sand percentage (69.5%) and low clay percentage (4.4%). Grain size shows polymodal distribution with the major mode at 185.6µm and secondary mode at 751.9 µm. The mean value (111.0 µm) of SKR1 falls within the very fine sand range of grain size. This location exhibits very fine skewed (0.345) and mesokurtic (0.94) in nature. In the case of cluster analysis based on Quartz grain microtexture, this sample constitutes Zone I, which is dominated by rounded to subrounded grain (sometimes featureless grains) with bulbous edges with small to medium conchoidal fractures, medium pits, imbricated grinding features, meandering ridges, arcuate steps, random scratches, and grooves. These features are mostly associated with the aeolian mode of deposition.

(B) Marginal area

(1) Nalapur

Samples from Nalapur marginal areas are marked by NP2 and NP3. In the case of diatom studies, the number of diatoms is comparatively higher in these samples (15500 per gm in NP2 and 13500 per gm in NP3). Both the samples constitute Zone II in diatom cluster analysis (Fig. 4) and are characterised by an increased abundance of Diploneis smithii, Encyonema minutiforme, Eunotia circumborealis, E. pectinalis, Pinnularia viridis and decreasing abundance of Achnanthidium exiguum, Caloneis silicula, Luticola mutica, Pinnularia latarea and Placoneis anglophila with a prominent abundance of Eunotia valida and Rhopalodia musculus. Sedimentologically NP2 is designated as very poorly sorted coarse silty medium sand with a comparatively lower sand percentage (65.8%) and higher clay percentage (6.16%). Grain size shows trimodal distribution with the major mode at 471.7µm and secondary mode at 28.73 µm. The mean value (88.32 µm) of NP2 falls within the very fine sand range of grain size. This location exhibits very fine skewed (0.517) and platykurtic (0.870) in nature. On the other hand, NP3 is designated as very poorly sorted coarse silty fine sand with the comparatively lowest sand percentage (57.0%) and the highest clay percentage (7.98%). Grain size shows bimodal distribution with the major mode at 169.1µm and secondary mode at 28.73 µm. The mean value (61.07 µm) of NP3 falls within the very coarse silt range of grain size. This location exhibits very fine skewed (0.422) and mesokurtic (0.994) in nature. In the case of cluster analysis based on Quartz grain microtexture, NP2 falls in Zone I (shared with NP1) while NP3 falls in Zone II (shared with NP4). The decrease in rounded to subrounded grain (sometimes featureless grains) with bulbous edges with small to medium conchoidal fractures, small to medium pits, imbricated grinding features, meandering ridges, arcuate steps, random scratches, and grooves, and increase in rounded to subrounded grain with meandering edges, straight steps, straight striations and scratches along with curved scratches. Some typical aqueous features, namely adhering particles, solution pits, solution channels, silica globules, and etched surfaces appear in these two samples. These features are mostly associated with the aeolian mode of deposition with some aqueous influences.

(2) Sukurbaria

The two samples SKR2 and SKR3 were collected from marginal areas. In case of diatom study, the number of diatoms in these samples is almost same as SKR1 (61000 per gm in SKR2 and 89000 per gm in SKR3). SKR2 and SKR3 constitute Zone II, which is represented by the substantial amount of Rhopalodia musculus, Pinnularia viridis, Stauroneis amphicephala, Placoneis anglophila, Nitzschia umbonata and Diploneis smithii with the minor occurrence of Eunotia valida, Pinnularia acrosphaeria, Pinnularia amabilis, Pinnularia latarea, and Stauroneis phoenicenteron. Sedimentologically both SKR2 and SKR3 are designated as very poorly sorted Fine Sandy Coarse Silt. SKR2 has a comparatively lower sand percentage (43.9%) and higher clay percentage (7.9%), while SKR3 has a comparatively higher sand percentage (74.2%) and lower clay percentage (3.6%). The grain size of SKR2 shows bimodal distribution with the major mode at 185.6µm and the secondary mode at 31.54 µm. The mean value (42.17 µm) of SKR2 falls within the very coarse silt range of grain size. This location exhibits finely skewed (0.174) and mesokurtic (0.904) in nature. On the other hand, the grain size of SKR3 shows trimodal distribution with the major mode at 154.0µm and the secondary mode at 28.73 µm. The mean value (40.61 µm) of SKR3 falls within the very coarse silt range of grain size. This location exhibits near symmetrical skewed (0.056) and mesokurtic (0.943) in nature. In the case of cluster analysis based on Quartz grain microtexture, SKR2 and SKR3 constitute Zone II (shared with SKR3). The decrease in rounded to subrounded grain (sometimes featureless grains) with bulbous edges with small to medium conchoidal fractures, small to medium pits, imbricated grinding features, meandering ridges, arcuate steps, random scratches, and grooves, and increase in rounded to subrounded grain with meandering edges, straight steps, straight striations and scratches along with curved scratches. Some typical aqueous features, namely adhering particles, solution pits, solution channels, silica globules, and etched surfaces appear in these two samples. These features are mostly associated with the aeolian mode of deposition with some aqueous influences.

(C) Open land area

(1) Nalapur

NP4, NP5, and NP6 are three samples collected from the open land area in Nalapur. In the case of diatom studies, the number of diatoms is comparatively much higher in NP4 and NP6 (441233 per gm and 488600 per gm, respectively) and the number is highest in NP5 with a value of 741800 per gm. All samples represent Zone II in diatom cluster analysis. Zone III is characterised by the major abundance of Diploneis smithii, Encyonema minutiforme, Eunotia circumborealis, Frustulia crassinervia, Gomphonema affine, Nitzschia umbonata, Pinnularia viridis and Rhopalodia musculus. Sedimentologically NP4, NP5, and NP6 are designated as very poorly sorted coarse silty medium sand. NP4 has a comparatively lower sand percentage (59.3%), while NP5 has little higher sand percentage (69.9%) and NP6 has a medium value of sand percentage among these three samples (62.6%). The grain size of NP4 shows polymodal distribution with the major mode at 269.5µm and the secondary mode at 568.4 µm. The mean value (85.9 µm) of NP4 falls within the very fine sand range of grain size. This location exhibits very fine skewed (0.418) and platykurtic (0.816) in nature. The grain size of NP5 shows bimodal distribution with the major mode at 471.7µm and the secondary mode at 28.73 µm. The mean value (136.7 µm) of NP5 falls within the fine sand range of grain size. This location exhibits very fine skewed (0.555) and platykurtic (0.842) in nature. The grain size of NP6 shows trimodal distribution with the major mode at 295.9µm and the secondary mode at 28.73 µm. The mean value (87.39 µm) of NP6 falls within the very fine sand range of grain size. This location exhibits very fine skewed (0.474) and platykurtic (0.838) in nature. In the case of cluster analysis based on Quartz grain microtexture, NP4 falls in Zone II (shared with NP3) while NP5 and NP6 fall in Zone III. These samples are characterised by less rounded to subrounded grain with bulbous edges with small to medium conchoidal fractures, small to medium pits, imbricated grinding features, meandering ridges, arcuate steps, random scratches and grooves and increase in rounded to subrounded grain with meandering edges, straight steps, straight striations and scratches along with curved scratches. Some typical aqueous features, namely adhering particles, solution pits, solution channels, silica globules, and etched surfaces are very prominent in these three samples. These features are mostly associated with an aqueous mode of deposition with less aeolian influence.

SKR4, SKR5, and SKR6 are three samples collected from the open land area in the Sukurbaria region. In the case of the diatom population, SKR4 and SKR5 show higher abundance (298000 per gm and 266500 per gm, respectively), while SKR6 shows the highest population (408000 per gm). In cluster analysis, all three samples fall in Zone III, which exhibits the major abundance of Rhopalodia musculus, Nitzschia umbonata, Pinnularia acrosphearia, P. amabilis, P. viridis, Placoneis anglophila with the substantial abundance of Diploneis smithii, Craticula cuspidata, Frustulia crassinervia, Stauroneis amphicephala, Pinnularia divergence, and P. latarea. Sedimentologically SKR4 is designated as very poorly sorted medium sandy medium silt, whereas SKR5 is designated as very poorly sorted very coarse silty fine sand, and SKR6 is designated as very poorly sorted fine sandy medium silt. All these samples have the comparatively lower sand percentage (45.4% in SKR4, 33.6% in SKR5, and 44.1% in SKR6) and higher silt percentages (48.6% in SKR4, 57.3% in SKR5, and 48.7% in SKR6). The grain size of SKR4 shows polymodal distribution with the major mode at 429.6µm and the secondary mode at 11.31 µm. The mean value (40.91 µm) falls within the very coarse silt range of grain size. This location exhibits near symmetrical skewed (0.011) and platykurtic (0.722) in nature. The grain size of SKR5 shows bimodal distribution with the major mode at 203.7µm and the secondary mode at 31.54 µm. The mean value (110.4 µm) falls within the very fine sand range of grain size. This location exhibits very fine skewed (0.476) and leptokurtic (1.340) in nature. The grain size of SKR6 shows trimodal distribution with the major mode at 185.6µm and the secondary mode at 12.41 µm. The mean value (25.31 µm) falls within the coarse silt range of grain size. This location exhibits near symmetrical skewed (0.039) and platykurtic (0.847) in nature. In the case of cluster analysis based on Quartz grain microtexture, SKR4 falls in Zone II (shared with SKR2 and SKR3) while SKR5 and SKR6 fall in Zone III. These samples are characterised by lesser rounded to subrounded grain with bulbous edges with small to medium conchoidal fractures, small to medium pits, imbricated grinding features, meandering ridges, arcuate steps, random scratches, and grooves and increase in rounded to subrounded grain with meandering edges, straight steps, straight striations and scratches along with curved scratches. Some typical aqueous features, namely adhering particles, solution pits, solution channels, silica globules, and etched surfaces are very frequently present in these three samples. These features are mostly associated with an aqueous mode of deposition with less aeolian influence.

5. Discussion

Diatoms are extremely sensitive to different ecological variables, such as temperature, salinity, nutrient availability, water quality (conductivity and pH), and water level. ^{26, 27} Hence diatoms can be used as a viable proxy for ecological and environmental studies. Based on their interaction with ambiance, diatom has been used to correlate with environmental and climatic parameters.

For the Nalapur region, the total absence of planktonic diatom indicates a very shallow water level with low energy. In Zone I, the low diatom frequencies indicate the drier condition of the area. Zone II is characterised by the substantial presence of epiphytic benthic diatom Pinnularia, Frustulia, and Eunotia, indicating the eutrophic condition of the region. ^{27, 28} In Zone III, the high frequencies of total diatom along with the presence of Eunotia and Gomphonema indicate the eutrophication as well as anthropogenic activities in the open land (lake) area. In the case of the Sukurbaria region, planktonic diatom is totally absent, indicating a very shallow water level with low energy. In Zone I, the low diatom frequencies indicate the drier condition of the area. Zone II shows the presence of Pinnularia, Frustulia, and Gomphonema indicating the eutrophic condition of the region. ^{27, 28} In Zone III, Pinnularia, Frustulia, and Nitzschia along with the high frequencies of the total number of diatoms indicate the eutrophication as well as anthropogenic activities in the open land (lake) area.

In the present study, grain-size parameters (also called texture of sediment) exhibit that the composition of sediment is mostly controlled mostly by sand and silt with a minor presence of clay. Inter-relation between different statistical parameters is globally used to determine the depositional condition of the sediment. Bimodal to polymodal distributions of grain size indicates the mixed deposition environmental conditions of aeolian to aqueous. Mean size indicates the overall condition of energy during deposition. ^{18, 29} The presence of sand-silt compositions indicates the decreasing water level along with high energy conditions. All the samples from both regions exhibit a very poorly sorted sediment type. The quartz grain microtexture shows a gradual decrease of the aeolian type of deposition and a gradual increase of aqueous deposition from forest areas towards open land (lake) areas.

6. Conclusions

• The results from this study of total diatom count and sand-silt-clay percentage show that the number of diatoms is low and the sand percentage is maximum within the forest area.

• In the margin area, the diatom number is increased and the sand percentage is decreased compared to the forest area.

• In the open land areas the diatom number is maximum and the sand percentage is low.

• The study infers that the diatom abundance & sand-silt-clay percentage varies with the different settings within the forest.

• Quartz grain microtexture exhibits the dominancy of aeolian transportation in the forest areas, while the aqueous effect on transportation prevails in the open land.

Acknowledgments

The authors wish to thank the Director, Birbal Sahni Institute of Palaeosciences, Lucknow for providing the laboratory and library facilities, and for permission to publish. The authors are grateful to Dr. Subodh Kumar, Technical Officer, Birbal Sahni Institute of Palaeosciences for taking the Scanning Electron Microphotographs. The authors also thank Mr. Rajaram Verma and Ms. Sakshi Shrivastava, Technical Assistants for processing the samples for this study. The authors are also thankful to the Department of Science and Technology (DST) for financial support (Grant No. EMR/2014/000233). This paper is submitted under BSIP Publication no. 51/2022-23.

References