Suman Sarkar

Palaeoecological implications of corallinacean red algae and halimedacean green algae from the prang formation of south shillong plateau, meghalaya

The southwestern part of south Shillong plateau (Meghalaya, N-E India), designated as Sylhet Limestone Group is sub-divided into three lithounits i.e., Lakadong, Umlatdoh and Prang formations in ascending order. The Prang Formation is the youngest lithostratigraphic unit of the Sylhet Limestone Group and has been dated as Middle to early Upper Eocene based on the benthic foraminifera studies.Thin section analysis of carbonate rocks from Prang Formation, exposed in the Bholaganj limestone quarry yielded a rich assemblage of calcareous algae. The coralline algal assemblage comprises both non-geniculate and geniculate forms. The green algae are represented by species of Halimeda belonging to the family Halimedaceae.Palaeoecological interpretation based on diversity, growth-form analysis and taphonomic aspects of the algal assemblage indicate that in all probabilities the deposition of Prang Formation occurred in shallow, warm, shelf environment of normal salinity within the transgressive phase.

Calcareous Algal-Rich Carbonate Sediments from Assam Shelf, N-E India: An Overview of the Palaeoenvironmental Implications

Assam Shelf in north-east India is one of the best studied sedimentary basins of the Indian subcontinent with respect to hydrocarbon exploration. Calcareous algae are common biogenic components of the Palaeogene carbonates outcropping in Meghalaya, Assam Shelf. They occur as encrusting and compact thalli, maerls and fragments in lagoonal to proximal outer shelf facies. The relative abundance of calcareous algae increase gradually from late Palaeocene to middle Eocene and is correlated to the evolution of a reefal environment also characterized by rich quantities of benthic foraminifera. The population dynamics of calcareous algae in the carbonate units of the Sylhet Limestone Group indicate an overall shallow bathymetry, mesotrophic nutrient regime and moderate species diversity. Abundance of mastophoroid and sporolithacean corallines points towards a tropical palaeoenvironment. The Palaeocene-Eocene time period featured critical, globally significant events such as the Indo-Asian plate collision and Palaeocene-Eocene Thermal Maximum (PETM). This factor adds to the importance of reef evolution and the constituent biogenic assemblages in NE India during this period.

Evaluation of coralline algal diversity from the Serravallian carbonate sediments of Little Andaman Island (Hut Bay), India

Palaeodiversity of coralline algae has been studied from the Serravallian carbonate sediments outcropping in the Little Andaman Island (Hut Bay). Little Andaman is the southernmost island of the Andaman Group, situated between South Andaman and Car Nicobar. The algae described in the present case study have been recovered from the fossiliferous limestone samples of Hut Bay Limestone Quarry belonging to the Long Formation and examined by means of thin-section microscopic analysis. Shannon–Weaver index has been applied for measuring the degree of algal diversity. A total of nine coralline algal genera, including three geniculate and six non-geniculate algae have been reported with the statistical data indicating a fairly conducive benthic environment. This is supported by the presence of a considerable number of growth-forms characterizing the concerned coralline algal taxa. The limited occurrence of taphonomic signatures observed in the algal thalli further strengthens the interpretation of a conducive environment. Greater diversity and abundance of benthic foraminifera present in close proximity to the coralline algae have resulted in possible failure of the latter in attaining higher levels of diverse assemblages. Factors of mild environmental disturbances, hydrodynamic conditions, and substrate availability also supposedly played a major role in determining the diversity of coralline algae.

Coralline algae and benthic foraminifera from the long formation (middle Miocene) of the Little Andaman Island, India: Biofacies analysis, systematics and palaeoenvironmental implications

Biofacies analysis is used to understand the palaeoenvironmental implications of late middle Miocene (Serravallian) algal-foraminiferal assemblages and carbonate deposition pertaining to the Hut Bay section of the Little Andaman Island, India. This study is based on the novel dataset of coralline algae and benthic foraminifera evaluated from the thin section analysis. Systematic descriptions of the abundant algal-foraminiferal taxa have been provided. The Long Formation in the studied succession contains rich assemblages of coralline algae and benthic foraminifera with moderate abundance of corals and sparse occurrences of echinoderms, gastropods, molluscs, bryozoans and barnacles. Based on the biogenic sedimentary components, Little Andaman Island was positioned in a tropical shelf environment at this time. The depth gradient of algal assemblages shows affinity to numerous other carbonate settings globally. The fossil benthic communities are indicative of thriving in a meso-oligotrophic regime. Dominance of coralline algae and larger benthic foraminifera indicates deposition in upper photic zone to the upper part of the lower photic zone. Frequent alternations of lower-energy, mud-supported wackestones with higher-energy, grain-supported grainstones and packstones suggest the likely incidence of cyclones/storms during the depositional period. The facies gradients and high incidence of taphonomic signatures such as abrasion and fragmentation support the likelihood of parautochthonous to allochthonous deposition for the bulk of the succession.

Upper Pliocene heterozoan assemblage from the Guitar Formation of Car Nicobar Island, India: palaeoecological implications and taphonomic signatures

The upper Pliocene Guitar Formation outcrops in Car Nicobar Island, a near-pristine locality of the Andaman-Nicobar Group of Islands, India. Tropical-subtropical photozoan and cool-water heterozoan assemblages have been broadly studied up to now, but little is known about the tropical heterozoans that require deeper analysis. The major objectives of the current assessment pertaining to the Guitar Formation are evaluation of biotic diversity, palaeoecological processes governing the depositional environment and general taphonomic signatures. The carbonate sediments rich in algal and benthic foraminiferal assemblages are dominated by lithophylloids and nummulitids, respectively. Secondary biotic constituents are corals, barnacles, bryozoans, echinoderms, molluscs and gastropods. In particular, a reefal environment is indicated after a thorough examination of the heterozoan assemblages including relative abundance of the algal-foraminiferal taxa. The results indicate that carbonate sedimentation occurred in warm, tropical waters under meso-oligotrophic conditions and shallow to mildly deeper bathymetric levels ranging close to or below the fair-weather wave base.

Early Eocene calcareous algae and benthic foraminifera from Meghalaya, NE India: A new record of microfacies and palaeoenvironment

Early Eocene carbonate sediments of the Umlatdoh Limestone (Meghalaya, N-E India) represent a shallow marine shelf environment. The major biotic components characterizing these carbonates are calcareous green algae and small to larger benthic foraminifera. Based on the biogenic associations and general sedimentological features, five major facies types (MFTs) are distinguished. They are dominated by poor to moderately sorted grainstones followed by packstones, rudstones and wackestones. Considerable abundance of Halimeda, scarcity of z-corals and poor to moderate occurrence of filter-feeding organisms imply mesotrophic to a slightly oligotrophic nutrient regime. Rare occurrence of geniculate coralline algae is probably due to the lack of suitable substrate and environmental conditions. High incidence of grainstones and packstones, fairly preserved microfossils and few reworked specimens indicate a parautochthonous mode of deposition. Preponderance of Alveolina and Nummulites indicate the possible advent of larger foraminiferal turnover (LFT) in the east Tethys during or even before early Eocene. A conceptual palaeoenvironmental model for the studied succession is provided to showcase various facies gradients, bathymetry levels and shelf zones pertinent to the Umlatdoh Limestone.

Diversity of corals and benthic algae across the shallow-water reefs of Andaman Islands: managing the valuable ecosystems

The Andaman Islands characterize one of the relatively unexplored coral reef zones of the Indian subcontinent. A few benthic biodiversity studies have been carried out to date, but several coral and algal species still remain to be evaluated in terms of their abundance, vigour and conservation priority. In absence of a comprehensive, reliable dataset, inter-relationships between anthropogenic impacts and climate alterations with the coral reefs cannot be assessed authentically. This paper addresses the coupled themes of biodiversity and reef management in the Andaman Islands by examining the percentage cover of corals and benthic algae from shallow-water ecosystems across the coasts of Havelock, Neil, Ross, Jolly Buoy Islands and Chidiyatapu beach proximal to Port Blair. Four major reef types were observed: coral-dominated, algae-dominated, neutral setting and algae overgrowing dead reefs. Jolly Buoy Island had the highest percentage cover of scleractinian corals and crustose coralline algae. Turf algae were most abundant at the Ross Island, while other macroalgae showed highest abundance at Chidiyatapu. Overall species diversity values for corals and algae were highest at the Jolly Buoy and Chidiyatapu, respectively. Given that corals and algae are critical reef components, management paradigms must consider the abundance and frequency of both these biogenic entities in the seascape. The needs, expectations and objectives of the people dependent on coral reef ecosystems also need to be considered. Long-term monitoring is imperative in understanding the natural typology of reefs and managing the possible algal encroachments. Refined management efforts that include greater thrust on development of marine protected areas and reserves; establishment of connectivity between various coral ecosystems of the region; control of invasive algae; and increasing awareness among the local people as well as tourists will ensure continued support of ecosystem to maintain healthy reefs. Collectively, the results are used to promote some strategies to conserve the Andaman coral reefs and cope with the detrimental anthropogenic and climate changes in these coastal habitats.

Coralline red algae from late Palaeocene–earliest Eocene carbonates of Meghalaya, N–E India: palaeocommunity and trophic-level implications

Coralline red algae are important skeletal components of the Sylhet Limestone platform deposits outcropping in Meghalaya, N–E India. These are highly abundant in the presently studied Late Palaeocene–earliest Eocene carbonate succession pertinent to the Lakadong Limestone. The coralline algae are mainly represented by melobesioid and sporolithacean taxa characterised by encrusting, warty, lumpy, fruticose, and unconsolidated growth forms. Larger benthic foraminifera are also abundant in the examined biotic assemblages. Herein, the aspects of palaeocommunity analysis and trophic-level reconstruction have been approached as well. Special emphasis has been given to the dominant fossil coralline algae by means of discussion on their systematic descriptions and contribution to the community structure as one of the primary producers. Palaeocommunity analysis through the evaluation of the fossil assemblages makes it possible to interpret trophic-group differentiation in primary producers and various consumer groups. A schematic food web organization supported by the recorded biogenic components has also been deciphered.