Ajai Rai | University of Allahabad (original) (raw)

Papers by Ajai Rai

Pteropod assemblages were studied at ODP Site 728A near the Oman margin (northwestern Arabian Sea... more Pteropod assemblages were studied at ODP Site 728A near the Oman margin (northwestern Arabian Sea) lying at a water depth of 1428 m, just below the present-day Oxygen Minimum Zone (OMZ) and aragonite compensation depth (ACD). Variations in the strength of OMZ over the past 175 kyrs in the northwestern Arabian Sea were reconstructed on the basis of changing abundances of pteropod at this site. The down-core abundance reveals pteropod spikes at the transitions of isotope stage MIS 6/5, MIS 2/1 and during glacial stages MIS 6 and MIS 2, reflecting deepening of ACD and relatively less intense OMZ in this region, possibly due to deep-sea mixing and thermocline ventilation, and the relative decline in surface productivity during winter monsoon. In general, the interglacial periods are largely devoid of, or marked with low pteropod abundances, indicating dissolution of aragonite due to increased intensity of OMZ in the northwestern Arabian Sea. The distinct peak of abundance of Limacina inflata and mesopelagic forms during transition of MIS 6/5 reflects relatively lower surface productivity compared to MIS 2/1 transition. However, both these transitions are marked with intense deep-sea ventilation. This explains that the pteropod preservation is mainly influenced by the fluctuations in the OMZ due to deep-sea ventilation and remains largely independent of variations in the surface productivity. Thus, the dissolution and not the lack of supply is mainly responsible for the low abundances of pteropod shells in the deep-sea sediments of the high-productivity regions like the Oman margin.

In the present study four distinctive species of the genus Favocassidulina are recorded from the ... more In the present study four distinctive species of the genus Favocassidulina are recorded from the Neogene deep sea sequences of seven DSDP Sites (Legs 22,23 and 24) in the Indian Ocean. Detailed systematic descriptions and stratigraphic ranges of these species in the Indian Ocean have been provided. In addition, on the basis of quantitative faunal analysis, an attempt has been made to assess the ecological significance of the genus Favocassidulina . Species of this genus are largely confined to lower bathyal and abyssal depths and exhibit restricted occurrences in Indian and, Pacific Oceans only. The intervals of higher relative abundances of genus Favocassidulina during Neogene possibly reflect low organic carbon input in the bottom waters.

The distribution pattern of deep sea bemhic foraminifera across the Miocene/ Pliocene boundary in... more The distribution pattern of deep sea bemhic foraminifera across the Miocene/ Pliocene boundary in the northern Indian Ocean DSDP sites 219, 237 and 238 were examined in order to determine their relationship with the bottom water circulation pattern. Of the 153 species of benthic foraminifera encountered at the Miocene/Pliocene transition Uvigerina spp. [mainly U. probosci-dw): Cibicides wuellerstorfi, C kullenbergi Globoaissidulinn subglobosa, Buli-mina alazanensis, Oridorsalis umbonatus and Stilostomella lepidula show marked changes in their relative abundances. The relative abundance curves of dominant bemhic foramimleral taxa. species diversity; H(SJ and equitahiltty; t' during Miocene/Pliocene transition at the DSDP situs 219, 237 and 238 were taken into account Lo explain bottom water history in the northern Indian Ocean. The latest Miocene and earliest Pliocene in all the examined sites are marked by fagh Uvigerina spp. abundances, bottom water cooling, increased upwellmg and intensified bottom water circulation. However, low abundance of Uvigerina spp. at 5.2 Ma (Miocene/Pliocene boundary) suggests a short interval of relative warming of bottom waters. The intermittent peaks of occurrence of Nuttallides umbonifera at site 238 relied recurrent incursions of Antarctic Bottom WalerjAABW) in this region.

Palaeogeography, Palaeoclimatology, Palaeoecology, 2012

Diversity parameters of Neogene deep-sea benthic foraminifera were measured at Ocean Drilling Pro... more Diversity parameters of Neogene deep-sea benthic foraminifera were measured at Ocean Drilling Program (ODP) Hole 752A, southeastern Indian Ocean (water depth of 1086.3 m) using Information Function (H), Equitability (E), number of species (S) and Sander's rarefaction values. These parameters combined with population abundance of dominant benthic foraminifera (Bulimina macilenta, Nuttallides umbonifera, Cibicides wuellerstorfi, Cibicides lobatulus, Bolivina pusilla, Ehrenbergina carinata, Gavelinopsis lobatulus, Cassidulina laevigata, Globocassiulina subglobosa) reveal significant paleoceanographic changes in the southeastern Indian Ocean during the Neogene. The values of all the diversity parameters show a decrease from 25 to 23 Ma and thereafter an increase with peak values at~13.5 Ma. The Late Oligocene to Earliest Miocene was an interval of more unstable conditions at Hole 752A dominated by species characteristic of low organic carbon, well-ventilated, carbonate corrosive high energy conditions. The highest values of diversity parameters coincide with the early Middle Miocene climatic optimum. All these parameters show a declining trend and gradual decrease from 13.5 to 4.5 Ma coinciding with the major build up of ice sheets in the Antarctic region. Major increase in B. pusilla and E. carinata population during this time suggests high nutrient levels and low oxygen conditions at Hole 752A. This interval corresponds with the so-called "biogenic bloom" and an intense Oxygen Minimum Zone as observed throughout the Indo-Pacific region. Deep waters were warmer from 4.5 to 3 Ma marked by an increase in species diversity values, coinciding with the early middle Pliocene warmth. The species diversity values abruptly decreased in the younger interval, contemporaneous with the major Northern Hemisphere glaciation. During this time species characteristics of high-energy bottom currents and relatively cold deep water were dominant.

Journal of Earth System Science, 2011

Changes in the abundance of selected planktic foraminiferal species and some sedimentological par... more Changes in the abundance of selected planktic foraminiferal species and some sedimentological parameters at ODP site 728A were examined to understand the fluctuations in the surface productivity and deep sea oxygenation in the NW Arabian Sea during last ∼540 kyr. The increased relative abundances of high fertility taxa, i.e., Globigerinita glutinata and Globigerina bulloides mainly during interglacial intervals indicate intense upwelling. Strong SW summer monsoon probably increased the upwelling in the western Arabian Sea during interglacial intervals and caused high surface productivities due to the lateral transport of eutrophic waters. Most of the glacial periods (i.e., MIS 2, 4, 6, 8 and 12) are characterized by higher relative abundances of Neogloboquadrina pachyderma and Neogloboquadrina dutertrei associated with Globigerinoides ruber. The more stratified condition and deep mixed layer due to increased NE winter monsoon are mainly responsible for the higher relative abundances of N. pachyderma during glacial periods. Some of the glacial intervals (i.e., MIS 6 and 8) are also characterized by pteropod spikes reflecting deepening of aragonite compensation depth (ACD) and relatively less intense oxygen minimum zone (OMZ) in this region due to deep sea mixing and thermocline ventilation, and relatively less intense surface productivity during winter monsoon. The interglacial periods are largely devoid of pteropod shells indicating more aragonite dissolution due to increased intensity of OMZ in the northwestern Arabian Sea. In general, the interglacial periods are characterized by low sediment accumulation rates than the glacial intervals. On an average, the total biogenic carbonate percentages were higher during interglacial and during periods of higher surface productivity. Most terrigenous material was trapped on shelf during intervals of high sea level stands of interglacial, whereas more erosion of shelf increased the sedimentation rates during glacial periods. In addition, the fragmentation record may be the result of changes in intensity and vertical distribution of the OMZ with time. During glacial intervals, the lower boundary of the OMZ probably was in a shallower position than during interglacial periods, when dissolution increased as a result of higher organic production. The higher rates of sinking organic matter result into a stronger OMZ as oxygen is used to disintegrate the organic matter. This process lowers the p H of water which results into increased dissolution of calcium carbonate.

Journal of Earth System Science, 2013

The DSDP Site 237 in the central Indian Ocean is suitable for the Miocene deep-sea benthic forami... more The DSDP Site 237 in the central Indian Ocean is suitable for the Miocene deep-sea benthic foraminiferal study because of continuity of sequence, good preservation and abundance of benrliic fauna. A total of 166 benthic foraminiferal species are recorded from the Miocene sequence at this site. Detailed analysis of quantitative benthic foraminiferal data integrated with stable isotope records reveal few marked changes in the phsico-chemical behaviour of the central Indian Ocean bottom waters during the Miocene.

Agglutinated benthic foraminifera were investigated from two ODP Sites 762B and 763A located on t... more Agglutinated benthic foraminifera were investigated from two ODP Sites 762B and 763A located on the Exmouth Plateau off north west Australia in the eastern Indian Ocean over an interval of Late Miocene to Late Pleistocene. Sixteen species of agglutinated benthic foraminifera are recorded from 152 samples from these sites. The foraminifera are generally well preserved. All the recorded taxa described and illustrated herein belong to ten genera and five families.

OCEANOLOGIA, 2012

Pliocene-Pleistocene deep sea benthic foraminifera from ODP Site 762B in the eastern Indian Ocean... more Pliocene-Pleistocene deep sea benthic foraminifera from ODP Site 762B in the eastern Indian Ocean were examined to understand the tectonically/climatically induced palaeoceanographic changes. In addition to already published data on this site by Rai & Singh (2001), some more faunal parameters were considered in the present work. Characteristic benthic foraminiferal assemblages as well as more diverse fauna during the early Pliocene (before 3.5 Ma) reflected relatively oligotrophic and warm bottom water conditions. At the beginning of the late Pliocene (i.e. ∼ 3 ± 0.5 Ma) relative abundances of Uvigerina proboscidea, infaunal taxa and high productivity taxa increased, whereas faunal diversity showed a distinct decline, suggesting the development of pronounced upwelling resulting in higher surface water productivity. The strongly reduced inflow of warm and oligotrophic water masses as the South Equatorial Current (SEC) from the South Pacific to the eastern Indian Ocean due to the effective closure of the Indonesian seaway increased the surface water productivity. The closing of the Indonesian seaway during the late Pliocene was also responsible for the cessation of the warm, southward-flowing Leeuwin Current (LC) and the greater influence of the cold, deep The complete text of the paper is available at http://www.iopan.gda.pl/oceanologia/ Open access under CC BY-NC-ND license.

Palaeogeography, Palaeoclimatology, Palaeoecology, 2001

In the present investigation, the Late Neogene deep-sea benthic foraminiferal diversity at ODP Si... more In the present investigation, the Late Neogene deep-sea benthic foraminiferal diversity at ODP Site 762B has been measured by using Hurlbert's diversity index (S m). This index measures the expected number of species to a common sample size (m). Here, m is put equal to 100. This method is relatively more advantageous than other commonly used diversity indices because it eliminates differences in sample size. The Late Neogene section at Site 762B is characterised by marked variations in species diversity (S 100) of the deep-sea benthic foraminiferal assemblages. Species diversity broadly exhibits an inverse relationship with the values of Uvigerina proboscidea abundance and percentage of infaunal taxa in the examined section of the site. This suggests that temporal variations in primary productivity of the overlying surface waters are re¯ected in the Late Neogene deepsea benthic foraminiferal diversity trends. The species diversity pattern explains that the trophic level of the bottom water changed at the beginning of the Late Pliocene (,3.5 Ma) due to an increased surface water productivity. The higher values of species diversity during the Early Pliocene was due to a stable deep-sea environment with low trophic resource levels. However, the Late Pliocene and Pleistocene interval is largely characterised by low diversity re¯ecting an unstable deep-sea environment with higher surface water productivity. A distinct increase in the values of species diversity along with low percentages of U. proboscidea and infaunal taxa during the latest Pliocene (,2.3±1.9 Ma) suggests low organic carbon in¯ux in the bottom waters due to a decrease in the surface water productivity.

Micropaleontology

of a JSTOR transmission must contain the same copyright notice that appears on the screen or prin... more of a JSTOR transmission must contain the same copyright notice that appears on the screen or printed page of such transmission. JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org. ABSTRACT: A detailed qualitative and quantitatve investigation of foraminifera from DSDP Site 219 (Arabian Sea) has enabled us to identify a new, distinctive benthic foraminiferal genus, Neopleurostomella, in the Middle Miocene to Early Pliocene deep sea sequences.

ABSTRACT A high resolution record of deep sea benthic foraminiferal diversity variations in the e... more ABSTRACT A high resolution record of deep sea benthic foraminiferal diversity variations in the eastern Arabian Sea for the last 30 ka BP was obtained from two sediment cores (SK17 and MD131) retrieved from the present day Oxygen Minimum Zone (OMZ) in the Indian margin off Goa. The benthic foraminiferal diversity is represented in terms of Sander&#39;s rarefaction number (S100), Shannon Wiener Index [H(S)], Equitability (E0) and Alpha Index (a). Records of diversity indices exhibit millennial scale changes during the late glacial and deglacial periods, corresponding to the Northern Hemisphere climatic events. We compared the faunal diversity with proxy records of primary productivity (Corg %) and bottom water oxygen (low- O2 taxa %). We suggest that benthic foraminiferal diversity in the eastern Arabian Sea OMZ is largely controlled by the primary productivity induced organic carbon flux and strength of bottom water oxygenation. The less diverse fauna along with increased percentages of Corg and low- O2 taxa during the last glacial maximum (18e22.5 ka BP) suggest eutrophic and oxygen-poor benthic environment, attributed mainly to a strong OMZ associated with intense winter monsoon wind induced productivity and a weak deep ocean circulation. The intervals of distinct increase in diversity closely correspond with North Atlantic cold Heinrich events, when eastern Arabian Sea experienced significant declines in monsoon driven productivity and better deep sea ventilation due to enhanced inflow of Antarctic Intermediate Water (AAIW).

Micropaleontology, 1992

ABSTRACT A detailed qualitative and quantitative investigation of foraminifera from DSDP Site 219... more ABSTRACT A detailed qualitative and quantitative investigation of foraminifera from DSDP Site 219 (Arabian Sea) has enabled us to identify a new, distinctive benthic foraminiferal genus, Neopleurostomella, in the Middle Miocene to Early Pliocene deep sea sequences.

Clay mineral assemblages of the terrigenous fraction of 39 samples from ODP site 728A, located on... more Clay mineral assemblages of the terrigenous fraction of 39 samples from ODP site 728A, located on the continental margin of Oman, northwestern Arabian Sea, have been analysed to understand the influence of sediment source, possible transport pathways and palaeoclimatic conditions during the late Quaternary. Illite, palygorskite and chlorite were the most dominant clay minerals, with minor amount of kaolinite and smectite. Quartz, plagioclase feldspar and dolomite were the other detrital minerals. The terrigenous content showed antithetic relationship with the biogenic carbonate, and was high during the glacial stages mainly due to low sea level and erosion of exposed continental shelf by wind and/or fluvial processes. The high biogenic carbonate during the interglacial periods has been correlated to intense monsoonal activity and surface productivity. The study suggests that the detrital minerals are largely aeolian derived from the adjoining Arabian Peninsula, Persian Gulf and Iran-Makran regions and transported to the northwestern Arabian Sea by northwesterly winds. It appears that the cold and dry periods during the glacial times were responsible for physical weathering and contribution of higher amounts of chlorite and dolomite. The warm and humid interglacial period caused extensive chemical weathering and contribution of kaolinite.

The present study is based on heavy mineral assemblages (HM) of top 104-cm thick section of gravi... more The present study is based on heavy mineral assemblages (HM) of top 104-cm thick section of gravity core SK 221 (Lat. 8 • 7.12 N; Long. 73 • 16.38 E and water depth-2188 m) located near the Chagos-Laccadive Ridge in the southeastern Arabian Sea to evaluate the provenance and paleoenvironmental changes during the last 32 kyr. The biogenic carbonate, acid insoluble residue, magnetic susceptibility, total organic carbon (TOC) and clay based humidity index, i.e., kaolinite/illite ratio are also utilized to correlate with the above paleoenvironmental changes. Ilmenite, garnet, staurolite, pyroxenes, andalusite and zircon are the dominant HM with moderate to low ZTR (zircon-tourmaline-rutile) index indicating instability of the sediments and rapid erosion in the source region. The characteristic HM suggest their mixed sources from the basic igneous, gneisses/granites, high grade metamorphic rocks and sandstones occurring mainly in the western and southwestern India. The temporal variations of HM, AIR (acid insoluble residue), MS (magnetic susceptibility), biogenic carbonate and C org (TOC) during preglacial and early Holocene suggest intensive weathering, erosion, and transportation of terrigenous detritus from continental region by fluvial processes and summer monsoon led high biogenic productivity, respectively. The convective mixing of waters due to intense winter monsoon resulted in very high biogenic carbonate content during the early stages of glacial period. The HM and associated proxies indicated that the winter monsoons of Heinrich (H3, H2, and H1) and Younger Dryas (YD) events and summer monsoons of Bølling/Allerød (BA) event were not strong enough to bring drastic changes in the above parameters.

JOURNAL-GEOLOGICAL SOCIETY OF INDIA, Jan 1, 2007

JOURNAL-GEOLOGICAL SOCIETY OF INDIA, Jan 1, 2004

Journal of the Geological Society of India. Vol: 63, No: 4, April 2004, pp: 415-429. Late Neogene... more Journal of the Geological Society of India. Vol: 63, No: 4, April 2004, pp: 415-429. Late Neogene Deep Sea Benthic Foraminiferal Biostratigraphy of ODP sites 762B and 763A (Exmouth Plateau), Eastern Indian Ocean. AJAI K. RAI and VB SINGH. ...