Browsing by Author "A.D. Singh"

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Annual report for the year 2019-2020
(Palaeontological Society Of India, 2020) A.D. Singh
[No abstract available]
Aragonite pteropod flux to the Somali Basin, NW Arabian Sea
(2008) A.D. Singh; S.M.-H. Conan
Aragonite export fluxes of pteropods (>250, 150-250 and 125-150 μm) in the Somali Basin were estimated using a 9-month time-series sediment trap (MST9-E) from June 1992 to February 1993. The trap with 23 time-series sediment collectors placed at 1032 m water depth collected settling particles over a total of 249 days. Pteropods showed large seasonal variations in both the numerical and mass fluxes (>125 μm) with their maxima at the end of the SW Monsoon during September and early October. It was also observed that records of pteropod fluxes varied in different size fractions. The average numerical flux and mass flux of pteropods (>125 μm) from the 9-month record of the sediment trap was estimated to be about 1830 shells m-2 d-1 and 18 mg m-2 d-1, respectively. We estimate that about 22.5% of the total CaCO3 in the Somali Basin was contributed by aragonitic pteropods (>125 μm). © 2008 Elsevier Ltd. All rights reserved.
Benthic foraminiferal biofacies on the shelf and upper continental slope off north Kerala (southwest India)
(Geological Society of India, 2012) N.R. Nisha; A.D. Singh
This study examines benthic foraminifera (>63 μm) both qualitatively and quantitatively, from 19 closely spaced surficial sediment samples covering 30 to 200 m water depths across the shelf and upper continental slope off north Kerala (SW India). A total of 59 species are recorded. The major constituents of benthic foraminiferal assemblages in the study area are fursenkoinids, bolivinids, nonionids, rotaliids, elphidiids, buliminids, miliolids, gavilinellids, amphestiginids, bagginids, vaginulinids, uvigerinids and various agglutinated taxa. Cluster analysis using Bray Curtis similarity index defines four sample groups, each typified by a characteristic assemblage representing a biofacies. The major benthic foraminiferal biofacies identified are: Biofacies I, Fursenkoina-Nonion-Ammonia beccarii s.I. (30-40 m); Biofacies II, Fursenkoina-Nonion (40-55 m); Biofacies III, Bolivina robusta-Hanzawaia-Cancris- Amphistegina and miliolids (55-115 m) and Biofacies IV, Bolivina persiensis-Uvigerina-Bulimina-Fursenkoina and agglutinants (115-200 m). Relict foraminifera, most commonly represented by shallow-water benthic taxa are concentrated on the outer shelf. The relict assemblage appears to be a product of late Pleistocene low sea level. The foraminiferal biofacies have a good correspondence with the bathymetrically distributed three major lithofacies across the shelf and the upper continental slope. We studied the distribution pattern of individual taxa constituting the biofacies. The study demonstrates a relationship between the pattern of distribution of major benthic fauna and the sediment-size and organic carbon content across the inner shelf to upper slope. Additionally, certain taxa appear to be sensitive to various hydrographic parameters, such as, Bulimina marginata to temperature and Fursenkoina complanata and Uvigerina ex gr. U. semiomata to dissolved oxygen level. © GEOL. SOC. INDIA.
Benthic foraminiferal diversity response to the climate induced changes in the eastern Arabian Sea oxygen minimum zone during the last 30 ka BP
(Elsevier Ltd, 2015) A.D. Singh; A.K. Rai; K. Verma; S. Das; S.K. Bharti
A high resolution record of deep sea benthic foraminiferal diversity variations in the eastern Arabian Sea for the last 30kaBP 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's rarefaction number (S100), Shannon Wiener Index [H(S)], Equitability (E') and Alpha Index (α). 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 (18-22.5kaBP) 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). © 2014 Elsevier Ltd and INQUA.
Environmental significance of raised rann sediments along the margins of Khadir, Bhanjada and Kuar Bet islands in Great Rann of Kachchh, Western India
(2011) N. Khonde; D.M. Maurya; A.D. Singh; V. Chowksey; L.S. Chamyal
[No abstract available]
Episodic preservation of pteropods in the eastern Arabian Sea: Monsoonal change, oxygen minimum zone intensity and aragonite compensation depth
(2007) A.D. Singh
The preservational record of pteropod shells (aragonite) for the last 30 kyr has been studied in a core (SK17) recovered from the eastern Arabian Sea margin at off Goa coast (depth 840m, lat. 15°15'N, long. 72°44'E). The chronostratigraphy of the core established on the basis of high resolution stable isotope record of a planktic foraminifera (Globigerinoides ruber) and several AMS radiocarbon ages demonstrates millennial scale variation in Δ 18O defining Younger Dryas and Heinrich like Events. Records of absolute abundance of pteropods (1g/dry wt >125 μm), abundance ratio of pteropod and planktic foraminifera, aragonite (wt %), organic carbon (OC) (wt %) and CaCO3 (wt %) show major changes during these isotopic events. Aragonite maxima and higher number of well-preserved pteropod shells, are noticed during cold stadial periods. The study indicates a negative correlation between aragonite and OC % (productivity index). On the other hand, total CaCO3 content (calcite and aragonite) is positively correlated with the aragonite. The variation patterns of pteropod shells and aragonite content in the sediment core are suggested to be controlled by the preservational conditions associated with the fluctuation in Aragonite Compensation Depth (ACD) and Oxygen Minimum Zone (OMZ) intensity. It is suggested that the high biological productivity during intensified summer monsoons in late Holocene and inter-stadial periods might have resulted in severe oxygen depletion (strong OMZ) leading to shallowing of the ACD. A weak summer monsoon and low productivity condition prevailing during cold stadial periods would have resulted in a weak OMZ and deepening of the ACD.
Fluctuations of Mediterranean Outflow Water circulation in the Gulf of Cadiz during MIS 5 to 7: Evidence from benthic foraminiferal assemblage and stable isotope records
(Elsevier, 2015) A.D. Singh; A.K. Rai; M. Tiwari; P.D. Naidu; K. Verma; M. Chaturvedi; A. Niyogi; D. Pandey
We studied variations in benthic foraminiferal assemblages and δ13C for the last 225kyr at IODP site U1387 which is currently bathed by upper core of the Mediterranean Outflow Water (MOW). The MOW paleocirculation and sea-floor environment (oxygen level, trophic condition, bottom current strength) have been inferred from faunal composition; species diversity, abundances of selected index species/groups, microhabitat preferences combined with δ13C record of the epifaunal Cibicidoides pachyderma. The faunal and isotope records indicate relatively better ventilation at sea-floor and low trophic condition during MIS 1, 5 and 7 possibly due to increased influence of upper MOW in the Gulf of Cadiz. Our multi-proxy record reflects significant and rapid changes during cold (stadial) and warm (interstadial) phases within the interglacials MIS 5 and 7 and at Termination II. The faunal and isotope records reveal strong MOW flow and better ventilated, oligotrophic bottom-water conditions during stadials MIS 5b, 5d, 7b and 7d. The study further demonstrates weakened MOW intensity associated with poor ventilation and increased trophic level at sea-floor during interstadials MIS 5a, 5e, 7a and 7c. MOW flow was relatively sluggish at Termination II, followed by its strengthening at the end of MIS 5e. The chronology of these events suggests that periods of weakened MOW correlate with sapropel layers of the Mediterranean Sea, implying strong coupling between glacial-interglacial climate and MOW circulation in the Gulf of Cadiz. © 2015 Elsevier B.V.
Impact of climate induced hypoxia on calcifying biota in the Arabian Sea: An evaluation from the micropaleontological records of the Indian margin
(2011) A.D. Singh; S. Das; K. Verma
High biological productivity combined with the poor ventilation produces severe oxygen depletion (hypoxia) in upper intermediate waters of the Arabian Sea. The naturally developed Arabian Sea oxygen minimum zone (OMZ) is one of the most pronounced low oxygen ocean environments known today. The OMZ impinges the Indian margin where oxygen concentration reaches values less than 0.05 ml/l leading denitrification. In recent studies, it has been observed that the OMZ strength has varied considerably in the past, in tune with the global climate change. But the effect of changes in natural mid-water hypoxic environment on the marine biota particularly of the eastern Arabian Sea is unknown. Here, we analyzed 30,000 yr record of temporal changes in two major groups of marine calcifying microfauna pteropods secreting aragonitic shells and foraminifera secreting calcitic shells in terms of abundance and diversity variations. This study will provide an insight into our understanding of potential impact of rising atmospheric CO 2 on marine ecosystem.
Inconsistent change in surface hydrography of the eastern Arabian Sea during the last four glacial-interglacial intervals
(Cambridge University Press, 2020) Rajeev Saraswat; S.R. Kurtarkar; R. Yadav; A. Mackensen; D.P. Singh; S. Bhadra; A.D. Singh; M. Tiwari; S.P. Prabhukeluskar; S.R. Bandodkar; D.K. Pandey; P.D. Clift; D.K. Kulhanek; K. Bhishekar; S. Nair
The eastern Arabian Sea is influenced by both the advection of upwelled water from the western Arabian Sea and winter convective mixing. Therefore, sediments collected from the eastern Arabian Sea can help to understand the long-term seasonal hydrographic changes. We used the planktonic foraminifera census and stable isotopic ratio (δ18O) from sediments drilled during the International Ocean Discovery Program Expedition 355 to reconstruct surface hydrographic changes in the eastern Arabian Sea during the last 350 kyr. The increased abundance of Globigerina bulloides suggests enhanced advection of upwelled water during the latter half of MIS7 and the beginning of MIS6, as a result of a strengthened summer monsoon. A large drop in upwelling and/or advection of upwelled water from the western Arabian Sea is inferred during the subsequent interval of MIS6, based on the rare presence of G. bulloides. The comparable relative abundance of Neogloboquadrina dutertrei, G. bulloides and Globigerinoides ruber suggests that during the early part of MIS5, hydrographic conditions were similar to today. The upwelling decreased and winter convection increased with the progress of the glacial interval. A good coherence between planktonic foraminiferal assemblage-based monsoon stacks from both the eastern and western Arabian Sea suggests a coeval response of the entire northern Arabian Sea to the glacial-interglacial changes. The glacial-interglacial difference in δ18Osw-ivc was at a maximum with 4-5 psu change in salinity during Termination 2 and 3, and a minimum during Termination 4. The significantly reduced regional contribution to the glacial-interglacial change in δ18Osw-ivc during Termination 4 suggests a lesser change in the monsoon. © Cambridge University Press 2019.
IODP Expedition 339 in the Gulf of Cadiz and off West Iberia: Decoding the environmental significance of the Mediterranean outflow water and its global influence
(2013) F.J. Hernández-Molina; D. Stow; C. Alvarez-Zarikian; G. Acton; A. Bahr; B. Balestra; E. Ducassou; R. Flood; J.-A. Flores; S. Furota; P. Grunert; D. Hodell; F. Jimenez-Espejo; J.K. Kim; L. Krissek; J. Kuroda; B. Li; E. Llave; J. Lofi; L. Lourens; M. Miller; F. Nanayama; Naohisa Nishida; C. Richter; C. Roque; H. Pereira; M. Goñi Fernanda Sanchez; F.J. Sierro; A.D. Singh; C. Sloss; Y. Takashimizu; A. Tzanova; A. Voelker; T. Williams; C. Xuan
IODP Expedition 339 drilled five sites in the Gulf of Cadiz and two off the west Iberian margin (November 2011 to January 2012), and recovered 5.5 km of sediment cores with an average recovery of 86.4 %. The Gulf of Cadiz was targeted for drilling as a key location for the investigation of Mediterranean outflow water (MOW) through the Gibraltar Gateway and its influence on global circulation and climate. It is also a prime area for understanding the effects of tectonic activity on evolution of the Gibraltar Gateway and on margin sedimentation. We penetrated into the Miocene at two different sites and established a strong signal of MOW in the sedimentary record of the Gulf of Cadiz, following the opening of the Gibraltar Gateway. Preliminary results show the initiation of contourite deposition at 4.2-4.5 Ma, although subsequent research will establish whether this dates the onset of MOW. The Pliocene succession, penetrated at four sites, shows low bottom current activity linked with a weak MOW. Significant widespread unconformities, present in all sites but with hiatuses of variable duration, are interpreted as a signal of intensified MOW, coupled with flow confinement. The Quaternary succession shows a much more pronounced phase of contourite drift development, with two periods of MOW intensification separated by a widespread unconformity. Following this, the final phase of drift evolution established the contourite depositional system (CDS) architecture we see today. There is a significant climate control on this evolution of MOW and bottom-current activity. However, from the closure of the Atlantic-Mediterranean gateways in Spain and Morocco just over 6 Ma and the opening of the Gibraltar Gateway at 5.3 Ma, there has been an even stronger tectonic control on margin development, downslope sediment transport and contourite drift evolution. The Gulf of Cadiz is the world's premier contourite laboratory and thus presents an ideal testing ground for the contourite paradigm. Further study of these contourites will allow us to resolve outstanding issues related to depositional processes, drift budgets, and recognition of fossil contourites in the ancient record on shore. The expedition also verified an enormous quantity and extensive distribution of contourite sands that are clean and well sorted. These represent a relatively untapped and important exploration target for potential oil and gas reservoirs. © Author(s) 2013.
Late Glacial–Holocene record of benthic foraminiferal morphogroups from the eastern Arabian Sea OMZ: Paleoenvironmental implications
(Springer, 2018) K. Verma; S.K. Bharti; A.D. Singh
The Arabian Sea is characterized today by a well-developed and perennial oxygen minimum zone (OMZ) at mid-water depths. The Indian margin where the OMZ impinges provides sediment records ideal to study past changes in the OMZ intensity and its vertical extent in response to the changes of monsoon-driven primary productivity and intermediate water ventilation. Benthic foraminifera, depending upon their adaptation capabilities to variation in sea floor environment and microhabitat preferences, develop various functional morphologies that can be potentially used in paleoenvironmental reconstruction. In this study, we analysed benthic foraminiferal morphogroups in assemblage records of the last 30 ka in a sediment core collected from the lower OMZ of the Indian margin (off Goa). In total, nine morphogroups within two broadly classified epifaunal and infaunal microhabitat categories are identified. The abundance of morphogroups varies significantly during the late Glacial, Deglacial and Holocene. It appears that monsoon wind driven organic matter flux, and water column ventilation governing the OMZ intensity and sea-bottom oxygen condition, have profound influence on structuring the benthic foraminiferal morphogroups. We found a few morphogroups showing major changes in their abundances during the periods corresponding to the northern hemisphere climatic events. Benthic foraminifera with planoconvex tests are abundant during the cold Heinrich events, when the sea bottom was oxygenated due to a better ventilated, weak OMZ; whereas, those having tapered/cylindrical tests dominate during the last glacial maximum and the Holocene between 5 and 8 ka BP, when the OMZ was intensified and poorly ventilated, leading to oxygen-depleted benthic environment. Characteristically, increased abundance of taxa with milioline tests during the Heinrich 1 further suggests enhanced ventilation attributed probably to the influence of oxygen-rich Antarctic Intermediate Water (AAIW). © 2018, Indian Academy of Sciences.
Modern pollen assemblages from Hamtah and Chhatru glaciers, Lahaul-Spiti, India: Implications for pollen-vegetation relationship in an alpine arid region of western Himalaya
(Elsevier Ltd, 2015) Ratan Kar; Ruchika Bajpai; A.D. Singh
The present study deals with the pollen analysis of 25 surface samples, 15 from the outwash plain of the Hamtah Glacier and 10 from the Chhatru glacial valley, Lahaul-Spiti, Himachal Pradesh, India. The samples were collected in a linear transect from the respective sites at an altitudinal range of ~3200masl to ~4000masl. The area is a high altitude cold desert in the western Himalaya, characterised by scrub type of vegetation. The pollen assemblages reveal an overall dominance of extra local arboreals (conifers) over the local non-arboreals. Though it is a vegetation sparse area, the alpine meadow is well represented by the presence of non-arboreal elements, such as Rosaceae, Asteraceae, Artemisia, Lamiaceae, Poaceae, Apiaceae and Chenopodiaceae/Amaranthaceae. However, predominance of Pinus has been observed along with moderate values of Abies and Picea. The overwhelming values of Pinus pollen can be attributed to its profuse production and efficient dispersal through wind currents from the temperate forests and eventual deposition around the study sites. Temperate broad-leaved taxa including Alnus, Ulmus, Betula and Corylus are present in low abundance. The consistent low frequencies of fern spores, algal and fungal elements, and absence of aquatic and marshy taxa reflect the dry climatic conditions in the region. Impact of anthropogenic activities can be observed in the form of degraded patches of tree-line and evidence of extensive grazing. Palynological data from the surface samples is not coherent with the actual vegetation around the study area due to the prolific over-representation of extra local pollen over the local herbaceous taxa. Palynological studies have been initiated in the vicinity of Hamtah and Chhatru glaciers on sub-surface sediment profiles, and this data will be helpful in developing modern analogues for understanding the past vegetational changes with respect to palaeoclimatic conditions in and around the study area. © 2015 Elsevier Ltd and INQUA.
Monsoon-influenced variations in productivity and lithogenic flux along offshore Saurashtra, NE Arabian Sea during the Holocene and Younger Dryas: A multi-proxy approach
(Elsevier B.V., 2017) Syed Azharuddin; Pawan Govil; A.D. Singh; Ravi Mishra; Shailesh Agrawal; A.K. Tiwari; Kamlesh Kumar
A sediment core from NE Arabian Sea was studied to reveal the variations in shelf environment of western continental margin of India. A multi-proxy analysis based on planktonic foraminifera abundance (PFA), benthic foraminifera abundance (BFA), planktonic-benthic foraminiferal ratio (P/B), CaCO3 percentage (wt%), Oxygen (δ18OG. ruber) and Carbon (δ18OG. ruber) isotopes of foraminifera as well as sediment X-ray studies were carried out on 3.4 m long core SK-240/485 recovered from offshore Saurashtra, NE Arabian Sea. The study aims to understand the monsoon variability in the NE Arabian Sea offshore Saurashtra since the late Pleistocene-Holocene transition and how monsoon climate coupled with sea-level change influenced paleo-productivity pattern. Six AMS radiocarbon dates place the core in the time interval 12–1 Ka. Our study reveals that there have been three periods of major changes in the monsoonal climate, productivity and sea level; viz. 12–8 Ka (Younger Dryas and early Holocene); 8–4 Ka (middle Holocene) and 4–1 Ka (late Holocene). The studied time period was also marked by weakened south west monsoon (SWM) during the cold events such as Younger Dryas (YD), Abrupt Event (AE) centring around 9.8 Ka as well as 8.2 and 4 Ka cold events. The Younger Dryas and early Holocene shows the low sea level stand and less Indus river discharge. Thereafter during middle and late Holocene the monsoon strengthened which resulted in raised sea level and increased Indus river discharge. The proxy records broadly suggest relatively low foraminiferal productivity at the core location during the Younger Dryas and early Holocene. We record a moderate foraminiferal productivity during the middle Holocene between 8 and 4 Ka, which suggest the mesotrophic condition. The foraminiferal productivity however was relatively high since 4 Ka suggesting the eutrophic conditions at the core site. © 2016 Elsevier B.V.
Monsoonal dynamics and evolution of the primary productivity in the eastern Arabian Sea over the past 30 ka
(Elsevier, 2014) E. Cabarcos; J.A. Flores; A.D. Singh; F.J. Sierro
This study presents a high-resolution primary productivity record for the last 30. ka in the eastern Arabian Sea (EAS) based on the coccolithophore assemblage at SK 17 core in the continental slope off Goa (India). Coccolithophores were proposed as indicators of primary productivity and nutricline position, and the results were used to reconstruct the monsoonal dynamics, which is a main factor controlling productivity changes in the EAS. Both seasons in the Indian monsoon (winter and summer monsoon) exert a strong control over the primary productivity in the Arabian Sea. An increased productivity was recorded during the late glacial period in the EAS, contrasting with records obtained in the western and northern Arabian Sea. This enhanced productivity was related to strengthened winter monsoon winds. Planktonic foraminifera data suggested a weakened winter mixing during the last deglaciation, which would reduce productivity in the area. However, our coccolithophore data reveal a high primary productivity during the deglaciation, provably maintained by the nutrient supply from the continental runoff due to the presence of enhanced summer monsoon. Regarding the Holocene, surface waters were highly stratified since 10.5. ka. Our data were compared with ice cores isotopic records from high latitudes of both hemispheres, allowing us to observe a good correlation between the stratification of the EAS and the climate variability over high latitude regions in the northern hemisphere. However, there has been some inconsistency between our coccolithophore data and planktonic foraminiferal records particularly during the short time intervals corresponding to the Heinrich cold events described in the north Atlanctic region. © 2014 Elsevier B.V.
Palaeoredox reconstruction in the eastern Arabian Sea since the late Miocene: Insights from trace elements and stable isotopes of molybdenum (δ98/95Mo) and tungsten (δ186/184W) at IODP Site U1457 of Laxmi Basin
(Elsevier B.V., 2022) M. Alam; M. Tripti; G.P. Gurumurthy; Y. Sohrin; M. Tsujisaka; A.D. Singh; S. Takano; K. Verma
The present study investigates the oxygenation history of the northeastern Arabian Sea since the late Miocene using redox sensitive elemental and metal stable isotopic signatures in the deep-sea sediments. To achieve this, the sediment core samples collected at Site U1457 (67°55.80′E, 17°9.95′N, water depth 3534 m) of Laxmi Basin in the northeastern Arabian Sea during the International Ocean Discovery Program (IODP) Expedition 355 were analysed for a suite of elemental (Mo, W, U, V, Ba, Cd and P) and stable molybdenum (Mo) isotope (δ98/95Mo relative to NIST SRM 3134 lot No. 130418) as well as stable tungsten (W) isotope (δ186/184W relative to NIST SRM 3163 lot No. 080331) composition. Sedimentary δ98/95Mo values (−0.70‰ to +1.18‰) at IODP Site U1457 in the northeastern Arabian Sea indicated partial authigenic Mo component. In contrast, the sedimentary δ186/184W values (−0.02‰ to +0.21‰) were in the range similar to that of lithogenous material suggesting dominance of detrital composition. The study reveals that the water column in the eastern Arabian Sea was oxic during the late Miocene and Pliocene while oxic to suboxic condition prevailed during the Pleistocene. The study also explores that under oxic to suboxic condition with limited particle shuttling, the W isotopes do not undergo significant fractionation, and their isotope ratios reflect the detrital source signature. This study reports the first results on isotopic compositions of Mo and W in sediments of the northeastern Arabian Sea since the late Miocene to investigate the palaeoredox conditions on a million-year time scale. © 2021 Elsevier B.V.
Planktic foraminiferal responses to orbital scale oceanographic changes off the western Iberian margin over the last 900kyr: Results from IODP site U1391
(Elsevier B.V., 2015) A.D. Singh; K. Verma; S. Jaiswal; M. Alonso-Garcia; B. Li; F. Abrantes
This paper presents planktic foraminiferal assemblage records of the last 900 kyr from the SW Iberian margin (IODP Site U1391). The faunal records show the history of surface oceanographic changes on glacial/interglacial scales before and after the Mid-Brunhes Event (MBE), a period when a major shift in the climate pattern was recorded in other regions. Temporal variations in relative abundances of characteristic species/groups are used to infer changes in the latitudinal position of the polar/Arctic water (% Neogloboquadrina pachyderma sinistral and Turborotalita quinqueloba), influence of the transitional subpolar water mass (% N. pachyderma dextral), and subtropical water (% tropical/subtropical species/group). Past changes in the upwelling intensity and productivity pattern associated with seasonal trade wind strength are inferred from the abundance variations of Globigerina bulloides and G. bulloides+. Globigerinita glutinata, respectively. Faunal data reveal the influence of cold water masses (polar/subpolar) at the examined site was more pronounced during glacial stages except for marine isotope stage (MIS) 14 and 16. The magnitude of the polar/subpolar water mass invading the study area was at maximum before the MBE during MIS 18, 20 and 22, resulting in a situation like the present day Arctic Front. Interglacial periods prior to the MBE were also relatively colder than those of the post-MBE. Our faunal based inferences are in agreement with the ice-rafted debris (IRD) concentration and N. pachyderma sinistral records of the subpolar North Atlantic sites. Based on faunal proxies, we recorded major and rapid changes in upwelling intensity and related productivity during glacial Terminations. Both the upwelling intensity and productivity significantly increased after the MBE, particularly during the interglacials MIS 7, 9 and 11. Our productivity record parallels the EPICA CH4 record suggesting teleconnections between trade winds induced productivity and the tropical climate through oceanic-atmospheric processes. © 2015 Elsevier B.V.
Quaternary climatic changes indicated by planktonic foraminifera of Northern Indian Ocean
(1993) A.D. Singh; M.S. Srinivasan
The relative abundances of planktonic foraminifera, the changes in their coiling and the isotopic record from deep sea cores of some crucial sites in Northern Indian Ocean indicate climatic fluctuations for the past 3.0 million years (Late Pliocene to Pleistocene). The abundance of temperate forms like Globigerina spp. together with sporadic occurrence of a polar species Neogloboquadrina pachyderma during specific time intervals suggest cooling episodes. Warmer intervals are marked by the dominance of tropical assemblages including Globigerinoides spp., Globorotalia s.l., Pulleniatina and Globoquadrina. On the basis of the planktonic foraminiferal frequency changes, four intervals of major climatic cooling have been recognized-one in the Late Pliocene (2-3 m.y.), three in the Pleistocene immediately above the Pliocene/Pleistocene boundary (1.6 m.y.), at 0.6 m.y., and during the late Pleistocene. Marked shift in coiling directions within Pulleniatina populations and Globorotalia s.l. is observed to coincide with the frequency changes. There was marked increase of δ l8O values in oxygen isotopic record of foraminiferal tests around 2-3 m.y. (Late Pliocene) and within the Pleistocene. Major faunal turnover accompanied by changes in coiling synchronous with the isotopic shifts, suggest these faunal changes to be climatically induced.
Recent advances in understanding neogene climatic evolution: Indian perspective
(Indian National Science Academy, 2020) A.D. Singh; A.K. Ghosh; R.C. Mehrotra; R. Patnaik; M. Tiwari
This article presents an overview of the research work done during 2015-2019 on the South Asian/Indian monsoon climate variability, based on the Neogene terrestrial and marine proxy records. The paleoclimatic and paleoceanographic records provide better insights into our current understanding of timing of initiation/intensification of the Indian monsoon system; its evolution and seasonal variability pattern through time and underlying mechanisms; and impacts on paleobiogeography of terrestrial fauna and flora and physico-chemical and biological processes in the northern Indian Ocean. The Neogene upliftment history of the Himalaya was reconstructed based on detailed analysis of fossil flora from Himalayan sequences and a link between step wise uplift of Himalaya-Tibetan plateau and Indian summer monsoon intensification was established. The rainfall pattern in the northeast India during the Neogene period was quantified; and the new data suggest the presence of rainfall in the region since 11.6 Ma. δ13C record of pedogenic nodules in Siwalik succession provides evidence of major shift in vegetation between ~2.8 Ma and 1.1 Ma due to large variation in temperature and precipitation. New results of the multiproxy based paleoceanographic studies carried out on sediment cores collected during recent IODP expeditions in the Arabian Sea and Bay of Bengal, have major implications on the evolution of the Indian monsoon and dynamics of its seasonal variations in the Neogene. The sedimentological and geochemical records from carbonate platform of the Maldives suggest an abrupt onset of modern monsoon circulation at ~ 12.9 Ma, which cannot be entirely explained by the Himalayan tectonics as many believed earlier. It appears that the global climate also controls Indian monsoon circulation, in addition to the tectonics. Record of past changes in the Arabian Sea denitrification driven mainly by monsoon-wind induced productivity and water column ventilation condition, reveals that the Indian monsoon intensified during ~2.8-3.2 Ma. Recently, based on the integrated multiple proxy records it was inferred that seasonal monsoon circulation was weak during 1.85-2.7 Ma, subsequently followed by the intensification of winter monsoon between ~1.65 and 1.85 Ma, attributed to the development of strong zonal and meridional circulations due to enhanced E-W Pacific temperature gradients. © 2020 Indian National Science Academy. All rights reserved.
The osteoinductive property of decalcified bone matrix. An experimental study
(1978) S.M. Tull; A.D. Singh
Demineralized homologous bone-matrix implant was used to bridge a large circumferential osteoperiosteal gap in the diaphysis of the ulna of rabbits. Periodic observations of the graft were made clinically, radiologically, histologically and by tetracycline fluorescene up to 42 wk. By the 12th wk after operation 81% of the animals revealed bone formation in the implant and complete bridging of the gap. The new bone was laid on the surface and in the substance of the matrix, suggesting that the inductive principle was acting locally. The bone, once formed, remodelled to the texture of a mature tubular bone and did not undergo absorption during a long follow-up period. Demineralized bone-matrix proved to be a highly osteoinductive and readily osteoconductive material. The graft did not evoke any appreciable local foreign-body or immunogenic reaction. The high degree of success in bridging massive bone defects justifies further serious studies and hopes for a useful substitute for massive autologous bone grafts.