Browsing by Author "K. Verma"

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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.
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.
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.
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.
Seasonal variation of pteropods from the Western Arabian Sea sediment trap
(2006) Rahul Mohan; K. Verma; L.P. Mergulhao; D.K. Sinha; S. Shanvas; M.V.S. Guptha
Sediment trap samples collected from the Western Arabian Sea yielded a rich assemblage of intact and non-living (opaque white) pteropod tests from a water depth of 919 m during January to September 1993. Nine species of pteropods were recorded, all (except one) displaying distinct seasonality in abundance, suggesting their response to changing hydrographical conditions influenced by the summer/winter monsoon cycle. Pteropod fluxes increased during the April-May peak of the intermonsoon, and reached maximum levels in the late phase of the southwest summer monsoon, probably due to the shallowing of the mixed layer depth. This shallowing, coupled with enhanced nutrient availability, provides ideal conditions for pteropod growth, also reflected in corresponding fluctuations in the flux of the foraminifer Globigerina bulloides. Pteropod/planktic foraminifer ratios displayed marked seasonal variations, the values increasing during the warmer months of April and May when planktic foraminiferal fluxes declined. The variation in fluxes of calcium carbonate, organic carbon and biogenic opal show positive correlations with fluxes of pteropods and planktic foraminifers. Calcium carbonate was the main contributor to the total particulate flux, especially during the SW monsoon. In the study area, pteropod flux variations are similar to the other flux patterns, indicating that they, too could be used as a potential tool for palaeoclimatic reconstruction of the recent past. © Springer-Verlag 2006.