Browsing by Author "Rajeev Saraswat"

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A revised chronostratigraphic framework for International Ocean Discovery Program Expedition 355 sites in Laxmi Basin, eastern Arabian Sea
(Cambridge University Press, 2020) Claire M. Routledge; Denise K. Kulhanek; Lisa Tauxe; Giancarlo Scardia; Arun D. Singh; Stephan Steinke; Elizabeth M. Griffith; Rajeev Saraswat
International Ocean Discovery Program Expedition 355 drilled Sites U1456 and U1457 in Laxmi Basin (eastern Arabian Sea) to document the impact of the South Asian monsoon on weathering and erosion of the Himalaya. We revised the chronostratigraphic framework for these sites using a combination of biostratigraphy, magnetostratigraphy and strontium isotope stratigraphy. The sedimentary section at the two sites is similar and we divided it into six units bounded by unconformities or emplaced as a mass-transport deposit (MTD). Unit 1 underlies the MTD, and is of early–middle Miocene age at Site U1456 and early Paleocene age at Site U1457. An unconformity (U1) created by emplacement of the MTD (unit 2) during the late Miocene Epoch (at c. 9.83–9.69 Ma) separates units 1 and 2 and is identified by a marked change in lithology. Unit 3 consists of hemipelagic sediment with thin interbeds of graded sandstone of late Miocene age, separated from unit 4 by a second unconformity (U2) of 0.5–0.9 Myr duration. Unit 4 consists of upper Miocene interbedded mudstone and sandstone and hemipelagic chalk deposited between c. 8 and 6 Ma. A c. 1.4–1.6 Myr hiatus (U3) encompasses the Miocene–Pliocene boundary and separates unit 4 from unit 5. Unit 5 includes upper Pliocene – lower Pleistocene siliciclastic sediment that is separated from unit 6 by a c. 0.45 Myr hiatus (U4) in the lower Pleistocene sediments. Unit 6 includes a thick package of rapidly deposited Pleistocene sand and mud overlain by predominantly hemipelagic sediment deposited since c. 1.2 Ma. © Cambridge University Press 2019.
Arabian Sea Monsoon: Deep sea drilling in the Arabian Sea: Constraining tectonic-monsoon interactions in South Asia
(2015) Dhananjai K. Pandey; Peter D. Clift; Denise K. Kulhanek; Sergio Andò; James A.P. Bendle; Sophia Bratenkov; Elizabeth M. Griffith; Gundiga P. Gurumurthy; Annette Hahn; Masao Iwai; Boo-Keun Khim; Anil Kumar; A. Ganesh Kumar; Hannah M. Liddy; Huayu Lu; Mitchell W. Lyle; Ravi Mishra; Tallavajhala Radhakrishna; Claire M. Routledge; Rajeev Saraswat; Rakesh Saxena; Giancarlo Scardia; Girish K. Sharma; Arun D. Singh; Stephan Steinke; Kenta Suzuki; Lisa Tauxe; Manish Tiwari; Zhaokai Xu; Zhaojie Yu
The Arabian Sea in the northern Indian Ocean pre-serves regional sedimentary records of rifting, tectonic subsidence, and paleoceanographic history, and also provides archives of long-term erosion of the Himalaya since the start of collision between In?dia and Eurasia. Investigations reveal that drilling in this region can provide erosion records through analyses of the sediment cores, along with providing age control for the regional seismic stratigraphy. It is only by quantifying the volume of sedi?ment deposited in the fan that researchers can mass balance the volume of bedrock eroded from the mountains, constrained by thermochronology, with the volume of eroded rock deposited in the offshore and in the foreland basin.
First evidence of denitrification vis-à-vis monsoon in the Arabian Sea since Late Miocene
(Nature Publishing Group, 2017) Shubham Tripathi; Manish Tiwari; Jongmin Lee; Boo-Keun Khim; Dhananjai K. Pandey; Peter D. Clift; Denise K. Kulhanek; Sergio Andò; James A.P. Bendle; Sophia Aharonovich; Elizabeth M. Griffith; Gundiga P. Gurumurthy; Annette Hahn; Masao Iwai; Anil Kumar; A. Ganesh Kumar; Hannah M. Liddy; Huayu Lu; Mitchell W. Lyle; Ravi Mishra; Tallavajhala Radhakrishna; Claire M. Routledge; Rajeev Saraswat; Rakesh Saxena; Giancarlo Scardia; Girish K. Sharma; Arun D. Singh; Stephan Steinke; Kenta Suzuki; Lisa Tauxe; Zhaokai Xu; Zhaojie Yu
In the Arabian Sea, South Asian monsoon (SAM)-induced high surface water productivity coupled with poor ventilation of intermediate water results in strong denitrification within the oxygen minimum zone (OMZ). Despite the significance of denitrification in the Arabian Sea, we have no long-term record of its evolution spanning the past several million years. Here, we present the first record of denitrification evolution since Late Miocene (∼10.2 Ma) in the Eastern Arabian Sea, where the SAM generates moderate surface water productivity, based on the samples retrieved during the International Ocean Discovery Program (IODP) Expedition 355. We find that (i) the SAM was persistently weaker from ∼10.2 to 3.1 Ma; it did not intensify at ∼8 Ma in contrast to a few previous studies, (ii) on tectonic timescale, both the SAM and the East Asian Monsoon (EAM) varied synchronously, (iii) the first evidence of denitrification and productivity/SAM intensification was at ∼3.2-2.8 Ma that coincided with Mid-Pliocene Warm Period (MPWP), and (iv) the modern strength of the OMZ where denitrification is a permanent feature was attained at ∼1.0 Ma. © The Author(s) 2017.
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.
Indian contributions to marine micropaleontology (2010-2015)
(Indian National Science Academy, 2016) Arun Deo Singh; P. Divakar Naidu; Rajeev Saraswat
An overview of the research contributions made during the last five years by the Indian scientists in the field of marine micropaleontology is presented. Large scale micropaleontological studies employing assemblages of various microfossil groups (mainly foraminifera, pteropods, ostracodes and nannoplanktons) and their geochemical proxies have been carried out on deep sea sediment cores from the Indian Ocean. These integrated studies provided better insights to our understanding of past changes in surface and deep ocean circulations on different time scales and their relationships to seasonal variations of monsoon climate. It was observed that the foraminiferal abundances and diversity in the Arabian Sea are mainly driven by the variation in monsoonal intensity. Within the present day oxygen minimum zone (OMZ), benthic diversity increased during the cold stadials when the monsoon circulation was weak, the productivity was low resulting to a weak OMZ. The enhanced inflow of Antarctic Intermediate Water (AAIW) ventilated the deep water column during this period. In the southern Bay of Bengal, the contribution of southern ocean deep water increased during the last glacial maximum, as compared to the Holocene with greater influence of North Atlantic Deep Water (NADW). For the first time boron isotopebased pCO2 of seawater was reconstructed from the eastern Arabian Sea. The results obtained have major implications in understanding the effect of monsoon on the efficiency of carbon pump in geological records. Numerous studies have reconstructed the variability of the Indian monsoon by using several parameters suggest a significant spatial variability of the Indian monsoon rainfall triggered by natural and anthropogenic perturbations over different time scales. © 2016 Printed in India.
Indonesian throughflow controlled the westward extent of the Indo-Pacific Warm Pool during glacial-interglacial intervals
(Elsevier B.V., 2019) Rajeev Saraswat; D.P. Singh; David W. Lea; A. Mackensen; D.K. Naik
The tropical atmospheric circulation is strongly influenced by the sea surface temperature (SST) of the warmest region of the world ocean, the Indo-Pacific Warm Pool (IPWP). The effect of recent anomalous Indian Ocean warming on the extent of the IPWP remains uncertain. Past SST records from the tropical Indian Ocean, associated with different temporal boundary conditions, can help shed light on how climate change impacts the IPWP and associated monsoonal processes. Here, we reconstruct the seawater temperature and precipitation changes (from Mg/Ca and δ18O of surface-dwelling planktic foraminifera Globigerinoides ruber) of the last ~184 kyr BP from the westernmost margin of the IPWP. The data is then used to reconstruct the spatial extent and intensity of the IPWP structure as well as its relationship with other regional phenomena. A consistently warmer (>28 °C) central equatorial Pacific Ocean and significantly cooler equatorial western Pacific and the Indian Ocean suggests that the warm pool extent reduced considerably during MIS6 as well as the last glacial interval, leading to a strong La-Nina like condition. The most enriched seawater δ18O (δ18Osw) during MIS6, was in the westernmost IPWP, suggesting weakened precipitation, despite La-Lina like conditions. The westernmost part of the IPWP became warmer than the threshold 28 °C during MIS5e and the early Holocene, suggesting the westward extension of the IPWP. The eastern equatorial Pacific Ocean also warmed considerably during MIS5e, suggesting a weaker Walker Circulation and a stronger El-Nino. The difference in δ18Osw decreased considerably during MIS5e and is attributed to increased precipitation/freshwater runoff in the central equatorial Indian Ocean. The increase in δ18Osw (~1.5‰) between ~55 kyr until the last glacial maximum, in a majority of the Indian Ocean records, suggests progressively weakened precipitation. A majority of the records have a uniform SST anomaly throughout the Holocene. A warmer equatorial Indian Ocean during interglacials suggests a strong modulation of the IPWP extent by the transport of warm water through the Indonesian throughflow region. The large drop in δ18Osw despite the comparable change in SST during the last glacial interval suggests that monsoon forcing has been much stronger in altering the surface hydrography in the Indian Ocean than the central Pacific Ocean. © 2019 Elsevier B.V.
Large freshwater-influx-induced salinity gradient and diagenetic changes in the northern Indian Ocean dominate the stable oxygen isotopic variation in Globigerinoides ruber
(Copernicus Publications, 2023) Rajeev Saraswat; Thejasino Suokhrie; Dinesh K. Naik; Dharmendra P. Singh; Syed M. Saalim; Mohd Salman; Gavendra Kumar; Sudhira R. Bhadra; Mahyar Mohtadi; Sujata R. Kurtarkar; Abhayanand S. Maurya
The application of stable oxygen isotopic ratio of surface-dwelling planktic foraminifera Globigerinoides ruber (white variety; δ18Oruber) to reconstruct past hydrological changes requires a precise understanding of the effect of ambient parameters on δ18Oruber. The northern Indian Ocean, with its huge freshwater influx and being a part of the Indo-Pacific Warm Pool, provides a unique setting to understand the effect of both the freshwater-influx-induced salinity and temperature on δ18Oruber. Here, we use a total of 400 surface samples (252 from this work and 148 from previous studies), covering the entire salinity end-member region, to assess the effect of freshwater-influx-induced seawater salinity and temperature on δ18Oruber in the northern Indian Ocean. The analysed surface δ18Oruber mimics the expected δ18O calcite estimated from the modern seawater parameters (temperature, salinity, and seawater δ18O) very well. We report a large diagenetic overprinting of δ18Oruber in the surface sediments, with an increase of 0.18‰ per kilometre increase in water depth. The freshwater-influx-induced salinity exerts the major control on δ18Oruber (R2Combining double low line0.63) in the northern Indian Ocean, with an increase of 0.29‰ per unit increase in salinity. The relationship between temperature- and salinity-corrected δ18Oruber (δ18Oruber-δ18Osw) in the northern Indian Ocean [TCombining double low line-0.59 (δ18Oruber-δ18Osw)+26.40] is different than reported previously, based on the global compilation of plankton tow δ18Oruber data. The revised equations will help create a better palaeoclimatic reconstruction from the northern Indian Ocean by using the stable oxygen isotopic ratio. The entire data set (newly generated and previously published) used in this work is available both as a Supplement to this article and at PANGAEA (10.1594/PANGAEA.945401; Saraswat et al., 2022). © 2023 Rajeev Saraswat et al.
Large-scale mass wasting on the miocene continental margin of Western India
(Geological Society of America, 2020) Sarah K. Dailey; Peter D. Clift; Denise K. Kulhanek; Jerzy Blusztajn; Claire M. Routledge; GérÔme Calvès; Paul O'Sullivan; Tara N. Jonell; Dhananjai K. Pandey; Sergio Andò; Giovanni Coletti; Peng Zhou; Yuting Li; Nikki E. Neubeck; James A.P. Bendle; Sophia Aharonovich; Elizabeth M. Griffith; Gundiga P. Gurumurthy; Annette Hahn; Masao Iwai; Boo-Keun Khim; Anil Kumar; A. Ganesh Kumar; Hannah M. Liddy; Huayu Lu; Mitchell W. Lyle; Ravi Mishra; Tallavajhala Radhakrishna; Rajeev Saraswat; Rakesh Saxena; Giancarlo Scardia; Girish K. Sharma; Arun D. Singh; Stephan Steinke; Kenta Suzuki; Lisa Tauxe; Manish Tiwari; Zhaokai Xu; Zhaojie Yu
A giant mass-transport complex was recently discovered in the eastern Arabian Sea, exceeding in volume all but one other known complex on passive margins worldwide. The complex, named the Nataraja Slide, was drilled by International Ocean Discovery Program (IODP) Expedition 355 in two locations where it is ~300 m (Site U1456) and ~200 m thick (Site U1457). The top of this mass-transport complex is defined by the presence of both reworked microfossil assemblages and deformation structures, such as folding and faulting. The deposit consists of two main phases of mass wasting, each consisting of smaller pulses, with generally fining-upward cycles, all emplaced just prior to 10.8 Ma based on biostratigraphy. The base of the deposit at each site is composed largely of matrix-supported carbonate breccia that is interpreted as the product of debris-flows. In the first phase, these breccias alternate with wellsorted calcarenites deposited from a high-energy current, coherent limestone blocks that are derived directly from the Indian continental margin, and a few clastic mudstone beds. In the second phase, at the top of the deposit, muddy turbidites dominate and become increasingly more siliciclastic. At Site U1456, where both phases are seen, a 20-m section of hemipelagic mudstone is present, overlain by a ~40-m-thick section of calcarenite and slumped interbedded mud and siltstone. Bulk sediment geochemistry, heavy-mineral analysis, clay mineralogy, isotope geochemistry, and detrital zircon U-Pb ages constrain the provenance of the clastic, muddy material to being reworked, Indus-derived sediment, with input from western Indian rivers (e.g., Narmada and Tapti rivers), and some material from the Deccan Traps. The carbonate blocks found within the breccias are shallow-water limestones from the outer western Indian continental shelf, which was oversteepened from enhanced clastic sediment delivery during the mid-Miocene. The final emplacement of the material was likely related to seismicity as there are modern intraplate earthquakes close to the source of the slide. Although we hypothesize that this area is at low risk for future mass wasting events, it should be noted that other oversteepened continental margins around the world could be at risk for mass failure as large as the Nataraja Slide. © 2019 Geological Society of America.
Mid-Pleistocene Transition altered upper water column structure in the Bay of Bengal
(Elsevier B.V., 2023) Sudhira R. Bhadra; Rajeev Saraswat; Sanjeev Kumar; Sangeeta Verma; Dinesh Kumar Naik
The unique water column structure and seasonal change in the hydrodynamics make the Bay of Bengal an interesting area to understand the effect of monsoon on the water column stratification. We have reconstructed a 1.45 Myr record of planktic foraminifera assemblages and a 1.16 Myr (MIS 47- MIS 8) record of the difference in the stable oxygen isotopic ratio (Δδ18O) between the mixed layer dwelling Globigerinoides ruber and thermocline dweller Neogloboquadrina dutertrei from the IODP Site U1446 to understand the change in water column stratification and its relationship with the monsoon. From the glacial-interglacial shift in Δδ18O and mixed layer and thermocline assemblages, we infer a stronger stratification during the warm interstadials and comparatively weaker stratification during the cold stadials. The strengthened summer monsoon induced high direct rainfall and riverine influx inhibited mixing in the upper layer and created a shallow thermocline during the interstadials. We report a large variability in Δδ18O prior to the mid-Pleistocene transition (MPT). Interestingly, the glacial-interglacial shift in Δδ18O significantly decreased post MPT. A stronger upwelling and deeper mixed layer during the stadial periods reduced the glacial-interglacial shift in Δδ18O post mid-Pleistocene transition. The strengthened summer monsoon during the post MPT period increased the stratification in the Bay of Bengal. © 2023 Elsevier B.V.