Virupaxa Banakar - Academia.edu (original) (raw)

Papers by Virupaxa Banakar

The deglacial transition from the last glacial maximum at ∼20 kiloyears before present (ka) to th... more The deglacial transition from the last glacial maximum at ∼20 kiloyears before present (ka) to the Holocene (11.7 ka to Present) was interrupted by millennial-scale cold reversals, viz., Antarctic Cold Reversal (∼14.5–12.8 ka) and Greenland Younger Dryas (∼12.8–11.8 ka) which had different timings and extent of cooling in each hemisphere. The cause of this synchronously initiated, but different hemispheric cooling during these cold reversals (Antarctic Cold Reversal ∼3∘C and Younger Dryas ∼10∘C) is elusive because CO2, the fundamental forcing for deglaciation, and Atlantic meridional overturning circulation, the driver of antiphased bipolar climate response, both fail to explain this asymmetry. We use centennial-resolution records of the local surface water 18O of the Eastern Arabian Sea, which constitutes a proxy for the precipitation associated with the Indian Summer Monsoon, and other tropical precipitation records to deduce the role of tropical forcing in the polar cold reversals. We hypothesize a mechanism for tropical forcing, via the Indian Summer Monsoons, of the polar cold reversals by migration of the Inter-Tropical Convergence Zone and the associated cross-equatorial heat transport

Earth and Planetary Science Letters, 2009

The Indian Ocean is an important part of the global thermohaline circulation system, receiving de... more The Indian Ocean is an important part of the global thermohaline circulation system, receiving deep waters sourced from the Southern Ocean and being the location of upwelling and surface-ocean current flow, which returns warm and salty waters to the Atlantic. It is also an ideal location to reconstruct the link between thermohaline circulation and deep-water nutrient contents. No mixing occurs between major deep-water masses along flow paths within the Indian Ocean, so changes in water-mass provenance reflect changes in deep-ocean circulation while nutrient contents reflect addition and dissolution of organic matter. We present neodymium (Nd) and carbon (C) isotope records, proxies of water-mass provenance and nutrient contents, respectively, from an equatorial Indian Ocean core (SK129−CR2) spanning the last 150 kyr. The Nd isotope record shows that an increased proportion of North Atlantic Deep Water (NADW) reached the Indian Ocean during interglacials (marine isotope stages, MIS 1 and 5), and a reduced proportion during glacials (MIS 2, 4, and 6), and also that changes occurred during MIS 3. The magnitude and timing of deglacial and some MIS 3 variability is very similar to those in the RC11−83/TNO57−21 South Atlantic deep Cape Basin Nd isotope record, suggesting that Atlantic meridional overturning circulation changes were effectively propagated from the southeastern Atlantic into the central Indian Ocean via the Southern Ocean. Comparison of the Nd and C isotope records shows that deep-ocean circulation was decoupled from nutrient-content changes on glacial−interglacial timescales, in particular suggesting higher productivity during MIS 5. Increased equatorial productivity during MIS 5 is supported by benthic infaunal−epifaunal foraminiferal δ 13 C gradients, as well as benthic foraminiferal δ 13 C gradients along deep-water flow. Concurrent warming, indicated by planktonic foraminiferal Mg/Ca during MIS 5 at the site is consistent with changing thermocline temperature and may indicate a link to surface-ocean hydrographic changes.

A detailed study of a nodule from the Somali Basin dated by 230Thexcess was correlated with the p... more A detailed study of a nodule from the Somali Basin dated by 230Thexcess was correlated with the paleoceanographic events recorded in Site 236 (Leg 24) Deep Sea Drilling Project (DSDP) cores. Tentative indications are that the phase of nodule accretion starting with the development of pillar structure at a depth of 20 mm in the nodule around 13 Ma coincides with increased Antarctic Bottom Water (AABW) flow and an elevated calciumcarbonate compensation depth (CCD).The Late Miocene lowering of the CCD is represented by the mottled zones between 8 and 18 mm in the nodule is characterised by an abundant silicate component (>20%) of aeolian origin. The Miocene/Pliocene boundary (5 Ma) occurs at a depth of about 8 mm and is represented by the development of pillar structure and a minimum of aeolian dust (10.3%).The increased biological productivity of the Somali surface water since the Middle Miocene is demonstrated by the increasing Corg content of the nodule (from 0.11 to 0.19%) towar...

Due to rapidly depleting land-based mineral resources, oceanic mineral deposits gain greater sign... more Due to rapidly depleting land-based mineral resources, oceanic mineral deposits gain greater significance. Ferromanganese deposits on the seabed (nodules) and seamounts (crusts) known for the enrichment of several transition metals were discovered during pioneering expeditions of H. M. S. Challenger during 1872-76. The metal contents in these deposits show large variations from basin to basin. For India, the Cu and Ni (∼1% each) in nodules and Co (∼0.7%) and Pt (∼0.5 ppm) in seamount crusts recovered from the Indian Ocean are important. The hydrogenous crusts are not only important as economically valuable deposits of Co and Pt, but also are potential paleoceanographic repositories. Ferromanganese nodule exploration by India began in 1981 and concluded with recognition by the International Seabed Authority as a Pioneer Investor in 1987. This exploration license provides India with exclusive exploration rights over an area of 150,000 km 2 (Pioneer Area) in the Central Indian Ocean. Nearly 700 million tonnes of nodule resources are estimated in this mine site, which are expected to contain around 14 million tonnes of combined Cu and Ni metals valued approximately over Rs. 1000 billion at current average market rate. Quantitative resource evaluation for seamount ferromanganese crusts is not yet available due to limited exploratory work. However, a promising area of Co-Pt enriched ferromanganese crust occurrence has been discovered on the Afanasiy-Nikitin Seamounts in the Eastern Equatorial Indian Ocean by NIO scientists, which contains Co upto 0.9 % and Pt upto 1 ppm.

... Shankar, D., Vinayachandran, PN, and Unnikrishnan, AS, 2002. ... Mahesh Badanal Male: born in... more ... Shankar, D., Vinayachandran, PN, and Unnikrishnan, AS, 2002. ... Mahesh Badanal Male: born in 1980: Doctoral Student at National Institute of Oceanography (HQ, Goa), a constituent laboratory of Council of Scientific and Industrial Research (New Delhi, India). ...

Marine Geology

ABSTRACT Transition metal fluxes to the top and bottom of two oriented manganese nodules (SS-657 ... more ABSTRACT Transition metal fluxes to the top and bottom of two oriented manganese nodules (SS-657 and SK-176) were determined by combining radiochemical and geochemical analyses. Distinct differences in transition metal fluxes, 230Th contents, and ratios and silicate mineralogy in the top and bottom of nodule SS-657 suggest that since the time of inception of oxyhydroxide accretion (< 1 m.y. B.P.) it has been growing with a fixed orientation on the seafloor. In nodule SK-176, in contrast, nearly uniform accumulation rates, 230Thexc and extrapolated to surface activities, transition metal fluxes and increasing diagenetic influence outwards in the top and bottom are thought to be suggestive of rotation of the nodule several times over time intervals which are smaller than the time resolution involved in U-Th isotope dating techniques.

Deep Sea Research Part I: Oceanographic Research Papers

Journal of Earth System Science

The deglacial transition from the last glacial maximum at ∼20 kiloyears before present (ka) to th... more The deglacial transition from the last glacial maximum at ∼20 kiloyears before present (ka) to the Holocene (11.7 ka to Present) was interrupted by millennial-scale cold reversals, viz., Antarctic Cold Reversal (∼14.5-12.8 ka) and Greenland Younger Dryas (∼12.8-11.8 ka) which had different timings and extent of cooling in each hemisphere. The cause of this synchronously initiated, but different hemispheric cooling during these cold reversals (Antarctic Cold Reversal ∼3 • C and Younger Dryas ∼10 • C) is elusive because CO 2 , the fundamental forcing for deglaciation, and Atlantic meridional overturning circulation, the driver of antiphased bipolar climate response, both fail to explain this asymmetry. We use centennialresolution records of the local surface water δ 18 O of the Eastern Arabian Sea, which constitutes a proxy for the precipitation associated with the Indian Summer Monsoon, and other tropical precipitation records to deduce the role of tropical forcing in the polar cold reversals. We hypothesize a mechanism for tropical forcing, via the Indian Summer Monsoons, of the polar cold reversals by migration of the Inter-Tropical Convergence Zone and the associated cross-equatorial heat transport.

Global Planet Change, 2005

Northward flowing coastal currents along the western margin of India during winter-spring advect ... more Northward flowing coastal currents along the western margin of India during winter-spring advect low-salinity Bay of Bengal water in to the Eastern Arabian Sea producing a distinct low-salinity tongue, the strength of which is largely governed by the freshwater flux to the bay during summer monsoons. Utilizing the sedimentary records of y 18 O G. sacculifer , we reconstructed the past salinity-gradient within that low-salinity tongue, which serves as a proxy for the variation in freshwater flux to the Bay of Bengal and hence summer monsoon intensity. The north-south contrast in the sea level corrected (residual)-y 18 O G. sacculifer can be interpreted as a measure of surface salinity-contrast between those two locations because the modern sea surface temperature and its past variation in the study region is nearly uniform. The core-top residual-y 18 O G. sacculifer contrast of 0.45x between the two cores is assumed to reflect the modern surface salinity difference of 1 psu and serves as a calibration for past variations. The residual-y 18 O G. sacculifer contrast varies between~0.2x at~75 ky B.P. (i.e., late-Marine Isotope Stage 5) and~0.7x at 20 ky B.P. (i.e., Last Glacial Maximum), suggesting that the overall salinity difference between the northern-and southern-end of the low-salinity tongue has varied between~0.6 and~1.6 psu. Considerably reduced difference during the former period than the modern suggests substantially intensified and northward-extended low-salinity tongue due to intense summer monsoons than today. On the other hand, larger difference (~1.6 psu) during the latter period indicates that the low-salinity tongue was significantly weakened or withdrawn due to weaker summer monsoons. Thus, the salinity-gradient in the eastern Arabian Sea low-salinity tongue can be used to understand the past variations in the Indian summer monsoons.

The top 1 m radiocarbon dated section of a 5.6 m long sediment core retrieved from the Equatorial... more The top 1 m radiocarbon dated section of a 5.6 m long sediment core retrieved from the Equatorial Indian Ocean is studied for productivity changes in response to climate variability that have taken place during the last~33 kyr. The robust indicators of marine productivity such as phytol and brassicasterol exhibit distinctly higher contents (av. 5.8 ng/g and 4.8 ng/g respectively) during the LGM (25-17 ky BP) as compared to the succeeding period (16-5 ky BP: av. 1.9 ng/g each) and preceding period (33-25 ky BP: av. 1.8 and 0.8 ng/g respectively) which suggest increased productivity during the cold and dry climate of the LGM. The C org content is also relatively higher during this period than the warmer Holocene. However, marginally higher C/N ratios (~7.8) and enrichment of δ 13 C org (~−19.8‰) during the LGM than the Holocene (C/N~6.4 and δ 13 C org~− 21.5‰), and also the higher carbon preference index (CPI) of n-alkanes and n-alkanols together suggest the presence of terrestrial organic matter in the sediment. The increased equatorial productivity and terrestrial organic matter input together during the LGM may indicate intensified inter-monsoon equatorial Westerly Jets resulting in elevated productivity.

Geochim Cosmochim Acta, 1999

Carbon-isotopes (δ 13 C) composition of benthic foraminifera has been extensively used to underst... more Carbon-isotopes (δ 13 C) composition of benthic foraminifera has been extensively used to understand the link between deep-water circulation and climate. Equatorial Indian Ocean δ 13 C records of planktic-and benthic-foraminifera together show an unexplained shift in the long-term mean oceanic-δ 13 C around the penultimate glacial termination (T2: 132 ka). The time-series planktic-and benthic-species δ 13 C records exhibit two distinct mean-δ 13 C levels. The low mean-δ 13 C characterises the pre-T2 period (250 ka-132 ka), while the post-T2 (~95 ka-Present) period records high mean-δ 13 C, generating a one-time shift of ~0.4 ‰ within the last ~250 kyr time-period. This shift is a result of consistently higher-δ 13 C in post-T2 glacial (and interglacial) periods as compared to the pre-T2 glacial (and interglacial) periods, and begins around the T2 (~132 ka), lasts until ~95 ka, and sustained through the T1. The normally observed glacial-interglacial δ 13 C variations of ~0.3 ‰ occur as secondary fluctuations around the long-term primary mean-levels in the Indian Ocean, as well as in other oceans. The T2-δ 13 C shift appears to be an inherent feature of the world oceans although with certain timing offsets. Therefore, it should represent a fundamental change in deep-ocean circulation (nutrient) dynamics. But, the leading hypotheses of circulation-driven oceanic distribution of δ 13 C fail to explain the observed mean-δ 13 C shift. Therefore it is proposed that, in addition to changes in deep-water circulation, the oceans before T2 were characterised by significantly lower-δ 13 C than after. Such low-δ 13 C mean-ocean during the pre-T2 period might have been the result of significantly increased transfer of terrestrial light-carbon to the ocean reservoir due to changes in global wind patterns.

1/6b. "Age Model": Ramesh and Tiwari (hereafter 'RT') have comments on ages and associated peaks ... more 1/6b. "Age Model": Ramesh and Tiwari (hereafter 'RT') have comments on ages and associated peaks reported by us (Banakar et al., 2005, hereafter 'Bea'). We had no 14 C ages available for the studied core, hence adopted tropical stack of oxygen isotope stratigraphy of Bassinot et al. (1994) to define climate stage boundaries and chronology as clearly been spelt out in first paragraphs of sections 2 and 3 (see also Bea Figure 2; please note that m should cm in this figure). Second (see RT-6b), we compared our data to published data with a slightly different age model. Therefore, the minimum δ 15 N at e.g. ~60 ka (Bea-Figure 5, GC08 and RC27-16) occurs at ~67 ka for NIO-464. However, corresponding intervals are clearly indicated by arrows in our figure. Thus no discrepancy or even anti-correlation occurs as suggested in RT-6. 2. The basis for assuming Holocene mean-SST as 28 o C has been described in the last paragraph of section 2 ('Material and methods'). If we would have had AMS-14 C dates and a few more SST data points between 12 and 11 ka, we might have interpreted the lowest SST at ~11.3 ka as combined effect of Younger Dryas and locally enhanced upwelling due to hydrographic perturbations caused during LGM-Holocene climate transition. Except for that one-indeed intriguing-data point, the lowest SSTs (~26 o C) in 'Bea' are concentrated around peak-LGM (~18 ka). Sonzogni et al. (1998) also recorded lowest LGM-SSTs between ~17 and 20 ky B.P. (see their Tables 3: ~25.7 o C and 4: ~24.7 o C), not at ~21 ka as

Paleoceanography, 2015

Changes in ocean circulation structure, together with biological cycling, have been proposed for ... more Changes in ocean circulation structure, together with biological cycling, have been proposed for trapping carbon in the deep ocean during glacial periods of the Late Pleistocene, but uncertainty remains in the nature and timing of deep ocean circulation changes through glacial cycles. In this study, we use neodymium (Nd) and carbon isotopes from a deep Indian Ocean sediment core to reconstruct water mass mixing and carbon cycling in Circumpolar Deep Water over the past 250 thousand years, a period encompassing two full glacial cycles and including a range of orbital forcing. Building on recent studies, we use reductive sediment leaching supported by measurements on isolated phases (foraminifera and fish teeth) in order to obtain a robust seawater Nd isotope reconstruction. Neodymium isotopes record a changing North Atlantic Deep Water (NADW) component in the deep Indian Ocean that bears a striking resemblance to Northern Hemisphere climate records. In particular, we identify both an approximately in-phase link to Northern Hemisphere summer insolation in the precession band and a longer-term reduction of NADW contributions over the course of glacial cycles. The orbital timescale changes may record the influence of insolation forcing, for example via NADW temperature and/or Antarctic sea ice extent, on deep stratification and mixing in the Southern Ocean, leading to isolation of the global deep oceans from an NADW source during times of low Northern Hemisphere summer insolation. That evidence could support an active role for changing deep ocean circulation in carbon storage during glacial inceptions. However, mid-depth water mass mixing and deep ocean carbon storage were largely decoupled within glacial periods, and a return to an interglacial-like circulation state during marine isotope stage (MIS) 6.5 was accompanied by only minor changes in atmospheric CO 2. Although a gradual reduction of NADW export through glacial periods may have produced slow climate feedbacks linked to the growth of Northern Hemisphere ice sheets, carbon cycling in the glacial ocean was instead more strongly linked to Southern Ocean processes. Evidence on past Atlantic Ocean circulation derived from carbon isotope reconstructions has been used to suggest that ocean circulation is primarily responding to, rather than driving, Pleistocene climate change on orbital timescales. For example, Imbrie et al. [1992] placed deep Atlantic ventilation within a "late response" group of variables, with increased ventilation occurring~8 kyr behind precessional maxima in Northern Hemisphere insolation. More recently, Lisiecki et al. [2008] similarly proposed a lag of 6-11 kyr WILSON ET AL.

Earth and Planetary Science Letters, 2015

We use reductive sediment leaching to extract lead (Pb) from the authigenic fraction of marine se... more We use reductive sediment leaching to extract lead (Pb) from the authigenic fraction of marine sediments and reconstruct the Pb isotope evolution of the deep central Indian Ocean over the past 250 thousand years at ~3 kyr resolution. Temporal variations define a binary mixing line that is consistent with data from ferromanganese nodules and which records mixing between two well-defined endmembers through time. The unradiogenic endmember appears to represent a widely-distributed Pb source, from mid-ocean ridges or possibly volcanic aerosols, while the radiogenic endmember coincides with the composition of Ganges-Brahmaputra river sediments that are indicative of the Himalayan weathering inputs. Glacial-interglacial Pb isotope variations are striking and can be explained by an enhancement of Himalayan contributions by two to three times during interglacial periods, indicating that climate modulates the supply of dissolved elements to the ocean. While these changes could accurately record variations in the continental chemical weathering flux in response to warmer and wetter conditions during interglacials, the relative proportions of Pb derived from the Ganges and Brahmaputra appear to have been constant through time. This observation may point towards particulate-dissolved interactions in the estuary or pro-delta as a buffer of short timescale variability in the composition (and potentially flux) of the fluvial inputs. In addition, the changes are recorded at 3800 m water depth, and with the lack of deep water formation in the Bay of Bengal, a mechanism to transfer such a signature into the deep ocean could either be reversible scavenging of dissolved Pb inputs and/or boundary exchange on the deep sea fan. Unless the mechanism transferring the Pb isotope signature into the deep ocean was itself highly sensitive to global climate cycles, and with the absence of a precessional signal in our Pb isotope data, we suggest that the Indian climate and its influence on basinscale chemical weathering were strongly modulated by glacial versus interglacial boundary conditions. Key words Indian Ocean, Ganges-Brahmaputra, Himalayan weathering, boundary exchange, lead isotopes, monsoon Highlights We reconstruct the Pb isotope evolution of the deep Indian Ocean 0-250 ka BP. Control by binary mixing between volcanic and Ganges-Brahmaputra endmembers. Reversible scavenging or boundary exchange transfer riverine Pb into deep ocean. Quaternary climatic variations modulate dissolved element supply to the oceans. Himalayan weathering flux enhanced by approximately 2-3 times during interglacials. in the Pb isotopic composition of the central Indian Ocean from 0-250 ka; (iii) assess the sources of Pb and the mechanisms for generating that temporal variability; and (iv) consider the implications of our high resolution record for past changes in the Himalayan weathering inputs. 2. Regional setting Our study is based on two marine sediment cores from the deep central Indian Ocean (Figure 1). Core SK129-CR2 (3° N, 76° E, 3800 m water depth) is located on the east side of the Chagos-Laccadive Ridge in the Central Indian Basin (Banakar, 2005) and core ODP 758 (5° N, 90° E, 2925 m water depth) is located on Ninetyeast Ridge (Farrell and Janacek, 1991). Deep waters at both sites are supplied by northward-flowing Circumpolar Deep Water (CDW), which is transported from the Southern Ocean to the Central Indian Basin after crossing the Southeast Indian Ridge, either directly via the South Australia Basin or indirectly via the West Australia Basin and gaps in the Ninetyeast Ridge (Mantyla and Reid, 1995; You, 2000). Both cores are located towards the distal limit of the Bengal Fan, but neither is directly influenced by the Bengal Fan sedimentation due to their elevated bathymetry, approximately 600 m and 1200 m respectively above the surrounding ocean floor. Therefore, they do not record (in any simple manner) the physical inputs of the Himalayan erosion, which are supplied via the Ganges-Brahmaputra river system to the Bengal Fan. However, because of their more proximal location, they appear well-placed to record an even greater influence of Himalayan weathering on the Pb isotopic composition of Indian Ocean seawater than previously observed in ferromanganese crusts SS663 and DODO-232D (Frank and O'Nions, 1998; Frank et al., 2006) (Figure 1). 3. Materials and methods 3.1. Sampling and age models Lead isotopes were measured on bulk sediment acid-reductive leachates at 94 depths between 8-518 cm (5-251 ka BP) in core SK129-CR2 (i.e. an average sampling resolution of ~3 kyr) and at seven depths between 67-487 cm (33-249 ka BP) in core ODP 758. Both cores are dominated by carbonate ooze, with carbonate contents between 35-70 % for SK129-CR2 and 50-70 % for ODP 758 (Farrell and Janacek, 1991). The age model for SK129-CR2 is constrained by radiocarbon dates for 0-33 ka BP, beyond which the benthic foraminiferal C. wuellerstorfi δ 18 O record is tuned to the LR04 benthic δ 18 O stack (Lisiecki and Raymo, 2005) at major marine isotope stage (MIS) boundaries (Supplementary Information; Tables S1-S3). For ODP 758, we use the most recently presented age model (Gourlan et al., 2010), which was also based on an orbitally-tuned δ 18 O record. 3.2. Sediment leaching The authigenic component of the sediment was extracted by acid-reductive leaching, as described in Wilson et al. (2013). Briefly, bulk sediment samples of ~5 cm 3 were leached in 30 mL 0.44 M acetic acid solution (buffered to pH 5 by sodium acetate) in 50 mL centrifuge tubes on a rotating wheel. This process was repeated until lack of reaction demonstrated that carbonate had been removed. Samples were then washed at least twice with de-ionised water. The authigenic fraction was recovered by acid-reductive leaching for 1 hour in 30 mL of a pH 2 solution of 0.02 M hydroxylamine hydrochloride (HH) in 4.4 M acetic acid. This HH leachate was centrifuged at 5000 rpm for 10 min and decanted three times in sequence, before chemical separation and mass spectrometry. We recently demonstrated that the reproducibility of sediment leachate Nd isotope data can be sensitive to the solution/solid ratios used during the leaching procedure (Wilson et al., 2013), due to the removal of authigenic components during the decarbonation step and progressive exchange with volcanic components, if present. To test the sensitivity of Pb isotopes to such a process in this location, we leached different sample sizes at two core depths in SK129-CR2 (328 cm within MIS 6 and 424 cm within MIS 7), and also analysed a subset of nine leachates from MIS 6-7 using smaller solution/solid ratios in order to prevent complete decarbonation before HH leaching. 3.3. Chemical purification and mass spectrometry Full analytical methods are contained in the Supplementary Information and summarised briefly here. The Pb fraction was separated using HBr-HCl chemistry on AG1-X8 anion exchange resin. The Pb isotopic composition was analysed on a Nu Plasma multicollector inductively-coupled plasma mass spectrometer (MC-ICP-MS) in the Department of Earth Sciences at the University of Cambridge, using thallium (Tl) as an internal standard to correct for mass fractionation (Hirata, 1996; Belshaw et al., 1998; Rehkamper and Mezger, 2000) according to an exponential law. Concentration-matched NIST-SRM-981 Pb standards were measured after approximately every five samples, and a linear correction was applied to all data measured in each analytical session to produce agreement with the accepted composition of NIST-SRM-981 Pb (Galer and Abouchami, 1998; Abouchami et al., 2000). Our external reproducibility for each analytical session, assessed from the standard deviation (2σ) of repeat measurements of NIST-SRM-981, is in the range of 30-180 ppm for 206 Pb/ 204 Pb, 60-240 ppm for 207 Pb/ 204 Pb and 50-250 ppm for 208 Pb/ 204 Pb. In addition, two internal standards (leachate samples that had been through column chemistry) were analysed in multiple analytical sessions over three years (n=14-15) and yield typical long term reproducibility (2σ) of 140 ppm for 206 Pb/ 204 Pb, 160 ppm for 207 Pb/ 204 Pb and 190 ppm for 208 Pb/ 204 Pb. Replicates of 11 samples analysed in two separate sessions give results that are consistent with that external reproducibility (Table S4). Full procedural blanks for the sediment leaching were 1.9 ± 0.7 ng (1σ, n=9). In comparison to leachate samples that contained 200-1000 ng Pb, the blank contribution represents only 0.2-1.0 % of the total Pb, which in the worst case would generate an error of ~100 ppm for 207 Pb/ 204 Pb and ~ 400 ppm for 206 Pb/ 204 Pb and 208 Pb/ 204 Pb. Since such errors are comparable to the external analytical reproducibility, and negligible in comparison to downcore variability, no blank correction has been applied. 4. Results and discussion 4.1. Reconstructing past seawater Pb isotopic composition Our reconstruction of past seawater Pb isotopic composition is based on acidreductive leaching of bulk marine sediments which is used to extract the authigenic phases (Gutjahr et al., 2007; Martin et al., 2010; Wilson et al., 2013). The authigenic fraction is operationally-defined, but considered to be dominated by ferromanganese oxides formed in bottom water or shallow pore waters. This approach has been used in relatively few studies for Pb isotope reconstructions (e.g. Gutjahr et al., 2009; Stumpf et al., 2010; Crocket et al., 2012) and we have therefore investigated some important issues of the method; in particular, (i) the possibility for anthropogenic Pb contamination, and (ii) possible analytical artefacts related to the selectivity of the leaching procedure (Wilson et al., 2013). We summarise here the major outcomes of that assessment (see Supplementary Information) and further discuss the origin of the signal recorded by the authigenic phases. 4.1.1. Anthropogenic contamination The leachate Pb isotope data...

Earth and Planetary Science Letters, 2009

Marine Geology

Deep Sea Research Part I: Oceanographic Research Papers

Journal of Earth System Science

The deglacial transition from the last glacial maximum at ∼20 kiloyears before present (ka) to th... more The deglacial transition from the last glacial maximum at ∼20 kiloyears before present (ka) to the Holocene (11.7 ka to Present) was interrupted by millennial-scale cold reversals, viz., Antarctic Cold Reversal (∼14.5-12.8 ka) and Greenland Younger Dryas (∼12.8-11.8 ka) which had different timings and extent of cooling in each hemisphere. The cause of this synchronously initiated, but different hemispheric cooling during these cold reversals (Antarctic Cold Reversal ∼3 • C and Younger Dryas ∼10 • C) is elusive because CO 2 , the fundamental forcing for deglaciation, and Atlantic meridional overturning circulation, the driver of antiphased bipolar climate response, both fail to explain this asymmetry. We use centennialresolution records of the local surface water δ 18 O of the Eastern Arabian Sea, which constitutes a proxy for the precipitation associated with the Indian Summer Monsoon, and other tropical precipitation records to deduce the role of tropical forcing in the polar cold reversals. We hypothesize a mechanism for tropical forcing, via the Indian Summer Monsoons, of the polar cold reversals by migration of the Inter-Tropical Convergence Zone and the associated cross-equatorial heat transport.

Global Planet Change, 2005

Geochim Cosmochim Acta, 1999

Paleoceanography, 2015

Earth and Planetary Science Letters, 2015