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Bristol, United Kingdom

Martin P.G.,University of Bristol | Griffiths I.,University of Bristol | Jones C.P.,University of Bristol | Stitt C.A.,University of Bristol | And 5 more authors.
Spectrochimica Acta - Part B Atomic Spectroscopy

Traditional methods to locate and subsequently study radioactive fallout particles have focused heavily on autoradiography coupled with in-situ analytical techniques. Presented here is the application of a Variable Pressure Scanning Electron Microscope with both backscattered electron and energy dispersive spectroscopy detectors, along with a micromanipulator setup and electron-hardening adhesive to isolate and remove individual particles before synchrotron radiation analysis. This system allows for a greater range of new and existing analytical techniques, at increased detail and speed, to be applied to the material. Using this method, it was possible to erform detailed energy dispersive spectroscopy and synchrotron radiation characterisation of material likely ejected from the Fukushima Daiichi Nuclear Power Plant found within a sediment sample collected from the edge of the 30 km exclusion zone. Particulate material sub-micron in maximum dimension examined during this work via energy dispersive spectroscopy was observed to contain uranium at levels between 19.68 and 28.35 weight percent, with the application of synchrotron radiation spectroscopy confirming its presence as a major constituent. With great effort and cost being devoted to the remediation of significant areas of eastern Japan affected by the incident, it is crucial to gain the greatest possible understanding of the nature of this contamination in order to inform the most appropriate clean-up response. © 2016 The Authors. Published by Elsevier B.V. Source

Lippold J.,University of Bern | Lippold J.,University of Heidelberg | Gutjahr M.,Bristol Isotope Group | Gutjahr M.,Leibniz Institute of Marine Science | And 12 more authors.
Earth and Planetary Science Letters

Reconstructing past modes of ocean circulation is an essential task in paleoclimatology and paleoceanography. To this end, we combine two sedimentary proxies, Nd isotopes (εNd) and the 231Pa/230Th ratio, both of which are not directly involved in the global carbon cycle, but allow the reconstruction of water mass provenance and provide information about the past strength of overturning circulation, respectively. In this study, combined 231Pa/230Th and εNd down-core profiles from six Atlantic Ocean sediment cores are presented. The data set is complemented by the two available combined data sets from the literature. From this we derive a comprehensive picture of spatial and temporal patterns and the dynamic changes of the Atlantic Meridional Overturning Circulation over the past ~25 ka. Our results provide evidence for a consistent pattern of glacial/stadial advances of Southern Sourced Water along with a northward circulation mode for all cores in the deeper (>3000 m) Atlantic. Results from shallower core sites support an active overturning cell of shoaled Northern Sourced Water during the LGM and the subsequent deglaciation. Furthermore, we report evidence for a short-lived period of intensified AMOC in the early Holocene. © 2016 Elsevier B.V. Source

Gutjahr M.,ETH Zurich | Gutjahr M.,Bristol Isotope Group | Hoogakker B.A.A.,University of Cambridge | Frank M.,Leibniz Institute of Marine Science | McCave I.N.,University of Cambridge
Quaternary Science Reviews

The strength of the North Atlantic Meridional Overturning Circulation during climatically highly variable Marine Isotope Stage (MIS) 3 has attracted much attention in recent years. Here we present high-resolution Nd isotope compositions of past seawater derived from authigenic Fe-Mn oxyhydroxides recovered from drift sediments on the Blake Ridge in the deep western North Atlantic (ODP Leg 172, Site 1060, 3481m water depth). The data cover the period from 45 to 35kaBP, tracing circulation changes during major Heinrich iceberg discharge event 4 (H4, ∼40-39kaBP). The Nd isotope record suggests that there was no northern-source water (NSW) mass like modern NADW at the deeper part of Blake Ridge at any time between 45 and 35ka. This is fundamentally different from the hydrographic situation during the Holocene where NADW extends below 4500m at this location. The e{open}Nd of past deep water recorded in the Blake Ridge sediments was least radiogenic during Dansgaard/Oeschger (D/O) Interstadial (IS) 8 (e{open}Nd=-11.3) and most radiogenic immediately preceding IS 9 (e{open}Nd=-9.8). More radiogenic compositions were also recorded during H4 (-10.2≤e{open}Nd≤-9.9). The Nd isotope variability in MIS 3 matches that of a physical bottom current strength reconstruction from the same location. Neither record follows the pattern of Northern Hemisphere D/O climatic cycles. In our record, reduced mixing with northern source waters started in stadial 12 and lasted until after H4 in stadial 9, followed by a rapid increase in NSW contribution thereafter. This major change in the Nd isotope record predates the iceberg discharge event Heinrich 4 by more than 3ka indicating a shallowing of the water mass boundary between Glacial North Atlantic Intermediate Water and Southern Source Water beneath. This early change in bottom water properties at the deep Blake Ridge suggests that North Atlantic deep water advection may already have decreased several thousand years before the actual iceberg discharge event and associated freshening of the surface waters in the North Atlantic. The change can thus not be attributed to climatic events in the North Atlantic but may be related to changes in flux of deep water from the South. © 2010 Elsevier Ltd. Source

Pogge Von Strandmann P.A.E.,Bristol Isotope Group | Pogge Von Strandmann P.A.E.,Birkbeck, University of London | Forshaw J.,Bristol Isotope Group | Forshaw J.,Kingston University | Schmidt D.N.,Bristol Isotope Group

Magnesium is an element critically involved in the carbon cycle, because weathering of Ca-Mg silicates removes atmospheric CO2into rivers, and formation of Ca-Mg carbonates in the oceans removes carbon from the ocean-atmosphere system. Hence the Mg cycle holds the potential to provide valuable insights into Cenozoic climate-system history, and the shift during this time from a greenhouse to icehouse state. We present Mg isotope ratios for the past 40 Myr using planktic foraminifers as an archive. Modern foraminifera, which discriminate against elemental and isotopically heavy Mg during calcification, show no correlation between the Mg isotope composition (δ26Mg, relative to DSM-3) and temperature, Mg/Ca or other parameters such as carbonate saturation (ΔCO3). However, inter-species isotopic differences imply that only well-calibrated single species should be used for reconstruction of past seawater. Seawater δ26Mg inferred from the foraminiferal record decreased from ∼0‰ at 15 Ma, to-0.83‰ at the present day, which coincides with increases in seawater lithium and oxygen isotope ratios. It strongly suggests that neither Mg concentrations nor isotope ratios are at steady state in modern oceans, given its ∼10 Myr residence time. From these data, we have developed a dynamic box model to understand and constrain changes in Mg sources to the oceans (rivers) and Mg sinks (dolomitisation and hydrothermal alteration). Our estimates of seawater Mg concentrations through time are similar to those independently determined by pore waters and fluid inclusions. Modelling suggests that dolomite formation and the riverine Mg flux are the primary controls on the δ26Mg of seawater, while hydrothermal Mg removal and the δ26Mg of rivers are more minor controls. Using Mg riverine flux and isotope ratios inferred from the 87Sr/86Sr record, the modelled Mg removal by dolomite formation shows minima in the Oligocene and at the present day (with decreasing trends from 15 Ma), both coinciding with rapid decreases in global temperatures. Source

Rae J.W.B.,Bristol Isotope Group | Rae J.W.B.,University of Bristol | Foster G.L.,Bristol Isotope Group | Foster G.L.,University of Bristol | And 4 more authors.
Earth and Planetary Science Letters

Accurate records of the state of the ocean carbonate system are critical for understanding past changes in pCO2, ocean acidification and climate. The chemical principles underlying the proxy of oceanic pH provided by the boron isotope ratio of foraminiferal carbonate are relatively well understood, but the proxy's reliability has been questioned. We present 76 new Multi-Collector Inductively-Coupled Plasma Mass Spectrometry (MC-ICPMS) δ11B measurements on a range of benthic foraminifera from 23 late-Holocene samples from the Atlantic that reaffirm the utility of the δ11B-pH proxy. Our boron isotope measurements on ~10 benthic foraminifera tests typically yield a precision of ~±0.25‰ at 2 s.d. (equivalent to ~±0.03 pH units). δ11B values of epifaunal species are within analytical uncertainty of those predicted from a simple model assuming sole incorporation of B(OH)4 - from seawater and no vital effects, using the independently determined fractionation factor of 1.0272 between 11B/10B of aqueous boron species. Infaunal foraminifera are consistent with this model, but record the combined effects of lower pore-water δ11B and pH. No influence of partial dissolution or shell size on δ11B is observed. We have also measured the B/Ca ratios of the same samples. For individual Cibicidoides species, B/Ca shows a good correlation with δ[CO3 2-], but the B/Ca of different co-occurring species morphotypes varies considerably. These effects are not seen in δ11B, which may therefore provide a more robust proxy of the ocean carbonate system. Whilst in theory δ11B and B/Ca can be combined to provide a quantitative reconstruction of alkalinity and dissolved inorganic carbonate (DIC), in practice this is precluded by propagated uncertainties. δ11B data give significant constraints on foraminifera calcification mechanisms, and seem most simply explained by incorporation of B(OH)4 - into a HCO3 - pool, which is then completely incorporated in foraminiferal CaCO3. Our demonstration of the predictable variation of δ11B with pH, across a wide range of species and locations, provides confidence in the application of MC-ICPMS measurements of foraminiferal δ11B to reconstruct past changes in the ocean carbonate system. © 2010 Elsevier B.V. Source

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