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Mikael Hult - One of the best experts on this subject based on the ideXlab platform.
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Timescales of hydrothermal scavenging in the South Pacific Ocean from 234 Th, 230 Th, and 228 Th
Earth and Planetary Science Letters, 2019Co-Authors: Frank Pavia, Robert Anderson, Erin Black, Lauren Kipp, Sebastian Vivancos, Martin Fleisher, Matthew Charette, Virginie Sanial, Willard Moore, Mikael HultAbstract:Hydrothermal activity in the deep ocean generates plumes of metal-rich particles capable of removing certain trace elements from seawater by adsorption and sedimentation. This removal process, known as scavenging, can be probed using the insoluble radiogenic isotopes of thorium (Th), which are produced at a known rate in the water column via the decay of soluble Uranium (234 Th, 230 Th) and radium (228 Th) isotopes. We present dissolved and particulate measurements of these three thorium isotopes in a hydrothermal plume observed in the southeast Pacific Ocean on the GEOTRACES GP16 section. Since their half-lives vary from days (234 Th) to years (228 Th) to tens of thousands of years (230 Th), the combination of their signals can be used to understand scavenging processes occurring on a wide range of timescales. Scavenging is a multi-step process involving adsorption and desorption onto particles, followed by particle aggregation, sinking, and eventual sedimentation. We use thorium isotopes to study how hydrothermal activity affects these steps. The rate constants for net adsorption of 234 Th determined here are comparable to previous estimates from hydrothermal plumes in the Atlantic and North Pacific Oceans. The partitioning of 234 Th and 230 Th between large and small particles is more similar in the hydrothermal plume than above it, indicating faster aggregation of particles within the hydrothermal plume at stations nearby the East Pacific Rise than in waters outside the plume. In addition to rapid scavenging and aggregation near the ridge axis, we also infer continuous off-axis scavenging from observations and modeling of 228 Th/ 228 Ra activity ratios. The degree of depletion of the three thorium isotopes increases in order of half-life, with total 234 Th activity close to that of its parent 238 U, but 230 Th showing nearly 70% depletion compared to expected values from reversible scavenging. By modeling the variations in depletion for the different isotopes, we show that much of the 230 Th removal is inherited from scavenging events happening long before the most recent hydrothermal inputs.
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Timescales of hydrothermal scavenging in the South Pacific Ocean from 234Th, 230Th, and 228Th
Earth and Planetary Science Letters, 2019Co-Authors: Frank J. Pavia, Sebastian Vivancos, Virginie Sanial, Robert F. Anderson, Erin E. Black, Lauren E. Kipp, Martin Q. Fleisher, Matthew A. Charette, Willard S. Moore, Mikael HultAbstract:Hydrothermal activity in the deep ocean generates plumes of metal-rich particles capable of removing certain trace elements from seawater by adsorption and sedimentation. This removal process, known as scavenging, can be probed using the insoluble radiogenic isotopes of thorium (Th), which are produced at a known rate in the water column via the decay of soluble Uranium (234 Th, 230 Th) and radium (228 Th) isotopes. We present dissolved and particulate measurements of these three thorium isotopes in a hydrothermal plume observed in the southeast Pacific Ocean on the GEOTRACES GP16 section. Since their half-lives vary from days (234 Th) to years (228 Th) to tens of thousands of years (230 Th), the combination of their signals can be used to understand scavenging processes occurring on a wide range of timescales. Scavenging is a multi-step process involving adsorption and desorption onto particles, followed by particle aggregation, sinking, and eventual sedimentation. We use thorium isotopes to study how hydrothermal activity affects these steps. The rate constants for net adsorption of 234 Th determined here are comparable to previous estimates from hydrothermal plumes in the Atlantic and North Pacific Oceans. The partitioning of 234 Th and 230 Th between large and small particles is more similar in the hydrothermal plume than above it, indicating faster aggregation of particles within the hydrothermal plume at stations nearby the East Pacific Rise than in waters outside the plume. In addition to rapid scavenging and aggregation near the ridge axis, we also infer continuous off-axis scavenging from observations and modeling of 228 Th/ 228 Ra activity ratios. The degree of depletion of the three thorium isotopes increases in order of half-life, with total 234 Th activity close to that of its parent 238 U, but 230 Th showing nearly 70% depletion compared to expected values from reversible scavenging. By modeling the variations in depletion for the different isotopes, we show that much of the 230 Th removal is inherited from scavenging events happening long before the most recent hydrothermal inputs.
Katharine Maher - One of the best experts on this subject based on the ideXlab platform.
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an evaluation of paired δ18o and 234u 238u 0 in opal as a tool for paleoclimate reconstruction in semi arid environments
Chemical Geology, 2017Co-Authors: Jessica L Oster, Kouki Kitajima, John W Valley, Bruce Rogers, Katharine MaherAbstract:Abstract Authigenic opal deposits are widespread in semi-arid and arid environments and often contain high Uranium concentrations that facilitate high-precision dating by U-series and U-Pb geochronology. However, despite their ubiquity and utility for placing temporal constraints on surficial processes, the potential for authigenic opal deposits to be used as archives of past climate variability remains largely unexplored compared with more frequently used authigenic carbonate deposits. Here we investigate the potential for authigenic opal deposits to provide records of paleoclimate and paleohydrologic variability through the development of a combined record of Uranium (( 234 U/ 238 U) 0 ) and oxygen ( δ 18 O) isotope variability using in situ ion microprobe methods on opal deposits from Pinnacles National Park, California that precipitated from approximately 16 to 3 ka. Pinnacles opal ( 234 U/ 238 U) 0 suggests a moderate decrease in net infiltration from the early to middle Holocene, coincident with the timing of increasing aridity inferred from Sierra Nevada lake records, while Pinnacles opal δ 18 O indicates a relative increase in subtropical storms at the same time. A shift to more North Pacific sourced storms commenced after ~ 5 ka coincident with the easing of aridity in Northern California and increased storminess in the Sierra Nevada as indicated by lake sediment records. These results suggest that useful insights about past hydrologic change can be gleaned from authigenic opal deposits, and that these deposits have the potential to enhance the spatial and temporal coverage of paleoclimate reconstructions in regions where other archives might be scarce. However, further study of the influence of water in opal on δ 18 O variability and additional in situ analysis of δ 18 O variability on longer-lived opal deposits with uniform growth histories are necessary to verify the controls on this potential proxy. Similarly, further investigation into short-term and spatial variability in ( 234 U/ 238 U) sw will improve qualitative and quantitative applications of ( 234 U/ 238 U) 0 in authigenic opal deposits, enhancing our ability to use this promising new paleoclimate archive.
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An evaluation of paired δ18O and (234U/238U)0 in opal as a tool for paleoclimate reconstruction in semi-arid environments
Chemical Geology, 2017Co-Authors: Jessica L Oster, Kouki Kitajima, John W Valley, Bruce Rogers, Katharine MaherAbstract:Abstract Authigenic opal deposits are widespread in semi-arid and arid environments and often contain high Uranium concentrations that facilitate high-precision dating by U-series and U-Pb geochronology. However, despite their ubiquity and utility for placing temporal constraints on surficial processes, the potential for authigenic opal deposits to be used as archives of past climate variability remains largely unexplored compared with more frequently used authigenic carbonate deposits. Here we investigate the potential for authigenic opal deposits to provide records of paleoclimate and paleohydrologic variability through the development of a combined record of Uranium (( 234 U/ 238 U) 0 ) and oxygen ( δ 18 O) isotope variability using in situ ion microprobe methods on opal deposits from Pinnacles National Park, California that precipitated from approximately 16 to 3 ka. Pinnacles opal ( 234 U/ 238 U) 0 suggests a moderate decrease in net infiltration from the early to middle Holocene, coincident with the timing of increasing aridity inferred from Sierra Nevada lake records, while Pinnacles opal δ 18 O indicates a relative increase in subtropical storms at the same time. A shift to more North Pacific sourced storms commenced after ~ 5 ka coincident with the easing of aridity in Northern California and increased storminess in the Sierra Nevada as indicated by lake sediment records. These results suggest that useful insights about past hydrologic change can be gleaned from authigenic opal deposits, and that these deposits have the potential to enhance the spatial and temporal coverage of paleoclimate reconstructions in regions where other archives might be scarce. However, further study of the influence of water in opal on δ 18 O variability and additional in situ analysis of δ 18 O variability on longer-lived opal deposits with uniform growth histories are necessary to verify the controls on this potential proxy. Similarly, further investigation into short-term and spatial variability in ( 234 U/ 238 U) sw will improve qualitative and quantitative applications of ( 234 U/ 238 U) 0 in authigenic opal deposits, enhancing our ability to use this promising new paleoclimate archive.
Yemane Asmerom - One of the best experts on this subject based on the ideXlab platform.
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The half-lives of Uranium-234 and thorium-230
Chemical Geology, 2000Co-Authors: Hai Cheng, John A. Hoff, R. L. Edwards, Christina D. Gallup, David A. Richards, Yemane AsmeromAbstract:Abstract We have re-determined the 234 U and 230 Th half-lives to be 245,250±490 years (2σ) and 75,690±230 years (2σ), respectively. Using high precision thermal ionization mass spectrometric (TIMS) methods, we measured 234 U / 238 U and 230 Th / 238 U atomic ratios in 4 different materials that were likely to have behaved as closed systems for 106 years or more: zircons with concordant 238 U – 206 Pb , 235 U – 207 Pb , and 232 Th – 208 Pb ages, Iceland Spar, Table Mountain Latite, and aliquots of a solution of Harwell uraninite (HU-1). We calibrated the TIMS multipliers using U-500, U and Th gravimetric standards, and U double spike. Consistent 234 U / 238 U values for all measured materials and consistent 230 Th / 238 U values for all materials with the exception of our HU-1 solution support the secular equilibrium status. The new half-lives agree within error with previously determined values; however, errors in our values are generally smaller than those in the earlier determinations. Our 234 U half-life is about 3‰ higher than that commonly used in 230 Th dating laboratories and our 230 Th half-life is about 4‰ higher. 230 Th ages calculated with the new half-lives are generally older than those calculated with the previously used half-lives. The difference in age, though, is small throughout the 230 Th age range because our revised 234 U and 230 Th half-lives are offset from earlier values in the same sense (both to higher values). In the case of dating materials older than 350 ka in laboratories that rely solely on gravimetric standardization procedures, use of our decay constants and their associated errors will considerably reduce the errors in age arising from uncertainty in the decay constants.
Bruno Hamelin - One of the best experts on this subject based on the ideXlab platform.
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new tims constraints on the Uranium 238 and Uranium 234 in seawaters from the main ocean basins and the mediterranean sea
Marine Chemistry, 2002Co-Authors: Doriane Delanghe, Edouard Bard, Bruno HamelinAbstract:An accurate knowledge of U isotopes in seawater is important for a number of applications in Earth Sciences including geochronology, paleothermometry and pollution budgets. The distribution of Uranium isotopes in seawater and modern corals has been revisited by Thermal Ionisation Mass Spectrometry (TIMS) in order to refine the limited availability of precise data (mostly the work by Chen et al. [Earth Planet. Sci. Lett. 80 (1986) 241] based on 20 seawater samples from 2 oceans). Our results for samples collected in the three main oceans allow us calculation of an average U concentration of 13.56 pmol/g normalized to 35 salinity (1SD of 0.28 pmol/g, n=20) in good agreement with the two previous estimates by α-spectroscopy [Ku et al. Deep-Sea Res. 24 (1977) 1005] and by TIMS [Earth Planet. Sci. Lett. 80 (1986) 241]. Similarly, we show that the distribution of δ234U is homogeneous in the ocean at an average of 149.6‰ (1SD of 1‰ in large samples, n=9 and of 3‰ on all samples analyzed, n=23), greater than previous estimation by TIMS (mean of 139.5‰, 1SD of 3.5‰, n=9 [Earth Planet. Sci. Lett. 80 (1986) 241]). The first data obtained by TIMS for Mediterranean Sea samples show no significant departure from conservative behavior for both U concentration and δ234U ratio. Finally, δ234U values measured in modern corals from the Atlantic and the Pacific Oceans average, 146.6‰ (1SD of 1.4‰, n=10), are very close but slightly lower than the seawater value.
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Uranium-234 anomalies in corals older than 150,000 years
Geochimica et Cosmochimica Acta, 1991Co-Authors: Edouard Bard, Bruno Hamelin, Richard G. Fairbanks, Alan Zindler, Chi Track HoangAbstract:We present new precise U-Th ages of well-preserved coral specimens collected from the island of Barbados, West Indies, and the atoll of Mururoa, French Polynesia. Our new data confirm the ages attributed to oxygen isotope stage 7 in the framework of the Milankovitch theory (BERGER, 1978; Mar-TINSON et al., 1987). By using thermal ionization mass spectrometry (TIMS), it is also possible to quantify precisely the 234U238U ratios in corals. Samples older than 150 kyr B.P. are shown to be characterized by significant excesses of 234U relative to the Uranium isotopic composition expected if the corals grew in present-day sea water. Assuming that the 230Th-ingrowth ages are accurate, these anomalies translate into high initial 234U238U ratios: about 1.2 at 200 kyr and up to 1.5 at about 450 kyr B.P. We propose that the anomalies result from both diagenetic addition and replacement of U and possibly from global changes in the 234U238U composition of sea water through time. The 234U anomalies cast doubt on the accuracy of the classical 230Th-ingrowth dating method in old corals, and in particular for the use of measured 234U238U ratios alone to date corals older than 150 kyr.
Jessica L Oster - One of the best experts on this subject based on the ideXlab platform.
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an evaluation of paired δ18o and 234u 238u 0 in opal as a tool for paleoclimate reconstruction in semi arid environments
Chemical Geology, 2017Co-Authors: Jessica L Oster, Kouki Kitajima, John W Valley, Bruce Rogers, Katharine MaherAbstract:Abstract Authigenic opal deposits are widespread in semi-arid and arid environments and often contain high Uranium concentrations that facilitate high-precision dating by U-series and U-Pb geochronology. However, despite their ubiquity and utility for placing temporal constraints on surficial processes, the potential for authigenic opal deposits to be used as archives of past climate variability remains largely unexplored compared with more frequently used authigenic carbonate deposits. Here we investigate the potential for authigenic opal deposits to provide records of paleoclimate and paleohydrologic variability through the development of a combined record of Uranium (( 234 U/ 238 U) 0 ) and oxygen ( δ 18 O) isotope variability using in situ ion microprobe methods on opal deposits from Pinnacles National Park, California that precipitated from approximately 16 to 3 ka. Pinnacles opal ( 234 U/ 238 U) 0 suggests a moderate decrease in net infiltration from the early to middle Holocene, coincident with the timing of increasing aridity inferred from Sierra Nevada lake records, while Pinnacles opal δ 18 O indicates a relative increase in subtropical storms at the same time. A shift to more North Pacific sourced storms commenced after ~ 5 ka coincident with the easing of aridity in Northern California and increased storminess in the Sierra Nevada as indicated by lake sediment records. These results suggest that useful insights about past hydrologic change can be gleaned from authigenic opal deposits, and that these deposits have the potential to enhance the spatial and temporal coverage of paleoclimate reconstructions in regions where other archives might be scarce. However, further study of the influence of water in opal on δ 18 O variability and additional in situ analysis of δ 18 O variability on longer-lived opal deposits with uniform growth histories are necessary to verify the controls on this potential proxy. Similarly, further investigation into short-term and spatial variability in ( 234 U/ 238 U) sw will improve qualitative and quantitative applications of ( 234 U/ 238 U) 0 in authigenic opal deposits, enhancing our ability to use this promising new paleoclimate archive.
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An evaluation of paired δ18O and (234U/238U)0 in opal as a tool for paleoclimate reconstruction in semi-arid environments
Chemical Geology, 2017Co-Authors: Jessica L Oster, Kouki Kitajima, John W Valley, Bruce Rogers, Katharine MaherAbstract:Abstract Authigenic opal deposits are widespread in semi-arid and arid environments and often contain high Uranium concentrations that facilitate high-precision dating by U-series and U-Pb geochronology. However, despite their ubiquity and utility for placing temporal constraints on surficial processes, the potential for authigenic opal deposits to be used as archives of past climate variability remains largely unexplored compared with more frequently used authigenic carbonate deposits. Here we investigate the potential for authigenic opal deposits to provide records of paleoclimate and paleohydrologic variability through the development of a combined record of Uranium (( 234 U/ 238 U) 0 ) and oxygen ( δ 18 O) isotope variability using in situ ion microprobe methods on opal deposits from Pinnacles National Park, California that precipitated from approximately 16 to 3 ka. Pinnacles opal ( 234 U/ 238 U) 0 suggests a moderate decrease in net infiltration from the early to middle Holocene, coincident with the timing of increasing aridity inferred from Sierra Nevada lake records, while Pinnacles opal δ 18 O indicates a relative increase in subtropical storms at the same time. A shift to more North Pacific sourced storms commenced after ~ 5 ka coincident with the easing of aridity in Northern California and increased storminess in the Sierra Nevada as indicated by lake sediment records. These results suggest that useful insights about past hydrologic change can be gleaned from authigenic opal deposits, and that these deposits have the potential to enhance the spatial and temporal coverage of paleoclimate reconstructions in regions where other archives might be scarce. However, further study of the influence of water in opal on δ 18 O variability and additional in situ analysis of δ 18 O variability on longer-lived opal deposits with uniform growth histories are necessary to verify the controls on this potential proxy. Similarly, further investigation into short-term and spatial variability in ( 234 U/ 238 U) sw will improve qualitative and quantitative applications of ( 234 U/ 238 U) 0 in authigenic opal deposits, enhancing our ability to use this promising new paleoclimate archive.
