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Bernhard Peuckerehrenbrink - One of the best experts on this subject based on the ideXlab platform.
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determining chondritic impactor size from the marine Osmium isotope record
Science, 2008Co-Authors: Francois S Paquay, Gregory E Ravizza, Tarun K Dalai, Bernhard PeuckerehrenbrinkAbstract:Decreases in the seawater 187Os/188Os ratio caused by the impact of a chondritic meteorite are indicative of projectile size, if the soluble fraction of Osmium carried by the impacting body is known. Resulting diameter estimates of the Late Eocene and Cretaceous/Paleogene projectiles are within 50% of independent estimates derived from iridium data, assuming total vaporization and dissolution of Osmium in seawater. The variations of 187Os/188Os and Os/Ir across the Late Eocene impact-event horizon support the main assumptions required to estimate the projectile diameter. Chondritic impacts as small as 2 kilometers in diameter should produce observable excursions in the marine Osmium isotope record, suggesting that previously unrecognized impact events can be identified by this method.
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the marine Osmium isotope record
Terra Nova, 2000Co-Authors: Bernhard Peuckerehrenbrink, Greg RavizzaAbstract:Over the past decade the marine Osmium isotope record has been developed into a new tracer in palaeoceanographic research. Several analytical developments, particularly in the past few years, have significantly increased our ability to study the behaviour of Osmium in the surficial environment. The 187Os/188Os and Osmium concentration of seawater, river water, rain, and hydrothermal vent fluids have been measured directly. Recently, the behaviour of Osmium in estuaries–critical for estimating the marine residence time of Osmium–has been studied. Our knowledge of the surficial Osmium cycle has thus significantly improved. In addition, reconstructions of past variations in the marine 187Os/188Os recently have been extended back into the Mesozoic. This review attempts to summarize our current understanding of the marine Osmium system–present and past. The 187Os/188Os of seawater during the Cenozoic to first order mimics the marine 87Sr/86Sr record. It is therefore tempting to interpret both records as reflecting increased input of radiogenic Osmium and strontium resulting from enhanced continental weathering regulated by climatic/tectonic processes. However, the marine Osmium isotope system differs fundamentally from the marine strontium isotope system. This review emphasizes three important differences. First, large impacts are capable of resetting the 187Os/188Os to unradiogenic values without significantly affecting the marine strontium system. Second, organic-rich sediments are characterized by high 187Re/188Os; resulting 187Os/188Os ingrowth-trajectories are similar to the average slope of the Cenozoic 187Os/188Os seawater record. Trends towards more radiogenic 187Os/188Os seawater therefore can be caused by weathering of organic-rich sediments at a constant rate. Third, the marine residence time of Osmium is sufficiently short to capture short-periodic (glacial-interglacial) fluctuations that are inaccessible to the buffered marine strontium isotope system. This offers the opportunity to discriminate between high-frequency (climatic) and low-frequency (tectonic) forcing.
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accretion of extraterrestrial matter during the last 80 million years and its effect on the marine Osmium isotope record
Geochimica et Cosmochimica Acta, 1996Co-Authors: Bernhard PeuckerehrenbrinkAbstract:The flux of particulate extraterrestrial (ET) matter to the deep-sea has been calculated using a four-component mixing model based on Osmium concentrations and isotope ratios in slowly accumulating pelagic sediments from the Pacific Ocean. Nineteen published bulk-leach Osmium isotope data pairs that cover the last 80 million years have been used for the calculation. The calculated annual particulate flux, averaged over several 100 ka by slow accumulation and bioturbation, ranges from ∼18,000 to ∼67,000 tons, with a mean value of ∼37,000 ± 13,000 tons. The data indicate no significant variability in the flux with time, except at the K-T boundary and are thus compatible with results based on Ir accumulation in deep-sea sediments. The inferred constancy of the ET matter flux is in contrast to recent results based on the ET 3He flux that integrates only a small size fraction of the entire ET matter flux. 3He data indicate variability by a factor of 6 over the past 70 Ma and a fivefold increase in the ET flux between 3 Ma and 1 Ma that is not seen in the bulk ET matter flux based on Osmium isotope data. The apparent constancy of the cosmic matter flux over the past 80 Ma stands in marked contrast to the dynamic nature of the marine Osmium isotope record and indicates that dissolution of cosmic matter does not drive changes in this record, except at the K-T boundary. A comparison between Osmium isotope and Ir data from two pelagic sediment cores from the Pacific (DSDP Site 596 and LL44-GPC3) indicates that the seawater-soluble fraction of ET Os cannot exceed ∼36 kg/a and most likely is significantly smaller. The maximum value of 36 kg/a can account for approximately one half of the seawater-soluble ET matter flux necessary to balance the radiogenic continental runoff of Os without any additional unradiogenic source. Simple mass balance calculations indicate that an additional unradiogenic source of Os to the oceans, most likely alteration of oceanic crust, is required to balance the present-day seawater Osmium-isotopic composition. This source is probably more important in balancing radiogenic continental runoff than is dissolution of cosmic matter in seawater.
Hiroki Sato - One of the best experts on this subject based on the ideXlab platform.
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a metallacycle fragmentation strategy for vinyl transfer from enol carboxylates to secondary alcohol c h bonds via Osmium or ruthenium catalyzed transfer hydrogenation
Journal of the American Chemical Society, 2015Co-Authors: Boyoung Y Park, Tom Luong, Hiroki SatoAbstract:A strategy for catalytic vinyl transfer from enol carboxylates to activated secondary alcohol C–H bonds is described. Using XPhos-modified ruthenium(0) or Osmium(0) complexes, enol carboxylate–carbonyl oxidative coupling forms transient β-acyloxy-oxametallacycles, which eliminate carboxylate to deliver allylic ruthenium(II) or Osmium(II) alkoxides. Reduction of the metal(II) salt via hydrogen transfer from the secondary alcohol reactant releases the product of carbinol C–H vinylation and regenerates ketone and zero-valent catalyst.
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A Metallacycle Fragmentation Strategy for Vinyl Transfer from Enol Carboxylates to Secondary Alcohol C–H Bonds via Osmium- or Ruthenium-Catalyzed Transfer Hydrogenation
2015Co-Authors: Boyoung Y. Park, Tom Luong, Hiroki Sato, Michael J. KrischeAbstract:A strategy for catalytic vinyl transfer from enol carboxylates to activated secondary alcohol C–H bonds is described. Using XPhos-modified ruthenium(0) or Osmium(0) complexes, enol carboxylate–carbonyl oxidative coupling forms transient β-acyloxy-oxametallacycles, which eliminate carboxylate to deliver allylic ruthenium(II) or Osmium(II) alkoxides. Reduction of the metal(II) salt via hydrogen transfer from the secondary alcohol reactant releases the product of carbinol C–H vinylation and regenerates ketone and zero-valent catalyst
Greg Ravizza - One of the best experts on this subject based on the ideXlab platform.
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the marine Osmium isotope record
Terra Nova, 2000Co-Authors: Bernhard Peuckerehrenbrink, Greg RavizzaAbstract:Over the past decade the marine Osmium isotope record has been developed into a new tracer in palaeoceanographic research. Several analytical developments, particularly in the past few years, have significantly increased our ability to study the behaviour of Osmium in the surficial environment. The 187Os/188Os and Osmium concentration of seawater, river water, rain, and hydrothermal vent fluids have been measured directly. Recently, the behaviour of Osmium in estuaries–critical for estimating the marine residence time of Osmium–has been studied. Our knowledge of the surficial Osmium cycle has thus significantly improved. In addition, reconstructions of past variations in the marine 187Os/188Os recently have been extended back into the Mesozoic. This review attempts to summarize our current understanding of the marine Osmium system–present and past. The 187Os/188Os of seawater during the Cenozoic to first order mimics the marine 87Sr/86Sr record. It is therefore tempting to interpret both records as reflecting increased input of radiogenic Osmium and strontium resulting from enhanced continental weathering regulated by climatic/tectonic processes. However, the marine Osmium isotope system differs fundamentally from the marine strontium isotope system. This review emphasizes three important differences. First, large impacts are capable of resetting the 187Os/188Os to unradiogenic values without significantly affecting the marine strontium system. Second, organic-rich sediments are characterized by high 187Re/188Os; resulting 187Os/188Os ingrowth-trajectories are similar to the average slope of the Cenozoic 187Os/188Os seawater record. Trends towards more radiogenic 187Os/188Os seawater therefore can be caused by weathering of organic-rich sediments at a constant rate. Third, the marine residence time of Osmium is sufficiently short to capture short-periodic (glacial-interglacial) fluctuations that are inaccessible to the buffered marine strontium isotope system. This offers the opportunity to discriminate between high-frequency (climatic) and low-frequency (tectonic) forcing.
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Re–Os isotope evidence for the composition, formation and age of the lower continental crust
Nature, 1998Co-Authors: Alberto E. Saal, Greg Ravizza, Roberta L. Rudnick, Stanley R. HartAbstract:Knowledge of the composition of the lower continental crust is important for understanding the formation and evolution of the crust as a whole, and the petrogenesis of continental basalts. Here we present rhenium–Osmium isotope data for two well characterized suites of lower-crustal xenoliths from North Queensland, Australia1,2,3,4,5,6,7, which have average major- and trace-element compositions similar to estimates of the bulk lower continental crust8,9. Our data indicate that the lower crust has 1 to 2 times as much Osmium, about half as much rhenium, and is less radiogenic than the upper continental crust10. We interpret the rhenium–Osmium isotope systematics to indicate that assimilation and fractional crystallization are important processes in the formation of the lower crust, and lead to dramatic changes in the Osmium isotopic composition of basalts that pond and fractionate there. A consequence of this is that the rhenium–Osmium isotopic system should not be relied on to yield accurate mantle extraction ages for continental rocks.
Mukul Sharma - One of the best experts on this subject based on the ideXlab platform.
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Osmium isotopes in hydrothermal fluids from the juan de fuca ridge
Earth and Planetary Science Letters, 2000Co-Authors: Mukul Sharma, Albrecht W Hofmann, G. J. Wasserburg, D. Anthony ButterfieldAbstract:We present Os data for axial high-temperature and off-axial low-temperature hydrothermal solutions from the Juan de Fuca Ridge. The high-temperature, H₂S-bearing axial fluids have unradiogenic Os isotopes pointing to a nearly complete domination of Osmium isotopes from the basalts during hydrothermal circulation. The ridge axis fluids typically do not show large enrichment in Osmium concentration over seawater although one high-temperature fluid has an Os concentration enhanced by a factor of 4 above seawater. It appears that the Os concentration of high-temperature hydrothermal fluids is typically buffered at roughly the seawater concentration. We suggest that subseafloor precipitation of pyrite from the high-temperature hydrothermal fluids controls the Osmium transportation. The axial hydrothermal activity does not supply significant amounts of unradiogenic Osmium to the deep oceans. In contrast, a low-temperature off-axis fluid is enriched in non-radiogenic Osmium over seawater by a factor of 9, showing much less precipitation of Osmium at low temperature. Because a large fraction of the cooling of oceanic lithosphere occurs on ridge flanks, Os from low-temperature, ridge-flank hydrothermal circulation may be a significant contributor to the balance of Os in the oceans. A detailed balance between Os contributions from dissolution of cosmic dust and hydrothermal fluids is still not possible. The low-temperature hydrothermal sample gives ¹⁸⁷Os/¹⁸⁸Os = 0.110 ± 0.001. This extremely unradiogenic Osmium can only come from a source that underwent depletion of Re over 2.6 Ga ago. Assuming no contamination during sample collection, this result suggests that the convecting upper mantle contains ancient depleted material that imparted unradiogenic Osmium to intruding basaltic melts.
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Osmium isotopes in hydrothermal uids from the juan de fuca ridge
2000Co-Authors: Mukul SharmaAbstract:We present Os data for axial high-temperature and off-axial low-temperature hydrothermal solutions from the Juan de Fuca Ridge. The high-temperature, H2S-bearing axial fluids have unradiogenic Os isotopes pointing to a nearly complete domination of Osmium isotopes from the basalts during hydrothermal circulation. The ridge axis fluids typically do not show large enrichment in Osmium concentration over seawater although one high-temperature fluid has an Os concentration enhanced by a factor of 4 above seawater. It appears that the Os concentration of hightemperature hydrothermal fluids is typically buffered at roughly the seawater concentration. We suggest that subseafloor precipitation of pyrite from the high-temperature hydrothermal fluids controls the Osmium transportation. The axial hydrothermal activity does not supply significant amounts of unradiogenic Osmium to the deep oceans. In contrast, a low-temperature off-axis fluid is enriched in non-radiogenic Osmium over seawater by a factor of 9, showing much less precipitation of Osmium at low temperature. Because a large fraction of the cooling of oceanic lithosphere occurs on ridge flanks, Os from low-temperature, ridge-flank hydrothermal circulation may be a significant contributor to the balance of Os in the oceans. A detailed balance between Os contributions from dissolution of cosmic dust and hydrothermal fluids is still not possible. The low-temperature hydrothermal sample gives 187 Os/ 188 Os = 0.110 ˛ 0.001. This extremely unradiogenic Osmium can only come from a source that underwent depletion of Re over 2.6 Ga ago. Assuming no contamination during sample collection, this result suggests that the convecting upper mantle contains ancient depleted material that imparted unradiogenic Osmium to intruding basaltic melts. fl 2000 Elsevier Science B.V. All rights reserved.
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himalayan uplift and Osmium isotopes in oceans and rivers
Geochimica et Cosmochimica Acta, 1999Co-Authors: Mukul Sharma, Albrecht W Hofmann, G. J. Wasserburg, Govind J. ChakrapaniAbstract:Previous studies have shown that ^(187)Os/^(188)Os in seawater has become increasingly radiogenic over the last 40 Ma in a manner analogous to strontium. This rapid rise in the marine ^(187)Os/^(188)Os over the last 17 Ma has been attributed to an increase in the bulk silicate weathering rates resulting from the rise of the Himalayas and/or selective weathering and erosion of highly radiogenic organic rich ancient sediments. The key test of this hypothesis is the ^(187)Os/^(188)Os and the total Osmium concentration of the Himalayan rivers. We report the concentration and isotopic composition of Osmium in the Ganges, the Brahmaputra, and the Indus rivers. The ^(187)Os/^(188)Os of the Ganges close to its source (at Kaudiyal, 30°05′N, 78°50′E) is 2.65 and [Os] = 45 fM/kg. A second sample of the lower reaches of the Ganges at Patna (25°30′N, 85°10′E) gives ^(187)Os/^(188)Os =1.59 and [Os] = 171 fM/kg. The ^(187)Os/^(188)Os of the Brahmaputra at Guwahati (26°10′N, 91°58′E) is 1.07 and [Os] = 52 fM/kg. A sample of the Indus (Besham, 34°55′N, 72°51′E) has a ^(187)Os/^(188)Os of 1.2 and [Os] = 59 fM/kg. We infer that the Himalayas do not provide either a high flow of Osmium or a highly radiogenic Osmium component to the oceans. The overall trend for Osmium and strontium could be explained by a regularly increasing input of global continental weathering sources but the Himalayas themselves appear not to be the dominant source.
Boyoung Y Park - One of the best experts on this subject based on the ideXlab platform.
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a metallacycle fragmentation strategy for vinyl transfer from enol carboxylates to secondary alcohol c h bonds via Osmium or ruthenium catalyzed transfer hydrogenation
Journal of the American Chemical Society, 2015Co-Authors: Boyoung Y Park, Tom Luong, Hiroki SatoAbstract:A strategy for catalytic vinyl transfer from enol carboxylates to activated secondary alcohol C–H bonds is described. Using XPhos-modified ruthenium(0) or Osmium(0) complexes, enol carboxylate–carbonyl oxidative coupling forms transient β-acyloxy-oxametallacycles, which eliminate carboxylate to deliver allylic ruthenium(II) or Osmium(II) alkoxides. Reduction of the metal(II) salt via hydrogen transfer from the secondary alcohol reactant releases the product of carbinol C–H vinylation and regenerates ketone and zero-valent catalyst.
