1,721,116 research outputs found
Investigation of Thallium isotope fractionation during sorption to Mn oxides (abstract of paper presented at: 18th Annual V. M. Goldschmidt Conference Vancouver, Canada, July 2008)
No full textRetreat of the Laurentide ice sheet tracked by the isotopic composition of Pb in western North Atlantic seawater during termination 1
Full text linkDuring the Last Glacial Maximum much of North America was covered by the Laurentide ice sheet. Its melting during termination 1 led to systematic changes in proglacial lake formation, continental runoff, and possibly North Atlantic Meridional Overturning Circulation. The accompanying change in chemical weathering rates in the interior of North America throughout the deglaciation resulted in a pronounced change in seawater Pb isotope composition in the western North Atlantic Ocean. Here we present the first high-resolution records of seawater Pb isotope variations of North Atlantic Deep Water extracted from authigenic Fe–Mn oxyhydroxides in three sediment cores (51GGC, 1790 m depth; 31GGC, 3410 m depth; 12JPC, 4250 m depth) from the Blake Ridge off Florida. These data reveal a striking excursion from relatively unradiogenic 206Pb/204Pb as low as 18.93 towards highly radiogenic Pb isotope compositions that was initiated during the Bølling–Allerød interstadial and was most pronounced in both intermediate and deep waters during and after the Younger Dryas (206Pb/204Pb as high as 19.38 at 8.8 ka in 4250 m). This pattern is interpreted to be a direct function of increased inflow of continent-derived radiogenic Pb into the western North Atlantic, supplied through chemical weathering of North American rocks that had been eroded and freshly exposed during the preceding glacial cycle. These sediment-derived data are complemented by new laser ablation Pb isotope data from a ferromanganese crust from the Blake Plateau at 850 m water depth, which show only small glacial–interglacial Pb isotope variations of the Florida Current (206Pb/204Pb between 19.07 and 19.16). The lack of change in the Blake Plateau record at the same time as the radiogenic excursion in the deeper sediments supports a northern origin of the pulse of radiogenic Pb. After the Younger Dryas, the deep western North Atlantic has experienced a persistent highly radiogenic Pb supply that was most pronounced during the first half of the Holocene and still lasts until today
Opening the gateways for diatoms primes Earth for Antarctic glaciation
No full textThe abrupt onset of Antarctic glaciation during the Eocene–Oligocene Transition (?33.7 Ma, Oi1) is linked to declining atmospheric pCO2 levels, yet the mechanisms that forced pCO2 decline remain elusive. Biogenic silicon cycling is inextricably linked to both long and short term carbon cycling through the diatoms, siliceous walled autotrophs which today account for up to 40% of primary production. It is hypothesised that during the Late Eocene a sharp rise in diatom abundance could have contributed to pCO2 drawdown and global cooling by increasing the proportion of organic carbon buried in marine sediment. Diatom and sponge silicon isotope ratios (?30Si) are here combined for the first time to reconstruct the late Eocene–early Oligocene ocean silicon cycle and provide new insight into the role of diatom productivity in Antarctic glaciation. At ODP site 1090 in the Southern Ocean, a 0.6‰ rise in diatom ?30Si through the late Eocene documents increasing diatom silicic acid utilisation with high, near modern values attained by the earliest Oligocene. A concomitant 1.5‰ decline in sponge ?30Si at ODP site 689 on the Maud Rise tracks an approximate doubling of intermediate depth silicic acid concentration in the high southern latitudes. Intermediate depth silicic acid concentration peaked at ?31.5 Ma, coincident with the final establishment of a deepwater pathway through the Tasman Gateway and Drake Passage. These results suggest that upwelling intensification related to the spin-up of a circum-Antarctic current may have driven late Eocene diatom proliferation. Organic carbon burial associated with higher diatom abundance and export provides a mechanism that can account for pCO2 drawdown not only at, but also prior to, Antarctic glaciation as required by a pCO2 ‘threshold’ mechanism for ice sheet growth
Controls on the incongruent release of hafnium during weathering of metamorphic and sedimentary catchments
No full textIt is well established that Hf weathers incongruently such that the isotopic compositions in seawater are offset from those of Nd relative to the correlation defined by bulk lithologies of the continental crust. Here we study this process in detail with new records of the seasonal variability of isotope compositions and concentrations of Hf and Nd in four Swiss rivers. The water has been filtered at a pore size of 0.45 mu m and therefore represents the truly dissolved and the colloidal pool of both elements. The studied rivers drain metamorphic (gneissic) or sedimentary (mixed carbonate/siliciclastic) lithologies. The dissolved isotope data are compared to the isotope compositions and concentrations of the suspended load and different fractions of the actual source rocks in the respective catchments, as well as to concomitant changes in the aqueous chemistry of the major elements.
Dissolved Nd concentrations span similar ranges for all rivers, whereas Hf concentrations are one order of magnitude lower in the rivers that drain gneissic catchments compared to those draining sedimentary rocks. This primarily results from the retention of most of the Hf in the gneissic zircons, as indicated by the Hf budget of the gneisses, whereas Hf in the sedimentary catchments is readily weathered from fine detrital silicates.
Large differences are found between the dissolved Hf isotope compositions of the rivers and those of the suspended load and the source rocks, consistent with the release of Hf from a radiogenic rock fraction during weathering. In the metamorphic catchments this primarily reflects that fact that zircons are barely accessible for weathering. The zircon-free portion of the rocks appears to weather congruently as the riverine Hf isotope compositions are similar to the zircon-free portion of the gneisses, rather than being distinctly more radiogenic. Leaching experiments performed to understand the riverine Hf budget in the sedimentary catchments reveal that the carbonate fraction of the sedimentary rocks is extremely radiogenic, yielding Hf isotope compositions up to epsilon(Hf) of + 208. However, the Hf concentrations in the carbonate fractions are too low to dominate the riverine Hf budget, which is instead controlled by the weathering of detrital silicate minerals.
Two of the catchments, a metamorphic and a sedimentary one, show relatively systematic changes towards more radiogenic dissolved Hf isotope compositions as discharge increases. This suggests that continental runoff conditions could be a relevant parameter for the control of the seawater Hf isotope composition, whereby more congruent weathering is achieved during low discharge when Hf is increasingly derived from weathering-resistant unradiogenic mineral
A chemical, isotopic and geochronological study of the Cameroon line, West Africa.
No full textThe Cameroon line is defined by a 1600 km long chain of intraplate volcanoes and older plutonic complexes that extend from the Gulf of Guinea into the continental interior of west Africa. A combined chemical, chronological and isotopic study of the Cameroon line has been undertaken to determine the compositions of the magma sources, how these have developed with time, the differences between oceanic and continental lithospheric mantle in this region and the causes of melting. The oceanic sector of the Cameroon line displays isotopic variations in time and space despite reasonably uniform trace element chemistry. The earliest magmatism within the oceanic sector decreases in age oceanward. This age progression is consistent with the suggested motion of the African Plate over the same period of time, which, combined with isotopic data, implies that the magmatism on each island was initiated by a deep mantle plume with PREMA isotopic signatures that has been gradually modified by a second "St. Helena type" HIMU component, yet to affect Pagalu. Most continental basalts from the Cameroon line show trace element and isotopic compositions similar to the oceanic basalts, implying negligible crustal contamination. Isotopic variations in the continental basalts are explicable by mixing between a plume derived "mixed PREMA-HIMU" component analogous to a mixture of the components resolvable in the oceanic sector and a second HIMU component with higher Sr and lower Nd, typically developed at the continent/ocean boundary ("c.o.b. type HIMU"). The continental lithosphere of the Cameroon line is characterized by very depleted isotopic compositions and was not involved in the genesis of the Cameroon line magmas. Some xenoliths display evidence of metasomatism, resulting U and LREE enrichments and the presence of trace amphibole. The timing of the metasomatism has been estimated as Mesozoic, similar in age to an event recorded in the Pb isotopic data for the Cameroon line basalts. This study has resulted in a new model for the Cameroon line and shed light on the manner in which heterogeneities are produced within the upper mantle.PhDGeologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/103985/1/9423242.pdfDescription of 9423242.pdf : Restricted to UM users only
A geochemical study of volcanism associated with the early stages of continental rifting in northern Tanzania.
No full textThe volcanic province of northern Tanzania lies at the southern end of the eastern branch of the East African Rift. In Kenya the underlying lithosphere is Proterozoic in age, while in northern Tanzania the rift intersects the Archean Tanzanian Craton. A petrological, isotopic and chronological study was undertaken to determine the source of the volcanic rocks and the relationship between rifting and volcanism. The isotopic compositions of the volcanic rocks indicate that the source is located in the chemically heterogeneous continental lithospheric mantle. Trace element concentrations in the northern Tanzanian lavas suggest that the continental lithospheric mantle in this area has been metasomatized by small degree partial melts from the asthenospheric mantle. Many of the mineral grains present in the samples were not in isotopic or chemical equilibrium with the magma represented by the whole rock chemical and isotopic compositions. This complex petrogenetic history is consistent with the small volume of lava produced in northern Tanzania. It is probable that many mantle-derived melts freeze before they reach the surface and these solidified magmas are the source of the xenocrysts in the erupted lavas. If the volcanic rocks in northern Tanzania are being generated in a rising mantle plume, the direction of the initiation of volcanism should be consistent with the movement of the African plate over this time. Ages obtained for the volcanoes in northern Tanzania show no consistent geographic trend; there is no evidence of a plume track. If the lavas are being generated in the continental lithospheric mantle, the heat source may be a mantle plume but geographic location of volcanism may be controlled by the location of low melting temperature material in the lithospheric mantle. Mantle xenoliths from the Labait cone display a range of isotopic compositions similar to those measured in the northern Tanzanian lavas. While melt from an individual xenolith has isotopic compositions that match those of the lavas, the xenoliths represent a potential heterogeneous source for the northern Tanzanian lavas.PhDGeologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/104847/1/9610217.pdfDescription of 9610217.pdf : Restricted to UM users only
Uranium-series disequilibrium dating using energy-filtered ICP magnetic sector multiple collector mass spectrometry.
No full textThe uranium-series disequilibrium dating of pure and impure carbonate has been investigated using energy-filtered MC-ICPMS (Multiple Collector ICP Mass Spectrometry). A new chemical procedure using TRU-Spec resin has been developed, which achieves both excellent chemical yields and a clean separation of Th and U from one another and other elements. High precision uranium and thorium isotopic analytical protocols have also been established using MC-ICPMS. Mass spectrometric dating results of microatolls from the South China Sea are obtained for the first time in this study. U-Th ages for these Holocene corals are consistently older than their corresponding 14C ages. These ages agree well with independently determined histories of local sea-level changes. High 232Th contents in these samples are common which is considered to be a result of mechanical entrapment of very fine-grained crustal materials inside coral skeletons. The derived average initial 234U/238U ratio is consistent with other reported values within analytical errors. Our results also show that the sea level history of the South China Sea region is complicated and shows high frequency variations on a timescale of 1,000 years. The total sample dissolution method for impure carbonate dating is theoretically and experimentally investigated. It is concluded that derivation of a valid age requires that the authigenic end member is completely free of any 232Th or that the other end member has a very large 232Th/ 238U ratio compared to that of the authigenic component. A good linearity for total sample dissolution regression lines is a necessary condition but does not guarantee the validity of a derived age. Fossil land snail shells collected from paleosols and loess in the Chinese Loess Plateau were found to contain enough uranium to allow the accurate determination of 230Th ages, indicating uranium uptake soon after burial. Large quantities of 232Th are commonly observed for all snail shell samples. Three correction models are proposed to estimate the ages for fossil snail shells from the Chinese loess plateau.PhDEarth SciencesGeochemistryPaleontologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/131718/2/9929890.pd
Reliable extraction of a deepwater trace metal isotope signal from Fe-Mn oxyhydroxide coatings of marine sediments
No full textThe extraction of a deepwater radiogenic isotope signal from marine sediments is a powerful, though under-exploited, tool for the characterisation of past climates and modes of ocean circulation. The radiogenic and radioactive isotope compositions (Nd, Pb, Th) of ambient deepwater are stored in authigenic Fe–Mn oxyhydroxide coatings in marine sediments, but the unambiguous separation of the isotopic signal in this phase from other sedimentary components is difficult and measures are needed to ensure its seawater origin. Here the extracted Fe–Mn oxyhydroxide phase is investigated geochemically and isotopically in order to constrain the potential and the limitations of the reconstruction of deepwater radiogenic isotope compositions from marine sediments.Our results show that the isotope compositions of elements such as Sr and Os obtained from the Fe–Mn oxyhydroxide fraction are easily disturbed by detrital contributions originating from the extraction process, whereas the seawater isotope compositions of Nd, Pb and Th can be reliably extracted from marine sediments in the North Atlantic. The main reason is that the Nd, Pb and Th concentrations in the detrital phase of pelagic sediments are much lower than in the Fe–Mn oxyhydroxide fractions. This is reflected in Al/Nd, Al/Pb and Al/Th ratios of the Fe–Mn oxyhydroxide fractions, which are as low as or even lower than those of hydrogenetic ferromanganese crusts. Mass balance calculations illustrate that the use of the 87Sr/86Sr isotope composition to confirm the seawater origin of the extracted Nd, Pb and Th isotope signals is misleading. Even though the 87Sr/86Sr in the Fe–Mn oxyhydroxide fractions is often higher than the seawater Sr isotope composition, the corresponding detrital contribution does not translate into altered seawater Nd, Pb and Th isotope compositions due to mass balance constraints. Overall the rare earth element patterns, elemental ratios, as well as the mass balance calculations presented here highlight the potential of using authigenic Fe–Mn oxyhydroxide coatings as paleoceanographic archives for the analysis of past seawater Nd, Pb and Th isotope compositions
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