Isotopic feature and uranium dating of the volcanic rocks in the Okinawa Trough

Volcanic rocks from the northern and middle Okinawa Trough were dated by uranium-series dating method. Differential fractions using magnetic procedure were designed to separate samples. New report on the ages and isotopic data of rocks in the northern trough (especially black pumice) was discussed. Based on the uranium dates and Sr-Nd isotopic ratio, magmatic evolution process of the Okinawa Trough was noted. Firstly, there have been wide silicic volcanic activities in the Okinawa Trough from late Pleistocene to present, and the volcanic rocks can be divided into three subgroups. Secondly, magma generally came from PREMA source area under the Okinawa Trough. Magmatic evolution in the northern trough was similar to the middle, but different to the south. Finally, volcanic activities indicated that opening of the southern Okinawa Trough did not happen due to the collision between Luson Arc and Eurasian Plate until the early Pleistocene.

(Table 2) Uranium series results of Villars Cave stalagmite Vil-stm14

We present a new high resolution speleothem stable isotope record from the Villars Cave (SW-France) that covers part of marine isotope stage (MIS) 3. The Vil14 stalagmite grew between ~52 and 29 ka. The d13C profile is used as a palaeoclimate proxy and clearly shows the interstadial substages 13, 12 and 11. The new results complement and corroborate previously published stalagmite records Vil9 and Vil27 from the same site. The Vil14 stalagmite chronology is based on 12 Th-U dating by MC-ICP-MS and 3 by TIMS. A correction for detrital contamination was done using the 230Th/232Th activity ratio measured on clay collected in Villars Cave. The Vil14 results reveal that the onset of Dansgaard-Oeschger (DO) events 13 and 12 occurred at ~49.8 ka and ~47.8 ka, respectively. Within uncertainties, this is coherent with the latest NorthGRIP time scale (GICC05-60 ka) and with speleothem records from Central Alps. Our data show an abrupt d13C increase at the end of DO events 14 to 12 which coincides with a petrographical discontinuity probably due to a rapid cooling. As observed for Vil9 and Vil27, Vil14 growth significantly slowed down after ~ 42 ka and finally stopped ~ 29 ka ago where the d13C increase suggests a strong climate deterioration that coincides with both North Atlantic sea level and sea surface temperature drop.

Secondary uranium mineralization in southern Finland and its relationship to recent glacial events

Uranium series dating has been carried out on secondary uranyl silicate minerals formed during sub-glacial and post-glacial weathering of Proterozoic uraninite ores in south west Finland. The samples were obtained from two sites adjacent to the Salpauselkä III ice marginal formation and cover a range of depths, from the surface to more than 60 m. Measured ages fall into three distinct groups, 70–100 ka, 28–36 ka and < 2500 yr. The youngest set is associated with surface exposures and the crystals display clear evidence of re-working. The most likely trigger for uranium release at depths below the surface weathering zone is intrusion of oxidising glacial melt water. The latter is often characterised by very high discharge rates along channels, which close once the overpressure generated at the ice margin is released. There is excellent correspondence between the two Finnish sites and published data for similar deposits over a large area of southern and central Sweden. None of the seventy samples analysed gave a U–Th age between 40 and 70 ka; a second hiatus is apparent at 20 ka, coinciding with the Last Glacial Maximum. Thus, the process responsible for uranyl silicate formation was halted for significant periods, owing to a change in geochemical conditions or the hydrogeological regime. These data support the presence of interstadial conditions during the Early and Middle Weichselian since in the absence of major climatic perturbations the uranium phases at depth are stable. When viewed in conjunction with proxy data from mammoth remains it would appear that the region was ice-free prior to the Last Glacial Maximum.

Uranium series disequilibria in ground waters from a fractured bedrock aquifer (Morungaba Granitoids-Southern Brazil): Implications to the hydrochemical behavior of dissolved U and Ra

Activity concentrations of dissolved U-234, U-238, Ra-226 and Ra-228 were determined in ground waters fromtwo deep wells drilled in Morungaba Granitoids (Southern Brazil). Sampling was done monthly for little longer than 1 year. Significant disequilibrium between U-238, U-234 and Ra-226 were observed in all samples. The variation of U-238 and U-234 activity concentrations and U-234/U-238 activity ratios is related to seasonal changes. Although the distance between the two wells is short (about 900m), systematic differences of activity concentrations of U isotopes, as well as of U-234/U-238, Ra-226/U-234 and Ra-228/Ra-226 activity ratios were noticed, indicating distinct host rock-water interactions. Slightly acidic ground water percolation through heterogeneous host rock, associated with different recharge processes, may explain uranium and radium isotope behavior. (c) 2008 Elsevier Ltd. All rights reserved.

Uranium-lead dating method at the Pará-Iso isotope geology laboratory, UFPA, Belém – Brazil

ABSTRACT: Analytical procedures for U-Pb isotope dilution analyses at the Pará-Iso isotope geology laboratory of the Federal University of Pará (UFPA) are described in detail. The procedures are applied to zircon, titanite, rutile, apatite, columbite-tantalite and whole rock. Reagent preparation and chemical processing are done in clean-room conditions. Samples are dissolved using Teflon™ microcapsules in steel jacket Teflon™ Parr Instrument™ bomb or Teflon™ screw cap containers. U and Pb are separated using anion exchange AG 1x8 resin columns. Typical blanks for mineral sample amounts of 0.01 to 1.0 mg are less than 1 pg U and 20-30 pg Pb. Isotope analysis of the U and Pb from the same filament are carried out using a Finnigan MAT 262 mass-spectrometer in static and dynamic modes. The current analytical level is demonstrated on analyses of international standard zircon 91500 with three different ²³⁵U-²⁰⁵Pb and ²³⁵U-²⁰⁸Pb isotope tracers and whole rock standards. Results of analyses of two zircon samples are also presented.

Uranium-Thorium dating and sea surface temperatures from corals from Hainan Island, South China Sea

Three mid-Holocene sea surface temperature (SST) records spanning more than 30 years were reconstructed for the northern South China Sea using Sr/Ca ratios in Porites corals. The results indicate warmer than present climates between circa 6100 yr B.P. and circa 6500 yr B.P. with the mid-Holocene average minimum monthly winter SSTs, the average maximum monthly summer SSTs, and the average annual SSTs being about 0.5°-1.4°C, 0°-2.0°C, and 0.2°-1.5°C higher, respectively, than they were during 1970-1994. Summer SSTs decrease from circa 6500 yr B.P. to circa 6100 yr B.P. with a minimum centered at circa 6300 yr B.P. The higher average summer SSTs are consistent with a stronger summer monsoon during the mid-Holocene, and the decreasing trend indicates a secular decrease of summer monsoon strength, which reflects the change in summer insolation in the Northern Hemisphere. El Niño-Southern Oscillation (ENSO) cycles were apparent in both the mid-Holocene coral and modern instrumental records. However, the ENSO variability in the mid-Holocene SSTs was weaker than that in the modern record, and the SST record with the highest summer temperatures from circa 6460 yr B.P. to 6496 yr B.P. shows no robust ENSO cycle. This agrees with other studies that indicate that stronger summer monsoon circulation may have been associated with suppressed ENSO variability during the mid-Holocene.

Dating sediments in coastal areas and potential applications for constraining the palaeo-environmental context of an archaeological site at Vohemar

We present an overview on different environmental zones within coastal areas and summarise the physical basis behind the three most important methods that are available to date Holocene coastal sediments. Besides radiocarbon and uranium series dating, Optically Stimulated Luminescence (Osl) has increasingly been applied for dating in coastal settings over the past decade. This is illustrated by a number of case studies showing that Osl can be applied to sediments from almost any kind of coastal environment, covering a potential dating range from some years up to several hundred thousand years. Osl dating may hence be the method of choice for deciphering natural environmental change along coasts as well as the presence and the impact of human occupation in such areas. In addition, we briefly show how and where these dating methods could be applied to constrain the palaeo-environmental context of an archaeological site at Vohemar in north-eastern Madagascar.

U-series dating of Locality 15 at Zhoukoudian, China, and implications for hominid evolution

This paper reports U-series dates on speleothem samples from Locality 15 at Zhoukoudian, one of the richest Paleolithic sites in northern China. The age of the lower part of Layer 2 is securely bracketed between 155,000 and 284,000 yr. The underlying Layer 3 dates back at least 284,000 yr. Layer 4, further below, should be older still, possibly by a cycle on the SPECMAP time scale before 284,000 yr ago. These ages, much greater than the previous estimates of 110,000-140,000 yr from U-series and electron spin resonance dating of fossil teeth, suggest that Locality 15 was broadly contemporaneous with Locality 4 (New Cave) and with the uppermost strata of Locality 1 (Peking Man site). The physical evolution and cultural development evidenced by the timing of the Zhoukoudian localities are in line with the opinion of Chinese anthropologists for a regional transition from Homo erectus to archaic Homo sapiens. (C) 2004 University of Washington. All rights reserved.