Uranium distribution in Baikal sediments using SSNTD method for paleoclimate reconstruction

Uranium distribution in Baikal sediments using SSNTD method for paleoclimate reconstruction

Radiation Moasurements Radiation Measurements31 (1999)657-660 PERGAMON URANIUM DISTRIBUTION IN BAIKAL SEDIMENTS SSNTD METHOD FOR PALEOCLIMATE RECO...

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Radiation Moasurements

Radiation Measurements31 (1999)657-660

PERGAMON

URANIUM

DISTRIBUTION IN BAIKAL SEDIMENTS SSNTD METHOD FOR PALEOCLIMATE RECONSTRUCTION

USING

S.M. ZHMODIK*, A.G. MIRONOV**,N.A. NEMIROVSKAYA* AND N.S. ZHATNUEV** "United Institute of Geology, Geophysics and Mineralogy of the Siberian Branch of Russian Academy of Science, Novosibirsk, Russia "*Geological Institute of the Siberian Branch of Russian Academy of Science, Ulan-Ude, Russia ABSTRACT First data on local distribution of uranium in the core of Lake Baikal floor sediments (Academician ridge, VER-95-2, St 3 BC, 53°113'I2'N/108°25'01'E) are presented in this paper. They have been obtained using (n,f)-radiography. Various forms of U-occurrence in floor sediments are shown, i.e. evenly disseminated, associated with clayey and diatomaceous components; micro- and macroinclusions of uranium bearing minerals - microlocations with uranium content 10-50 times higher than U-concentrations associated with clayey and diatomaceoaas components. Relative and absolute U-concentration can be determined for every mineral. Signs of various order periodicity of U-distribution in the core of Lake Baikal floor sediments have been found. Using (n,f)-radiography method of the study of Baikal floor sediment permits gathering of new information that can be used at paleoclimate reconstruction. KEYWORDS (n,f)-radiography; uranium; Lake Baikal; floor depositions; paleoclimate. INTRODUCTION Prognosis of the changes of the climate and environment for the next millenium is one of the most important tasks in natural sciences. The only method to verify prognostic models of the climate is their application to climate records of the past, first of all to the climate Holocene and Pleistocene. In recent years complex study of Lake Baikal has significantly improved and extended due to implementation of international projects on climate deciphering of the Earth (Grachev et al., 1997). Lake Baikal is the deepest (over 1.600 meters deep), largest (23.000 cubic kilometers of fresh water) and oldest (20 My BP) lake in the world. The bottom sediments of Lake Baikal are more than 8.000 m. It has been found from the results of these studies that the most sensitive indicators of paleoclimate reconstruction is the microelement distribution, primarily the uranium distribution in Lake Baikal floor depositions. It has been inferred that diatomaceous water-plants which are used as indicators of a warm climate as well as of biogenic silica and organic carbon are in particular positively correlated with uranium and molybdenum (Gavshin et al., 1993). In addition, direct association of U-anomalies in Baikal deep-sea depositions with diatomaceous oozes enriched in humic acids have been established (Gavshin et al, 1993; Edgington et al., 1996). At the same time, analyses of clayey, diatomaceous and terrigenons fractions of Baikal floor sediments extracted by aerodynamic separation method, testify to the sterility of a diatomaceous component related to uranium. 1350-4487/99/$ - see frontmatter© 1999 ElsevierScienceLtd.All rights reserved. PII: S1350-4487(99)00162-6

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The necessity of applying position sensitive methods to the study of the distribution and occurrence of elements in Lake Baikal floor sediments without destruction of the samples becomes evident in spite of thoroughness involved by the use of modem analytical techniques. (n,f)-radiography fully satisfies all these demands, allowing the determination of both U - concentration and the probable form of U-occurrence, due to high spatial resolution and high sensitivity (10-5-10 -6 wt 0/0) and the density of the induced fission tracks (Fleischer et al, 1964). METHODS The first results of U-distribution study in deep-sea sediments of Lake Baikal were obtained by the (n,f)-radiography method. The core from VER-95-2 St 3 BS station, H 265 m, 53°113'12'N / 108°25'01'E, was drilled and Baikal deposits on Academician ridge peak have been taken for study (Grachev et al., 1997). In the given interval of depositions the transition from diatomaceous ooze to clayey deposition with insignificant amounts of diatoms has been observed. A sample 35 iron wide and 15 mm thick was cut out of the core. It suffered cryogenic vacuum drying, followed by epoxidic compound saturation under vacuum. The two polished sample sections have been made from the central part of the cemented floor deposition material. Quartz glass has been used as underlying. Synthetic fluorine-phlogopite and lavsan have been used as solid state nuclear track detectors. The polished sections have been irradiated by 1016 ] c m 2 thermal neutrons in the reactor at the Tomsk Institute of Nuclear Physics. Detectors have been etched by well-known techniques: Lavsan in 40% KOH solution, fluorine-phlogopite in 20% HF solution. JENAVAL microscope has been applied, when studying the distributional character of 235U fission tracks in SSNTD and in counting. The best resolution quality of the tracks have been obtained with synthetic fluorine-phlogopite as SSNTD. For relative U-concentration determination tracks have been counted with 1000x magnification in an area of 0,005 mm 2, out of a 1 mm interval from the central part for relative U-concentration determination. Highly active inclusions have not been considered in the counting of tracks correlating with the disseminated form of U-occurrence. The track number of such mineral-concerttrators and accumulators has been counted particularly to present diagrams of their distribution in the core. These diagrams naturally have a qualitative character and do not reflect the U-concentration in the core of floor sediments as it is done by the distribution of uranium in its disseminated form. RESULTS OF MEASUREMENTS

Fig. 1. Microphotos of the core material of Baikal lake floor sediments. Sections with high concentrations of evenly distributed ("dispersed") uranium represent diatomites with a mixture of clayey minerals and iron hydroxides. Relatively large "fragments" are terrigenous minerals, i.e. magnetite, quartz, feldspars and others. Magnification: a - 250x, b - 500x, c - 1000x. The analyzed material from the Lake Baikal floor deposits was taken at intervals from the Academician ridge and represented itself 5 cm upper of diatomaceous ooze and 10 cm lower of clayey sediment with admixture of diatoms to 10-20 vol.%. The diatomaceous horizon is composed of 85-95%

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diatomaceous water-plants (Fig. 1) and the clayey horizon is predominantly composed of dispersed clayey minerals (perhaps with iron hydroxides) which are brown and less than 50 microns in size. Somewhat large, rare fragments of terrigenous minerals of quartz, feldspar, and ore minerals (magnetite) occur, although the amount is generally greater in clayey part of the core.

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Fig. 2. Concentrations of'dispersed" uranium m core of Baikal Lake sediments (1 ram step) from track data obtained by (n,f)-radiography (curve 3 - uranium concentration sum m 1 mm strip of cross section, curve 4 - tracks quantitative in 1 mm2 of cross section). The cycling in uranium distribution with a period of about 1 thousand year has been observed. The linear trend (curves 1 and 2) can attribute to periodicity of higher order in uranium distribution. The variation of relative U-concentrations (in tracks/mm2) in floor depositions with 1 step mm is presented in Fig. 2. These data testify the presence of both spatial variations of U-concentrations and of those conditioned by continuously periodical factors. Uranium with various types of spatial distribution that can be related to various forms of U-occurrence is fixed along the interval of study. The most characteristic type of U-distribution is even distribution without layering. Such character of Udistribution has been observed both in diatomaceous oozes and diatomaceous clay sediments with 510 times variation of U-concentration (Fig. 3). Another forth also occurs. It is associated with uranium accumulator minerals such as zircon, apatite, ilmenite and others. They occur with high track densities or peculiar fission track "stars" (Fig. 3). Uranium mineral accumulator and uranium-rich microinclusions are more often detected in diatomaceous- clayey and clayey layers of the core, and rarely occur in diatomaceous oozes. In addition, high track densities have been found in clayey-diatomaceous sediments corresponding to regularly distributed U-concentrations exceeding by ten times the ones registered in clayey and diatomaceous components (Fig. 3b). For the present it is established that such "fields" occur over sections rich in diatom valves and can represent formations originating the process of deposition. The presented investigations gave the possibility to get newer information on U-distribution and forms of U-occurrence in Baikal floor sediments using (n,f)-radiography method. The uranium terrigenous component in floor sediments can be clearly observed and counted. Variations of Uconcentration in the process of sedimentation can be determined with accuracy of years and months in

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the studied part of Lake Baikal Academician ridge. The sedimentation rate is 4-6 cm/1000 years (Grachev et al., 1997; Kuzmin et al., 1997) and located by (n,f)-radiography method resolution approximates to the first microns. It permits determination of the form of U-association with diatoms and also their role in U-accumulation by Baikal depositions and in U-distribution in Lake Baikal water. It should be noted that spatial distribution not only of uranium, but boron can be studied by neutron induced alpha-autoradiography method, as well as other elements-using 13-radiography. There is the possibility to use beta-radiography after irradiation of core sample for the study of such element distribution in sediments as cobalt, europium, cesium and scandium. Changing the order of locations ("horizons" in the core) a depletion of the excess minerals has seen discovered.

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Fig. 3. (n,I)-radiographs fixing spatial U-distribution in clayey-diatomaceous (a), clayey (b),(c) sediments. (n,f')-radiography of section with high concentrations of U within clayeydiatomaceous material, (d) - in ilmenite and magnetite, (e),(f),(g) - microinclusions in uraniumbearing minerals. Magnification: 400x - 450x, g - 1000x, neutron fluence 1016n/cm2 .

Acknowledgment -- We would like to thank Prof M.A.Grachev for discussion and practical assistance. The work has been carried out with a financial support of RFBR ; grant - 97-05-96379.

REFERENCES Grachev M.A., Likhoshvai E.V., Vorobyova S.S. and Khlystov A.M. (1997). Signals of Upper pleistocene paleoclimates in Lake Baikal depositions. Geol. and Geophys. 38, N_o5,957-980. Gavshin V.M., Bobrov V.A. and Bogdanov Yu.A. (1993) Uranium anomalies in Lake Baikal deepsea sediments. Reports ofRus.Acad, of Sci. 334, No3, 356-359. Kuzmin M.I., Grachev M.A. and Williams D. (1997). Continuous chronicle of paleoclimates from Lake Baikal for the last 4,5 Ma. Geol. and Geophys. 38, No_5, 1021-1023. Fleischer R.L., Price P.B. and Walker R.M. (1964). Fossil records of nuclear fission. New Scientists 21, No_ 1-2, 29-42. Edgington D.N., Robbins J.A. and Colman S.M. {1996). Uranium-series disequilibirium, sedimentation, diatom frustules and paleoclimate change in Lake Baikal. Earth and Planet.Lett. 142, No_l2, 29-42.