Preliminary Uranium Series Dates on Speleothem in the Kinta Valley and Its Significance in the Karst Landscape Evolution

Geological Sociery of Malaysia Annual Geological Conference 2001 Jun e 2-3 2001 , Pangkor island, Perak Darul Ridzuan, Ma laysia Preliminary uranium series dates on speleothem in the Kinta Valley and its significance in the karst landscape evolution 1 2 1 R6s FATIHAH , PETER SMART AND YEAP EE BENG 1Department of Geology, University of Malaya, 50603 Kuala Lumpur 2Department of Geographical Sciences, University of Bristol Abstract: Uranium series dating technique has been used in dating cave materials and give estimation of up to 500 ka . The 230'fh/ 234U method has been proven to be the most versatil e and useful of all the uranium series methods and has been appli ed to a wide range of materials including speleothems in whi ch the optimal range being around 350 ka using the alpha spectrometer and 500 ka for mass spectrometer. This technique has been used in dating speleothem samples from Kinta Val ley caves. The preliminary ages obtai ned show some in dications that it can be correlated to the rate of denudation in this area. These ages when combined with th e rate of denudation and studi es of slope processes will help in better understanding the evolution of karst land scape. Abstrak: Tekni k siri uranium te lah digunakan dalam penentuan umur bahan-bahan enapan gua dan anggaran umur sehingga 500 ribu tahun diberi. Kaedah 230'fh/ 234U telah dibuktikan ya ng paling versatil dan berg una dalam semua kaedah siri uranium dan telah diaplikasikan kepada pelbagai bahan termas uk speleothem di mana julat optimum adalah sekitar 350 ribu tahun menggunakan alpha spektrometer dan 500 ribu tahun menggunakan mass spektrometer. Teknik ini telah digunakan dalam menentukan umur speleotherm dari gua-gua di Lembah Kinta. Keputusan awal menunjukkan umur umur yang didapati memberi tanda yang ianya boleh digunakan untuk korelasi dengan kadar denudasi di kawasan ini. Umur-umur ini bila digabungkan dengan kadar denudasi dan kajian proses cerun boleh menolong dalam memahami evolu si landskap karst. INTRODUCTION crystal lattice. Fission track studies of such precipitates show that the uranium is homogeneously di stributed Uranium series dating methods have been of great throughout the calcite crystal and not preferentially bound importance in Quaternary geology. This technique is not to crystal in terfaces or are entrapped in the detritus. only applicable to a wide range of geological materials but Uranium is readily oxidised from the 4+ state, in which it also can provide reliable dates of up to 500,000 years in it is fo und as primary U-minerals (such as uraninite, absite, age. Many caves contain layers or concretions of pitchblende and davidite) in igneous rocks, to the 6+ state )2+ inorganically precipitated calcium carbonate or speleothem in the form of the soluble uranyl ion (U02 (Langmui r, as cave floor material or as stalactites and stalagmites. 1978). The oxidized U6+ state is the usual form in carbonate These precipitates can, in many instances, be dated by rocks. This ion is readily soluble, and frequently fo rms measurements of the disequilibrium between the isotopes ani on complexes in natural waters with carbonate and of uranium and their various radioisotopic daughters, over phosphate (S mart, 1991). the time range from about 5,000 to 500,000 years (Schwarcz, Whereas uranium is readily transported in 1980) groundwaters, thorium and protactinium are quickly The 230Th I 234U method has been proven to be the hydrolyzed on dissol ution of their primary host rock (e.g. most er ati1e and useful of all the uranium series methods limestone) and trapped in clay minerals and hydrox ides in and has been applied to a wide range of materials including the soil ; both these elements are present in negligible !jle.leothems in which the optimal range being around 350 concentrations in groundwaters. Therefore calcareous ka using the alpha spectrometer and 500 ka for Mass deposits that are formed by precipitation from solutions Spectrometry (Smart, 1991). (for example speleothems) can contain uranium but are 230 The uranium series age determination of the free of its long-lived daughter isotopes, Th (t 112 = 75,200 23 1 speleothems collected from the limestone hills of the Kinta y) and Pa (t 112 = 32,500 y) . Valley was canied out to bracket so me crucial dates in the With regards to the U-isotopes, the activity ratio of 234 formation of the karstic features. UJ2 38 U is generall y found to be greater than unity in ground waters due to the tendency of 234 U to be selectively GENERAL PRINCIPLES leached from rocks during incipient weathering (Cherdyntsev, 1971). Because of this, therefore the 234 U/ 238 When CaC03 is precipitated from natural waters, U ratio in precipitated carbonate is initially > 1.0. With uranium ions that are present in solution are co-precipitated the passage of time after deposition, the decay of excess 234 with the calcium ions and are trapped in the carbonate U, into secular equilibrium with 238 U will then produce 23 1 296 Ros FATIHAH, PETER SMART AND YEAP EE BENG Pa, 230 Th daughter isotopes. speleothem samples were collected where they would At the time of the formation, speleothems can contain provide a good age constraint on the cave. In practice thi , traces (about 1 ppm) of uranium but are devoid of the meant sampling the basal portion (where possible) of the daughter isotopes 230Th and 231 Pa. Any content of the oldest speleothem in the cave. Although it is impossible to 230Th and 231 Pa that are found subsequently in the always pick the oldest speleothem, often a good idea can be speleothem is due to the decay of the 234U with time. Thus obtained based on the physical condition (a compact the age of the speleothem deposit can be determined from spleothem is preferred), stratigraphic position an the content of these daughter isotopes that are in secular geomorphic location. However, where possible clean, non equilibrium with the uranium. The age of the speleothem porous uncontaminated insitu speleothem was collected. I sample can be calculated based on the equation below order not to cause unnecessary damage to the cave (lvanovich and Harmon, 1992): environment, wherever possible, only broken, damaged 230Th/234U = 1 - e-A.230t/(234U/238U) + and unaesthetic speleothem specimens were taken. A {l-l/(234U/238U)} A.230/(A.230 - A.234) { 1 - minimum of 500 g per sample of the speleothem was e-(A.230-A.234)t} collected using hammers and chisels. For large size where A. denotes decay constant of a given isotope. The speleothem growth in the caves, a portable light weight 230Th half-life is of75.38 ka, giving the 230ThJ234U method corer was used to core to depths of 20 em. The cored holes a potential range of350 ka. Note that a conection is applied are sealed with the drilled powder and white cement. for the presence of 238U, which may maintain the activity Generally the speleothems are rather fragile and using this of 234U, affecting 23 ~hJ2 34 U ratio. method the 25 mrn diameter cores are usually rather broken Detrital contaminants in the deposit may contribute and the longest core is only about 10 em long. For samples 2 3~h and 231Pa as well as uranium all of which could be that are obtained using the corer about 30 em aggregate leached from old detrital clay particles. Other sources of core length is collected per sample and this give weights enor include leaching of U or Th from late recrystalization ranging from 200 to 300 gram per sample. All these and cementation of the deposit by later generations of speleothem samples were packed and hand-carried from carbonates. Methods are proposed to conect these errors. Kuala Lumpur International Airport to the University of Bristol. METHODOLOGY Samples for analysis were prepared in the Soil Laboratory of the Department of Geographical Sciences, Thompson (1973 in Farrant, 1995) was the first to University of Bristol. In the laboratory these samples were demonstrate the validity of the 23 ~hP 3 4 U method and cleaned and cut with a rock saw to select the best portion since then, the technique had been applied to many problems for the analysis. Some samples needed to be crushed and in the earth sciences. The chief dating method that uses the handpicked to obtain the cleanest material. The procedure decay of excess 234U to 230Th has a normal limit of 350 ka of further sample preparation in the laboratory is listed i1l (or approximately ten times the 14C range) by the standard Appendix 1. method of counting the a disintegration of the these isotopes (lvanovich and Harmo~, 1992). Substitution of atom SAMPLE LOCATION AND SIGNIFICANCE counting by mass spectrometry may permit the extension of the detemination limit (Ford and Williams, 1989). Out of the 21 samples of speleothem from Kinta Valley Developments in mass spectrometric U-series techniques and Lenggong area that were brought over to the University has enabled the age to be extended to about 500 ka with of Bristol, only 14 samples were considered suitable for considerable increase in precision (Li et al. , 1989). Age testing. Samples of the speleothems were collected determinations using the U-seiies methodology that have according to their geological and geomorphological been published for sites in Europe and Asia, that have a significance. These speleothems are mainly found in the range between 400,000 years to 12,000 years B.P are cave systems of the limestone hills. The caves and the consistent with estimates based on archeological and containing speleothems were mapped prior to any sampling. geological evidence (Schwarcz, 1980) Where possible, samples were collected from where the The dating of speleothem by the Th/U disequiblibrium stratigraphic relationship between the geomorphological method is now routine and is the method used here.

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