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Licensed Unlicensed Requires Authentication Published by De Gruyter (O) September 25, 2009

A statistical study of 238U and 234U/238U distributions in coral samples from the Egyptian shoreline of the north-western Red Sea and in fossil mollusk shells from the Atlantic coast of High Atlas in Morocco: Implications for 230Th/234U dating

  • A. Choukri , J.-L. Reyss , O.K. Hakam and J. C. Plaziat
From the journal Radiochimica Acta

Summary

In this work, radiochemical analysis results of 126 unrecrystallized coral samples from the Egyptian shoreline of northwestern Red Sea and 120 fossil mollusk shell samples from the Atlantic coast of Moroccan High Atlas at the North of Agadir City in Morocco are presented and discussed. The coral samples were collected in Egypt from the emerged coral reef terraces over 500 km from The Ras Gharib-Ras Shukeir depression (28°10') in the north to Wadi Lahami (north of Ras Banas, 24°10') in the south. The fossil mollusk shells were collected in Morocco from Agadir-Harbour in the south to Tamri village in the north extending over about 50 km. The statistical distributions of results (238U content, 234U/238U activity ratio and ages) obtained on the dated materials in the two different regions were compared for three fossil sea levels corresponding to three different climatic stages (Holocene, 5e, 7 and/or 9) in the aim to establish methodological criteria for judging validity of the measured ages.

For corals, 238U content varies in narrow interval around the same average value of 3 ppm for the three sea levels, the calculated initial 234U/238U values are in agreement with the actual sea water ratio (1.15) with some values slightly higher than for the older sea levels. The obtained ages are in good agreement with the ages reported previously for the three emerged fossil sea levels on unrecrystallized corals by alpha spectrometry and by mass spectrometry.

For mollusk shells, except for Holocene sea level, 238U and initial 234U/238U activity ratios vary for the older levels in wide intervals, independent of species and calcite contents of samples. The high 238U contents and 234U/238U activity ratio are due eventually to a post-incorporation of secondary uranium from sea water or from continental waters drained away rivers. This incorporation leads to a rejuvenation of mollusk shell ages and is responsible for the wide dispersion of their apparent ages according to the mode and the timing of uranium introduction.

Received: 2001-7-2
Accepted: 2002-1-28
Published Online: 2009-9-25
Published in Print: 2002-6-1

© Oldenbourg Wissenschaftsverlag, München

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