Radiochim. Acta 90, 329–336 (2002)  by Oldenbourg Wissenschaftsverlag, München A statistical study of 238 U and 234 U/ 238 U 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 230 Th/ 234 U dating By Abdelmajid Choukri 1 , ∗ , Jean-Louis Reyss 2 , Oum-Keltoum Hakam 1 and Jean-Claude Plaziat 3 1 Laboratoire des Faibles Radioactivit´ es et d’Environnements, UFR: Faibles Radioactivit´ es, Math´ ematiques physiques et environnement, Facult´ e des Sciences, P.B 133, K´ enitra, 14000, Maroc 2 Laboratoire des Sciences de Climat et de l’Environnement,Domaine du CNRS, Avenue de la Terrasse 91, F-91198, Gif sur Yvette, France 3 Universit´ e de Paris-Sud, D´ epartement des Sciences de la Terre, URA 723, Bâtiment 504, F-91405,Orsay, cedex, France (Received July 2, 2001; accepted in final form January 28, 2002) U-series dating / Sea level / Coral / Mollusc shells / Quaternary / Atlantic coast of High Atlas / Egyptian shoreline of Red Sea 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 col- lected in Morocco from Agadir-Harbour in the south to Tamri village in the north extending over about 50 km. The statistical distributions of results ( 238 U content, 234 U/ 238 U activity ratio and ages) obtained on the dated materials in the two different regions were compared for three fossil sea levels correspond- ing to three different climatic stages (Holocene, 5e, 7 and/or 9) in the aim to establish methodological criteria for judging va- lidity of the measured ages. For corals, 238 U content varies in narrow interval around the same average value of 3 ppm for the three sea levels, the calcu- lated initial 234 U/ 238 U 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, 238 U and initial 234 U/ 238 U activity ratios vary for the older levels in wide intervals, independent of species and calcite contents of sam- ples. The high 238 U contents and 234 U/ 238 U 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 ap- parent ages according to the mode and the timing of uranium introduction. *Author for correspondence (E-mail: choukrimajid@yahoo.com). 1. Introduction The Quaternary period of the geological record is character- ized by episodic fluctuations in global climate. The physical processes accompanying these climatic changes have pro- duced a wide variety of geological and geomorphological features which have been studied in detail and are used to reconstruct the paleoclimatic history. Several methods based on nuclear phenomena have been developed and applied to date fossils in the deposits occurred in the past [1–3]. Among these methods, we cite particularly the 230 Th/ 234 U method based on the measurement of uranium and thorium radioisotope activities and activity ratios. After the establishment of the relationship between 230 Th/ 238 U ratio activity and age in coral samples [4, 5] and after the discovery of the 234 U/ 238 U activity ratio disequi- librium in natural waters [6] and in sea water [7], Broecker derived the 230 Th/ 234 U age equation taking into account that the initial 234 U/ 238 U activity ratio uptaken by corals from the sea water differ from unity [8]. After considerable develop- ment of alpha-spectrometry [9–12] and mass spectrometric dating techniques [13, 14], they are applied up to now to fossil corals as a means of determining the timing of Pleis- tocene sea level fluctuations. In case of absence of corals and in spite of the pa- per by Kaufman et al. [15] demonstrating that U-series ages derived from fossil mollusk are extremely unreli- able, these samples have been used to determine the sea level fluctuation ages and to establish the stratigraphic scale of palaeoenvironmental change for some coasts in the world. In the Western Mediterranean Basin, follow- ing an early study by Stearns and Thurber [16] in Mo- rocco, a large body of U-series data, derived from sev- eral species of mollusk, has accumulated as a result of work by Bernat et al. [17], Goy et al. [18], Hillaire- Marcel et al. [19, 20], Causse et al. [21], Hearty et al. [22], El Gharbaoui et al. [23] and Choukri [12]. In contrast to unrecrystallized corals regarded as ideal dating material in the marine environment, chronological Brought to you by | New York University Bobst Library Technical Services Authenticated Download Date | 5/28/15 3:22 PM