March 2001 A. Choukri et al, Phys. Chem. News 1 (2001) 91-96 PCN 91 INFLUENCE OF URANIUM POST-INCORPORATION ON THE FOSSIL MOLLUSK SHELL AGE REJUVENATION APPLICATION TO THE STUDY OF THE MARINE LEVEL VARIATION IN THE PAST A. Choukri 1* , E. Jahjouh 1 , S. Semghouli 1 , O.-K. Hakam 1 , J.-L. Reyss 2 1 L.F.R.E, Université Ibn Tofail, Faculté des Sciences Département de physique, B. P. 133, Kénitra, Morocco 2 L.S.C.E, Avenue de la Terrasse 91198, Gif sur Yvette Cedex, France * Corresponding author. E-mail : choukrimajid@yahoo.com Received 20 October 2000 Abstract We discuss two models based on episodic and continual uranium migration into mollusk shells after death and give a relationship between the apparent and real ages. For the episodic model, we discuss this relationship according to the uranium post-incorporation rate, the initial 234 U/ 238 U activity ratio and the time of incorporation. We test both models on some of mollusk shells taken from four fossil marine shorelines on the Atlantic coast of the Moroccan High Atlas assumed formed during the last interglacial stage about 122 000 years ago. We try to draw a general conclusion concerning the unreliability of mollusk shell samples for uranium series dating. Keywords: Uranium series dating; Open system; Continual uranium post-incorporation model into fossil mollusk shells; Episodic uranium post-incorporation model into fossil mollusk shells. 1. Introduction U-series measurements of reef corals and fossil mollusk shells by α-particle counting and thermal ionization mass spectrometry have been used to unravel the time scale and regional specificity of sea level changes during the last 350 000 years. In contrast to corals, mollusk shells contain little authigenic U: their bulk U content represents essentially early diagenetic uptake. Several authors [1-3] concluded that the 230 Th/ 234 U ages are generally unreliable compared to those of unrecrystallized coral samples. Kaufman et al. [1] ascribed this failure to the post-mortem migration of U into mollusk shells. The U concentrations of fossil shells are usually higher than in living shells and the 234 U/ 238 U ratios are, in most cases, higher than would be possible if their U were incorporated from seawater. A number of models based on certain geochemical assumptions have been used to correct rejuvenated fossil mollusk shell or fossil bone and teeth ages, but the results obtained showed that these models can not be applied to samples taken from the same site [4-9]. In this paper, we show that U is generally trapped by the mollusk shell system after death and fossilization. Then we discuss the relationship between real and apparent ages in the three following cases: • The ideal case where U is assimilated into system soon after burial, with the mollusk shell remaining a closed system thereafter. • The case of a continual and linear U post- incorporation. This model assumes that the U concentration increases proportionally with fossilization time. • The case of an episodic U uptake by the mollusk shell during short period of time between fossilization and the present time. In this case, we study the influence of the incorporated U / authigenic U rate (Ke) and the U uptake age / real age ratio (D) on the relationship between the real and apparent ages. In the three cases, we determine the influence of 234 U/ 238 U activity ratio on this relationship. 2. Evidence of U post-incorporation by fossil mollusk shells We have analyzed 47 samples of mollusk shells taken from four sites assumed to be geologically developed during the last interglacial period (isotopic stage 5e). The sampled sites are situated on the Atlantic Coast of the Moroccan High Atlas north of the city of Agadir. The 5e stage has been dated by several authors in most marine shorelines at the world on unrecrystalized coral samples at about 122 000 years [10,11]. Fig. 1 shows that the ages obtained are in most cases rejuvenated compared to the expected age of 122 000 years for the sampled marine shorelines. Because of the great stability of Th behavior in natural conditions, the rejuvenation of fossil mollusk shell ages can be explained only by an early uptake of U by their systems after death. This post-incorporation could occur at any moment during the burial history.