C-13 mineralogy and their relevance to C-14 dating of carbonates in a lacustrine system: Tigalmamine lakes, Morocco
by
Abdelfattah Benkaddour
Université Cadi Ayyad, Faculté des Sciences et Techniques, Marrakech, MAROC

Accurate dating is of fundamental importance to paleoclimatic reconstruction. It is self-evident that a contaminated sample will give an erroneous date. At the lacustrine site of Tigalmamine, where old carbonates (Liassic dolomite) are present, contamination is one of the important problems facing paleohydrology and paleoclimatic reconstruction. Two sources of contamination are dominant: 1) Detrital carbonates. These cases are clearly shown in the ¹³C and ¹⁸O profiles, in the mineralogical nature of the sample (containing some dolomite) and by the incompatibility of the dates obtained on both carbonates and organic matter from the same levels. Knowing the amount of the detrital component we can consider the sample analysed as a mixing of authigenic carbonate with detrital carbonate. The calculation of the faction (x) of detrital member in a given sample may be calculated as following : xAD + (1-x)AC = AM

Where: x : detrital component, AD : ¹⁴C activity of the detrital carbonate, which is about 0 pmc, AC : ¹⁴C activity of the authigenic carbonate, AM : Measured ¹⁴C activity. For estimating (x), two methods are used: a) assessment of the dolomite amount, b) using the ¹³C balance. 2) Dissolved HCO^3- derived from old, inert sources, and incorporated into authigenic calcite. TDIC (Total Dissolved Inorganic Carbon) from Tigalmamine gave an age of about 650 to 750 years BP. In this case we can deduct about 700 years from the apparent ages. However, this method does not take in account CO² exchanges between lake water and the atmosphere. Authigenic calcite precipitated in isotopic equilibrium with the water would have ¹³C around +2 ‰vPDB. A supposed authigenic calcite from Tigalmamine shows low ¹³C contents, from -6.35 to -9.41 ‰ vPDB. These variations are due to the influence of different sources of carbon in the lake. Thus, it is excluded from the interpretetation of ¹⁴C ages without taking the ¹³C values into account. Here, we adopt Fontes's model, which includes variation in CO² exchange between the lake and the atmosphere through time.Corrected ages are systematically younger than the measured ages. The differences vary between 300 to 700 years. They also are also in agreement with ages obtained on terrestrial organic matter.

Date received: April 24, 2001

Copyright © 2001 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # cahi-09.