U-Th isochron dating of pedogenic impure carbonates: a first attempt of heavy liquor separation.
Résumé
Pedogenic carbonates and their geochemical composition are pertinent proxies of paleoclimates, paleoenvironments,
and past Earth surface processes [1]. But dating pedogenic carbonate remains a challenge due to re-precipitation
processes [2]. Radiocarbon dating is limited to 55 ka, making attractive the use of U-Th dating method (up to 500 ka).
However, pedogenic carbonates are impure by nature making U-Th dating a methodological and analytical issue. The Total
Sample Dissolution technic has been proved to be the only method capable to properly correct for the detrital Th
component [3]. An alternative method based on physical separation of detrital and authigenic phases using heavy
liquor has been performed successfully on non-consolidated carbonate marine sediments [4]. Nevertheless, no attempt was
made on indurated impure carbonates. We tested for the first time the heavy liquor separation on two Holocene pedogenic
carbonate nodules from Far North Cameroon, where the geological settings and the processes that have led to their
formation are well-constrained [5]; this area is thus ideal to test a new method and to compare it with existing
radiocarbon ages.
Pedogenic carbonate nodules include three main components: i) authigenic calcite; ii) granite saprolite; and iii)
Saharan dust. The conventional method of isochrones is based on a two end-member mixing (i.e. authigenic phase and
detrital phase). This model is no longer applicable in the present case. We attempt to separate the different
mineralogical phases by using heavy liquor in order to reduce the system to a two-pole mixture. Results from a first nodule
do not provide a plausible age. However, in a second nodule, an isochrone could be applied giving a calculated age quite
consistent with radiocarbon data.