Dating of sediments in the Biscay bay: Implication for pollution chronology

Abstract

Important historical informations on the temporal changes of anthropogenic pollution in marine environment can be assessed using sediment analysis. Dating is a crucial prerequisite to reconstruct pollution events, to calculate fluxes, and thus to allow comparison between different sites. This work presents estimates of accumulation rates of sediments in the Bay of Biscay. Fives cores were collected during RIKEAU 2002 cruise on board o/v Thalia in order to study temporal changes in PAH and organohalogens compounds content of sediment. We compare chronostratigraphic estimates on cores derived from the natural radionuclide ²¹⁰Pb in excess with estimates from the known times of introduction of the artificial radionuclide ¹³⁷Cs to the environment. ²¹⁰Pb, ²²⁶Ra and ¹³⁷Cs were measured directly by non-destructive gamma spectrometry using a well type γ-detector. Total ²¹⁰Pb and ²²⁶Ra activities vary from 30 to 150 mBq g^-1, and 20 to 36 mBq g^-1 respectively; ¹³⁷Cs presents lower levels (< 5 mBq g^-1). Profiles of ²¹⁰Pb$_{\scriptsize\textrm{xs}}$ in three cores present a well mixed layer, from 2-3 to 10 cm, in the uppermost sediments, followed by an exponential decrease of activities, suitable for the determination of sedimentation rates. Under constant flux and sedimentation rate assumptions, vertical accretion rates derived from ²¹⁰Pb$_{\scriptsize\textrm{xs}}$ present a large range from nearly 0.1 cm yr^-1 up to almost 0.3 cm yr^-1. Differences are mainly due to relative position of studied cores regarding the muddy patch. Although the moderate level of ¹³⁷Cs limits the accuracy of this dating method, profiles of ¹³⁷Cs with depth strengthen mean rates derived from ²¹⁰Pb$_{\scriptsize\textrm{xs}}$ data. The implication of this dating on pollutant inputs in sediments of the Bay of Biscay is briefly discussed.