The behavior of Al, Mn, Ba, Sr, REE and Th isotopes during in vitro degradation of large marine particles

The extent and the time constant of dissolution of a set of inorganic tracers during the decomposition of large marine particles an estimated through in vitro experiments. Large marine particles were collected with in situ pumps at 30 m and 200 m in the Ligurian Sea at the end of summer. They were subsequently incubated under laboratory conditions with their own bacterial assemblage for 20 days in batches under oxic conditions in the dark. Some samples were initially sterilized in order to observe possible differences between biotic and abiotic samples. Particulate (> 0.2 mum) and dissolved(< 0.2 <mu>m) concentrations of Al, Sr, Ba, Mn, Rare Earth Elements (REE) and Th isotopes were determined over time. We obtain percentages of dissolution in agreement with the general knowledge about the solubility of these tracers: Th approximate to Al < Heavy REE < Light REE < Mn < Ba < Sr. For Mn and Ce, precipitation/adsorption occurs at the end of the experiment probably due to their oxidation as insoluble oxides. Particulate residence time of the tracers ranged from less than 1 day to 10-14 days. During the experiment, biological activity has a control on the dissolution process through the remineralization of particulate organic carbon, In the 30 m experiment, the observed dissolution of aragonite indicates that the pH of the incubation solution significantly decreases in response to the CO2 respiration. Speciation calculations suggest that this pH shift leads to a decrease of the complexation of dissolved REE by carbonate ions. Th isotope data are consistent with an irreversible dissolution of Th and they do not support a rapid particle-solution chemical equilibrium. (C) 2001 Elsevier Science B.V. All rights reserved.