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Anthropic acceleration of a natural clay mineral reaction in marshland soils (Atlantic Coast, France)

¹ Centre Littoral de Géophysique, avenue M. Crépeau, 17042 La Rochelle cedex 01, France, and² Laboratoire HydrASA, CNRS-UMR 6532, 40 avenue du Recteur Pineau, 86022 Poitiers cedex, France

^* E-mail: vmathe{at}univ-lr.fr

(Received 15 May 2005; revised 12 September 2006)

Soil clay minerals in recent natural polders react on a human timescale in response to local environmental conditions. With increasing age, the mineral reaction leads to the dissolution of the chlorite component and a composition change of the different illite-smectite mixed-layer minerals (I-S MLMs): i.e. smectite layer content decreases and illite content increases. The process of oxidation, which is proven by magnetic susceptibility to trigger clay mineral reaction, changes the mineralogical composition of the sediment above the redox front. The mineral changes appear to be a non-linear function of time. In natural conditions the process lasts >1000 y. However, anthropoic forcing such as artificial drainage accelerates the oxidation reaction to complete the whole process in a few tens of years.

KEYWORDS: illite, smectite, anthropic impact, drainage, marshland soils, Holocene, X-ray diffraction