Fractionation of organic C, nutrients, metals and bacteria in peat porewater and ice after freezing and thawing.

To better understand freezing - thawing cycles operating in peat soils of permafrost landscapes, we experimentally modelled bi-directional freezing and thawing of peat collected from a discontinuous permafrost zone in western Siberia. We measured translocation of microorganisms and changes in porewater chemistry (pH, UV absorbance, dissolved organic carbon (DOC), and major and trace element concentrations) after thawing and two-way freezing of the three sections of 90-cm-long peat core. We demonstrate that bi-directional freezing and thawing of a peat core is capable of strongly modifying the vertical pattern of bacteria, DOC, nutrients, and trace element concentrations. Sizeable enrichment (a factor of 2 to 5) of DOC, macro- (P, K, Ca) and micro-nutrients (Ni, Mn, Co, Rb, B), and some low-mobile trace elements in several horizons of ice and peat porewater after freeze/thaw experiment may stem from physical disintegration of peat particles, leaching of peat constituents, and opening of isolated (non-connected) pores during freezing front migration. However, due to the appearance of multiple maxima of element concentration after a freeze-thaw event, the use of peat ice chemical composition as environmental archive for paleo-reconstructions is unwarranted.