MICROBIAL INFLUENCE IN FORMATION OF FERROMANGANESE STRUCTURES IN LACUSTRINE SEDIMENTS FROM THE HEADWATERS OF THE CONNECTICUT RIVER, NEW HAMPSHIRE

Morphologically distinct ferromanganese-rich (Fe-Mn) sedimentary structures (microbialites) positioned at the sediment/water interface (3-15 m water depth) were discovered in two (Second and Third Connecticut Lakes) of the four headwater lakes of the Connecticut River, Pittsburg, New Hampshire, USA (Asikainen and Werle 2007). Similar structures, known from the literature, are called concretions, metalliferous sediments, microbialites, stromatolites, among other terms and have been described from other freshwater systems such as in Lake Oneida, New York and Lake Vermillion, Minnesota. Most of the specimens develop by accretion of concentric rings of iron and manganese hydroxides that nucleate around a central object such as a stone or shell (Moore et al. 1980; Sommers 2002).

Morphologic study revealed features suggesting microbial precipitation of the Fe and Mn oxides therefore we adopt the term microbialites for these structures. The New Hampshire microbialites consist of mineral grains cemented by Fe/Mn oxides and clay minerals that form pustulate, lattice, chimney, concentric and non-concentric rings morphotypes. Examination by SEM and Micro-probe of both whole material and thin-sections showed bio-herm type zonation of Fe and Mn resembling microstromatolites (Boston et al. 2001). The matrix shows high concentrations of rods, short filamentous and coccoid shapes ranging from 0.5 – 5 microns in diameter and identifiable protists. This observation supports the hypothesis that Fe and Mn oxide is actively precipitated by a microbial community that supports the formation of these disparate morphotypes.