Recognition of
Biotically Induced Precipitates and a Proposed Geochemical Biosignature
Chafetz, Henry S.1 (1)
Hot spring precipitates, e.g.,
Clumps of bacteria at the centers of
aragonite splays clearly indicate that bacterial catalytic action can overcome
the physicochemical inhibition of aragonite to nucleate, and once initiated,
growth can succeed by abiotic precipitation. Thus, individual crystals can be
initially biotically induced whereas later growth of the same crystals can be
abiotic.
Accumulations of amorphous Mn-oxides,
which include microscopic oncoids, coatings on cyanobacterial filaments, and
dense featureless laminae, occur adjacent to the aragonite splays. These
bacterially induced Mn-oxides precipitated from spring waters with less than
0.2ppm Mn, i.e., waters highly undersaturated with respect to manganese.
Significantly, all three distinctly different bacterially induced Mn-oxides
have a high oxidation state, equal to or above 3.7, whereas laboratory produced
abiotic Mn-rich precipitates tend to have oxidation states of 3 or lower. Older
Mn-oxide shrub deposits from
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California