Recognition of Biotically Induced Precipitates and a Proposed Geochemical Biosignature

Chafetz, Henry S.¹ (1) University of Houston, Houston, TX

Hot spring precipitates, e.g., Yellowstone National Park, can be biotically induced, abiotic, or combinations of both. Recognition of these precipitates may be only evident at the SEM and/or geochemical level. Examples include aragonite and Mn-oxide precipitates.

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 New Mexico and Morocco corroborate the high oxidation state of bacterially induced deposits. That is five different bacterially induced Mn-oxide deposits display Mn oxidations states between 3.7 and 4.0. Thus, it is proposed that the high oxidation state is a biosignature, a chemical indication of microbial influence, and therefore may be very valuable in the recognition of the previous existence of life where other evidence is obscure. This may have profound significance in the search for extraterrestrial life, e.g., Mars.