Black shrubs (i.e., arborescent forms) are present within hot-water travertine deposits from Morocco. The shrubs are commonly a few centimeters high and laterally grouped along individual laminae. Other than in mineral composition, the black shrubs closely resemble the calcite shrubs from other hot-water travertine deposits (e.g., Bagni di Tivoli, Italy, and Mammoth Hot Springs, Yellowstone National Park). They are composed of a variety of minerals that are Mn-rich and also have a significant Fe-oxide component. Similar Mn- and Fe-rich shrub-like forms have been described from desert varnish, deep-sea Mn nodules, and marine carbonates (Frutexites). SEM analyses show that bacterial bodies are densely packed within the Mn-rich black travertine shrubs whereas no bacterial bodies are evident within the enclosing aragonite and calcite laminae. The bacterial bodies range in shape and size from rods (commonly 0.8 micrometers long by 0.1 micrometers in diameter), to spherical forms (commonly 0.3 micrometers in diameter) that form chains up to 1 mm long, to nanobacterial spheres (150 to 200 micrometers in diameter) that make up densely packed chains commonly 15 micrometers in length. It is well documented within the microbiological literature that, not only do bacteria have the ability to induce the precipitation of Mn and Fe, they can concentrate Mn in mineral precipitates from waters with very low to essentially undetectable concentrations of Mn. The black Mn-rich shrubs within the Moroccan travertines, other hot-water travertines, and related features such as desert varnish, deep-sea Mn nodules, and Frutexites are postulated to also be the result of bacterially induced precipitation.

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