Biomineralization and Magnetization of Sediments

KIRSCHVINK, JOSEPH L., California Institute of Technology, Pasadena, CA

Although nearly 60 mineral-like compounds are known to be synthesized biochemically by living organisms, only two [magnetite Fe(3)O, and greigite Fe(3)S(4)] have ferromagnetic properties capable of providing a stable remanent magnetization to sediments. In particular, the magnetotactic bacteria are now known to provide a significant supply of fine-grained, single-domain magnetite to surface sediments in environments ranging from deep-sea sediments to terrestrial soils. In many such environments, including the deep-sea sediments shallow-water platform carbonates, and perhaps soils, the fossilized bacterial magnetosomes are clearly the source of the stable remanent magnetization (NRM) upon which paleomagnetic studies and magnetic polarity stratigraphies are based. The crystal structures of these bacterial magnetofossils, revealed through the use of high-resolution TEM techniques, confirms both their biological origin and

documents some of the postdepositional dissolution and diagenetic effects to which they are subjected. These biogenic magnetites are distinguished easily from magnetic minerals produced by igneous, metamorphic, or authigenic processes.

Recent discoveries have revealed an amazing diversity of magnetite-precipitating micro-organisms, many of which produce up to several hundred times more fine-grained magnetite per cell than known previously. These organisms include some of the largest bacterial cells yet found, which are up to 15 micrometers in length and contain several thousand magnetite crystals in their magnetosome chains. The contribution of these organisms to the magnetization of sediments is as yet unknown. However, a variety of ultrastructural and evolutionary arguments suggest that these large magnetotactic bacterial cells may be the co-descendants of the ancestral eukaryote host cells that evolved nearly 1.5 billion years ago. If so, the magnetotactic bacteria may be the common ancestor to all higher forms o life on earth.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)