Rock magnetic criteria for the detection of biogenic magnetite

We rcpmt results on the magnetic properties of magnetites produced by magnetotaetie and dissimilatory iron-reducing bacteria. Magnetotactic bacterial (MTB) strains MS I. MV J and MV2 and dissimilatory iron-reducing bacterium ,train GS-15. grown in pure cultures, were used in this study, Our results suggest that a comhination of room tempnature coercivity analysis and low temperature remanence measurements provides a characteristic magnetic signature for intact chains of single domain (SD) particles of magnetite from MTBs. The most useful magnetic property measurements include: (1) acquisition and demagnetization of isothermal remanent magnetiza­ tion (IRM) using static, pulse and altcrnating fields: (2) acquisition of anhysteretie remanent magnetization (ARM); and (3) thermal dependence of low temperature (20 K) saturation IRM after cooling in zero field (ZFC) or in a 2.5 T field (Fe) lrom 300 K, Howcver, potentially the most diagnostic magnetic parameter for magnetosome chain identification in bulk sediment samples is related to the difference hetween low temperature zero-field and field cooled 51 RMs on warming through the Verwey transition (T z J00 K). Intact chains of unoxidized magnetite magnetoSl)ll1eS have ratios of D,.c/t5ZFC greater than 2, where the parameter D is a measure of the amount of remanenCl' lost by warming through the Verwey transition. Disruption of the chain structure or conversion of the magnetosuilles to maghemite reduces the.) FC/OLl-C ratio to around L similar to values observed for some inorganic magnetite. rnaghemitc. greigitc and GS-15 particles. Numerical simulations of t5,.c/Dzvc ratios for simple binary mixtures of magnctosome chains and inorganic magnetic fractions suggest that the 15 njDzFc parameter can he a sensitive indicator of biogenic magnetite in the form of intact chains of magnetite magnetosomcs and can he a useful magnetic technique for identifying them in whole-sediment samples. The strength of our approach lies in the comparati\e case and rapidity with which magnetic measurements can be madc, compared to techniques such as electron III ic roscopy.