2:00 PM - 2:15 PM
[SEM17-13] DRM lock-in depth and magnetofossils
Keywords:rock magnetism, magnetofossil, DRM
Recently it has become recognized that biogenic magnetite is a major constituent of magnetic mineral assemblages in global deep-sea sediments (e.g., Roberts et al., 2011; 2012, Yamazaki and Ikehara, 2012). Biogeochemical remanent magnetization carried by magnetofossils may contribute to remanence acquisition processes of sediments, but details are not understood yet. The abundant occurrence of magnetofossils with equant octahedral morphology in oxic red clay indicates that some species of magnetotactic bacteria (MTB) that produce equant octahedral magnetosomes do not avoid oxic conditions and may be aerotorelant (Yamazaki and Shimono, 2013). The population of such MTB may be largest near the sediment-water interface because of the highest availability of nutrients there. On the other hand, it is revealed that MTB that yield bullet-shaped magnetosomes prefer chemical conditions near the oxic-anoxic transition zone (Yamazaki et al., in prep.).
Remanent magnetization of sediments is considered to be a mixture of detrital remanent magnetization and biogeochemical remanent magnetization (BRM). If the floc model is true, the remanence carried by the detrital component does not show delay. The BRM carried by magnetofossils with equant morphology may also have no delay, whereas the BRM of bullet-shaped magnetofossils may contribute to delayed remanence acquisition. Lock-in depth of sediments may be determined by balance of these three components. The delayed component may have higher coercivity due to larger shape anisotropy of bullet-shaped magnetofossils.