[S-EM16] Geomagnetism, paleomagnetism, and rock magnetism

[SEM16-12] Potential of marine sediments suffering from reductive dissolution of magnetic minerals for recovering relative paleointensity

*Toshitsugu Yamazaki^1,2, Jiaxi Li ¹, Yuhji Yamamoto² (1.Atmosphere and Ocean Research Institute, The University of Tokyo, 2.Marine Core Research Institute, Kochi University)

Keywords:relative paleointensity, reductive diagenesis, rock magnetism, marine sediments

Deep-sea surface sediments become anoxic with increasing depth within a sedimentary column associated with the decomposition of organic matters. Magnetic minerals start to dissolve at a certain depth below the Fe-redox boundary. In this process, unprotected magnetites including magnetofossils are lost, whereas silicate-hosted magnetic inclusions and hematite survive. The reductive magnetic dissolution can be detected from a sudden decrease in S-ratio, indicating an increased proportion of high-coercivity magnetic minerals like hematite, as well as a sudden decrease in the kARM/SIRM ratio, suggesting a reduced proportion of magnetofossils. Magnetic mineral dissolution leads loss of paleomagnetic records. For studies of relative paleointensity (RPI) estimations, such sediments were discarded so far without detailed examination because their magnetic properties are far from the empirical criteria for reliable RPI estimations such as homogeneous magnetic mineralogy and magnetic grain size.

In this study, we conducted paleo- and rock magnetic analyses of sediment cores taken from the Ontong-Java Plateau and northwest Pacific Ocean. These cores have intervals that suffered from reductive dissolution of magnetic minerals. A large drop of the kARM/SIRM ratio in the dissolution intervals implies that NRM intensities normalized by ARM and IRM have large differences. We found that variations of NRM intensity normalized by ARM mimic the PISO-1500 stacked RPI curve for both cores, which means that RPI estimations are possible even in the sediments with strongly heterogeneous magnetic properties caused by selective dissolution of magnetic minerals. This rather unexpected result demonstrates the potential of legacy cores for further paleomagnetic studies.