Rock magnetic study of sediments from Site 808, Leg 131

Shipboard magnetic investigation showed two zones of weak natural remanent magnetization (NRM): 675-925 and 1080-1243 meters below sea floor. Within these two sections, NRM intensity decreases by one to two orders of magnitude. Additional magnetic maesurements onshore have shown similar fluctuations for NRM and saturation magnetization (J_s). J_s becomes almost zero in the low-NRM intensity zones, indicating a small magnetic content. The variation of relative magnetic grain sizes have been examined by the ratio of ARM/K₀ (anhysteretic remanent magnetization to low magnetic field susceptibility) and by partial ARM (pARM) measurements. Both results indicate difference in magnetic grain sizes between the high- and low-intensity zones. For these Site 80 sediments, no correlation was found between magnetic intensity variation and sediment units. Because the changes in sediment sources and compositions cannot explain these unusual magnetic phenomena, the low-intensity zones may be interpreted in terms of deep-seated diagenetic alteration. -from Authors