(Table 1) Rock magnetic study of sediments from ODP Site 131-808

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 measurements onshore have shown similar fluctuations for NRM and saturation magnetization (Js). Js 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 808 sediments, we have not found any correlation between magnetic intensity variation and sediment units. In addition, the calcite content varies only by a factor of 1.5 at the most, rather than 1 to 2 orders of magnitude, and therefore calcite dilution can not account for the low intensities. Because the changes in sediment sources and compositions can not explain these unusual magnetic phenomena, we suggest that the low-intensity zones may be interpreted in terms of deep-seated diagenetic alteration. Our study points out that (1) it may be difficult to establish a magnetostratigraphy for deep sediments due to extremely low NRM intensity, and (2) sudden decreases in NRM intensity may not result from an unusually low paleomagnetic field intensity.