Rock magnetic property of gravity core CSH1 from the northern Okinawa Trough and the effect of early diagenesis

Detailed rock magnetic investigations were undertaken at 2-4 cm interval for the gravity core CSH1 (with a length of 17.36 m) from the northern Okinawa Trough. Time-scale of the core was constructed by two characteristic tephras and foraminferal assemblages, indicating an age of 50 ka for the bottom of the core. Except for three tephras and abrupt decrease in surface, there are little changes in all kinds of rock magnetic parameters that can be correlated to the climate change for the last 50 ka. Different from the common sediments, most S-ratios (S equals the negative ratio of IRM_-0^.3T to SIRM, which is an indicator of low coercivity content) of the sediments are smaller than 0.9, which implies a substantial amount of magnetic minerals with high coercivity. The existence of iron sulphide (greigite or pyrrhotite) is revealed by representative susceptibility - temperature curves showing 200-350 °C Curie temperature in addition to 580 °C of magnetite, and also by awful smell during heating and dark heating products. Both the occurrence of authigenic iron sulphide and quick decrease of magnetic parameters near the surface clearly show that sediments from Core CSH1 have undergone early diagenesis. The featureless magnetic changes of the whole core except for three tephras mean that the post-depositonal alteration is so strong that most original signals have been destroyed. For the same reason, the organic matter in sediment and sulphate in pore water must have been consumed along with dissolution, precipitation of iron and manganese happening sequencially during the redox reaction series. Great caution must be taken when using these altered chemical parameters for the interpretation of climatic changes.