Inconsistent magnetic polarities between greigite- and pyrrhotite/magnetite-bearing marine sediments from the Tsailiao-chi section, southwestern Taiwan

Abstract

To establish a magnetobiostratigraphy for a 620-m-thick middle to late Pleistocene mudstone sequence in the Lower Gutingkeng Formation of the Tsailiao-chi (TLC) section in southwestern Taiwan, we conducted paleomagnetic and mineral magnetic measurements, together with sediment granulometry and calcareous nannofossil identification. Paleomagnetic samples from 65 sites revealed two types of thermal demagnetization (25–400°C) behavior: (1) single-component stable characteristic remanence in magnetite- and pyrrhotite-dominated samples (Type S), and (2) abrupt changes in polarity when samples with significant greigite concentrations were heated above 320–340°C (Type C). The characteristic polarities derived from Type S samples and from magnetite-dominated Type C samples (obtained above 340°C) are consistent with those determined from nannofossil biostratigraphy. This implies that the NRM carried by magnetite and pyrrhotite is reliable. The essentially antiparallel remanence components in Type C samples below 340°C are attributed to greigite. The almost antiparallel direction could have resulted from delayed formation of greigite, but in this case, the different direction of this component must have resulted from variable remanence lock-in times. Alternatively, the opposite polarities may result from self-reversal, which warrants further investigation. Pyrrhotite and greigite may have both formed authigenically, but there is no clear explanation for the observed differences in direction.