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Preliminary palaeomagnetic investigation of riverine tufa: is riverine tufa a reliable recorder of the geomagnetic field direction?



Riverine tufas are freshwater carbonate deposits that precipitate in springs and streams distributed in limestone areas. They are noteworthy for their annual laminations that contain both dense and porous laminae and their fast precipitation rate. These features indicate that riverine tufa may be an excellent material for recording the secular variation of the geomagnetic field. Part of recently deposited tufas in Nagaya in Okayama Prefecture, Japan have a stable natural remanent magnetization (NRM), which is a reliable record of the average geomagnetic field direction. However, only the dense laminae in the recent tufa, however, carry a faithful record of the geomagnetic field direction; the porous laminae do not. The NRM of the porous laminae seems to be affected considerably by the dip direction of the slope on which the tufa is deposited. Based on the results of isothermal remanent magnetization acquisition experiments and microscopic observations, the dominant magnetic mineral in the recent tufa was determined to be magnetite. A Holocene palaeotufa, which was formed 2000–2800 years ago, was found to have no stable remanent magnetization. This is possibly due to the absence of dense layers where magnetic particles are tightly fixed. Since it is very important in palaeomagnetic secular variation studies to be able to reconstruct much older, continuous records of the variation of the geomagnetic field, tufa deposits are not ideal materials and their applicability for this purpose is rather low, although the further investigation is needed.