Paleomagnetism reveals the emplacement age of tsunamigenic coral boulders on Ishigaki Island, Japan

Abstract

We use temperature-dependent viscous remanent magnetization to estimate the emplacement age of tsunamigenic coral boulders along the shorelines of Ishigaki Island, Japan. The boulders consist of the hermatypic coral Porites, and the time of their deposition by tsunamis has been established using radiocarbon dating. Recently deceased corals at reef edges around Ishigaki Island record the Earth's current magnetic fi eld (present Earth fi eld, PEF) as a remanence parallel to the fi eld in the skeleton. Since the time when the coral skeletons were emplaced on the shorelines as boulders by destructive tsunami waves, a new viscous magnetization was partially overprinted in the boulder parallel to the PEF. The results of thermal demagnetization indicated that the boulders were rotated at least once, and their emplacement ages determined from L. Néel's relaxation theory for single-domain magnetite agree well with the radiocarbon ages, although there are traces of multidomain magnetites. New application of Néel's theory to tsunamigenic coral boulders gives us an opportunity to ascertain the age and transportation mode of individual tsunamigenic coral boulders in this area.

Stretched exponential relaxation of viscous remanence and magnetic dating of erratic boulders

“…The reworked age can be determined through experimental time‐temperature combinations for removing viscous remanence at ambient temperature [e.g., Borradaile , ]. Sato et al [] used viscous remanence dating to estimate reworked ages of two coral boulder moved by tsunamis. The timing of tsunamis estimated from unblocking temperatures of viscous remanence was compared with 14 C accelerator mass spectrometry ages obtained from the surface of well‐preserved coral boulders [ Araoka et al ., ].…”

Testing the use of viscous remanent magnetisation to date flood events

“…Instead, we apply Equation (4) directly to the determination of VRM age, thereby estimating absolute, independent ages. There have been only two previous VRM-dating studies of geological processes (Borradaile, 1998;Sato et al, 2014).…”

“…Both previous VRM-dating studies of geological processes (Borradaile, 1998;Sato et al, 2014), relied on step-wise thermal demagnetisation rather than continuous thermal demagnetization as used in this study; this means that: (1) the temperature step sizes were larger purely for practical reasons, and (2) the heating rates are not identical, i.e., static versus continuous heating. Borradaile (1998) used a calibrated system based on archeological masonary to date rock slumps <1000 years in age; he primarily used the protocol to put events into chronological sequence.…”

“…Borradaile (1998) used a calibrated system based on archeological masonary to date rock slumps <1000 years in age; he primarily used the protocol to put events into chronological sequence. Sato et al (2014) examined two large paleotsunami erratics. They used the same equation as us to determine the age of erratics, i.e., Equation (4).…”

“…This secondary magnetisation is termed viscous remanent magnetisation (VRM), and is logarithmically time dependent (Dunlop, 1973(Dunlop, , 1983. This time-dependency of the intensity of the VRM, has been applied successfully to the dating of archeological events (e.g., Heller and Markert, 1973;Borradaile, 1996Borradaile, , 2003Borradaile et al, 1999), however, this same method has not been used in widely in geological studies (i.e., Borradaile, 1998;Sato et al, 2014).…”