Not just salt - the 11 March 2011 Tohoku-oki Tsunami and the significance of geochemical proxies

Researchers most often rely on the extent of sandy deposits to assess the magnitude and extent of palaeotsunamis, assuming that the landward limit of the deposit approximates the maximum runup of the tsunami. While it has been reported in many modern examples, it is not always the case. One such example is the 11 March 2011 Tohoku-oki tsunami, which inundated c. 4.5 km inland in the Sendai plain. However, a recognisable (>0.5 cm thick) sand deposit was recorded only up to c. 2.8 km inland (~ 62% of the inundation distance). Further inland, the deposit was dominated by mud containing at least a one-grain thick sand lamina up to the inundation limit. Therefore, relying on the extent of sandy deposits may lead to a gross underestimate of the generating event, and extent of tsunami inundation. The magnitude of the 869AD Jogan earthquake, which was based on the extent of the sandy deposits reported in the Sendai Plain, was estimated at ~8.4. Data acquired after the Tohoku-oki tsunami suggest it was an underestimate. Then the question arises - how will one be able to distinguish mud deposited by a tsunami from terrestrial mud and soil? Geochemical markers might provide the answer. We reported brackish and saline ponded water up to 2.6 km inland, despite >60 mm of precipitation in the two months since the tsunami, and we also observed salt crusts on numerous paddy fields up to the limit of tsunami inundation, where the water had evaporated. Elevated concentrations of chloride (salt), sulphate, magnesium, calcium and other associated elements were measured not only in mud deposits, but also in sandy deposits, where seawater had stagnated and then evaporated. The preservation of the marine geochemical signature (mostly salt but also sulphur) is problematic in sandy deposits, mainly due to leaching and post-depositional diagenetic processes. On the other hand, it has been proven in organic-rich sediments, such as mud, and has been used as an additional proxy to identify palaeotsunami deposits. Other geochemical markers, which are specific to each source area, can also be used. Geochemical markers could be used in conjunction with other proxies, such as microfossil data (e.g. marine diatoms), which have also been found to occur beyond the limit of recognisable tsunami deposits. They can provide the clues to identify the maximum inundation limit of palaeotsunamis that extends beyond the limit of the recognisable sandy deposits. The marine geochemical signature of the 2011 Tohoku-oki tsunami was found up to the limit of tsunami inundation near the Tobu Highway, and even beyond, where the waves reached through the underpasses. As geochemical markers show a good preservation potential in fine-grained and organic-rich sediments, we could use them to estimate the inundation distance of the Jogan tsunami beyond the preserved sand layer. © American Geophysical Unio