Information about past tsunami hazards, such as their recurrence interval and magnitude, is needed for future disaster prediction and mitigation. We examined radiocarbon ages of the surfaces of massive coral boulders cast ashore by past tsunamis in the southern Ryukyu Islands, Japan, where few historical and geological records of past tsunamis are available. We selected only non-eroded Porites coral boulders along the shoreline, because their characteristics make it possible to determine the probable timing of their deposition by tsunamis, and we applied a dating method that uses the cumulative probability distributions of large numbers of radiocarbon measurements of those boulders to determine the timing of past tsunamis. The results demonstrate that the southern Ryukyu Islands have repeatedly experienced tsunami events since at least 2400 yr ago, with a recurrence interval of ~150-400 yr. The largest Porites tsunami boulder that we studied (long axis, 9 m), which is probably the largest single-colony tsunami boulder in the world, was displaced by the A.D. 1771 Meiwa tsunami. Although the 1771 Meiwa tsunami was likely the largest event in at least the past 700 yr, calculations of current velocity show that all identifi ed tsunamis occurring before 1771 were probably large enough to cause considerable damage to human-built structures and loss of life. This study demonstrates that by reliably dating large numbers of selected coastal boulders it is possible to ascertain the timing, recurrence interval, and magnitude of past tsunamis in a location where few adequate survey sites of sandy tsunami deposits exist.
The Li isotope ratio (δ7Li) is expected to be a useful tracer of silicate weathering in river and groundwater systems, which is an important contributor to the seawater compositional changes that accompany the evolution of the Earth's surface environment. To obtain accurate estimates of continental Li fluxes to the ocean, we determined δ7Li values of dissolved Li in the lower Ganges‐Brahmaputra river system in both the dry and rainy seasons, and in deep groundwater in the Bengal basin. Dissolved Li and δ7Li values in the lower reaches of the rivers (0.04–0.66 µmol kg−1 and +19.1‰ to +34.2‰, respectively) were predominantly derived from silicate weathering, as is the case in the upper parts of these rivers. We observed large changes in δ7Li over a distance of more than 1000 km downstream that were due mainly to Rayleigh‐type removal of Li from river water. Extremely high Li concentrations (1.15–1.67 µmol kg−1) and low δ7Li values (+5.1‰ to +11.6‰) in groundwater samples indicate congruent isotope leaching and dissolution of silicate minerals in the deep aquifer, where the water residence time is long. In the rainy season, Li concentrations and δ7Li values were lower than in the dry season, owing to the shorter residence time of river water and the substantial input of local subsurface flow through lowland alluvium. These results suggest that accurate estimation of continental Li fluxes to the ocean should take account of downstream and seasonal changes, as well as aquifer depth variations, in δ7Li values.
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