Saori Ito, Tsuyoshi Watanabe

Natural disasters like earthquakes and tsunamis significantly affect coral reefs and marine ecosystems. The Ryukyu Islands, including Kikai Island and the surrounding coral reefs, face the potential risk of experiencing a significant earthquake with Mw > 8. While historical records offer insights into past tsunami occurrences, there is scarce observation or quantitative data on the impacts of seismic events on the reef ecosystems. Here, we show the environmental impact of the 1911 Kikai Island Earthquake and Tsunami on coral reef ecosystems using coral skeletal geochemical proxies with weekly-to-bi-weekly temporal resolution, particularly focusing on the coral barium/calcium ratios (Ba/Ca). Analysis of a sub-fossil Porites coral specimen collected from Shiomichi Bay, eastern Kikai Island, revealed an anomalous peak in Ba/Ca ratios following the earthquake and tsunami event, indicating increased sediment load in seawater. This anomaly persisted for approximately two months post-earthquake, highlighting the prolonged effect of the tsunami on coral reef environments. Other coral geochemical proxies, such as coral strontium/calcium ratios (Sr/Ca), magnesium/calcium ratios (Mg/Ca), oxygen (δ18Ocoral) and carbon stable isotopes (δ13Ccoral) and oxygen isotopes in seawater (δ18Osw) showed no anomalous change corresponding to the event. Despite the disturbance, the coral growth rate did not show an exceptionally low value, suggesting that the event may not have been catastrophic enough to have a severe impact. Our findings underscore the utility of coral Ba/Ca ratios in assessing historical variations in sediment concentrations associated with seismic events and provide valuable insights into the environmental impacts of tsunamis on coastal ecosystems.