HIGH-RESOLUTION STABLE-ISOTOPE PROFILES FROM BIVALVES: A PROXY FOR THE TIMING OF TSUNAMI EVENTS

Tsunamis are catastrophic events that dramatically affect coastal and shelf areas. The ability to link tsunami deposits with specific offshore earthquakes or submarine slide events could aid in (a) determining periodicity and risk assessment; (b) link tsunami deposits in areas with limited historical records (sparsely populated areas) with events recorded elsewhere; and (c) qualitatively assess the magnitude of prehistoric events. In this study we use the environmental histories archived as variability in δ¹⁸O_shell values of bivalves as a proxy for the seasonal timing of deposition of a tsunami deposit. Bivalves are known to faithfully record seasonal environmental information that includes variability in seawater temperature. Therefore δ¹⁸O_shell variability can be used to determine the season of death for a particular bivalve, based on the isotopic composition during the final week of growth (i.e. at the commissure). Articulated bivalves are targeted, as they are assumed to have undergone transport and deposition alive and therefore have recorded environmental conditions up until the transportation (tsunami) event. It is assumed that the high-energy of the tsunami wave(s) will dislodge any previously expired articulated valves transported by the event. Bivalves which survive the event undergo transportation to a different environment which will be reflected in the isotope record as a shift in water chemistry and/or food sources. Stable-isotope profiles were obtained from multiple specimens of three bivalve species from a tsunami deposit in Sur, Oman. δ¹⁸O_shell profiles indicate that all specimens died within a similar season. A comparison of local sea surface temperatures and the δ¹⁸O_shell at the commissure suggests that the time of death occurred in the late fall. This timing corroborates well with an 8.1 magnitude earthquake that occurred off the coast of Pakistan on November 28^th 1945, triggering a tsunami that caused extensive damage on the coasts of Pakistan, India, and Iran. Stable-isotope profiles of bivalves derived from tsunami deposits can be a powerful tool in investigating the timing of tsunami events, or other single-event deposits, especially in areas lacking a long-term historical record.