Diatoms in the 1946 and 1964 Historical Tsunami Deposits in California (USA)

Abstract:

Diatoms, a class of aquatic microalgae that secrete silt-sized siliceous hard parts, are commonly entrained and transported along with other sedimentary particles by climatically driven flooding or tsunamis. Because different taxa are indicative of preferred growth habitats in marine to freshwater environments, the presence of certain diatom taxa in sedimentary deposits, including tsunami deposits, can provide valuable insight as to the provenance of the deposit. In coastal California (USA), biostratigraphic and chronological evidence exists for two historical tsunami deposits, both from trans-Pacific tsunamis in the 20^th century: the 1946 tsunami deposit at Pillar Point Marsh (Half Moon Bay) and the 1964 tsunami deposit at Crescent City. A possible third tsunami deposit at Crescent City, though not as well verified as the 1964 deposit, may record inundation by the farfield tsunami from the M9.5 Chile earthquake in May 1960. For the 1946 and 1964 tsunamis, damage to coastal communities at the point of greatest impact was severe, yet the geological record—the distribution and preservation of the sedimentary deposits—is very limited, with the best preservation confined to locations where the deposits settled out and were preserved in shallow standing water. Diatoms in the deposits differ in that the assemblages in the 1946 deposit indicate an offshore origin from the shallow shelf off Pillar Point, whereas the assemblages in the 1964 deposits at Crescent City indicate nearshore and beach deposits comparable to what is observed in storm deposits at the same locations. The results of the study highlight the variability of diatom assemblages found in different historical deposits, and emphasize that the limited sedimentological record for inundation from trans-Pacific tsunamis in California underestimates the past history and potential future hazard.