Tsunamis - Extreme Events: A Physical Reconstruction and Risk Assessment - page 127

benthonic (sea floor dwelling) species are transported shoreward as bedload
by fair weather waves and deposited onto the beach. The smaller benthonic
and planktonic (sea surface dwelling) species are moved offshore by backwash,
particularly so during storm waves, and back onshore by winds blowing the
surface of the water. The difference between storm and tsunami assemblages
is that the former have smaller diatoms and foraminifera with some of the
larger species incorporated from the beach, whereas tsunami deposits have many
larger species amongst a chaotic mixture of species from different environments
including freshwater settings. Hemphill-Haley (1996) has warned that microfos-
sil assemblages cannot by themselves differentiate between tsunami and storm
deposits. And Minoura et al. (2000)found quite the opposite sort of assemblage
in a study of deposits from the Minoan tsunami generated by the Santorini vol-
canic eruption ∼ 3600 years BP. This deposit showed very high numbers of plank-
tonic foraminifera species and an absence of species that dominate the modern
shore.
Seawater is rich in chlorine (Cl), sodium (Na) and sulphates (SO 4 )andsuch
elements are much less common in terrestrial waters. Na, potassium (K), cal-
cium (Ca) and magnesium (Mg) also occur in seawater with the latter two high
in marine shells and foraminifera. Geochemical studies of sediments thought to
have been deposited by tsunami are frequently high in these elements. Minoura
and Nakaya (1994)found such trends in their study of tsunami sediments in
a Japanese coastal lake and Minoura and Nakaya (1991)found similar levels of
these elements in sediments deposited by the AD 1993 Japan Sea tsunami. Goff
et al .(2004)also analysed geochemical signatures in the sediments of Okarito
Lagoon, New Zealand (Fig. 5.9). Here they found that iron (Fe), sulphur (S), tita-
nium (Ti), strontium (Sr), barium (Ba) and Na concentrations increased markedly
in a core which at the same level showed a distinct change in sediment grain
size. Goff et al .(2000, 2004)suggestthat these increases in the chemical composi-
tion of the sediments reflect the influence of saline water (tsunami inundation)
into the normally brackish to freshwater lagoon.
Identification of tsunami deposits is best achieved when all of these criteria
are employed within a study. Goff et al .'s (1998)study of the AD 1855 tsunami
deposit near Wellington, New Zealand examined the sediment textural charac-
teristics, the diatom assemblage, the roundness and sphericity of lithic clasts, the
fabric of clasts (i.e. alignment of a axes), and the presence and nature of pumice
and marine shells. Likewise, Goff and Chague-Goff's (1999)study incorporated
these same elements plus geochemistry and radiocarbon and 137 Cs analysis to
determine the age of the tsunami events in Abel Tasman National Park, New
Zealand.