Signatures of Tsunami in the Coastal Landscape - Tsunami - page 37

et al. 2008 ). Each of the beds could be linked to an his-
torical tsunami event going back in antiquity to 395. In the
Gargano region of southeastern Italy, multiple sandy
deposits in marsh areas suggest an average recurrence
interval of 1200-1700 years for tsunami events since 3500
BC (De Martini et al. 2003 ). While very infrequent, tsunami
in this region are still a distinct possibility given enough
time. Finally, dating of sand layers in the Bay of Palairos-
Pogonia, northwest Greece shows that five strong tsunami
have occurred with a recurrence interval of 500-1000 years
over the past 4000 years (Vött et al. 2011 ).

freshwater species Pinnularia (Shi et al. 1995 ). The Burin
Peninsula Tsunami of November 18, 1929 deposited a
distinct diatom signature throughout the 25 cm thick sand
units in peat swamps (Dawson et al. 1996 ). While the peats
contain only freshwater assemblages, the tsunami sands
contain benthic and intertidal mudflat species such as
Paralia sulcata and Cocconeis scutellum. Freshwater spe-
cies are incorporated in the lowermost part of the sands,
indicating that material was ripped up from the surface of
the bogs as the tsunami wave swept over them. Finally,
tsunami deposits in eastern Scotland attributable to the
Storegga slide 7950 years ago contain Paralia sulcata,
which is ubiquitous in the silty, tidal flat habitats of eastern
Scotland (Dawson et al. 1996 ). The freshwater species
Pinnularia is also present, indicating that bog deposits were
eroded in many locations by the passage of the tsunami
wave. These results must be tempered with the results
obtained from sediments deposited by the recent T ¯hoku
Tsunami, March 2011 in Japan. While extensive sand and
mud deposits—fining both inland and upwards—were
found, these did not consist of marine sediments and hence
contained little if any offshore marine diatoms and foram-
inifera (Szczuci ´ ski et al. 2012 ). Smedile et al. ( 2011 )
overcame this type of problem by sampling a marine core
from 72 m depth in water in Augusta Bay, southeast Sicily,
for foraminifera—dominated by Nubecularia lucifuga and
Neocorbina posidonicola—associated with seaweed and sea
grass growing on the inner shelf. These foraminifera are
presumably transported seaward by tsunami backwash.
Using cluster analysis, they found anomalous inshore
foraminifera in 12 layers, three of which could be radio-
carbon dated to historic tsunami events affecting eastern
Sicily in 1169, 1693, and 1908. One layer possibly repre-
sents a tsunami originating from Crete in 365, while an
older one is associated with the eruption of Santorini about
3600 years BP. Tsunami along this stretch of coast occur
every 330-370 years. This study is rare in that it uses both
foraminifera and the detection of anomalous sediment lay-
ers—albeit on the seabed—to identify tsunami events. It
also overcomes conflicting interpretations between tsunami
and storms for the deposition of boulders and sediment on
land, because storms are unlikely to influence sedimentation
at these depths on the outer shelf.

3.2.2 Foraminifera and Diatoms

Silty sand units deposited inland by tsunami can also con-
tain a distinct signature of marine diatoms or foraminifera.
Foraminifera are small unicellular animals, usually about
the size of a grain of sand, that secrete a calcium carbonate
shell. On the other hand, diatoms are similarly sized, single-
celled plants that secrete a shell made of silica. Both
organisms vary in size and live suspended either in the
water column (planktonic) or on the seabed (benthic).
Under fair-weather conditions, only the larger benthonic
species are transported shoreward as bedload under wave
action and deposited on the beach (Haslett et al. 2000 ).
Smaller benthonic and the suspended planktonic species are
moved offshore in backwash or transported alongshore in
currents to quiescent locations such as estuaries or the low-
energy end of a beach. Storm wave conditions tend to move
sediment, including diatoms and foraminifera, offshore in
backwash, undertow, or rips. However, winds can blow
surface waters to shore. A storm assemblage includes very
small diatoms or foraminifera diluted with larger, benthonic
foraminifera reworked from pre-storm beach sediments.
Tsunami assemblages are chaotic because a tsunami wave
moves water from a number of distinctive habitats that
include marine planktonic and benthic, intertidal, and ter-
restrial environments. A high proportion of the forams and
diatoms are broken, with spherical-shaped species being
overrepresented because of their greater resistance to ero-
sion. Storm waves are incapable of flinging debris beyond
cliff tops, whereas tsunami can override headlands more
than 30 m high. In the latter case, the occurrence of coarse,
inshore, benthonic species in the debris indicates a marine
provenance for the sediment. Where this material is mixed
with gravels or other coarse material, it forms one of the
strongest depositional signatures of tsunami.
Foram and diatom assemblages have been studied for a
number of historic and paleo-tsunami events. For example,
the 1 m thick sand layers deposited by the Flores tsunami of
December 12, 1992 contain planktonic species such as
Coscinodiscus

3.2.3 Boulder Floaters in Sand

Boulders are not usually transported along sand beaches
under normal wave conditions. Their presence as isolated
floaters within a sand matrix is therefore suggestive of
rapid, isolated transport under high-energy conditions. In
many cases, deposits containing boulders are less than
1.5 m thick and lie raised above present sea level along and Cocconeis scutellum as well as the