Shinozaki et al.
Progress in Earth and Planetary Science(2022) 9:29
https://doi.org/10.1186/s40645-022-00491-6

RESEARCH ARTICLE

Progress in Earth andPlanetary Science

Open Access

Identifying tsunami traces beyond sandy
tsunami deposits using terrigenous biomarkers:a case study of the 2011 Tohoku‑oki tsunami
in a coastal pine forest, northern Japan
Tetsuya Shinozaki1* , Yuki Sawai1 , Minoru Ikehara2 , Dan Matsumoto1 , Yumi Shimada1 ,Koichiro Tanigawa1   and Toru Tamura1,3   

Abstract
The distributions of sandy tsunami deposits do not reflect the true extents of tsunami inundation areas, leading tounderestimates of inundation by past tsunamis and thus the magnitudes of their associated tsunamigenic earthquakes. To archive the sedimentological and geochemical features of the 2011 Tohoku-oki tsunami deposit, we performed visual observations and computed tomography, grain-size, water content, and organic geochemical analysesof sediments from a coastal forest at Oirase Town, northern Japan. Stratigraphic observations revealed the 2011
tsunami deposit to be a landward-thinning interbedded sand and soil layer that became ambiguous in landward
locations. The sediment samples from the inundated area did not contain marine-sourced biomarkers; instead, peakconcentrations of isolongifolene, an organic compound derived from Pinus in the forest, were observed within or justabove the sandy tsunami deposits in sediment sections. Peak isolongifolene concentrations were also detected inlandward soils inundated by the tsunami in which no sand layer was observable, but were not observed beyond theinundation limit. Although this characteristic biomarker is unique to this and similar depositional environments, theseresults suggest that lateral changes of the concentrations of environment-specific biological proxies in the sedimentary column may record tsunami inundation.
Keywords: 2011 Tohoku-oki tsunami, Tsunami deposit, Inundation limit, Grain-size analysis, Biomarker analysis,
Coastal forest, Aomori Prefecture
1 Introduction
Large tsunamis triggered by subduction-zone earthquakes occur on time scales of 100–1000 years (e.g., Pariset al. 2020), representing a low-frequency catastrophic
natural disaster. Current instrumental records date backonly about 100 years and historical records not morethan 1000 years in most parts of the world (Satake andAtwater 2007). Tsunami deposits are thus particularly*Correspondence: tetsuya.shinozaki@aist.go.jp
1
Geological Survey of Japan, National Institute of Advanced Industrial
Science and Technology, Central 7, Higashi 1‑1‑1, Tsukuba 305‑8567, Japan
Full list of author information is available at the end of the article

important because they record events predating historical records. Indeed, the spatial distributions and chronologies of tsunami deposits have been used to reconstruct
the magnitudes and recurrence intervals of tsunamis andearthquakes in tectonically active margins (e.g., Atwater
et al. 2005; Jankaew et al. 2008; Fujiwara et al. 2020; Goffet al. 2020; Sawai 2020; Costa et al. 2021).
Studies comparing the inundation area of the 2011
Tohoku-oki tsunami with the maximum landward
extent of the associated sandy deposits indicate thatsandy tsunami deposits are not always preserved ininundation areas (Goto et al. 2011; Abe et al. 2012,2020). For example, Abe et al. (2012) reported that

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Shinozaki et al. Progress in Earth and Planetary Science(2022) 9:29

sandy tsunami deposits ≥ 0.5 cm thick are distributed
over only 57–76% of inundated areas, especially inareas where the tsunami ran up > 2.5 km inland. Therefore, tsunamis do not transport sand up to the landward inundation limit and the distributions of sandy
tsunami deposits likely underestimate the inundation
limits of past tsunamis and thus the magnitudes of theirassociated tsunamigenic earthquakes (Sawai et al. 2012;
Namegaya and Satake 2014). To more precisely reconstruct past tsunami events, it is therefore necessary toestablish a method of accurately identifying their inundation areas.
Only a handful of previous studies have sought geochemical proxies to reveal tsunami traces lacking observable sand deposits. For example, high concentrationsof seawater components such as chloride and sulfate
were detected in coastal lowlands beyond the extents
of sandy deposits from the 2010 Maule (Chagué-Goff
et al. 2015) and 2011 Tohoku-oki tsunamis (Goto et al.
2011; Chagué-Goff et al. 2012). Moreira et al. (2017) also
detected salinity indicators such as Cl and Br beyond
the inland limits of sandy deposits from the 1755 Lisbon
tsunami and accordingly re-estimated the inundation
distance. Nonetheless, other geochemical proxies of tsunami traces beyond the landward limits of sandy deposits remain largely unexplored and further case studies
should explore various approaches that are appropriate
for different coastal settings and inundation events.
In this study, we focused on biomarkers as a proxy to
identify tsunami traces beyond the distribution limit ofsandy tsunami deposits. Biomarkers are molecular fossils derived from living organisms, which are stable ona geological time scale and variable with sedimentary
environments (Peters et al. 2007). Analyses of biomarkers
and anthropogenic markers have been applied to such as
modern tsunamis (Shinozaki et al. 2015, 2016; Bellanova
et al. 2020a, 2021) and paleotsunamis (Alpar et al. 2012;
Bellanova et al. 2020b; Konechnaya et al. 2022) as proxies to identify tsunami deposits. Results from the modern
tsunami indicated that marine-sourced biomarkers are
more likely to be adsorbed in fine-grained and organicrich sediments rather than in sandy sediments (Shinozaki
et al. 2015), which may be useful for detecting tsunami
traces beyond distribution of sandy tsunami deposits.
A coastal forest in Oirase Town, northern Japan, waspartly inundated by the 2011 Tohoku-oki tsunami andthus provides an opportunity to apply geochemical proxies for detecting tsunami inundation events. We therefore
performed grain-size and organic geochemical analysesof sediment samples collected from this coastal forest in
search of such proxies. The results of this study lead tothe more comprehensive reconstruction of inundation
area of paleotsunami.

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2 Study site
2.1 Geomorphology

Our study site is a coastal forest facing the Pacific Ocean
in Oirase Town, Aomori Prefecture, northern Japan(Fig. 1a, b). Landward from the ocean, the Oirase area
consists of a sandy beach 100–150 m wide and a coastal
pine forest 100–200 m wide (Fig. 1c). The beach andcoastal forest were separated by a seawall approximately
6 m high before the 2011 Tohoku-oki tsunami. Later,for greater protection from inundation, the height ofthe seawall was subsequently increased to 7 m (Fig. 1d,e). The former coastal forest was heavily damaged by thetsunami associated with the M 8.1 1933 Showa-Sanriku
earthquake (Utsu 2004) and pine trees were replanted
after that event (Nakamura et al. 2012).
2.2 The 2011 Tohoku‑oki tsunami and associated tsunami
depositThree tsunami waves struck the study area during the2011 Tohoku-oki tsunami, the second being the largest(Koiwa et al. 2014). The run-up and inundation heights
in this area were 5.3 and 4.5 m, respectively (Fig. 1c; The
2011 Tohoku Earthquake Tsunami Joint Survey Group2011). Several studies have investigated the associatedtsunami deposit in this area. Nakamura et al. (2012)
reported local variations in tsunami inundation and thesedimentary characteristics and mineral component of
the 2011 Tohoku-oki tsunami deposit. They found thatthe tsunami deposits were mainly affected by coastal
topography and the extent of erosion at any one point,
rather than by flow height. Koiwa et al. (2014) examined the tsunami behavior based on interpretations ofsequential digital photographs of the tsunami, the spatial
distribution of tsunami deposits, and topography reconstructed using a digital elevation model. Both studies
found that the thick tsunami deposits comprise multiple unlaminated layers with inverse and normal grading.
These layers probably formed by repeated accelerations
and decelerations during the several minutes between thearrival of the tsunami and the start of the backwash flow
(Koiwa et al. 2014).
Tanigawa et al. (2018) conducted a field survey aroundthe Oirase coastal forest several months after the 2011
tsunami and examined the source of transported sandbased on diatom assemblages. They reported that thesand in the deposit may have sourced from supra-, inter-,and sub-tidal locations (including beach and dune) sediments. In addition, they reported the tsunami inundation
limit based on eyewitness accounts and the distributionof debris (Fig. 1c).
Bellanova et al. (2021) traced a wood- and organicbearing deposit directly atop the sandy deposit; it
extended further inland than the sandy deposit, andPage 3 of 10

c

N

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5.3
Japan

Transect A

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Fig. 2bN

Aomori Prefecture

a

a’

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Oirase(Detail in c)

4.5

Inundation limit(Tanigawa et al. 2018)all

Coastal forest
Reference point (R)bFig. 2a

b

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Transect A

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250

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Distance (m)

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Coastal forest

100
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Transect B
B5 B4 B2
B3
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0

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Coastal forest

16

4m

4

Seawall20

8Inundation height (m)Photo direction

A6
A7 A5 A4 A2
A3 A1
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Run-up height (m)Transect B

Elevation (m)ab(2022) 9:29

Elevation (m)Shinozaki et al. Progress in Earth and Planetary Science

2

b
150

100
Distance (m)

50

0

0

Fig. 1 Location maps of the study site and topographic profiles of the studied transects. a Map of Japan. b Detail of east Japan showing theepicenter of the main shock of the 2011 M 9.0 Tohoku-oki earthquake (black star) and the associated coseismic slip distribution (dashed lines;
Ozawa et al. 2011). c Aerial photograph of the study site in the coastal pine forest of Oirase. The inundation limit is inferred from eyewitness
accounts and the debris distribution reported by Tanigawa et al. (2018). The run-up height (the elevation at the most inland point inundated by
the tsunami) and inundation height (the maximum inundation depth at a given point) are from The 2011 Tohoku Earthquake Tsunami Joint Survey
Group (2011). d, e Topographic profiles and sampling locations along Transects A and B

they accordingly re-estimated the extent of the tsunami inundation area. They also determined sedimentsources based on biological and anthropogenic proxies and depth profiles of the terrigenous/aquatic ratio,
which indicates relative amounts of terrestrial versus
aquatic n-alkanes in a sediment (Peters et al. 2007).
They found that the wood- and organic-rich tsunami
trace showed less onshore and more marine signals
than the soil and dune sediments. In addition, the 16
priority pollutant polycyclic aromatic hydrocarbons,
which are anthropogenic markers, were present at highconcentrations in the tsunami sand deposit and pretsunami soil and dune sediments.

3 Methods
In October 2020, we collected sediment samples within
the area of the coastal forest inundated by the 2011 tsunami along two transects roughly perpendicular to theshoreline (Figs. 1c–e, 2). Sediment samples were taken
directly from the walls of excavated pits using a 18-cmlong and 10-cm-wide plastic box (Fig. 2c, d) at sevenlocations along Transect A (A1–A7) and five along Transect B (B1–B5) (Fig. 1d, e). The sediment samples werevisually described and photographed. We also collected areference sediment sample (R, Fig. 1c) beyond the known
inundation limit in June 2021 using the same sampling
approach. In addition, pine bark, needles, and cones on

Shinozaki et al. Progress in Earth and Planetary Science(2022) 9:29

Fig. 2 Photographs of the field survey. a Photograph of the mostseaward side of the coastal forest. b A representative photographof the study site, taken looking toward the ocean. c Excavated pit
at location A2. d Sediment sample collected directly from the wallof the pit using a plastic sampling box. The photograph in a wastaken in September 2017, and the photographs in b–d were taken in
October 2021

the ground surface were collected for geochemical reference data. Topography was measured using a Leica Viva
TS15 imaging total station (Leica Geosystems, St. Gallen,
Switzerland) and a Leica Viva GS10 high-precision GNSS
receiver.
In the laboratory, sediment samples were scanned using
a computed tomography (CT) scanner (Supria Grande,
Hitachi) in the Geological Survey of Japan laboratory(National Institute of Advanced Industrial Science andTechnology). They were then subsampled in 1-cm-thickvertical intervals. The subsamples were dried at 60 °C for
24 h, and their water contents (%) were calculated fromthe sample weights before and after drying. A portion of
the dried samples was then homogenized with an agate
mortar and pestle, weighed, and ashed at 550 °C for 4 h toestimate loss on ignition (LOI550, %), which is indicativeof organic content (Dean 1974; Santisteban et al. 2004).
Before grain-size analysis, organics and carbonates
were, respectively, removed from the remaining driedsamples by dissolution in hydrogen peroxide (­H2O2)
and hydrochloric acid (HCl). The samples were thensieved at 63 μm (4 phi) to remove the mud fraction, andPage 4 of 10

the sand content (%) was calculated from the sample
weights before and after sieving. Grain-size analysis was
performed on the residual sand samples using an image
analyzer (Camsizer, Retsch Technology GmbH, Haan,
Germany) with an effective measuring range of − 5.25 phi(pebble) to 6.25 phi (silt) at 0.25-phi intervals. The mean
grain size of each sample was then calculated from themeasurements using the logarithmic graphical method of
Folk and Ward (1957).
We kept about 1.9–2.4 g of each subsample from location A6 for radioactive cesium measurements using a
gamma spectrometer at the Geological Survey of Japan(GCW2022, Canberra Industries Inc., USA). The 137Cs(t1/2 = 30.2 years) data were used to detect horizons ofspecific ages (e.g., 1950–1964 CE; He and Walling 1997;
Wren and Davidson 2011).
Biomarkers were measured in samples A1, A4, A6, A7,
and the reference sample, as well as in the pine materials
taken from the ground surface. Because large plant fragments constitute a large portion of the organic matterpresent in these sediments (Shinozaki 2021), terrestrial
plants were mostly removed from each subsample using
a 250-μm sieve, and fraction finer than 250 μm in eachsubsample was then dried and homogenized. Lipids wereextracted from the homogenized subsamples with dichloromethane and methanol using an accelerated solvent
extractor (ASE350, Dionex, Sunnyvale, USA). Hydrocarbons (N1 fraction; locations A1, A2, A4, A6, R, and pinematerials), ketones (N3; locations A4 and A6 only), andalcohols and sterols (N4; locations A4 and A6 only) wereextracted as described by Shinozaki et al. (2015). These
organic fractions were measured using a gas chromatograph with flame ionization detection (Agilent 7890B or
Agilent 6890 N, Agilent Technologies Inc., USA) and an
Agilent DB-5HT nonpolar column (30 m long, 0.25 mm
internal diameter). Individual compounds were identified
using an Agilent 5973 network mass selective detector
gas chromatograph/mass spectrometer (GC–MS).

4 Results
4.1 Sedimentological characteristics
4.1.1 Transect A

At most locations, the sediment samples consisted ofan upper organic soil layer that contained sand particles
and a thick lower fine- to medium-grained sand layer(A2–A7, Fig. 3). Based on visual observations, fine- tomedium-grained sand layers 1.5–4.0 cm thick were intercalated into the soil layer at locations A1, A2, A4, andA5, but could not be identified within the soil layer at
locations A3, A6, and A7 (Fig. 3). The intercalated sandlayer thinned landward (Fig. 3). The contacts between thesand and soil layers were vague, especially at the inland
locations A4 and A5. No sedimentological structure wasShinozaki et al. Progress in Earth and Planetary Science(2022) 9:29

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Fig. 3 Lithologic profiles along Transects A and B and at the reference location R. At each site, the stratigraphy is presented as (left) interpreted
stratigraphic columns based on visual observations, and (center) photographs and (right) CT images of the sampled sediment columns

observed in the intercalated sand layer. At location A1,
the intercalated sand contained abundant colorless minerals, such as quartz. The soil, intercalated sand layer, andupper part of the bottom sand layer contained fine roots(Fig. 3).
In the CT images, the sand layers at locations A1, A2,
and A4 were easily identifiable, but those at locations
A3 and A5–A7 were difficult to identify, if present at all(Fig. 3). At locations A1, A2, and A4, the positions of theintercalated sand layers in the CT images corresponded
to those observed visually.
Water content and LOI550 showed similar trends,
both being relatively high in the soil layer and low in thebottom and intercalated sand layers (Additional file 1:
Fig. S1). Notably, both were highest at the top of the soil
and decreased with increasing depth (Additional file 1:
Fig. S1). At locations A1 and A4, these values markedly
decreased within the intercalated sand layer, but at locations A2 and A5, these values only slightly decreased
within the intercalated sand layer. As with the CT
images, the water content and LOI550 depth profiles did
not show any remarkable features in the soil layer at the
landwardmost locations (A6 and A7, Additional file 1:
Fig. S1).
Sand content was generally low at the top of the soil
layer and tended to increase with depth (Additional file 2:
Fig. S2). The bottom sand layer contained > 80% sand
(Additional file 2: Fig. S2). Sand contents in the soil layer
at seaward locations were > 50%, but that at the mostinland location (A7) was only 12%. Mean grain size was

generally finer and more variable in the soil layer than in
the bottom sand layer (Additional file 2: Fig. S2), although
there were some differences among the trends at different
locations. For example, we observed small variations of
the mean grain size in the soil and sand layers at location
A3 and large variations in the lower sand layer at location
A4 (Additional file 2: Fig. S2).
The soil layer was poorly sorted, especially in the upper
part (Additional file 3: Fig. S3), whereas the bottom sandlayer was well sorted with no significant variation withdepth (Additional file 3: Fig. S3). At all locations, it wasdifficult to discriminate the intercalated sandy layer fromthe soil layer based on the depth profiles of the grain-size
distribution (Additional file 3: Fig. S3).
4.1.2 Transect B

Sediments from Transect B also consisted of an upperorganic soil layer atop a thick fine- to medium-grained
sand layer (B1–B5, Fig. 3). Fine- to medium-grained
sand layers 0.5–3.0 cm thick intercalated into the soil
layer were visually identifiable at locations B1–B4, but
we could not identify an intercalated sand layer at location B5 (Fig. 3). At locations B3 and B4, the intercalated
sand was not layered, but only partially contained within
the soil. The intercalated sand layer thinned landward(Fig. 3). No sedimentological structure was observed in
the intercalated sand layer. Fine roots were present in thesoil, the intercalated sand layers, and the upper part ofthe bottom sand layer (Fig. 3).

Shinozaki et al. Progress in Earth and Planetary Science(2022) 9:29

In CT images, the intercalated sand layers could be
identified within the soil at locations B1 and B2, but those
at locations B3–B5, if present, were difficult to identify(Fig. 3). At locations B1 and B2, the positions of the intercalated sand layers in the CT images corresponded tothose observed visually.
Within the soil layer, water content and LOI550
were highest at the top and gradually decreased withinincreasing depth (Additional file 1: Fig. S1); values in thebottom sand layer were stable and low. The water content
and LOI550 values in the intercalated sand layers at locations B1 and B2 were similar to those in the bottom sandlayers (Additional file 1: Fig. S1).
Sand contents were generally low at the top of the soil
layer and increased with increasing depth toward thebottom sand layer (Additional file 2: Fig. S2), which contained > 80% sand (Additional file 2: Fig. S2). The mean
grain size was fine at the top of the soil layer, graduallycoarsened with depth, and was fairly stable in the bottom
sand layer (Additional file 2: Fig. S2).
The soil layer, especially its upper part, was poorly
sorted (Additional file 3: Fig. S3), whereas the bottom
sand layer was consistently well sorted (Additional file 3:
Fig. S3). At all locations, it was difficult to identify theintercalated sand layer based on the depth profiles of thegrain-size distribution (Additional file 3: Fig. S3).
4.2 Biomarkers

We illustrate the N1 (hydrocarbon) chromatographs forlocation A4 (Fig. 4), which are generally representativeof those at other locations. At locations A1, A4, A6, A7,
and R, long-chain n-alkanes ­(C23–C33) were detectedin every measured layer. The peaks of odd-numberedn-alkanes ­(C23–C33) were larger than those of even-numbered n-alkanes ­(C24–C32), primarily reflecting contributions from vascular plants (Eglinton and Hamilton 1963).
None of the samples contained detectable short-chain
n-alkanes ­(C15–C19).
A significant peak was observed at a retention time ofca. 6.3 min in all layers and at all measured locations;analysis by a GC–MS identified this peak as isolongifolene ­(C15H24), a main sesquiterpene component derived
from an essential oil of pine. For example, sesquiterpene
in the essential oil of Pinus thunbergii heart wood contains 30.5% longifolene (Mukai et al. 2017). Indeed, theanalyzed pine bark, needles, and cones taken from theground surface contained isolongifolene at concentrations on the order of ­104–105 ng/g. Background isolongifolene concentrations in the soil layers at locations A1,
A4, A6, A7, and R were typically less than 2.5 × ­103 ng/g
(Fig. 5), consistent with the location of the study site ina coastal pine forest (Figs. 1c, 2a). Peaks at locations A1
and A4 occur just above and within the sandy tsunami

Page 6 of 10

Fig. 4 Gas chromatography of hydrocarbons (N1 fraction) at location
A4. Numbers indicate n-alkanes ­(CnH2n+2) and ‘IS’ represents the
5α-cholestane internal standard. The retention time is the elapsed
time at which the compound was detected. The flame ionization
detector (FID) response represents signal intensity

deposits, respectively, whereas concentrations do not
exceed the background at the reference site (Fig. 5).
At locations A4 and A6, alkenones (long-chain ­C37–
C39 unsaturated methyl and ethyl ketones derived fromseveral species of haptophytes, primarily oceanic phytoplankton including the widely distributed coccolithophorids Emiliania huxleyi and Gephyrocapsa oceanica;
Volkman et al. 1980; Marlowe et al. 1984; Conte et al.
1994) were not detected in any samples. Sterols (cholesterol and phytosterols such as stigmasterol andβ-sitosterol) were detected as components of the N4 fraction in every measured layer, but no marine-sourced sterols, such as dinosterol or brassicasterol, were detected in
any layer at these locations.

5 Discussion
5.1 Discrepancy between tsunami inundation area
and the distribution of sandy tsunami deposits

We interpret the sand layer intercalated in the soil layer
as the 2011 Tohoku-oki tsunami deposit. A sandy organic
soil has developed at the study site after the replantingof pine trees in 1933 (Nakamura et al. 2012). Therefore,
the tsunami sand deposited on the ground surface in2011 (Tanigawa et al. 2018) is easily recognizable as theintercalated sand layer, which has since been covered

Shinozaki et al. Progress in Earth and Planetary Science(2022) 9:29

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Fig. 5 The thickness of the sandy tsunami deposit (bars), the presence (filled stars)/absence (open star) of isolongifolene concentration peaks (n.a.,
not analyzed), and stratigraphy based on visual observations and depth profiles of isolongifolene concentrations at locations A1, A4, A6, A7, and thereference site R (white circles), and 137Cs concentrations at location A6 (black circles). The gray shaded area indicates our stratigraphic correlationof the tsunami trace, including the sand deposit and the isolongifolene peaks. The black arrow representing the inundation area of the 2011
Tohoku-oki tsunami is based on Tanigawa et al. (2018)

by organic soil that developed under normal depositional conditions. We could only trace the sandy tsunami
deposits as far inland as locations A5 and B4 (222.3 and217.6 m from the present shoreline, respectively, Fig. 3),
about 78% of the total inundation distance (280 m fromthe present shoreline according to Tanigawa et al. 2018).
5.2 Terrigenous biomarker distribution consistent
with the inundation area

Sedimentological and standard geochemical approaches
did not provide any evidence for a marine incursion as far
inland as the known limit of tsunami inundation. Beyond
the sandy tsunami deposit (locations A6, A7, and B5; Figs.
S1–S3), it was difficult to identify the event layer fromsedimentological approaches alone (CT images, sandcontent, mean grain size, and grain-size distribution).
Furthermore, our biomarker analysis failed to detectany marine-sourced biomarkers, such as short-chain
n-alkanes, alkenones, dinosterol, and brassicasterol, bothat locations where the sandy tsunami deposit was (A1,
A4) and was not recognized (A6, A7; Fig. 4). The sand in
this area is interpreted to have originated in the supra-,inter-, and sub-tidal locations (Tanigawa et al. 2018),
suggesting that there should be a sufficient potential formarine-sourced biomarkers to be present. One possibility for the failure of marine-sourced biomarkers is thatthey tend to be adsorbed in organic-rich, fine-grained
particles (Shinozaki et al. 2015), and it is therefore likelythat these biomarkers were too scarce to be detected in
the coastal forest zone near the shoreline. Or anotherpossibility is that the study locations are relatively inlandand most of the materials incorporated into the tsunami
deposits are from the local, and thus, it may have become
difficult to find marine-sourced biomarkers.
One striking result of our geochemical analysis, however, is that elevated isolongifolene concentrations wereobserved at every location within the inundation area
(Fig. 4). At location A1, the peak concentrations may

Shinozaki et al. Progress in Earth and Planetary Science(2022) 9:29

be due to the accumulation of isolongifolene in a mud
cap generated by the tsunami waves and/or the tsunami waves physically impacting the trees and knocking loose pine materials to the ground. Particularly, trees
on the seaward side of the coastal forest were found to
be downed or dead (Fig. 2a), and it is possible that theconstituents leached from these destroyed trees andpenetrated inland within the tsunami inundation area.
Although it is not clear why the isolongifolene concentrations increased, we note that the peak concentrations
surpassing normal deposition clearly correlate with the
sandy tsunami deposit, when present.
The peak isolongifolene concentration at location A6(ca. 1 × ­104 ng/g at 1–2 cm depth), where the sandy tsunami deposit is absent, probably represents the trace ofthe 2011 Tohoku-oki tsunami inundation (Fig. 5). At thatlocation, we also observed elevated 137Cs concentrationsin the soil layer at 0–1 and 3–4 cm depth (Fig. 5), which
we attribute to the aftermath of the Fukushima Daiichi
Nuclear Power Plant incident in March 2011 and fallout
from nuclear weapons testing in 1963, respectively (e.g.,Marshall et al. 2007). Because the 2011 Tohoku-oki tsunami is the only large tsunami to have inundated this area
since 1963, it is reasonable to conclude that the isolongifolene peak observed above the 1960s Cs peak at location
A6 was caused by the tsunami. Similarly, we conclude
that the peak isolongifolene concentration in the uppermost soil at location A7 also correlates with the 2011
Tohoku-oki tsunami inundation (Fig. 5).
5.3 Implications for precise estimation of tsunami
inundation areas

Observations of the 1992 Flores (Shi et al. 1995), 2004
Sumatra (Chadha et al. 2005), and 2006 Kuril tsunamis(MacInnes et al. 2009) have demonstrated that seawater generally inundates areas beyond the distributions ofassociated visually identifiable sandy tsunami deposits. In
Japan, Goto et al. (2011), Abe et al. (2012, 2020), ChaguéGoff et al. (2012), and Iijima et al. (2021) reported that the
distributions of sandy tsunami deposits ≥ 0.5 cm thickwere less extensive than the observed seawater inundation areas in the Tohoku region. Some studies haveattempted to bridge this gap via geophysical approaches(Sugawara et al. 2011, 2013; Namegaya and Satake 2014),
but it has proven too difficult to reconstruct past seawater inundation areas solely based on observations ofstratigraphic records.
Our results indicate that geochemical approaches may
be effective for identifying paleotsunami traces in geological layers beyond the limits of visually identifiabletsunami deposits. As with isolongifolene in the Oirase
coastal pine forest, if an appropriate biogeochemicalcompound can be identified at a given study site, it could

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be used to reconstruct past tsunami inundation events(Fig. 5). Marine-sourced markers are known to be effective in identifying tsunami traces, and our results thateven terrestrial-sourced markers can be used to identify
tsunami traces add to the arsenal of techniques available.
Although isolongifolene is likely restricted to pine forestsand may not be useful in pine forests in other regions,
our results suggest that it may be possible to detect tsunami inundation events by tracing depth and lateralchanges of geochemical proxies specific to a given depositional environment, even if marine-sourced proxies are
not present. If specific organic compounds can be foundby reconstructing paleovegetation, there is a possibility
that terrigenous biomarker can be applied in paleotsunami research. In addition, although the present study
was conducted on a relatively simple topography (flat togentle slope, Fig. 1d, e), it is necessary to examine in the
future how the characteristic biomarker behaves in various topographic settings that greatly affect tsunami inundation and deposition.

6 Conclusions
We performed sedimentological and geochemical investigations in a coastal pine forest that was inundated by
the 2011 Tohoku-oki tsunami. Of the sites within the
known inundation area, the sandy tsunami deposit wasrecognized at seaward sites, but was not visually or sedimentologically identifiable at the most landward sites.
For comparison, we also studied a reference site beyond
the known inundation limit. Marine-sourced biomarkers were absent at all locations, but isolongifolene, anorganic compound derived from Pinus, was detected at
all analyzed locations. Elevated isolongifolene concentrations clearly above the background soil concentration were observed not only within the sandy tsunami
deposit, but also at similar depths in soil samples at
locations further inland but still within the known inundation area; no peak isolongifolene concentration wasobserved at the reference site. These results suggest thatthe incorporation of this terrigenous biomarker into thesoil was enhanced by or during the 2011 Tohoku tsunami
inundation.
Our results show that site-specific terrigenous biomarkers can provide evidence of tsunami inundationeven when no marine-sourced biomarkers are found.
Although the biological marker analyzed herein is probably unique to pine forests in Oirase, the reconstruction of
paleovegetation may reveal environment-specific organic
compounds. By following the lateral and depth changesof these unique organic compounds, it may be possible tofind traces of tsunami inundation beyond the sandy tsunami deposits, even in paleotsunami research.

Shinozaki et al. Progress in Earth and Planetary Science(2022) 9:29

Abbreviations
CT: Computed tomography; LOI: Loss on ignition; GC–MS: Gas chromatograph/mass spectrometer.

Supplementary Information
The online version contains supplementary material available at https://​doi.​
org/​10.​1186/​s40645-​022-​00491-6.
Additional file 1: Figure S1. Stratigraphy compared to depth profiles ofwater content (%) and loss on ignition (%).
Additional file 2: Figure S2. Stratigraphy compared to depth profiles ofsand content (%) and mean grain size (phi).
Additional file 3: Figure S3. Stratigraphy compared to vertical changes in
the grain-size distribution of the sand fraction.
Acknowledgements
We acknowledge Kazumi Ito for his useful advice during Cs analyses. The
biomarker analysis was performed under the cooperative research programof the Center for Advanced Marine Core Research (CMCR), Kochi University(Accept No. 20B067). CT image analysis was performed at the Geological Survey of Japan, AIST. We thank Pedro J.M. Costa and an anonymous reviewer forproviding valuable suggestions and comments. We also thank Masaki Yamada
for kindly editing our paper.
Author contributions
TS and YSa conceived and designed the study. TS, YSa, and DM conducted
the field survey, and KT assisted with planning the field survey. TS and MI conducted biomarker analysis. TS conducted grain-size analysis, and DM analyzed
the data. YSh conducted Cs analysis, and TT assisted with the interpretations. TS led the writing of the main text and preparation of the figures, with
contributions from the other authors. All authors read and approved the final
manuscript.
Funding
This research was supported by Geological Survey of Japan (AIST) to YSa, DM,
YSh, KT, and TT and a JSPS KAKENHI (20J00423) to TS.
Availability of data and materials
Data sharing is not applicable to this article as no datasets were generated oranalyzed during the current study. Please contact the corresponding author
for data requests.

Declarations
Competing interests
The authors declare that they have no competing interests.
Author details
1
Geological Survey of Japan, National Institute of Advanced Industrial Scienceand Technology, Central 7, Higashi 1‑1‑1, Tsukuba 305‑8567, Japan. 2 Center
for Advanced Marine Core Research, Kochi University, Monobe 200, Nankoku 783‑8502, Japan. 3 Graduate School of Frontier Sciences, The Universityof Tokyo, Kashiwa 277‑8561, Japan.
Received: 3 March 2022 Accepted: 20 May 2022

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