Article Cite This: ACS Earth Space Chem. 2019, 3, 1346−1352
http://pubs.acs.org/journal/aesccq
Mussel Shell Geochemical Analyses Reflect Coastal Environmental Changes Following the 2011 Tohoku Tsunami Naoko Murakami-Sugihara,*,† Kotaro Shirai,† Masako Hori,†,‡ Yosuke Amano,† Hideki Fukuda,† Hajime Obata,† Kiyoshi Tanaka,† Kaoruko Mizukawa,§ Yuji Sano,† Hideshige Takada,§ and Hiroshi Ogawa† †
Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa, Chiba 277-8564, Japan Natural Sciences, Osaka Kyoiku University, Kashiwara, Osaka 582-8582, Japan § Tokyo University of Agriculture and Technology, Tokyo 183-0054, Japan
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‡
S Supporting Information *
ABSTRACT: Coastal areas are socioeconomically important but are susceptible to disturbances by both natural and manmade events. On March 11, 2011, the eastern coast of Japan was seriously inundated by a massive tsunami following the 2011 Tohoku earthquake. The tsunami caused a major disturbance around the coastal area. However, an understanding of the consequences of such an event is often hampered by a lack of knowledge of prior conditions. Furthermore, field observations during and immediately after the event are often particularly difficult. The present study demonstrates that environmental reconstruction by geochemical and growth pattern analyses of mussel shells successfully revealed transitional (daily) environmental changes caused by the Tohoku tsunami. A pronounced surge in shell Mn/Ca ratios observed immediately after the tsunami implies a drastic emission of pore water following sediment disturbance as well as a large input of terrestrial material through backwash. The subsequent decrease of the high Mn/Ca peak indicates a prolonged tsunami disturbance effect over ca. 40 days, with the stabilized shell Mn/Ca ratios observed thereafter (being higher than that prior to the tsunami) suggesting that the latter had altered the coastal environment, allowing for greater susceptibility to terrestrial input following ground subsidence and loss of coastal levees. Shell Mn/Ca patterns provide evidence for the tsunami-generated release of materials stored in sediments, such as organic, nutrient, and pollutant materials, which are then suspended in the water column for sufficient periods to allow for incorporation into geochemical cycles. Although the greatest environmental disturbance occurred immediately after the tsunami, the effects lasted for longer than several months thereafter. KEYWORDS: tsunami, coastal environment, natural hazard, bivalve shell, trace element, Tohoku earthquake, retrospective environmental monitoring
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INTRODUCTION Natural geological processes, including storms, floods, earthquakes, tsunamis, and volcanic eruptions, may cause extensive ecological and socioeconomic damage.1,2 On March 11, 2011, the eastern coast of Japan was impacted by a massive tsunami following the magnitude 9.0 Tohoku earthquake off the Pacific coast of Tohoku.3 The earthquake and tsunami not only caused tremendous social and economic damage but also greatly altered coastal ecosystems and biogeochemical cycles,4−8 including the possibility of heavy metals and other chemical pollutants contained in disturbed marine sediments being released and contaminating the coastal environment.9 Even though tsunamis constitute one of the most catastrophic natural events affecting coastal environments over a very short period of time (from a human perspective), our understanding of the detailed dynamics of environmental and ecosystem © 2019 American Chemical Society
changes resulting from such tsunamis is limited. To accurately understand the impact of such a natural hazard on the coastal environment, continuous environmental information from both before and after the event is essential. However, data covering the period prior are often lacking. Furthermore, field observations are often difficult during and immediately after such an event. In fact, during the 2011 Tohoku event, most automatic observation systems in the affected coastal areas were destroyed by the tsunami and vessels and research facilities were rendered essentially unusable. For example, Fukuda et al. 8 noted that the earliest oceanographic Received: Revised: Accepted: Published: 1346
February 25, 2019 April 27, 2019 May 23, 2019 June 11, 2019 DOI: 10.1021/acsearthspacechem.9b00040 ACS Earth Space Chem. 2019, 3, 1346−1352
Article
ACS Earth and Space Chemistry
Figure 1. (a) External view of the M. galloprovincialis shell. (b) Photograph of the shell cross section along the maximum growth axis. (c) Photographs of the etched shell section (upper image, 11OTWMYT-01; lower image, 11OTWMYT-09). Growth increments and chemical composition of both shells were determined analytically. Triangular arrows indicate spring tide.
of the Supporting Information), opening to the western North Pacific Ocean. The mussel bed had likely been positioned in the intertidal zone before the earthquake in light of the common habitat but was found in the subtidal zone when collected, likely because of land subsidence induced by the earthquake. With a local maximum height of 15.1 m,18 the 2011 Tohoku tsunami inundated approximately 6.68 km2 of land around Otsuchi Bay, causing 5600 buildings to collapse.19 Detailed information on the sampling location can be found elsewhere.4,8,20 After collection, soft tissue was removed from the shells, which were then cleaned and dried at room temperature. One valve from each specimen was embedded in epoxy resin and sectioned along the axis of maximal growth (Figure S2a of the Supporting Information) using a low-speed diamond saw equipped with a wafering thin blade (Isomet 1000, Buehler, Lake Bluff, IL, U.S.A.). Water was used as a lubricant during the sawing process. A half section was used for element analyses (stable isotope and trace element analyses; panels b and c of Figure S2 of the Supporting Information) and the other half for growth pattern analyses (Figure S2d of the Supporting Information). Both sections were mounted on glass slides, ground with diamond cup whetstones of 70 and 13 μm, polished with alumina powder and a rotary polisher, washed with deionized water, and dried. Two individuals (11OTWMYT-01 and 11OTWMYT-09) were used for the analyses. Growth Pattern Analyses. To assign a chronology to shell geochemical profiles, shell growth patterns were examined by sclerochronological methods. The polished cross sections of the shells were immersed in Mutvei’s solution (500 mL of acetic acid, 25% glutaraldehyde, and 7.5 g of alcian blue
observations using scientific instruments (at the same site as the present study) were made more than 2 months after the earthquake, with observations limited to basic environmental parameters. High-resolution reconstruction of past environmental parameters using biological hard tissues, such as bivalve shells, is an effective method for solving the above problem.10 During their growth, bivalves reflect the external environment in the isotopic and chemical composition of their shells. Furthermore, bivalve shells function as a calendar, forming tidal growth patterns.11 A combination of these factors enables the reconstruction of environmental parameters existing during mollusc shell formation, at a temporal resolution of several days or less.12−15 Mussels are particularly suitable for reconstructing environmental conditions as a result of their wide geographical and environmental distribution16 and high tolerance to pollution.17 Furthermore, being sessile organisms, mussels reflect the hydrological changes over time in the same place. This study demonstrates that retrospective environmental monitoring using shells of live-collected mussels enables the reconstruction of environmental changes, brought about by catastrophic events with daily temporal resolution, which are inherently difficult to monitor by conventional methods.
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MATERIALS AND METHODS Sample Collection and Preparation. On September 6, 2011, mussels (Mytilus galloprovincialis, with an average shell height of ca. 4 cm; Figure 1a) were randomly collected alive from the middle of a mussel bed attached to the quay walls of the quay in 7 km long, semi-enclosed Otsuchi Bay (Figure S1 1347
DOI: 10.1021/acsearthspacechem.9b00040 ACS Earth Space Chem. 2019, 3, 1346−1352
Article
ACS Earth and Space Chemistry
Figure 2. Mussel shell geochemical profiles. Vertical gray lines indicate spring tides. (a) Oxygen stable isotope data along the growth axis of the shell. The green (11OTWMYT-01) and yellow green (11OTWMYT-09) lines indicate oxygen stable isotope values. The yellow line indicates the 3 year average seawater temperature (2008−2010). (b) Mn/Ca (red and pink lines) and Mg/Ca (dark blue and bright blue lines) ratios along the shell growth axis. (c) Expanded section of panel b.
powder) at 37−40 °C for 50 min,21 rinsed with deionized water, and dried. Images were taken with a digital microscope (KEYENCE VHX-2000). Portions with narrowly spaced growth lines alternate regularly with portions of widely spaced (and less distinct) growth lines, because the length of time that sessile intertidal organisms are exposed to air at low tide varies with the tidal cycle.22 Accordingly, it was possible to add a time axis to the shell section, starting with the most recent period and working backward. First, the central portions with narrowly spaced growth lines were assigned to the calendar date of the corresponding spring tide. Second, on the basis that the growth rate between consecutive spring tides remains constant, the time axis was calculated by dividing the interval between consecutive spring tides by the number of days elapsed within that interval. Although a time axis error of several days may exist, such would not exceed 2 weeks. Because most in situ tide recorders were destroyed by the tsunami, spring−neap tidal cycle information (days−weeks) was obtained from the free astronomical data software package TIDEforWIN (http://fmie.cside7.com/program/tide.html). As a result of the likelihood of earthquake-induced subsidence having occurred, finer resolution tide data (minutes−hours) may have been inaccurate and was not used. Oxygen Stable Isotope Analyses. To validate the accuracy of the sclerochronological time scale, the shell portion of the winter/summer peak was identified by oxygen
stable isotope profiles as temporal anchor points. Powdered samples were obtained from the outer calcitic shell layer (Figure S2d of the Supporting Information) of the same shell section range for both oxygen isotope profiles and trace element analyses. For each sample, a triangular tungsten carbide drill bit (SHOFU) was used to produce triangular grooves of 1−1.5 mm length and 300 μm depth, parallel to growth lines, with intervals of 1−2 mm. A total of 17 and 26 samples were obtained from 11OTWMYT-01 and 11OTWMYT-09, respectively. δ18O values were measured using a continuous flow isotope ratio mass spectrometer (Delta V Plus, Thermo Fisher Scientific), equipped with an automated carbonate reaction device (GasBench II, Thermo Fisher Scientific), at the Atmosphere and Ocean Research Institute, The University of Tokyo. All isotope values are reported with respect to Pee Dee Belemnite (PDB) based on a NBS 19 value of −2.20‰. Repetitive reproducibility of NBS 19 was better than 0.19‰ (1σ). Water temperatures from 2008 to 2010, recorded at the nearby International Coastal Research Center of The University of Tokyo (Figure S1 of the Supporting Information), were averaged and compared to δ18O (Figure 1b). Trace Element Analyses. Concentrations of trace elements (Mg, Ca, and Mn) in the shell were analyzed by laser ablation inductively coupled plasma mass spectrometry (LA−ICP−MS). Analyses were performed with an eximer laser 1348
DOI: 10.1021/acsearthspacechem.9b00040 ACS Earth Space Chem. 2019, 3, 1346−1352
Article
ACS Earth and Space Chemistry
Figure 3. Photographs and schematic images (a and d) before the tsunami, (b and e) just after the tsunami, and (c, f, and g) some more time after the tsunami. Arrow sizes denote the amount of material flux.
calculations were based on the combined analyses of NIST SRM 612, standard glass, and reproducibility [percent relative standard deviation (% RSD)], measured at
