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題目(和文)

実験室培養実験に基づく陸生カタツムリ炭酸塩殻の同位体シグナルがもつ環境情報

Title(English)

Environmental implication from isotopic signatures of land snail shellcarbonate based on laboratory culturing experiment

著者(和文)ZHANGNAIZHONG

Author(English)

Naizhong Zhang

出典(和文)

学位:博士（理学）,学位授与機関:東京工業大学,
報告番号:甲第9679号,授与年月日:2014年12月31日,
学位の種別:課程博士,
審査員:吉田 尚弘,大河内 直彦,山田桂太,豊田栄,上野Citation(English)

Degree:,Conferring organization: Tokyo Institute of Technology,
Report number:甲第9679号,
Conferred date:2014/12/31,
Degree Type:Course doctor,Examiner:,,,,

学位種別(和文)

博士論文

Category(English)

Doctoral Thesis

種別(和文)

要約

Type(English)

OutlinePowered by T2R2 (Tokyo Institute Research Repository)雄一郎

Environmental implication from isotopic signatures of land snail shellcarbonate based on laboratory culturing experimentLand snails are strictly local indicators of terrestrial environment and are considered asarchive materials in studying the paleo environment, especially in the Quaternary period. Upto now, isotopic tools such as δ13C, δ18O and clumped isotope thermometry (Δ47) are widely
applied in reconstructing the environment conditions such as paleo-climate changes, shift ofvegetation distribution, rainfall amount, and so on. However, the knowledge of therelationships between these isotopic signatures and relative environment conditions are still
very poor, which could limit their application in better understanding the paleo environment.
Therefore, laboratory cultivation of land snails under various controlled environment isnecessary.
In this study, eight adult land snails of Acusta despecta sieboldiana, which is widelydistributed around Japan and Korea, were collected from Yokohama, Japan. Their offspringwere cultured from eggs to adults around 6–8 months under various controlled environments.
Briefly, the snails grew up at three different temperatures (20 °C, 25 °C, and 30 °C), twokinds of diet (Cabbage, δ13C = -28.4‰; Corn, δ13C = -12.0‰), three kinds of water (Tapwater, δ18O = -8.2‰; Canadian ice water, δ18O = -12.9‰ and Oceanic deionized water, δ18O= -0.1‰), and two kinds of calcium sources (calcium carbonate and calcium phosphate).
Finally, shell δ13C, δ18O and clumped isotope values were measured by isotope ratio massspectrometry (IRMS, MAT 253 or Delta XL); organic δ13C and water δ18O values weremeasured by two different cavity ring-down spectroscopy (CRDS, Picarro), or by IRMS
(Delta V).
For carbon isotopic composition (δ13C) of land snail shell carbonate, we confirmed it derives
from three sources: diet, atmospheric CO2, and ingested carbonate (limestone). Herein, we
consider the influences of metabolic rates and temperature on the carbon isotopic compositionof the shell carbonate. Based on previous works and on results obtained in this study, a simple
but credible framework is presented for discussion of how each source and environmental
parameter can affect shell carbonate δ13C values. According to this framework and somereasonable assumptions, we have estimated the contributions of different carbon sources foreach snail individual: for cabbage (C3 plant) fed groups, the contributions of diet, atmospheric
CO2 and ingested limestone respectively vary as 66–80%, 16–24%, and 0–13%. For corn (C4plant) fed groups, because of the possible food stress (lower consumption ability of C4 plant),
the values vary respectively as 56–64%, 18–20%, and 16–26%. Moreover, we present newevidence that snails have discrimination to choose different plants as food. Therefore, wesuggest that food preferences must be considered adequately when applying δ13C in paleoenvironment studies.
For oxygen isotopic composition (δ18O), we discussed the effectiveness of flux balance model
raised by Balakrishnan and Yapp (2004). However, the complex and variable environmental
parameters, such as relative humidity and ingested water isotope composition, can restrict its
application in the modern and paleo studies. Secondly, we observed a fairly good andcommon relationship (slop = 0.69; R2 = 0.98) between snail shell δ18O values and body water
δ18O values at various temperatures, suggesting the shell is directly precipitated from the body
fluid. Nevertheless, the deviation of this slope from unity (slope = 1; isotopic-equilibrated

precipitation), indicates that potential non-equilibrated precipitation or vital effect exists. In
addition, these relations seem to be only controlled by the environmental parameter such astemperature and evaporation, rather than the ingested limestone, input water (e.g. differentdiet) and growth rate. Thirdly, snails living in some non-optimal conditions (e.g. lowtemperature) can adapt their biological behaviours to be accustomed to the environment,
which will affect the oxygen isotope compositions in body fluid and shells. Additionally, weobserved that evaporation effect can largely enrich 18O in snail body water, and in further
produce a more enriched δ18O values than expected values of isotopic equilibrated
precipitation.
For the clumped isotope (∆47), we present the first empirical calibration of the clumped
isotope thermometer based on land snails in-laboratory cultured in a temperature range of
20 °C to 30 °C. The data were normalized into the absolute reference frame by the empiricaltransfer function (ETF) suggested by Dennis et al. (2011). The slope is consistent with thepublished ones digested at 25 °C; and different to those digested at 90 °C. Besides, our data
show a discrepancy (~0.02‰) to those re-calibrated data using two-point secondary transferfunctions (e. g. Ghosh et al., 2006; Dennis and Schrag, 2010), however, the values are
consistent with those directly referenced to the ETFs (e.g., Henkes et al., 2013; Wacker et al.,2014), suggesting the absolute reference frame based on isotopic equilibrated CO2 gas atvarious temperatures is effective. Therefore, we encourage more studies about empiricalcalibration curves should be carried out based on directly referencing to the absolute frame.
Moreover, we confirmed that land snail shells are precipitated in isotopic equilibrium
condition (for clumped isotopes), and the clumped isotope data are not related to the various
bulk isotopic sources such as different food, or water sources, and also has no relation to theingested carbonate or snail growth rate. In addition, we confirmed the size effect reported byWacker et al. (2013), that is, to those digested in phosphoric acid at 25 °C, the sample size
less than 6.5 mg will present more positive data, which sometimes can be as large as 0.07‰.
In summary, this study permits us to comprehensively understand the relation between
isotopic signatures of land snail shells and their living environment. The next step is to test
these relations from studying the living snails in natural and then try to apply theseimplications in the Quaternary land snail fossils. All in all, our researches prompt the field of
paleo environment reconstruction from studying the isotopic composition of land snail fossils.