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国立大学法人SHIMANE
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島根大学

144th Estuaries Open Seminar

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East Asian winter monsoon intensity in the Pleistocene basedon seasonal land snail shell stable isotope ratios
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Dr. David L. Dettman
University of Arizona, USAZP^X\ ZX[_e (QTWcOdf!"UeVf!")Monsoon climates are characterized by strong seasonal oscillations between a very wet season andvery dry conditions. The study of ancient variation in monsoon climate strength and its impact onregional environments is difficult because geological materials rarely record individual seasons. Onesolution is to use high-resolution sampling of land snail shells as monitors of wet-season and dry-season
differences. We subsampled shells along the spiral growth axis, producing 15 to 35 samples per shell todocument maximum and minimum oxygen and carbon stable isotope ratios. The most robust signal inland snail shell oxygen isotope records is connected to evaporation. The arid dry-season leads tosignificant evaporation in the land snail body water, which elevates the d18O values recorded in theshell. Wet-season oxygen isotope ratios are more negative and can be tied roughly to the d18O of wet
season precipitation and summer temperatures. Carbon isotope ratios are tied to the diet of theanimals, and reveal the proportion of C3 and C4 plants in the landscape.
Larger land snail (> 1.5 cm diameter) taxa have behaviors that can lead to significant evaporation ofbody water, as they are willing to range away from local water sources. Modern samples show that thetotal amplitude in oxygen isotope variation in a shell can be related to the intensity and duration of thedry season. This suggests that amplitudes in ancient shells are indicators of the winter monsoon
strength. The most negative d18O values are not correlated with the intensity of the summer monsoon,
suggesting that d18O values are not good indicators of summer monsoon strength.
Data from one genus of land snail (Cathaica species) living on the Chinese loess plateau throughout thelate Pleistocene allow us to make some generalizations about monsoon climate in this region. In general,
the oxygen isotope amplitude in shells from glacial periods are larger than those from the interglacial
periods. This suggests that the East Asian Winter Monsoon was strongest during glacial intervals on theLoess Plateau. The largest d18O amplitude observed was 19.2 ‰. Marine Isotope Stages 2, 6, and 8
have the largest amplitudes and, by inference, the strongest winter monsoons in our record. Olderglacial intervals seem to have weaker winter monsoons. However, glacial amplitudes were not always
larger, just as climate undergoes significant short-term variation in glacial intervals (such as D-O cyclesand Heinrich events) oxygen isotope amplitudes show large amounts of variability in glacial times. In
contrast, interglacial intervals tend to have smaller d18O amplitudes, pointing to warmer and wetter
winters in interglacial times. But like the glacial data, some interglacial amplitudes are larger than someglacial records. These data show that the differences in high frequency climate variability is as large asthe differences between glacial and interglacial climates.

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