OXYGEN ISOTOPE EVIDENCE FOR A WARMING-INDUCED INCREASE IN SOIL WATER EVAPORATION DURING THE PLIOCENE ON THE CHINESE LOESS PLATEAU

Rainfall in eastern Asia provides water to nearly one fifth of the world’s population. How does warming affect the availability of water in this region? We measured the δ¹⁸O values and, for select samples, the triple and clumped isotope compositions of paleosol carbonate nodules exposed in the late Miocene-Pliocene Red Clay at Jiaxian near the northern margin of the Chinese Loess Plateau in northern China. We measured the δ¹⁸O values of as few as 1 to as many as 16 (mean = 6) nodules every 25 cm (there are a few gaps where nodules did not occur) through 60 m of section in order to reduce uncertainty on mean values and to inform the selection of samples for clumped and triple oxygen isotope analyses. The clumped isotope temperatures (which for soil carbonates are probably between mean annul and maximum monthly air temperatures) show late Miocene cooling (as low as 16°C) and a warm Pliocene (as high as 33°C), consistent with trends in mid, high and some low latitude sea surface temperatures. The stratigraphic mean carbonate nodule δ¹⁸O values are lowest (-9.5‰ PDB) during the coldest period (late Miocene) and are highest (-8.5‰ PDB) during the warm Pliocene from 5 to 3.5 Ma. This trend occurs also in the calculated soil water δ¹⁸O values which are lowest (-8.5‰ SMOW) during the late Miocene and highest (-6‰ SMOW) during the warm early- to mid-Pliocene. An intensified East Asian summer monsoon circulation during warmer climates is expected to result in the opposite trend. We explore the cause of the observed trend using Δ’¹⁷O values of soil water, calculated from clumped and triple oxygen isotope measurements of carbonate, which are primarily sensitive to evaporation. Δ’¹⁷O values of water range from -20 to -60 per meg and decrease with increasing δ¹⁸O values, consistent with an evaporation control by which soil water is more evaporated in warmer climates. We therefore conclude that the effect on soil water availability on the Chinese Loess Plateau of any increase in rainfall that might occur with warming is moderated or reversed by increased evaporation.