Isotope amount effects in hydrologic and climate reconstructions of monsoon climates: Implications of some long-term data sets for precipitation

• Seasonal and longer Isotope amount effects are weak to non-existent in a 32-year record for precipitation in Tucson, Arizona.
• Seasonal and long-term amount effects are weak to absent in records from New Delhi and Hong Kong, but may exist at Guam.
• Seasonal mean isotope data at Tucson, New Delhi and Hong Kong are not simply related to region-wide monsoonal intensity.
• The data analysis raises serious questions about published paleoclimate reconstructions based on isotopes in speleothems.

Many studies of Quaternary climate make use of terrestrial stable isotope records which are interpreted based on seasonal patterns of stable isotopes in modern precipitation. Multi-decade records of isotopes in rainfall allow testing of the assumed behavior of isotope signals used for this interpretation on multi-year to decadal scales. A 32-year record of stable O and H isotopes in precipitation in Tucson, Arizona permits a detailed examination of stable isotope amount effects, at time scales ranging from individual events to decades, in a location with summer monsoonal and winter frontal rainy seasons. Amount effects are weak to non-existent in Tucson at seasonal and longer time scales, and are not useful for discriminating either wetter or drier rainy seasons or wetter or drier decades. Amount effects are also weak to non-existent in published data for annual and multi-year amount-weighted averages for monsoonal precipitation in New Delhi and Hong Kong, but an annual amount effect appears to be present on Guam (U.S. Territory). In addition, site-specific amount effects do not correlate with measures of regional monsoon intensity. This data analysis challenges the general validity of paleoclimate reconstructions based on short-term (sub-annual) relationships observed in precipitation isotope data when applied to long-term records such as speleothem studies.

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