WHAT IS ISOTOPE SEASONALITY?

Records of oxygen isotope (δ¹⁸O) variability in fossil teeth, invertebrate shells, and speleothems have been used to infer changes in climate seasonality through time. During their accretionary growth, these substrates record seasonal fluctuations in environmental water δ¹⁸O values. These fluctuations reflect seasonal variation in δ¹⁸O values of precipitation, either delivered locally or integrated over a catchment, which in many cases is correlated with seasonal temperature or precipitation cycles.

The isotopic seasonality method is one of a limited number of methods for reconstructing terrestrial paleoclimate seasonality and promises to become more widely applied as δ¹⁸O analysis of fossil organic substrates is further explored. Many assumptions and intermediate processes underlie the method, but at its foundation is the assumption that the δ¹⁸O seasonality of precipitation tells us something meaningful about climate parameters. This assumption has been explored in a phenomenological context, but never assessed in a global analysis. Here, I use global grids of monthly precipitation δ¹⁸O, mean temperature, and precipitation amount to investigate the degree to which the amplitude and phasing of seasonal δ¹⁸O cycles reflect climate seasonality. There is a striking degree of variability in the relationship between δ¹⁸O of precipitation and both climate parameters worldwide, but strong correlation between temperature and δ¹⁸O dominates north of 30° N latitude. The analysis reaffirms classic concepts, such as the shift from temperature-controlled isotope seasonality at high latitudes to precipitation-controlled seasonality in the tropics, but also highlights a number of regions in which there is significant ambiguity between temperature- and precipitation-controlled isotope seasonality or no strong relationship with either climate parameter. The products of this global analysis reveal where and how δ¹⁸O seasonality might be useful for paleoclimate study, with the caveat that they are features of the modern climate state. The mechanisms underlying observed δ¹⁸O/climate seasonality relationships must be explored in the context of dynamic climates to allow robust interpretation of the δ¹⁸O seasonality proxy.