Shells and Paleo-Isotope Hydrology In the Freshwater/terrestrial Realm: Looking for the Monsoon Using Seasonal Aridity Records

The use of oxygen isotope ratios in biogenic freshwater shell for the study of paleotemperature has proven very difficult due to uncertainties in the isotopic composition of the water in which the shell was produced. By inverting the question, however, it is possible to be precise about the isotopic composition of ancient surface waters. An empirical comparison of bulk shell δ¹⁸O and average growing-season river water δ¹⁸O leads to a linear correlation:

Shell δ¹⁸O_VPDB = 0.89 x water δ¹⁸O_VSMOW – 0.98 (R²=0.975, n=28)

This relationship should be applicable in temperate seasonal climates across a broad range of mean annual temperatures.

In the study of the history of the Asian Monsoon, evidence for strong seasonal aridity is a reliable geochemical indicator of monsoon intensity. This has been applied using freshwater mollusks in bodies of water that change in δ¹⁸O on a seasonal basis, but many river systems in the monsoon environment do not undergo a large cycle in oxygen isotope ratios. The short residence time of water in river systems and groundwater sources lead to a greatly reduced response in river δ¹⁸O to seasonal aridity. In the shells of terrestrial mollusks, however, there is a strong oxygen isotope response to a dry season. A comparison of seasonal δ¹⁸O cycles of unionid bivalves with that of land-snails in humid climates and monsoonal climates demonstrates a unique monsoon relationship. All land snails have minima that are within the δ¹⁸O cycle present in contemporary unionid shells. In humid climates (Texas, Japan) the maximum δ¹⁸O value is within 2‰ of the associated aquatic shell maximum. In contrast, shells from a monsoon system (central China) have maxima that are offset by +7 to +9‰. The δ¹⁸O range in land-snail shell is much smaller in humid environments (2 to 4‰) than in monsoon climates (9 to 11.5‰).