2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 8
Presentation Time: 9:45 AM

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


DETTMAN, David L., Department of Geosciences, University of Arizona, Tucson, AZ 85721, dettman@email.arizona.edu

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 δ18OVPDB = 0.89 x water δ18OVSMOW – 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‰).