A NEW PALEOCLIMATE PROXY FOR GROWING SEASON PRECIPITATION USING PALEOSOL GEOCHEMISTRY: APPLICATIONS FOR RECONSTRUCTING SOUTH AMERICAN MONSOON CONDITIONS IN NORTHWEST ARGENTINA

Proxy based paleoclimate reconstructions are important for developing models to predict future climate change; however, most paleoclimate proxies only provide mean annual conditions. To gain a more detailed understanding of major hydroclimate changes in the past, as well as how climate may change in the future, it is important to be able to reconstruct seasonal climate conditions which influence dominant vegetation and growing season conditions. Chemical weathering in soils is controlled in part by precipitation and temperature, and previous studies have related the geochemical composition of modern soils to mean annual precipitation (MAP; Sheldon et al., 2002) and mean annual temperature (MAT; Gallagher and Sheldon, 2013) to develop proxy reconstructions of past climates. However, chemical weathering in soils may be more strongly controlled by seasonal conditions because of seasonal temperature and precipitation variability and thresholds for hydrolysis. Here, we use previously published modern soil datasets to determine the relationship between bulk soil geochemistry (B-horizons) and seasonal climate variables. These seasonal climate variables are compared to two established weathering indices: the paleosol weathering index (PWI) and the chemical index of alteration without potash (CIA-K). Preliminary results show that growing season precipitation (GSP), defined as the sum of precipitation in all months meeting a 5℃ and 25mm minimum threshold, has a significantly stronger relationship (R2=0.7) with PWI than the previously defined relationship with MAT. The stronger relationship of GSP with PWI compared to both MAP and MAT proves the significance of seasonal conditions for controlling chemical weathering, and provides an important new paleoclimate proxy. As an example of its broad applications, we use the relationship between GSP and PWI to reconstruct changes to GSP in Northwest Argentina (Salta Province) from the Late Miocene-Pliocene to assess changes to the strength of the South American Monsoon (SASM). We find that GSP steadily increases by ~75% from 6.5 to 5.5Ma, suggesting that the SASM may have intensified over this period (as previously suggested by models).