The carbonate clumped isotope geothermometer is a potentially powerful new stable isotope proxy that, among other uses, could be applied in reconstructing land surface temperatures at the Earth’s surface. Knowledge of prior land surface temperature has various applications, from reconstructing the elevation of ancient mountain ranges, to providing constraints to evaluate the output of climate simulations in the recent past of deeper in Earth’s history. Pedogenic, or soil carbonates, are abundant over Earth’s arid to semi-arid climates and provide an ideal proxy material for constraining earth surface conditions over broad geographic areas. Critical to unleashing the potential of the clumped isotope temperature proxy is constraining how the temperatures derived from the pedogenic carbonate proxy material relate to actual surface temperatures. Without this solid framework for deriving surface temperatures, their application will be fraught with misinterpretations. 

The goal of this NSF-sponsored study was to determine the how the variables of climate, seasonality and elevation influence clumped isotope temperatures by exploiting the unique juxtaposition of environments available in the Andes mountains of Argentina and Chile. To achieve this goal, the study was executed through the collaborative efforts of early career faculty in Earth science and their graduate students at Syracuse University and the University of Washington. In addition, we leveraged the principal investigator's network of collaborators in Argentina and Chile to participate in the study. Our foreign colleague's backgrounds brought unique and complementary perspectives to the study. To simulate different climates in a winter dominated precipitation regime, we excavated, sampled and instrumented active modern soils over a 4,300 m (14,190 ft) of elevation and ~ 150 km to the Agua Negra mountain pass with Argentina. In the lowlands on the eastern flank of the Andes, we explored the potential for site to site variability at a low elevations over ~ 1.5° of latitude (120 miles) and different ecological niches. A total of 10 sites were instrumented for 2 years of in-situ measurements of soil moisture and soil temperature, along with 6 of the same sites measuring soil carbon dioxide concentrations and above ground meteorologic conditions. To improve our understanding of the isotopic context of the pedogenic carbonates, we executed field campaign collections of river water samples and monthly precipitation samples at selected weather stations previously established and maintained by the Chilean and Argentine governments. All of these data were necessary to determine the conditions under which pedogenic carbonate formed at each pit, and how that compared with simple metrics like mean annual air temperature, hottest or coolest month temperatures.  

The major results of our study were published in the peer-reviewed, scientific literature (doi’s 10.1016/j.epsl.2016.02.033 and 10.1016/j.epsl.2016.02.003). We identified the infiltration of water into the soil profile after the largest rainfall events as the likely mechanism that results in observation of isothermal temperatures with depth in both the clumped isotope temperature record and in-situ instrumentation. This observation also requires that pedogenic carbonate form during the initial drying of the soil profile, in the summer, and not towards the near complete loss of soil moisture as previously assumed. Our monitoring efforts in Argentina, combined with information on soil water chemistry, suggest that carbonate would always be supersaturated in the soil solution, thus carbonate formation should be relatively frequent in the upper meter of the soil, where wetting is frequent, and less so with depth since wetting events only happen once or twice a year. In Chile, our work was identified a major temperature shift in clumped isotope temperatures that can only be caused by the average lowest elevation extent of perennial snowfall. We also identified the soil carbonate under near or sub-zero conditions in our highest elevation site where mean annual temperatures are near the freezing point or water. Despite a strong winter seasonality, clumped isotope temperatures on the western flanks of the Andes also reflected summer temperatures. Unfortunately, our in-situ monitoring of soil conditions at the Chilean sites were hampered by drought conditions in this part of Chile which did not abate until the end of the study, thus we have limited context for interpreting these hotter than expected temperatures. Together our results represent a major advance in our understanding of how to apply clumped isotopes to ancient soils in continental settings and provide additional insight into how pedogenic carbonate forms. 

This NSF award supported the national and international collaboration in the Earth Science at two domestic institutions and two institutions in Latin America. This award directly supported the MS work of a female graduate student at Syracuse University. The results of this study have been an continue to be presented at national conferences, incorporated into research presentations at other universities and used by the PI in teaching at the undergraduate and graduate levels at Syracuse University.  

Last Modified: 10/10/2016
Modified by: Gregory D Hoke