The information available from stable isotope data of pedogenic carbonates has proven invaluable in our understanding of the terrestrial environment. DeltaC13 values of pedogenic carbonate are influenced by the ratio of C3 to C4 biomass in soils, which varies with changes in atmospheric p(CO2) and the seasonality of precipitation. DeltaO18 values reflect local meteoric water, which is directly tied to climate. The amount of information that can be extracted from pedogenic carbonates is limited, however, because certain assumptions regarding their formation have never been tested. Questions include: 1) Do carbonates form during a particular season or following discreet storm events?; 2) Do carbonates form in isotopic equilibrium with soil CO2 and soil water?; 3) Are there distinct characteristics of different biomes (in addition to C3/C4 ratios) that can be used to better model carbon isotope measurements of ancient pedogenic carbonate? We address these questions by measuring the isotopic composition of soil CO2 and water as a function of depth and comparing these data with measured values of neoform carbonate in modern (late Holocene) soils. Several other parameters that control deltaC13soil CO2 will also be monitored throughout the year to identifying variability in response to changes in temperature and moisture. The results of this study will allow for more detailed and accurate reconstruction of paleoclimate using soil carbonate geochemistry.
We have identified two sites for the study: 1) the piedmont of the Sandia mountains east of Albuquerque and 2) the Sevilleta National Wildlife Refuge (SNWR), located approximately 1 hour south of Albuquerque. The SNWR is ideal because it encompasses the convergence of four distinct biomes. These biomes have migrated over the last 100 years in response to climate change, allowing us to evaluate the degree of dissolution/reprecipitation that occurs in pedogenic carbonate. All analytical techniques required for this study have been fully vetted. We have developed a simple technique for extracting and measuring the deltaC13 and deltaO18 values of soil CO2, and another for analyzing the deltaO18 value of sub-millimeter amounts of soil water. We have demonstrated our ability to measure the deltaC13 and deltaO18 values of neoform carbonates in soil profiles. The project will benefit from the expertise of the two PIs, Sharp for stable isotope geochemistry, and McFadden for soil genesis and modeling of soil carbonate formation.
Broader impacts: This work involves a significant student-research component at graduate, undergraduate and K-12 levels. It forms the basis of a Ph.D. study by Dan Breecker and will expose him to a wide range of geochemical analytical techniques in addition to the extensive field work combining pedology with geochemistry. UNM is designated as a Minority Serving Institution and is one of only four Research/Doctoral-Extensive institutions in the country to also be designated as a Hispanic Serving Institution. As in the past, we will recruit several minority undergraduate students to undertake senior honor's theses as part of their education and for the benefit of the project. Finally we expect to have several NASA SHARP students (NASA funded summer internships for minority/women students) at the 11th/12th grade level working on an aspect of this project. PI Sharp has already had five successful NASA SHARP students working on various projects.
