Effects of elevated CO2 levels in soils

Interactions of CO2 in Aqueous Solutions and at Mineral-Water Interfaces

Most scientists agree that global warming is an urgent issue that will have serious consequences on the Earth’s environment. Long term observations and climate modeling studies have indicated that the concentration of carbon dioxide (CO2), one of the main greenhouse gases, is increasing at alarming rates and plays a key role in modifying global climates. In an attempt to control CO2 emissions into the atmosphere, researchers have been trying to identify different ways to collect and store/dispose of CO2 in geological systems, such as deep ocean waters, sediments, and aquifers. Research is in progress at several institutions to explore the biogeochemistry of these “CO2-mixed” systems, and their short-, and long-term affects on the ecosystem. We are examining the molecular interactions of CO2and its soluble species with other chemical species in aqueous solutions and at mineral-water interfaces under a variety of environmental conditions. The specific objectives of our research group are to evaluate the:

  • Coordination chemistry of CO2 and its soluble species at a range of temperatures and pressures in aqueous solutions
  • Dissolution of minerals in the presence of CO2 and their influence on soil solution chemistry
  • Influence of elevated CO2 on soil mineral weathering and biogeochemical reactions in the rhizosphere.

Currently we are building an apparatus for examining the molecular chemistry of CO2in aqueous solutions and at mineral surfaces using vibrational and X-ray spectroscopy methods.

Role of Elevated CO2 on Biogeochemical Reactions in the Rhizosphere

Several recent investigations have shown that elevated CO2 significantly influences biogeochemical reactions in soils, and in soil fauna and flora. Our research is focused on variations in soil mineralogy, mineral surface chemistry, and soil pore water chemistry as a function of elevated CO2 in soil pore spaces, and the influence of these variations on vegetation and metal uptake by plants. Mammoth Mountain in California has provided us with a natural laboratory by which we can study CO2-soil mineral interactions and their influence on metal uptake by plants.

Geological History of Mammoth Mountain

Mammoth Mountain is a volcanic region in mid-eastern California, at the southwest corner of the Long Valley Caldera. Due to high volcanic activity (Inyo volcanic chain, shown in the figure below), the area around Mammoth Mountain is covered by volcanic flows of different composition ranging from andesitic to rhyolitic. These flows are covered with an approximately 500 hundred year old thick layer (varies from location to location) of pumice—light, porous rock that forms when lava cools quickly, thus trapping air bubbles inside. Mammoth Mountain also lies in a seismologically active area and is plagued by frequent earthquakes. A large earthquake in 1989 was accompanied by a magma intrusion that seems to have tapped a source of carbon dioxide.

Since the 1989 earthquake there has been a steady flux of CO2 from a relatively shallow (within 20km of the surface) magma reservoir. The CO2 flux occurs along fault lines and was identified as the main reason for large areas of tree-kill in the vicinity of fault/ fracture zones. This CO2 flux has been carefully recorded by researchers at the US Geological Survey, and we plan to take advantage of the consistent CO2 flux to determine how soil minerals have reacted to prolonged (~12 years) exposure to elevated CO2.
 

Figure 1
Figure 1: CO2 flux at the Horseshoe Lake tree-kill area, Mammoth Mountain, CA (Taken from Farrar et al. 2001)

 

Due to its young age, the soil profile is not well developed. We recently collected pumice samples from different locations of the tree-kill areas and also from unaffected regions (about 12" deep). We are in the process of examining the chemical makeup of soils using x-ray diffraction, IR, Raman, electron microscopy, and X-ray absorption spectroscopy. By comparing the affected and unaffected areas, we would like to determine the role of CO2on mineral weathering processes. Since a majority of reactions involve dissolution and precipitation on mineral surfaces, we will be focusing on the surface chemistry of silicate glasses in the pumice.

 

Figure 2
Figure 2: Influence of high CO2 efflux on the pines and redwoods in the vicinity of Horseshoe Lake, Mammoth Mountain, CA.
 
Figure 3
Figure 3: White pumice layer and soil profile at Mammoth Mountain sampling site.

 

Mammoth Mountain is a unique area from the perspective of carbon sequestration research. Since many researchers are hoping to slow the accumulation of CO2 in the atmosphere by injecting it into underground reservoirs, it is important to know how this injection will affect the surrounding environment. The situation at Mammoth Mountain is an example of what might happen if CO2 escapes from these underground reservoirs. If we can determine the level of CO2 that is detrimental to plants we can in turn determine how careful we must be about containing the injected CO2.

Spectroscopic Investigation of Carbonate at the Mineral-Water Interface

Chemical reactions occurring at the mineral water-interface determine the mineral surface properties and the mobility of mineral-forming elements as well as other dissolved species. Carbonate is a pervasive anion in aquatic systems with the potential to strongly influence the properties of the mineral-water interface. The ubiquitous nature of dissolved carbonate species makes carbonate an important species in environmental systems despite its relatively low affinity for most mineral surfaces. Dissolved carbonate species arise from the dissolution of carbonate minerals and from the dissolution of gaseous CO2. Carbon dioxide concentrations significantly above the atmospheric level in subsurface systems can result from biological respiration, magmatic outgassing, and anthropogenic storage as part of a carbon sequestration strategy.

A fundamental spectroscopic study of dissolved carbonate species at the surfaces of a suite of magnesium-containing silicate minerals is being undertaken. While silicate minerals have relatively low specific surface areas, they are the most abundant mineral phases encountered in almost all subsurface environments. At a fundamental level, the nature of carbonate binding to mineral surfaces will be examined as a function of total concentration and pH. Baseline mineral surface properties will first be determined in the absence of dissolved carbonate, and then with dissolved carbonate through both the dissolution of COas well as the addition of carbonate and bicarbonate salts. The effects of CO2-enhanced dissolution on the surface will be examined at low temperature and pressure. In a collaborative effort, the effects of dissolution at high pressure and slightly elevated temperature will also be investigated. A set of experiments will be conducted to investigate the behavior of heavy metal nutrients and contaminants at the silicate mineral-water interface at elevated CO2 concentrations. At such elevated concentrations, the formation of ternary surface complexes and the precipitation of uncommon metal carbonate solid phases may be encountered.

This study will use spectroscopic techniques to gain molecular-scale information for the reactions at the mineral-water interface. Fourier transform infrared spectroscopy will be performed in attenuated total reflectance mode for the investigation of suspensions and in external reflection mode for the investigation of single crystals. Many experiments will involve the in situ spectroscopic investigation of interfacial reactions, but specimens generated externally will also be analyzed. For the work involving heavy metals, extended X-ray absorption fine structure (EXAFS) spectroscopy and X-ray absorption near-edge structure (XANES) spectroscopy will be performed at synchrotron facilities. Additional solid characterization will be accomplished with X-ray diffraction, electron microscopy, and other routine mineralogical techniques.