Atmospheric CO2 Sequestration by Soil CaCO3 in Jordan and New Mexico

Proposal Abstract

Soil carbonate refers to the population of CaCO₃ particles residing in soil. The overall goal of this study was to increase our understanding of soil carbonate in the context of carbon sequestration. That is, to advance our understanding about whether or not soil carbonate in arid and semiarid soils is a sink for atmospheric CO₂.

The primary question that provided the focus for this study was "Has soil carbonate in arid soils of Jordan, New Mexico, and other arid regions sequestered atmospheric CO₂?" Based on the premise that CO₂3- and that HCO₃- is the chemical species that combines with Ca₂+ to form CaCO₃, then the answer is yes. However, before a general question like the one above can be answered, it is helpful to name and classify the components of the system. Thus, an important part of this study was to develop a classification system. The one we present has been revised several times, and probably will be revised further. As of the completion of this report, there are three major categories of our classification scheme with several subcategories that are based on whether the carbonate formed in calcareous or igneous parent material, whether it formed in situ or ex situ, and whether it formed biotically or abiotically. An outline of this carbonate classification is presented below:

For over a century the amount of total soil carbonate has been measured by applying acid to soil, dissolving the carbonate, and measuring weight loss or CO₂ generation. Although this method measures the amount of total soil carbonate, it does not provide information about the types of carbonates listed above. Therefore, the specific goal of this project was to determine if analytic methods could be used to distinguish between the soil carbonate types.

To this end we examined four analytical techniques: (1) x-ray diffraction, (2) calcium isotopes, (3) thin section petrography, and (4) scanning electron microscopy. From these methods we have reached the following conclusions.

1. X-ray diffraction: Based on the level of data gathered so far, the most significant conclusion derived from the XRD investigations is that all Pedogenic carbonate is calcite. Regardless of whether it formed in limestone alluvium, igneous alluvium, as pendants on basalt, or as pendants on dolostone, all pedogenic carbonate measured in this study was calcite. Therefore, XRD is not a promising technique for distinguishing carbonate types.
2. Calcium isotopes: The potential for using Ca isotopes, however, is promising, especially for 42Ca, 43Ca, and 44Ca. First, there are no isotopes or compounds with similar masses to 42Ca and 43Ca that would interfere with their detection. For 44Ca, there is a potential interference of CO₂, but this interference can be eliminated by boiling the solution to drive out CO₂ before ICP-MS analysis. Second, the concentrations of Ca isotopes occur in sufficient quantities to be detected by ICP-MS. Third, differences between Ca isotopic ratios are as great as, or greater than, differences between Sr isotopes that are routinely used as proxy indicators of Ca sources.
3. Thin section petrography: As an analytic technique, thin section petrography can be used to identify non-pedogenic carbonate in the gravel and sand fractions if these particles contain fossils. This technique is also useful for identifying biogenic in situ pedogenic carbonate. But this technique is not useful for distinguishing between non-pedogenic and pedogenic carbonate of the silt and clay fraction.
4. Scanning electron microscopy: With SEM microscopy, in situ forms of pedogenic carbonate can be identified based on appearance of delicate, unbroken structures and euhedral crystals that could not have survived transport without being destroyed. SEM analysis is also able to identify many biotic in situ forms of carbonate-- most confidently for calcified roots and fungal hyphae, less confidently for calcified spheres, elliptical rings, and crystals on roots.

In order to verify these conclusions statistically, more analytic work is needed (which is the topic of a current Ph.D. study at NMSU). While more analytic work is needed, it is now evident that all soil carbonate contains sequestered atmospheric CO₂, even limestone detritus. For even limestone detritus contains CO₂-generated HCO₃- resulting from soil formation. At this stage of our investigation, the primary question has proceeded from "Has soil carbonate sequestered atmospheric CO₂?" to "When was the atmospheric CO₂ in soil carbonate sequestered?"

In addition to evaluating techniques for identifying carbonate types, progress was made in soil and geomorphic mapping and soil characterization for the Jordan study areas of (1) Azraq basin, (2) Qa Shubeika and (3) Qa Suwaed. These basin areas are an important source of grazing as well as providing limited areas suitable for barley cropping on residual moisture. Most of the area of the northern Badia is covered with large basalt boulders. Wind and water erosion has washed some of the soils into low areas. Some of the low areas are known as qa which tend to be covered with deposits of very limited permeability. Other deposits are linear, fed by and drained by wadis. They usually represent zones where channeled wadis spread laterally over more extensive ground surface area. The deposits are seldom saline or are of low salinity and are known locally as marab.

1. Non-Pedogenic Carbonate (carbonate formed in marine or other aquatic environment)
   1. in situ disintegration
   2. ex situ disintegration
      1. eolian detrital
      2. alluvial detrial
2. Pedogenic Carbonate (carbonate formed in soil)
   1. Calci-pedogenic carbonate (calcareous parent material)
      1. in situ calci-pedogenic carbonate
         1. biotic in situ calci-pedogenic carbonate
         2. abiotic in situ calci-pedogenic carbonate
      2. ex situ calci-pedogenic carbonate
         1. biotic ex situ calci-pedogenic carbonate
         2. abiotic ex situ calci-pedogenic carbonate
   2. Igneous-pedogenic carbonate (igneous parent material)
      1. in situ igneous-pedogenic carbonate
         1. biotic in situ igneous-pedogenic carbonate
         2. abiotic in situ igneous-pedogenic carbonate
      2. ex situ igneous-pedogenic carbonate
         1. biotic ex situ igneous-pedogenic carbonate
         2. abiotic ex situ igneous-pedogenic carbonate

Outcome

Articles in Journals

Kraimer, R.A., H.C. Monger, and R. Stiener. "Mineralogical distinctions of carbonates in desert soils." Soil Science Society of America. (in review).

Monger, H.C. and B.T. Bestelmeyer. "The soil-geomorphic template and biotic change in deserts." Journal of Arid Environments. (in review).

Monger, H.C. and B.T. Bestelmeyer. "Broad scale landscape, soil, and wind influences on fine scale dynamics." In Proceedings of the 6th Symposium on the Natural Resources of the Chihuahuan Desert, ed. C. Hoyt. Chihuahuan Desert Research Institute, Alpine, TX. (in review).

Serna-Perez, A., H.C. Monger, J.E. Herrick, and L. Murray. "CO₂ emissions from exhumed petrocalcic horizons (calcrete)." Geochimica et Cosmochimica Acta. (in review).

Lindemann, W.C., H.C. Monger, and R.A. Kraimer. "Carbon isotope fractionation by soil bacteria during biogenic carbonate formation." Soil Science Society of America. (in revision).

Kraimer, R.A. and H.C. Monger. "Carbon isotopic and micromorphologic distinctions of carbonate in desert soils." Soil Science Society of America. (in preparation).

Liu, X., H.C. Monger, and W.G. Whitford. "Carbonate in termite galleries, Chihuahuan Desert, USA." Biogeochemistry. (in preparation).

Michaud, G., H.C. Monger, and D. Anderson. "Geopedological-vegetation relationships in the northern Chihuahuan Desert." Journal of Arid Environments. (in preparation).

Monger, H.C., D.R. Cole, and R.A. Gallegos. "Carbon isotopic fractionation during carbonate biomineralization by desert plants." Geology (in preparation).

Books

Monger, H.C., J.J. Martinez-Rios, and S.A. Khresat. 2005. "Arid and semiarid soils". In Encyclopedia of soils in the environment, 182-187, ed. H. Hillel. Elsevier Ltd., Oxford, U.K.

Monger, H.C. 2003. "Millennial-scale climate variability and ecosystem response at the Jornada LTER site." In Climate variability and ecosystem response at long-term ecological research sites, 341-369, eds. D. Greenland, D.G. Goodin, and R.C. Smith. Oxford Univ. Press, New York.

Monger, H.C. 2002. "Arid soils." Encyclopedia of soil science, p. 84-88. Marcel-Dekker, New York.

Monger, H.C. and L.P. Wilding. 2002. "Inorganic carbon: composition and formation." Encyclopedia of soil science, 701-705. Marcel-Dekker, New York.

Monger, H.C. and J.J. Martinez-Rios. 2001. "Inorganic carbon sequestration in grazing lands." In The potential of U.S. grazing lands to sequester carbon and mitigate the greenhouse effect, 87-118, eds. R.F. Follett et al. Lewis Publishers, Boca Raton, FL.

Monger, H.C. and R.A. Gallegos. 2000. "Biotic and abiotic processes and rates of pedogenic carbonate accumulation in the southwestern United States relationship to atmospheric CO₂ sequestration." In Global climate change and pedogenic carbonates, 273-289, eds. R. Lal et al. Lewis Publishers, Boca Raton, FL.

Ph.D. Dissertation

Kraimer, R.A. 2002. Mineralogical distinctions of carbonate in desert soils. Ph.D. Thesis. New Mexico State University, Las Cruces, New Mexico.

Funding