GEOL 6338 - Thermodynamics of Geological Processes
List of Publications [PDF]
The solid Earth, atmosphere, liquid water and living organisms interact in surface and near-surface environments termed the "critical zone". These interactions occur on time scales that range from fractions of a second to hundreds of thousands of years or more. Our research on the geochemistry of the critical zone has included study of regolith minerals, continental paleoclimates, ancient biological activity, the hydrologic cycle, and modern and ancient atmospheric chemistry.
A Rusting Earth
The coexistence of abundant iron, liquid water, and free atmospheric oxygen on Earth is, as far as we know, unique in the modern solar system. In surface and near-surface environments, chemical weathering reactions that result from this remarkable association of materials commonly produce iron (III) oxide and oxyhydroxide minerals ("rusts"). On Earth, the two most abundant such "rusts" are goethite (a-FeOOH) and hematite (a-Fe2O3). Natural variations of the stable isotope ratios of structural oxygen and hydrogen in goethite indicate variations of ancient temperatures and isotopic compositions of waters, thus providing information on paleoclimates. In addition, all natural goethites analyzed thus far contain small amounts of an Fe(CO3)OH component in solid solution. Exchange experiments indicate that this Fe(CO3)OH component remains a closed system as long as the goethite structure remains intact. Therefore, the amounts and d13C values of the Fe(CO3)OH in different goethite samples preserve information on variations of the partial pressure and d13C of ambient CO2 among ancient environments of goethite crystallization.
Although goethite is extremely insoluble in oxidizing environments at values of pH characteristic of most low temperature terrestrial environments, our research on a modern system indicates that there is apparently continuous, biologically-mediated dissolution and re-precipitation of goethite in active, wet, oxidized soils. Upon removal from this biologically active zone (e.g., burial), the insoluble nature of goethite (and hematite) favors preservation of the isotopic signatures acquired during weathering. Therefore, in active soil environments, pedogenic goethites are continually "updating" their isotopic record of soil conditions. Subsequent burial of the goethite locks in information on continental temperatures, rainfall, and atmospheric and soil CO2 pressures. These proxy records are found in goethites throughout much of the Phanerozoic (i.e., the last ~570 million years). For example, data from a weathering system formed 440 million years ago indicate that the Earth's atmosphere contained 16 times more CO2 than it does today, while temperatures in wet, continental, tropical environments were only about 23°C. Inferred values of the oxygen isotope ratios of the ancient tropical waters suggest intense seasonal, perhaps monsoonal, rainfall. Interestingly, this high concentration of atmospheric CO2 was contemporaneous with continental scale glaciation on Gondwanaland (portions of which were, at that time, near the South Pole). Data from goethites in Paleozoic and Mesozoic deposits pose interesting questions about (1) the dynamics and energy balance of climates in the Earth's past and (2) the role of various boundary conditions in shaping those climates. Our current research on iron (III) oxides, paleoclimates, and ancient atmospheres has expanded to include Cenozoic deposits of Paleocene and Eocene age.
Concentrations of the Fe(CO3)OH component in goethites from a Late Ordovician soil indicate that even prior to the widespread advent of vascular plants there was significant biological activity on the continents. Data from goethite formed in that ancient, wet, tropical environment, indicate that fluxes of soil CO2 arising from biological activity (respiration) may have been as high as 10 millimoles of CO2 per square meter per hour. Soil respiration rates of this magnitude are comparable to those measured in modern, wet, tropical soils.
Mixing of CO2 from different sources in oxidizing surficial environments occurs as a consequence of a number of processes. The Fe(CO3)OH component in goethite can serve as a recorder of many of these processes. For example, formation of goethite as a consequence of pyrite oxidation in the presence of ambient limestone represents a circumstance in which mixing between the CO2 from "upstream soils" and CO2 from local dissolution of limestone is recorded by the Fe(CO3)OH component in the goethite. Evolution of the pH in the system may be deduced from the pattern of variation of concentrations and carbon isotope ratios of the Fe(CO3)OH. Such a pattern appears to be present in goethites which were formed in the Late Cretaceous or Earliest Tertiary as pseudomorphs after pyrite. These data for goethites from the Lucero Mountains of New Mexico indicate how information on groundwater chemistry and the ancient, contemporaneous, upstream soil may be constrained with results from appropriate "natural experiments".
The Interface between Lithosphere and Atmosphere
For the most part, terrestrial gastropods spend the active portions of their lives immediately at the interface between the solid Earth and the atmosphere. Consequently, they can be important indicators of present and past conditions in this critical boundary layer. Our research has shown that the oxygen isotope composition of land snail shells is controlled by ambient temperature, relative humidity, and the oxygen isotope ratio of rain present at the time of snail activity (i.e., the times of aragonitic shell formation). The stable carbon isotope ratios of land snail shells reflect the carbon isotope composition of plants used by the snails for food as well as the flux of carbon associated with normal snail metabolism. Therefore, the carbon isotope composition of these shells records information on the relative importance of C3 versus C4 photosynthesis in the ambient environment. Oxygen and carbon isotope data from modern land snail populations calibrate these paleoenvironmental indicators. These modern data serve as a basis for interpretation of ancient snail populations. A modern calibration is part of a Ph.D dissertation project in this laboratory intended to use isotopic measurements of ancient land snail shells from northeastern New Mexico to address questions about local environmental change at the time of human occupation of the Folsom, NM, site ~11,000 years ago.
Urban Atmospheric Carbon Dioxide
Anthropogenic inputs of CO2 to the atmosphere are superimposed on natural cyclical inputs and outputs associated with soil/plant respiration, photosynthesis, and ocean-atmosphere exchange. The effects of all of these processes must be evaluated on large spatial scales to understand the overall global evolution of CO2 concentrations in the Earth's modern atmosphere. It might be expected that at the smaller spatial scales of "CO2 factories" (i.e., urban environments) anthropogenic inputs would dominate the local atmospheric CO2 budget in a relatively simple manner. However, we found, using measurements of the concentrations and carbon isotope ratios of CO2, that during warmer months, in the midst of the Dallas metropolitan area, photosynthesis has a measurable influence on the local near-surface budget of atmospheric CO2 and may on occasion locally dominate the CO2 flux balances. The effects of this photosynthetic "complication" can be accounted for and separated from anthropogenic effects. The result is that seasonally changing patterns of fossil fuel use are clearly detectable in the Dallas metropolitan area when the concentration and stable carbon isotope composition of atmospheric CO2 are considered in combination. However, our research highlights the need for accurate assessment of regional scale net photosynthetic carbon isotope fractionation factors to enable more precise calculations of relative magnitudes of natural and anthropogenic fluxes of CO2 in this urban setting.
Clark-Thorne, S.T. and Yapp, C.J. (2003) Stable carbon isotope constraints on mixing and mass balance of CO2 in an urban atmosphere: Dallas metropolitan area, Texas, USA, Applied Geochemistry 18, 75-95.
Yapp, C.J. (2004) Fe(CO3)OH in goethite from a mid-latitude North American Oxisol: estimate of atmospheric CO2 concentration in the Early Eocene “climatic optimum”. Geochim. Cosmochim. Acta 68, 935-947.
Tabor, N.J., Yapp, C.J., and Montañez, I.P. (2004) Goethite, calcite, and organic matter from Permian and Triassic soils: carbon isotopes and CO2 concentrations. Geochim. Cosmochim. Acta 68, 1503-1517.
Balakrishnan, M. and Yapp, C.J. (2004) Flux balance models for the oxygen and carbon isotope compositions of land snail shells. Geochim. Cosmochim. Acta 68, 2007-2024.
Balakrishnan M., Yapp, C.J., Theler J.L., Carter B.J., and Wyckoff D.G. (2005) Environmental significance of 13C/12C and 18O/16O ratios of modern land snail shells from the southern Great Plains of North America. Quaternary Research 63, 15-30.
Balakrishnan M., Yapp C.J., Meltzer D.J., and Theler J.L. (2005) Paleoenvironment of the Folsom archaeological site New Mexico, ~10,500 14C years B.P. as inferred from the stable isotope composition of fossil land snail shells. Quaternary Research 63, 31-44.
Tabor, N.J. and Yapp, C.J. (2005) Coexisting goethite and gibbsite from a high paleolatitude (55°N) Late Paleocene laterite: concentration and 13C/12C ratios of occluded CO2 and associated organic matter. Geochim. Cosmochim. Acta 69, 5495-5510.
Yapp, C.J. (2007) Oxygen isotopes in synthetic goethite and a model for the apparent pH dependence of goethite-water 18O/16O fractionation. Geochim. Cosmochim. Acta 71, 1115-1129.
Yapp, C.J. (2008) 18O/16O and D/H in goethite from a North American Oxisol of the Early Eocene climatic optimum. Geochim. Cosmochim. Acta 72, 5838-5851.
Feng, W. and Yapp, C.J. (2009) 18O/16O and D/H ratios of pedogenic kaolinite in a North American Cenomanian laterite: paleoclimatic implications. Geochim. Cosmochim. Acta 73, 6249-6263.
Yanes Y. and Yapp C.J. (2010) Indoor and outdoor urban atmospheric CO2: stable carbon isotope constraints on mixing and mass balance. Applied Geochemistry 25, 1339-1349.
Yanes Y., Yapp C.J., Delgado A., Ibáñez M., Alonso M.R., De-la-Nuez J., Castillo C., Quesada M.L. (2011) Pleistocene-Holocene environmental change in the Canary Archipelago: a land snail isotopic record. Quaternary Research 75, 658 - 669.
Yapp C.J. and Shuster D.L. (2011) Environmental memory and a possible seasonal bias in the stable isotope composition of (U-Th)/He-dated goethite from the Canadian Arctic. Geochim. Cosmochim. Acta 75, 4194 - 4215.