CRAYTON J. YAPP
Ph.D., California Institute of Technology
Professor

• Stable Isotope and Minor Element Geochemistry of Low Temperature Systems
  
• Paleoclimates from Geochemical Proxies
  
• The Water Cycle
  
• Goethite-bearing Deposits and Ancient Atmosphere

Low Temperature Geochemical Systems

rayton Yapp's research has focused on the study of paleoclimates, modern and ancient soils, modern and ancient atmospheres, and the hydrologic cycle using the methods of stable isotope geochemistry. This includes study of the isotopic properties of minerals which form in low temperature environments dominated by the combined interactions of the atmosphere, hydrosphere, and biosphere with the solid earth. Such authigenic minerals are the principal (commonly the only) sources of quantitative information on ancient climates, waters and gases.

Carbonates in surface and Subsurface Environments
Many carbonate-bearing systems of Cretaceous and younger age have yielded isotopic data for which there is a consensus that original oxygen and carbon isotope ratios are preserved. Therefore, carbonate minerals are widely used in paleoenvironmental studies on time scales of about 140 million years or less. One current project being conducted by PhD student Meena Balakrishnan involves isotopic study of the aragonitic shells of several species of modern terrestrial gastropods in a transect extending from Oklahoma to Colorado and New Mexicco. These modern data provide a basis for the interpretation of paleoenvironments recorded in gastropod shells that date from the Late Wisconsin ice age at the Folsom archeological site in northeastern New Mexico. Preliminary results indicate the possibility of significant climatic differences between Folsom time and the present with a rather monotonic change of climate.

Ancient atmospheres, Climates
Current research also includes study of the isotopic systematics of low temperature iron(III) oxides and hydroxides. These minerals commonly precipitate in near-surface, oxidizing environments at pH values less than 6. They are extremely insoluble at higher, near-neutral, pH values. Consequently, their most stable and common representatives, goethite (FeOOH) and hematite (Fe2O3) may often preserve original oxygen isotopic ratios in ancient deposits. This supposition is supported by exchange experiments carried out in Yapp's laboratory. Additional experiments have determined the temperature dependence of the oxygen isotope partitioning between goethite and the water from which it precipitated. These results indicate that goethite is a viable paleothermometer.

Our research on oxygen and hydrogen isotopes led to the discovery of an Fe(CO3)OH component which seems to be in solid solution in goethite. The amount of Fe(CO3)OH in goethites appears to be a function of the ambient CO2 pressure and temperature at the time of mineral formation. This geochemical parameter is being used to quantitatively investigate some elusive aspects of environments recorded in goethites formed in ancient subaerial weathering systems (e.g., soils). Under the right circumstances, such systems can preserve information on ancient temperature, rainfall and the partial pressure of carbon dioxide in the Earth's atmosphere. Research on systems ranging in age from 440 million years to the present suggests that atmospheric CO2 levels have been as much as 16 to 18 times higher at different times in the ancient past than they are today. However, surface temperatures of ancient, low altitude, tropical sites appear to have varied only from about 23 degrees C to 32 degrees C. Interestingly, these temperatures do not seem to covary directly with ancient atmospheric CO2 pressures. In particular an inferred CO2 pressure 16 times higher than modern coincides with continental scale glaciation on Gondwanaland, parts of which were in a near-polar position in the southern hemisphere at that time. In addition, the biological productivity of this 440 million year old soil was quantitatively inferred from measurements of the Fe(CO3)OH component in the goethite. That ancient biological system, which existed on the continents before the widespread advent of vascular plants, seems to have had levels of productivity comparable to modern tropical soils. A quantitative approach was formulated that suggests it is possible to put lower bounds on the amount of molecular oxygen in the Earth's atmosphere at different times in the Phanerozoic. The existing data indicate that atmospheric oxygen levels were not lower than 13 percent of modern over the past 440 million years.

Modern Atmospheres
M.S. graduate student Shannon Clark is studying the concentration and isotopic composition of atmospheric carbon dioxide in the Dallas metropolitan area. This research is exploring the role of combustion of different fossil fuel types in the modification of CO2 concentrations in an urban atmosphere and possible relationships to seasonality, atmospheric dynamics, etc.

Modern Soils
Jean Hsieh and Yapp published carbon isotope data from the Fe(CO3)OH component in goethites from an active soil in eastern Texas. Their results indicate that the carbon isotope budget of CO2 in that soil was influenced by both oxidation of organic matter and dissolution of relict marine calcite inherited from the parent rock. Moreover, the study suggests that goethite is probably dissolved and reprecipitated in the soil as a consequence of biological mediation. This result supports an assumption that ancient pedogenic goethites can preserve information on steady-state soil CO2 systematics.

SELECTED PUBLICATIONS
Yapp, C.J., and Poths, H., (1992), Ancient atmospheric CO2 pressures inferred from natural goethites, Nature, 355, 342-244.

Yapp, C.J., (1993), Paleoenvironment and the oxygen isotope geochemistry of the Upper Ordovician Neda Formation ironstone, Geochim. Cosmochim. Acta, 57, 2319-2327.

Yapp, C.J., and Poths, H., (1993), The carbon isotope geochemistry of goethite (alpha-FeOOH) in the Upper Ordovician Neda Formation ironstone: implications for Early Paleozoic continental environments, Geochim. Cosmochim. Acta,57, 2599-2611.

Yapp, C.J., (1993), The stable isotope geochemistry of low temperature Fe(III) and Al "oxides" with implications for continental paleoclimates, in: Amer. Geophys. Union Monograph 78, Climate Change in Continental Isotopic Records (Swart P.K., Lohman, K.C., McKenzie, J., and Savin, S., eds.), 285-294.

Yapp, C.J., and Poths, H., (1994), Productivity of pre-vascular continental biota inferred from the Fe(CO3)OH content of goethite, Nature, 368, 49-51.

Yapp, C.J., and Poths, H., (1996), Carbon isotopes in continental weathering environments and variations in ancient atmospheric CO2 pressure, Earth Planetary Science Letters, 137, 71-82.

Yapp, C.J., (1996), The abundance of Fe(CO3)OH in goethite and a possible constraint on minimum atmospheric oxygen partial pressures in the Phanerozoic, Geochim. Cosmochim. Acta, 60, 4397-4402.

Yapp, C.J., (1997), An assessment of isotopic equilibrium in goethites from a bog iron deposit and a lateritic regolith, Chemical Geology, 135, 159-171.

Yapp, C.J., (1998), paleoenvironmental interpretations of exygen isotope ratios in oolitic ironstones, Geochimica et Cosmochimica Acta 62, 2409-2420.

Hsieh, J.C.C. and Yapp, C.J., (1999), Stable carbon isotope budget of CO2 in a wet, modern soil as inferred from Fe(CO3)OH in pedogenic goethite: Possible role of calcite dissolution, Geochimica et Cosmochimica Acta 63, 767-783.