Crayton J. Yapp
Ph.D., Caltech
Professor
Phone 214-768-3897
Email cjyapp@smu.edu
- Stable Isotope and Minor Element Geochemistry of Low Temperature Systems
- Paleoclimates from Geochemical Proxies
- The Water Cycle
- Goethite-bearing Deposits and Ancient Atmosphere
Courses Taught
GEOL 6338 - Thermodynamics of Geological Processes
Related Links
List of Publications [PDF]
Research Statement
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.
Selected Publications
Yapp, C.J. and Poths, H. (1992) Ancient atmospheric CO2 pressures inferred from natural goethites, Nature 355, 342-344.
Yapp, C.J. (1993) Paleoenvironment and the oxygen isotope geochemistry of ironstone of the Upper Ordovician Neda Formation, Wisconsin, USA, Geochim. Cosmochim. Acta 57, 2319-2327.
Yapp, C.J. (1993) The stable isotope geochemistry of low temperature Fe(III) and Al "oxides" with implications for continental paleoclimates, Climate Change in Continental Isotopic Records, Geophys. Monograph 78, 285-294.
Yapp, C.J. and Poths, H. (1993) The carbon isotope geochemistry of goethite (a-FeOOH) in ironstone of the Upper Ordovician Neda Formation, Wisconsin, USA: implications for early Paleozoic continental environments, Geochim. Cosmochim. Acta 57, 2599-2611.
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 Plan. Sci. Lett. 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, Geochimica et Cosmochimica Acta 60, 4397-4402.
Yapp, C.J. (1998) Paleoenvironmental interpretations of oxygen isotope ratios in oolitic ironstones, Geochimica et Cosmochimica Acta,62, 2409-2420.
Hsieh, J.C.C. and C.J. Yapp (1999) Stable carbon isotope budget of CO2 in a wet, modern soil as inferred from Fe(CO3)OH in goethite: possible role of calcite dissolution, Geochimica et Cosmochimica Acta 63, 767-783.
Yapp, C.J. (2000) Climatic implications of surface domains in arrays of dD and d18O from hydroxyl minerals: goethite as an example, Geochimica et Cosmochimica Acta 64, 2009-2025.
Yapp, C.J. (2001) Rusty relics of Earth history: iron (III) oxides, isotopes, and surficial environments, Annual Review of Earth and Planetary Sciences 29, 165-199.
Yapp, C.J. (2001) Mixing of CO2 in surficial environments as recorded by the concentration and d13C values of the Fe(CO3)OH component in goethite, Geochimica et Cosmochimica Acta 65, (in press).