Research interests: impact of climate and life on earth processes, soils in biogeochemical cycles, human impacts on soils and ecosystems

The Earth has entered the Anthropocene, a geologic epoch where humans are altering the face of the planet on a scale rivaled only by past meteor impacts or glacial cycles. These are therefore exciting and important times for earth scientists, a period requiring multidisciplinary research to understand the functioning of the present global system, a need to project the Earth’s future, and the skill to conveying this message effectively and clearly to the public.

 

The Physical and Chemical Imprint of Life on Landscapes

Chile is a natural long running outdoor experiment, where rainfall declines continuously from the south to the north. My research in Chile follows a series of sites stretched along the northern end of the country, sites ranging from semi-arid, to nearly lifeless, climate zones. We have discovered that soil and ecological processes change non-monotonically as rainfall declines and life disappears, and the dry, abiotic region attains properties unexpected from extrapolations from the rest of our largely biotic planet.

For example, soil chemical processes shift from net chemical losses to large chemical gains (from retention of atmospheric solutes) with this decline in rainfall (Ewing et al. 2006). We discovered that the vast accumulations of nitrate in the Atacama desert are the result of the cessation of the soil nitrogen cycle due to aridity and a lack of microbial activity (Ewing et al. 2007), and that without microbial processes that reduce N back to nitrogen gas in soils and sediments , the Earth’s atmospheric composition would (Capone et al. 2006) be largely different than today.

We discovered that variations in the stable isotopes of the element calcium, recently considered to be largely a mark of biology, undergo large changes do to entirely abiotic processes, as do isotopes of sulfur and oxygen in sulfate (Ewing et al. 2008). These fascinating observations, combined with a provocative recent paper by Dietrich and Perron (Dietrich and Perron 2006), have provided a unique insight into how profoundly different how planetary processes behave without the stamp of life. The role of life on geological processes is a question at the forefront of modern earth science. Yet establishing the experimental abiotic “controls” in field studies is exceedingly challenging since most of the Earth’s surface has been exposed to life since the Precambrian.

 

Biotic Effects on Geologic Processes

In a similar vein, we are also studying the role of burrowing animals on the formation of soils and the vernal pools of the Merced River region of California. The formation of vernal pools, and adjoining Mima mounds, is a long standing question in California geomorphology and ecology. This project, part of the PhD of Sarah Reed, combines the use of natural radioactive isotopes, and GIS modeling, to understand the rates at which animals can reconfigure the land surface (Reed and Amundson, 2007).

 

Effect of Climate on Earth’s Nitrogen and Sulfur Cycles

One of the important, and poorly integrated, parts of the soil N cycle is the impact of climate on the production of N2O, a trace gas with more greenhouse gas strength than CO2. Additionally, an even lesser understood feature is the N and O isotope composition of N2O from soils (and its geographical distribution), a chemical signature important to determine the present and past sources of N2O in the atmosphere. We are presently developing numerical models to allow us to understand how variations in different soil processes affect both the quantity and isotope composition of soil N2O. Additionally, we are planning field studies to examine the geographical and temporal variations in California ecosystems.

We have just begun a large collaborative project to examine the climate effect on the S cycle. Unlike C and N, the soil S cycle is much less studied, yet it is an important ecosystem nutrient, and an important pollutant from the burning of coal. We are presently in the very initial stages of our work on this problem, one that involves 4 PIs and numerous students and post docs. The research sites in this project extend from the wet tropics of Puerto Rico to the dry limits of the Atacama Desert.

 

The Soils of Mars

In 2004, we began to analyze the growing data of set of Martian soil chemical analyses using geochemical models commonly used on Earth. These calculations suggested that the Mars soils had lost significant quantities of water-soluble elements relative to the rock from which they formed. This loss, on Earth, can be achieved only by rain/snowfall and the subsequent downward movement of this water and the reaction products. The well-known accumulation of sulfate and chloride salts in Martian soils then appears to have been overlain on this earlier stage of chemical weathering by downward moving water.

This interpretation of the soil chemistry suggests a much wetter (in a relative sense) earlier Mars climate, with a long and protracted aridification. We have recently published the results of this work, and have discussed the nature of Mars soils, in a recent paper (Amundson et al. 2008), This initial foray into planetary geochemistry is part of a larger interest in Mars research. I am an associated earth scientist involved in the development of the “Urey instrument”, a proposed amino acid and organic compound analyzer that is projected to be on the payload of an upcoming European Space Union mission to Mars (Aubrey et al. 2008).

 

The Human Imprint on Earth

The longest running theme in my research is the impact of humans on soil and ecological processes. My concern now extends, like most scientists, to the global climate system and to the earth as a whole. I have discussed the role and impact of humans in two recent articles/essays (Amundson et al. 2007)(Amundson 2008).