I studied soils, past and present.

For my Ph.D. research, I was interested in how life has been sustained on Earth deep in its past as well as here and now—and what that means for our future and search for life on other planets.

Soil nutrient cycling

If we want to know more about how ancient nutrient cycles worked, we need to understand modern processes as best as we can. I collected hundreds of soil samples to study spatial and climatic trends in modern soils. Their geochemistry provides a critical baseline for spatial variability in terrestrial ecosystems which we can use to improve interpretations of "fossil" soils' geochemistry.

Paper: Dzombak & Sheldon (2020), Soil Systems 73(4)

Data: on Mendeley Data


Sampling thin, rocky soils from an active streambed in Iceland, in July 2018.


I was also interested in biological soil crusts, which are symbiotic communities of bacteria, algae, and fungi that thrive all over the world. (You may be familiar with them from "Don't Bust The Crust!" signs dotting many trails in the southwestern U.S.) I studied how these small but powerful communities store and transport biologically-important elements (like phosphorus), which is useful not only for understanding modern ecosystems, but also in looking for signs of ancient life on Earth.

Technical talk: Paired geochemistry and metagenomics in biological soil crusts (abstract here)

Biological soil crusts are a critical component of aridlands ecosystems, preventing erosion and serving as little oases with water and nutrients. Utah, May 2019.

The evolution of terrestrial biogeochemical cycles

The same nutrients that are important in terrestrial systems today were important billions of years ago, too. I compiled a huge dataset of geochemistry from paleosols ("fossil" soils preserved in the rock record) to constrain how one these nutrients—phosphorus—changed over time, and how it relates to some of the most dramatic changes in Earth's history (like the evolution of animals and Snowball Earth). I used the same geochemical data to quantify weathering intensity on the continents over the past three billion years. Using these records, I showed that on geologic timescales, both weathering intensity and the concentration of P in soils lack discrete state changes—contrary to prevailing hypotheses. I also demonstrated how the evolution of land plants helped create what is today the biggest carbon sink: soils.


Technical talk: Three billion years of continental weathering

Public talk: my Ph.D. defense!

One of my favorite spots so far has actu

Thick mosses cover a recent volcanic landscape in Iceland. This scene stretches for miles and miles, making it easy to imagine a misty, primordial landscape!

Exploring past climates and atmospheres


Sampling Eocene (~50 million year old) soils in the Red Desert of Wyoming in September 2019.


We can also use paleosols to reconstruct past climates and environments. Since soils form at the intersection of the biosphere, atmosphere, and the solid Earth, they provide a unique opportunity for us to understand what was happening right at Earth's surface. Using their chemistry, we can estimate how hot it was, how much it rained, what plants were growing in the soil, and even how much carbon dioxide and oxygen were in the atmosphere when they formed.

My research using paleosols focuses on two aspects: reconstructing climate change during key times in Earth's history (e.g., mass extinctions and greenhouse periods), and statistical analyses of paleosol geochemistry to explore their heterogeneity and constrain paleosol-based proxies.

This type of reconstruction is of interest beyond academia. Constraining how climate changed in the past is critical for understanding how the climate (and ecosystems) might change in the future. For example, my work using Eocene paleosols from the Faroe Islands well help us understand how high-latitude regions respond to greenhouse warming, like our planet is undergoing today.

Review paper: Reconstructing atmospheric CO2 and O2 using paleosols

Paper: Stable terrestrial climate and environment in India across the K-Pg

Technical talk: Lateral heterogeneity in paleosols and implications for paleoclimate proxy uncertainty