I studied soils, past and present.

As a geochemist, I'm 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 have collected over 400 modern soils (from mostly North America) to study spatial and climatic trends in modern B horizons, which are the horizons typically used in paleoclimate or paleoenvironment reconstructions. Improving our understanding of modern nutrient cycles on land is also useful when thinking about how land use and fertility may shift under a changing climate.

In particular, I'm interested in potential relationships between the iron content of soils and regional climates/seasons, and how biological soil crusts (symbiotic desert communities of microbes, moss, and fungi) cycle phosphorus, an essential nutrient that's typically biolimiting in terrestrial ecosystems.

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.

Photo by Dr. Emily Beverly.

Spending my Friday afternoon sieving som

Modern soils with a range of iron oxides make a red and yellow rainbow!

Terrestrial early Earth

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!

The same nutrients that are important in terrestrial systems today were important two and a half billion years ago, too. I'm interested in constraining what these nutrients were doing - specifically, phosphorus and iron (and how they relate to the all-important carbon cycle). I'm also interested in how weathering has changed over time, delivering different amounts of nutrients to the oceans. Knowing how essential nutrients behaved is critical for reconstructing past oxygen levels and understanding the conditions in which early life evolved.

Speaking of early life...

By studying the chemistry of fossil soils (paleosols), we can look for signs of terrestrial life in the rock record - life that probably looked a lot like the biological soil crusts I also study. They leave chemical traces, called 'biosignatures,' in the soil, and this is what I look for in paleosols. These same terrestrial communities likely contributed oxygen; the question now is, how much?

Review paper: Reconstructing atmospheric CO2 and O2 using paleosols

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

Technical talk: Three billion years of continental weathering


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

Photo by Katie Seguin.

Using past climates to predict the future

We can also use these 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.

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

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


Sampling Eocene (~50 million year old) soils in the Red Desert of Wyoming in September 2019. We traced a single fossilized soil across 3km to see how much it changed spatially, in order to improve how we interpret its geochemistry and the proxies that rely on it.

Photo by my labmate, Dr. Rebekah Stein.