Congratulations to Rebecca Dzombak, who successfully defended her dissertation on Thursday, May 20, 2021

Advisor: Nathan Shedon


The Earth has undergone major changes over the past three billion years, including the evolution of microbial life, the atmosphere filling with oxygen, supercontinents forming and breaking apart, and complex multicellular life spreading on land and in oceans. These events are intimately connected to each other. Because these major Earth systems (the biosphere, atmosphere, and geosphere) are so interconnected, it is critical to know how their relationships with each other have evolved through time if we want to understand how some of the biggest changes in our planet’s history, like the oxygenation of the atmosphere or the rise of eukaryotic life, happened.

A lot of what we know about geologic time comes from marine records like shales, which tell us reliably what was happening in the oceans. But that’s only half of the question. Without looking at land, we cannot fully understand how the relationships between the biosphere, atmosphere and geosphere have evolved. What was happening on land that sent those sediments into the sea to begin with?

Soils help us answer that question. My dissertation focuses on how some key nutrients behave on land today, how they may have behaved in the past, and how weathering (breaking down rocks) may have controlled the transport of nutrients from the continents to the oceans. By analyzing the chemical composition of soils and their fossilized counterparts (called paleosols) through time and comparing soil chemical trends to important factors like climate and tectonics, I connect changes on the continents to changes in the oceans and atmosphere. In my dissertation defense talk, I will focus on three major questions:

  • Did the concentration of phosphorus, an essential nutrient for life, change in soils through geologic time?
  • Did the intensity of weathering on land change through time, as many geologists have hypothesized?
  • How does the distribution of fossil soils over 3 billion years relate to the formation and break-up of continents?