Scientists look at pyrite isotopes on the ocean floor in a whole new light
“Fool’s gold” stores information about local environmental conditions
Beneath the ocean floor, layers of sediment tell a story of hundreds of millions of years of environmental change. However, this story has taken a surprising turn. Scientists for the past several decades assumed that sulfur isotopes in pyrite (commonly referred to as “fool’s gold”) could be used to transcribe the history of the oxidation state of the Earth’s oceans. But in a shocking twist, scientists have learned that local conditions on the sea floor are what really controls pyrite sulfur isotopes.
“We found that bulk pyrite sulfur isotopes are dominantly controlled by local conditions on the seafloor, such as the rate of sediment accumulation, porosity and permeability, and organic matter concentration,” says Dr. Roger Bryant from Purdue University College of Science. “This discovery should fundamentally change how bulk pyrite sulfur isotopes are used by the scientific community.”
Bryant and his team published their findings in Science in a publication titled, “Deconvolving microbial and environmental controls on marine sedimentary pyrite sulfur isotope ratios.” Bryant is the lead author and an Assistant Professor at the Purdue University Department of Earth, Atmospheric, and Planetary Sciences.
“Marine sedimentary rocks are like time capsules that record information about Earth’s surface environment from whenever they were deposited,” he explains. “We can use sedimentary rocks to study the past four billion years of Earth’s history. For me, the most exciting thing about that is that we can tackle big questions like ‘How and why did life first evolve?’ or ‘Why are we here?’ Our best answers to these questions relate to the chemical makeup of our oceans and atmosphere, which we believe have changed substantially in the past. For example, we suspect that rising atmospheric oxygen levels paved the way for the evolution of complex life – including humans – on Earth. Testing this hypothesis requires creative thinking about the controls on geochemical variations in marine sediments (e.g., pyrite sulfur isotopes), so we can interpret the sedimentary rock record with higher degrees of certainty.”
According to the publication, the team compared recent glacial and interglacial sediments and found that, even though there was significant environmental change, some data and elements unexpectedly remained roughly the same. Bryant explains that their process led the team to discover that microbial activity was not the main driver in bulk pyrite sulfur isotopes as was previously thought.
“One of the main benefits of single-grain sulfur isotopic analyses is that you can generate a histogram of sulfur isotopic variability within the population of pyrite grains contained in a sediment sample (as opposed to traditional ‘bulk’ analyses that just give you a single data point for a sediment sample),” he explains. “Different features of these histograms can be used to infer different environmental information. For example, the minima of histograms can be used to infer ‘εmic’, which is the magnitude of sulfur isotopic fractionation associated with the metabolic activity of microbes called sulfate reducers. We noticed that even when bulk pyrite sulfur isotopes changed a lot (between glacials and interglacials), the minima of histograms (and therefore εmic) remained broadly constant. This tells us that microbial activity was not the thing driving the changes in bulk pyrite sulfur isotopes and refutes one of our major hypotheses.”
Getting a core sample from the bottom of the ocean is no easy feat. The core samples used in Bryant’s discovery were sourced back in 2004 as part of a French ship-based ocean drilling project called PROMESS 1 (PROfiles across MEditerranean Sedimentary Systems). The ship used boring devices typically used by the oil and gas industry to dig hundreds of meters into the ocean floor. This drilling process produced core samples that provide a pristine and untouched glimpse into the history of the Earth.
Bryant started this work during his PhD at Washington University in St. Louis, then continued his collaborative work with that team when he became an assistant professor at Purdue.
“Several groups of researchers have studied these samples, most notably my coauthor Virgil Pasquier, who published a seminal paper on the glacial-interglacial bulk pyrite sulfur isotope oscillations, in PNAS, in 2017,” says Bryant. “Soon after that, my PhD advisor David Fike let Virgil know that I was developing microanalytical methods to measure the sulfur isotopic composition of individual pyrite grains, and Virgil very graciously agreed to let me have some of his samples to test out my methods. Back then, we had no idea it would lead to this long and fruitful collaboration. Itay Halevy got on board with the project even earlier and began constructing his numerical model to simulate pyrite formation in marine sediments. Eventually, we were able to show with Itay’s model that my data, Virgil’s earlier data, and all published bulk pyrite sulfur isotope data, could be interpreted in a self-consistent manner.”
Halevy’s work is detailed in a companion paper in the same issue of Science.
According to Bryant, this discovery was made possible by a globally unique Secondary Ion Mass Spectrometry (SIMS) facility at Washington University in St. Louis, funded in part by grants to David Fike from the DOE and NSF. Bryant’s research group at Purdue will luckily have continued access to that facility, in addition to a world class collection of new instruments in West Lafayette. These instruments include several mass spectrometers and a laser ablation system that will allow his team to do similar work but also branch into several other applications of isotope geochemistry. He says the research is ongoing and the team at Washington University in St. Louis will continue to refine the methods he used with SIMS and hopes that it will stimulate several new research avenues among the broader academic community.
How does a scientist studying the ocean floor end up at a landlocked university in northern Indiana? Bryant explains that the land under Northern Indiana is surprisingly similar in that the rock beneath the area was formed in a sedimentary fashion due to its oceanic history.
“Purdue has shown a commitment to growth in the Earth, Atmospheric and Planetary Sciences, and I was excited to join a thriving and ambitious community. Since arriving, I have seen broad potential for collaboration across the Colleges of Science and Agriculture,” he says. “In addition, although West Lafayette is far from the modern ocean, we are surrounded here by sedimentary rocks that formed in a warm ocean hundreds of millions of years ago. Many of these rocks are full of fossils that I plan to use to piece together how some of North America’s earliest animals made a living on the ancient seafloor.”
RNB was supported by a McDonnell Center for the Space Sciences Graduate Fellowship. This work was supported by DOE/BER (#DE-SC0014613) and NSF awards (EAR-1229370 and EAR-2048986) to DAF. IH acknowledges a European Research Council Starting Grant (#337183). The drilling operation was conducted within the European project PROMESS 1 (contract EVR1-CT-2002-40024).
About the Department of Earth, Atmospheric, and Planetary Sciences at Purdue University
The Department of Earth, Atmospheric, and Planetary Sciences (EAPS) combines four of Purdue’s most interdisciplinary programs: Geology & Geophysics, Environmental Sciences, Atmospheric Sciences, and Planetary Sciences. EAPS conducts world-class research, educates undergraduate and graduate students, and provides our college, university, state and country with the information necessary to understand the world and universe around us. Our research is globally recognized, our students are highly valued by graduate schools, employers, and our alumni continue to make significant contributions in academia, industry, and federal and state government.
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Writer: Cheryl Pierce, Communications Specialist
Contributor: Dr. Roger Bryant, Assistant Professor for the Purdue University Department of Earth, Atmospheric, and Planetary Sciences