Collaborative Research: Trace Elements in Pyrite—Validation and Calibration of a Novel Paleoenvironmental Proxy

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Life in the oceans is often limited by the availability of nutrients, such as nitrogen and phosphorus. Other elements occurring in far lower abundances are also bioessential and potentially limiting—particularly trace metals such as iron and molybdenum. These metals are co-factors in many biological processes, and their distributions through time and space are strongly coupled to oxygen levels in the oceans; weathering of the continents; evolving atmospheric composition; and biological evolution. All of these factors tie intimately to climate change as a critical driver and consequence of metal cycling on land and in the oceans. Yet, despite extensive study of metals in modern and ancient oceans, basic gaps remain in our knowledge because of inadequate testing of certain chemical methods, particularly in the modern oceans. These methods, once refined, would allow for more confident exploration of a vast array of past conditions at Earth’s surface—far beyond those seen today. From that reinforced vantage, researchers can begin to imagine more rigorously and comprehensively what may lie in our future. One of the most promising methods for ‘time travel’ through Earth’s evolving oceans is elemental analysis of pyrite, an iron sulfide mineral common today and in the past that may provide easily accessible and highly preservable historical archives. Surprisingly, despite the advantages that may lie with this approach and hints of success already, no previous effort has attempted to validate and calibrate the pyrite tracer in modern oceans where present environmental conditions, local and global, can be tied directly to the composition of the mineral. This study is the first comprehensive investigation of these relationships in modern systems, strengthened by targeted experiments and novel analytical techniques. The principal expected outcome is an improved understanding of the controls and consequences of change at Earth’s surface as expressed in evolving ocean chemistry—past, present, and future—and the cause-and-effect relationships with co-evolving life.Many studies are now using a technique called laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) to measure trace metal contents of pyrite as a tracer for past marine conditions, and the initial results are encouraging. That said, there is still little mechanistic understanding of how and when trace elements are incorporated into pyrite and how these patterns and controls vary across environmental gradients. These uncertainties weaken their utility. The relationships among local controls and the capture of potentially global signals remain largely unknown. Thus motivated, this study is designed around a two-pronged approach: (1) sampling in five classic, well-studied modern marine environments with a well characterized diversity of primary environmental conditions and (2) complementary, carefully conceived laboratory simulations designed to provide unprecedented insight into the mechanisms of trace metal uptake by iron sulfide minerals and their sensitivity to environmental backdrops. The ultimate goal is to expose the strengths and weaknesses of the proxy and best practices while revealing new opportunities, such as the possibility of tracking the availability of bioessential trace metals in the ancient deep biosphere. Plans include analyzing pore waters and sediments from diverse settings in modern oceans and interpreting those data within the framework of experimental results to reveal how trace metal content scales with conditions in the surrounding environment. The utility of the approach will be explored further using novel, highly sophisticated analytical methods applied at high resolution to characterize experimental and natural samples. This will be the first systematic study that correlates pyrite metal contents with metal availability in surrounding fluids and relates such data more generally to the wide range of environmental parameters and proxies often included in studies of Earth’s ancient biosphere. The impacts of this study will extend broadly through outreach opportunities targeting middle and high school students and UCR undergraduates placed in the Lyons lab, while championing diversity and inclusion at all levels. That community footprint will be expanded through a recurring public lecture series and an on-campus science festival presented entirely in Spanish, among other already tested community-directed efforts. At CMU, plans include a geochemical study at two middle schools in Flint (MI) emphasizing water quality and use of those data to construct a new, much-needed community-driven water quality database for Flint.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.