60th Annual Report on Research 2015

For much of Earth’s early history the atmosphere was oxygen depleted, with significant changes in ocean-atmosphere oxygen levels both catalyzing and responding to some of the most important evolutionary events in the history of life, including mass extinctions. On the modern Earth, global warming has begun to intensify the development of anoxia in coastal areas worldwide, placing renewed emphasis on understanding the historical dynamics of anoxia. Within the last few years, some breakthrough studies have sought to use Cr systematics (measurements of total Cr concentration and/or stable isotope distributions) in banded iron formations and ancient soil profiles in order to refine our understanding of oxygen abundances in the ocean-atmosphere system over time. However, an incomplete understanding of Cr geochemistry remains, impeding the full realization of its use as a proxy for reconstructing paleoredox conditions.

The processes responsible for Cr incorporation into anoxic sediments are unknown. Only thermodynamic calculations and laboratory experiments have been conducted to explore the fate of Cr at the sediment water interface and below. Information regarding Cr burial in reducing aquatic environments and anoxic sediments is scarce. Even the most recent studies using Cr as a proxy largely rely on ancient data from batch experiments under nitrogen atmosphere between pH 2 and 6. These pH values are not representative of any marine environment. To our knowledge, Cr speciation in natural anoxic sediments has never been determined. Once incorporated in anoxic sediments or black shales, the main permanent host phase(s) also remain unidentified. Although basic Cr systematics have been explored within anoxic systems covering more than 3.5 billions years of age, the evolution of the Cr redox state and its molecular environment through geological time remain unexplored, notably in black shales. The full potential for Cr as a paleoredox tracer thus cannot be realized until these parameters are known.

We plan to address these gaps in our understanding of Cr geochemistry by answering these questions:

1. What is the Cr oxidation state in black shales and modern marine anoxic sediments? How has the Cr oxidation state evolved over geological time?
2. What is the molecular environment of Cr in black shales and modern marine anoxic marine sulfidic sediments? How has the molecular environment of Cr evolved over geological time?
3. How is Cr distributed spatio-chemically within black shales and modern marine anoxic sediments? What phase(s) (mineral or organic) act as the main host(s)?
4. How must we refine our understanding of the Cr system to accommodate this new more detailed information on Cr speciation?

Project Objectives

1. Determine the evolution of Cr oxidation state in a selection of ancient black shales of Archean, Proterozoic and Phanerozoic age and modern marine anoxic sediments using X-ray Absorption Near Edge Spectroscopy (XANES).
2. Explore the molecular environment of Cr in a selection of ancient black shales of Archean, Proterozoic and Phanerozoic age and modern marine anoxic sediments and its potential transformation over time using Extended – X-ray Absorption Fine Structure (EXAFS).
3. Identify the main host phase(s) for Cr in these euxinic settings by using an elemental mapping approach with Laser – Ablation High – Resolution Inductively Coupled Plasma Mass Spectrometry (LA-HR-ICP-MS) and Synchrotron-based X-Ray Fluorescence Microprobe (SXRFM).
4. Propose an updated geochemical model for using Cr as a paleoredox proxy based on these new findings.

Accomplishments and current progress

Most of our proposed research requires having access to a synchrotron light source such as the Advanced Photon Source (APS) at Argonne National Lab. EXAFS can provide information on the distance, coordination number, and nearest atoms to the Cr atom (S, O, Cr, Fe, and C); while XANES is sensitive to oxidation state and coordination chemistry of the Cr atom. Access to these techniques is limited and consequently competitive. Research proposals (for beamtime only) can be submitted three times a year and must be fared very well (top 10%) during the peer review process.

This project is still in its early stage since the Ph.D. student (Heidi Babos), involved in this project, just started 9 months ago. Yet, after reading the Cr literature and began learning about the basics of XANES and EXAFS, we submitted our first proposal to get beamtime at APS last summer. In August 2015, we were awarded 5 days of beamtime for December 2015, meaning that data collection for objectives 1 and 2 will be achieved by the end of the year. This fall we are running preliminary tests for LA-HR-ICP-MS analyses that will be conducted next year.

Impact on Dr. Chappaz’s career

Heidi Babos is my first Ph.D. student. Without the support of the ACS-PRF, I could not have recruited her. Graduate students are vital for young assistant professors. I defined myself as a molecular geochemist specialized in studying trace metal speciation in both modern and ancient aquatic systems with the motivation to elucidate how trace element biogeochemistry has evolved over geologic time, as well as the impact of human activities. This project will promote the importance of considering trace element speciation (molecular geochemistry) and help establish my leadership in this field.

Impact on Heidi Babos’s career (Ph.D. candidate)

Heidi will receive a comprehensive training in state of the art techniques used in molecular geochemistry, and will certainly publish a series of high impact papers in geochemistry journals. Her objective is to work in academia and I am confident this project will contribute to achieve her goal.