Reports: DNI254583-DNI2: Chromium Speciation and Distribution in Modern and Ancient Anoxic Settings: Advancing the Paleoproxy

Anthony J. Chappaz, PhD, Central Michigan University

Project goals and impacts

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. 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 in sedimentary records 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. 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)?

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).

Accomplishments and current progress

Most of our proposed research requires having access to a synchrotron light source such as the Advanced Photon Source (APS). Access to these beamlines is highly competitive. Research proposals can be submitted three times a year and must be fared very well (top 10%) during the peer review process.

We submitted our first proposal at APS during the summer 2015. Luckily, we were awarded 5 days of beamtime in December 2015. Unfortunately, quickly after having started our first measurements, we run into an unexpected issue. The manganese present in the sample matrix interfered with the Cr signal so we could not collect any data. To not lose beamtime, I decided to run molybdenum, iron and manganese speciation from our samples. With the data collected in December 2015, I was able to published four articles in EPSL (2), GCA (1) and STOTEN (1).

To deal with that interference issue, I applied to use a different type of XAFS technique called micro-probe XAFS. I was awarded 3 days of beamtime in February 2017 that allowed me to specifically target the Cr phases present in our samples unlike the bulk XAFS used previously (and deal with the Mn interference). During this second visit at the APS, we were able to collect excellent data for both Cr XANES and EXAFS from six groups of ancient sedimentary records, guaranteeing us to achieve objectives 1 and 2. My PhD student and I are still processing the data, yet we are confident that two articles will be submitted soon to highly quality journals in geochemistry.

Additionally, my PhD student presented her preliminary results related to this project at three international conferences (Goldschmidt, GSA and AGU) and received positive reviews!

To study Cr distribution within our samples and complete objective 3, I have been developing a collaboration with the University of Tasmania (Drs. Gregory and Danyushevsky). Our samples are currently being analyzed using their LA-HR-ICP-MS facilities (CODES). I have already collaborated with that group and we have developed a good and productive relationship.

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 Heidi. Graduate students are vital for the success of my research program. I defined myself as a molecular geochemist specialized in studying trace metal speciation in both modern and ancient aquatic systems. This project promotes the importance of considering trace element speciation (molecular geochemistry) and contributes to establish my leadership in this new field. This ACS-PRF award also contributed significantly to obtain my tenure last year!

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

Heidi has been trained 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.