Reports: DNI255392-DNI2: Understanding the Role of Iron Availability during Early Diagenetic Uptake and Preservation of Molybdenum in Organic Rich Carbonates
Achim Herrmann, PhD, Louisiana State University
Molybdenum concentrations and isotopic values of siliciclastic sedimentary rocks (mostly black shale lithologies) have long been used as proxies for reconstructing the oxidation state of organic-rich depositional environments. Recently, it has been suggested that the molybdenum geochemistry of carbonates could also provide this information. As carbonates are spatially and temporally more widespread than black shales this would give a more complete record of environmental changes through time. This research project aims at understanding the processes and products of molybdenum authigenesis in a range of depositional environments in the Bahamas.
Towards understanding the processes controlling the early diagenesis of molybdenum in organic-rich carbonate settings: Application of a newly developed sequential extraction technique
One aspect of this project is to develop and characterize an extraction method for the evaluation of molybdenum distribution in soils and sediments by analyzing various chemical phases of the carbonate sediment. Sequential extraction methods have been used in various studies to determine the distribution of heavy metals within sediment phases. We faced the problem that many existing sequential extraction techniques are not applicable to calcium carbonates as important phases (e.g. apatite phase) are either not included in existing methods or not adequate (e.g., able to digest large amounts of calcium carbonate phase under buffered conditions). We overcame these issues and applied the new technique to a range of samples.
We also conducted fieldwork in The Bahamas to collect samples and applied the newly developed sequential extraction technique to these samples (Figure 1).
Figure 1. Preliminary results applying the modified sequential extraction technique to a set of carbonate core samples from The Bahamas. Most of the Molybdenum appears to be tied up in an exchangeable form. Undergoing speciation testing aims at understanding the mineralogy of this Molybdenum (unpublished results from MS thesis project).
In addition, significant progress has been made in developing a scalable approach to analyze stable isotopes (i.e., nitrogen, carbon and sulfur isotopes) of the organic matter to probe for the importance of organic matter geochemistry on the uptake of molybdenum into carbonate sediments.
impact of the research including the impact on students who participated in the project at LSU
Three graduate students (two PhD students and one MS student) were working on this project over the last year. In addition, one undergraduate student worked on this project as part of his undergraduate thesis project and one student worked as a student helper on this project. All students gained valuable experience conducting research, testing scientific hypotheses, and operating advanced instrumentation. Two students (one graduate and one undergraduate student) participated in the fieldwork campaign. Two graduate students (one PhD and one MS student) will defend their thesis projects in the spring of 2018. The undergraduate student will defend his undergraduate thesis in the fall of 2017.
Additional fieldwork will be conducted in the spring of 2018. This fieldwork is guided by the result of the first field campaign in the spring of 2017. Method development has progressed and multiple sediment cores from The Bahamas are currently studied for the molybdenum geochemistry. We will conclude sample analyses in the summer of 2018 and results will be written up for publication in peer-reviewed journals.