61st Annual Report on Research 2016

Background

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: Method development

This year was spent on method development for multiple geochemical analyses needed for this project in preparation for fieldwork and sample analysis.

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. Our sequential extraction will target the following phases in order: exchangeable ions, carbonates, apatite, oxides, organics and residual matter (Table 1). Two graduate students worked on this project and now that the sequential extraction technique is set and we know the sample size requirements to get good data for all phases, we will conduct fieldwork in the fall of 2016 in the Bahamas.

Table 1. Expanded section of the modified sequential extraction procedure illustrating the various extraction reagents and specific preparation for each phase.

In addition, significant progress has been made in developing a scalable approach to precisely date the calcium carbonate rich sediments using ²¹⁰Pb and analyzing nitrogen isotopes.

impact of the research including the impact on students who participated in the project at LSU

Two graduate students are currently working on this project. Both students have gained valuable experience conducting research, testing scientific hypotheses, and operating advanced instrumentation (laser ablation ICPMS). One graduate student successfully presented some of his research results at the AAPG student expo this fall where he won 3^rd place in the poster competition. He is on track to defend his MS thesis later this semester. The other student is on track to defend his dissertation during the next academic year.

Outlook

Sampling this summer was unfortunately not possible to due scheduling issues with the field station and the beginning of hurricane season. Fieldwork will be conducted in the fall of 2016.