Weathering of sedimentary rocks rich in organic matter

Supervisors:
Dr. Bernhard Peucker-Ehrenbrink, Department of Marine Chemistry and Geochemistry, WHOI
In collaboration with:
Dr.Tim Eglinton , Department of Marine Chemistry and Geochemistry, WHOI
Dr. Ken Sims , Department of Geology and Geophysics, WHOI
and Dr. Steven Petsch, UMASS Amherst

Rationale: Weathering of sediments rich in organic matter (i.e., gray and black shales) is of global importance for the cycling of organic carbon, atmospheric oxygen, and redox-sensitive elements such as molybdenum, rhenium, osmium, and uranium. Redox-sensitive elements are sequestered from seawater during the deposition of sediments rich in organic matter, a process that constitutes the long-term sink of atmospheric carbon dioxide and source of oxygen to the atmosphere.
It is thus a key process in regulating atmospheric carbon and oxygen levels on geologic time scales. Uplift, erosion and weathering of sediments rich in organic matter completes the global cycle in that oxidation of organic carbon consumes atmospheric oxygen and releases carbon dioxide as well as redox sensitive metals sequestered during deposition of these sediments. As some of these metals are toxic in groundwater (V, Se, Pb, U, etc.), weathering of organic-rich sediments is also of environmental interest.

Methodology: This project aims at 1) quantifying the rates of weathering by applying uranium- and thorium-series decay series techniques to naturally occurring weathering profiles (see above picture of a weathering profile in ~365 Myr old Devonian black shale in Kentucky), and 2) by experimentally studying weathering of gray and black shales using custom-designed weathering reactors. The first component entails intense work in clean laboratories to separate and purify U-, and Th-series nuclides, primarily by ion-exchange chromatography, and subsequent analysis of nuclides of interest with single and multi-collector plasma ionization mass spectrometers (ThermoFinnigan's ELEMENT and NEPTUNE). The second, experimental component involves modifying the design of outdoor weathering reactors for the use of organic-rich rocks. This will be done in collaboration with engineers in the AOPE Department at WHOI. The aim is to monitor the temporal changes in the chemical and isotopic composition of the rainwater interacting with organic-rich sediments and tracking temporal changes in the chemical composition of the bulk sediments. These experiments will be performed under oxic as well as anoxic conditions to gain insight into the change in the style of weathering of such sediments that occurred during the oxygenation of the atmosphere in the early Proterozoic, about 2 billion years ago.

Wider Implications: Deposition and weathering of sediments rich in organic matter is a key regulator in the global cycle of organic carbon and oxygen. It is of environmental interest because groundwater contamination has been documented in areas dominated by the occurrence of sediments rich in organic matter (e.g., upstate New York).

Suitable For: Students with strong analytical and technical skills.

Training: In addition to acquiring generic skill, the student will learn fundamental techniques in isotope geochemistry, clean laboratory protocols, and the use of state-of-the-art plasma ionization mass spectrometry techniques. The project also involves fieldwork (drilling, water sampling) in the U.S. (and possibly northern Canada) and collaboration with engineers in designing custom-made weathering reactors.

Additional Relevant Links:

References:

  • Peucker-Ehrenbrink B and RE Hannigan (2000) Effects of black shale weathering on the mobility of rhenium and platinum group elements. Geology, 28, 475-478.
  • Jaffe LA, B Peucker-Ehrenbrink and S Petsch (2002) Effects of weathering of black shales on the mobility of Re, PGE and Corg. Earth Planet. Sci. Lett., 198, 339-353.
  • Peucker-Ehrenbrink B and GE Ravizza (2000) The marine Os isotope record. Terra Nova, 12(5), 205-219.