Research Feature: Molybdenum isotopes in marine sediments

Link to the Article

Poulson, Rebecca, Siebert, C., McManus J., Berelson, WM, 2006, Authigenic Molybdenum Isotope Signatures in Marine Sediments, Geology, vol.34, no. 8, p. 56-59.

Posted: February 6, 2007

For geologists, observations of modern processes are often the key to understanding the past.  That is a guiding principal for Rebecca Poulson, a Subsurface Biosphere IGERT student and Oceanography Ph.D. candidate.  She is studying molybdenum isotopes in modern marine sediments and hopes they will be a tool for unlocking secrets about the ancient ocean.

Rebecca published some of her findings in the journal Geology in August 2006.  Co-authors on the paper were Christopher Siebert, her faculty advisor James McManus, and William Berelson. “One of the fun things about this project is that I’m studying pre-rocks – sediments that a million years from now will be rocks.” says Rebecca, “We’re hoping that our studies will teach us something about how to interpret the rock record.”

Molybdenum is a nutrient essential for life and it is the most abundant trace metal dissolved in modern sea water. Microbial activity in marine sediments causes it to precipitate and form deposits. Rebecca has been studying how the isotopic composition of molybdenum in modern marine sediments varies with the biogeochemical processes that produce them. The hope is that if different processes produce unique isotopic signatures, then the ratios found in ancient molybdenum deposits could be used in reconstructing the chemistry of the ancient ocean.

For example, previous studies had shown that molybdenum deposited in aerobic settings has a more negative isotopic composition than that found in modern sea water.  The biogeochemical processes that cause molybdenum to precipitate in these settings fractionate the isotopes.  Rebecca compared this isotopic signature with that found for molybdenum deposited in anaerobic settings where the dominant microbial processes are sulfate reduction and methanogenesis.  She found that molybdenum deposited in these settings is less fractionated and has isotopic compositions much more similar to that found in modern sea water.  In their paper in Geology, Rebecca and her co-authors suggest that there may be four unique isotopic signatures for molybdenum that each correspond to different biogeochemical processes and sea water chemistry.

Rebecca Poulson (left) in Africa..

Rebecca Poulson (left), sampling core on Lake Tanganyika in Tanzania.

Rebecca says that her research has benefited from the interdisciplinary emphasis of the IGERT program.  Her microbiology classes have helped her understand the microbial processes controlling molybdenum geochemical behavior and she has enjoyed interacting with graduate students in different departments. “We have Chalk Talks where we try to explain our projects.  The questions are always different from the people with different backgrounds and that really makes you think.” She also comments that researchers with different expertise tend to focus on different aspects of her project: “The oceanographers are thinking about geochemistry and about the importance of chemical gradients while the biochemists are thinking about how microbes control the environment.”

Rebecca had the opportunity to continue her studies of isotopes last summer when she went to Lake Tanganyika in Tanzania, Africa, the second deepest lake on the planet.  The lake provides a unique system for study, with oxygenated surface waters and sulfidic bottom waters.  She sampled sedimentary deposits in the lake’s freshwater, system for comparison with the marine environment.  Rebecca’s work in Tanzania was funded by the IGERT program’s support for international research experiences, and was the highlight of her graduate program.

For more information, Rebecca can be contacted through the College of Oceanic and Atmospheric Sciences.