Reports: UR856079-UR8: Sulfur Isotope Chemostratigraphy as a Potential Tool for Correlation in Paleozoic Dolomite

David S. Jones, PhD, Amherst College

Scientific Progress

The project began in Summer 2016 with lab work on rock samples collected prior to the start of the grant year, rather than with new field work. This was a deviation from the originally proposed field work for that summer. Sulfur isotope ratios were measured on sulfate and pyrite for the Upper Ordovician Ely Springs Dolostone, a well-studied rock unit for which the PI had previously published a carbon isotope record and for which sequence stratigraphic and biostratigraphic correlations exist. The results showed that the sulfur isotope records of multiple sections along a paleo-depth transect were different, lacking clear tie points for stratigraphic correlation. However, differences in the sulfur isotope records appear to be linked to the early diagenetic history at each location, which has opened new questions about how early diagenesis influences sulfur isotope ratios in dolostones.

In order to gain further insights into the geochemical effects of early diagenesis on these rocks, calcium and magnesium isotope ratios were analyzed. These data are useful for identifying the extent to which early diagenetic fluids flushed the sediment in a chemically open or closed system and can yield constraints on the chemistry of the diagenetic fluid. Our initial results demonstrate a pelogeographic pattern in the amount of open system fluid flushing during early diagenesis that we tentatively attribute to the effects of glacioeustatic sea level change during the Late Ordovician ice age.

The new Ca and Mg isotope data, paired with the sulfur isotope data, suggest a new means to assess the effects of early diagenesis on carbon, oxygen, and sulfur isotope records in ancient carbonate strata. In order to test this hypothesis further, the project will analyze time equivalent units from other sedimentary basins. Field work in Summer 2017 targeted the Bighorn Dolomite in the Bighorn Mountains, Wyoming. These strata were deposited on the same continental margin as the Ely Springs Dolostone but experienced a different history of diagenetic alteration. Additional material from well-preserved limestones from Anticosti Island, Quebec, will provide an undolomitized end member.

Work in the 2017-18 project year will focus on geochemical measurements of the newly collected Bighorn Dolomite samples, as well as finishing measurements of the Ely Springs. A new collaboration with André Desrochers (University of Ottawa) will facilitate work on new material from Anticosti Island during the academic year.

Impact on PI

The grant supported the PI for a portion of Summers 2016 and 2017. The PI was able to recruit a cadre of undergraduate students to work on the project for extended periods during the summers and in the academic year. Grant funds were used to perform field work in a new field location, the Bighorn Mountains of Wyoming. The grant also facilitated collaboration between the PI and two collaborators, David Fike (Washington University in Saint Louis) and John Higgins (Princeton University).

Impact on Undergraduate Students

The grant funded the research activities of five undergraduate students from Amherst College during the reporting year.

Brian Beaty worked on the project for eight weeks in Summer 2016, which was the summer between his junior and senior year. He worked in the lab on sulfate and pyrite extractions for samples of the Upper Ordovician Ely Springs Dolomite. He visited David Fike’s sulfur isotope lab at Washington University in Saint Louis, where he learned how to analyze the samples for stable isotope ratios on a dual inlet gas source mass spectrometer. Brian graduated from Amherst College in May 2017. He plans to apply to PhD programs in geoscience with a target start data of Fall 2018.

Ian Petty worked on the project for one month in Fall 2016, during his sophomore year. Ian’s lab work involved preparing samples of the Ely Springs Dolomite for calcium and magnesium stable isotope analysis. He traveled to John Higgins’s lab at Princeton University, where he learned about the operation of the ion chromatograph and the multicollector inductively coupled plasma mass spectrometer.

Three students worked on the project as a team for two months in Summer 2017. Emma Swislow, Heather Grotzinger, and Raina Chinitz joined the PI for two weeks of field work in the Bighorn Mountains (Wyoming) and six weeks of lab work at Amherst College. All three were in between their first year and sophomore years. In the field, the students learned how to measure stratigraphic sections, make graphic logs, and take samples. In the lab, the students learned how to prepare the samples for carbon and oxygen isotope analysis, as well as sulfate extractions for sulfur isotope analysis. All three students will continue their work during the 2017-18 academic year and are currently enrolled in additional geology courses.