Reports: ND252457-ND2: Examining Evidence for a Dynamic Sulfur Cycle During the Early Triassic with Isotopic Measurements of Carbonate Associated Sulfate

Jonathan L. Payne, Stanford University

The objective of this project is to develop and interpret a high resolution sulfur isotope record for the Early Triassic and to assess the evidence for a gradient with water depth in the sulfur isotope composition of seawater sulfate.

The post-doctoral fellow funded through this project traveled to Northwestern University in September of 2012 to learn the methods for extracting carbonate associated sulfate (CAS) from limestones in the lab of our collaborator, Professor Matthew Hurtgen. 

Soon after this visit, the post-doctoral fellow took a seven month leave for personal reasons (childbirth and illness in the family).  This situation has necessarily slowed progress on the project as there was not sufficient time to identify another qualified individual to conduct the project and the duration of the post-doctoral fellow’s leave was uncertain.  The post-doctoral fellow returned to the lab in July 2013 and we are now resuming work on the project at the initially planned pace.

We were able to make some progress in the post-doctoral fellow’s absence.  A graduate student from Stanford traveled to the lab of Professor Alex Sessions at Caltech in Dr. Meyer’s place during October 2012 to learn an alternative technique for analyzing CAS sulfur isotopes via inductively coupled plasma mass spectrometry (ICP-MS) and to obtain pilot data via this technique.  The advantage of this technique over more traditional methods of gas phase isotope ratio mass spectrometry is that it requires substantially less sample material – 1 mg vs 100 g of rock – and therefore allows for more targeted sampling of the carbonate phases within a given sample that are most likely to record primary seawater sulfur isotope values. Our goal is to analyze a suite of samples to compare the results of the traditional, bulk rock method with results of the newly developed ICP-MS approach. 

The post-doctoral fellow will begin sample preparation work during October 2013 and will generate our sulfur isotope record over the next six months.  We have an agreement in place with Professor Hurtgen to measure the sulfur isotope composition of samples prepared by the traditional method at Stanford and are in the process of setting up logistics that may allow us to analyze a subset of samples via ICP-MS at Caltech.

To aid in the interpretation of our sulfur isotope data, the post-doctoral fellow and a graduate student have been developing a quantitative model of the global carbon and sulfur cycles.  This model will allow us to quantitatively interpret secular variation in the CAS sulfur isotope record within the context of known variation in the global carbon and calcium cycles.  Because these cycles are tightly linked via their effects on seawater alkalinity and the coupling of the carbon cycle to the alkalinity flux via silicate weathering, it is essential that any interpretation of sulfur isotope variation be carried out in light of constraints from these other isotope systems.

As discussed above, progress in preparing and measuring samples for sulfur isotopes has been significantly slower than planned due to the unforeseen circumstances of the post-doctoral fellow’s leave from the lab.  With the post-doctoral fellow back at work on a full time basis and we anticipate that we can make substantial progress in generating our sulfur isotope record in the next six months, leaving us time to interpret and write up our findings during the second year of the grant.