60th Annual Report on Research 2015

Porphyrins found in ancient shales are the degradation products of chlorophylls that were once the centerpiece of the primary producer photosynthetic apparatus. These biomarkers preserve reasonably well and provide a wealth of information on the nature of ancient marine life and geochemical conditions. Porphyrins are of particular interest to those studying the ancient nitrogen cycle because they provide one of the only opportunities for compound-specific isotope analyses in ancient rocks and sediments. The work funded by ACS-PRF under this DNI has focused on developing further the utility of compound specific nitrogen isotopes in porphyrins. Our efforts address questions regarding the nature of ancient marine conditions during Oceanic Anoxic Events and how they relate to the genesis of black shales.

Progress on this DNI grant continues with the work centering on the Ph.D. project of Ben Uveges (B.S. McGill). In all aspects this project is maturing and we expect Ph.D. student Ben Uveges to have completed these projects within 1.5 years. He has driven the developing aspects of this proposal and is focusing on three central areas of research:

1. The central project focuses on paleoceanographic conditions that prevailed during the Frasnian-Famennian extinction (FF) during the Late Devonian, with a focus on black shales from the American mid-continent. This work now encompasses several sites from Western NY to Iowa and has revealed striking similarities between FF black shales and those of the Mesozoic and elsewhere. The nitrogen isotope values are typically negative. This has remained a point of contention for many of us studying the ancient nitrogen cycle and has yet to ultimately be resolved. Porphryin studies were a hopeful target to resolve outstanding issues in this regard, but they are not preserved in the interbedded gray shales. This has resulted is using a combination of nitrogen isotopes of porphyrins and whole extracts, which are easily extracted from the low-organic content shales or where higher maturity has reduced porphyrin content. This may ultimately allow us to simply use bulk extracts for higher resolution, organic-N analyses where porphyrins are not present. This is a significant simplification but does lose some of the richness that is provided by the biomarker and metal speciation provided by porphyrin studies.
2. We recognized significant differences the nitrogen isotopic composition of porphyrins with different chelating metals but the same structure. This suggests that there is an isotope effect during chelation. We are using a synthetic porphyrin to investigate the isotope effect of chelation on nitrogen. This is extremely important because most porphyrins in shales are present as metal complexes (VO, Ni, Zn, Fe, Cu). If there is a strong effect in most metals, our ability to interpret nitrogen isotope signals in porphyrins may be limited. An additional outgrowth of this work is using the complimentary metal system to investigate potential effects on stable metal isotopes with collaborators at other institutions.
3. The final aspect of this project is simply understanding the range of isotope heterogeneity (N and C) of porphyrins in single samples from multiple environments. This important aspect provides a broader understanding of the range of source organisms and isotopic differences between structures and chelating metals.

The importance of these projects focuses clearly on understanding the role of primary producers during intervals of widespread anoxia and black shale deposition. However, given that porphyrins are important constituents of oils, these studies can help assist in source rock-oil correlations and the speciation of metals within oils.

Over the course of this project we have also worked to develop methods for reliable, rapid analysis of the stable isotopic composition of N and C in porphyrins and organic extracts at nanomolar quantities of N nitrogen isotopes coupled with conventional (micromolar) or nanomolar quantities of carbon for carbon isotopes. The Nano/MicroEA system is a cryo-trapping, capillary-focusing system. This system has several benefits beyond many of the current, similar systems employed in a few stable isotope labs. I have automated the connection between our Elementar Isotope Cube elemental analyzer (EA) and the Trace Gas analyzer; this enhances the long-term reproducibility of results and enhances ease of use such that little training is necessary to operate the system (suitable for undergraduates).  A unique feature of the Elementar Cube EA is that it uses a chemical trap/purge system that retains carbon dioxide but allows nitrogen to flow freely through the system to the IRMS (or Trace Gas/LN trap); carbon dioxide is released when trap is heated. I have constructed a valve arrangement using two Valco multi-port valves that allows for automated switching between the N₂-trapping system and the full flow of the EA. Analysis of focused N₂ and convention C on the same sample is now possible. This overcomes one of the principal issues that limited coupled C and N analysis of organic material with a high C:N, such as kerogen or total lipid extracts, on the standard trapping systems, even when He-dilution was maximized. This arrangement is ideal for the coupled carbon and nitrogen analysis of very precious or labor-intensive samples such as porphyins. It also provides the additional carbon isotope data that are currently not possible with the N₂O-denitrifier methods.