61st Annual Report on Research 2016

The general topic of this proposal is the study of nitrogen isotopes in chlorins, here broadly defined as chlorophylls and their diagenetic products (i.e. dihydroporphyrins that are functionalized precursors of sedimentary porphyrins). Thus, measurement of nitrogen isotopes in marine sediments provide a unique way of understanding the cycling of the major nutrients between geologic and biologic reservoirs. The general objective is to evaluate the postdepositional sedimentary alteration of the nitrogen isotopic signatures of chlorins, from the early stages of the diagenetic alteration of chlorophylls to the initial stages of the formation of porphyrins. In this project we will validate one of the assumptions of the use of δ¹⁵N in tetrapyrroles, which is that chlorins and porphyrins retain the isotopic signature of the source photoautotrophs, despite extensive postdepositional diagenetic transformations. This assumption, which to our knowledge has not been validated, justifies that the measurement of δ¹⁵N may be undertaken, not just in individual tetrapyrrolic compounds, but in a pooled fraction of chlorins or porphyrins. This poses considerable analytical advantages, particularly given the increasing diversity of porphyrins with age of the sediment. Porphyrins are common in oils and sediments, and this proposal is design to contribute to the understanding of the diagenetic processes that lead to their formation, as well as to constrain the interpretation of nitrogen isotopic signature derived from tetrapyrroles in sediments to reconstruct depositional conditions related to biogeochemical processes in ancient environments. Such information can be of use to understand the complexity and nature of porphyrin mixtures and also of use for oil-source rock and oil-oil correlation studies.

To date we have concentrated in the setting up of the methodologies that are new to the lab. For this purpose we have established a collaboration with Prof. Nao Ohkouchi from JASMTEC, Japan, that have pioneered in their group the study of δ¹⁵N in pigments. As part of this collaboration to date we have analyzed a suite of modern sediments and are now in the process of interpreting the δ¹⁵N pigment data. In addition we have been working on the analysis of maleimides and their isotopic composition. In the next phase of the project we will apply the new isotopic methodologies to the targeted sediment cores in the Benguela upwelling. The results obtained so far have been presented in the Goldschmidt 2016 meeting in Yokohama, and the 12th International Conference in Paleoceanography in Utrecht.

The ACS-PRF New Directions (ND) grant has enabled me to pursue a new research line with my graduate students which we are applying to several of our current projects. The project is allowing us to focus in the setting up the required analytical techniques, which pose a considerable number of challenges, both in terms of acquiring the required skills in wet chemistry analytical work, and the application of instrumental developments which are not provided by the instrument manufacturers, such as nano-IRMS. This investment in the development of the analytical know-how should allow us to constrain further the processes that lead to the sedimentary signals of δ¹⁵N in chlorins, and ultimately achieve the goals of the project. The three students that are working in the framework of the project are acquiring a considerable expertise in cutting edge analytical techniques that will allow them progress further in their careers as geochemist in environmental and Earth sciences once they graduate.