[ESS] Lubna Shawar (California Institute of Technology)

“Intramolecular Isotope Structure of Biomarkers: A Frontier Tool for Source Attribution”

Biomarkers, molecular fossils derived from once-living organisms, preserve biosynthetic signatures that reflect biological sources, environmental conditions, and depositional settings. Their structural fidelity makes them powerful tools in geochemical reconstructions, petroleum exploration, and environmental forensics. However, structurally similar or identical compounds can originate from taxonomically diverse organisms, and post-depositional (diagenetic) processes may further alter biomarker structures, complicating source attribution.

In this talk, I will present two complementary research efforts aimed at addressing these challenges. The first draws on classical organic geochemistry: identifying novel biomarkers
through structural analysis, source comparison, and model compound synthesis. Specifically, I will share results from my postdoctoral work at MIT, where I identified two previously unrecognized C31 steranes (namely: 24-n-butylcholestane and 24-sec-butylcholestane) in Neoproterozoic sedimentary rocks. Their structures were confirmed through synthesis of model compounds, coinjection with geological extracts, and analysis using GC-QQQ-MS. These steranes were traced to C31 sterol precursors found in modern demosponges, reinforcing the value of C31 steranes as molecular fossils of early animal life.

The second part of my talk introduces a new analytical frontier for biomarker-based source attribution, developed during my research at Caltech. This work presents the first successful application of site-specific isotope analysis (SSIA) to steroidal compounds, providing a foundation for extending this approach to steranes preserved in the geological record. SSIA uses isotopically resolved molecular fragments to recover position-specific isotope information within a molecule, revealing source and transformation signals that are obscured in conventional bulk and compoundspecific isotope analyses. As a proof of concept, we selected androsterone, a structurally simple steroidal androgen well suited to controlled experiments, to evaluate whether SSIA can distinguish isotopic patterns arising from different precursor sources and chemical transformations.

Our findings demonstrate that SSIA can resolve position-specific isotopic patterns obscured in conventional analyses. This capability establishes a high-dimensional isotopic fingerprinting framework, enabling more precise interpretation of biomarker sources and transformation pathways. Extending this approach to geological biomarkers opens a new dimension in molecular geoscience, with the potential to improve source attribution in complex petroleum systems, refine oil–source correlations, and enable more robust reconstructions of organic matter origins, paleoenvironmental conditions, and thermal histories.

Earth Science Seminar —

Lecture portion of the EAPS graduate-level class 12.571, covering current research in geophysics, geology, geochemistry, and geobiology. All members of the MIT community are welcome to join for presentations by guest speakers, held approximately every two weeks during the term.

Contact: earth-science-seminar-info@mit.edu