Strain induced molecular heterogeneity in ancient sedimentary organic matter mapped at nanoscales using optical photothermal infrared spectroscopy

Here we report ultra-high resolution infrared mapping of organic matter functional group distribution in Tasmanites (algal microfossils) from the Upper Devonian Ohio Shale using optical photothermal infrared spectroscopy (O-PTIR). O-PTIR is capable of rapidly measuring the vibrational response of samples in situ with ~500-nm spatial resolution, well below the infrared diffraction limit. Our results indicate that organic matter within the fold apices regions (zones of greatest deformation) of relatively large Tasmanites is more aliphatic and less oxygen-rich than organic matter in the surrounding Tasmanites body. This contrasts with data from relatively thinner Tasmanites which show statistically invariant chemical differences between the fold apices and body regions. These findings indicate that deformation strain due to mechanical stress during burial can alter the molecular structure of sedimentary organic matter to different degrees across nanometer to micron length scales leading to chemical heterogeneity. However, chemical differences induced to organic matter by this mechanism appear be minor compared to those from thermal alteration. Finally, the data presented here represent the first application of O-PTIR to study sedimentary organic matter highlighting the great promise this analytical approach has for future studies evaluating the molecular composition of geologic materials.