FLUORESCENCE SPECTROSCOPY OF TASMANITES MICROFOSSILS IN RELATION TO THERMAL MATURITY AND CHEMICAL COMPOSITION: APPLICATION OF CONFOCAL LASER SCANNING MICROSCOPY

We used confocal laser scanning microscopy (CLSM) to characterize spectral fluorescence properties of Tasmanites microfossils in Devonian shale from the Appalachian Basin. Spectral scans of the same field of view with different Ar laser lines (405, 458, 476, 488 and 514 nm) showed progressive red-shift in emission maxima with longer excitation wavelengths. Emission maxima varied as a logarithmic function of excitation wavelength with moderate to high positive correlation (r² 0.64-0.99) for regions with high emission intensity. This result suggests steady-state Tasmanites fluorescence emission is an overlapping combination of emission from multiple distinct fluorophore functions, similar to extant observations of crude oil fluorescence. Stokes shift generally decreased with increasing excitation wavelength, further suggesting presence of multiple fluorophore functions with different S₁ → S₀ transition energies. This observation also indicates that at higher excitation wavelengths, less absorbed light energy is dissipated via collisional transfer (radiative and non-radiative) than at lower excitation wavelengths, also similar to results from crude oils. In most cases, emission intensity increased with increasing excitation wavelength. This observation also may suggest that fluorophores absorbing at higher wavelengths experienced less non-radiative energy loss. This preliminary result is contradictory to observations from crude oils, where emission intensity decreases with increasing excitation wavelength, and may depend more on relative power delivered to the sample than on differences in fluorophore function and concentration. Emission spectra from regions in individual Tasmanites having high fluorescence intensity are blue-shifted relative to other areas in the same body with lower fluorescence intensity. We interpret this to be due to quenching moiety dilution and minimization of non-radiative energy transfer. Finally, Tasmanites CLSM spectra show robust correlation in progressive red shift from samples artificially and naturally elevated to progressively higher levels of thermal maturity. These results suggest CLSM has broad potential in characterization of sedimentary organic matter.