RAMAN spectroscopy for the evaluation of thermal maturity in Jurassic clam shrimps

JIMÉNEZ, VICTORIA C.; COPELLO, GUILLERMO J.; GALLEGO, OSCAR F.; MONFERRÁN, MATEO D.; PELLERANO, ROBERTO G.; CABALERI, NORA G.

Corrientes (Cap)

Congreso; XI Congreso Argentino de Química Analítica; 2021

Asociación Argentina de Químicos Analíticos

n palaeontology, the use of RAMAN spectroscopy is a relatively new field (1, 2). This direct and non-invasive technique allows a determination of the chemical phases and specific mineralogy at the molecular level, as well as the thermal maturity of fossils. Investigations in the remains of fossil clam shrimps have focused mainly on the study of the mineralogical and elemental constitution of the carapaces while the study of preservation of carbonaceous material (possibly organic) has been neglected. In this work, Jurassic clam shrimp carapaces of the La Matilde Formation (Middle Jurassic) were analyzed by RAMAN spectroscopy to identify the presence of organic remains and determine the thermal maturity of the sample. RAMAN spectroscopic analysis revealed the presence of amorphous carbon in the carapace, as evidenced by graphitic ‘G’ and disordered ‘D’ bands. In addition, bands ‘D‘ and ‘G‘ were deconvoluted into Gaussian-Lorentzian component bands representing vibrational modes that become active RAMAN due to structural disorder (3). These bands include band D2, band G, band D3, band D1, and band D4 (3). The deconvolution of amorphous carbon peak yields full width at half maximum (FWHM), area (peak intensity), and height of each band. It has been shown that the ratio (R) of the band area of the D1 band to the sum of the areas of the D1, the G, and the D2 bands acts as a proxy for the total amount of thermal alteration experienced by the sample (4), so it can be used as a geothermometer (2, 4). In this study, the presence of amorphous carbon was revealed in fossilized clam shrimp carapace. In addition, the relative peak intensity between the bands and the resulting parameters allowed us to roughly estimate the paleotemperature inscribed in the carapace. Based on the geothermometer (4), the estimated paleotemperature ranged between 380-400 °C data that conforms with cited by (2). However, we must consider that although this detected carbonaceous material may be the rest of the organic matter of the invertebrate conserved in situ. Nonetheless, it could also derive from inorganic carbonaceous material or the fossil diagenetic microbial activity. RAMAN spectroscopy by itself does not provide all the necessary details to know the origin of the carbonaceous material. Nevertheless, it would allow to determine the taphonomic history experienced by the fossil. The sedimentological characteristics and the paleoenvironment (5) of the sequence from which the fossils come would confirm that the carbonaceous material detected in the sample would come from bacterial activity.