CARBON CHARACTERIZATION IN THE SIRIUS PASSET BIOTA AND A GEOTHERMAL GRADIENT THROUGH CAMBRIAN LAGERSTÄTTEN

Mechanisms that delay the degradation or destruction of organisms are viewed as one of the most important taphonomic factors that lead to the exquisite preservation seen in Lagerstätten deposits. However, perhaps of equal importance, yet rarely explored, is the subsequent metamorphic history and peak thermal paleotemperature of Lagerstätten deposits. Raman spectroscopy is a non-destructive technique sensitive to molecular vibration, which allows the investigation of the organic and inorganic chemical structure of samples. The structure and the degree of disorder of the organic matter can then be used to estimate peak metamorphic temperature and assess heterogeneity between-samples. The Sirius Passet Lagerstätte is an exceptionally preserved early Cambrian faunal community that contains a diverse array of stem-group euarthropods, lobopodians, worms, sponges and the iconic Halkieria. Material collected in situ during recent expeditions has yielded a range of fossil specimens that are preserved as two-dimensional, reflective films. Here we investigate structure of carbonaceous matter with these films using Raman spectroscopy, comparing results obtained from other prominent Cambrian Lagerstätten. Raman analysis confirms that these reflective films are kerogenous and resemble, at least superficially, a similar mineralization mode to that seen in Burgess Shale-Type deposits. A relatively higher D and G band intensity and band area of disordered carbon differentiate the carbonaceous matter of the fossils from the carbonaceous material in the surrounding matrix. The spectral characteristics of the fossils are indicative of moderately ordered kerogenous matter and indicate that the transitional Buen Formation that hosts the Sirius Passet biota was thermally altered at a peak temperature of 409 °C (±50°C). This paleotemperature is much higher when compared to other Cambrian Lagerstätten deposits, such as the Burgess Shale (335 °C), Spence Shale (327 °C) and the Guanshan Biota (300°C). The comparative lack of anatomical detail in the Sirius Passet fossil films may be a result of a higher peak paleotemperature having effaced some morphological detail from the fossil films, suggesting that the morphological fidelity of fossils in Lagerstätten deposits may be correlated with the thermal maturity of the host strata.