GEOCHEMICAL CHARACTERIZATION OF THE LOWER-MIDDLE CAMBRIAN TRANSITION IN WESTERN GONDWANA

The Cambrian period is of fundamental importance in the history of life. Nevertheless, due to the general endemism, the scarcity of high-precision radioisotopic dates, and the lack of well-preserved and exposed strata, the Cambrian time scale remains one of the least precise in the Phanerozoic. This hampers a precise understanding of the bio- and geochemical processes and their integration in the global Cambrian correlation framework. However, a set of global carbon isotope excursions are recorded during the Cambrian which make δ¹³C an extremely powerful tool to correlate sections and events globally. In this study, we conducted a high-resolution sampling on the well-known section of Ferrals-les-Montagnes (Southern France) to resolve the identification of both the lower-middle Cambrian boundary (LMC) and the ROECE event in western Gondwana. To do so a total of 22 fossils and 141 samples for TOC and δ¹³C were collected. Combination of our newly collected dataset with the literature indicates that the ROECE occurs slightly earlier in time than the LMC boundary in western Gondwana. Indeed, the ROECE is defined as a negative carbon excursion occurring within the LMC boundary interval and is associated with a major extinction amongst trilobites. Identification of the collected fossils have also confirmed that the studied section extends until at least the late Drumian but the absence of reliable δ¹³C_carb and TOC values in this interval does not allow to identify the middle Cambrian DICE event with confidence. δ¹³C_org is being measured for the whole section in aims to pinpoint the DICE and to build a robust geochemical dataset for western Gondwana where isotopic data are strongly lacking for the middle Cambrian interval. The earlier onset of the ROECE in Gondwana but also in Laurentia could be explained as followed: 1) The ROECE is slightly asynchronous and came later in the centre of the Iapetus Ocean, 2) The fossil index taxa first appeared in Gondwana or Laurentia and then migrated, leading to an erroneous age of the onset of the ROECE and 3) The earliest age of the ROECE in Gondwana/Laurentia could be due to a combination of asynchronous onset of the event and migration of the fossil index taxa.