Progress in the evaluation of U-Pb dates of late Ediacaran carbonate rock from drill cores through simultaneous Sr isotope analysis (Laser Ablation – Split Stream – Inductively Coupled Plasma – Mass Spectrometry)

Marine carbonates are archives of geochemical proxies, such as e.g. Sr and U-Pb isotopes, which potentially can be utilized in the reconstruction of past climate conditions, ancient seawater composition and/or their alteration during burial and fluid interaction, if interpreted accurately. The ability to confidently reconstruct environmental conditions in the past times is of great importance since they can be linked with changes in the biosphere. For example, the Ediacaran-Cambrian transition was a period where significant evolutionary change modified the biosphere towards appearance of extant animal clades and the establishment of metazoan-dominated ecosystems. Any contribution towards an intact, continuous record of environmental conditions through multiple proxies will help to understand better the timing, nature and sequence of events that preceded or accompanied such changes in biodiversity. However, carbonate rocks are susceptible to numerous post-depositional processes (such as: oxidative weathering, diagenesis, burial, lithification, deformation, dissolution and reprecipitation), which may alter the geochemical record. Additionally, detrital components may increase the complexity of the geochemical signature and the carbonate composition, particularly in shallow marine settings with variable continental run-off.

Thus, we have to understand and identify the presence or absence of such processes, before extracting meaningful geological information from these archives. Laser Ablation – Inductively Coupled Plasma – Mass Spectrometry (LA-ICP-MS), is a tool that offers spatial resolution when performing geochemical analyses, which may help to interpret the geochemical data more confidently. In this work, we combine observations from Sr and U-Pb isotopic systematics supported by trace element abundances to identify domains that are indicative of post-depositional processes, over protracted time and variable in their extent. The sample material is taken from drill cores in Namibia, obtained within the frame of the GRIND project, which aims to investigate the late Edicaran period. Situated in the south of Namibia, the Gariep belt was geologically active during the Pan African orogenesis and possibly affected by the Atlantic Ocean opening.