EVIDENCE FOR ELEVATED FLUX OF IRON TO THE EARLY PHANEROZOIC OCEAN

Iron is a limiting nutrient in many settings in the world ocean and is important to biogeochemical processes including both the fixation and oxidation of organic carbon. In this study, we analyzed five Late Cambrian weathering profiles developed in the Pike’s Peak Granite in the vicinity of Manitou Springs, Colorado, to evaluate the hypothesis of Driese et al. (2007) that elevated pCO₂ in the Cambrian led to lower pH of soil waters and liberation rather than retention of Fe³⁺ during chemical weathering of basement rock. In the study area, the Pike’s Peak Granite was exposed to the atmosphere at the Great Unconformity prior to deposition of the overlying Sawatch Formation. We collected five vertical transects through the upper 4m of the Pike’s Peak Granite below the Great Unconformity. Samples were analyzed for major element chemistry by XRF. We find that iron was preferentially lost from these profiles, rather than accumulated in the way that occurs under modern weathering conditions. These results, in agreement with those of Driese et al. (2007), imply an elevated weathering flux of Fe³⁺ to the Cambrian ocean. Due to unprecedented exposure of crystalline basement rock at the Great Unconformity during the terminal Neoproterozoic-Early Ordovician, our findings and those of Driese et al. suggest an elevated weathering flux of Fe³⁺ to the oceans prior to and during the deposition of the Cambrian-Early Ordovician Sauk Sequence, during which time crystalline basement rocks were progressively covered by sedimentary rocks and protected from the processes of chemical weathering. This elevated Fe³⁺ flux helped to promote widespread precipitation of glauconite and Fe-ooids in Cambrian sands, and would have had several additional consequences, including the stimulation of primary productivity in some settings and enhanced burial efficiency of C-org by Fe-chelation (Lalonde et al., 2012). In combination with an enhanced sedimentary reservoir for C-org storage in epicratonic seas, and with enhanced basement weathering and production of clay minerals, which also act to protect organic carbon against oxidation (Kennedy et al., 2002), our results suggest that enhanced burial of organic carbon may have contributed to a rise in pO₂ during the Terminal Neoproterozoic-Early Ordovician.