Numerical simulations have suggested that a stable (non-reversing) geodynamo may be more efficient at generating stronger dipole fields, and that this condition can arise from a favorable pattern of laterally varying heat flux across the core-mantle boundary. Thus it is important both for understanding how the geodynamo works and for inferring past lower-mantle conditions to find out whether or not the average field strength really was unusually high during the Cretaceous Normal Superchron (CNS), the span of time from around 121 to 83 Ma during which the geodynamo stopped reversing. The investigators propose a major effort to characterize the paleomagnetic field intensity and directional dispersion during CNS. They plan field work to collect lava samples in Madagascar, Korea, Costa Rica, N. Greenland, and Svalbard erupted during, just before, and just after the CNS. They will also collect basalt of mid-Miocene age (a time of high reversal rate) from the Columbia River Plateau. This research seeks answers to the following important questions that are still unresolved: (i) whether the average intensity during the CNS was unusually high, low or unexceptional, (ii) whether there were long-term intensity trends within the CNS, and (iii) whether the fluctuation of intensity on time scales of order 104 yr resembled that of the recent field or was distinctly different. An alternative conjecture for the occurrence of the CNS is that the geomagnetic field can switch abruptly into and out of a non-reversing state, simply by virtue of the highly nonlinear geodynamo processes operating in the core. The investigators will test this hypothesis by seeing whether there is an identifiable temporal trend in paleointensity or dispersion leading up to the beginning or end of the CNS. This international collaborative project includes bringing senior scientists, postdocs and students to work together and share their knowledge and ideas. The scientific results will be made available in the comprehensive paleomagnetic database currently under development with NSF support and will be disseminated in journal articles. The character of the geomagnetic field during the Cretaceous Normal Superchron also has societal relevance and interests a broad scientific community trying to understand mantle behavior and determine potential causal relationships between plumes, large igneous provinces formation and climatic changes.
