The ultimate science objective of this project is clarification of the distribution of water and other volatiles in the deep interior of the Earth. Uniquely among the planets, Earth has oceans at its surface that are key to the origin and support of life. It has been postulated that mantle rocks at depths between 410 and 660 km store about 10 times as much water as is in the world oceans, and that the global water cycle extends from the atmosphere down to the mid-mantle. For example, Earth's mantle may serve as a reservoir for water, buffering the oceans from variations that might be induced by surface geological processes. In this project we are using novel electromagnetic geophysical methods to image electrical conductivity variations in the mantle deep beneath Earth's surface. Because the conductivity of mantle rocks is highly sensitive to even small amounts of water, these images will allow us to constrain the distribution of water in the deep Earth, and improve our understanding of deep Earth water cycles. Results of this research have the potential to impact our understanding of the evolution of the oceans, and perhaps ultimately life itself.
To improve resolution of upper mantle and transition zone electrical conductivity we will exploit large scale electric currents induced by the flow of periodic ocean tidal currents through the Earth's main magnetic field. We are using an extensive base of seafloor data collected over the past 30 years to estimate the tidal electromagnetic field in the semidiurnal and diurnal bands, and then combining these (along with terrestrial geomagnetic data) with a 3D tidal simulation to invert for mantle electrical structure. The seafloor database consists of both cable and point electric and magnetic field measurements that are heavily concentrated in the Pacific basin, which is thus the focus of our study. Key to the success of the effort is the recent great improvement in our knowledge of open-ocean tidal currents that has resulted from modern satellite altimetry, and sophisticated data assimilation schemes. Combination of models of the tidal forcing function with 3D electrical conductivity models, and then constraining them with measurements, is the technical approach that will lead to the science objective.
This project is supported by the Geophysics and Marine Geology & Geophysics Programs.
Results from laboratory experiments demonstrate convincingly that electrical conductivity in upper mantle minerals is greatly increased by small amounts of volatiles such as water, by silicate melts, and, most dramatically by carbonatite melts. Geophysical determination of deep electrical conductivity can thus provide critical constraints on the presence of volatiles and melting processes (or their remnants through low electrical conductivity), all of which have major implications for the dynamics and evolution of Earth. However, probing electrical conductivity at upper mantle/transition zone depths (200-400 km) requires geoelectromagnetic data with periods of a few to one cycle/day. This is a challenging frequency band for induction studies due to the spatially complex and poorly constrained ionospheric sources that dominate at these periods, and the difficulty in obtaining reliable electric field data on land at long periods. Resolution of these data is limited further by the restriction to toroidal electric source currents flowing parallel to Earth's surface implicit from the use of external sources. An alternative large scale EM source is the electric currents induced by oceanic tidal flow through Earth’s main field. This project produced a data analysis methodology to automatically extract the long period, diurnal, semidiurnal and overtide components from EM data that test as significant using a likelihood ratio criterion. It also supported the coding of a semi-global inversion method combining a 3D electromagnetic code with a data assimilation tidal model constrained by satellite altimetry and seafloor pressure data that yields the tidal velocity field. An extensive base of seafloor EM data collected over the past forty years consisting of both cable and point electric and magnetic field measurements that are heavily concentrated in the Pacific basin has been produced. Inversion of the data using the inversion code is onging as of this writing. Give that the work supported under this grant is ongoing and not yet complete, there are no firm conclusions about the earth that can be specified at this time.
