Understanding processes in the deep earth over geological time scales
The fundamental goal of DeepDyn is to exploit the exceptionally long paleomagnetic record for reconstructing the evolution of the core-mantle system through geologic time.
Our geomagnetic field is generated by the geodynamo in the liquid iron core. One of the most striking manifestations of the geodynamo are complete field reversals of its dipole field. Paleomagnetic studies show that the frequency of these field reversals changes over geologic time periods, which are also typical for mantle convection. Numerical simulations, in turn, show that the lower mantle has various effects on the geodynamo: in particular, the absolute value and the pattern of heat flow through the core-mantle boundary influence the field strength, field geometry and reversal rate. However, neither the long-term evolution of the lower mantle and the geodynamo nor the coupling between the two are well understood. Furthermore, field strength and reversal rate likely influence the survival and evolution of magnetoreceptive organisms, particularly magnetotactic bacteria. We invite contributions aimed at understanding the long-term evolution of the geomagnetic field and the Earth's core dynamics, the dynamics of the deep mantle and their influence on the geodynamo. This interdisciplinary collaborative project will bring together researchers in paleomagnetism, dynamo modeling, mantle dynamics, seismology, materials science and biology.
Research priorities and scientific goals of DeepDyn
Figure 1
Geomagnetic reversal frequency over the Phanerozoic, highlighting superchrons and hyper- reversing (>10/Myr) times (after Gallet et al., 2019). Red arrows signify the world’s most voluminous volcanic flows (traps) that presumably originated from mantle plumes, which originated from the core-mantle boundary 10-20 Myr before their emplacement on the surface. Cz, Mz, Pz and pC represent the Cenozoic, Mesozoic, Paleozoic and Precambrian eras. (click ⤢ to expand figure)
Complete reversals of the magnetic field polarity are fascinating events in Earth’s history. Although paleomagnetism has documented several hundred, and although reversals can be simulated on a computer, the process remains poorly understood. Reversals seem to happen randomly, with a likelihood that changes over ten-million-year time scales (Figure 1). While the last reversal happened 780,000 years ago, a rate of four to five reversals per million years is more typical over the past 15 Ma. There have also been periods, called superchrons, with no reversals occurring for several tens of millions of years. Episodes with more than ten reversals per million years, so called hyper-reversing epochs, have likewise been documented. Long-term records of other magnetic field properties like its mean strength, geometry, and stability exist, but are associated with much larger uncertainties.
Ten-million-year time scales are characteristic of mantle dynamics. Numerical simulations suggest an influence of the lower mantle structure on the dynamo process operating in the liquid iron core. Though highly intriguing, the coupling is not well understood. The lower mantle imposes a thermal boundary condition on the core that can be incorporated in dynamo simulations. Using mantle reconstructions of the lower mantle structure in dynamo simulations to explain the observed variation in the reversal frequency has had only limited success so far (Olson et al., 2014; Olson and Amit, 2014; Olson and Amit, 2015). While mantle reconstructions reach back several tens of million years, the reversal record spans several 100s of million years. Although sparse, some paleomagnetic observations provide information about the magnetic field some four billion years into Earth’s past. If we knew how the lower mantle impacts the dynamo process, we could use this outstanding paleomagnetic record to constrain not only the dynamo process but also the history of Earth’s mantle.
There is also a connection to the history of life. Numerous organisms detect and utilize the geomagnetic field for navigation (Nordmann et al., 2017). This implies that variations in the geomagnetic field can trigger evolutionary processes. An understanding of this fascinating aspect could lead to additional information on the ancient geomagnetic field based on the type and abundance of magnetic-sensing species preserved in the fossil record. So far, however, the influence of the magnetic field strength, geometry and reversals on magnetoreceptive organisms remains unclear. There seems to be another link between the evolution of species and the geodynamic and geomagnetic evolution of the Earth. Two of the world’s largest extinction events, the ones that mark the end of the Paleozoic and Mesozoic eras, are linked to giant flood basalt eruptions (Deccan and Siberian traps) from mantle plumes originating from the core-mantle boundary (CMB) (Courtillot and Besse, 1987; Courtillot, 1999) (Figure 1). The initial rise of the plume from the CMB marks the end of the superchrons some 10-20 Myr before the plumes cause giant volcanic eruptions on the surface that lead to mass extinction.
The fundamental goal of DeepDyn is to exploit the exceptionally long paleomagnetic record for reconstructing the evolution of the core-mantle system through geologic time. This puzzle requires the collaboration of five distinct scientific disciplines, each being represented by leading international experts in the extended programme committee:
- Paleomagnetism provides information about the long-term evolution of Earth’s magnetic field. This theme is represented by Stuart Gilder, a paleomagnetist at LMU München, and Monika Korte, a geomagnetist at GFZ Potsdam.
- Dynamo models explore how this record can be explained, using input from material scientists and mantle dynamicists. This field is represented by Johannes Wicht, a dynamo-modeller at MPS Göttingen.
- Mantle dynamics define the long-term evolution of the mantle and in particular reconstruct the lower-mantle state based on input from material scientists and seismologists. This field is represented by Peter Bunge, a geodynamicist at LMU München, and Christine Thomas, a seismologist at WWU Münster.
- Mineral physics constrains the material properties relevant to the lower mantle and core. This field is represented by Ronald Redmer, an expert on material properties at Uni. Rostock.
- Biomagnetism supplements the geomagnetic record by understanding the sensitivity of magnetoreceptive organisms to magnetic fields and using magnetofossils from magnetotatic bacteria as a proxy. This field is represented by Dirk Schüler, a microbiologist at Uni. Bayreuth.
Ensuring close integration between these five themes is a challenging task that lies at the heart of this priority program. Scientifically, the goal is to link the geodynamo and geodynamic models to produce testable hypotheses concerning the material properties and structure of the lower mantle and outer core and how particular scenarios bear on the characteristics of reversals or field strength. In parallel, targeted efforts to produce more realistic input parameters for the models will be provided. One example here concerns the development of magnetofossils as proxies for paleointensity.