Projects

Core Formation in the Lab
Sensitivity of a Coupled Earth System Model to Isopycnal Stirring
Climate interactions forced by continent assembly and breakup
The Young Inner Core
F-layer formation at the inner core boundary
How increasing mixing warms the polar regions
Modeling the Geodynamo
Global Paleobathymetry and Ocean Sediment Reconstruction
Magnetic Reversals and the Earth's Core
Geomagnetic Superchron Cycles Driven by Mantle Convection
Mantle convection, plates, and the Earth system
Geodynamic Carbon Cycling


Mantle convection, plates, and the Earth system


Crust Mantle

The dynamics of the Earth's interior is controlled by mantle convection On the largest scales, the present-day lower mantle convective structure includes a spherical harmonic degree two heterogeneity, with large low seismic velocity structures beneath Africa and the central Pacific, the so-called superplumes, with higher seismic velocity structures forming a ring that circumscribes the Pacific. Evolution of these structures drives global change events in the mantle plate tectonics, surface vertical motions including continental flooding and sea level change. Open Earth Systems aims to investigate how the evolution of mantle convection has affected the ocean, the atmosphere, and even the core.

The history of global tectonics includes repeated phases of continental aggregation and dispersal and plate re-organization. Two well-documented geodynamic events in the Phanerozoic (0 to 545 Ma approximately) were the assembly and breakup of the supercontinent Pangea around 330 Ma and 180 Ma, respectively. Pangea assembly is linked to major mountain building, and later Pangean evolution and its breakup are linked to volcanic and magmatic activity including flood basalts (eruptions that formed large igneous provinces, LIPs) such as the Central Atlantic Magmatic Province. There is also significant global sea level change and large-scale continental flooding associated with Pangea assembly and its breakup and significant variability in the geomagnetic field, most importantly the two and possibly three long, constant polarity magnetic superchrons. It has been argued that mantle convection has alternated between spherical harmonic degree 1 and 2 patterns as shown in the Figure below, representing pre-breakup and pre-aggregation phases of the continents. The Pacific superplume likely predates Pangea whereas the African superplume is more enigmatic possibly being older than 500 Ma or forming as a consequence of Pangea.


Figure 1. Snapshots of lower mantle temperature anomalies (red=positive,hot; blue=negative,cold) at 2750 km depth at various times from a numerical simulation of Phanerozoic mantle history by Zhang et al. (2010) driven by thermo-chemical convection and constrained by paleogeographic reconstructions (Scotese, 1997). The solid and dashed black lines in images a-e represent reconstructed continental and oceanic plate boundaries, respectively, and present-day continent outlines are shown for reference. The dashed circles in image a highlight the change in thermal regime in the African hemisphere.

Plate dynamics coupled to mantle convection plays major roles in controlling long-term core and surface processes. The driving of core activity by variations in CMB heat flux is well known, and the influence on volcanic activity is of course implicit. The negative feedback of mineral carbonation through tectonically driven erosion and weathering and volcanism provides long-term stability for the climate system. Plate reorganization, especially those inducing continental break-up speeds mineral carbon sequestration. Second, the positions of continents has a significant effect on ice-albedo feedback during cooling periods, an extreme example of which are Proterozoic Snowball events. Lastly, continental positions and movement have a profound effect on ocean circulation and transport of heat. For example, the long-term cooling of the Cenozoic is thought to have occurred because of the Australian sub-continent departing from Antarctica, thus opening up the Antarctic circumpolar current that thermally isolated Antarctica thus promoting permanent ice cover.

Large igneous provinces provide another illustration of how the evolving mantle dynamics shown in Fig. 1 affects the whole Earth system, not only through their eruption products, but also in the climate and the biosphere (through mass extinctions). Despite a long history of study, the origin of LIPs remains controversial, with many open issues, including their relationship to deep-seated mantle plumes, the geomagnetic field, timing, rates and volatile distributions during eruptions, and the scale of perturbations to the atmosphere, ocean, and biosphere.

Deciphering the role of LIPs in disrupting climate and the biosphere requires understanding numerous aspects of the eruptions, including the volumes of individual eruptions, time interval between eruptions, volume and rates of gas loss to the atmosphere, interaction with the substrate and role of the geological setting.

Open Earth System integration of mantle convection, plate dynamics and magma dynamics with Earth history allow us to test the following hypotheses:

1. The coevolution of large-scale mantle convection and plate dynamics governs the history of global sea level, continental uplift, and large igneous provinces.

2. The slow carbon cycle is governed by mantle convection and plate dynamics, thus controlling Earth's climate variations on geologic time scales.



References

Scotese, C.R., 1997. Continental Drift, 7th ed., PALEOMAP Project, Arlington,
       Texas. 79pp.

Zhang, N., S.J. Zhong, W. Leng, and Z.X. Li, 2010. A model for the evolution of
       the Earth's mantle structure since the Early Paleozoic, J. Geophys. Res., 115,
       B06401, doi:10.1029/2009JB006896.