M. Ritzwoller, N. Shapiro, S. Zhong, & J. Wahr University of Colorado at Boulder

1. Thermal structure of the upper mantle beneath Antarctica:Implications for heat flux and visco-elastic rebound M. Ritzwoller, N. Shapiro, S. Zhong, & J. Wahr University of Colorado at Boulder • Why mantle temperatures beneath Antarctica? • Information about tectonic history. • Surface heat flux -- boundary condition for ice sheet and ice • stream modeling. • Temperature tied to rheology -- affects solid earth’s response to icesheet loading/unloading, with possible feedbacks to ice • sheet stability, sea level and climate change.

2. Visco-Elastic Response to Plausible Glacial Loading/Unloading 10 ky BP “West Antarctic” Rheology “East Antarctic” Rheology • Maximum difference in total depression ~300 m. • Magnitude depends mostly on lithospheric thickness. • Rate of change depends mostly on absolute viscosity.

3. Extent of Basal Water 6 My into Coupled Climate - Ice Sheet Simulation “W. Antarctic” Heat Flux (75.4 mW/m2) “E. Antarctic” Heat Flux (37.7 mW/m2) David Pollard, Robert DeConto, Andrew Nyblade, Sensitivity of Cz Ant. Ice Sheet Variations to Geothermal Heat Flux, Submitted to Global and Planetary Change, 2004.

4. Inferring Upper Mantle Temperatures Problem 1: Seismic models alone do not reveal temperatures faithfully. Approach 1: Add heat flux information to calibrate upper mantle temperatures -- works well for other continents. Problem 2: No heat flow data for Antarctica. Approach 2: Extrapolate heat flux data from other continents -- works well elsewhere. Use seismic model to guide extrapolation. Problem 3: Poor horizontal resolution. Approach 3: “Antarctic Array” & new observing methods.

5. Problem 1: Mantle temperatures from seismic models alone don’t agree well with physical models of mantle temperature structure Typical cratonic temperature profile from thermal modelers from Jaupart and Mareschal (1999)

6. Approach 1. Constraining seismic inversions to fit surface heat flux Data: surface wave dispersion maps 100 sec Rayleigh wave group speed

7. Local dispersion curves All dispersion maps: Rayleigh and Love wave group and phase velocities at all periods

8. Inversion of dispersion curves All dispersion maps: Rayleigh and Love wave group and phase velocities at all periods Monte-Carlo sampling of model space to find an ensemble of acceptable models

9. Heat flux: Constraint in Uppermost Mantle seismically acceptable models

10. Inversion with the seismic parameterization seismically acceptable models

11. Inversion with the seismic parameterization seismically acceptable models

12. Simple thermal parameterization of the continental uppermost mantle

13. Lithospheric thickness and mantle heat flow in Canada Power-law relation between lithospheric thickness and mantle heat flow is consistent with the model of Jaupart et al. (1998) who postulated that the steady heat flux at the base of the lithosphere is supplied by small-scale convection.

14. Problem 2: Little heat flow data for Antarctica Heat flow data base: Pollack et al., 1993

15. Approach 2: Extrapolate heat flow measurements to Antaractica Extrapolation is guided by a global seismic model. Produces a distribution of values on a 2 deg x 2 deg grid world-wide. Works well elsewhere in the world. Two points in Antarctica Mean and st dev much higher in West Antarctica.

16. Mean of the Extrapolated Distribution

17. 100 km depth Results: Vs and temperature across Antarctica W. Ant. Rift So. Pole E. Ant. Craton A A’ temp • W. vs E. Antarctica: @100 km > 1000 deg difference. @ 300 km. > 400 deg difference. • Along Transantarctic Mtns: @ 100 km, > 1deg/km laterally. • E. Antarctic cratonic core > 300 km thick, but much thinner nearer the coast.

18. Results: Lithospheric thickness compared with other continents Other cratons lithospheric thickness

19. Results: Lithospheric thickness vs mantle heat flow compared with other continents Other Continents Antarctica

20. Results: Lithospheric thickness vs mantle heat flow compared with other continents Other Continents Antarctica

21. Anomalous Mantle Structure Beneath Antarctica? Locations with relatively thin lithosphere but low heat flux. Cause? Erosion of the continental roots caused by Mesozoic rifting? or Simply poor lateral resolution?

22. Problem 3: Low resolution.Approach 3: Improve instrumentation and methods. • Improving instrumentation: “Antarctic Array” -- • a vision for seismology on an ice-bound • continent. In the planning stages now, • for initial deployment during IPY. • www.antarcticarray.org • Develop seismic methods to extract information about earth structure without using earthquakes as the source -- needed because significant seismicity is remote to Antarctica. Use surface waves emanating from microseisms and atmospheric fluctuations to estimate Green functions between receivers. :

23. Green Functions by Cross-Correlating Ambient Noise in Antarctica? Record section: Cross-correlate 1 month of ambient noise, Z Bandpass centered on: 20 sec 20 sec period Rayleigh wave

24. Summary and Conclusions • Understanding mantle temperatures beneath Antarctica is particularly important, due to potential ties to ice sheet/stream stability and sea level & climate change through heat flux and mantle rheology. • Combining information about surface wave dispersion with heat flow information extrapolated from other continents is providing new information about the temperature structure of the mantle beneath Antarctica. • New methods of surface wave analysis based on non-earthquake sources promise improved resolution. • Great advances will require a new generation of seismic instrumentation such as that being proposed as part of the new Antarctic Array initiative.

26. Results: Mantle heat flow compared with other continents

27. conversion between seismic velocity and temperature computed with the method of Goes et al. (2000) using laboratory-measured thermo-elastic properties of main mantle minerals and cratonic mantle composition non-linear relation

28. Monte-Carlo inversion of the seismic data based on the thermal description of model

29. Monte-Carlo inversion of the seismic data based on the thermal description of model a-priori range of physically plausible thermal models

30. Monte-Carlo inversion of the seismic data based on the thermal description of model a-priori range of physically plausible thermal models constraints from thermal data (heat flow)

31. Monte-Carlo inversion of the seismic data based on the thermal description of model a-priori range of physically plausible thermal models constraints from thermal data (heat flow) randomly generated thermal models

32. Monte-Carlo inversion of the seismic data based on the thermal description of model a-priori range of physically plausible thermal models constraints from thermal data (heat flow) randomly generated thermal models converting thermal models into seismic models

33. Monte-Carlo inversion of the seismic data based on the thermal description of model a-priori range of physically plausible thermal models constraints from thermal data (heat flow) randomly generated thermal models converting thermal models into seismic models finding the ensemble of acceptable seismic models

34. Monte-Carlo inversion of the seismic data based on the thermal description of model a-priori range of physically plausible thermal models constraints from thermal data (heat flow) randomly generated thermal models converting thermal models into seismic models finding the ensemble of acceptable seismic models converting into ensemble of acceptable thermal models

35. 3D seismic model

36. Where the cratons are? Geological data (Goodwin, 1996) Geophysical data Heat flow (Pollack et al, 1993) No information about mantle structure Un-evenly distributed Over Earth’s surface

37. Where the cratons are? Geological data (Goodwin, 1996) Geophysical data Inversion of heat flow (Artemieva and Mooney, 1998) No information about mantle structure Un-evenly distributed Over Earth’s surface

38. Seismic surface-waves • Provide homogeneous coverage in the uppermost mantle • Provide sensitivity to the thermal structure of the uppermost mantle 1. Data 2. Two-step inversion procedure global set of broadband fundamental-mode Rayleigh and Love wave dispersion measurements (more than 200,000 paths worldwide) Surface-wave tomography: construction of 2D dispersion maps Inversion of dispersion curves for the shear-velocity model Group velocities 18-200 s. Measured at Boulder. Phase velocities 40-150 s. Provided by Harvard and Utrecht groups

39. Where the cratons are? Geological data (Goodwin, 1996) No information about mantle structure

40. Where the cratons are? Geological data (Goodwin, 1996) No information about mantle structure