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[SIT20-23] Lateral heterogeneity of stable stratification at the outermost outer core estimated by a radial one-dimensional model
Keywords:Core convection, Stable stratification, Lateral heterogeneity, Radial one-dimensional model
Analyses of geophysical data have suggested the existence of a stable stratification at the top of the Earth's outer core. The stable layer thickness is estimated to be between 90 and 300 km for seismic waves [1,2] and up to 140 km from geomagnetic secular variations [3]. While these analyses potentially assumed a globally homogeneous layer, the outer core surface flow based on geomagnetic variations suggested the possibility of a laterally heterogeneous stable layer [5].
Mound et al. (2019) [6] showed that stable layer may occur locally by numerical simulations of thermal convection in a three-dimensional rotating spherical shell with a thermally heterogeneous boundary condition at the CMB. However, their model did not include the effect of compositional convection driven by the light element release from the inner core boundary (ICB). Furthermore, their model uses Boussinesq fluid with a constant mean density and thermal conductivity.
In this study, we address with the lateral variations in the thickness of a stable layer at the top of the outer core by locally applying a one-dimensional thermal and compositional convection model [6]. By assuming an isentropic temperature and uniform compositional profiles and giving the CMB heat flux, the growth rate of the inner core and the thermal and compositional convective fluxes are calculated. Furthermore, diagnosed profile of the kinetic energy production rate is used to assess the thickness of the stable layer [7]. The heat flux distributions at the CMB are converted from the seismic velocity anomalies in the lowest mantle [8]. Given the heat flux at each location on the CMB, one-dimensional thermal and compositional convective structure can identify the emergence and thickness of the stable layer there.
First, we perform 1D model calculations in the Boussinesq system to determine the thickness of the stable layer using the same settings as in Mound et al. (2019) [5]. The obtained results sufficiently explain the existence and thickness of the stable stratification by 3D calculations, indicating the validity of the 1D model for the present problem.
Next, in order to include the effect of compositional convection, the compositional flux caused by the inner core growth is converted and added to the thermal boundary flux at the ICB. The results show that the stable layer tends to be thinner compared to the cases without the effect of compositional convection. When the compositional flux is five times larger than the averaged CMB heat flow, the stable layer about 100 km thick appears just below Africa.
In addition, considering uncertainties in the core thermal conductivity, the calculations with a more realistic density structure of the outer core indicate that the thickness of the stable layer ranges from 300 to 850 km in low seismic velocity anomalies, such as the Pacific and Africa. On the other hand, stable stratification is not expected in faster seismic anomalies, such as the subduction zones.
Such a lateral heterogeneous structure of stable layer would have an impact on the geomagnetic fluctuation on a multi-decadal scale (e.g., Huguet et al., 2016).
References
[1] Tanaka, S. (2007) Earth Planet. Sci. Lett., 259, 486-499.
[2] Kaneshima, S. (2018) Phys. Earth Planet. Int., 276, 234-246.
[3] Buffett, B. A. (2014) Nature, 356, 329-331.
[4] Huguet, L., Amit, H., Alboussiere, T. (2016) Geophys. J. Int., 207, 934-948.
[5] Mound, J., et al. (2019) Nature Geosci., 12, 575-580.
[6] Nakagawa, T. et al. (2023) in Core-Mantle Co-evolution: An Interdisciplinary Approach, AGU monograph series, 276, 145-163.
[7] Takehiro, S., Sasaki, Y. (2018) Front. Earth Sci. 6:192.
[8] Masters, G. et al. (2000) in Earth's deep interior. Mineral physics and tomography from the atomic to the global scale, Geophysical Monograph series, 117, 63-87.
Mound et al. (2019) [6] showed that stable layer may occur locally by numerical simulations of thermal convection in a three-dimensional rotating spherical shell with a thermally heterogeneous boundary condition at the CMB. However, their model did not include the effect of compositional convection driven by the light element release from the inner core boundary (ICB). Furthermore, their model uses Boussinesq fluid with a constant mean density and thermal conductivity.
In this study, we address with the lateral variations in the thickness of a stable layer at the top of the outer core by locally applying a one-dimensional thermal and compositional convection model [6]. By assuming an isentropic temperature and uniform compositional profiles and giving the CMB heat flux, the growth rate of the inner core and the thermal and compositional convective fluxes are calculated. Furthermore, diagnosed profile of the kinetic energy production rate is used to assess the thickness of the stable layer [7]. The heat flux distributions at the CMB are converted from the seismic velocity anomalies in the lowest mantle [8]. Given the heat flux at each location on the CMB, one-dimensional thermal and compositional convective structure can identify the emergence and thickness of the stable layer there.
First, we perform 1D model calculations in the Boussinesq system to determine the thickness of the stable layer using the same settings as in Mound et al. (2019) [5]. The obtained results sufficiently explain the existence and thickness of the stable stratification by 3D calculations, indicating the validity of the 1D model for the present problem.
Next, in order to include the effect of compositional convection, the compositional flux caused by the inner core growth is converted and added to the thermal boundary flux at the ICB. The results show that the stable layer tends to be thinner compared to the cases without the effect of compositional convection. When the compositional flux is five times larger than the averaged CMB heat flow, the stable layer about 100 km thick appears just below Africa.
In addition, considering uncertainties in the core thermal conductivity, the calculations with a more realistic density structure of the outer core indicate that the thickness of the stable layer ranges from 300 to 850 km in low seismic velocity anomalies, such as the Pacific and Africa. On the other hand, stable stratification is not expected in faster seismic anomalies, such as the subduction zones.
Such a lateral heterogeneous structure of stable layer would have an impact on the geomagnetic fluctuation on a multi-decadal scale (e.g., Huguet et al., 2016).
References
[1] Tanaka, S. (2007) Earth Planet. Sci. Lett., 259, 486-499.
[2] Kaneshima, S. (2018) Phys. Earth Planet. Int., 276, 234-246.
[3] Buffett, B. A. (2014) Nature, 356, 329-331.
[4] Huguet, L., Amit, H., Alboussiere, T. (2016) Geophys. J. Int., 207, 934-948.
[5] Mound, J., et al. (2019) Nature Geosci., 12, 575-580.
[6] Nakagawa, T. et al. (2023) in Core-Mantle Co-evolution: An Interdisciplinary Approach, AGU monograph series, 276, 145-163.
[7] Takehiro, S., Sasaki, Y. (2018) Front. Earth Sci. 6:192.
[8] Masters, G. et al. (2000) in Earth's deep interior. Mineral physics and tomography from the atomic to the global scale, Geophysical Monograph series, 117, 63-87.