Poster number: PP13B-2115
AGU Fall Meeting
Dec. 3-7, 2012
Sea Level Variations During Snowball Earth Formation: A Preliminary Analysis
Yonggang Liu1,2, W. Richard Peltier1
Department of Physics, University of Toronto, Toronto, ON, Canada
2
Woodrow Wilson School of Public and International Affairs, Princeton University, Princeton, NJ, USA
[email protected]
1
Sea Level Variation for Realistic Continental Config.
Motivation
Mean sea level should have dropped by ~1000 m1,2 during the global glaciation
(snowball Earth) events3,4 occurred during late Neoproterozoic (1000 Ma - 540 Ma) due
to the extraction of water from ocean to form ice sheets on the continents. However, this
ubiquitous large drop of sea level has not been observed in geological settings. Probably
the only observation was that by Hoffman et al.5 in Northern Namibia, which indicates a
sea level change of ~500 m. To resolve this issue, here we make numerical predictions of
spatial patterns of sea level variations during snowball Earth events, and demonstrate
why the large (~1000 m) sea level drop cannot be directly observed in geological settings.
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Bedrock displacement (m)
Ocean surface lowering (m)
point A
point B
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lowering (m)
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B370
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B'
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h (m)
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Figure 3 Timescale at which the ocean surface height change (at
point A in Figure 2c) and bedrock deformation (at point B in Figure
2c) reach equilibrium. The dashed part of the lines indicate the equilibrium values.
Both the ocean surface height and bedrock elevation are close to
equilibrium a few tens of thousands of years after the ice sheets are
emplaced on the continent.
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B’
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Main Conclusion
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Timescale of Sea Level Adjustment
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A’
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Longitude
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Figure 2 a) Ice thickness distribution on a circular super-continent during snowball Earth event calculated by an ice sheet model coupled with energy balance model (EBM)2, b) topview of bedrock
depression (contour lines) by the ice sheet and ocean surface lowering (filled contours) relative to its initial position 5 Myr after the ice sheet is emplaced on the continent, c) sideview of b), where
the pink and orange patches are the ice sheet and solid Earth, respectively. Blue solid line is the new position of the ocean surface, grey and blue dashed lines are the initial position of ocean surface
and ocean floor, respectively. Ocean surface lowering shown in b) is the distance between grey dashed line and blue solid line in c). Ocean surface near the ice sheet edge is much higher than that
far from the ice sheet mainly due to the gravitational attraction of the ice sheet.
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Longitude
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• Local ice thickness around the coastline; it determines the local bedrock (i.e. land surface) elevation (see Figure 5).
h (m)
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• Large scale ice sheets distribution over the continents; it determines the ocean surface
height change (see Figures 2 and 4).
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Radius (km)
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Radius (km)
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Major Factors Affecting Freeboard in a Snowball Earth Event
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Figure 5 Sideview of transections AA' and BB' as indicated by solid black lines in the lower right panel of Figure 4. This figure
demonstrates that even though the ocean surface lowering along the coastline may be similar, the difference in local ice thickness can produce much different freeboard.
Ocean surf.
lowering (m)
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Ice thick.
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Sea Level Variation for an Ideal Circular Super-continent
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A
• Rotational feedback not considered.
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• Transition between continent and ocean is sudden, i.e. the continental boundaries are
cliff-like.
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Longitude
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Figure 4 Similar to Figure 2a and 2b except that here the continental configurations are more realistic. The top and bottom panels are
for 720 Ma9 and 570 Ma10 continental configurations, respectively. The numbers along the coastlines indicate the freeboard, i.e. the
distance between the land surface and ocean surface, which may be inferred from and compared with geological sediments..
• Both the continents and ocean floor are flat, and are 10 m above and 4000 m below
initial ocean surface, respectively.
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• The ice sheets that would exist in a snowball Earth state is emplaced on the continents
instantaneously at time t = 0.
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Assumptions/simplifications
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Ice thick.
(km)
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Figure 1 Radial profile of a) density (from PREM7) and b) viscosity of the mantle (ICE6G-VM5a8)
a
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Viscosity (log10 Pa S)
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Density (kg m-3)
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We calculate the sea level change due to ice
sheet formation by solving the Sea Level
Equation gravitationally consistently using
normal mode theory6. The Earth is assumed
to be spherically symmetric and behaves
like a Maxwell body. The radial structures
of density and elastic properties are taken to
be that of PREM7 and the radial profile of
viscosity to be employed is that of the
VM5a model8 (Figure 1).
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Method
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b
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Ocean surf.
lowering (m)
Ice thick.
(km)
Our calculation with two realistic continental
configurations show that the freeboard along
coastlines during a snowball Earth event was generally between 300 - 750 m, although the ice
volume is equivalent to approximately 800 m and
1000 m of eustatic sea level for the 720 Ma and
570 Ma continental configurations, respectively.
Therefore, extreme sea level changes (~1000 m)
may not be observed directly from geological settings; the single estimation of sea level change of
~500 m currently available from North Namibia
is not inconsistent with the occurrence of snowball Earth events.
References
1. Hyde et al. (2000), Nature, 405, 425-429
2. Liu and Peltier (2010), J. Geophys. Res. - Atmos, 115, Doi 10.1029/2009jd013082
3. Kirschvink (1992), in The Proterozoic Biosphere: A multi-disciplinary study, eds. Schopf, J. W.
, C. Klein and D. Des Maris, Cambridge University Press, Cambridge, 51-52.
4. Hoffman et al. (1998), Science, 281, 1342-1346.
5. Hoffman et al. (2007), Earth Planet. Sci. Lett., 258(1-2), 114-131.
6. Peltier (2007), in Treatise on Geophysics, edited by G. Schubert, 243-‐293.
7. Dziewonski and Anderson (1981), Phys Earth Planet In, 25(4), 297-‐356
8. Peltier and Drummond (2008), Geophys. Res. Lett., Doi 10.1029/2008gl034586
9. Li et al. (2008), Precambrian Res., 160(1-2), 179-‐210.
10. Dalziel, I. W. D. (1997), Geol Soc Am Bull, 109(1), 16-‐42.