Teaching Slides: Sea-glacier dynamics and tropical sea-ice thickness


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9.2:Three models of snowball earth with sea-ice dynamics. Top: tropical oasis ("slushball") model after Hyde et al. (2000) and Peltier et al. (2004). Middle: thin equatorial marine ice model after McKay (2000) and Pollard & Kasting, 2005). Below: thick tropical sea glacier model ("hard snowball") after Warren et al. (2002) and Goodman & Pierrehumbert (2003). The three models require increasingly high levels of radiative forcing to deglaciate. Differences between the models illustrate their sensitivity to parameters like ice albedo in sublimation zones.



9.3: Steady-state sea-glacier dynamics with ice lines close to the critical latitude (top) and in snowball earth (bottom), based on the model results of Goodman & Pierrehumbert (2003).



9.4: Pole-to-pole cross-section of dynamic sea glaciers on an all-ocean snowball planet according to the thin equatorial marine ice model of Pollard & Kasting (2005). Equatorial ice is <2 m thick, allowing healthy rates of photosynthesis, but the model is stable only when dark marine ice is exposed. If bubbly meteoric ice encroaches into the equatorial zone, the ice rapidly thickens due to higher albedo (see next slide).



9.5:Pole-to-pole cross-section of the dynamic sea glaciers on an all-ocean snowball planet according to the thick tropical sea glacier model of Goodman & Pierrehumbert (2003). Equatorward flow of sea glaciers is balanced by sublimation-condensation and melting-freezing. Flowage causes thicker ice in the tropics and thinner ice elsewhere, compared with static ice.



9.6: Hysteresis in CO2-radiative forcing associated with a complete snowball "freeze-fry" cycle assuming the thin equatorial marine ice solution of Pollard & Kasting (2005). The amplitude in radiative forcing is 35x less than for the thick tropical sea glacier model.



9.7: Three hypothetical ice-mass types—ice-sheet ice, sea-glacier ice, and multiannual landfast ice (sikkusak)—on a low-latitude continental margin of a snowball earth (Halverson et al., 2005). Sikkusak thickness is equilibrated to local surface air temperature (see slide 9-5) whereas sea-glacier ice is thicker because of flowage from higher latitudes.



9.8: Sea-glacier flowage means that crack systems will perpetually exist wherever the moving ice impinges on the sea floor or landfast ice. Cracks propagate through thick ice due to water pressure and freezing produces juvenile (<5 years old) sea ice hosting brine channels populated by a variety of prokaryotic and eukaryotic, phototrophic and heterotrophic organisms. Because oxygen cannot diffuse from isolated brine channels, the organisms must be tolerant of hyperoxia, a potentially useful pre-adaptation if atmospheric oxygen levels rose following the younger Cryogenian snowball earth.



9.9: Cryogenian and early Ediacaran successions in the East Greenland (EG) and East Svalbard (ES) Caledonides. Glacial sedimentary interpretation after Halverson et al. (2005).



9.10: Cryogenian and early Ediacaran succession in northeastern Svalbard. Note the glacial marine Petrovbreen Member, the separate glacial terrestrial Wilsonbreen Formation, and the MacDonaldryggen argillite and Slangen evaporitic dolostone between the glacials. Conventional interpretion involves two separate glaciations, possibly correlated with the older and younger Cryogenian glaciations. Photo of section near Dracoisen, Ny Friesland, Spitsbergen.



9.11: Upper Wilsonbreen glacial diamictite and basal Dracoisen cap dolostone in the Ditlovtoppen section, Ny Friesland, Spitsbergen. The cap dolostone and overlying strata are virtually indistinguishable from younger Cryogenian (Marinoan) cap dolostones in East Greenland and NW Canada.



9.12: Storeelv glacial diamictite and basal Canyon cap dolostone in the Andrée Land section, Fjord Region, East Greenland.



9.13: Stelfox glacial diamictite and Ravensthroat cap dolostone in the Arctic Red River section, Mackenzie Mountains, northern Canadian Cordillera.



9.14: Carbon isotopic records of the basal Dracoisen cap dolostone and the Russøya Formation of the Polarisbreen Group, NE Svalbard, and apparent correlative units in NW Canada, NW Namibia and South Australia. Correlation of the Russøya and Trezona anomalies implies that both glacial horizons (and inter-glacial strata) in Svalbard belong to the younger Cryogenian (Marinoan) glaciation. Alternatively, a Trezona-like anomaly preceded the older Cryogenian (Sturtian) glaciation in Svalbard.



9.15: Characteristic laminated argillite (suspension deposits) with early diagenetic calcite nodules (glendonites, pseudomorphic after ikaaite) interpreted as a sub-sikussak deposit sourced from basal ice-sheet meltwater plumes (see slide 9-8). Ikaite is the preferred phase in cold phosphate-rich waters, implying significant organic productivity possibly associated with crack systems or clear marine ice <20 m thick.



9.16: Idealized three ice-mass types on a low-latitude continental margin on a snowball earth. Paleomagnetic data (Maloof et al., in press, Bull. Geol. Soc. Amer.) place East Svalbard in low southerly latitudes on the then southest-facing (windward) margin of Laurentia.



9.17: Carbon and oxygen isotopic records for two sections of the Slangen Member, an evaporitic carbonate parasequence between the two Polarisbreen glacials. Contrasting δ13Ccarb records suggest diachronous deposition associated with tidal flat progradation in an enclosed basin with a rapidly changing isotopic composition.



9.18: Development of snowball “oases” (Halverson et al., 2005) in areas of former sikkusak (low-latitude silled basins and inland seas). Sikkusak disappears when sea-surface air temperatures reach the melting point, while sea-glaciers continue their invasion of tropical ocean basins. Opening of oases may alter vapor transport and hence the mass-balance of adjacent ice sheets.



9.19: Possible areas of oasis development on a hypothetical snowball earth with present geography. Oases might occupy a few percent of global surface area. Note sources of dust in low-lying ice-free terrestrial areas situated beneath the descending arms of the atmospheric Hadley cells.



9.20: Isotopic composition of δ13Ccarb in a snowball oasis, compared with the present ocean. In the present ocean, depends on the isotopic composition of volcanic C input (~-5‰) and on steady-state organic burial forg. Present atmospheric δ13C is equilibrated by the larger oceanic C reservoir, and the equilibrium fractionation (at ~15C) is ~-8‰. On a snowball, atmospheric δ13C might be equilibrated with seawater through air-sea gas exchange in cracks or with volcanic CO2 emissions, which are ~-5‰, or ~3‰ higher than the present atmosphere. In a snowball oasis, δ13Ccarb will equilibrate to the larger atmospheric C reservoir. With surface temperature ~0C, the equilibrium fractionation is ~10‰ (12‰ for CaCO3), giving d13Ccarb as high as 7‰. As sikussak disappears, the isotopic composition of oasis water might evolve rapidly if equilibration between the atmosphere and ocean was not previously maintained through cracks.



9.21: Sikussak-oasis model for the Polarisbreen Group, in which the two glacial horizons express the onset and termination respectively of a single snowball epoch, the Marinoan. The model is being tested by comparative studies in the more landward correlative succession in East Greenland. This work is funded through grants from the U.S. National Science Foundation, Arctic Natural Science Division.