Earth School Dynamics - 101 (ESD-101)
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More than 10 million titles spanning every genre imaginable, at your fingertips. New titles added every day! A Summer seasonal cycle of a cryoconite hole in the sublimation zone on Canada Glacier Fig. In early summer, cryoconite melts to an equilibrium depth, after which it maintains constant depth relative to the sublimating surface. Air is evolved in the hole from melting of glacial ice containing bubbles of air and from photosynthetic O 2 production.
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Meltwater refreezes in winter and is covered by an ice cap in summer. Cryoconite is suffused with filamentous cyanobacteria and extracellular mucilaginous polysaccharides, and the holes are also inhabited by green algae, fungi, protists, and certain bilaterian animals—nematodes, rotifers, and tardigrades B Postulated diurnal cycle hour of a cryoconite hole on the low-latitude sublimation zone of a sea glacier on Snowball Earth. Relatively high CO 2 allows the nocturnal ice cap to melt away in midafternoon.
Cryoconite holes and ponds provide supraglacial habitats for Cryogenian cyanobacteria and eukaryotic algae and heterotrophs once the surface becomes sufficiently warm to retain dust exposed by sublimation — Snowball Earth is essentially an oceanographic phenomenon. Its onset is defined when the tropical ocean freezes over, and its termination is defined when the equatorial ice shelf finally divides and collapses. Small areas of open ocean are unsustainable because the sea ice becomes hundreds of meters thick within a few thousand years, due to the albedo-driven cold surface temperatures.
Consequently, the ice spreads gravitationally and fills in any area that is not physically restricted , , , , The most favorable conditions for thin oceanic ice exist in hydrothermal areas and marine embayments into low-albedo ice-free land areas — Thin oceanic ice or open water is not a prerequisite for phototrophy if meltwater existed on the ice surface. Moreover, those source areas of terrigenous dust are situated in the same zone where surface winds associated with the Hadley cells are strongest Fig.
The Snowball troposphere was dusty, and dust trapped anywhere on the sea glacier or on ice sheets feeding the sea glacier would be carried by glacial flow to the sublimative surface of meteoric ice in the equatorial zone Fig. Whereas cryoconite holes and ponds in the polar regions freeze solid in winter Fig. We begin the assessment of supraglacial refugia hypothesis by Vincent and co-workers — by briefly reviewing what is known from molecular and body fossil evidence about pre-Sturtian and pre-Marinoan marine life, with emphasis on purported crown groups, which, by definition, are lineages that survived the cryochron s , and all subsequent vicissitudes.
We then review attempts to find geologically acceptable climate-model states in which the tropical or equatorial ocean remains ice-free. Next, we sketch sea-glacier dynamics and its response to surface warming, based on 2D and 3D models. Finally, we consider the timing and extent of cryoconite accumulation and its potential climatic, geochemical, and evolutionary consequences.
Cellular fossils and molecular phylogenetics indicate that cyanobacteria, including those with cellular differentiation, evolved more than 10 9 years before the Cryogenian — Low ratios of eukaryotic-to-bacterial biomarkers from indigenous bitumens and oils imply that bacteria were the dominant primary producers in pre-Sturtian oceans 49 , As for eukaryotic primary producers, red algae and possibly green algae, including multicellular forms, are known from the pre-Sturtian cellular fossil record , — Molecular sterane biomarkers suggest that green algae supplanted red algae as the dominant eukaryotic phototrophs sometime between the late Tonian and late Cryogenian 48 , 49 , Among eukaryotic heterotrophs, vase-shaped microfossils VSMs resembling extant amoebozoans and rhizarians are widely preserved in pre-Sturtian strata around Ma — , and various protistan morphotypes including VSMs are found in nonglacial strata between the cryochrons — Molecular clocks predict that stem-group metazoans predated the Sturtian cryochron 47 , and sterane biomarkers suggest that a metazoan crown group, demosponges, evolved before the Marinoan cryochron [ 46 — 48 ; but see the study by Brocks and Butterfield ].
The fossil record in total is too coarse to correlate extinctions or originations with cryochrons, but the Cryogenian stands out as an anomalous period of low total and within-assemblage eukaryotic diversity After the Cryogenian ended, acritarch diversity increased sharply , , as perhaps did that of benthic macroalgae , , The fossil record and molecular phylogeny together indicate that multiple clades of eukaryotic algae and heterotrophs, both single-celled and multicellular, not only survived the Cryogenian glaciations but may have significantly evolved during that period — There have been concerted efforts to find climate-model solutions that satisfy basic inferences from Cryogenian geology—dynamic ice sheets that reach sea level in the paleotropics Fig.
The modeling task is a difficult one. First, the solutions, by their nature, lie close to the Snowball bifurcation Fig. This is a tall order, given stochastic, orbital, tectonic, and paleogeographic forcings Second, a large hysteresis must exist between the Waterbelt and nonglacial states to satisfy the geologically observed abrupt deglaciations and attendant geochemical anomalies—cap carbonates 27 , 29 , 84 , , , , proxy indicators of high CO 2 84 — 88 , 90 , and spikes in weathering 60 , 89 , Third, as applied to the Sturtian cryochron, solutions must be compatible with deep-ocean ferruginous anoxia, given widespread synglacial nonvolcanic iron formations Figs.
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This is a challenge because a narrow tropical ocean will experience intense wind-driven ventilation , , and the remote ice-covered regions will lack organic productivity, reducing the demand for oxygen by aerobic respiration in those areas. However, the model solutions are interesting in their own right, independent of the needs of Cryogenian geology.
The most-cited Waterbelt solution, HCBP00 , emerged from a 2D energy-balance model coupled to a dynamic ice-sheet model, with a paleogeography in which a high-latitude supercontinent has promontories and large islands that extend across the deep tropics. Long integration times allow orbital forcing to be included.
By incrementally lowering the CO 2 radiative forcing, a Snowball bifurcation is found at which ice sheets abruptly extend to all latitudes.
The fully glaciated response from the coupled model was then prescribed in an atmospheric GCM Genesis 2 with a mixed-layer ocean that is, no ocean dynamics and nondynamic sea ice. The solution was criticized for lacking sea-ice dynamics , Sea-ice dynamics facilitate ice-line advance in the mid-latitudes where the Coriolis effect drives sea ice equatorward under the influence of westerly winds. The concern was left unresolved because the wind field in the dynamic sea-ice model was imported from a GCM FOAM response to Cryogenian paleogeography in the absence of ice.
A low-latitude ice margin would produce a much stronger wind field , , in which Coriolis forcing under the influence of the trade winds might actually retard sea-ice advance. Another concern with the HCBP00 solution is that the mid- to high-latitude sea-ice caps are arbitrarily limited to 10 m of maximum thickness, and therefore, gravitational flow , , , is excluded. Moreover, an artificial heat source was introduced to limit ice thickness , and this heat source arbitrarily retards ice-cap growth.
Additional simulations showed that the sea-ice margins in the HCBP00 solution retreat poleward in response to even modest increases in CO 2 , simulating the loss of weathering by the ice-covered continents. The hysteresis demanded by the records of Cryogenian deglaciation is not present. It was found that by prescribing a large difference in broadband albedo between ablative 0. As the floating ice margins enter the tropics in response to weakened radiative forcing, the ablation zones widen as they encroach upon the subsiding limbs of the intensified Hadley cells.
This lowers the zonal and planetary albedos, stabilizing the ice margins. The narrow seaway migrates nearly its own width back and forth across the equator with the seasons. Ice sheets develop on elevated continents in the equatorial zone where, unlike Snowball Earth, there is a large excess of precipitation over evaporation , Strong hysteresis between Jormungand and nonglacial states has been found in aquaplanet atmosphere-only GCMs, although less so than for the Snowball state , but more work is needed to clarify the impact of ocean dynamics and continents on Jormungand hysteresis.
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Orbital forcing and sea-glacier flow have yet to be investigated in the Jormungand state. As with Jormungand, equatorial ice sheets are compatible with the BR15 solution A potential destabilizing process involves boundary-layer temperature inversions in the winter hemisphere, where the surface becomes very cold through radiation 81 , — , Inversions decouple the ocean from winds, disabling the wind-driven negative feedback. Atmospheric GCMs with high vertical resolution and realistic paleogeography are needed to resolve this issue. A Snowball Earth ensues when sea ice, having reached a critical latitude, advances uncontrollably to the equator through ice-albedo feedback Fig.
Where the cold ice is ablating by sublimation, the hydrohalite crystals accumulate as a surface lag, with an albedo higher than fresh snow , , However, when produced in the laboratory, the hydrohalite lag is a loose powder that would be susceptible to dispersal by winds After a few millennia, the oceanic ice cover outside the tropics is expected to reach thicknesses of several hundred meters, built up from above by frost deposition and snowfall and from below by freezing of seawater.
The sea glaciers would flow into the tropics and displace the thinner sea ice. They are composed of two ice types Fig. Without continents, dynamic steady state requires that sublimation and accumulation of meteoric ice are in balance, as are melting and freezing of marine ice Fig. Sublimation of marine ice cannot occur in steady state in the absence of a return flux to the ocean Fig.
If continents are present, then the situation is reversed. Sublimation of marine ice must occur Fig. Sublimation of marine ice is observed in some models 81 , , , and geological evidence supports meltwater discharge at tidewater grounding lines of Cryogenian ice sheets 96 , Surface exposure of marine ice strongly influences climate and sea-glacier dynamics because of its low albedo A Non—steady-state Snowball aquaplanet on which low-albedo marine ice outcrops in the sublimation zone.
Sublimation of marine ice magenta arrow is not balanced by a return flux to the seawater—marine ice subsystem B Steady-state Snowball aquaplanet on which only meteoric ice is exposed. C Non—steady-state Snowball Earth with continents. Meteoric ice-sheet meltwater enters the ocean at ice grounding lines magenta arrows but is not balanced by a return flux to the atmosphere—meteoric ice subsystem, because only meteoric ice is exposed.
D Steady-state Snowball Earth with continents. Ice-sheet meltwater injected into the ocean is balanced by sublimation of outcropping marine ice. Cryoconite meltwater flushing Fig. The high albedo of bare meteoric ice is the result of light scattering by air bubbles and cracks , and that of bare sea ice is caused by brine inclusions, air bubbles, and cracks Consequently, it has a low bare-ice albedo 0. Both bubbles and brine inclusions migrate downward, toward the warm end of a temperature gradient.
However, laboratory experiments show that, for the expected rates of sublimation on Snowball Earth Fig.
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Steady-state sea-glacier ice thicknesses and meridional velocities in different GCM-ice model couplings with 0. Continents are neglected, and the ice albedo is set to 0. The treatment was extended to spherical geometry and two horizontal dimensions, with a Cryogenian paleogeography Fig. The models all indicate thick ice at the equator with modest thickening toward the poles Figs. With temperature-dependent ice viscosity, low-latitude ice thicknesses actually increase slightly as the surface warms The thinnest ice is found in low-latitude embayments Fig.
With uniform surface albedo, embayment ice remains too thick to allow sub-ice photosynthesis Fig. Low-latitude embayments may have been critical for phototrophs in early stages of cryochrons, when sea ice elsewhere was too thick for sub-ice photosynthesis and too cold for dust retention Fig. Steady-state ice thickness scale bar in meters and velocity field arrows, in meters per year, with every fourth velocity vector shown with surface temperatures smoothly fitted to NCAR GCM results assuming a surface albedo of 0.
Paleogeography from the study by Li et al.