20.7: Starting and stopping a Snowball
- Page ID
- 22764
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)The Snowball Earth glaciations have important implications for our understanding of the role of greenhouse gases in regulating Earth’s climate. As you have learned, the planet’s surface temperature depends on several key variables, including the amount of energy the Sun emits, the planet’s position relative to the Sun, the ability of the planet to redistribute heat on its surface (mainly via ocean currents), and the ability of the planet’s atmosphere to retain heat that would otherwise be radiated out into space.
It is in this last category that greenhouse gases matter. They allow visible light from the Sun to pass through them unimpeded, but block infrared radiation – the kind that the planet’s rocks and oceans re-emit after basking in the sunlight for a while.
The most accepted model for the initiation of the Snowball Earth glaciations depends exquisitely on the level of the greenhouse gas carbon dioxide, \(\ce{CO2}\), in Earth’s atmosphere. The explanation goes like this: in the late Proterozoic, a supercontinent called Rodinia formed in Earth’s equatorial region. Because of its direct rays of incoming sunlight, the tropics are today home to (1) warm temperatures and (2) a humid climate with lots of rain. Lots of water vapor can be stored in warm air. Assuming that these same warm, wet conditions applied to the tropical belt during the Neoproterozoic, there would have been enhanced chemical weathering of the rocks in that belt. The reactions of chemical weathering consume \(\ce{CO2}\), such as the process by which feldspar (the most common mineral in Earth’s crust) is converted to clay (a widespread constituent of sedimentary rock). This means that as chemical weathering proceeded, \(\ce{CO2}\) levels in the atmosphere would have fallen. Typically, weathering-induced \(\ce{CO2}\) drawdown is balanced by volcanic \(\ce{CO2}\) emissions. But if the rate of tropical-weathering-induced \(\ce{CO2}\) drawdown was greater than the volcanoes could compensate for, atmospheric \(\ce{CO2}\) levels would have fallen. And because that means there would be less of a greenhouse effect, the temperature would have gotten colder.
The next stage of the story takes our attention away from the equator and focuses it instead on the polar regions, where incoming sunlight is much less direct. As a general rule, these are the coldest parts of our planet. As global \(\ce{CO2}\) was drawn down, ice would have begun to grow at the poles, forming floating sea ice that grew and grew through time, reaching into progressively lower and lower latitudes.
Sea ice has a high albedo: about 92% of incoming sunlight is reflected away. Very little energy is absorbed from the incoming sunlight. In contrast, ocean water has an extremely low albedo: only about 8% of incoming sunlight is reflected away, and the rest is absorbed as heat energy.
As sea ice built up in the polar regions, its high albedo reflected away more and more of the energy the planet was receiving from the Sun. This constantly-increasing albedo was an amplifying feedback system that took the \(\ce{CO2}\)-induced cooling and made it even more pronounced. Eventually, when the ice reached about 33\(^{\circ}\) of latitude, the albedo-induced cooling effect was unstoppable, and the remainder of the planet’s surface, including the equator, froze over in as little time as a decade.
The Snowball Earth had begun. The average global temperature at this time is estimated to have been -50 \(^{\circ}\)C.
Sea ice sealed off gas exchange between the liquid ocean below and the atmosphere above, resulting in anoxic waters developing in much of the sea. It was at this time that banded iron formation was resurrected. Up above, during the unbelievable cold of the Snowball, much of the the carbon cycle would no longer be functional. Chemical weathering rates would be effectively nil. The planet was locked in an icy death grip, so reflective it could never warm up again… right?
Wrong! (Obviously, since we’re here to talk about it.) You will recall that plate tectonics is not driven by energy from the Sun. Instead, it operates from Earth’s internal heat, driven in part by radioactive decay. So under the sea ice and under the glaciers, the plates continued their slow lateral movement. More to the point, subduction continued and seafloor spreading continued, and both of these processes resulted in volcanism. During the Snowball Earth, volcanoes continued to erupt, producing lava and ash but more importantly, emitting gases. These gases included water vapor (about 60% by volume) and carbon dioxide (about 15% by volume). The water vapor would have frozen, but the \(\ce{CO2}\) would not.
As the volcanoes continued to erupt, \(\ce{CO2}\) levels in the atmosphere began to build up again. With no weathering to extract the \(\ce{CO2}\) from the air, the amount of this greenhouse gas began to increase unchecked. Eventually, after about 5 million years or so, enough \(\ce{CO2}\) would have accrued that the resulting greenhouse heat retention would have overcome the albedo’s cooling effects. A few patches of sea ice would have melted –probably at the equator — and the exposed ocean water would have efficiently absorbed incoming solar energy. This melted more ice, which exposed more ocean water, and once again an amplifying feedback loop took over, rapidly melting off the sea ice (and glaciers) in short order.
The Snowball Earth was over; the post-glacial hothouse had begun. With an atmosphere chock full of \(\ce{CO2}\) and a low albedo allowing maximum absorption of incoming solar radiation, the average global temperature at this time is estimated to have been +50 \(^{\circ}\)C.
The \(\ce{CO2}\)-charged atmosphere and pulverized glacial sediment (full of calcium ions) then equilibrated, reacting to produce a tremendous amount of carbonate sediment that rapidly precipitated in the oceans. As swollen, warm seas transgressed onto the continents, the cap carbonates were laid down, rapidly and perhaps violently. Gradually, \(\ce{CO2}\) was drawn down again.
Overall, this story is thought to have repeated at least twice (Sturtian and Marinoan), and perhaps with lesser intensity a third time (Gaskiers). Because of its repetition, some geoscientists have suggested it as a kind of cycle: the Snowball cycle.
Did I Get It? - Quiz
What is the sequence of geological events/processes that is thought to have initiated the Snowball Earth glaciations?
a.
1) \(\ce{CO2}\) drawdown through intense weathering of the exposed crust in the tropical weathering belt,
2) Global cooling causing sea ice build up in polar regions and growing into lower latitudes,
and finally,
3) Runaway albedo-driven amplifying feedback, tipping the planet's surface into a totally frozen state.
b.
1) Global cooling causing sea ice build up in polar regions and growing into lower latitudes,
2) Runaway albedo-driven amplifying feedback, tipping the planet's surface into a totally frozen state,
and finally,
3) \(\ce{CO2}\) drawdown through intense weathering of the exposed crust in the tropical weathering belt.
c.
1) Runaway albedo-driven amplifying feedback, tipping the planet's surface into a totally frozen state,
2) Global cooling causing sea ice build up in polar regions and growing into lower latitudes,
and finally,
3) \(\ce{CO2}\) drawdown through intense weathering of the exposed crust in the tropical weathering belt.
- Answer
-
a.
1) \(\ce{CO2}\) drawdown through intense weathering of the exposed crust in the tropical weathering belt,
2) Global cooling causing sea ice build up in polar regions and growing into lower latitudes,
and finally,
3) Runaway albedo-driven amplifying feedback, tipping the planet's surface into a totally frozen state.
How do we think that Earth escaped from the Snowball Earth glaciations?
a. Meteorite impact
b. Being warmed by lots of volcanic lava
c. Volcanic ash deposits darkening snow cover and reversing the albedo-induced cooling effect
d. Volcanic \(\ce{CO2}\)-induced global warming
- Answer
-
d. Volcanic \(\ce{CO2}\)-induced global warming
Problems with the traditional Snowball Earth model
The Snowball Earth is a controversial topic among some geologists. The evidence for widespread glaciation is incontrovertible, but whether the entire planet froze over entirely (a “hard” Snowball) is less agreed-upon. Perhaps it was cold, but not quite -50 \(^{\circ}\)C. After all, somehow life survived through the Snowball, and it is hard to imagine how that would work. Perhaps there were “refugia,” protected places or warmer-than average places: adjacent to deep-sea hydrothermal vents, perhaps, or polynyas that are maintained by currents flowing from beneath. So there are ways of getting around that objection.
A harder problem to surmount is the way that glaciers flow. In the modern world, where we observe glacial ice, it flows downhill, from its zone of accumulation toward its zone of ablation. We have ample evidence that there was glacial flow, glacial erosion, and glacial deposition. But in order for that all to happen, a functional hydrological cycle is probably a requirement. Glaciers only flow in the modern world because they get fresh ice forming at their upstream end, due to precipitation of snow. But that precipitation wouldn’t be possible without evaporation of water from somewhere else. If the planet were truly frozen over during the Snowball Earth glaciations, there should have been dramatically reduced potential for evaporation, not only because of the low temperatures (meaning low amounts of energy to drive evaporation), but more importantly because of the impact of the sea ice acting as a physical barrier: sealing off potential exchange between the ocean and the atmosphere. If the Earth experienced a true Snowball, then how did the glaciers keep flowing?
For this reason, a substantial number of geoscientists prefer a milder “Slushball Earth” model as their interpretation for Neoproterozoic glaciation.
Did I Get It? - Quiz
A major argument against a "hard Snowball" model for Neoproterozoic glaciation is that...
a. ...no life survived these ice ages.
b. ...glaciers need a functional water cycle in order to continue to flow. The fact that we have plenty of evidence they did flow (and erode rock) during the Neoproterozoic suggests that there must have been open water somewhere to allow evaporation.
c. ...it just seems so hard to accept that the world could change that much.
- Answer
-
b. ...glaciers need a functional water cycle in order to continue to flow. The fact that we have plenty of evidence they did flow (and erode rock) during the Neoproterozoic suggests that there must have been open water somewhere to allow evaporation.