Tag Archives: Sea level rise

Sea-level rise following a Snowball Earth

The Cryogenian Period (850 to 635 Ma) of the Neoproterozoic is named for the intense glacial episodes recorded in strata of that age. There were two that palaeomagnetism  in glaciogenic sedimentary rocks indicates that ice covered all of the continents including those in the tropics, and a third, less extreme one. These episodes, when documented in the 1990s, became dubbed, aptly enough, as ‘Snowball Earth’ events. But evidence for frigidity does not pervade the entire Cryogenian, the glacial events being separated by long periods with no sign anywhere of tillites or glaciomarine diamictites shed by floating ice. Each Snowball Earth episode is everywhere overlain by thick carbonate deposits indicating clear, shallow seas and a massive supply of calcium and magnesium ions to seawater. The geochemical change is a clear indicator of intense chemical weathering of the exposed continents. The combination of Ca and Mg with carbonate ions likewise suggests an atmosphere rich in carbon dioxide. For frigidity episodically to have pervaded the entire planet indicates a distinct dearth of the greenhouse gas in the atmosphere during those events. The likely explanation for Snowball Earths is one of booms in the abundance of minute marine organisms, perhaps a consequence of the high phosphorus levels in the oceans during the Neoproterozoic when seawater was alkaline. The carbon-isotope record suggests that there were periodic, massive bursts of organic matter that would have drawn down atmospheric CO2, which coincide with the evidence for global frigidity, although marine life continued to flourish.

Artist’s impression of the glacial maximum of a Snowball Earth event (Source: NASA)

Under such ice-bound conditions the build-up of continental glaciers would have resulted in huge falls in global sea level, far exceeding the 150 m recorded during some late-Pleistocene glacial maxima. The end of each Snowball Earth would have led to equally dramatic rises and continental flooding. Such scenarios are well accepted to have occurred when accumulation of volcanic CO2 during full ice cover reached a threshold of global warming potential that could overcome the reflection of solar radiation by the high albedo of ice extending to the tropics. That threshold has been estimated to have been between 400 to 500 times the CO2 content of the atmosphere at present. Yet it has taken an intricate analysis of sedimentary structures that are commonplace in marine sediments of any age – ripple marks – to quantify the pace of sea-level rise at the end of a Snowball Earth event (Myrow, P.M. et al. 2018. Rapid sea level rise in the aftermath of a Neoproterozoic snowball Earth. Science, v. 360, p. 649-651; doi:10.1126/science.aap8612).

The Elatina Formation of South Australia, deposited during the Marinoan (~635 Ma) glaciation, is famous for the intricacy of its sedimentary structures especially in the clastic sedimentary rocks beneath the cap carbonate that marks the end of glacial conditions. Among them are laminated silts and fine sands that were originally thought to be the equivalent of modern varved sediments that form annually as lakes or shallow seas freeze over and then melt with the seasons. Since they contain ripple marks the laminates of the Elatina Formation clearly formed as a result of current flow and wave action – the sea surface was therefore ice free while these sediments accumulated. Careful study of the larger ripples, which are asymmetrical, shows that current-flow directions periodically reversed, suggesting that they formed as a result of tidal flows during the bi-monthly cycle of spring and neap tides in marine deltas. Data from experiments in wave tanks shows that the shapes (expressed as their amplitude to wavelength ratio) of wave ripples depend on the orbital motion of water waves at different depths. The smaller ripples are of this kind. So Myrow and colleagues have been able to tease out a time sequence from the tidal ripples and also signs of any variation in the water depth at which the smaller wave ripples formed.

Ripples on a bedding surface in the Elatina Formation, South Australia. They formed under the influence of tidal current flow. (Credit, University of Guelph, https://atrium.lib.uoguelph.ca/xmlui/handle/10214/9367?show=full)

Just over 9 metres of the tidal laminate sequence that escaped any erosion was deposited in about 60 years, giving a sedimentation rate of 27 cm per year. This is extremely high by comparison with those in any modern marine basins, probably reflecting the sediment-charged waters during a period of massive glacial melting. Throughout the full 27 m sequence smaller, wave ripples consistently show that water depth remained between 9 to 16 m for about a century. Over such a short time interval any tectonic subsidence or sag due to sediment load would have been minuscule. So sea-level rise kept pace with deposition; i.e. at the same rate of 27 cm per year. That is at least five times faster than during any of the Pleistocene deglaciations and about a hundred times faster than sea-level rise today that is caused by melting of the Greenland and Antarctic ice caps and thermal expansion of ocean water due to global warming. It has been estimated that the Marinoan ice sheets lowered global sea level by between 1.0 to 1.5 km – ten times more than in the last Ice Age – so deglaciation to the conditions of the cap carbonates, shallow, clear seas at around 50°C, would have taken about 6,000 years at the measured rate.

To read more on the Snowball Earth hypothesis and other early glacial epochs click here

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When did the Greenland ice cap last melt?

The record preserved in cores through the thickest part of the Greenland ice cap goes back only to a little more than 120 thousand years ago, unlike in Antarctica where data are available for 800 ka and potentially further back still. One possible reason for this difference is that a great deal more snow falls on Greenland so the ice builds up more quickly than in Antarctica. Because ice flows under pressure this might imply that older ice on Greenland long flowed to the margins and either melted or calved off as icebergs. So, although it is certain that the Antarctic ice cap has not melted away, at least in the last million years or so, we cannot tell if Greenlandic glaciers did so over the same period of time. Knowing whether or not Greenland might have shed its carapace of ice is important, because if ever does in future the meltwater will add about 7 metres to global sea level: a nightmare scenario for coastal cities, low-lying islands and insurance companies.

Margin of the Greenland ice sheet (view from p...

Edge of the Greenland ice sheet with a large glacier flowing into a fjiord at the East Greenland coas  (Photo credit: Wikipedia)

One means of judging when Greenland was last free of ice, or at least substantially so, is based on more than a ice few metres thick being opaque to cosmic ‘rays’. Minerals, such as quartz, in rocks bared at the surface to ultra-high energy, cosmogenic neutrons accumulate short-lived isotopes of beryllium and aluminium – 10Be and 26Al with half-lives of 1.4 and 0.7 Ma. Once rocks are buried beneath ice or sediment, the two isotopes decay away and it is possible to estimate the duration of burial from the proportions of the remaining isotopes. After about 5 Ma the cosmogenic isotopes will have decreased to amounts that cannot be measured. Conversely, if the ice had melted away at any time in the past 5 Ma and then returned it should be possible to estimate the timing and duration of exposure of the surface to cosmic ‘rays’. Two groups of researchers have applied cosmogenic-isotope analysis to Greenland. One group (Schaefer, J.G. et al. 2016. Greenland was nearly ice-free for extended periods during the Pleistocene. Nature, v. 540, p. 252-255) focused on bedrock, currently buried beneath 3 km of ice, that drilling for the ice core finally penetrated. The other systematically analysed the cosmogenic isotope content of mineral grains at different depths in North Atlantic seafloor sediment cores, largely supplied from East Greenland since 7.5 Ma ago (Bierman, P.R. et al. 2016. A persistent and dynamic East Greenland Ice Sheet over the past 7.5 million years Nature, v. 540, p. 256-260). As their titles suggest, the two studies had conflicting results.

The glacigenic sediment grains contained no more than 1 atom of 10Be per gram compared with the 5000 to 6000 in grains deposited and exposed to cosmic rays along the shores of Greenland since the end of the last ice age. These results challenge the possibility of any significant deglaciation and exposure of bedrock in the source of seafloor sediment since the Pliocene.  The bedrock from the base of Greenland’s existing ice cap, however, contains up to 25 times more cosmogenic isotopes. The conclusion in that case is that there must have been a protracted, >280 ka, exposure of the rock surface in what is now the deepest ice cover at 1.1 Ma ago at most. Allowing for the likelihood of some persistent glacial cover in what would have been mountainous areas in an otherwise substantially deglaciated Greenland, the results are consistent with about 90% melting suggested by glaciological modelling.

Clearly, some head scratching is going to be needed to reconcile the two approaches. Ironically, the ocean-floor cores were cut directly offshore of the most likely places where patches of residual ice cap may have remained. Glaciers there would have transported rock debris that had remained masked from cosmic rays until shortly before calved icebergs or the glacial fronts melted and supplied sediment to the North Atlantic floor. If indeed the bulk of Greenland became ice free around a million years ago, under purely natural climatic fluctuations, the 2° C estimate for global warming by 2100 could well result in a 75% glacial melt and about 5-6 m rise in global sea level.

Read more about glaciation here and here.