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geo117 ice sheets
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- 1Present Problem 1:Ice sheets are losing mass.
These are measurements of the mass balance of ice sheets.
1Gt = 1,000,000,000,000
(one thousand billion)
Like your bank balance. If it's negative, then you are losing money. If positive, you are gaining money, 0 means spending = income.
If ice sheet mass balance is negative, water stored as ice is being lost to the ocean. If ice sheet mass balance is positive, more water is being stored in the ice sheet than is melting. - 2Present Problem 2:Ice sheets are losing mass... at an accelerated rate
Mass balance is becoming more and more negative each year
West Antarctica is losing a lot of mass, however getting slightly bigger in East Antarctica, however west is losing more than east is gaining (overall losing). - 3Present Problem 3:Ice sheet mass loss leads to sea level rise
All the ice that is lost from the ice sheets goes into the ocean, causing sea level to rise - 4Present Problem 4:We still don't fully understand ice sheets and how they work...
How thick is the ice?
How do icebergs form?
What role does the ocean play?
How does water flow beneath ice?
How do they flow over bumps?
What's beneath an ice sheet? - 5Present Problem 5:We therefore don't know how much ice is going to be lost in the future - hard to make predictions
Because we're unsure how ice sheets work, it's hard to make predictions on what they are going to do - 6Present What is an ice sheet?An ice sheet is a large block of ice
Ice sheet is a large mass of ice, so large flow in the interior is independent of underlying bed topography (mountains)
A mass of ice larger then 50,000 km squared - 7Present Where are the ice sheets? How many ice sheets are there today?Ice sheets exist today in Antarctica and Greenland
There are 3 ice sheets in the world today:
1) Greenland ice sheet
2) West Antarctic ice sheet
3) East Antarctic ice sheet - 8Present Why study ice sheets?Make conditions on Earth conducive to life - reflect sun away (regulate global temperatures)
Dynamic parts of a global system
Rapidly changing right now
If melted completely, global mean sea level would rise by 65 m on average (cannot be sure of exact rise) and have already contributed to sea level rise
Have had an influence on recent Earth history (2.6 million years and more) - 9Present Ice on the move - slowlyIce flows away from the centre of an ice sheet toward the ocean (1km/yr)
Ice in the interior of an ice sheet moves slowly (1m/yr to 10m/yr)
The middle of an ice sheet is called an ice divide
All ice flows from here, but these regions flow slowly - 10Present Ice on the move - slowlyMore snow falls in the interior of the ice sheet than the margins
Accumulated snow compacts over 100s of years turning into ice
If ice didn't flow, snow and ice would build up and up and up...
Also, more ice melts further down the ice sheet than is replenished by snowfall
So if ice didn't flow into this area, there would be no ice
The pressure pushes out air bubbles in the snow and compresses the snow which is why ice sheets are blue and not white because there's no air in it (this process can take 100s of years)
The weight of the snow and ice pushes the surrounding ice downhill
Ice flows due to gravity pulling ice downhill
Ice therefore flows from areas where there is a lot of accumulated snowfall, into areas where there is less snowfall - 11Present Slowly - a bit like syrup flow...The weight of the syrup pushes the surrounding syrup outward
Gravity pulls the syrup toward the centre of the Earth, but it doesn't get there because a pancake is in the way. It therefore has to go sideways, across the pancakes
Ice is much more viscous (stickier) than syrup and therefore flows slower - 12Present Slowly - Glen's Flow LawSlow ice flow can be expressed as an equation
This is known as Glen's Flow Law
John Glen (1952) took some ice from a glacier, and pushed it in the lab to see what would happen
Equation:
How fast the ice is flowing =
Temperature dependent constant (value that changes with temperature) Cold ice flows slower than warm ice. Really warm ice becomes water, flows much faster!
How hot is the ice?
x
How much pressure is being put on the base of the ice at any one time?
(Basal shear stress)
->
To the power of: Constant (90% of the time n = 3)
---------------------------
Basal shear stress (pressure at the bottom of the bed: how hard is the ice pushing downwards?)
=
Ice Density (ice is less dense than water, and therefore floats: how heavy is ice for a given unit of volume?)
x
Gravity (ice is pulled down by gravity)
x
Thickness (thicker ice pushes more onto the base of the ice)
x
Surface slope (thicker ice will push away thinner ice downhill) - 13Present Ice on the move - quickly Ice streamsIce streams (between the centre and the border of the sheet) can move between hundreds to thousands of meters per year (up to 2km p/yr)
Regions of fast flowing ice- likened to the arteries of ice sheets. 90% of ice discharge from an ice sheet goes through an ice stream - export most of the ice.
PROBLEM: Ice streams are flat and thin (can't apply Glen's Flow Law anymore)
Flat = low surface slope = less driving stress
Thin = low driving stress
Two processes that must occur to contribute to velocity of the ice stream:
1) slippery bed: glacial sliding e.g. water between the bed and the ice sheet meaning the ice sheet is detached
2) sediment deformation: (similar to pizza dough) ice sheets don't just sit on bedrock, tend to sit on sediment called TILL, when till is full of WATER and pressure from overlying ice, it is squidgy and can moves. This movement can push an ice stream along. - 14Present Ice on the move - quickly Ice shelvesIce shelves float into the ocean. No friction here from the bed.
Ice flows across the grounding line (point where the ice starts to float).
Ice shelves stop slow ice that is further upstream - buttressing - like a traffic jam, the car behind cannot pass until the one in front goes.
Ice cannot flow into the ocean as rapidly, because the ice shelf holds it back.
If the ice shelf doesn't move, the ice behind it (at the grounding line) cannot move.
BUT ice shelves are prone to rapid breakup.
Larson B collapsed in 2002 in just 6 weeks (size of Lancashire). Glaciers behind sped up as a result causing Antarcticas contribution to sea level rise. - 15Present Water: On top - Supraglacial waterSupraglacial lakes form in similar positions each year on the Western Greenland ice sheet. Can drain in a couple of hours where the water falls into the ice sheet.
Distribution of water on the surface of Antarctica. Previously, people didn't think there was much water. Water on the surface of Shackleton Glacier - found at 1200 meters above sea level.
So much water on top of Antarctic ice sheet that it falls off the ice shelf into massive waterfalls. - 16Present Water: Inside - Englacial waterWater rapidly drains to the bed of an ice sheet through englacial drainage through moulins on top of the ice sheet.
We don't know much about the water here.
How does the drainage system evolve?
How rapidly does water get to the bed?
Can water be stored englacially? - 17Present Water: Underneath - Subglacial waterWater gets to the bed through englacial drainage through moulins.
Zwally et al., 2002 showed that water can speed up the ice once the water reaches the bed - water draining to the bed of an ice sheet can speed it up - lubricating the bed. - 18Present Pressure MeltingWater at base of ice often at pressure melting point pushing down from above, means that colder ice can melt - pushes all the molecules in ice together so that it is easier to form water.
Ice is colder near the bed however because there is so much pressure this causes the ice to melt - takes less energy for ice to turn into water.
The pressure causes the melting point of ice to go down (i.e. can form water at -2 degrees) - 19Present Subglacial lakesWater is stored in subglacial lakes.
Pressure of ice can cause water beneath an ice sheet to flow uphill.
Flows in subglaical cavities and is drained by subglacial channels.
Lake Vostok (160 miles long) largest subglacial lake under 2km of ice in Antarctica. - 20Present Implications of water on top of Ice SheetsWater is bad news for ice shelfs...
Water gets into crevasses (causing them to expand) and breaks up ice shelfs, potentially leading to ice shelf collapse (e.g. lots of lakes formed on top of the ice sheet before Larson B collapsed)
Water could lead to rapid collapse of ice shelf causing glaciers behind it to speed up leading to sea level rise. - 21Past Geologic Time ScaleIce Ages have occurred 5 times in Earths history
2.7 billion years ago was the first ice age
2.5 million years ago last recent ice age
Technically, earths climate at the moment is in an ice age - 22Past What is an ice age?A period of time over which Earth's temperature reduces such that continental and polar ice sheets expand
This temperature is recorded in sediments on the sea floor
Cooling that started around 3 million years ago lead to the ice age climate we are in today - 23Past What is the Quaternary?The most recent ice age starting around 2.58 million years ago. We are in the Quaternary period now - this is the time that continental ice sheets started appearing in the Northern Hemisphere.
Divided into the Pleistocene (2.6 to 0.01 million years ago.
Last 10,000 years is called the Holocene - age when man influenced climate.
Currently, continental ice sheets are much reduced.
An ice age is different to a glaciation...
In an ice age you will have a fluctuation in temperature which causes the ice sheets to expand and shrink, this expansion and shrinking of the ice sheet is called a glaciation.
Information on past climate is stored in ice sheets - gasses/air trapped when snow falls (as it is compacted into ice through pressure).
Most recent glacial cycle - 120,000 years ago to now.
A glacial period - a long cold period during which ice sheets expand.
Interglacial - warm period (now) during which some ice sheets are not present.
Stadial - a cold period within a glacial.
Interstadial - a warm period within a glacial - 24Past The Last Glacial Cycle30,000 years ago = last glacial maximum;
Called the Laurentide ice sheet:
- Covered nearly all of Canada
- Flowed down to USA (New York state)
- Drained by several ice streams
Cordilleran ice sheet - out to Alaska
Innuitian ice sheet - Canada to Greenland
Greenland ice sheet and Icelandic ice sheet also bigger at the time
Antarctic ice sheet expanded.
Not a lot of data (all underwater today), but most people agree grounded ice out to continental shelf break.
Patagonian ice sheet:
- Covered the Southern Andes (Argentina and Peru) an ice sheet
- Drained by Fjords on the Western side
- Drained by lobes on the Eastern side
Palaeo ice sheets of Eurasia:
- Barents sea ice sheet
- Fennoscandian ice sheet
- British-Irish ice sheet
Extra ice on land in ice sheets
Lowered sea level by around 120 to 140 m at the last glacial maximum - 25Past The importance of Palaeo-ice sheets- Shaped the landscape we see today
- Important analogues for what might happen in the future
- Enable us to calibrate ice sheet models; longer record than can be directly observed
- We can get to the bottom of these ice sheets (important regarding sliding, till deformation and other processes) - 26Past Palaeo-ice sheet ReconstructionIce sheets leave behind a legacy on the landscape in the form of glacial landforms.
These landforms can be used to reconstruct past ice sheet extent, geometry and flow conditions.
For Britain, we have very detailed information regarding glacial landforms. - 27Britice TerminologyMoraine: An accumulation of sediment with a topographic expression recording an ice margin position. Record former margin positions of an ice sheet. Tell you where the end of the ice sheet was (but don't tell you when the ice sheet stopped)
(How to find) Lake Extent: The extent of proglacial lakes inferred from combination of lake deposit distribution and topography.
Subglacial Rib Area: Ridges formed at the base of the ice sheet, transverse to flow direction. Previously known as ribbed or Rogen moraine, the word moraine being misleading as they are known not to record ice marginal positions.
Erratic (pathways): Erratics are rocks or clasts of a different type than the surrounding native bedrock, presumed to have been transported by the flow of ice.
Subglacial Lineations: Streamlined landforms primarily composed of drift, formed at the sole of the ice sheet, oriented in the direction of ice flow. - 28Past Palaeo-ice sheet reconstruction cont.Subglacial bedforms:
Landforms created at the ice-bed interface from subglacial sediments
Drumlins:
Ribbed moraines
Mega-scale glacial lineations
Streamlined subglacial hills which point in the direction of ice flow
The formation of drumlins (and other subglacial bedforms) has been debated for over 100 years
Drumlins can be grouped into flowsets - drumlins that are thought to record the same phase of ice flow.
Flowsets can cross cut: this mean that some drumlins are printed on top of each other. From this we can work out the sequence of ice flow
Mega-scale glacial lineations:
Really long drumlins.
Typically 1-2km (as opposed to normal drumlins 600m) and reported up to 180 km long.
Again, indicate former ice flow direction.
Indicate the presence of palaeo-ice streams. Great length thought to indicate the presence of fast ice flow. - 29Past Palaeo-ice sheet reconstruction cont.Subglacial ribs:
Transverse to flow direction.
Thought to indicate former slow ice flow conditions.
Can also be grouped into flowsets.
Subglacial bedforms:
May be genetically related.
All formed by the movement of sediment, organised by the ice sheet.
Different "types" determined by ice velocity.
Meltwater channels:
Can be subglacial (beneath) or lateral (sides).
Record water passage and former ice sheet location.
Lateral channels - might have an ice sheet below mountains. If it rains on the mountains it can't go into the ice so it forms a channel into the ice instead. Record the former elevation of an ice surface.
Subglacial channels - we can work out the pressure gradient beneath the ice sheet (which way it was pushing the water) and this water carves channels in the ice sheet.
Eskers: subglacial channels infilled with sediment. Record the former passage of water beneath an ice sheet. - 30Past Why we need dating techniquesMoraines record former margin positions of an ice sheet. They tell you where the end of an ice sheet was... but not when.
In order to understand the timing of ice sheet retreat - need dating techniques.
Methods for deriving the timing of sediment/rock deposition involve sampling in the field and lab measurements. - 31Past Dating techniques1) Optically stimulated luminescence (OSL) dating - deposition of sand, last time it saw light.
2) Cosmogenic nuclide dating - last time a rock was eroded.
3) Radiocarbon dating - death of an animal (typically a shell). - 32Past The last British-Irish Ice sheetThe last British-Irish ice sheet is one of the most studied palaeo-ice sheets. Over 100 years of study on it.
Louis Agassiz first to notice glaciation of Britain in 1840s, Darwin also had interest.
Along the north-west margin, the BIIS reached the continental shelf break and was marine terminating.
Drained by several ice streams, the best constrained is the Minch ice stream (about 27,000 years ago)
Western extent onto Porcupine Bank.
Series of Moraines/grounding line features found on Porcupine Bank.
Ice flow through 320m deep Slyne trough.
Interpreted to be either terminal moraine and/or re-grounded ice shelf (ice rise/rumple)
Confluence of BIIS and Fennoscandian ice lead to saddle collapse over North Sea.
Separation of the two caused a surge along the east coast of Britain.
All of this happened in about 500 years.
BIIS also dammed water & several proglacial lakes - changing the direction/shape of the lake e.g. Proglacial lake Pickering - 33Future A Warming WorldHumans have started an experiment with the climate system - 'lets see what happens if we pump the atmosphere with CO2'
Levels of CO2 are larger than they have been for 650,000 years and changing at a much faster rate than naturally occurs
Burning fossil fuels releases CO2 (which traps heat in the atmosphere by absorbing outgoing radiation. More CO2 means more heat trapped in atmosphere. All the heat that is omitted from the Earth is absorbed by the CO2 in the atmosphere (warmer atmosphere = warmer oceans). Some heat gets absorbed by oceans, heating up the surface waters and changing ocean circulation patterns. - 34Future Predicting ClimateThe Intergovernmental Panel on Climate Change (IPCC) makes predictions as to how much the climate may change in the future.
Several different groups of scientists with computer models to try and understand what might happen in the future.
Biggest range of uncertainty - not sure how much fossil fuels humans will burn. The IPCC develops potential different scenarios for this called RCPS. - 35Future Ice Sheets in a Warmer AtmospherePerhaps most obvious effect of warmer atmosphere is more ice sheet melt. This is perhaps already happening (extreme melt events) e.g. extreme melt in 2012 across the Greenland Ice Sheet - at one point this year nearly all of the surface of Greenland had some melting.
More melting = more supraglacial lakes (which can rapidly drain to the bottom of the ice sheet and accelerate the ice sheet flow).
This is perhaps already happening i.e. 2012 melt on Greenland ice sheet led to "extraordinary" run off discharge - destroyed a bridge.
Bendixen et al (2017) studied how this discharge collects sediments and dumps them in the ocean = bigger deltas. Changes the geomorphology (of Greenland). - 36Future Ice Shelves in a Warmer AtmosphereSurface melt also bad news for ice shelves.
Remember that these provide a buttressing force, holding back ice flow:
a) Water gets onto the ice shelf, and weighs it down in a pond.
b) When it drains, the ice shelf rebounds (lifts back up again) due to floating, cracking some ice.
c) Repeated action of this leads to crevassing and ice shelf weakening.
- This process is called HYDROFRACTURING.
This has already happened (Larson A and B) - both had meltwater ponds on their surfaces just before collapse. - 37Future Ice Sheets with Warmer Oceans (Greenland)Greenland somewhat shielded from warm waters because of fjords.
Fjords have their own circulation pattern, shielding them from open ocean.
But warm water is coming in, changing circulation pattern, and warm up the ocean right at the face of the ice sheet. - 38Future Ice Sheets with Warmer Oceans AntarcticaOceans getting warmer is particularly a problem for (West) Antarctica which actually sits below sea level. Large area of West Antarctica is in contact with the ocean, and therefore is prone to ocean warming.
Pine islands glacier and Thwaites glaciers - two of the largest ice streams in West Antarctica.
Warm currents are eating away at their ice shelves - nearby Dotson Ice Shelf has a subglacial channel cut out by the ocean.
Much of the Antarctic ice sheet is below sea level. Loss of ice shelf means that there is loss of buttressing. This means that the grounding line (the point where ice starts to float) retreats.
Ice flow is highly sensitive to ice thickness at grounding line.
If grounding line retreats, where ice is flowing uphill, ice thickness at grounding line increases.
This causes the ice sheet to further accelerate, and thin further.
This leads to a positive feedback of grounding line retreat.
Marine ice sheet instability:
ice velocity at the grounding line = the ice temperature x how much ice is passing the grounding line to the power of 4.75 - 39Future Modelled Ice SheetsTo predict future ice loss, we need a model.
Model is a representation of all the known physics of ice sheets in order to determine what's going to happen if we change the climate.
Put in likely climate scenarios, see how ice sheet respond.
Ritz et al (2015) ran a model and predicts there is likely to be 20cm of ice sheet contribution to sea level rise from Antarctica by 2200 - however predictions vary widely e.g. DeConto and Pollard (2016) said possibly 5m instead of Ritz 20cm.
But models disagree - they are improving all the time (as we learn more) i.e. new physics, new ice sheet conditions, new modelling techniques. Biggest problem is, we don't know what will happen with climate in the future.
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- Geography Fichas sobre geo117 ice sheets, creado por Emilia Verney el 10/01/2018.
- Creado por
- Emilia Verney · ene 2018
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- 6 abr 2018
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