Development of the theory Flashcards
1
Q
Why was ‘paleoclimate’ not appreciated earlier?
A
- ‘Climate’ was essentially ‘weather’ on human timescales
- No instrumental records of weather
- No ‘proxy’ records of climate
- Evidence for glaciations usually interpreted as deposits from ‘Great Flood’
- ‘Young Earth’ – not enough time for massive climate swings
- This led to widely accepted paradigm which was not questioned
- Logistical difficulties associated with travel in the 1700s and 1800s.
- Continental glaciers were essentially unknown
- World centers of learning only familiar with Alpine glaciers
- Difficult to interpret features of continental glaciation in currently ice-free areas
- Climate is a ~30-year period of average weather conditions
2
Q
Glacial Structures:
A
- General erratics: Boulders transported by ice and then left when glacier recedes. Often of different rock type to local rock.
- Glacial erratics: Too large to be moved by water. Later used as evidence against ‘Great Flood’ theory
- Glacial striations and grooves: rocks entrained in base of ice scratch out parallel grooves in bedrock – show direction of glacier
- Moraines: chaotic rock and sediment left behind by glacier
o Jumbled up rock and soil on side of glacier
o moraines: become less obvious the older they are – vegetation develops, slopes erode
Different types:
o Lateral
o Medial
o Terminal
o Recessional
Roche moutainnees: asymmetrical rock outcrops once covered by ice where one side is smoothed and the other is abrupt
o often have striations on the top surface
3
Q
Two different types of glacier
A
o ‘Cold-based’ glacier: ice at glacier base is frozen and does not move
o ‘Warm-based’ glacier: ice at the base semi-liquid and allows glacier to move
4
Q
History of Ice Ages?
A
- Louis Agassiz was the first to popularise the concept of ‘Ice Ages’ and therefore palaeoclimate.
- Jean de Charpenier convinced him there were more massive Alpine Glaciers
- Agassiz saw erratics, moraines, and glacial grooves everywhere when he returned home
- Eventually he became convinced that the entire Earth had been affected by times of global frigidity and that glaciers extended MUCH further than at present
- Presented his ‘Ice Age’ theory at Natural History Society meeting in Switzerland in 1837 – few believed him at the time
- In 1840 Agassiz published Studies on Glaciers:
- Suggested ice ages had caused extinctions
- Suggested ice had once covered most of Europe, northern North America and northern Asia.
- Eventually found abundant evidence for glaciation in Britain and Ireland.
- Based on rigorous field mapping: most scientists believed that a series of 4 ice ages of uncertain timing and duration (the “Günz,” the “Mindel“, the “Riss,” and the “Würm.“) had occurred
- Knew very little about them
- Evidence for older glaciations was destroyed by younger glaciations (depth of geologic time not appreciated)
- meant that the ‘Great Flood’ theory, and ‘Young Earth’ theories in general, were probably wrong
5
Q
1860s theories on the causes of glaciation?
A
- The Earth’s pole of rotation had shifted, so that now temperate areas were in higher latitudes.
- The Earth traveled through alternating cold and warm regions of space.
- The Sun is a variable star and at times gives out more heat than at other times
- Land mass distribution was different in the past, and glaciations occurred when land was distributed at poles.
6
Q
James Croll and what he proposed
A
- Most credible mechanism is called the “Astronomical Theory of the Ice Ages” proposed by James Croll in 1875
- Croll eventually obtained a position as a caretaker at the Andersonian College (Glasgow):
Orbital forcing - The Earth’s orbit varies from circular to elliptical (with the Sun at one of the two foci) because of interactions with the gravity of other planets and the Sun.
- Croll calculated the variations in the Earth’s orbit, using the location of each planet through time and its influence on the Earth (building on ideas proposed by Joseph Adhemar)
- Determined that there were three features to the Earth’s orbit that could influence climate: eccentricity, obliquity, and precession
7
Q
Eccentricity
A
- change in shape of Earth’s orbit, varies on 100,000-year cycles
- Affects the distance from the Earth to the Sun at different points in the orbit (seasons)
Eccentricity = 0 when perfect circle and no seasons
Eccentricity = 5 when oval and north and south hemispheres have opposite summer months
8
Q
Obliquity
A
- Variability in the tilt of Earth’s axis, varies on 41,000 year cycles
- 22.1 and 24.5 degrees
• Affects how much solar radiation is received by the Earth at different latitudes
• Higher obliquity means more INcoming SOLar RadiATION (insolation) at higher latitudes (during summer).
• Will lead to ablation (melting of glacier)
9
Q
Precession
A
Precession (of the equinoxes): wobble of Earth’s axis, affects timing of N hemisphere Summer, varies on 21,000 year cycles
- Earth wobble between pointing toward North Star and Vega
10
Q
Combining eccentricity and precession?
A
- Eccentricity combined with precession changes timing of solstices
- Affects at what position in the Earth’s orbit the seasons occur
- The Northern Hemisphere winter solstice is currently when the Earth is closest to the Sun.
- In 10,500 years the Northern Hemisphere winter solstice will be when the Earth is furthest from the Sun.
11
Q
How did Croll predict glaciations and what were some problems of his theories?
A
- When North Pole pointing away from Sun coincides with the greatest distance from the Sun, glaciers would grow in N. Hemisphere
- Earth’s climate was influenced by changes in its orbit around the sun
- He hypothesized that ice sheets would grow during severe winters resulting from the interacting effects of precession and eccentricity.
- Importantly, Croll realised that the variations in insolation implied by his calculations were not enough to trigger glaciation.
- Introduced the concept of a “climatic feedback” specifically the Ice-Albedo Feedback.
- Hypothesized that glaciations could change ocean circulation
- Major problem: suggests alternating glaciations between different hemispheres while geological evidence suggests simultaneous glaciation
- Secondary problem: Croll’s theory suggested that the maximum ice accumulation should have occurred around 80,000 years ago. There was no way to date the glaciations directly
- calculations based on the geomorphology suggested a younger age.
12
Q
Milankovitichs, who was he and what was his work?
A
- Serbian mathematician and engineer
- From 1914 to 1920 re-calculated Croll’s work but for different latitudes…and extending back 600,000 years
- Whereas previous scientists postulated that winter T was the key to an ice age Milankovitch realised summer T was more important
- If summer temperatures became low enough, glaciers would not melt, accumulation would outbalance ablation, and the glacier would continue to grow, potentially causing continental glaciation.
- The same colder summer T meant warmer winter T, this meant more evaporation from oceans and more snowfall onto glaciers
o Need moisture in air for snow - Calculated that obliquity was very important, more important than previously appreciated
- Realised that because most of the land is in the Northern Hemisphere, it is the insolation at 65ºN that is most important
- Therefore glaciations were hypothesized to occur simultaneously in both hemispheres.
- Timescale in agreement with geomorphological evidence – so much so that his timescale was used as an absolute chronology for the ice ages
13
Q
What were two problems with Milankovitch’s reworking of Croll’s theory?
A
1) Timing: Once again the timing was not right. The geomorphological work that once backed up the Orbital Theory was proven to be completely wrong (specifically, the timescale for glaciation of Penck and Bruckner based on river terraces)
Carbon dating: A new technique developed by Libby in the late 1940s. 14C dates of sediment left after the last glaciation in N. America showed many glacial advances, not just one at 25,000 year BP as suggested by Milankovitch
2) Orbital variations, even with increased albedo, still were not enough to account for changes.
14
Q
Orbital Forcing Strikes Back! What was measured?
A
- Validation of the Orbital Forcing Theory came from an unlikely source – deep sea sediment cores
- Measurements are generally not taken on whole core, but on individual foraminifera
- Proxies measured include d18O, d13C, and Sr/Ca
- Measurements taken of individual foraminifera
- Forams that live near the surface are called pelagic
- Forams living near the ocean floor are called benthic
- Local water conditions recorded in shell
- Coiling direction records temperature
o Right coiled (dextral) species = warm water conditions
o Left coiled (sinistral) species = cold water conditions - Foraminiferal oxygen isotope composition
o Vast majority of oxygen isotope signature due to the composition of the water
o Isotopic composition of water reflects global ice volume - d18O of forams increases when 16O is increasingly locked in continental ice sheets, i.e, during glaciations
15
Q
Orbital Forcing Strikes Back! What were the findings?
A
that there were three cyclicities present, superimposed on top of each other:
- 100,000 years
- 41,000 years
- 20,000 years
- The close match between the values calculated by Croll and Milankovitch is unlikely to be a coincidence. Most of the remaining skeptics then accepted orbital forcing
