Discovering the Ice Age - University of Iceland Geology pdf/Fyrirlestur 12... · Once widespread acceptance of the concept of ice ages was in place, the stage was set for a remarkable

Discovering the Ice Age

Jarðsaga 2

Ólafur Ingólfsson

Háskóla Íslands

Defining the QuaternaryThe Quaternary Period is a subdivision of geolo-gical time which covers the last 1.8 MY, up to thepresent day. The Quaternary can be subdivided into two epochs; the Pleistocene (1.8 MY-10 ka) and the Holocene (10ka-present)

The Quaternary Period has been one of extraordinary changes in global environ-ments as well as the period during which much of the evolution of Man took place.

The PleistoceneThe term Pleistocene ("most recent") was coined by Lyellin 1839, on the basis of a section of type strata in easternSicily, according to the proportion of extinct to livingspecies of mollusk shells in the sediment. Strata with 90 to 100% present day species were designated Pleisto-cene. Clearly this is a somewhat arbitrary arrangement, and in any cases many strata do not contain mollusk shells.

• The present definition of the Pleistocene is based onradiometric dating of 1.8 MY or more recent, thepresence of cooler water mollusks and foraminifers, and on land the fossil remains of modern horses and true elephants (in the past more widespread than they are today).

A Pleistocene journey towardsever harscher environments

Conspicuous trends through the Quaternary:

• Ice volumes increase and global sea levels fall

•The amplitude of change during interglacial-glacial cycles increases. Duration of Glacial Cycles increases.

• Interglacials are very brief periods in an overall cool-cold climate

• 2,6-0,8 MY ago: Glacial cycles ~ 40,000 years. Periods of glacials and interglacials of similar duration

• After 0,8 MY ago: Glacial cycles ~ 100,000 years. Glaciation periods increase in duration, interglacial periods remain brief periods in an overall cool-cold climate.

The Ice Age conceptSince the middle of thel9th century geologistsrealised that during themost recent period of geological time ("The Ice-Age") there had been large changes in the landscape and the environment.

It was widely accepted that the action of glaciers had pro-foundly altered the surface of the earth over wide areas, including most of northern Europe and North America.

Glacial erratics – a key to the Ice Age theory

Erratics are onevery obvious signature of former glaciations: Scandinavian rockson NW Europeansediments; Canadian rocks on the great plains of USA; Alpine rocks on European lowlands, etc

The track of a glacier is as unmistakable as that of a man or bear.John Strong Newberry (1870).

Theories cooked up to explain erratics

Few early naturalists had experience with modern glaciers. Early theories to explain large erratic blocks include:• 1815 - “Rollstein” or mudflow theory (von Buch, von Humboldt, Sefström)• 1823 - Diluvium theory (Buckland)• 1833 - Drift theory (Lyell, Darwin)

Discovery of the Ice Age• 1787 - Kuhn, a Swiss minister, attributes erratic boulders near Grindelwald to glacier contraction.• 1795 - Hutton, English geologist, publishes Theory of the Earth, where he describes how ice transported granite erratics to the Jura Mountains.• 1795 – Sveinn Pálsson’s “Book on Glaciers” recognizes more extensive glaciations in the past (not published until 1945).• 1815 - Perraudin, a Swiss mountaineer and hunter, argues that glaciershad at one time extended well into the Val de Bagnes in the Alps.• 1823 - Goethe, a German poet, takes note of findings by scientists oferratics on the North German Plains and publishes the novel WilhelmMeister in which he promotes the idea of “Eiszeit” or ice age.• 1824 - Esmark, a Norwegian naturalist, argues that the glaciers of Norway had once been more extensive.• 1829 - Venetz, a Swiss engineer, argues that glaciers once covered the Jura and Swiss plain.• 1837 - Agassiz, a Swiss zoologist and president of the Swiss Society of Natural Sciences, presents a lecture explaining the origin of erratics by glacier transport.

Louis Agassiz (1807-1873)

• Observing the glaciers of his native Switzerland, Agassiz noticed the marks that glaciers left on the Earth: great valleys; large glacial erratic boulders; scratches and smoothing of rocks; moraines pushed up by glacial advances.

• He realized that in many places signs of glaciation could be seen where no glaciers existed. Previous scientists had explained these features as made by icebergs or floods; Agassiz integrated all these facts to formulate his theory that a great Ice Age had once gripped the Earth, and published his theory in Étude sur les glaciers (1840). Later book, Système glaciare (1847), presented further evidence for his theory, gathered all over Europe.

• Like many of the 19th century naturalists, Agassiz was educated in the medical tradition and qualified as a physician. Received the Ph.D. in the spring of 1829 from the Univ. of Erlangen.

• In 1832 he accepted a position as professor of natural history at the Lyceum of Neuchâtel in Switzerland, where he developed his ideas on continental glaciation and earth history.

• In January 1848, Agassiz took a professorship in zoology and geology at Harvard College.

Agassiz always regarded himself as a zoologist, rather than physicistor geologist. Founded the Museum of Comparative Zoology at Harvard University. http://www.mcz.harvard.edu/Departments/Fish/

“I have devoted my whole life to the study of Nature, and yet a singlesentence may express all that I have done. I have shown that there is a correspondence between the succession of Fishes in geological times and the different stages of their growth in the egg...” (1869). Interestingly enough, Agassiz was a creationist and opposed Darwins ideas...!

European scientists debated theGlacial Theory for 25 years!

• The Swedish pioneer of polarresearch, Otto Torell (1828-1900) visited Iceland and Svalbard in the mid-19th Century to study glaciers and glacial deposits.

• In 1875 he convinced theGermans that bits and pieces of Scandinavia had been scattered over continental NW Europe by the Scandinacian Inland Ice.

A church built of Scandinaviangranites – brought in by the ice

3) Siljan-Granit; 4)Växjö-Granit; 5 & 11) Småland-Granit; 12) Åland-Granit

The 12th Century church StMarcus, in Marx, southwest of Vilhelmshafen, Friesland, is built of assorted granite blocks. These were brought to the area by glaciers advancing across the Baltic Sea. http://www.nihk.de/index.php?id=134

The glacial fingerprintingA. Morphological data:

1. Landscapes shaped by glacial action

2. Glacial striae on bedrock and boulders, glacial drift on surface

3. Terminal moraines and sandur areas

4. Isostatic fingerprinting: Distributionand altitude of raised beaches

The glacial fingerprinting...

B. Stratigraphical data:

1. Lithostratigraphical data: tills, glaciofluvium, glaciomarine sediments.

2. Biostratigraphical data: fossil shells, pollen, macrofossils

3. Seismostratigraphical data from the shelf areas

Glacial erratics

Glacial drift and striated surfaces

Glacial landscapes

Valleys and fjords shaped by glacial action

U-shaped valleys

Horns and arrêtes

Raised beaches and isostasyEvidence of Isostatic Rebound:In areas formerly covered by icesheets (around the Baltic Seaand Hudson Bay, for example), sea cliffs and beach ridges arenow found nearly 300 m abovesea level! 14C ages on marineshells and driftwood show that these features are postglacial (less than 14,000 years old). They were formed at sea level and, even though eustatic sea level has risen, they have risen far more from isostasy.

OngoingGlacioisostatic

Recovery:An example of reboundcan be found in Scandi-navia. The northern Baltic Sea is rising nearly 1 cm/ year, or 1 m century.

Isostatic recovery on Iceland very fast...

A huge glaciation in the not too distant past...

Enormous discussion as to the nature of an Ice Age

Monoglacialists vs multiglacialistsOnce widespread acceptance of the concept of ice ages was in place, the stage was set for a remarkable discovery: the Earth has been subject to numerous ice ages over the course of its existence. Geikie theorized five interglacial periods had occurred in Britain. Penck and Bruckner, in 1909, noted remnants of four sets of river terraces in the outwash gravels in the northern foothill valleys of the Alps.

Monoglacialists accepted that there had been an Ice Age, but did not accept that the Ice Age had consisted of a number of glaciations, seperated by ice free interglacials.

A very bitter dispute in IcelandHelgi Pjetursson (1872-1949)was a pioneer in developing thetheory of Ice Ages. In his doktoral thesis from 1905 he proposed that Iceland had been subjected to more than one major glaciation.

The leading Icelandic naturalist at the time, ThorvaldurThoroddsen (1855-1921), was a staunch monoglacialist and he basically went out of his way to destroy Helgi Pétursson’s reputation. Despite being the first Icelandic scientist ever to write a doctoral thesis in geology, Helgi Pjetursson never got a job doing research in Iceland.

Fingerprinting of older Ice Ages discovered...

Following establishment of an ice age in Europe and N America, geologists looked for evidence of ice ages in older rocks.

1856 - Permian glacial sediments described from India1859 - Permian glacial sediments located in Australia1868 - Permian glacial sediments (Dwykka tillite) found in South Africa1891 - Precambrian glacial sediments described from Scotland and Norway

Stratigraphy suggestedmore than one glaciationDistinct zonation in terminal morainessuggested repeated glaciations. This was confirmed by till stratigraphical studies.

Glacial stratigraphy: repeated glaciations

Classicaldivision of

the Quaternary

From: Lowe & Walker1998: ReconstructingQuaternary Environments. Harlow, Longman.

Once glaciations were accepted, their cause was up for discussion

In the 1870s the Scottishscientist James Croll wasfirst to come up with a comprehensive theoryexplaining glacial cycles. His theory was based on the amount and distribut-ion of energy received by Earth from the sun.

Crolls theory...• While the total amount of insolation received at a given latitude did not vary from year to year, theamount received in a given season for a given latitude could vary significantly from year to year because of changes in the earth's orbit.

• These seasonal variations were caused by twoorbital phenomena known as precession(“möndulvelta”) and eccentricity (“hjámiðja”) .

• While the initial climatic effect of changes in theearth's orbit might be rather small, but that thesechanges were amplified significantly by climatic feedback mechanisms in the earth's climate system.

http://www.ngdc.noaa.gov/paleo/slides/slideset/11/index.html

Orbital changes

Today’s situationViewed in the present, the tiltedearth revolves around the sun onan elliptical path. The orientationof the axis remains fixed in space, producing changes in the distri-bution of solar radiation over the course of the year.

These changes in the pattern of radiation reaching earth's surface cause the succession of the seasons. The warmweather of summer comes to the northern hemisphere, for instance, because during these months the northern hemisphere is tilted towards the sun (at the same time, the southern hemisphere experiences winter because it is tilted away from the sun).

Precession (“möndulvelta”)

Precession

It takes 19.000-23.000 years tocomplete one precession cycle

Like a spinning top, the earth's orbit wobbles so that overthe course of a precessional cycle, the North Pole traces a circle in space. The position of the equinoxes (“jafndægur”) and solstices (“sólhvörf”) shifts slowly around the earth's elliptical orbit. Precession changes the date at which the earth reaches its perihelion (“sólnánd”) serving to amplify or dampen seasonal climatic variability.

Effects of Precession through timeThe earth currently reaches its perihelion on January 3, close to the Northern Hemisphere's winter solstice. Thistiming of the perihelion and Northern Hemisphere's winter solstice reduces seasonal differences in insolationin the Northern Hemisphere because the hemisphere iscloser to the sun in winter and hence relatively warmer. On the other hand, the earth is further away from the sun and relatively cooler during the Northern Hemisphere's summer, reaching its aphelion on July 5. However, 11,000 years ago, the reverse was true: the earth reached its perihelion during the northern summer, increasing the seasonal variability of earth's climate.

Eccentricity“hjámiðja”

The shape of the earth's orbitvaries from nearly circular. Thesevariations occur at a frequenciesof 100,000 years and 400,000years. Variations in orbitaleccentricity have a small impacton the total amount of radiationreceived at the top of earth's atmosphere (on the order of 0.1%), but the eccentricity cycle modulates the amplitude of the precession cycle. During periods of high eccentricity (a more elliptical orbit), the effect of precession on the seasonal cycle is strong.

MilankovitchCroll's arguments provoked a great deal of debate and research. In the 1910’s MilutinMilankovitch began a series of calculationsthat would eventually revive the orbital theory of climate change. Milankovitch's main contributions were threefold:

1) He used new astronomical calculations that took intoaccount a 3rd cyclical variation in the earth's orbit: tilt (obliquity – “möndulhalli”).2) He reasoned that summer rather than winter tempera-tures were the main contributors of ice sheet oscillations.3) He calculated summer radiation curves for the keylatitudes of 55, 60, and 65 degrees N that correlated well with evidence then available from the geologic record.

Axial tilt (“möndulhalli”)

Effects of changesin the axial tilt

Earth's axial tilt varies from 24.5° to 22.1° over thecourse of a 41,000-year cycle. Changes in axial tiltaffect the distribution of solar radiation received at the earth's surface. When the angle of tilt is low, polarregions receive less insolation. When the tilt is greater, the polar regions receive more insolation during the course of a year. Like precession (“möndulvelta”) and eccentricity (“hjámiðja”), changes in tilt thus influence the relative strength of the seasons, but the effects of the tilt cycle are particularly pronounced in the high latitudes where the great ice ages began.

The combined effect...

http://www.rt.is/ahb/sol/mbl.html

Insolation curve for 65°N

Marine evidencefor numerous

glaciations

Marine geological studies onsediment cores provide a much more continuous recordof glaciation than terrestrialdeposits do. The reason for this being that subsequent waves of glaciation often erased or altered traces of earlier glacial and interglacial periods.

Evidence from sediment coresSediment cores from the ocean floorcontain information on fluctuationsof global climate. Important infor-mation comes from the microscopic shells of forams:

• Different species of forams prefer different oceantemperature and nutrient conditions. Much about theclimatic conditions of a core site in the past can be learned by by looking at which species once inhabited the area.

•The shells of forams lock in the oxygen and carbon isotop-ic composition of the waters in which they formed. Becausepast periods of glaciation changed the relative quantities of 18O and 16O, the isotopic composition of foram shells as a proxy signal for past changes in global ice volume.

Climate forcing on glacial-interglacial timescales

This figure summarizes ourcurrent understanding of theclimate forcing, and the climateresponse that we observe in thegeologic record on glacial –interglacial timescales. The toppanel is June insolation at 65°N, in watts/m2.The three lower panels are all geologic records of glacial-interglacial change. δ18O in foraminifer skeletons is affected by both temperature and the amount of 16O locked away in ice sheets.

Insolation and global ice volume fluctuations

When the data are filtered for solar insolation and global ice volume over thepast 400,000 years, we see that insolation and global ice volume fluctuated at the same major frequencies: the precession (“möndulvelta”) cycle of 23,000 years and 19,000 years, the tilt (“möndulhalli”) cycle of 41,000 years, and the eccentricity (“hjámiðja”) cycle of 100,000 years

How many Pliocene-Quaternary glaciation cycles?

Pherhaps >30!

Changingorbital

dominance

The relative importance of thethree orbital cycles – eccentric-ity (“hjámiðja”), obliquity(“möndulhalli”), and precession(“möndulvelta”) - has not beenconstant in the past. The 18O curves from a pelagic sedimentcore spanning the last 2.6 MY show that the 41 ka obliquity cycle dominates the early part, with the result that glacials and interglacial were of roughly equal length. For the last 800 ka, the 100 ka eccentricity cycle has dominated, with glacials 5-10 times longer than interglacials. The reason for that is not clear.

The orbital factors too small tomatter in a hothouse world...

The amount of heat received at higher lati-tudes is less than that at lower latitudesfor three reasons. 1) a ray of solar radiationthat strikes Earth at a high latitude isspread over more area than an equal raythat is perpendicular to Earth's surface at a lower latitude, 2) the high-latitude ray also passes through a greater thickness of atmoshpere, and 3) more of the ray's energy is reflected due to the low angle at which it strikes Earth's surface.

...but of crucial importance in an ice-house world where global tectonics restrict global distribution of energy

Brief summary• Ideas on a past Ice Age started developing in the late 18th Century.

• Agassiz theory of Ice Ages (1840) was based on empirical data, but did not become generally accepted until the 1870’s.

• It was soon realized that Earth had been subjected to a number of major glaciations, and that the last Ice Age had consisted of a number of glacial and interglacial periods.

• Croll and later Milankovitch described the orbital parameters that cause periodic fluctuations in nergu from the sun.

• Geological data (deep sea sediment core data, ice core data etc) strongly support the periodicity suggested by the Milankovitch theory.

• Feedback effects in Earth’s complex atmospheric and oceanic systems modify insolation changes.

References used for this lectureStanley: Earth System History. Arnold, London

http://www.ormstunga.is/islenska/itarefni/jon_eyth_um_thoroddsen.htm

Lowe & Walker 1998: Reconstructing Quaternary Environments. Harlow, Longman.

http://www.emporia.edu/earthsci/student/sedlacek1/website.htm

http://www.palaeos.com/Cenozoic/Quaternary.htm

http://www.ucmp.berkeley.edu/index.html

http://www.ngdc.noaa.gov/paleo/slides/slideset/11/index.html

http://www.rt.is/ahb/sol/mbl.html

http://www.geo.oregonstate.edu/people/faculty/clark_publications/clarketal.-science1999.pdf

http://www.mcz.harvard.edu/Departments/Fish/

http://www.oulu.fi/~spaceweb/textbook/crays.html

http://pubs.giss.nasa.gov/docs/2001/2001_ShindellSchmidtM1.pdf

http://www.ucmp.berkeley.edu/history/agassiz.html

http://www.owlnet.rice.edu/~echollet/introduc.html