Quaternary: Lectures 14-19 Flashcards
(34 cards)
Outline key characteristics of the Quaternary.
Last 2.6 million years
Divided into Pleistocene and Holocene (11.5k - now)
Characterised by glacial-interglacial cycles (the cyclic growth & decay of huge continental ice sheets)
We are currently in the Holocene, an interglacial
How has ppm changed over time? Why is it significant today?
280ppm before industrialisation
400pm passed in 2015
Projected 1200ppm by 2100AD; not seen in last 1000 or more yrs
Define catastrophism and uniformitarianism/gradualism
Catastrophism = all land features a result of successive, individual catastrophic events
Uniformitarianism = land features a result of gradual, small-scale events
Name key geological academics, what did they say?
- Charles Lyell - “present is the key to past”
- James Hutton - “past is key to future”
- Luis Agassiz - first to theorise processes/cycles of cooling and warming (glacials and interglacials)
- Milankovitch - cycles used to explain mechanism behind waxing and waning ice sheets
What are Milankovitch Cycles?
Orbital forcing on the Earth that create solar variability, altering the Earth’s climate:
- Eccentricity - roundness of orbit (100kyrs, and also 400kyrs!)
- Obliquity - tilt of axis, currently at 23.44 (41kyrs)
- Precession - axial wobble (25kyrs)
Perihelion = closest to Sun
How were the first records for palaeoclimatology taken?
Marine records:
- 1947 seismology, shallower sediments
- 1950s paleomagnetism
- 1960s Deep Sea Drilling
- 1970s John Imbrie analysis of G. bulloides O-isotopic composition in shells
Ice Core records:
- 1820 1st continental ice shelf sighting
- Greenland ice cap not confirmed until late 1800s
- 1950s ice sheet drilling
Describe ice cores - how far back do they take us and what can they tell us?
High resolution, up to 3km long, across both hemispheres…
Antarctic ice sheet - contains oldest ice cores (800,000 yrs, potentially further now to 2 Ma!); 4.5km thick
Greenland ice sheet - only 130,000 yrs; 2.4km thick
Isotopic composition indicates temperatures: 18O-enriched ice core suggests warmer temps
Can be correlated with marine cores: 18O-enriched core suggests cold because less evaporation
What is radiocarbon dating and why can’t it be used all the time? What other proxies are used?
Radiocarbon dating (14C) - radioactive carbon indicate age but loses precision and accuracy after 50k/60k yrs
Speleothems = caves; bands, isotopes indicate climate
Trees (dendroclimatology) = high resolution bands, indicates moisture, climate, temp
Pollen/plants = in peat/lakes, indicator species of temp and climatic conditions
Chironomids = non-biting midges; specific temp, O, pH requirements
Beetles = warmer temp-specific species (Russell Coope)
Forams = O and C composition of shells, indicates water temp
Mammals = dating, marks…
Coral = SST
Testate amobea = peatland water table
Name the 4 continental ice sheets of the Northern Hemisphere Quaternary that would have experienced cyclic growth and degrowth
N. America - Laurentide and Cordilleran
Europe - British and Scandinavian
Russia - Barents and Kara
Greenland
What/when was the last glacial?
Last glacial period Devensian
- Interstadials identified within this period (smaller warm periods within a glacial)
(Not the LGM which was 22ka!)
Describe the British Ice Sheet during the Devensian or LGM.
Covered all of Scotland and Wales (800,000 km2, 1.5km thick)
- Diagonal maximum extent (so didn’t cover places like Bristol, Oxford or Cambridge)
Evidence:
- Terminal moraines
- Contested e.g. small glaciers on Exmoor, Dartmoor, erratics in N. Devon
Describe sea level change in the Quaternary and resulting landforms?
Eustatic (with global ice volume) and Isostatic (localised, ice loading like Britain)
125m lower during LGM = Doggerland, land bridges = plant, animal, human dispersal
- Rapid change, 16-25m rise in just 400-500 yrs
Evidence: wave-cut platforms, raised beach deposits, marine fossils
- Hope’s Nose Peninsula, S. Devon
- Huon Peninsula, Papua New Guinea
What caused the end of the last ice age (last glacial Devensian)?
- Increased summer insolation at higher N latitudes; maximum obliquity and perihelion
- Increased atmospheric CO2 from 190-280ppm; ocean degassing, biomass feedbacks (increased CO2 respiration)
What made the end of the Devensian complex? What evidence do we have of deglaciation?
Not smooth into Holocene e.g. 15ka warming and other interstadials/stadials
- ‘Late glacial’
- Younger Dryas
Biome shifts - expansion of oak woodland, contraction of tundra
Doggerland flooded, submerged 8,200 yrs ago (freshwater pulses)
Animals indicate fertility of the land e.g. mammoth teeth, lions, human tools
What were the two ice sheets either side of Rocky Mountains called?
Laurentide (3km thick, 33 M km3)
Cordilleran (merged with Laurentide in LGM)
What is significant about research on the North Atlantic and Northern Hemisphere?
Dominated research in past - historical bias
Teleconnections - is the NA a driver of global change?
Significant processes within this region - NAO and AMOC
Outline NAO and AMOC in the context of ice ages.
North Atlantic Oscillation:
- Air pressure gradient between subpolar low and subtropical high
- N. Atlantic jet stream strength/position influenced
- Affects temp & precipitation
- Positive (strong gradient, jet stream N) / Neg (weak)
Atlantic Meridional Oceanic Circulation
- N flow of warm, saline surface water keeps Europe warm
- S. flow of colder deep water, transports heat energy from tropics and SH to N. Atlantic
- Causes disruption during deglaciation (freshwater, less deep water formation etc)
What are Dansgaard-Oeschger events and Heinrich events? How many in late Quaternary and what are the potential drivers?
D-O events:
- Abrupt/rapid warming (interstadials)
- Last 500-2000 yrs
- 25 in last glacial
- 5-8C temp rise in just 40 yrs!!??
- More muted in SH
Heinrich:
- Cooling before D-O in some cases
- Cooling every 10k yrs with southward dloating ice discharges
- 6 in late Quaternary
- Transportation of ice-rafted debris into N. Atlantic
Links between them possible…
Internal forcing - NAO changes from freshwater pulse, ice sheet dynamics/changes
External forcing - solar, orbital, periodicity
Outline deglaciation in N. America. What have such discoveries meant for studies into human dispersal and megafauna in N. America?
- Proglacial lakes formed (terminal moraine dams)
- Rapid Cordilleran melt = megafloods (17M m3/sec) in Missoula and Columbia
- Staged catastrophic collapse of Lake Agassiz = 20-40cm global sea-level rise
- Freshwater pulse = NAO disruption?
Movement of humans into N. America?
- N. America 15ka
- 125m lower sea-levels, Alaksa-Russia land bridge
- Somehow ppl moved to N. America; before Cordilleran/Laurentide fusion? - ice-free corridor route vs. pacific coast route hypotheses
Megafauna mass extinction 11ka
- Sabre tooth tiger, Smilodon, Mastadon, Mammathus, Megatherium
- Caused by human hunters or climate changes?
Why is it important to study Asia and the tropcis?
Population: >3 bn ppl
Climate: NAO, SAM, SWWB, ACC, ENSO, Monsoons, ITCZ
No continental ice sheets - elsewhere e.g. NZ, Patagonia, E. Africa, Papua New Guinea
New archives/proxies
Hemispheric differences
Hominid evolution, agriculture
Hurricanes
Biggest portion of world’s wetlands (GHGs)
Importance of the Himalayas?
Indian-Eurasian collision - mountain building
- Si weathering increase –> C sequestration in deep ocean –> reduced atmospheric CO2 –> cooling climate = Quaternary ice age 2.6 Ma
Describe the Asian monsoon and its 3 main components.
Seasonal reversal of wind direction - dramatic precipitation changes
- Driven by strong presure gradients = heating in Asian summer, L to H, bring moist SW Pacific air
- Pressure gradients reverse = H to L pressures, cooling Asia
- East Asian Summer Monsoon (EASM)
- Indian Summer Monsoon (ISM)
- Western North Pacific Summer Monsoon (WNPSM)
What are Loess deposits - what do they show us?
Fine-grained, wind-blown material
- Largest in N. China, Lanzhou
- 10-50m thick deposits; thickest in cold periods, breaks down in warmth so analysis can indicate glacial-interglacials
- Size of grains indicate wind strength = monsoon winds
- Loess and GROP 18-O records over shorter (sub-Milankovitch) timescales; connected to N. Atlantic?
Quaternary loess in discontinuous belt in S. Asia, small amounts in Kent and Cornwall
What new proxies are available from Asia?
Ice cores from Tibetan plateau (3rd pole) = 4 ice caps, 600,000 yrs old
Speleothems from Dongee & Hulu caves = high resolution
- Uranium-Thorium dating
- O isotope proxy for monsoon changes
- Weakening of Asian monsoon 8ka-2ka
