history of our planet week 8

ice age

Glacial-interglacial cycle:

Evidence of the quaternary glacial-interglacial is strong

Characterised by the waxing and waning of vast, continental ice sheets

Quaternary stratigraphy:

‘stratigraphy’ – process of assigning order to geological, climatic, and environmental events

Researchers have identified series of ice ages based of glacial geomorphology

Different names in different parts of the world

Marine isotope stages (MIS) – a universal ‘quaternary timeline’ based on oxygen isotope records from deep sea sediments

A note on terminology:

Glacial and interglacial periods have different names across the world

E.g.:

  Current interglacial (MIS 1): Holocene (EU/US); Flandrian (UK)

Last glacial (MIS 3): Devensian (UK); weichselian (EU); wisonsinan (US)

  Last interglacial (MIS 5e): ipswichian (uk); eemian (EU); Sangamonian (us)

  ‘best analogue’ interglacial (MIS 11): hoxnian (UK); holsteinian (EU);  pre-illonian (US)

Quaternary ice sheets: 1

Europe (british, scandanavian)

North America (Laurentide, cordilleran)

Russia (Barents, kara)

Greenland

Increased global ice volume

Lower sea levels

Quanternary ice sheets: 2

Maximum extent of ice sheets through the quaternary

Each glacial left behind ‘evidnece’: glacial geomorphology, till, erratics etc

‘law of superpositions’: within wider geological context

Allow us to ‘date’ the evidence and form glacial stratigraphy for each area

Last glacial ice sheets in Europe:

European continent at the end of the last glacial period

ice sheet covering most of northern Europe

Extensive glaciation in alpine regions as far south as southern spain, Greece, and Georgia (Morocco?)

18^th and 19^th C – glacial geomorphological evidence led to glacial-interglacial cycle theory

British ice sheet:

Maximum extent of british-irish ice sheet during Devensian glacial

Last glacial maximum (LGM) c. 21,000 years ago

Covered Scotland, Ireland, wasles, and northern England

800,000km ^3

Up to 1.5km thick

One third size of modern day Greenland

Significant pressure system over ice sheet

Extent of glaciation is contected

Saunton sand, north devon – pink granite erratic from Scotland

Small glaciers on Exmoor?

Independent mini-ice sheet on Dartmoor?

North American ice sheets:

two independent ice sheets either side of the continental divide: rocky mountains

1. Laurentide ice sheet: C. 3km thick, 33 million km^3

2.Cordilleran ice sheet: Merged with Laurentide during LGM

Combined = 1.5 x modern Antarctic ice sheets

Research focus – on north Atlantic region

Sea level in the quaternary:

Eustatic sea level change: global average sea level fluctuates with global ice volume

Sea level closely correlated with glacial-interglacial cycle

Sea level at LGM c. 120/125m lower than present

Crustal rebound following deglaciation can cause isostatic (localised) sea level rise

Compounds anthropogenic sea level rise

Geomorphological and sedimentological evidence for past sea level change

Wave cut platforms, raised beach deposits, marine fossils

Hope’s nose peninsula, south devon

Huon peninsula, papua new giunea

Sea level since the LGM:

LGM c. (20-25k years ago) – sea level 125m lower

Significant effect on coastline compared to modern day

Land-bridges – dispersal of plants, animals, humans

end of the last glacial: causes:

Two major driving factors:

1. Increased summer insolation at higher northern latitudes

2. Increase in atmospheric CO2 from 190 to 280ppm:

Increased seasonality

Reduce ice volume, sea-ice cover

end of the last glacial: causes: 1. Increased summer insolation at higher northern latitudes

Obliquity (tilt) reached maximum

Precession (wobble) brough earth closer to sun

end of the last glacial: causes: 2. Increase in atmospheric CO2 from 190 to 280ppm:

Ocean degassing

Biomass feedback

The end of the last glacial: structure:

Greenland ice-core (GISP2) temperature record

No smooth transition from LGM to Holocene

Punctuated by series interstadials, stadials

The end of the last glacial: deglaciation:

‘late glacial’ characterised by relatively rapid deglaciation of northern hemisphere continental ice sheets

Beginning of the Holocene, just a small ice cap on Scotland’s Trossachs and highland range

Glacial re-advance during the loch Lomond stadial, also known as the Younger Dryas

Landscape change in the quaternary:

Periglacial (edge of ice) landscapes and processes

Extensive permafrost

Dramatic shifts In biome range:

Vegetation, Animals, humans (not just h. sapiens)

The end of the last glacial: biome shifts:

temperate biomes expand northward

  Glacial ‘refugia’

Tundra contracts

Implications for fauna (including humans)

The end of the last glacial: sea level rise:

Doggerland: low lying ‘country’ stretching from Britain to Netherlands

Sea level rise at the end of last glacial began flooding

Submerged c. 8200 years ago (‘8.2 kyr event’)

Archeology: mammoth, lion, human tools and weapons

Bathymetric studies: vast flood-plains of europes major rivers

Fertile land, prime for human habitation

The end of the last glacial: British isles:

Lake Windermere, Cumbria

Late-glacial ‘type site’

Coope and pennigton (1977)

Changing sedimentology indicated change in geomorphological processes in catchment (ice action, organics)

Found in lake sediments across NW Europe – sharp transitions, indicates rapid climate change

Beetle records provided temperature reconstruction: sharp temp increases, c. 1*C per decade, rapid climate change

The end of the last glacial: terminology

north America – the Devensian is known as the wisconsinan

key: the overall pattern of glacial-interglacial transition is very similar across the northern hemisphere

the north Atlantic:-              regional focus on north Atlantic region:

historic bias due to distribution of researchers in NW Europe

genuine driver of global change – teleconnections

the north Atlantic: -              potential drivers for change:

atmospheric circulation change, via north Atlantic oscillation (NAO)

oceanic circulation change, via Atlantic meridional overturning circulation (AMOC) – not the same as ‘Atlantic multidecadal oscillation’ – AMO

the north Atlantic: NAO:

air pressure gradient between:

subpolar (Icelandic) low

subtropical (Azores) high

influences strength and position of north Atlantic jet stream, northern westerlies

modulates patterns of zonal and meridional (i.e. latitudinal and longitudinal) heat and moisture transport

affects changes in temperature and precipitation across region

positive = strong gradient, jet stream forced north

negative = weaker gradient, enhanced trough-peak-trough in jet stream

the north Atlantic: AMOC:

northward flow of warm, salty surface water, includes the gulf stream

southward flow of colder, deep water, part of the thermohaline circulation system

transport of heat (energy) from tropics and southern hemisphere, to north Atlantic, where heat is lost to atmosphere

  impacts on global climate system: sea surface temperature, arctic sea ice, moisture supply

potential for disruption during deglaciation

the north Atlantic: in the quaternary: 1

• glacial and interglacials punctuated by a series of interstadials and stadials

• millennial-scale abrupt climatic shifts

• dansgaard-oeschger events: very common in glacials, weak in interglacials

o   25 events during last glacial, 120k years ago

• In the northern hemisphere, characterised by rapid warming to interglacial temperatures, then slower cooling

• 5-8*C increases in c. 40 years in Greenland

• In southern hemisphere, slow warming and smaller temperature fluctuations

• Suggestion of periodicity (c. 1470 years) – debated

• Heinrich events: Occur in cold spells preceding some D-O events, 6 identified in late quaternary,  Icebergs break off from ice sheets, Transport terrestrials, minerogenic ‘ice rafted debris’ into north Atlantic, deposited on ocean floor

the north Atlantic: in the quaternary:2

Drivers of change – link between dansgaard-oeschger and Heinrich events

Still largely uncertain

Difference between northern and southern hemisphere response, although globally synchronous

Internal forcing? Changes in north Atlantic circulation, freshwater pulses, thermohaline, Ice sheet dynamics: binge-purge cycle, size of oce sheets (control on atmospheric, oceanic circulation)

External forcing: Solar activity, orbital cycles – periodicity

Deglaciation in north America:

• Proglacial lake formed along margin of Laurentide and cordilleran ice sheets

• Glacial lake Agassiz: At maximum extent covered most of Manitoba, Ontario, and part of Quebec, lake Winnipeg modern remnant

• staged but catastrophic collapse

• one single collapse evet resulted in global sea level rise of 20-40cm

• freshwater into north Atlantic – potential global climate impact

summary: the quaternary in the northern hemisphere:

• historical focus on quaternary climate and environmental change in northern hemisphere (nw europe, north America)

• continental ice sheets dominate landscapes

• several important climate-system components:NAO, AMOC

• driver of change?

• Teleconnections?

• Important to consider global climate system as whole, vital components else where: Tropics, Southern hemisphere

Quaternary across the world:

• quaternary history of the southern hemisphere is very different

• North characterised by was and wane of continental ice sheets

• Patagonian ice sheet

• Glaciers elsewhere:

•   New Zealand

• Tasmania

• East Africa (mt. Kilimanjaro, mt. Kenya, Rwenzori range)

• Papua new guinea

• Antarctica, sub Antarctic islands

• Spatial patterns in quantity of quaternary palaeo-records:  Historical bias, Geographical distribution of land

• Effort to expand this spatial network, to answer crucial questions about earth climate system

 

The southern ocean:1

• Ocean south of 60*S latitude

• Antarctic convergence: Cold water meets warmer water, High productivity

• Longest and strongest ocean current on earth (Antarctic circumpolar current, ACC)

• Inter basin exchange between pacific, Atlantic, Indian

• Relatively poorly understood

The southern ocean: 2

• Global ocean is largest carbon reservoir in earth system

• Co2 rich deep water

• 80% of earths deep water resurfaces in southern ocean

• Southern westerlies create strongly divergent surface flow over southern ocean, promoting upwelling of deep water

Southern westerly wind belt:

• Northern westerlies, land masses, season gyres

• Southern westerlies, uninterrupted, strongest time-average winds on the world

• Strength and position of SWWB determines whether southern ocean acts as a source or sink of atmospheric co2

• Value in reconstructing past variability in SWWB

Southern annular mode (SAM):

• Pressure gradient – similar to NAO

• Positive SAM – strong lows over Antarctica, stronger high s further north

• Linked to increased upwelling in the s. ocean, increased atmospheric co2

• Recent years has seen increasingly positive SAM – natural, anthropogenic?

• Shift in strength and position of SWWB

• Terrestrial records, less ice – but not much land

• Current research focus: new Zealand, Patagonia, Antarctic peninsula

The monsoonal systems:

• Seasonal reversal of wind direction

• Dramatic seasonal changes in precipitation patterns

• Summer monsoon provide 90% of Asian precipitation

• Complex: 3 x Asian monsoons, African monsoon, south American monsoon

Loess deposits:

• Fine grained, wind blown sediments – associated with strength of Asian monsoon

• Biggest deposits are found in Loess Plateau, middle reaches of yellow river, north china

• Covers c. 450,000km^2 , twice the size of the UK

• 10-50m thick are common, forming near continuous drape over landscape – deepest 505m

• Winds are strong in glacials (thick deposits), weak in interglacials (palaeosol formation

• Sedimentological changes reflect glacial-interglacial cycle (i.e. marine isotope stages – MIS)
  
  

A green Sahara:

• Archaeological evidence in Sahara that environment was very different in recent past, within Holocene

•   Abundant tropical fauna and flora (e.g. giraffes, elephants, baboons and other primates)

• Palaeolithic cave art shows animals

• Dwarf crocodiles still present in isolated southern Sahara water holes

Asian ice cores:

• Antarctic/ Greenland records valuable but polar only

• Asian/ sub tropical ice cores improve understanding of earth climate system

• Four ice caps on Tibetan plateau (aka, the ‘third pole’)

• 2017: gulyia ice cap, oldest ice core from non polar region, c. 600,000 years old. Asian monsoonal reconstruction

Asian speleothems:

• Several key speleothem records from caves across Asia/ subtropics

• Potentially high resolution records

• Stable isotope records can inform understanding of Holocene/ quaternary climate dynamics (e.g. Asian monsoon)

• Dongge and hulu cave speleothem records amongst most famous, both in China

• Dongge: solar facing, north Atlantic (Heinrich events?)

El nino – southern oscillation (ENSO):

• Poorly understood periodic variation in tropical pacific wind, ocean currents

• El nino – warming phase, high in western tropical pacific

• La nina – cooling phase, low in western tropical pacific

• Climatically and economically vital

Porites corals:

• Living stony corals can provide records c. 200 years

• Like tree rings can be combined – live and fossil

• Delta 18O record = SST  ENSO?

Summary – quaternary across the world:

Very different quaternary when compared to northern hemisphere:

Lack of contintenal ice sheets

Potentially imporat climate- system components about which very little is known:

SAM and SWWB: Climate change and southern ocean as carbon store/ sink

 Asian/ African monsoon

ENSO

• Valuable new records and climate proxies being developed – more needed