history of our planet: week 7

the quaternary

What is the quaternary:

Ice age

Glacial-inter-glacial cycle (cyclic growth and decay of continental ice sheets – particularly in northern hemisphere)

Current interglacial – Holocene

2.6 million years ago to present day

Holocene – current interglacial period (11.5 thousand years)

Pleistocene – 2.6Ma to 11.5 Kyr

Why study the quaternary: 1

Putting Anthropogenic climate change in context

Atmospheric CO2 passed 400ppm in 2016

Marker was once a target for stabilisation

Polar ice sheets (Greenland, antarctica), annual layers of snow, trap air bubbles

800,000 year record of carbon dioxide and methane concentrations from EPICA ice-core, antarctica

IPCC CO2 and CH4 projections for 2100 AD

Anthropogenic climate change:

Anthropogenic composition, temperature, sea level rise

By 2100, global average temperatures will probably be 5 to 12 standard deviations above the Holocene temperature mean…

Are we entering the Anthropocene

Why study the quaternary: 2

Geologically recent:

Distribution off continents and oceans comparable

  Wealth of geological archives to provide records of past climate and environmental change

Coincides with the evolution of hominids and the birth of ‘modern society’

Provides longer term context for understanding of the earth system:

Improve predictions of future climate and environment

Improve ability to mitigate and adapt to future change

Ice sheets, mammoths, extinctions, human evolution, volcanoes, meteorite impacts and mega floods

Still have lots of archaeological records as it is fairly recent

 

Where is the evidence: for quaternary

quaternary:

Multiple glacial and interglacial periods occurring over the 2.6 million years

Growth and decay of vast continental ice sheets covering much of the north hemisphere (and glaciation elsewhere)

Origins of the quaternary science:

The church vs. new science of geology

Differing interpretation of geomorphological features in the landscape:

Erratics – rock that differs in size and type (geology) from those in the area, often very large

E.g. Yeager rock, Waterville plateau, Washington – 400 tonnes

Till – unsorted sediments; variety of rock types and sizes, found across northern hemisphere

Glanllynnau, north Wales

Other large scale geomorphological features

U-shaped valleys and the ‘parallel roads’ of glen roy, scotland

Catastrophism (religion/ church??):

Rocks and sediments covering landscape – product of biblical flood

Earth’s surface the result of succession of catastrophes

Features form during individual events

Uniformitarianism (science):

Physical, chemical, biological laws that operate today also operated in the past

Earth history dominated by small-scale events and processes, ‘gradualism’

Catastrophes happen but are rare

Scottish enlightenment produced modern geologists:

James Hutton. 1726-97. ‘the past is the key to the future’

Charles Lyell. 1797-1875. ‘the present is the key to the past’

Lyell went to Switzerland 1780-90s:

Glaciers observed in Switzerland ‘dumping’ erratics

Parallel roads of glen Roy:

Charles Darwin (1809-82): 1839: ‘shorelines are raised beaches of marine origin’

Louis Agassiz (1807-1873): 1840: ‘shorelines were cut by freeze-thaw processes of an ice-damned loch, created by a glacier during a period of extensive glaciation in the past’

Louis Agassiz:

Hugely influential geologist: glacial lake Agassiz; ‘mount agassiz’ x 5; Agassiz x 2, even crater Agassiz on mars

Creationist, polygenist, believed in inferiority of black slaves he encountered in the US

Views fed into ‘scientific racism’ which gave racism ‘validity’ by association

1837: lectured on ‘ice ages’ at Swiss society of natural sciences

Most scientists though the earth had cooled gradually from molten state

1840: wrote book ‘study on glaciers’ and lectured with Lyell on cyclic ‘ice age theory’ at the geological society, London

Lack of plausible mechanism to explain climatic changes required to drive ice sheet formation

Drivers of the ice ages:

1842: joseph Adhemar introduced concept of orbital ‘eccentricity’

Shape of the earths elliptical orbit oscillates from more to less circular

Variations in eccentricity affect seasonality (i.e. mild winters, cool summers; cold winters, hot summers)

1864: James Croll wrote a paper suggesting that variations in eccentricity could drive cyclic ice ages (i.e. cool summers - year round ice – glaciers/ ice sheets)

Insolation = solar radiation that reaches earth’s surface

Milutin milankovic (1879-1958)

Developed mathematical explanation for climate change

1920 – published calculations of heat changes at different latitudes and periodicities of these changes

Identified 65*North as place where biggest insolation changes occur

From theory to fact:

Some evidence: glacial geomorphology and erratics  ice age theory

Mechanism: Milankovitch cycle theory

Detail?

How many glacials-interglacials

Was there a periodicity/ cyclicity

What were the wider effects on the earth system

Answer: paleoclimatic and paleoenvironmental reconstruction

Dawn of palaeoclimatology:

• marine records: WW2: technology and data boom,   Oceanography was revolutionised, Tectonic theory: deep-ocean sediments were shallower than expected, Magnetic signals stored in marine sediments were mirrored in different areas, 1960s: concerted and coordinated international effort to examine marine sediments, 1970s: chemical composition of the sediments and the biological remains preserved in them

• Ice core records

Dawn of palaeoclimatology:  

Marine records:

• Chemical composition (δ¹⁸O) of foram shells (‘tests’) reflected composition of ocean water

• Chemical composition (δ¹⁸O) of ocean water reflects global ice volume and temperature

• foram tests are preserved in marine sediments over time

• analysing preserved foram (δ¹⁸O) reveals changes in global ice volume and temperature over time

Dawn of palaeoclimatology:  ice core records: 1

were still in the ‘ice age’ – two major ice sheets – Greenland and Antarctica

multiple other glaciated areas (including tropics and high altitude areas)

high resolution, annual snow bands often preserved

analyses of both the ice, and the air bubbles trapped inside

provides records of precipitation chemical composition (δ¹⁸O) – temperature

greenhouse gas concentrations (co2 and ch4) – global atmospheric composition

insight into how the earth system operates over time

critically: across both hemispheres

dawn of palaeoclimatology:  ice core records: Antarctic ice sheet, c. 4.5km thick

Preserves record of snowfall through quaternary

Formed 35 million years ago

dawn of palaeoclimatology:  ice core records: Greenland ice sheet

c. 3km thick

Formed 18 million years ago

dawn of palaeoclimatology:  ice core records: Oldest continuous ice core:

Greenland: 130,000 years (NGRIP)

Antarctica: 800,000 years (dome C)

Potentially older ice in other parts of antarctica (e.g. 2.7 million years old, near Taylor dome, but continuous? Glaciers in Himalaya?)

using ice core records

10cm diameter but up to 3km long

Can be cored by hand, up to 40m

Any deeper requires machinery

Ice age: from theory to fact:

Correlations exist between ice cores (EPICA, Vostok)

Correlations extend to marine core records of global ice volume

Multiple archives, multiple lines of evidence, thus showing that glacial-interglacial cycle exists

ice age and milankovic cycles

Evidence that solar facing (i.e. Milinkovic cycle) can explain the timing of glacial-interglacial cycle

Milankovic was right. They explain the timing of the cycle (but not the magnitude)

But earth climate system is extremely complex, more detail is needed

Instrumental records:

Max out at 100-150 years if that:

Meteorology

Remote sensing

Instrumental records: Historical records:     

Weather observations

Ship logs

Books and stories

Longer term perspectives required to improve understanding and validate methods

Climate proxy records:

Longer-term perspective provided by proxy data from range of palaeoenvironmental archives

Proxy – a substitute or deputy

Physical, biological, and chemical properties of archives act as a substitute for direct measurement of variety of environmental parameters:

Temperature, Precipitation, Atmospheric circulation, Ecological change

Choice of archive and proxy technique depends very much on question to be

answered (timing, location etc)

lake cores

Chemical and biological techniques

Proxies often specific to each archive

Each proxy provides different information

ice core

Biological proxy records:

Plants and animals can be sensitive environmental indicators

E.g. temperature, salinity, moisture availability

Their remains are deposited and often preserved in sedimentary sequences

E.g. peatlands, lakes, marine sediments, caves, river terrace deposits

Limited evolutionary change in quaternary period, so taxonomy is strong – same species

Environmental reconstructions based on fossil assemblages using knowledge of current distribution, ecological preferences etc.

‘present is the key to the past’

Pollen analysis:

Pollen preserved in archive (peatlands, lakes), sourced from surrounding area

Reconstruct vegetation change – climate/human/both

Chironomid analysis:

Diverse group of non-biting midges

C. 1200 palaeoarctic species

Specific ecological requirements: Mainly temperature, Nutrient and oxygen conditions, ph

   Ubiquitous, well preserved and identifiable as fossils

1360m depth in lake Baikal, Russia

 Dendroclimatology

Climate information in tree rings

Annual resolution (early vs late wood)

Absolute dating (i.e. age control)

Spatially extensive, often fossilised

Factors affecting growth (stress from):

Temperature

Moisture availability

Fire, volcanic eruptions etc

Ring width: narrow tree ring events

Exceptionally high resolution climate records – annual, sometimes seasonal

tree rings have very important role in radiocarbon dating – method providing age estimates for sediments deposited over last 50,000 yrs

Geochronology (science of what happened when)

Before advanced dating techniques were available, reliance on basic geological laws

Law of superposition

Sediments higher in sequence were younger than those below

Rapid advances in palaeoclimatology and paleoenvironmental science is enabled by development of new dating techniques – otherwise dating is meaningless

geochronology provides a framework to estimate:

timing – when events happened

Duration  - how long event lasted

Rate – how quickly the change occurred

Cause and effect – how events related to one another (spatial variance, inter-site comparison, explanatory mechanisms)

dating techniques

Most dating techniques are only suitable in specific environments or on specific materials

All dating techniques have specific time intervals over which they operate

Each dating technique operates over a different time scale

All techniques can very in terms of their precision, accuracy = uncertainty

Error associated with analysis, sampling, and archive = uncertainty

Precision: Reproducibility of a results, Uncertainty (+- error margin), E.g. 10,000 +- 200 cal. Year BP, 10,000 +- 1200 cal year BP

Accuracy: Relationship between age estimate and true age of sample

Dencrochronology:

Patterns of tree ring widths can be matched between trees to produce dendrochronologies

Can include live, dead, foddil trees

Absolute dating tool extending back 1000s years

Dendrochronologies are crucial tools in accurate radiocarbon dating

Tree rings and 14^C dating

• Radiocarbon (14^C) is an unstable isotope, decays over time at a known rate

• Decay = parent nuclide into daughter nuclide

• Rate of decay = half life (i.e. time taken for quantity of parent nuclide to reduce by 50%) = 5730+- 30 years

• All living things absorb radiocarbon

• Death stops process, radiocarbon begins to decay

• Measuring amount of radiocarbon remaining in organic (living) sample determine time of death, based on understanding half life

• Radiocarbon dating is principle dating technique in paleoenvironmental science, up to c. 50,000 years ago – samples must be organic

• But as solar activity has varied over time, atmospheric 14^C has not remained constant

• 14^C measurements must be calibrated (converted) to reveal calendar age

• Can count tree rings absolutely

• Compare 14^C measurement from tree rings of ‘known age’ and see what he offset is – radiocarbon calibration curve

Tree rings and 14^C dating -              Complicating factors result in uncertainty

2354 +- 80 cal. Yr BP

2314 – 2394 cal. Yr BP

Tree rings and 14^C dating - in literature

14^C year bp = uncalibrated

Cal. Years bp = calibrated