Paleoclimate: past climate

1
Paleoclimate past climate Like being a
detective looking for evidence and reconstructing
past climate If you are interested in what
happened prior to written historical records, you
look at proxy data Information that indirectly
tells you what the climate was like in a certain
area in the past
Dust storm blowing off the Saharan desert in W
Africa
2
A note on proxy data Accuracy of your
information depends on your ability to look at
detailed time increments Like a digital camera
higher resolution gives you a better picture
And also to be able to date the information you
have Getting an actual age of your data is
better than just making relative
comparisons This field has bloomed in the last
few decades since technology becomes available to
date material, to make very detailed images of
material, and to store and manipulate large
quantities of data (by computer)
3
Most obvious feature of past climate are the ice
ages Spreading of glaciers over the surface of
Earth and the corresponding deglaciation (melting
of glaciers)
4
Glaciation presence of glaciers over large areas
of land that do not now have glaciers Evidence
of glaciation 1. U-shaped valleys 2. erratics
3. moraines 4. location of evidence
5
Evidence of glaciation 1. U-shaped valleys A
valley that is cut by a river will be V-shaped
Why? River cuts down into the rock making a
cliff face, which over time will slump as the
river cuts deeper Angle of rockslides makes
cliffs into V shape
San Juan River valley
6
A glacier, however, is a big sheet of ice that
moves very slowly over the land plowing up the
dirt and rock in front of it It makes a much
wider valley that has a U shape
Misty Fjords National Monument
How many people saw Lord of the Rings and
remember Edoras? How many people have been to
Mount St Helens National Park?
7
Evidence of glaciation 2. Erratics What are
they? Very large boulders (up to the size of a
car) that litter a landscape that has been
glaciated
In comparison Rivers can only move large rocks
when the water is flowing very fast, on rare
occasions of massive flooding Rocks moved by
water tend to be sorted so that large boulders
are found in the same place as other large
boulders Erratics are solitary extremely large
boulders - too big for water to move - that are
sitting in much finer sediments like sand or silt
Joshua Tree National Park,
8
Erratics are solitary extremely large boulders -
too big for water to move - that are sitting in
much finer sediments like sand or silt Why? As
ice moves over land it breaks up the rock under
it - partly from weight and partly by motion of
ice Boulders get trapped inside the ice and do
not fall out until the ice melts Meanwhile, the
bottom of the glacier is pulverizing everything
in its path The rest of the sediment in that
area has been ground up very fine When the
glacier melts and drops its largest boulders,
they sit in a bed of fine sediments.
9
Evidence of glaciation 3. Moraines What are
they? Ridges of dirt, sediment, rocks forming
fairly continuous lines across the land When the
climate warms and melting of the ice begins, by
definition a glacier has moved as far as it can
It has accumulated as much sediment and rocks as
it can in its lifetime As the ice melts, the
sediment and rocks drop out onto the land
wherever the edge of the glacier is located
Bylot Island, Canada
10
The glacier has been pushing a huge load in front
of it More sediments are trapped in the ice
right at the leading edge than anywhere else in
the glacier So much material drops out at the
leading edge that it makes a ridge across the
land, leaving a marker of the furthest extent
that the glacier moved The moraine! Example
Cape Cod would not exist except for a glacier
that dumped all of its load in the moraine that
is now Cape Cod
Map showing relationship of moraines on Long
Island to those of southern Connecticut, Rhode
Island and Massachusetts
11
Evidence of glaciation 4. Location of
evidence Glacial features on Earth's surface
have been found far south of the Arctic Circle
over continents and sometimes all the way to the
oceans In US, glaciers also moved as far south as
Iowa
Louis Agassiz (Swiss) was the first to
hypothesize that these features were formed by
huge ice sheets that had moved south from the N
Pole all the way to the Mediterranean Agassiz
called it a glaciation
12
How do we get information about glaciation and
deglaciation? Also known as Ice Ages or glacial
and interglacial periods 1. historical record
2. ice cores 3. ancient sediments 4. sea
level change 5. past plant life
13

• Information about glaciations
• Historical record
• Easiest to find but only goes back about 6000
  years
• Records generally mention exceptional weather
  events like storms or floods, but do not usually
  mention the prevailing day-to-day conditions
• Example freezing of the Thames River (London)

Between 800 and 1500 AD only 1-2 freezings are
recorded each century But 1500-1600 AD it froze
4 times The next century 8 times And the next
century 6 times
14
1. Historical record Hypothesis Progressive
cooling from 1600-1800 would explain the
increases in freezing events Example
Reconstructing climate from Icelandic historical
record 1864 - present meteorological
instruments record weather 1781-1845
reconstruction from records of severity and
frequency of drift ice around Iceland 1591-1780
historical records about weather combined with
incomplete records of drift ice 900-1590
incomplete records taken from Icelandic sagas
that mention severe weather and/or famines
15
Information about glaciations 2. Ice cores
Record goes back 110,000 years (Greenland ice
sheet) to 160,000 years (Antarctic ice
sheet) Ice sheets are formed by accumulating
snowfall each year that is compressed into ice as
it is buried This happens once snow gets deeper
than 80m
16
Information about glaciations 2. Ice cores Ice
cores are taken by drilling hollow tubes into the
ice and the cores are stored at below freezing
temperatures Often ice contains trapped air
bubbles By measuring chemistry of the ice and
the trapped air bubbles, we get both direct and
indirect proxies of climate
17
Measuring chemistry of the ice
Back to basics (review) Water in the form of ice
has two H atoms and one O atom Both of these
atoms have 2 different isotopes Isotope atoms
have different number of neutrons, same number
of protons and electrons
Hydrogen 1H and 2H (called deuterium), with 2H
being heavier and more rare on Earth Oxygen
16O and 18O, with 18O being heavier and more rare
on Earth
18
Isotopes What do they tell us? Physical
separation of isotopes When water evaporates,
greater amounts of the lighter isotope are
evaporated into water vapor
The water left behind gets "heavier" has more
of the heavy isotope When it rains or snows,
greater amounts of the heavy isotope are rained
out Ice in the ice sheet is slightly more "heavy"
than the water vapor it condensed from
19
The separation of these isotopes is called
fractionation and is a function of
temperature Ratio of 2H/1H or 18O/16O in water
is preserved in the ice of the ice sheet By
measuring those ratios, you have a proxy of
temperature You are not actually measuring
temperature but are measuring the effect of
temperature indirect proxy
Isotopes What do they tell us? Chemical
separation of isotopes
20
Information about glaciations 2. Ice cores (more
about)
Measuring chemistry of the trapped air
bubbles Trapped air bubbles in the ice contain a
little bit of the atmosphere at the time the snow
fell Can measure chemicals such as greenhouse
gases (CO2, CH4, N2O)
21
A direct measurement of the concentrations of
these gases in the atmosphere at that time, at
least in that location
Greenhouse gases and pollutants measured in ice
cores have all increased over the last 200 years
22
Information about glaciations
2. Ice cores (more about) Dust layers in the
ice Can give an idea of frequency of storms or
volcanic activity Also these are the only
materials in the ice cores that can be dated
(By measuring various isotopes or by comparison
to the historical record in the case of volcanic
eruptions)
23
(No Transcript)
24
Information about glaciations 3. Ancient
sediments Type of sediment, source of sediment,
and final locations that you find that sediment
Can tell you about dominant climate conditions
or prevailing wind directions
Dust storm blowing off the Saharan desert in W
Africa
25
Information about glaciations 3. Ancient
sediments Evaporite deposits Evaporites are
salt deposits (instead of dirt the sediment is
made almost entirely of salt crystals) Form in
an area where rate of evaporation exceeds rate of
precipitation (arid conditions) Water in a
closed lake (usually) evaporates leaving behind
the salt that used to be dissolved in the
water Eventually these might be buried by more
sediments and preserved in the rock record
Mono Lake, CA
26
Information about glaciations 3. Ancient
sediments Evaporite deposits
Water in a closed lake (usually) evaporates
leaving behind the salt that used to be dissolved
in the water
There are evaporite deposits forming right now in
western US (Great Salt Lake in Utah), Germany,
and central Asia
Great Salt Lake, Utah
27
Information about glaciations 3. Ancient
sediments Sand dune deposits Structure and
location of dunes can tell about prevailing wind
direction
28
Information about glaciations 3. Ancient
sediments Loess deposits - covering Memphis!
You know the way clouds of fine dirt kick up
every time someone mows their lawn (even if the
lawn is covered in grass)? That flour-like dirt
is loess. Forms usually at the front edge of
glaciers where the ice is grinding the dirt
into fine powder and collecting it as it pushes
along in front of the ice Then winds blowing
down off the ice pick up the loess and distribute
it many miles from its source
2.3 million-year-old section of loess deposits
(light brown) and interstratified soils (reddish)
near Xi'an, China
29
Information about glaciations 3. Ancient
sediments Ocean sediments
(1) Types of organisms found in them (skeletons)
and (2) isotopes of sediments (1) Can tell about
temperature of water in certain levels of the
ocean (generally surface waters) and (2) about
general trends in evaporation and precipitation
30
Explosion of Mt Pinatubo
Information about glaciations 3. Ancient
sediments Volcanic ash deposits
Can be pinpointed to a specific eruption in
specific location Where you find ash from a
certain eruption, you can say something about
prevailing wind direction that brought it to the
location you found it in the rock record
Cloud from Mt Pinatubo eruption sulfur dioxide
levels
31
Information about glaciations 4. Sea level
change As glaciers and ice sheets form they
take up more water from oceans and lock them
into ice (sea level falls) As ice melts during
deglaciation, water flows back into ocean (sea
level rises) This causes the most rapid change in
sea level of anything that could change sea
level Example If the present day Antarctic ice
sheet were to totally melt, the additional water
added to the oceans would cause an estimated 200
ft rise in sea level
32
Information about glaciations 4. Sea level
change Most common way to detect sea level
change in the past Location of coral reef
terraces Coral reefs can only grow in very
shallow warm regions in ocean (not fresh water)
Need lots of sun and are very sensitive to
temperature changes in water So when you find
an ancient coral reef exposed in a cliff face or
up the side of a mountain on a Carribean island,
you know where sea level was and what surface
ocean temperatures were like when it grew.
33
Information about glaciations 5. Past plant life
Plants require specific climates (temperature,
precipitation, soil conditions) Example
Elevation of tree lines on mountains As
mountain glaciers expand, tree line moves further
down the mountain Example Plants that only grow
in marshes or bogs Can tell you about extent
and locations of these warm, wet environments on
the planet Example Shape of leaves on plants
Leaves with lots of serrations occur on plants
in colder climates, plants in tropical climates
make smooth edged leaves with little or no
serrations
34
Information about glaciations 5. Past plant
life Pollen analysis (palynology) Lots of
plants make and distribute pollen Every plants
type has a different shape and size of pollen so
it is very diagnostic of the plants Pollen gets
everywhere and is found in sediment deposits, in
the bottom of lakes, trapped in ice etc. If you
have an area where you can core vertically into
the sediment or ice and compare pollen in
different layers then you can compare the change
in climate in that one location over time
35
Information about glaciations 5. Past plant life
Pitfalls! A plant has to be in a certain
abundance to make sure that the pollen from that
type of plant is trapped in the sediments Small
and rapid changes may not be picked up in this
record. Also only works for years before humans
began artificially burning and planting Human
intervention does not equal climate change Tree
ring studies Thickness of rings can tell you
about growing seasons, harshness of winters,
large forest fires Also very useful in relating
climate information with actual years and dates
36

• Climate since the last ice age (deglaciation)
• 18,000 years ago deglaciation began
• By 12,000 years ago, in N America only small
  isolated ice masses remained in US
• Most of Canada still covered in ice
• By 7,000 years ago most ice gone - only little
  patches left
• 5,500 years ago - peak in warm temperatures
• Only Greenland ice sheet and Arctic ice remained
  (like today)
• Evidence suggests temperatures in midlatitudes
  (like US) were 4.5?F higher on average than today

37

• Climate since the last ice age (deglaciation)
• 950-1250AD were years of peak temperatures and
  warmest conditions - called Little Climatic
  Optimum
• Vikings settled Greenland
• Ice sheet had withdrawn from southern coast and
  ground was not frozen so they could farm
• 1250-1450AD climate deteriorated around the
  globe
• Europe became excessively wet and damp leading
  to increased diseases
• By 1450 colonies in Greenland failed due to
  increasing permafrost (frozen ground) and cold

38

• Climate since the last ice age (deglaciation)
• 1450-1880AD known as Little Ice Age
• Long cold winters caused freezing of lakes and
  rivers that seldom used to freeze (River Thames
  in London)
• During American Revolution rivers froze and
  British could slide cannons across from
  Manhatten to Staten Island
• 1816 the year with no summer
• May temperatures fell (Indiana had snow for 17
  days), seedlings were killed
• Snow fell again in June killing crops in Ohio
  and northward (meat became the main food in diet
  that year)
• By turn of 20th century climate began improving
• Since 1975-1980 temperatures have been soaring
  leading people to talk about global warming