Climate Proxies

swedishstreakMécanique

22 févr. 2014 (il y a 3 années et 3 mois)

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Climate Proxies

How can you measure the climate
of the past?

Learner outcomes

At the end of this lecture you should be able to

1.
Describe how proxies differ from observations

2.
Describe how tree rings, corals, fossils, lake ice
and lake duration are used to estimate local
climate

3.
Describe how ocean sediment and ice cores are
used to estimate global climate

4.
The difference between stable and radiometric
isotopes and what type of information they tell use
about past climates

Proxies

Unlike instrumental records that tell us only
about the most recent century, proxy
records (natural archives of climate
change) enable us to place recent climatic
change in the context of the last several
hundred to thousand years.

Temperature

(Northern Hemisphere)

CO
2

Concentrations

1000 Years of CO
2

and

Global Warming


How do we develop proxies?


Assumptions


Observations of phenomenon today


Link current observations to past records

Assumptions


Observation: Solar radiation varies but
overall decreases


Therefore solar radiation in the past was
higher

Proxies


Corals*


Tree rings*


Pollen*


Fossils*


Sea level


Lake ice duration*


Ocean sediments


Ice Cores

*
indicative more of local climate change than global
climate change

Corals


Shells made of Calcium Carbonate
(CaCO
3
)


Shell in equilibrium with the ocean water


Band width provided evidence of
temperature the coral grew in


Growth rates change with ocean
temperatures, pH

levels


Local climate


Why should we hug trees?


Dendrochronology

is the study of the annual variability
of tree ring widths, which can be extended back to
8000 years ago.


The study of trees provides climate information
regarding temperature, runoff, precipitation, and soil
moisture.


Local climate


Date of last ring is

year tree was cut

1930

1950

1970

1910

1890

1870

Tree Rings

Growth conditions recorded in rings


Wide ring
-
warm days sufficient water


Narrow
-
cold days/drought

What can plant and animal fossils tell us
about ancient climates?


Certain plants and
animals live only in
specific environments,
so their presence is a
clue to local climate.

These 350 Ma fossil ferns were most likely the
oldest on land, and likely required high pCO
2

levels.

A trilobite, the
three
-

lobed king
of warm, shallow
Cambrian seas

Soft
-
bodied
Waptia
, an
arthropod from the
Cambrian Burgess
Shale

Lake Monona Ice Duration 1855-2005
0
20
40
60
80
100
120
140
160
180
1855
1875
1895
1915
1935
1955
1975
1995
Seasons
Duration of Ice (days)
Source: Wisconsin State Climatology Office

Examples of Climate Proxies


Pollen



Lake Ice Duration


Tree Rings


Lake Ice Thickness


Ice Cores

Global Proxies


Sea level


Ocean sediment


Ice cores


Layers (
varves
) in ice cores


Gases in ice cores


Stable Isotopes: O
-
16 to O
-
18 ratio in ice
cores


Radiometric
I
sotopes:Carbon

dating of
sediment in the ice cores or glacial deposits

Sea Level


Glaciation

low sea level

Ocean Sediment Cores: 3
-
3.5km



Thick levels of
sedimentation can
indicate heavy
weathering, warmer
temperatures


Volcanic sediments


Loss of sediment
layers through
erosion


55 mya

Frozen Core Some cores go 3 km deep!


Vostok, Antarctica

78
°
28' S, 106
°
48'E: Coldest
Places on Earth





















Vostok Station

Nationality:

Russia

Location:

Vostok


-

an outpost if there
ever was one
-

is located near the
South Geomagnetic Pole, at the center
of the East Antarctic ice sheet, where
the flux in the earth's electromagnetic
field is manifested.


The coldest
recorded
temperature on
Earth,
-
128.6
°
F (
-
89.2
°
C) was
measured here on
July 21, 1983.


Ice core drilling 3.4
km to go ½ million
years into past
climate




Ice Cores:
Varves


A
varve

is an annual layer of
sediment

or
sedimentary rock

Section of Greenland Cores

Dozen Ice Ages going back 1 billion years

How can ancient greenhouse gases be
trapped?


Atmospheric gases (CO
2
, CH
4
, SO
2
, etc.) can be trapped
in glaciers as frozen water metamorphoses from
snow

to
firn

to recrystallized
ice
.

The record of atmosphere
CO
2

since the Industrial
Revolution

During the Last Glacial
Maximum pCO
2

is
estimated at 180 ppm

Ice Core Thermometer

Isotopes


Stable Isotopes
-
temperature


Radiometric dating
-
rate and date


How can oxygen isotopes used as
paleoclimate proxies?


isotope

--

atoms of the same element with the same
atomic number (chemical properties) but differing
atomic weight (physical properties). Differ in number of
neutrons.


Oxygen is composed mostly of
16
O

and
18
O
, which as
part of water molecules are separated by physical
processes.

A typical carbon
atom with 6
protons

and 6
neutrons
and
6
electrons
.

Fractionation


To divide or separate into parts


Ocean water is made up of both O
-
16 and
O
-
18

There is a standard or average ratio of O
-
18
to O
-
16 (
standard mean ocean water as
the
baseline, SMOW)


Certain physical and biological processes
change the ratio (this is fractionation)


Oxygen Isotopic Ratios or
Amounts

O
18
/O
16

ratio in glacial ice indicate the
atmosphere temperature in which the
snow that made up the ice formed

Extent of isotopic difference (
fractionation
) is
dependent on the temperature.

So they form a temperature proxy!


Oxygen Isotopic Ratios vs.
Amounts


O
18
/O
16

ratio versus O
-
18 and O
-
16

Usually described as a ratio


Oxygen Fractionation Summary


If ratio O
-
18 to O
-
16 is higher than
expected in the ocean, colder
temperatures


If ratio O
-
18 to O
-
16 is lower than
expected in the ocean, warmer
temperatures


Oxygen Isotopes in Glacier Ice


Polar ice is preferentially enriched with O
-
16 relative to the ocean (O
-
16 locked in
glacier ice). So especially during glaciation
ocean water is “heavy”


Why is glacier ice “light”?


The water source is from precipitation which
is preferentially light.


So during a glaciation you would expect
remaining ocean water to be heavy

Radiometric Isotopes


Isotopes that decay (Carbon) can tell us
the approximate date of an event or the
rate at which an event took place


Glacial retreat

Carbon Dating: Rate of Glacial Retreat


Greenland Ice Sheet and Arctic:
Northern Hemisphere

Antarctica: South Pole