Last Glacial Maximum (LGM)!

•! Dramatically different from today!

•! Climate state steady for several thousand years!

•! Primary boundary conditions (Geography, orbital configuration, pCO2) very well known!

•! Within range of both 14C and U-Th dating!

•! Extensive data sets ford land and ocean!

CLIMAP (1981)!

•! Land Ice!

•! Sea ice!

•! Sea Surface Temperature!

•! Vegetation and Land surface albedo!

Today!

•! Land Ice and Sea Level reconstructions!

•! Land Surface Changes (Dust-Vegetation!

•! When was the LGM? Radiometric dating

techniques.!

Physically model ice flow!

(input T, bed characteristics, etc)!

Physically model!

isostatic rebound!

(input timing of retreat,!

Viscosity of mantle, etc.)!

How much ice?"

a) Ice Sheet reconstructions!

Red sea sill depth: 137 m!

How much ice?"

b) Changes in Ocean !18O!

!18Oo,LGM (Vo,today-Vice) + !18Oi,LGM Vice = !18Oo,todayVo,today!

Total amount of water and isotopic composition of water

doesn’t change!

Where Vice , !18Oi,LGM refer to the EXTRA ice during LGM!

What are the uncertainties in !18Oo,LGM , !18Oi,LGM ?!

How much ice? "

c) Changes in sea level from corals!

Some corals (A. palmata) live

only within a few meters of

sea level.!

Lower sea levels will result in

deeper corals!

Have to correct for tectonic

and isostatic uplift!

Sea level change >=120 m!

Fairbanks (1989) Barbados Sea level Curve!

LGM Sea-Level dropped by

about 120 m consistent with:!

•! Ice margin reconstruction (wet bed)!

•! Glacial Isostasy!

•! Sea level from corals!

•! Red sea constraint!

•! Oxygen isotopes in benthic foraminifera!

Why more arid?!

Hydrologic cycle?!

Plant response to CO2?!

http://www.pmip2.cnrs-gif.fr/pmip2/share/synth/glsdb/lakes.png!

Dust and Aridity!

•! More dust in atmosphere (ice cores)!

•! Fine grained material from glacial outwash

(loess deposits)!

•! More sand dunes in mid latitudes!

–!CO2 effect on vegetation?!

•! Higher Lake levels in mid latitudes!

–! In a warm atmosphere, wets are wetter, dries

are drier!

Overpeck, J.T., R.S. Webb, and T. Webb III. 1992. Mapping eastern North America vegetation change over the past 18,000 years: no-analogs and the future. Geology 20:1071-1074.

Vegetation!

•! Vegetation is reconstructed from pollen!

•! Vegetation responds to temperature and

precipitation changes, hard to untangle.!

•! Vegetation zones shifted southward!

When was the LGM?!

•! Different ice sheets have maxima at different

times!

•! Maximum cooling not always in phase with

maximum ice volume!

•! Two potential methods:!

–! Coral sea level record (still need to be sure LGM has

been reached)!

–! Oxygen isotope record (Does extremes in deep ocean T

coincide with ice volume extreme?)!

Radioactive Decay

In radioactive elements, the configuration of

protons and neutrons in the nucleus is not

stable, and will eventually spontaneously

change into another element either by

emitting and electron, capturing and electron

or emitting an entire He nucleus (2 protons

and 2 neutrons).

The transformation is a matter of statistical

probability, and given a reasonable number

of atoms, radioactive decay can be

expressed by:

dN/dt = -!N

where N is the number of atoms present and

! is the decay constant.

The decay constant is sometimes expressed

as a half-life (time it would take for half the

atoms to decay (T1/2 = ln(.5)/! = .693/!).

If there are no other sources and sinks of N,

N = N0e-!t

If the daughter product of the radioactive

decay is stable, the amount of daughter

product produced by decay is:

D* = N0 – N = N0 – N0e-!t

= N (e!t –1)

If there is no daughter product at time= 0,

then can invert this to get time:

e!t –1 = D*/N

e!t = D*/N + 1

!t = ln(D*/N+1)

t = !"1 ln(D*/N+1)

d230Th/dt = "234234U - "230

230Th!

d234U/dt = "238238U - "234

234U!

Initial conditions:!238U doesn’t change abundance by much (long half life)!

Can assume [238U/234U]0 same as in seawater!

[230Th]0 = 0!

Uranium-Thorium Dating of Corals!

14C Radiocarbon!

Produced in the atmosphere:!

ultimate source of neutrons: cosmic rays!

Decays everywhere:!

(beta decay)!

half life = 5730 years!!

o

1n+

7

14N" # "

6

14C+

1

1H

!

14C" # "

14N + e

$

What is 14Co?!

14C = 14Coe-"t!

•! 14C/12C in the atmosphere depends on production rate (and to a lesser extent on rate of mixing between ocean and atmosphere)!

•! 14C/12C in oceanic carbon will depend on the degree of equilibration with the atmosphere!

•! Initial 14C/12C will also depend on the isotopic fractionation between the phase being dated (e.g. shell, wood) and the carbon pool (e.g. ocean, atmosphere) from which it formed (correct using !13C)!

Atmosphere after 1950’s has extra radiocarbon from bomb testing!

The CO2 added to the atmosphere from land use

changes and fossil fuel burning is radiocarbon dead.

“Seuss effect” caused atmospheric #14C to decrease

before bomb testing!

Surface Ocean Reservoir Effect!

•! Pre-bomb measurements and shell

collections suggest seawater about 400

years old in most of the surface ocean!

•! Up to 800 years in upwelling areas!

Raw 14C age -> Reservoir corrected 14C age!

Production Rate Changes!

14C Age -> Calendar Age!

Beck et al

(2001)!

Can

reconstruct

how

atmospheric 14C has

changed

through time if

you have an

independent

clock: Tree

rings, varves,

U/Th clock!

Radiocarbon dating!

•! Can measure amount of 14C remaining in

shell or coral!

•! To determine initial 14C amount need to

account for:!

–!That the surface ocean has less 14C than the

atmosphere!

–!Changes in atmospheric 14C concentrations due

to production changes !

Why use radiocarbon?!

•! Can be used to date many different

materials (foraminifera, shells, wood, plant

remains) !

•! Can be very accurate once corrections are

made!

•! Only way of directly dating deep sea record!

LGM : 19-23 kyr (calendar) BP!