The history of Earth climate In order to understand the history of …mjm/climatestuff/climate-history.pdf · The history of Earth climate In order to understand the history of the

The history of Earth climate

In order to understand the history of theEarth's climate, we must first understandsomething about the age of the Earth andhow various events have been dated.

Some fundamental questions:

.How old is the Earth?

. How do we know the age of the Earth??

. What was the origin of the Earth'satmosphere?. What was the Earth's early climate like?. How has the Earth's climate changedover geologic time?

The Radiometric Time Scale: Key to ageof Earth and geologic time

. 1896, discovery of natural radioactivedecay of Uranium by Henry Becquerel,French physicist

. 1905 British physicist Lord Rutherforddescribed the structure of the atom andsuggested radioactive decay formeasuring geologic time

. 1907 Yale Prof. B.B. Boltwood publishedfirst chronology of Earth based onradioactivity

. 1950... first more-or-less accurate agedating.

Principles of radiometric dating

. elements distinguished by number ofprotons in the nucleus. Proton + neutroncombination = nuclide

. same no. protons but different no.neutrons = isotopes. e.g., isotopes of Cwith 6, 7, or 8 neutrons designated 12C,13C, 14C.

. radioactive decay of less stable nuclidesis statistically determined.

. The number of radioactive nuclei thatdecay in a given unit of time is directlyproportional to the number of nucleipresent at that time—first-order kinetics.

∂∂

λ

∂∂λ

N

tN

where

N

trateofchangeovertimeofradioactivenuclei N

decayconst uniqueforeachradioactivenuclide

= −

=

=

( )

.,Re :

:

ln( / )

arrangeto

N

Nt

Integratefromtimeot timet toget

N N t

N

Ne

o

t o

t

o

t

∂ λ∂

λ

λ

= −

= −

= −

2

2

2

takingnatural ofbothsides

N N tt o

log :

ln ln2 = − λ

A plot of ln N versus time will give astraight line with slope = -λλλλWith radioactive nuclides, we often speakof half-life, the length of time required todiminish the original # of radioactivenuclei by 1/2:

N = 1/2No

1/2 = e-λt1/2

2 = eλt1/2

t1/2 = ln2 = 0.693/λ λ

By various rearrangements, we can use ameasured concentration of a radioactiveelement at the current time to determine theage. After a time t has elapsed, N atoms ofthe parent (p) will be left and No - N atomsof the daughter (d) will be formed.

td

p= +

11

λln

where d = number of daughter atoms presenttoday (N-No)p = number of parent atoms remaining today(N)

Some commonly used radioactiveelements, their decay products, andcurrently accepted 1/2 lives:

Source: U.S. Geological Survey

Parent Isotope Stable Daughter Product Currently Accept. 1/2-LifeUranium-238 Lead-206 4.5 billion yearsUranium-235 Lead-207 704 million yearsThorium-232 Lead-208 14.0 billion yearsRubidium-87 Strontium-87 48.8 billion yearsPotassium-40 Argon-40 1.25 billion yearsSamarium-147 Neodymium-143 10.6 billion years

Based on different groups of rocks overgeologic time and radioactive dating, aGeologic Time scale has been developed:

Geologic time scale figure here.

Problem set handed out in class dueMarch 28th in class

So, how old is the Earth?Ancient rocks exceeding 3.5 billion yearsin age are found on all of Earth'scontinents. Oldest rocks on Earth foundto date = Acasta Gneisses innorthwestern Canada (4.03 billion yrs)and the Isua Supracrustal rocks in WestGreenland (3.7 to 3.8 billion yrs). Theseancient rocks are not from any sort of"primordial crust" but are lava flows andsediments deposited in shallow water, sothat Earth history began before theserocks were deposited. In WesternAustralia, single zircon crystals found inyounger sedimentary rocks haveradiometric ages 4.3 billion years. TheEarth is at least 4.3 billion years old.

The best age for the Earth (4.54 Ga) isbased on the Canyon Diablo meteorite. Inaddition, mineral grains (zircon) with U-Pb ages of 4.4 Ga have recently been

reported from sedimentary rocks in west-central Australia.

The oldest dated moon rocks have agesbetween 4.4 and 4.5 billion years andprovide a minimum age for the formationof the moon. The moon formed when aMars-sized body collided with theprimitive Earth.

Origin of the atmosphere:

. As Earth accreted, it trapped gases inroughly the proportion found in the sun.

(Note: Sun's main gases: H, He, O, Fe, N,Mg, C, Si, plus other gases)

. Some of these gases were light enough toaccumulate in the atmosphere, but heavyenough to be held by gravity.

.However, any early atmosphere wouldhave been burned off by the collisionmentioned above (temperatures to as highas 16000oK). The lightest gases werepreferentially lost. However, the deeperEarth contained volatile elements insimilar proportions to their originalsource (i.e., same as the sun).

The overall effect of the impact was toalter the mass fractionation of theatmosphere, with lighter elements burned

off/lost and heavier gases left behind, butmany in solid Earth.After impact: "Runaway Greenhouse".Under early conditions of high temp. atEarth's surface, main gas = water vapor.. Radiative cooling, water condensed intooceans.. Main source of warmth = sun, whichradiated less 4.5 billion years ago, by~30%

. water turned to ice, further cooling.

. carbon-containing gases received fromcarbonaceous chondrite meteorites.

. meteorite-related carbon gases causedgreenhouse warming.

. Initial high CO2 concentration inatmosphere diminished gradually bychemical weathering and formation of

carbonate in the oceans.. This uptake of CO2 allowed temperatureto drop enough that life could evolve.

. Eventually, bacteria-like organismsdeveloped, including cyanobacteria(stromatolites). Photosynthesis producedOxygen.

. There is some question as to when lifeand especially photosynthetic life firstevolved. The oldest stromatolites areabout 3.45 billion years old, westernAustralia.

Stromatolites are laminated structures

built mainly by cyanobacteria. They arestill found today, but were once muchmore common. They dominated the fossilrecord between about 1-2 by ago. Today,they are found mainly in saline lakes orhot spring environments. The best

example of living stromatolites is atHamelin Pool, Shark Bay, WesternAustralia.

The bacteria precipitate or trap and bindlayers of sediment to make accretionarystructures (domical, conical or complexlybranching). Hence, make for excellentfossil record. Range in size from cm tomany meters.

. Oxygen was used up as oxidation of Fe-Sminerals to form banded Fe formations,which are ubiquitous in early(precambrian) rocks.

. Eventually, photosynthesis won out overoxidation, and atmosphere evolved tomore oxygen-rich.

. multicellular organisms appeared atleast as far back as 550 million years ago.

. multicellular organisms in the oceanproduced shells which took up alot ofcarbon... eventually forming carbonatesedimentary rocks and organic-richdeposits.

Figure on change in atmosphere goes nearhere.

. Let's compare Earth to other nearbyplanets:Gas Early Earth Venus Mars Atmos. TodayCO2 98% 0.033% 96.5% 95.3%N2 1.9% 78% 3.4% 2.7%O2 trace 21% trace 0.13%Ar 0.1% 0.93% 0.01% 1.6%oC 290 16 477 -53Press. 60 1 92 0.006(bars)

Venus is the second planet from the sunand receives intense solar radiation. Itshigh CO2 concentration provides for arunaway greenhouse.

Mars is the fourth planet from the sunand receives less solar radiation thanEarth. However, the CO2 in theatmosphere does keep the planet frombecoming excessively cold.

The atmospheres of Venus and Mars arethought to be little changed over the last4+ billion years, but Earth has changedprimarily because of evolution of life.

Figure on climate changes over geologictime goes here.

A few notes:

Early Earth (Pre-Cambrian) very hot;eventually cooled as runaway greenhousereplaced by more oxygen-containingatmosphere.

In the Late Paleozoic, glacial episodesmade for a cold, wet climate.

Early Eocene: Considerably warmer andwetter than today.

Late Eocene: Global cooling began,leading to glaciation and the interglacialperiod we now are in.