In order to understand the history of the Earth's climate, we must first understand something about the age of the Earth and how 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's atmosphere? . What was the Earth's early climate like? . How has the Earth's climate changed over geologic time? The Radiometric Time Scale: Key to age of Earth and geologic time
. 1896, discovery of natural radioactive decay of Uranium by Henry Becquerel, French physicist
. 1905 British physicist Lord Rutherford described the structure of the atom and suggested radioactive decay for measuring geologic time
. 1907 Yale Prof. B.B. Boltwood published first chronology of Earth based on radioactivity
. 1950... first more-or-less accurate age dating. Principles of radiometric dating
. elements distinguished by number of protons in the nucleus. Proton + neutron combination = nuclide
. same no. protons but different no. neutrons = isotopes. e.g., isotopes of C with 6, 7, or 8 neutrons designated 12C, 13C, 14C.
. radioactive decay of less stable nuclides is statistically determined.
. The number of radioactive nuclei that decay in a given unit of time is directly proportional to the number of nuclei present at that time—first-order kinetics. ∂N =−λN ∂t where ∂N = rateofchangeovertimeofradioactivenuclei() N ∂t λ = decayconst., uniqueforeachradioactivenuclide Rearrangeto : ∂N =−λ∂t N
Integratefromtimeoto timet2 toget : =−λ ln(NNto2 / ) t
N −λ t2 = e t No takingnaturallog ofbothsides : =−λ lnNNtto2 ln A plot of ln N versus time will give a straight line with slope = -λ With radioactive nuclides, we often speak of half-life, the length of time required to diminish the original # of radioactive nuclei by 1/2:
N = 1/2No λ 1/2 = e- t1/2
λ 2 = e t1/2
t1/2 = ln2 = 0.693/λ λ By various rearrangements, we can use a measured concentration of a radioactive element at the current time to determine the age. After a time t has elapsed, N atoms of the parent (p) will be left and No - N atoms of the daughter (d) will be formed. 1 d t =+ln 1 λ p
where d = number of daughter atoms present today (N-No) p = number of parent atoms remaining today (N)
Some commonly used radioactive elements, their decay products, and currently accepted 1/2 lives:
Source: U.S. Geological Survey
Parent Isotope Stable Daughter Product Currently Accept. 1/2-Life Uranium-238 Lead-206 4.5 billion years Uranium-235 Lead-207 704 million years Thorium-232 Lead-208 14.0 billion years Rubidium-87 Strontium-87 48.8 billion years Potassium-40 Argon-40 1.25 billion years Samarium-147 Neodymium-143 10.6 billion years Based on different groups of rocks over geologic time and radioactive dating, a Geologic Time scale has been developed:
Geologic time scale figure here.
Problem set handed out in class due March 28th in class
So, how old is the Earth? Ancient rocks exceeding 3.5 billion years in age are found on all of Earth's continents. Oldest rocks on Earth found to date = Acasta Gneisses in northwestern Canada (4.03 billion yrs) and the Isua Supracrustal rocks in West Greenland (3.7 to 3.8 billion yrs). These ancient rocks are not from any sort of "primordial crust" but are lava flows and sediments deposited in shallow water, so that Earth history began before these rocks were deposited. In Western Australia, single zircon crystals found in younger sedimentary rocks have radiometric ages 4.3 billion years. The Earth is at least 4.3 billion years old.
The best age for the Earth (4.54 Ga) is based on the Canyon Diablo meteorite. In addition, 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 ages between 4.4 and 4.5 billion years and provide a minimum age for the formation of the moon. The moon formed when a Mars-sized body collided with the primitive Earth. Origin of the atmosphere:
. As Earth accreted, it trapped gases in roughly 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 to accumulate in the atmosphere, but heavy enough to be held by gravity. .However, any early atmosphere would have been burned off by the collision mentioned above (temperatures to as high as 16000oK). The lightest gases were preferentially lost. However, the deeper Earth contained volatile elements in similar proportions to their original source (i.e., same as the sun).
The overall effect of the impact was to alter the mass fractionation of the atmosphere, with lighter elements burned off/lost and heavier gases left behind, but many in solid Earth. After impact: "Runaway Greenhouse" .Under early conditions of high temp. at Earth's surface, main gas = water vapor. . Radiative cooling, water condensed into oceans. . Main source of warmth = sun, which radiated less 4.5 billion years ago, by ~30% . water turned to ice, further cooling. . carbon-containing gases received from carbonaceous chondrite meteorites. . meteorite-related carbon gases caused greenhouse warming.
. Initial high CO2 concentration in atmosphere diminished gradually by chemical weathering and formation of carbonate in the oceans.
. This uptake of CO2 allowed temperature to drop enough that life could evolve. . Eventually, bacteria-like organisms developed, including cyanobacteria (stromatolites). Photosynthesis produced Oxygen. . There is some question as to when life and especially photosynthetic life first evolved. The oldest stromatolites are about 3.45 billion years old, western Australia. Stromatolites are laminated structures
built mainly by cyanobacteria. They are still found today, but were once much more common. They dominated the fossil record between about 1-2 by ago. Today, they are found mainly in saline lakes or hot spring environments. The best example of living stromatolites is at Hamelin Pool, Shark Bay, Western Australia.
The bacteria precipitate or trap and bind layers of sediment to make accretionary structures (domical, conical or complexly branching). Hence, make for excellent fossil record. Range in size from cm to many meters. . Oxygen was used up as oxidation of Fe-S minerals to form banded Fe formations, which are ubiquitous in early (precambrian) rocks.
. Eventually, photosynthesis won out over oxidation, and atmosphere evolved to more oxygen-rich. . multicellular organisms appeared at least as far back as 550 million years ago. . multicellular organisms in the ocean produced shells which took up alot of carbon... eventually forming carbonate sedimentary rocks and organic-rich deposits.
Figure on change in atmosphere goes near here.
. Let's compare Earth to other nearby planets: Gas Early Earth Venus Mars Atmos. Today
CO2 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 -53 Press. 60 1 92 0.006 (bars)
Venus is the second planet from the sun and receives intense solar radiation. Its high CO2 concentration provides for a runaway greenhouse.
Mars is the fourth planet from the sun and receives less solar radiation than
Earth. However, the CO2 in the atmosphere does keep the planet from becoming excessively cold. The atmospheres of Venus and Mars are thought to be little changed over the last 4+ billion years, but Earth has changed primarily because of evolution of life.
Figure on climate changes over geologic time goes here.
A few notes:
Early Earth (Pre-Cambrian) very hot; eventually cooled as runaway greenhouse replaced by more oxygen-containing atmosphere.
In the Late Paleozoic, glacial episodes made for a cold, wet climate. Early Eocene: Considerably warmer and wetter than today.
Late Eocene: Global cooling began, leading to glaciation and the interglacial period we now are in.