First concept of relative dating: principle of primary horizontal nature of sedimentary layers and superposition (Nicolas Steno, 1669). Rate of Earth's Cooling – Based on measuring the cooling of metal spheres and heated rocks, he calculated an age of 75,000 years for the Earth. Sedimentary chronology – from the estimate of sedimentation time, a hill “1,000 toise high” corresponds to 14,000 years of sedimentation.
The "Quaternary" will then be divided into a lower, or Pleistocene, established in 1939 by the English geologist Charles Lyell, and an upper, or Holcene, created by the French Paul Gervais. One century later, publication of the evolutionary theory by Charles Darwin - which suggests that faunal compositions change with time. Period of Saint-Acheul or Acheulean - Period of Moustier or Mousterian - Period of Solutré or Solutrean.
The principle of continuity: the same layer is everywhere and along its entire length it is of the same age, which means it was deposited at the same time. When different layers overlap, the lower layer is the oldest and the upper layer is the youngest.
Principles of Stratigraphy
The principle of paleontological identity: Two layers with the same fossils have the same age.
Dendrochronology
Varves
Paleomagnetism
The magnetic grains are then randomly oriented within the rock and it is their overall magnetization that gives a fair indication of the ambient magnetic field. This magnetization, simultaneous or not simultaneous with the deposition, is acquired by the material orientation of the magnetic grains during the sedimentation: for example, if the sedimentation occurs in a quiet environment, the particle reacts as small magnets on which the Earth's Magnetic Field acts by aligned their magnetic moment in its own direction. The previous magnetic history of the grains is no longer important here, while many other external factors can change the orientation of the magnetic particles – hydrodynamics,.
Archaeomagnetism
Isotopic stratigraphy
Excenticity of terrestrial orbit
Few contracted seasons: Cold summers are not enough to melt the snow and ice accumulated during the winter. Then there is progressive accumulation of snow and ice and progressive increase in the albedo of the earth leading to a cooling.
Excenticity of terrestrial orbit Glacial configuration
Albedo is the measurement that defines the capacity of a body to reflect light energy from a given source such as the sun.
Excenticity of terrestrial orbit Interglacial configuration
Obliquity is the inclination of Earth axis regarding the ecliptic
Terrestrial axis obliquity
Maximum slope: strong polar insolation, opposite seasons Minimum slope: low polar insolation, ice accumulation.
Precession of equinoxes
Comment sont enregistrées les variations climatiques 5 –Chronologie isotopique
Levogyre : deviating from the plane of polarization of light on the left side Dextrogyre : on the right side.
Amino acid racemisation
Obsidian hydratation
An atom is identified by the composition of its nucleus, which means the number of protons Z, called the atomic number, and the number of neutrons N. The nucleus of the atom in question is identified by the symbol X, which are associated with A and Z, as written by convention. All nuclei of a given element X have the same number of protons Z, but may have different numbers of neutrons N (and thus mass number A).
Isotopes and radioactivity
Alpha emission: the nucleus emits an alpha particle (He nucleus, 2 protons, two neutrons): both the atomic number and that of the nucleus decrease by two.
Beta ray
The radiocarbone (carbone 14)
In addition to this small proportion of 14C, we can calculate that there are 13.56 disintegrations per minute per gram (dpmg) of carbon in each organic matter of which it is composed. The calculation of the 14C date is then the calculation of the date when the system closed, meaning that there are no more exchanges between the carbon gas and the individual or material. It then calculates the ratio between the number of radioactive carbon at the beginning (N0), at the death of the animal, and the current number of radioactive carbon (Nt) at time t.
But this method is not possible for radiocarbon, since the resulting stable isotope from 14C decay is 14N, which is too common in nature so that a very small variation in its amount cannot be measured in the material to be dated. This sample mass should be the largest to increase the number of counterfeits detected over a relatively long interval (2 to 3 days).. gt; then the method is very drastic. Then, to separate them, it is necessary to accelerate the ions much more than we want to sift.
Thus, we can calculate a 14C age for any carbonate that is sufficiently recent (less than 50,000 years) that Nt is measurable. The essential condition for calculating this age is that N0, i.e. the amount of radioactive carbon in the atmosphere, has always been constant.
BUT…
In principle, the production of 14C in the upper atmosphere depends on the flux of cosmic neutrons (most of which are emitted by the Sun). The concentration of 14C in the atmosphere also depends on changes in carbon stocks on Earth (e.g. oceans) due to climate change.
Calendar date AD
14C in the atmosphere over time have to correct the conventional 14C ages, to calibrate directly to the subscribers in ages obtained by another method.
How do we calibrate 14 C dates ?
Uranium series method
Fission tracks
Fission is the spontaneous splitting of a large nucleus into two fragments of a large nucleus (in nature: 235U). A heavy and charged particle leaves an area of linear damage during its trajectory in a non-conducting solid: The Latent Tracks (Young, 1958). Their size does not exceed 15 μm. - Have a dangerous distribution. - Their size depends on the VT/VG ratio characteristic of the solid.
The accumulation of traces of spontaneous fission in a mineral follows the general law of radioactive decay: d[N] = - λ [N] dt. Where is the average number (dN) of atoms (daughter element. Here spontaneous fissions) that decay in a time interval (dt). The number of spontaneous fissions of U-238 recorded in a mineral or glass at a given time.
Uranium concentration determined from the number of U-235 fission tracks induced by thermal neutron irradiation. Pi : Number of induced fission tracks of thermal neutrons φ : Flux of thermal neutrons (neutrons/cm2). Pi : Number of induced fission tracks of thermal neutrons φ : Flux of thermal neutrons (neutrons/cm2).
