The Geologic Time Frame

Fission-Track Dating

-"Tracks" formed by spontaneous fission of Uranium-238 in naturally occurring glasses and minerals -Tracks are annealed (erased) when a substance is heated to a high temperature, and they reform when it cools -Age of the last cooling event can be determined from the ratio between the density of tracks and the amount of U-238 a sample contains -Applicable to volcanic extrusives (lava, ash)

Hipparion

-3 toed horse -Key time marker for biostratigraphy -Evolved in North America 15.5 Ma, spread through Eurasia 12-11 Ma, dispersed through Africa 10.5 Ma -Together with the pig taxa, was crucial in demonstrating a large numerical (radiopotassium) dating discrepancy between the Koobi Fora and the Lower Omo sites

Varve

-A varve is a distinctive band of sediment, often made up of two subbands, that is laid down each year on the floor of a lake or other relatively calm body of water -The coarse particles settle out first, forming the lower subband of a varge, the finer particles settle out later, forming the upper subband -The floor must be anoxic so that the sediments are not disturbed by burrowing organisms -Most useful varves are ones that form in quiet glacier-fed lakes -Varves of known age have also been used to calibrate the C-14 dating, though they are less useful for this purpose than tree rings

Electron Spin Resonance

-Applied mainly to dental enamel where formation produced initially empty traps

Optically Simulated Luminescence

-Applied mainly to quartz sand grains in which the traps were emptied by exposure to sunlight -Relies on intense light to release and measure trapped electrons

Thermoluminescence

-Applied mostly to potsherds and occasional flint artifacts in which the crystalline traps were emptied by intense heat before burial -Relies on intense heat to release and measure trapped electrons

Isotope Half-Life

-Average amount of time necessary for half the radioactive atoms in a sample to decay

Elephas Recki

-Based on molar change, can be used to bracket sites within an average range of about 500 ky

Problems with Uranium Series Dating

-Some of the intermediate products decay too rapidly to be useful for dating geologic events; those with longer half-lives tend to decay at rates about equal to the rates at which they are produced (which means they can be used for dating only when they are transferred from the system in which they were produced to another, where their removal or introduction sets the clock to zero.) -Decay Products in Water Include: Th-230, Pa-231 -Terrestrial erosion may introduce uranium to ancient lake or sea deposits and then produced some of the Th-230 and Pa-231 -The rate at which Th-230 and Pa-231 settle out need not remain constant over time

Problems with Electron Spin Resonance

-Teeth tend to absorb large quantities of uranium after burial

3 Key Physical Findings of Carbon-14 Dating

1. Cosmic ray bombardment transforms N-14 into C-14 in the upper atmosphere 2. For every atom of C-14 in the atmosphere, there are about one trillion atoms of C-12 3. C-14 is radioactive, and C-12 is not

4 Assumptions of Carbon-14 Dating

1. The atmospheric ratio between C-14 and C-12 has remained constant throughout time 2. Atmospheric C-14 and C-12 are equally likely to be oxidized into carbon dioxide 3. CO2 mixes rapidly in the atmosphere, so that the C-14/C-12 ratio is about the same every where 4. Most organisms do not discriminate between C-14 and C-12 when they build their tissues

Advantages of Using Fossils vs. Artifacts for Biostratigraphic Dating

1.) Can be used to date sites were artifacts do not occur or that formed before artifacts were made 2.) With regards to earlier stages of human evolution, they often define stratigraphic units that cover larger areas and shorter timespans than do units defined by artifacts

Assumptions of C-14 Dating

1st: The C-14/C-12 ratio in the atmosphere has varied through time due to the Industrial revolution (which reduced the ratio) and nuclear explosions (which increased the ratio) 2nd: a sample obtained all its carbon from the atmosphere; some mollusks and water plants are known to assimilate carbon from old rocks, thus they will be enriched in C-12 relative to the atmosphere

Most Important Isotopic Dating Techniques

According to the paleoanthropological perspective are: -Radiopotassium (K/Ar) -Fission-Track Method -The Uranium-Series Method(s) -The Radiocarbon Method

Cementum

Calcified tissue that forms on the surface of tooth roots -Along with high-crowned molars, allowed more efficient feeding on grasses (for microtines)

Shifts in Earth's Polarity (Geomagnetic Stratigraphy)

Occur clearly on 2 scales: -Long periods characterized by essentially the same polarity are punctuated by much shorter intervals of opposite polarity -The long periods, lasting hundreds of thousands or even millions of years, are called polarity chrons (formerly epochs) -The shorter intervals, lasting no more than a few tends of thousands of years, are known as polarity subchrons (formerly events) -Ancient polarity can be detected most readily in volcanic rocks and in fine-grained sediments that settled into place relatively slowly (Ex: ocean floor) -Affect the entire globe simultaneously, and given appropriate deposits, it should be detectable everywhere (which makes it attractive for defining boundaries between time periods) -Useful for bracketing sites in time that cannot be dated more directly

Why is the C-14/C-12 ratio in animals closely related to that of the atmosphere?

Since most plants obtain all their carbon from atmospheric CO2, and since most animals obtain their carbon directly or indirectly from plants, the C-14/C-12 ratio in plants and animals closely approximates the ratio in the atmosphere

Mammoths

Successive species or subspecies of mammoths tended to evolve more complex molars: -Enamel that surrounded individual molar plates became thinner -Plates became more tightly packed -The number of plates per molar increased through time *Thus, mean enamel thickness and the mean number of plates can often date mammoth-rich sites relative to each other

Amino Acid Racemization

The amino acids that make up proteins can exist in two mirror-image molecular forms: L (left-handed) and D (right-handed). In general, only L-amino acids occur in the tissues of living animals, but they are converted after death to D-amino acids until an equilibrium 50/50 mix of L and D forms is reached -The ratio of L-to-D conversion depends on ambient temperature, moisture, and pH, and it differs for each amino acid -The reaction that produces the conversion is known as racemization (or epimerization) -If the postmortem temperature and moisture history of a specimen are known, the D/L ratio for a particular amino acid is a measure of the time since death

Dendochronology

Tree ring dating

Seashells

With carbonates, there is virtually no (insoluble) Th-230 or Pa-231, which means that activity ratios in a shell should therefore reflect the time elapsed since the shell formed

Uranium Series Dating

-Based on the parent isotopes: U-238, U-235, Th-232; all 3 decay through a series of intermediate radioisotopes to stable isotopes of lead -Depends on the high solubility of uranium in water -May be used to correct the radiocarbon timescale to 23.5 ka and beyond

Problems with Ca-40/K-40 in Radiopotassium Dating

-Ca-40 cannot be distinguished from the original Ca-40, which is abundant in nature

Mimomys, Arvicola, & Microtus

-Can be used to construct biostratigraphies that are directly relevant to human evolution and prehistory

Major Importance of Fission-Track Dating

-Checking radiopotassium dates at the important east African sites of Olduvai Gorge, Koobi Fora, Middle Awash, and Hadar

Red Deer

-Close relative of the American elk -Simple double-pointed terminal fork evolved into a multipronged crown -Crowned antler tips indicate an age of less than 500 ka for the Swanscombe site in England -Imply a time before 500 ka for the well-known Ambrona and Toralba hand ax (Acheulean) sites in Spain

Most Useful Fossils (for Biostratigraphic Purposes)

-Come from taxa that: spread quickly and widely, appear to have died out over larger areas at about the same time, or taxa that were evolving rapidly so that their stage of evolutionary development is a clue to their relative age Most productive biostratigraphy developed: -Ex: Microtine rodents in Eurasia -Ex: Elephants, pigs, and horses in Africa

Microtus

-Common vole -Associated increase in the number of triangular prisms or angles -Perhaps evolved in Asia from a species of Mimomys and then migrated to Europe

Problems with Paleomagnetism

-Complications can be introduced by secondary heating of a volcanic rock and by postdepositional chemical processes that alter the behavior of ferromagnetic particles

Problems with C-14 Dating

-Degree of underestimation grows with time -The greatest discrepancy occurs before 20ka, when C-14 ages are 3,000-3,500 years younger than calendar ages

Problems with Fission-Track Dating

-Depending on the substance, some track loss can occur at low temperatures -Particularly likely in natural glasses, therefore tend to produce ages that are "too young" -If there is too much U-238, the tracks are too closely packed for counting -Used less than radiopotassium because it is more tedious

Isotopic (Radiometic) Mehods

-Developed in the 1940s and 1950s -In the 1960s, first radiopatassium dates disproved the guesstimate that the timespan for human evolution was the last million years or so

Why did Amino Acid Racemization fall out of favor?

-Due in part to the erroneously ancient dates it provided for several prehistoric human skeletons from California -Can provide mistaken ages when foreign amino acids contaminate a buried bone, and the application of AAR to bone has been largely abandoned

"Messinian Salinity Crisis"

-During a cold period, the Mediterranean periodically dried up, leaving behind vast salt deposits -Repeated glacier growth could have been responsible for this if it lowered sea level below the sill that separates the Atlantic and the Mediterranean, but tectonic (upward) displacement of the sill may have played the main role

Hydroxyproline

-Especially sought for C-14 dating because it is restricted to bone and does not occur in plants, soil microorganisms, or other likely contaminants

One-Toed Horses

-Evolved in North America about 3.7 Ma, dispersed through Eurasia around 2.5 Ma, reached Africa shortly before 2 Ma -Overlapped with 33 toed species for 1.2-1.3 my

Biostratigraphic Dating

-Fossils can be used only when the biostratigraphy of a region has been worked out in advance

Cooling Events

-Generally associated with a decline in atmospheric CO2 -Debate over whether and when CO2 was mainly cause or consequence

Microtine Biostratigraphy in Europe

-Includes voles, the lemmings, and their relatives -Branch of Cricetids (hamsters), characterized by high-crowned molars -Heartland of evolution in Asia -Most successful rodent group in the northern hemisphere

The Biostratigraphic Implication of Large Eurasian Mammals

-Less helpful for biostratigraphy than small ones because their remains are less abundant and they do not permit such fine stratigraphic subdivision -Define 2 stages: the late Villafranchian and the succeeding Galerian

Extraordinarily Long Half-Life of K-40

-Means that the radiopotassium method has no practical maximum limit (it can be used to estimate the age of the earth) -In most cases, it cannot be used to date rocks younger than a few hundred thousand years old because they contain too little Ar-40

Lemmus & Dicrostonyx

-Microtines -Both migrated to Europe during periods of exceptionally cold climate, among the clearest biological evidence for repeated glaciation

The Biostratigraphy of Elephants, Pigs, and Horses in Africa

-Microtines play no role in African biostratigraphy because they only occur in Eurasia and North America -All 3 were characterized by important first and last appearances, and also by lineages for which directional evolutionary trends have been established -Basis for dating many important African sites relative to each other, and cross-checked the validity of absolute dates

"Astronomical Theory"

-Milutin Milankovic -Suggested that glaciation began at times when Northern Hemisphere summers became relatively cool -Insolation (solar heat) reaching high latitudes in summer oscillates regularly in correspondence with: the tilt of the Earth's axis, the the wobble of the earth's spin axis (the precession of the equinoxes), and the shape (eccentricity) of the earth's orbit

Arvicola

-Modern water vole -Evolution of rootless molar is used to define the appearance of Arvicola -Increases in crown height and cementum packing useful for characterizing successive stages in its evolution from it's immediate ancestor, Mimomys -Successive evolutionary stages are recognized primarily by a tendency for the enamel to become thinner on the rear surfaces and thicker on the fore surfaces of the molar prisms

C-14/C-12 Fluctuation

-Most ancient fluctuations probably reflect past variation in the strength of the earth's magnetic field and the sun's electromagnetic field, both of which deflect cosmic rays -Paired C-14 and tree ring dates allow the radiocarbon chronology to be corrected over the past 12,400 calendar years

Nonradiometic Numerical Dating Methods

-Most important are tree dating (dendochronology) and varve dating -Highly accurate, but can be applied only in certain restricted geographic regions -Cover only the past 8,000-10,000 years -Additional numerical dating method: amino acid racemization

Charcoal

-Most useful for C-14 dating because it's less likely to be contaminated and skew the dating -Inorganic components of shell and bone are generally not useful because they tend to exchange carbon with their burial environment

Horses

-Most useful with regards to first appearances since major events in horse evolution took place outside Africa and immigrant taxa then dispersed rapidly through much of the continent

Paleomagnetism

-Paleomagnetic reversals recorded in volcanic extrusives and nonvolcanic sediments -Can be used to provide rough dates at fossil or archeological where direct radiopotassium dating is impossible

Cenozoic Climate Change

-Probably most important in the naturally selectively forces that drove human evolution -Early Cenozoic: global climates were relatively warm, wet, and nonseasonal, and there was little difference between the equator and poles; glaciers and sea ice barely existed; forests were the dominant vegetation type -World temperatures rose slightly during the paleocene to a peak in the early Eocene before sporadically declining

Glacial Periods

-Progressive appearance of distinctly cold-adapted or cold-resistant species -The recurrent appearance and disappearance of these in midlatitude Europe counts as some of the strongest evidence for glacial/interglacial alternation after 300 ka

Continental Drift

-Redirected oceanic currents which transfer heat between latitudes -Promoted mountain building, which altered atmospheric circulation

General Features of Isotopic Dating

-Rely on the decay of naturally occurring radioactive elements or isotopes -Radioactive atoms are transformed from one isotope to another (not necessarily of the same element) -Ex: Carbon-14 decays to Nitrogen-14 -For each isotope, rate of decay is constant and unaffected by ordinary environmental variables such as temperature and humidity -Isotopes decay rate expressed as its half-life

Why is Ar-40/K-40 useful in Radiopotassium Dating?

-Rocks heated to a very high temperature tend to lose any original argon they contain, when they cool, radiogenic argon begins to accumulate again -Ar-40/K-40 in a rock is a direct function of the time elapsed since the rock cooled

2 Kinds of Traps for Luminescence Methods

-Shallow ones from which the electrons are easily released --Optically Simulated Luminescence is better than Thermoluminescence for shallow traps -Deeper ones from which they escape less readily, more likely to provide a date that is "too old" because they are less likely to have been fully emptied before burial

Problems with Shell Dates

-Shells tend to absorb uranium from their burial environment (bones commonly do too) -Different bones in the same layer can absorb different amounts of uranium, resulting in different parts of large bones producing different dates

The Luminescence Methods and Electron Spin Resonance

-The aggregate number of trapped electrons can be measured, and the rate at which they accumulated can be estimated from the level of background radioactivity to which a substance was exposed -Apply primarily to objects where heat or light emptied the traps before burial -Applicable to materials that are a few thousand and a few hundred thousand years old

Ar-40/Ar-39 Dating

-The amount of resulting Ar-39 is directly proportional to K-40, and it can be measured more precisely -Can be used to measure K-40/Ar-40 ratio in extremely small samples (including single grains of volcanic origin -Relies on lasers for the intense heat that is required to release argon from an irradiated sample -Not useful for samples older than 50ky because very young samples usually contain too little radiogenic Ar-40

Holocene

-The most recent interglacial -Might better be known as the Present Interglacial

Optically Simulated Luminescence vs. Thermoluminescence

-Thermoluminescence is more problematic with regard to sand because it generally requires many thousands of grains for a single date; this advantage makes optically simulated luminescence the method of choice

Mechanisms that Drove Late Cenozoic Cold/Warm Alternation

-Vulcanism may have played an occasional role --If the climate change extinguished many human populations, it could account for the lack of genetic diversity in people today

Issues with Radiopotassium Dating

-Works best on rocks (or rather minerals within rocks) -Some minerals tend to lose their argon under physical or chemical stress, independent of heating -Secondary heating may cause some, but not all radiogenic argon to escape

Numeric Dating Methods

-a.k.a. "Absolute" dating methods

Radiocarbon Dating

-a.k.a. Carbon-14 Dating -Most celebrated of all isotopic techniques because of its widespread application in archeology

Paleomagnetism

-a.k.a. Paleomagnetic Stratigraphy -Past fluctuations in the intensity and direction of the earth's magnetic field -Generally much less precise than those of other numerical dating methods -A cross between numerical and relative dating -Most important changes are in polarity; compass would sometimes point north or south

Paleomagnetism

-a.k.a. Paleomagnetic stratigraphy -A nonisotopic numerical method

Radiopotassium Dating

-a.k.a. Potassium/Argon Dating -K-40 makes up 0.01% of naturally occurring potassium in rocks -K-40 decays into Ar-40 and Ca-40 in a known ratio -Either Ca-40/K-40 or the Ar-40/K-40 could be used for dating -Generally requires high temperatures to set the clock (daughter/parent ratio) to zero -Its use is restricted mainly to volcanic lava and ash falls

How to Obtain an Isotopic Date

1. The object to be dated must contain a radioactive isotope with a known half-life 2. At the time of formation, the object should in general have contained only the radioactive "parent" and none of the "daughter" into which it decays (daughter: parent ratio should be 0:1) 3. Must be possible to measure the amount of parent and daughter in the object today (establish the modern daughter/parent ratio) -If these conditions are met, the half-life of the parent may be used to calculate the time that has elapsed since the daughter/parent ratio was zero

Most Important Application of U-series Dating

From a paleoanthropological perspective: -Atapuerca (Sima de los Huesos) -Neandertal and modern human lines had diverged by 500 ka

4 Main Glaciations

From younger to older: -Gunz -Mindel -Riss -Wurm