Establishing Timing in the Landscapes:

Weathering often affects only the surface of a rock and a cross-section will expose a very different material inside. Weathering affects seismic velocity of clasts. More weathering = slower velocity -- therefore older rock

Thin slabs or concentric sheets can peel off of this granite during the weathering process known as exfoliation or spalling.

This obsidian cobble has a frosted surface due to weathering, except where the large chip was knocked off. Thickness of rinds can be used as an age indicator.

Secondary carbonate accumulation in the soil profile is primarily due to calcium carbonate supplied by airborne dust, dissolved in infiltrating rainwater, and precipitated in the soil.

 Pedogenic carbonate horizons typically are approximately parallel to the land surface, their upper boundaries are within the range of the depth of wetting, and have distinct morphology.  The age of the geomorphic surface is related to the age of the carbonate horizon.  Gravelly soils can develop significant carbonate accumulation and reduced permeability within 10,000 years.

Maximum diameter of lichens can be used as an age indicator.

 Dendrochronology  Counting and measuring the widths of the annual rings.  Good growing seasons produce more growth and thicker rings, while thin rings occur in less favorable seasons.  The inner portion of a growth ring is formed early in the growing season, when growth is comparatively rapid (hence the wood is less dense) and is known as "early wood" or "spring wood".  The outer portion is the "late wood" (and has sometimes been termed "summer wood", often being produced in the summer, though sometimes in the autumn) and is more dense.

1. The species studied must only produce one ring per growing season or year. 2. Only one dominant environmental factor can be the cause of hindered or increased growth. 3. The dominant environmental factor should vary each year so we can see the changes clearly in every ring. 4. And lastly, the environmental factor must affect a small or large geographic area.

Dendrochronolgy has been useful for calibration of 14 C ages with calendar ages. Trees growing at latituded with seasonal variation in temperature will produce distinct growth rings during the spring-summer (light color) and fall- winter dark color). The bristlecone pine record in the White Mountains, CA has been extended back >10,000 years.

 The width of tree rings depends on their growth rate - in particularly bad years, they may not generate any rings.  Tree ring cores taken from 65 trees along the fault in Wrightwood, Calif.  30 trees in the Wrightwood area but not on the fault  Dramatic and extended' growth suppression in trees along the fault beginning in 1813  Something had happened between the 1812 and 1813 growth seasons.  1812 EQ on SAF Victim of the 1857 Fort Tejon earthquake on the San Andreas fault, this tree near Wrightwood had it's top snapped off, causing lower branches to grow vertically.

Radiocarbon dating Naturally occurring isotope carbon-14 ( 14 C) to determine the age of carbonaceous materials up to about 50,000 years Raw (uncalibrated) radiocarbon ages are usually reported in radiocarbon years "Before Present" (BP) "Present" IS defined as AD Such raw ages can be calibrated to give calendar dates.

14 C ----> 14 N 5730 year ½ life Useful between 100 and about 50,000 years old Can date things that contain organic carbon (Used to be living): bones, shells, wood, charcoal, plants, paper, cloth, pollen, seeds) Radiocarbon methods

Radiocarbon dating The radiocarbon clock is based on the known decay rate of the unstable isotope of carbon, 14 C, which is formed when cosmic rays interact with nitrogen in the atmosphere. The radiocarbon combines with oxygen to form a radioactive form of carbon dioxide. When a living organism dies, the carbon exchange stops. Measuring the 14 C concentration in organic samples, and provided they have not been contaminated by younger material, one can calculate the time elapsed since the material was originally formed.

Radiocarbon dating A raw BP date cannot be used directly as a calendar date, because the level of atmospheric 14 C is not constant in the past 50Ka. The level is affected by variations in the cosmic ray intensity which is affected by variations in the earth's magnetosphere caused by solar storms. The level has also been affected by human activities, it was changed during atomic bomb tests in the 1950s and 1960s.

Radiocarbon dating Raw radiocarbon dates, in BP years, are calibrated to give calendar dates. Comparison of radiocarbon dates of samples that can be independently dated by other methods such as examination of tree growth rings, ice cores, deep ocean sediment cores, lake sediment varves, & coral samples.

Effect of sea level rise on coral reefs. The coral in the first diagram is growing 5-7 m below sea level. As sea level rises, the coral dies and a new, younger coral grows 5-7 m below the new sea level. U-Th methods U/Th dating requires distinguishing between a sample’s radiogenic 230Th (produced by in situ 238 U decay) and its non-radiogenic 230 Th (derived from the surrounding environment). Levels of non-radiogenic 230 Th (230 Th) are small or negligible

 X-rayed thin slab reveals a clear record of annual growth bands expanding radially outward (from left to right) at about a cm per year.  The Highest Level of Survival (HLS) of the coral during the past 35 years is recorded in the topography of the coral's upper surface.  The arrows track the rise of sea level in the 1960s and its subsequent fall. U-Th methods Cross-section of a coral microatoll.

Life requires a certain composition and shape of amino acid molecules in order to complete their function. Living organisms on earth keep their amino acids in the L position, with a notable exception found in certain bacterial cell walls, and their sugars in the D position. When the organism dies, control ceases, and the ratio of D/L moves slowly toward equilibrium (racemic). Measuring the ratio of D/L of a sample can allow calculations of how long ago the specimen died. Amino acid racemization

 The rate at which racemization proceeds depends upon the type of amino acid, average temperature, humidity, acidity, alkalinity, and enclosing matrix.  Also, D/L concentration thresholds appear to occur as sudden decreases in the rate of racemization.  These effects restrict amino acid chronologies to materials with known environmental histories and/or relative intercomparisons with other dating methods.

 Measures the energy of photons being released.  In natural settings, ionizing radiation (U, Th, Rb, & K ) is absorbed and stored by sediments in the crystal lattice.  This stored radiation dose can be evicted with stimulation and released as luminescence. Luminescence dating

The calculated age is the time since the last exposure to sunlight or intense heat. The sunlight bleaches away the luminescence signal and resets the time 'clock'. As time passes, the luminescence signal increases through exposure to the ionizing radiation and cosmic rays. Luminescence dating

The intensity of the luminescence is calibrated in the laboratory to yield an equivalent dose, which is divided by an estimate of the radioactivity that the sample received during burial (dose rate, Dr) to render a luminescence age.

The selection and sampling of sediment is crucial. A luminescence age is a measure of the time since the last sunlight exposure (or heating) event of the sediment. The reduction in luminescence, usually by sunlight, must be related to an event (e.g. fault- generated colluvium) for a luminescence age to be meaningful. The luminescence dating of sediment that was exposed to little or no light during a specific event will yield a spurious age.

The preferred sediment for OSL dating has had > 1 hr sunlight exposure, accumulated as relatively homogeneous stratigraphic unit, >30 cm thick and has not undergone significant water-content variations or diagenetic changes during burial. In many sedimentary environments, particularly eolian and littoral, coarse- grained ( µm) quartz grains is best; it is an abundant particle size that is easily solar reset. In other sedimentary settings the fine-grained (4- 11 µm) polymineral or quartz fraction is best, particularly for loess, colluvial, eusturine, and many lacustrine and fluvial environments.

 Cosmogenic isotopes are created when elements in the atmosphere or earth are bombarded by high energy particles that penetrate into the atmosphere from outer space.  Some cosmic ray particles reach the surface of the earth and contribute to the natural background radiation environment. Cosmogenic Surface Exposure Ages

 Cosmic ray interaction with silica and oxygen in quartz produced measurable amounts of the isotopes Beryllium-10 and Aluminium- 26.  The accumulation of these isotopes within a rock surface could be used to establish how long that surface was exposed to the atmosphere. Cosmogenic Surface Exposure Ages

With constant rate of production, the number of atoms of Be-10 and Al-26 that accumulate in a rock surface will be proportional to the length of time the rocks were exposed to cosmic ray bombardment. The amount of each nuclide would be an estimate of the minimum time that the particular surface had been exposed.

Cosmogenic Surface Exposure Ages  Rocks exposed to cosmic rays contains “exotic” short-lived isotopes.  Only rocks near the surface (upper few meters) effected.  The older the surface, the higher the concentrations of CRN isotopes.  CRN’s produced in quartz grains by cosmic-ray bombardment of Si, O nuclei  Production rate variable with altitude, latitude  Cosmic-ray flux decreases exponentially with depth below the surface.  If a previously exposed surface is buried, nuclide production ceases.