Oxygen Isotope Stages (OIS)

Deep sea cores & the analysis of oxygen

isotope levels: 18O and 16O

Based on the trapping of oxygen isotopes in

calcium salts taken from deep-sea

Foraminifera

Record of temperature changes on a large

scale (regional).

More O^16 means warmer climate, b/c of evaporation and rain washing the O^16 back into the ocean.

More O^18 means colder climate

Oceans are not enriched in O18 as O16 returns relatively quickly to the oceans to maintain the balance of 18^O/16^O.

Oceans become enriched in 18^O bc of storage of 16^O in ice sheets

Catastrophism – Cuvier– Found that mammoth bone differed from modern elephants

• He concluded that the mammoth species had gone extinct

– Believed there were multiple periods of mass extinction around the world (30 in total)

– After which new species were “created” to replace the now extinct counterparts

• Preserving the fixity of species ideals

-Lyell

-assumes that the same natural laws and processes that operate in the universe now, have always operated in the universe in the past and apply everywhere in the universe

-Confirming the earths antiquity 4.6 billion years old

-Explains the processes of evolutionary change according with environmental adaptation and extinction

Relative: based on comparisons, does not give a precise age

Absolute: dates that are measured, calibrated

-Aging by features, condition or fossilization

- Often used; the original dating technique

-“Ancient” /“primitive” vs “modern”

-Can be very misleading, incorporating biases

-Ex. Eurocentrism, notions of primitive morphology. Comparisons with Neanderthals overused

-Ex. Associations made between tool types & type of hominid.

-Faunal correlations- presence/absence of species or species abundance may indicate a specific period/phase/date. Sometimes  referred to as index fossils.

-Need to use key species that show changes in their origins, terminations & ranges. Ex. Nestoritherium in China, Hipparion (prehorses), trilobites

-
Ex. Suid (pig) species in Africa important

-Placement in a series of layers that may encompass several localities or geological events. (River terraces, etc.)

-Fossil or site located below or above dated layers.

Tuffs - volcanis ash layer

-Age is determined by some form of counting

• Dates are measured in years before present (BP), must be later tied to calendar years (if need be)

• Dates are recorded as years, +/- one standard deviation (SD); One standard deviation means that there is a 67%

probability that the date is within that range.

• The reliability of the date, and the size of the SD, depends on many factors

• The quality of the sample: is it pure, or contaminated by more recent or older material?

• The age of the sample: bigger SD with older material

• Use of the half-life concept

• Length of time it takes for 1/2 of the existing isotopes in the material to decay, decay happens exponentially

• Particular to each isotope

Ex. 40 grams of material. After one half life--20 grams remain undecayed; after two half lives--10 grams remain of original 40. Exponential reduction.

• Over long periods of time, only extremely small fractions of the original substance may remain to be counted. This causes difficultly in dating old samples

-Used on volcanic materials older than 100k and upto 5m

• 40K (potassium) decays to 40Ar (argon)

• Half-life of 1.3 billion years

• 40Ar/39Ar refinement is new method.

sample is irradiated to convert all the 39K (more common, stable) to 39Ar.

• Easier counting, better with contaminated or smaller samples- can date single grains so multiple volcanic events aren’t mixed.

-Potas. turns into argon gas within the rock, remelt rock to count

Radiometric techniques –Radiocarbon, Potassium/Argon, Uranium series, Fission track

Trapped Electron Techniques -

Electron Spin resonance, Thermoluminesence, Optically stimulated luminesence

Techniques based on radioactivity; the decay of unstable nuclei (isotope decay)-- break into smaller, more stable nuclei

• Count of number of sample decays over a short time in a lab to estimate how much of the original material is left undecayed

• Or, count of actual decay since a fixed point in time (e.g., the death of the organism)

-Tuffs, or volcanic ash layers, in Africa’s Rift Valley are key to the hominin sequence

-Ex. KBS tuff- difficult to identify across long distances, many components.

-Two ages argued-- 2.5 & 1.87; later 1.87 is verified. Many important discoveries referred to as occurring below or above KBS tuff. Sequence-- oldest- Tula Bor 

-Then KBS, then Okote Tuff, then Karari tuff.

-Fossils: 1470 is between Tula Bor & KBS; 406

& 3733 bet. KBS & Okote