2 Fossil: any preserved trace left by an organism E.g. footprints, burrows, faeces, bones shells, teeth, impressions of an organismArtefacts: objects deliberately made by humansE.g stone tools, beads, carvings, charcoal from cooking, cave paintingsFossil FormationGenerally organisms are decayed by other micro organismsOrganisms may be fossilized if it’s buried rapidly by drifting sand, mud deposited by rivers, volcanic ash, and intentional burial.Wet, acidic soil minerals in bone are dissolvedNo O2, (peat) preservation of soft tissue and bones may occur.Alkaline soils are ideal petrification occurs: new minerals (lime/iron oxide) are deposited in pores of bone, replacing organic matter becoming a rock.
3 Four conditions are required for fossilization to occur: Rapid burialPresence of hard body partsAbsence of decay organismsLong period of stability
4 Fossil Discoverysometimes found by chance(due to erosion) but generally result of excavationsurface discoveries often lead to excavationsSteps:Area surveyed and sectionedSmall hand tools to gently remove soilRemoved soil is sievedPhotographs taken at every stageItems retrieved are labeled and cataloguedIn lab, fossils are scraped clean and repairedPlaster casts/latex moulds are made
5 Fossil Dating Absolute dating: actual age of specimen in years Relative dating: age relative to other fossils (older or younger)BP: Before Present
6 Absolute Dating Potassium-Argon Dating Dating of a rock sample (assuming there is a fossil in/near the rock of the same age)Decay of K Ca + ArIsotopes of Potassium are K-39, k-40, K-41. (diff number of neutrons, same protons)K-40 is radioactive and decays into Ar-40 + Ca-40.Decay is slow but constantComparing K-40 to Ar-40 can provide an age for a rock.Used to date rocks 100,000 years and aboveDates before 100,000 produce to little Ar-40 (0.0053%)Half life: time taken for any given quantity of a radioactive substance to halfK Ar Half life = 1.25 x 109 years. (1,250,000,000)
7 Absolute Dating Carbon-14 / Radiocarbon Dating Dating of actual organic sample (must contain Carbon)Requires minimum of 3g organic materialUsed on samples up to 60,000 years oldDecay of radioisotope C-14 into NitrogenC-14 is produced in upper atmosphere by cosmic radiation converting it from Nitrogen.In atmosphere ratio of C-14 to C-12 is 1: 1012 (one trillion)Green plants use atmospheric CO2 to photosynthesise, ratio is maintainedWhen animal eats plant, ratio is maintainedAnimal dies, C-14 continues to decayComparing C-14 to C-12 can provide an age for a sample.
9 Absolute Dating Carbon-14 / Radiocarbon Dating Half life: time taken for any given quantity of a radioactive substance to halfRatio between C-14 to C-12 is 1:1012Ratio decline to 0.5:1012 after years.Ratio of N-14 to C-14 in atmosphere appears to fluctuate so results may vary
10 Absolute DatingAccelerator Mass Spectrometry (AMS) Radio carbon DatingUsed on samples as small as 100 micrograms.Breaks sample into atoms so they can be individually accounted for.E.g. cave paintings
11 Absolute Dating Dendrochronology (Tree ring Dating) Each ring on the surface of a cut tree trunk represents one years growth (due to seasonal changes)Width of ring demonstrates how favourable the period of growth was.Living trees can be dated by drilling a small core from the trunk and counting the ringsMarker rings: rings which reveal years of exceptional growthCorrelation of marker rings allows different samples to be compared and dated.E.g Bristle cone pine (US): living trees dated at over 4500 years oldCorrelation of marker rings in dead pines date them back as early as 8600 yearsUsed up to 9000 years.
12 Absolute DatingDendrochronology (Tree ring Dating)
13 Relative Dating Stratigraphy Study of layers (strata) Principle of superposition: layers at the top are younger than those at the bottomDistortions of Earth’s crust may occur turn layers upside downSpecimens may have been deliberately buried (younger than surrounding Earth)Correlation of rock strata: matching layers from different areasIndex fossils: fossils which were widely distributed and only existed for a brief period of time
15 Relative Dating Fossilised pollen grains Indicates amount and type of vegetation existing at specific time.Can be used to support or refute relative dates from other sourcesFluorine DatingWhen bone is left in soil, F- from water, diffuse into bone. (depends on original F- level in soil)Older fossils will contain more Fluorine ionsFossils from same specimen should have similar Fluorine levels.