Sea Level Lecture 5- Reconstructing sea-level change Flashcards
What are potential issues with instrumental records?
Biased both spatially and temporally
Only a very limited snapshot of behaviour
Are instrumental records most precise and accurate?
Yes
What are challenges with reconstructing sea-level change?
Consistent framework is needed
- Very few sea-level indicators occur exactly at mean tide level
- Need to understand modern distribution with respect to tidal levels
- Concept known as the ‘indicative meaning’
Can we only reconstruct relative sea level?
Yes
How can we work out RSL in terms of reconstruction?
RSL = sample elevation - RWL
Sample elevation - sampling error as never know the core top and depth perfectly
RWL- indicator error as sea-level indicators form over a range of elevations not just at one (indicative range)
What information is needed to reconstruct sea level change?
Age
Location
Elevation (height of where sample found today and relationship of the indicator to a tidal level - indicative meaning)
Tendency of change and abruptness of change can be useful
What is a sea level proxy called and what must it have?
Sea-level indicator
Must have a systematic and quantifiable relationship to tides
What are the 4 types of sea-level indicators?
Physical
Biological
Archaeological
Geochemical
What is an example of a raised shoreline?
Forth valley
Discuss the surveying of palaeoshorelines?
In the field, relict shorelines are surveyed along their entire length to establish altitude (mOD) and also gradient
The gradient and orientation of the shoreline relates to the amount of isostatic uplift that has occurred
The features can then be plotted and correlated with other similar features
The age can then be established using radiocarbon or luminescence methods
What is an example of a physical sea level indicator?
Surveying of palaeoshorelines
What is an example of a biological sea level indicator?
Sedimentary evidence
Discuss sedimentary evidence.
Undertake detailed stratigraphic analyses using coring methods
Survey and map the area
Collect samples and undertake detailed analysis in the laboratory
Often uses microfossils
Typically dated using radiocarbon or other isotopic methods
Corals can be used
Discuss geochemical indicators
Bulk sediment ->
Sediment fraction ->
Compund specific
What are the three main groups of archaeological dating methods?
Age estimation techniques
Age equivalent techniques
Relative chronology
Discuss age estimation techniques.
Radiometric methods - based on rate of decay such as radiocarbon dating, uranium series dating
Incremental methods based on growth of an organism such as varves or dendrochronology
Discuss age equivalent techniques
Dates extended to similar distinctive horizons elsewhere such as pollution, tephra and stratigraphy
Discuss relative chronology.
Chemical reactions of weathering and organic decomposition that are time dependent
Relative dates estimated from the state of rock/sediment diagenesis
What is radiocarbon (14C) dating?
Developed by Libby (1949)
Least common of three isotopes - 12C, 13C and 14C
Half life of 5730 years
Used to date very recent events
What is the 14C principle?
Organisms take up 14C while they are alive, in a known proportion to other isotopes
When organism dies 14C is lost through radioactive decay
Time since death can be determined by 14C content
What can 14C be used to date?
Can be used to date wood, charcoal, bones, shells and pottery
Oldest dates around 50000 years
Calibration required as rate of production hasn’t been constant
Ages relative to 1950 AD
What are sources of error in 14C dating?
Problems of sample selection and contamination
Young carbon effects
Old carbon effects
Variation in 14C content in the ocean reservoir
Fractionation effects
Radiocarbon plateau
What are the principles of luminescence dating?
Light emitted from a mineral crystal (usually quarts or feldspars) when exposed to heat or light
The light emitted is related to the amount of ionizing radiation that the sample has been exposed to from sediment
The clock is set to zero by heating or optical bleaching
Wind and water transported sediments are good for luminescence dating
What are continuous cores?
Geologic tide gauges
