1 Flashcards
What is the Quaternary?
The most recent period of the geological record extending to the present. It is climatically, sedimentologically and ecologically distinct from earlier periods, and has had variable warm and cold cycles.
When did the Quaternary start?
It is internationally recognised as starting at 2.588 +/- 0.005 million years ago. (Gibbard et al. 2007).
What is the Holocene?
The current warm period within which major human development and modification of the Earth has taken place. (Last 10,000 years of Quaternary).
Evolution of Ideas of Earth’s Past Environment
James Ussher (16th Century) concludes Noah's flood created the Earth we see today. Lyell (18th Century) Argued observable geological processes could explain geographical history. Hutton (1726-97) observed rocks are constantly renewed and recycled, envisaged there was no begging or end, everything is explainable by processes currently operating on Earth. (Uniformitarian Theory).
How do we reconstruct past events (simple)
We need to find evidence, understand these and their distribution to reconstruct the past.
Huttons Maxim
The present is the key to the past.
What is ‘proxy data’ or ‘proxy record’
An indirect measurement of climate or environmental evidence.
What is a modern analogue?
This is how you unlock a proxy record by looking at modern processes or distributions etc.
‘When knowledge of a current system/process is used to interpret proxy data for past conditions, we are treating the current system/process as an analogue.
What does the use of a modern analogue assume?
- The same processes occur today in the same way as they did in the past.
- Through the quaternary, plant / animals haven’t evolved/adapted to different conditions.
- ‘all other things being equal’ e.g. assuming tree rings are dominated by climate cycle, not squirrel population.
Key assumptions of Uniformitarianism (these underpin the use of proxies and analogues)
- Controlling environmental factors are known (if beetle is temp sensitive or veg sensitive).
- Ecological affinities remain constant through time (dung beetle always ate dung)
- Populations past and present are/were in equilibrium.
- Modern analogues exist for past species/environments.
- Process of fossilisation (taphonomy) is known.
- No contamination or differential preservation.
- Fossil species can be identified.
Sources of Evidence
Glaciological (ice cores) Marine (sediment cores) Terrestrial (landforms, sediments). Biological (tree rings, insects, fossils). Historical (images, documents).
proxy conditions we can interpret from evidence
Temperature Precipitation Flows (atmos, ocean, ice) Events (timing & magnitude) Relationships between events (correlation)
Advantages of Oceanic Evidence
- Long records, continuous deposition means no gaps.
- Several different lines of evidence
- Reflects global hydrological and temperature changes,
Disadvantages of Oceanic Evidence
- Can lack spatial detail.
- Low temporal resolution (sedimentation is very slow as little as 1cm/1000yrs)
- Cant identify short lived events.
- Time and money consuming
Advantages of terrestrial Evidence
- Many different lines of evidence across multiple disciplines.
- Plenty of evidence (geomorphology, sediment, biological)
- Can reconstruct different elements of climate change, temperature, wind, precipitation etc.
Disadvantages of terrestrial evidence
- Temporal and spatial discontinuity due to erosion (except ice core records).
- The best records may only cover short time periods.
- Lack of good analogues.
What makes a good proxy?
- Good time span
- Real representation
- Continuity
- Un-ambiguity
- High sensitivity / fast response to climate change.
- Dating control
- High resolution
- Low cost
Why do environmental reconstructions use multiple proxies?
Most proxies only meet a few of the ‘good proxy’ criteria, so we use multiple to combine their interpretations, and use different dating methods for verification of a reliable picture of the past.
How can lag cause issues when comparing proxies?
Different proxies can have different lag times to respond to climate change. E.g. Coleoptera records may respond quicker to climate change than tree-pollen count, hence we must consider lag times when reconstructing the past.
What is relative dating?
This type of dating compares whether something is older or younger than something else. This is powerful as it places events/sediments in order, on an ordinal scale.
What is incremental dating?
This is when we know the rate of change per year, e.g. one tree ring grows per year, from this we can measure how many events have occurred and get an idea of the age of something. E.g. A tree with 100 rings we know is 100 years old.
What is numerical/absolute dating?
This produces the years of when an event occurred in the past, including luminescence and carbon dating. However there is usually some uncertainty within the result.
What are the two principles of relative dating?
Relative Position: based on stratigraphical principles, oldest sediment at bottom, youngest at top. E.g. Lancaster et al., 2002 confirmed subsequentuality of overlapping dunes in Western Sahara, using luminescence dating.
Relative change is based on physical and chemical changes, e.g. soils weathering. You can compare rocks with the same geology but different erosional features to get a sense of change and age.
What is a floating chronology?
A chronology of events that have been relatively dated, but there is no identified start or end point. An example would be if you aged a tree that had been cut down but didn’t know when it was cut.
What are lake varves? and who developed the principles of using these?
Varves are annually-deposited sediments in lakes. Principle developed by O. Heer (1865), in 1912 Baron G. Degeer developed the Swedish varve chronology that was the first accurate and precise dating of the late glacial and Holocene (14,000 years ago).
What layers are found in lake varves?
Each year there is sediment accumulation of 2 layers: Coarser (often lighter) summer unit and a finer (often darker) winter unit. The sizes of these vary depending on input volume etc. The sequence of these units allows us to trace the age of the sediments. Long sequences are possible via coring, from which we can trace deglaciation.
Terminology: what is a couplet?
A pair of adjacent different layers, e.g. a summer and winter unit in a lake varve.
Terminology: What is lamination?
Layers of sediment differing in composition.
Terminology: What are rhythemites?
A geoglacial deposit containing regularly alternating layers of different composition, but we don’t know whether they indicate an annual layer or decadal or any other frequency???
What is a clastic varve?
A Lake varve containing coarse material brought in by rivers in summer, and in winter the lake freezes over so fine clays in still water settle out to form the dark layer.
What are organic varves?
These rely on some biological plumes of organisms in the summer months, and in the winter months they die off. This results in a layer of calcite crystals or diatms relating to the summer, and a lack of this layer in winter.
What are mechanical varves?
These are discontinuous varves as there is some interruption by slumping and storms.
What are biological varves?
These are discontinuous varves as deposition only occurs with activity of bottom dwelling organisms being suppressed, and is interrupted by layers of homogenized sediment. Layers are often indistinct, thin and hard to count.
WHat might cause issues of multiple varves per year?
Flood events, multiple freezes.
Who developed the principle of dendrochronology?
AE Douglass, Arizona, early 1900’s. Principle: Annual tree growth adds rings of new material to the trunk. These rings can be counted to determine the age of the tree.
How long a record can dendrochronology provide?
Some long lived trees, e.g. Bristlecone Pine (SW USA) provide a record of up to 5,000 years. These trees are also sensitive to climate change and can give ‘control’ to other dating methods.
How do we make a continuous record using dendrochronology?
Cross dating of different tree ring samples, correlate the patterns within each tree to create a continuous record. This can be done with living, dead and human-used wood.
What considerations must be made when cross-dating tree ring samples?
You must compare the same tree species only. An Oak wouldn’t correlate with a Yew.
Also ‘noise’ factors exist affecting individual tree growth, e.g. bear removing bark, flood washing away soils, hence often studies take an average of 10 trees to remove the noise.
Principles of Lichenometry
We can work out how quickly lichens grow through time, and then use their diameter as a proxy for the age of the rock surface, larger = older. We use the mean largest size of lichens on a rock to calibrate this into absolute date using known age surfaces such as a local gravestone.
What is the most commonly used lichen in lichenoometry?
The Rhizocarpon Georaphica (grey/green).
What are some limits of lichenometry?
Lichens die and erode, there can be error in taking mean diameter or largest diameter, lag effect between rock exposure and establishment of lichens?
