Climate archives in coral skeletons Flashcards
How have measurements of SST changed over time?
- Began with wooden buckets
- Switched to canvas buckets when the ships got too fast/tall
- Engine room measurements in a steam ship (especially in 1945 - with war no lights on deck)
- Argo floats and satellites
What happens to uncertainty of SST measurements the further back in time we go?
It increases
What are some limitations of measuring SSTs using buckets?
- The type of bucket would influence heat loss (between hauling and measuring)
- Wind and depth would also affect heat loss
What can be used to fill gaps in SST records over time?
Corals
Why are corals important for long-term records?
- Generally exhibit high temporal resolution
- Skeletons are annually banded (shows growth even monthly)
- Exhibit regional scale climate change as they show temporal variability within a region (unlike satellites, can show small-scale variations across a reef)
- Understanding conditions during coral life might help predictions (such as bleaching susceptibility)
How can temperatures vary within a reef? What is an example of where variations in bleaching occurred?
Shallower regions are typically warmer.
For example, the lagoon can be several degrees warmer than the forereef.
South China Sea 2015 - Forereef was much colder than lagoon -> the lagoon experienced significant bleaching, unlike the forereef
What kind of corals are typically avoided when using corals for past climate research? Why?
Branching corals:
- tend to form new branches randomly
- Inconsistent growth
Which corals are typically used for past climate research?
Massive colonial corals
Which collection method is used to identify banding in corals for past climate research?
- A pneumatic drill with air tanks collects a coral core (a concrete plug is used to fill this hole after to aid recovery)
- CT scanning is carried out on the whole core
- A flat slab (~7 mm thick) is removed
- This slab is the X-rayed/CT scanned
This will show the rings that form annually over time.
How is the coral skeleton ideal for distinguishing different periods of growth?
Each year there is a light and dark band (means years can be distinguished)
Which other method can be used to produce thermal climate records in corals with limited seasonal variation (and therefore limited banding)?
Luminescence intensity under fluorescent light can be analysed to show the layers of humic acids from the land.
This is especially useful when there a clear seasonal rainfall patterns despite limited seasonal variations, with each band representing a rainy event.
Why can coral skeletal bands be visualised using X-rays?
The less dense bands transmit X-rays, whilst the denser bands don’t.
Which type of bands in coral skeletons are associated with summer typically?
Higher density bands.
When there is a bleaching event, growth is typically slowed, and a high-density band is laid down – these denser bands are therefore associated with paleo-stress events.
What kind of bands in coral skeletons are typically associated with bleaching?
High-density bands are laid down during bleaching as growth slows down.
What two techniques can be used to sample chemical/isotopic composition of coral?
- Microsampling with a microdrill
- Microsampling with laser ablation
What are the main differences in micro-sampling when using a micro-drill vs laser ablation?
Micro-drill:
- Resolution: 1 sample per month
- Powdered sample is cleaned and dissolved for analysis
- Limited spatial resolution
- Slow but precise and accurate
Laser ablation:
- Resolution: daily sampling is possible
- Slabbed sample is analysed directly (either with spots or line)
- Higher spatial resolution
- Very fast but lower precision and accuracy
- Can produce spatial 2D maps of the chemical compositions across the skeleton
What is microsampling?
A technique for coral skeleton composition analysis.
A trench is produced by sequentially drilling into the slab, before analysis via either a micro-drill or laser ablation
What can be measured from corals as a proxy for temperature?
Li/Mg ratio and Sr/Ca ratio
These are impurities.
How are impurities incorporated into the skeletal lattice? What are these common impurities?
Impurities in the ECM can be incorporated into the CaCO3 structure, although how this occurs varies by impurity:
- strontium replaces calcium in calcium carbonate (hence Sr/Ca).
- Li and Mg are thought to incorporate into aragonite via different mechanisms, so Li/Mg is used as an impurity ratio to study physiological and kinetic effects in coral growt
Why is looking at CaCO3/geochemical impurities a useful temperature proxy?
At higher temperatures, the ratios of impurities are lower.
This means that if we have a time period of unknown temperatures, this can be calculated using the impurity ratios from coral skeletons after temperature calibration.
Note: this varies depending on the driver analysed.
List some reasons as to why there is a strong relationship between impurities in aragonite and temperature?
- Inorganic crystallographic processes (lower temp = less time for crystal to equilibrate with solution, so more impurities).
- The calcifying space is semi-isolated (corals can modify ion concentrations before calcification, and since these modifications are temperature-dependent, this leads to temperature-sensitive incorporation of impurities like Mg, Li, and Sr into the coral skeleton)
- Precipitation rates (the faster the precipitation, the more likely impurities will be incorporated into crystals - note that this has the opposite effect of the others described above)
How can you calculate temperature from known impurities (calculation steps)?
- Plot a calibration curve of impurity ratios to temperature (temperature – x, ratio – y)
- Use a line of best fit in the “calibration” plot
- The equation of this line can be used (y = mx +c)
- Therefore, can rearrange to get x alone: x = (y-c)/m
How quick do coral skeletons grow?
1-10 mm per year
Which sampling techniques for climate records allow sub-annual scales?
Micro-drilling and laser ablation.
