The talk was about studying past sea level rises and falls using proxies from Greenland. The data relates to the climate changes that occurred during the past 1000 years and how these affected ice behaviour. The study wanted to gather sea level data to try and understand how ice and climate will interact with future climate changes.
Blog Theme: Sea level changes due to Greenland ice sheet surges and retreats over the last millennium - how climate changes affected ice behaviour
Ongoing Blog Theme: climate change on Earth: past and present – the things that have and will change and what we can do about it
Why study sea level changes in Greenland?
During the last 1000 years, the planet has experienced two small periods of warming and cooling – the Medieval Warm Period (MWP) and the Little Ice Age (LIA) (see Information Box 1 and 2).
These periods had effects on the ice extent in Greenland that subsequently affected sea level rise and fall in the Northern hemisphere.
Leanne and her team wanted to understand the sea level changes that occurred around Greenland over the last millennium as improving the current understanding of how ice sheets respond to climate change is very important. One day, future changes in land ice and sea level will be able to be predicted based on our understanding of how ice and climate interacted in the past (Wake et al, 2012).
The causes of sea level change
Sea level changes can be induced my multiple sources (see Figure 1). Ice is removed from the land during periods of glacier retreat which results in isostatic recovery of the land (see Information Box 3). Due to the land rising, the sea level falls.
The converse of this is ice loading on land due to glacial surges which causes the land surface to lower and the sea level to rise.
Ocean circulation can also cause small and localised sea surface rises and falls because currents are moving bodies of water, this results in changes at the sea surface (NASA, 2019). Weather changes across the ocean surface also result in changes in the sea surface level (see Figure 2).
The main focus of the talk was on the ice melting and vertical land motion changes that affect sea level. This is summarised below in Figure 3:
How do you study sea level changes in Greenland?
The rise and fall of sea level in close quarters to Greenland can give us information about which parts of the ice sheet are melting. The various proxies and observations that can be studied around Greenland can be used to create a sea level fingerprint (see Information Box 4).
The main proxies that can be used to create a fingerprint are:
· Tide gauges
· Corals
· Archaeology
· Salt marshes
· Isolation basins
However, there are very few tide gauges around Greenland and so these were omitted from Leanne’s study. There are also no corals in Greenland and the archaeological sites there don’t give much data about sea level, and so these too were omitted. Leanne used isolation basins and salt marshes (see Information Boxes 5 and 6) to gather data about sea level changes in Greenland over the last 600 years.
Methods
This section details the two main methods that Leanne’s team used to gather data about sea level changes around Greenland.
Isolation basins
Cores were taken from sediments in Greenland isolation basins. Studying the retrieved cores will give data about the shifts from marine to freshwater. From the cores taken, the marine influence stopped due to land uplift/sea level fall which cut off the basin from the sea. Dating the cut offs from the cores will give the age when the sea level fall occurred (see Figure 4).
Core cut off dates can be identified using the sedimentary sequence to note changes occurring through the core or through the changes in diatom populations through the core (see Information Box 7).
Leanne’s team also wanted to gather data from recent sea level changes (the last 100 years roughly) in Greenland and so took cores from lower elevation isolation basins too.
Salt marshes
Leanne’s team took cores from several salt marshes in Greenland. Salt marshes in Greenland (see Figure 5) are small but yield good data on sea level changes based on the diatom concentrations contained within the sediment cores.
Results
At sites in west Greenland, relative sea level (RSL) rise slows from 3 mm/year to 0 mm/year at 400 years BP and becomes stable thereafter (Wake et al, 2012). This can be seen in Figure 6. Basically, sea levels were rising up until the year 1600 A.D. and then began to level out to show a period of steady sea level.
In the south of Greenland, a similar RSL plateau occurred but it happened 200 years later than in the west, around the year 1800 A.D (Wake et al, 2012). This sea level fall and subsequent plateau 400 years ago is attributed to ice loss and thinning due to climate amelioration after the LIA which caused land to rise and RSL to fall. The south-west sector of the Greenland ice sheet was the largest contributor to this process (Wake et al, 2012).
How suitable were the techniques?
Isolation basins
The technique relies upon microfossil assemblages preserved within the sediment cores but there can be issues with this.
The main issues are:
· counting and identifying microfossils is time consuming
· microfossil abundance may be poor or under/over represented in layers
· preservation may be poor
· certain species have wide salinity tolerances and may not give specific results
However, on the whole, the technique is very suitable for gleaning sea level changes around Greenland for the last millennium as the typical diatom transitionary sequence from marine, to brackish, and then freshwater species is very telling (Balascio et al, 2011).
However, isolation basin data alone is not enough to give reliable RSL changes. Isolation basin data also has high uncertainties in Leanne’s work when compared to the salt marsh data More data points, or more isolation basin cores, need to be taken and studied to increase the reliability of the results (Wake et al, 2012).
Salt marshes
Salt marsh sediment samples also rely on the use of foraminifera and their limitations can be seen in the isolation basin section (above).
However, the technique is reliable for constraining sea level changes in Greenland and yields more reliable results than isolation basin data. To make the results for the study more reliable, Leanne’s team should focus on gathering more salt marsh samples to constrain their data as they only visited a total of three sites (Wake et al, 2012).
How did the talk advance our knowledge?
Climate shifts are set to cause many changes to the world that we are used to living in. Leanne's talk was trying to unpin the effects that past climate changes have had on sea level rises and falls in the last millennium. The talk was so relevant as the Greenland ice sheet is shrinking, and future warming could accelerate this process and cause an influx of water into the ocean, affecting current sea levels.
To read about Leanne's research please follow these links: https://www.northumbria.ac.uk/about-us/our-staff/w/dr-leanne-mary-wake/ and https://www-sciencedirect-com.ezproxy.is.ed.ac.uk/science/article/pii/S0012821X11005498#f0025
Thank you for reading my final blog!
I hope that the importance of the scientific studies and the underlying theme of studying past and current climate changes to help protect our future was effectively communicated through these blog. Hopefully there weren't too many rocks ;)
References
Balascio N.L, Zhang Z, Bradley R.S, Perren B, Dahl S.O, Bakke J. (2011). A multi-proxy approach to assessing isolation basin stratigraphy from the Lofoten Islands, Norway. Quaternary Research, 75, 288-300.
Easterbrook D.J. (2011). Medieval Warm Period. Available at: https://www.sciencedirect.com/topics/earth-and-planetary-sciences/medieval-warm-period [Accessed: 31/03/2019]
Lallensack R. (2017). Global fingerprints of sea-level rise revealed by satellites. Available at: https://www.nature.com/news/global-fingerprints-of-sea-level-rise-revealed-by-satellites-1.22588 [Accessed: 31/03/2019]
Long A.J, Woodroffe S.A, Roberts D.H, Dawson S. (2011). Isolation basins, sea-level changes and the Holocene history of the Greenland Ice Sheet. Quaternary Science Reviews, 30, 3748-3768.
Long A.J, Woodroffe S.A, Milne G. A, Bryant C.L, Simpson M.J.R, Wake L.M. (2012). Relative sea-level change in Greenland during the last 700yrs and ice sheet response to the Little Ice Age. Earth and Planetary Science Letters, 315, 76.
NASA. (2009). NASA Satellites Capture Sea Surface Heights Around the World. Available at: https://www.nasa.gov/mission_pages/hurricanes/features/seasurface_heights.html [Accessed: 31/03/2019]
NASA. (2019). Ocean Surface Topography. Available at: https://science.nasa.gov/earth-science/oceanography/physical-ocean/ocean-surface-topography [Accessed: 31/03/2019]
NOAA. (2018). What is a salt marsh? Available at: https://oceanservice.noaa.gov/facts/saltmarsh.html [Accessed: 01/04/2019]
NSIDC. (2019). Isostatic Rebound. Available at: https://nsidc.org/cryosphere/glossary/term/isostatic-rebound [Accessed: 31/03/2019]
Sefton J. (2018). SeftonGeo. Available at: https://twitter.com/SeftonGeo [Accessed: 01/04/2019]
Smith D.E. (2016). Isolation Basins. Available at: http://www.landforms.eu/ScottishSeaLevels/isolation%20basins.htm [Accessed: 1/04/2019]
Wake L.M, Milne G.A, Long A.J, Woodroffe S.A, Simpson M.J.R, Huybrechts P. (2011). Century-scale relative sea-level changes in West Greenland — A plausibility study to assess contributions from the cryosphere and the ocean. Earth and Planetary Science Letters, 315-316, 86–93.
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