A talk has been given by Dr Rachael H. Rhodes, University of Cambridge on October 30, 2018. I found really interesting to review this talk as it gives a brief introduction on what proxies tell us about changes in sea ice extents over the geological timescale. This post is a good way for me to introduce you to the study of ice cores and how those can enhance our knowledge of past and future Climate Changes.
What is an ice core?
An ice core refers to cylinders of ice drilled out of an ice sheet or glacier. These samples are great paleo-environmental archives as they record changes in temperatures, Co2 dust in the atmosphere and Na elements variabilities.
3 proxies have been discussed for past sea ice reconstruction
MSA or Methane Sulfonic Acid
These acid molecules are produced by phytoplankton then transported in the atmosphere by evaporation and finally trapped in the ice sheets where those can be found in ice cores. (molecule formula linked on the side)
According to studies on ice cores from Norwegian (Svalbard), the MSA increases with temperature and then productivity, and decreases with the sea ice extent. This is a really accurate parameter for studying past primary productivity and ice sea content.
But this proxy is more dedicated for small timescale changes (seasonal changes in sea ice) and within small areas as these MSA molecules can’t be transported on long-distance.
The sea salt Na
This chemical element is particularly rich in the open ocean (this source needs to be discussed!) and has a big potential on large changes of sea ice extent. Then, its concentration is useful to study glacial and interglacial event. Concentrations of sea salt within the core increase with the sea ice extent.
Bromine enrichment
This is a good proxy for the “first-year sea ice extent”. First-year sea ice is the ice that has been accumulated during Fall and Winter but that is totally melted during Spring and Summer. This kind of ice is different from the multi-year sea ice that can survive more than one year period of melting.
Bromine is a good proxy because it is trapped in the first-year sea ice starting Fall and released into the atmosphere during Spring. This proxy is really good to trace seasonability within a year. When ice melts, Bromine interacts more with the atmosphere to produce more Bromine via the Bromine explosion process. When ice is more stable (Winter), Bromine tend to stay trapped in ice and concentrations in the ice core tend to be lower.
Sea salt proxy is not fully understood yet!
Recent studies have been run in the Antarctic to better understand the sources of sea salt and how those control the concentration of sea salt in the atmosphere and then in ice cores.
It has been found, using a Cambridge p-TOMCAT model, that meteorology controls sea salt concentrations rather than sea ice extent. Past studies were not able to link high concentrations of sea salt in the cores to the open ocean source. According to a recent case study, it is believed that "blowing snow" (snow lifted by the wind from the ice surface) would be the main source of sea salt deposition, rather than the open ocean.
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