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Archive for January, 2013

Not so recently an article was sent to me that discusses how in areas of Sweden the relative sea level is falling and land bridges that did not exist within the last century are starting to appear. For some this could seem as counterintuitive with what is known about rising eustatic sea level caused by climate change. But, the process behind this relative falling sea level is simply isostatic rebound from the Last Glacial Maximum.

During the LGM, the massive Eurasian Ice Sheet depressed the land, and since deglaciation it has been recovering from that depression. What is happening specifically is that the rate of Swedish rebound is higher than the rate of sea level rise.

The process of isostatic rebound is becoming a concept of large importance for me to work with in a couple of ways. One of the ways we can reconstruct the glacial history of a region is by looking into the relative sea level changes experienced there. Using those changes along with an understanding of the global sea level can suggest the presence or lack of overlying ice sheets.

In a 2007 paper, Marshall McCabe et al discuss the relative sea level changes in northeast Ireland experienced after the LGM and up to the Younger Dryas. By using stratigraphic relationships between dated beach deposits and glacial diamictites, McCabe reconstructed a relative sea level curve for the region.

From McCabe et al 2007

At a Kilkeel outcrop (point 2 in the figure), beach notches are found 30 m above current sea level and are infilled with glaciomarine muds. This suggests that at the time of formation of the notches the coastline of Ireland was isostatically depressed 30 meters. Considering that eustatic sea level was 130 m below current during the LGM means the coastline was depressed ~160 m below present. Deglaciation is suggested by a subsequent fall in relative sea level as the unburdened coastline experienced uplift.

Another aspect of isostatic uplift that I have to consider has to do with the surface exposure dating I will use, and is a topic I have discussed at length with fellow students. Glossing over a whole bunch of details, to get as accurate of a date as possible we have to consider not just how long a particular sample was exposed to cosmogenic rays, but also at what altitude the sample was at while being exposed. Objects at higher altitudes will be exposed to more cosmogenic rays than those at lower altitudes. This comes into play where a glacial erratic may be deposited on a moraine that was 160 m below its current altitude during the LGM. Depending on what altitude the erratic was at would effect the calculated age of exposure. Do we use the current altitude in our calculations or the depressed altitude? Or do we compensate for the uplift by trying to adjust the increasing exposure with uplift? If so, do we assume a linear uplift, or the rebound curve that shows high initial rates followed by lower rates of uplift? Also, how significant of a difference does it make on the final calculation?

I understand that these are a lot of questions to consider, and ones I am sure I will have to tackle in the coming months. The truth is I welcome the discussion that will come, and if anyone reading this has thoughts or suggestions please feel free to chime in on the comments section.

 

McCabe, a. M., Cooper, J. a. G., & Kelley, J. T. (2007). Relative sea-level changes from NE Ireland during the last glacial termination. Journal of the Geological Society, 164(5), 1059–1063.

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