Dating Deglaciation of the Laurentide Ice Sheet

The calculated rate of retreat for the bulk sediment data provided was calculated to be 0.06337 km/year. Using the macrofossils data, the calculated rate is 0.06780 km/year. Finally, the cosmogenic data provided a rate of 0.02381 km/year. The macrofossil and bulk sediment calibrated ages are somewhat similar when comparing the two data sets to the third (cosmogenic exposure). This can be seen if you look at the calibrated age on Martha's Vineyard for macrofossils, which was calibrated to be 18,300 years while the cosmogenic exposure data states age is 27,500 years ago. The rates for the first two data sets are also very similar compared to the significantly slower rate calculated by using the cosmogenic exposure data.

The reason for this difference could be derived from error in my calculations, since I choose a representable distance, there could be inaccuracies involved in the bulk sediment and macrofossils data sets. The bulk sediment and macrofossils have very similar rates and calibrated ages. For bulk sediments there might be uncertainty within the ages due to organic material needing to develop for a period of time after the deglaciation. When looking at macrofossils compared to cosmogenic exposure data there is a significant change in time when the Laurentide ice sheet is to be said at its maximum on Martha's Vineyard. This is most likely due to some uncertainty within the macrofossils. This uncertainty is probably from the fact that after the ice sheet retreated, there was a certain amount of time that it took for organisms to inhabit the previously glaciated landscape. This means that there will be a degree of uncertainty for macrofossils compared to the cosmogenic exposure calculated retreat age because the cosmogenic data is based on the dating rocks based on comic ray exposure. Therefore, the cosmogenic exposure data is most likely the most accurate and would be the one I would choose to use if researching this further. Nevertheless, it’s important to understand where the uncertainty is coming from, that’s why the bulk sediment and macrofossils are good to have when showing these types of data. When considering the Lamback et al. paper, specifically figures 1 and 4, we can see the change in sea level rise over the period of deglaciation. When comparing figure 1 with the cosmogenic exposure, you can see that around 27,000 to 29,000 years ago there was still a lot of ice frozen and sea levels were down 120 meters. As time goes on, and the Laurentide ice sheet retreated, more water was released from meltwater and delivered to the ocean. However, it wasn't until 20,000 years ago where we can actually see a start in a great discharge of freshwater into the oceans. (Lamback et al., 2014) This could mean that the terminal moraine (Martha's Vineyard) took a long time to actually recede back North and for large volumes of water to be released.