You could take soil cores in Lake Ontario and run those for various proxies. As a timescale, you can generally count the seasons by the different layers. You could take leaves in those cores and count the stomata.
http://www.researchgate.net/publication/...
http://web.udl.es/usuaris/x3845331/inves...
http://web.udl.es/usuaris/x3845331/inves...
It could probably be calibrated to modern temperature data.
The problem with using any one biological proxy is that there are too many degrees of freedom. You could study multiple proxies, such as tree rings and stomata and come to a conclusion which is consistent. For example, if your samples have thin tree rings and closed stomata, tree rings could be thin because of a lack of water, which would cause the stomata to close to conserve water.
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