Much like trees, clam shells have growth rings. The chemistry of these rings can be used as a proxy for ocean chemistry. Recently, an international team of scientists used the growth rings found in shells of Arctica islandica to produce an annual absolutely dated marine δ18O record for the last millennium which was published in Nature Communications. The record represents the first fine scale archive longer than ~100 years. Additionally, it has higher resolution, and less age uncertainty than δ18O records produced from sediment cores.
To read more into what this record means, and the full results of the study see D.J. Reynolds et al, 2016.