USING SHIP SOUNDING DATA TO RECONSTRUCT LOCAL SEA LEVEL CHANGE NEAR MIDDLETON ISLAND, ALASKA

The Gulf of Alaska is a data sparse region, leaving sea-level reconstructions to be skewed or nonexistent in a region that possesses coastlines vulnerable to the effects of sea-level rise. The region however, is home to long-term sea-level data taken from historical ship sounding depths. Using charts of ship sounding depth from 1933, 1969 and 1983 we have reconstructed local sea-level changes near Middleton Island, Alaska. This new reconstruction approach tracks local sea-level change over time and allows for impact assessment on sea-level changes, in part caused by the 1964 Magnitude 9. 2 Alaska earthquake. This earthquake resulted in Middleton Island’s uplift of 3.05 m (Brocher et al., 2014), and we aim to differentiate the uplift from sea-level changes due to other processes. Using these records in comparison with local and global reconstructions, we aim to identify the processes that are contributing to local changes and quantify their impacts. By digitizing physical depth soundings from each year, we have created new spatial reconstructions of sea level for Middleton Island. Through these reconstructions, we are able to see clear sea-level changes throughout our years of data. In the 1933, the area where depth to the ocean floor was ≥ 365.76 meters is strictly north of 59.5°N, but this area of depth grew continuously larger, indicating changes in sea level between 1933-1964 and 1964-1983. With the recorded uplift impact from 1964, we can isolate known contributions to sea-level change, and discuss how different processes are specifically impacting the area. Investigating the causes of sea-level changes around Middleton Island and the surrounding area is crucial, as it could be due to several factors, including the 1964 earthquake, glacier melt, glacial isostatic adjustment, and thermal expansion. Although uninhabited, we hope that Middleton Island can be used as a proxy for the impacts that different climatic and non-climatic factors, including seismic deformation, have on sea level. Furthermore, this new method of reconstructing sea level using historical shipping records could be a very useful tool in building sea-level data records prior to modern observations.