Using ship sounding data to reconstruct local sea level changes near Middleton Island, Alaska

Abstract

The Gulf of Alaska is a data sparse region, leaving sea level reconstructions skewed or nonexistent. These coastlines are particularly vulnerable to large sea level changes and are home to long-term sea level data from historical ship soundings. Using ship sounding data from 1933, 1969 and 1983, reconstructions of local sea level changes near Middleton Island, Alaska were created, which is a technique not attempted prior to this work. By tracking local sea level change over time, we are able to make an impact assessment of sea level change forcings; including the 1964 Magnitude 9.2 Alaska Earthquake. This earthquake resulted in Middleton Island’s uplift of 3.05 m, and we aim to differentiate sea level changes caused by this uplift and other processes (Brocher et al., 2014). Using these records in comparison with local and global reconstructions, the processes that contribute to local sea level changes, we attempt to quantify their impacts. In digitizing physical depth soundings from each year and adjusting these reconstructions to a common reference, we have produced sea level change maps between the years 1933-1969 and 1969-1983. Accounting for GMSL changes (Hay et al. 2015), a reconstruction of local sea level changes in the area surrounding Middleton Island was created. Investigating the causes of sea level changes around Middleton Island is crucial, as changes can be attributed to several factors, including the 1964 Earthquake, glacier melt, and glacial isostatic adjustment. We find that there is clear sea level rise south of the island between 1933-1969 by about 10 m, which could be attributed to the Earthquake and/or local tectonic uplift. Middleton Island could be used as a proxy for the impacts that different forcings have on sea level in the Gulf of Alaska if there is more research done surrounding the Island. Our research demonstrates that creating sea level reconstructions from historical shipping records and depth data is workable and important, as it can assist in attaining local sea level trends in the Gulf of Alaska and could be a very useful tool in building sea level data records prior to the technological advancements of the past half century.