Freshening of the Alaska Coastal Current recorded by coralline algal Ba/Ca ratios

Abstract

Arctic Ocean freshening can exert a controlling influence on global climate, triggering
\nstrong feedbacks on ocean‐atmospheric processes and affecting the global cycling of the
\nworld’s oceans. Glacier‐fed ocean currents such as the Alaska Coastal Current are
\nimportant sources of freshwater for the Bering Sea shelf, and may also influence the Arctic
\nOcean freshwater budget. Instrumental data indicate a multiyear freshening episode of
\nthe Alaska Coastal Current in the early 21st century. It is uncertain whether this freshening
\nis part of natural multidecadal climate variability or a unique feature of anthropogenically
\ninduced warming. In order to answer this, a better understanding of past variations in
\nthe Alaska Coastal Current is needed. However, continuous long‐term high‐resolution
\nobservations of the Alaska Coastal Current have only been available for the last 2 decades.
\nIn this study, specimens of the long‐lived crustose coralline alga Clathromorphum
\nnereostratum were collected within the pathway of the Alaska Coastal Current and utilized
\nas archives of past temperature and salinity. Results indicate that coralline algal Mg/Ca
\nratios provide a 60 year record of sea surface temperatures and track changes of the Pacific
\nDecadal Oscillation, a pattern of decadal‐to‐multidecadal ocean‐atmosphere climate
\nvariability centered over the North Pacific. Algal Ba/Ca ratios (used as indicators of coastal
\nfreshwater runoff) are inversely correlated to instrumentally measured Alaska Coastal
\nCurrent salinity and record the period of freshening from 2001 to 2006. Similar multiyear
\nfreshening events are not evident in the earlier portion of the 60 year Ba/Ca record.
\nThis suggests that the 21st century freshening of the Alaska Coastal Current is a unique
\nfeature related to increasing glacial melt and precipitation on mainland Alaska.