Alkenone production in the East Sea/Japan Sea

• Alkenones were produced a year around except winter in the East Sea/Japan Sea.
• Alkenones were most likely produced at surface mixed layer in the East Sea/Japan Sea.
• Coretop alkenone temperature at deep site is close to annual-averaged SST.
• Coretop alkenone temperature at shallow site is close to summer to fall SST.

To test the applicability of alkenones as a proxy for past sea surface temperature (SST) in the East Sea (Japan Sea), this study investigated the season and depth of alkenone production in the area. Surface and subsurface seawater samples were collected from the East Sea during cruises carried out by the National Fisheries Research and Development Institute of Korea in 2008–2010. Surface samples were filtered for suspended material at two-month intervals. Subsurface samples were collected at water depths of 20, 50, 70 and 100 m by CTD bottle casts at two stations, one a coastal station and the other an offshore station. The results of alkenone analysis show that the concentration of total C37 alkenones was generally high in the surface mixed layer and decreased with depth, indicating that alkenones were most likely produced in or close to the surface mixed layer. Alkenone concentration varied seasonally: high in spring to fall and significantly reduced in winter. Comparisons of alkenone-based temperatures with in situ seawater temperatures show that alkenone temperatures measured from suspended particles in the surface waters were close to in situ SST in summer but were lower in winter. During winter, when alkenone production is significantly reduced, alkenones may be suspended for relatively long times and are likely to be advected from the north by eddies from Subpolar Front meanders. In summer when new production of alkenones increases, the settling velocity of alkenones appears to increase and residence time becomes shorter than in winter, suggesting that particles are less likely to be significantly advected at that time. Importantly, at the offshore station, coretop alkenone temperature corresponds to annual-averaged SST, while at the coastal station it corresponds to summer-to-fall averaged SST.