Alkenone production in the upper 200 m of the Pacific Ocean

To investigate the depth of alkenone production in the Pacific Ocean, seawater samples were collected and filtered for suspended material from the surface mixed layer, as well as from the water depths of 100, 150 and 200 m during the May–June 2002 IOC cruise aboard the R/V Melville and September 2002 Daeyang cruise by the R/V Onuri. The concentrations of total C37 alkenones were measured and compared to those of chlorophyll a measured during the same cruise. Results show that the vertical distribution of alkenone abundance appears to be consistent with changes in chlorophyll concentration. In the western boundary current and equatorial regions, where the chlorophyll maximum occurs near the surface, the alkenone concentration maximum exists near the surface. In contrast, in the subtropical gyre, where the chlorophyll maximum occurs in the subsurface, the alkenone concentration maximum occurs at the depth of the chlorophyll maximum (∼100 m). Comparison of alkenone-based temperature estimates with CTD in situ temperatures reveals that they are mostly consistent with each other. Our results suggest that in the western boundary and equatorial regions alkenones seem to be produced at or close to the sea surface, whereas in the subtropical gyre center a subsurface production could be of importance for the flux from the surface to the bottom.