Vertical flux of hydrocarbons as measured in sediment traps in the eastern North Pacific Ocean

Abstract

Sediment trap experiments were conducted at three stations (Stn. 5 at 4750 m depth, 7 at 4330 m depth and 11 at 3880 m depth) in the California Current and its adjacent areas in the eastern North Pacific from December 1982 to January 1983 to collect sinking particles, which were analyzed for organic carbon and hydrocarbons. The vertical fluxes of organic carbon, total nitrogen and lipid carbon at any depth decreased in the following order: Stn. 5 > Stn. 11 > Stn. 7, the same trend as the standing stocks of chl. a at these station, which suggests that regional variation of organic matter flux is considerably influenced by the primary productivity in the surface water layer. Hydrocarbons of the sinking particles consisted of n-C 15–C 20 with a maximum of n-C 17, n-C 21–C 32, n-C 21:6 and three branched C 25 alkenes (br-C 25:3, br-C 25:3, and br-C 25:4), but any odd or even carbon number predominance in n-C 21–C 32 was not observed. n-C 21:6 and n-C 17 were the most abundant throughout the depths at Stn. 5, while only n-C 17 was a major component of the sinking particles from the intermediate layers, but not from the deep layers, at Stns. 7 and 11. Thus, there were clear regional and vertical variabilities of hydrocarbon composition in the sinking particles. The vertical flux of various hydrocarbons at the three stations tended to decrease exponentially with depth. The attenuation constants of the hydrocarbon fluxes were calculated by analysis of the relationship between the fluxes and depths. Half-depth of the flux calculated from the attenuation constant showed extensive variability for the hydrocarbon species and the location of the station. The values of the half-depths of n-C 17, Σn-C 15–20, n-C 21:6 and pristane were lower than those of Σn-C 21–32 and Σbr-C 25, indicating that hydrocarbons derived from phytoplankton are more susceptible to biological degradation than those from zooplankton. The half-depths of the hydrocarbon fluxes tended to increase as follows: Stn. 7 < Stn. 11 < Stn. 5. This trend was the same as the size of the sinking particles collected at these stations, which suggests that the sinking rate of the particles tends to increase in the order of Stns. 7, 11 and 5. Thus, these data indicate that the sinking rate of the particles is a primary factor in determining the hydrocarbon compositon of the particles in the intermediate and deep waters.