Chapter 5 Settling Particles in the Central North Pacific

Abstract

Abstract Sediment-trap experiments were conducted in the central North Pacific from the subtropical to the subarctic water mass (30°N–46°N) in order to understand the characteristics of settling particles as they relate to the biological pump. Fluxes of total mass, organic matter (OM), carbonate, biogenic opal, and lithogenics showed definite seasonal changes. In particular, there was a steep gradient in export production from June to December 1993 at the studied sites along a transect at 175°E, while the gradient was quite uniform from January to April 1994. Annual mean total mass flux varied from 47.4 mg m−2 day−1 at the southernmost station (Site 6; 30°N) in subtropical waters to 207.7 mg m−2 day−1 at the subarctic station (Site 8; 46°N). The transition zone showed intermediate fluxes of 41.1 and 94.6 mg m−2 day−1 (Sites 5 and 7; 34°N and 37°N, respectively). Correlations between OM, carbonate, and biogenic opal fluxes suggest that siliceous plankton was mainly responsible for the OM flux via settling particles at the subarctic station (Site 8), while carbonaceous plankton was responsible at the subtropical station (Site 6). Compared with the mean particle flux at mid-latitudes in the Atlantic, the Pacific showed significantly high biogenic opal flux due to nutrient enrichment in Pacific surface waters. The injection of aerosol-derived iron from the Asian continent may play a role in enhancing primary and export production in this area. An interesting feature observed in this study area is that a significant time lag was present between the dust storm peaks in the Asian continent source region and peaks of lithogenic flux at Site 7. This time lag suggests that eolian dust is being transported in the following manner: atmospheric transport brings dust from the source region to the ocean, where it remains suspended in the upper layer of the Kuroshio Current and Kuroshio Extension for up to 2 months; the dust is then removed by incorporation into biogenic pellets or amorphous aggregates when primary production is active in the upper layers.