Silica fluxes in the northeast Atlantic frontal zone of Mode Water formation (38°–45°N, 16°–22°W) in 2001–2002

Abstract

The biogenic (BSi) and lithogenic (LSi) silica export fluxes were investigated in the northeast Atlantic (38°–45°N, 16°–22°W) as part of the Programme Océan Multidisciplinaire Méso Echelle (POMME) program in 2001–2002. They were measured at four stations located on both sides of a frontal zone (40°–42°N) by means of permanent moorings of sediment traps deployed at 400 and 1000 m depth. Averaged over the area, the annual BSi fluxes (corrected from 230Th trapping efficiencies) ranged between 0.240 mmol m−2 d−1 at 400 m to 0.316 mmol m−2 d−1 at 1000 m. The bulk annual BSi fluxes are comparable to bulk BSi export fluxes recorded for oligotrophic areas. The annual export flux of LSi (range 0.029 mmol m−2 d−1 at 400 m to 0.054 mmol m−2 d−1 at 1000 m) was lower than BSi and accounted for 10% of the total silica export flux. Results show a strong coupling between the two siliceous particulate fractions, which is interpreted as reflecting LSi scavenging by BSi and limitation of BSi production in surface water by lithogenic (trace metals) inputs. BSi export was maximum at the beginning of the productive season during the spring bloom. However, annual BSi export fluxes in 2001 were quite higher at 400 m in the southern area (e.g., 0.249–0.288 at the southeast station versus 0.211 mmol BSi m−2 d−1 at the northeast station) contradictory to the classical south‐north surface production increase. We suggest an advective lateral transport within the upper 400 m of siliceous particles from the northern, more productive area to the southern region.