The chemistry, biology, and vertical flux of particulate matter from the upper 400 m of the equatorial Atlantic Ocean

Abstract

Particulate matter, divided into <1, 1 to 53, and > 53- μm size fractions, was obtained in profile from the upper 400 m at 2°47′N, 8°51′W in the equatorial Atlantic by large volume in situ filtration. The samples were analyzed for Na, K, Mg, Ca, carbonate, Si, Sr, Fe, C, N, P, organic δ 13C, 7Be, 214Bi, 214Pb, ( 226Ra), 210Po and 210Pb and were studied by light and scanning electron microscopy to determine the size and morphological distributions of the particles. The chemical distributions in particulate matter are controlled largely by biological production, respiration, predation, aggregation, and fragmentation. The largest particulate elemental and mass concentration gradient occurs between 50 m (the base of the mixed layer, particle and organism maximum) and 113 m. The organic fraction was enriched with bound Mg and ion-exchangeable Ca and Sr. Dissolution of SrSO 4 from Acantharia Sp. was pronounced below 188 m. Large particles could be classified as crustacean appendage and carapace material, mucold material, hyaline or sheetlike material, fecal pellets, and fecal matter. The latter two types of fecal material, containing coccoliths, diatom fragments and other fine particles, become increasingly important with depth in the >53- μm size fraction with depth. A model for particle settling constructed from the chemical and microscopic data indicates that 99% of the vertical mass flux through 388m is carried by fecal matter and fecal pellets which contribute only 4% to the total suspended mass concentration. The sinking material has three major phases (flux in brackets): organic matter [94 mmoles C cm −2(1000 yr) −1]; carbonate [10.9 mmoles CaCO 3 cm −2(1000 yr) −1]; and opal [12.1 mmoles Si cm −2(1000 yr) −1]. Associated minor and trace components are transported vertically by a common mechanism but their regeneration depends on the chemistry of their carrier phases.