Particle-associated dissolved elemental fluxes: revising the stochiometry of mixed layer export
Résumé
Sinking particles carry substantial loads of dissolved elements in their interstitial spaces that contribute to the vertical transport of elements out of the euphotic zone. Elemental fluxes as traditionally measured by sediment traps underestimate total export when this particle-associated dissolved flux is not considered. The errors introduced are variable and alter both the absolute levels of flux as well as the stochiometry of export. Using samples from sediment traps in the North Atlantic and measuring excess dissolved carbon, nitrogen, phosphorus, silica and calcium in the supernatant of the collection cups, it is possible to quantitatively assess the total flux in the sample. At the base of the winter mixed layer, up to 90±6% of phosphorus fluxes are found as excess phosphate whereas for carbon and nitrogen dissolved concentrations account for between 30% and 47% of total fluxes respectively. Particle-associated dissolved silica fluxes are a mean of 61% of total flux. Little (<10%) of calcium fluxes are in dissolved form. The proportion of dissolved to total flux decreases with trap deployment depth. Calculations of the C:N:P ratios for particles only are well above the Redfield Ratio of 106:16:1 (Redfield et al., 1964), although the mid-water dissolved N:P and N:Si values as well as the C:N:P ratios of remineralisation along isopycnals conform to the Redfield Ratio at this site. Accounting for dissolved fluxes of all these elements brings the estimates in agreement with the Redfield Ratio and with other geochemical estimates of the stochiometry of winter mixed layer export. A factor of 3 to 4 higher ratios of organic: inorganic carbon export also implies that the net atmospheric CO2 sequestration by the biological pump is about 50% higher at this site when the particle-associated dissolved elemental fluxes are considered.
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