Biogenic barium in suspended and trapped material as a tracer of export production in the tropical NE Atlantic (EUMELI sites)

Abstract

Biogenic barium (bio-Ba) was measured in trapped and suspended particles at both the mesotrophic (M) and oligotrophic (O) EUMELI sites, in the tropical NE Atlantic. Trap data (2500 m depth) were used to calculate the exported production (ExP), using the relationship established by François et al. [François, R., Honjo, S., Manganini, S., Ravizza, G., 1995. Bio-Ba fluxes to the deep sea: implications for paleoproductivity reconstructions. Global Biogeochem. Cycles 9, 289–303.]. We find 12±4 and 1.6±0.4 gC/m 2 /year at sites M and O, respectively. These values are consistent with: (1) the measured total primary productions at both sites and (2) the measured exported carbon estimated by the “instantaneous” sampling of drifting traps in two seasons. At site O, our estimate compares well with the ExP deduced from measured particulate organic carbon (POC) at 2500 m [François et al., 1995; Sarnthein, M., Winn, K., Duplessy, J.-C., Fontugne, M., 1988. Global variations of surface ocean productivity in low and mid-latitudes: influence on CO 2 reservoirs of the deep ocean and atmosphere during the last 21,000 years. Paleoceanogr. 3, 362–399.]. At site M, there is an offset in flux timing and estimated ExP fluxes do not compare well with primary production observations. These observations suggest that advected refractory carbon is present in the traps and/or that organic matter yielding large POC fluxes is different from that favoring bio-Ba formation and transport. In particular, fecal pellets might play a main role in the bio-Ba synthesis and vertical transfer. In suspended particles, the vertical distribution of bio-Ba could be related to phytoplanktonic and/or heterotrophic activity. Based on the meso-pelagic bio-Ba maxima, we estimated that 1.4–1.5 and 0–0.1 gC/m 2 of exported carbon is remineralized during the month of June, just below the mixed layer at sites M and O, respectively. The ExP calculated using trapped bio-Ba flux (François et al., 1995) is discussed with regards to the fraction of this ExP remineralized at mesopelagic depths.