Carbon sequestration in the deep Atlantic enhanced by Saharan dust

Abstract

11ocean can potentially increase carbon uptake and sequestration at depth. Nutrients can 12 enhance primary productivity, and mineral particles act as ballast, increasing sinking 26Flux of airborne desert dust into the surface ocean can increase the amount of 27 photosynthetically fixed carbon dioxide (CO2) by reducing nutrient limitation of primary 28 production and thus increase the flux of particulate organic carbon (POC) to the deep ocean 1 . 29Dense dust-derived lithogenic particles can also increase particle size through aggregation and (Fig.1). During this period, NOG was subjected to, on average, ten-fold higher dust deposition 65 compared to SOG (Fig. 2a), as inferred from dust concentration measurements over Barbados 13 66for NOG and modelled data 14, 15 for SOG (Methods). At both sites, the average surface 67 production rates derived from a Vertically Generalised Production Model (VGPM) 16 were 68 lower than much of the global ocean 17 , and on average 23% higher at NOG than at SOG ( (Fig. 2d, 3). The POC fluxes at NOG and SOG were significantly lower than the (refs 21, 22, 23, 24, 25 ). This coincides with higher fluxes of aerosol iron in autumn than 136We estimated the contribution of different nitrogen sources to δ 15 NPN at NOG and SOG 137 using a two-end member nitrogen mass-balance model 29 (see Methods and references therein). 138We assumed that the isotope budget of the mixed layer in the permanently oligotrophic gyres 145The dust-derived nitrogen endmember was assigned δ 15 N of -3.1‰ based on the average for a possible origin of particles from a specific trophic level (e.g. faecal pellets) and alteration 158 of δ 15 NPN due to isotopic fractionation during particle remineralisation and transformation in 159 the mesopelagic. However, regardless of these uncertainties, the isotope budgets suggest a large 166The unique presence of intact Trichodesmium colonies in the deep particles at NOG 167 (Fig. 4) during late summer at NOG (Fig. 3a). 180 Ballasting effect of dust 181Higher dust input significantly altered the composition of particles at NOG compared 182 to SOG (Fig. 2e). Dust-derived lithogenic material was the second largest contributor 183 (34.3±11.6%) to the total mass at NOG after calcite, whereas at SOG this value was 4.7±2.3%, 184 consistent with the difference in the amount of dust being deposited at each site (Fig. 2a). 185Although the seasonal signal of elevated dust flux at both sites was largely lost at 3000 m depth, following high dust input (Fig. 3a). Assuming that this temporal coherence was not accidental, Table 2). 197Overall, lithogenic material appears to be a more important ballast for POC in the central Table 2). The ballasting ability of lithogenic particles at NOG appears to be 215 confined to the summer-autumn period (Fig. 3a) when the surface fertilisation by dust was Jickells, T., An, Z., Andersen, K. K., Baker, A., Bergametti, G., Brooks, N., et al. 244 Global iron connections between desert dust, ocean biogeochemistry, and climate.

Coccolithophore fluxes in the open tropical North Atlantic: influence of thermocline depth, Amazon water, and Saharan dust

“…therein;Okin et al, 2011). The fertilizing potential of Saharan dust is supported by previous studies in the Amazon Basin (Mahowald et al, 2008(Mahowald et al, , 2009Bristow et al, 2010), the Gulf of Mexico and the coast of southern Florida (Walsh et al, 2006;Lenes et al, 2012), and the North Atlantic subtropical gyre (Pabortsava et al, 2017). In addition to Saharan dust inputs, N 2 fixation by marine diazotrophs (Carpenter et al, 1999) and the seasonal discharge and eastward advection of the nutrient-enriched Amazon River Plume (e.g.…”

“…High fluxes of organic material recently observed in a sediment trap in the North Atlantic subtropical gyre (23 • N, 41 • W) have been associated with enhanced phytoplankton productivity resulting from stimulated nitrogen fixation by Trichodesmium species following the deposition of dust-derived nutrients (Pabortsava et al, 2017). The role of aeolian dust for nitrogen fixation in subtropical and tropical oligotrophic regions has been previously reported by several authors (e.g.…”

“…In spite of all this evidence, mineral dust deposition is also thought to increase carbon sequestration to the deep ocean by acting as a mineral ballast of sinking particles (Pabortsava et al, 2017). Van der Jagt et al (2017) report more abundant and faster-sinking aggregates when formed from a natural plankton community that has been exposed to Saharan dust deposition compared to less abundant and slowersinking aggregates when formed without dust.…”

“…In the tropical North Atlantic, meteorological conditions are mostly controlled by seasonal variations in the trade winds and the Intertropical Convergence Zone (ITCZ), the latter being a zone of low pressure and increased cloudiness and precipitation near the Equator (e.g. Oschlies and Garçon, 1998 Mann and Lazier, 2006;Haidar and Thierstein, 2001), the highest temperature values during the fall of 2012 and from mid-summer to the fall of 2013 suggest that stratification was the strongest under the influence of the ITCZ, probably reflecting the weakening of Figure 7. Spatio-temporal variation in the scores obtained from factor analysis.…”

The ballasting effect of Saharan dust deposition on aggregate dynamics and carbon export: Aggregation, settling, and scavenging potential of marine snow

“…So far, no studies have observed an influence from ballast minerals on microbial degradation (Ploug et al a,b ; Iversen and Ploug ; Iversen and Robert ), suggesting that the increased aggregate formation and higher sinking velocities may lead to an increased POC flux in dusty regions. This effect was directly shown in mesocosm experiments in the Mediterranean Sea, where an input of Saharan dust resulted in two to six times higher POC flux (Bressac et al ), and indirectly with sediment traps recording increased POC fluxes with dust deposition (Ternon et al ; Fischer et al ; Pabortsava et al ). Our results show that dust deposition caused a 10‐fold increase in total aggregated volume and two‐fold higher size‐specific sinking velocities of the dust‐ballasted aggregates.…”