Transport of airborne lithogenic material down through the water column in two contrasting regions of the eastern subtropical North Atlantic Ocean

Downward particle fluxes were measured using deep-moored sediment traps deployed in two regions of contrasting primary productivity levels (mesotrophic and oligotrophic) of the eastern subtropical North Atlantic Ocean. The high percentage of lithogenic material (similar to 20-30% on average) in the particulate matter collected shows the regional significance of the atmospheric dust inputs originating from West Africa. The magnitudes of lithogenic and biogenic fluxes decrease similar to 5-6 and similar to 8-9 fold, respectively, from near the African margin (mesotrophic region) to the remote open ocean (oligotrophic region). These trophic differences seem to give rise to differences in the characteristics of the downward transport of lithogenic material. At the oligotrophic site, the relatively low and slow export of biogenic matter apparently limits and delays the removal of lithogenic particles delivered to surface waters from the atmosphere. In contrast, the higher biological activity in the mesotrophic region seems to provide persistent conditions for an efficient and faster downward transport of the deposited lithogenic particles, and the temporal variability of lithogenic fluxes largely reflects that of the atmospheric dust inputs. Thus whether the temporal variability of the exported lithogenic flux in the water column follows that of the atmospheric deposition appears to depend on the trophic status. In the mesotrophic region the oft-observed linear relationship between lithogenic and particulate organic matter (hereinafter POM) fluxes breaks down at high POM fluxes. This observation adds weight to the idea that linear relationships between POM fluxes and some candidate proxies for POM transfer cannot be assumed when POM export is large. A high mesoscale variability of biogenic, but not lithogenic, fluxes in the water column of the mesotrophic region underscores the relevance of mesoscale studies for regional estimates of export of biogenic material.