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

Abstract

Abstract. 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 (~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 ~5-6 and -•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.

Atmospheric and oceanic dust fluxes in the northeastern tropical Atlantic Ocean: how close a coupling?

“…Atmospheric data result from a combination of satellite-derived observations of the dust load in the air column and model-derived deposition rates, and the oceanic data consists of time series mea- sured by sediment traps at several depths in the water column. Temporal matches and mismatches of the atmospheric and oceanic data sets are presented to complement previously published data (Bory and Newton, 2000) in terms of understanding the dust transfer processes from the atmosphere to the ocean water column, depending on the biological activity in the surface waters. Quantitatively, atmospheric and oceanic fluxes show significant discrepancies.…”

“…Ratmeyer et al, 1999a), and the variable efficiency of biologically-mediated downward dust transport (e.g. Bory and Newton, 2000). Due to the dearth of dust deposition measurements close to ocean time-series sites, only a few studies in the Mediterranean Sea have quantitatively explored the direct coupling of atmospheric dust deposition with fluxes in the water column Dulac et al, 1996).…”

Quantification of the lithogenic carbon pump following a simulated dust-deposition event in large mesocosms

“…All of these analyses, performed at an annual timescale, reflect average fluxes during both productive and oligotrophic periods missing important temporal variability within the flux relationships. Therefore, we focused on specific dust-deposition events on timescales on the order of a week, representative of the residence time for dust within surface waters estimated from days to month Bory and Newton, 2000;Croot et al, 2004;Frew et al, 2006). During this artificial seeding experiment in LNLC ecosystems, the lithogenic fluxes were high and the algal biomass was likely too low to drive large POC flux events.…”

Rates of dissolved organic carbon production and bacterial activity in the eastern North Atlantic Subtropical Gyre during summer

“…Duarte et al (2001) proposed lateral inputs from the productive upwelling areas in the NW African coast as a potentially relevant allochthonous carbon source in the subtropical NE Atlantic. Other allochthonous sources of organic matter that should also be taken into consideration would be atmospheric deposition of DOM through marine rain (Willey et al 2000) or dust storms (Cornell et al 1995, Bory & Newton 2000, and photochemical production of labile DOC from more refractory molecules (e.g. Mopper et al 1991, Bushaw et al 1996, Moran & Zepp 2000.…”