Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> Mesoscale eddies are abundant in the eastern tropical North Atlantic and act as oases for phytoplankton growth due to local enrichment of nutrients in otherwise oligotrophic waters. It is not clear whether these eddies can efficiently transfer organic carbon and other flux components to depth and if they are important for the marine carbon budget. Due to their transient and regionally restricted nature, measurements of eddies' contribution to bathypelagic particle flux are difficult to obtain. Rare observations of export flux associated with low-oxygen eddies have suggested efficient export from the surface to the deep ocean, indicating that organic carbon flux attenuation might be low. Here we report on particle flux dynamics north of the Cabo Verde islands at the oligotrophic Cape Verde Ocean Observatory (CVOO; approx. 17<span class="inline-formula"><sup>∘</sup></span>35<span class="inline-formula"><sup>′</sup></span> N, 24<span class="inline-formula"><sup>∘</sup></span>15<span class="inline-formula"><sup>′</sup></span> W). The CVOO site is located in the preferred pathways of highly productive eddies that ultimately originate from the Mauritanian upwelling region. Between 2009 and 2016, we collected biogenic and lithogenic particle fluxes with sediment traps moored at ca. 1 and 3 km water depths at the CVOO site. From concurrent hydrography and oxygen observations, we confirm earlier findings that highly productive eddies are characterized by colder and less saline waters and a low-oxygen signal as well. Overall, we observed quite consistent seasonal flux patterns during the passage of highly productive eddies in the winters of 2010, 2012 and 2016. We found flux increases at 3 km depth during October–November when the eddies approached CVOO and distinct flux peaks during February–March, clearly exceeding low oligotrophic background fluxes during winter 2011 and showing an enhanced particle flux seasonality. During spring, we observed a stepwise flux decrease leading to summer flux minima. The flux pattern of biogenic silicate (BSi) showed a stronger seasonality compared to organic carbon. Additionally, the deep fluxes of total mass showed an unusually higher seasonality compared to the 1 km traps. We assume that BSi and organic carbon/lithogenic material had different sources within the eddies. BSi-rich particles may originate at the eddy boundaries where large diatom aggregates are formed due to strong shear and turbulence, resulting in gravitational settling and, additionally, in an active local downward transport. Organic carbon associated with lithogenic material is assumed to originate from the interior of eddies or from mixed sources, both constituting smaller, dust-ballasted particles. Our findings suggest that the regularly passing highly productive eddies at CVOO repeatedly release characteristic flux signals to the bathypelagic zone during winter–spring seasons that are far above the oligotrophic background fluxes and sequester higher organic carbon than during oligotrophic settings. However, the reasons for a lower carbon flux attenuation below eddies remain elusive.

Highlights

  • 40 The eastern tropical North Atlantic hosts one of the major Oxygen Minimum Zones (OMZ) of the world oceans (Karstensen et al 2008; Brandt et al, 2015)

  • 440 A multiyear monitoring of the biogeochemical particle fluxes of passing eddies north of the Cape Verde Islands revealed the following major findings: - during eddy passages in the winters of 2010, 2012 and 2016, particle fluxes showed a consistent sinusoidal flux pattern with increasing fluxes in fall, a decrease at the turn of the year and maxima around February-March, - biogenic silicate (BSi) showed a stronger seasonal pattern with distinct peaks compared to organic carbon and the lithogenic

  • BSi flux patterns at 3 km depth revealed a higher seasonality compared to the 1 km depth level which was due to passages of low oxygen eddies, - the diatom assemblage during eddy passages in winter was dominated by small species and the flux was not related to the organic carbon or BSi fluxes

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Summary

Introduction

40 The eastern tropical North Atlantic hosts one of the major Oxygen Minimum Zones (OMZ) of the world oceans (Karstensen et al 2008; Brandt et al, 2015). The specific oxygen consumption and ventilation pattern creates two oxygen minima in the eastern tropical North Atlantic, the shallow one at about 80 m depth mostly associated with regional processes, and the deep one at about 450 m water depth (minimal oxygen about 40 μmol kg-1) associated with the gyre scale ventilation (e.g. Karstensen et al 2008, Fiedler et al, 2016; Brandt et al 2015). 45 flow features in the region are the boundary current system (Mittelstaedt, 1991; Brandt et al, 2015) and, at a local scale, westwards propagating mesoscale eddies (e.g. Schütte et al, 2016a; Pietri and Karstensen, 2018). The two features are connected as the mesoscale eddies spin off instabilities of the boundary current and transport water of coastal origin offshore, with elevated nutrients and high organic carbon. The combination of high biomass, high respiration of organic matter, and the physical isolation of the eddy core 55 water masses leads to low oxygen concentration in the eddies where even suboxic conditions (O2

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