Scientific background

Recovery of sediment traps onboard RV Meteor during expedition M140 FORAMFLUX. Video: J.-B. Stuut

Abbildung zeigt — The locations of the Cape Blanc sediment trap moorings.

Overview

The MARUM Cape Blanc sediment trap observatory was started in 1988 to study the fluxes of marine particles in the productive waters off Cape Blanc in Mauretania, West Africa. It consisted of a single mooring with sometimes just one but mostly two sediment traps with collection depths of around 3500 m for the deep and between 700 m and 1300 m for the shallow mounted sediment trap. In 2003 the mooring CB (simply for Cape Blanc) was joined by a second mooring in shallower, even more productive waters closer to the coast. This mooring, CBi (the i for inner) also consisted of two sediment traps, the shallower in a directly to CB comparable depth of around 1300 m, the deep one in around 1900 m water depth. In 2013, an autonomous dust-collecting buoy was deployed near to CB by the Royal Netherlands Institute for Sea Research (NIOZ). This buoy, named Carmen, collects high resolution meteorological data and physical atmospheric Saharan dust samples, similar to a sediment trap. This integrated approach of sample collection at sea offers the possibility to study not only the properties of Saharan-dust particles but also and foremost allows a direct data comparison between terrigenous inputs at the sea surface and the fluxes intercepted in the sediment traps at the very same location. Altogether, the Cape Blanc sediment trap observatory represents one of the longest continuous sediment trap time series world-wide and now approaches lengths that allow quantitative investigation of decadal-scale changes in productivity, lithogenic flux and the composition of marine plankton.

The material and data derived from the Cape Blanc sediment trap observatory is available to a consortium of researchers at MARUM (and also other institutes via collaboration) and has so far been employed in dozens of publications covering a broad range of flux associated topics such as flux variability in terms of mass and composition, particle advection, sinking rates, degradation or ballasting. Particularly in a time marked by unprecedented environmental challenges, sediment trap records serve as a crucial indicator of change.

Regional setting

The Cape Blanc region is part of the Canary Upwelling Ecosystem, and exhibits active upwelling both seasonal and year-round in different sub regions. The dominant feature of the Canary Upwelling Ecosystem's hydrography is the Cape Verde Front, which forms at the southern boundary of the North Atlantic Subtropical Gyre. The Canary Current, which forms the eastern boundary of the North Atlantic Subtropical Gyre, flows southwest along the African coast until it detaches from the coastline at around 20° and merges with the North Equatorial Current. The south-westerly trade winds promote year-round upwelling between 32° and 20° N with extended filaments and eddies caused by the region's geographical heterogeneity. South of the Cape Verde Front, the currents and upwelling intensity vary seasonally due to a complex interaction of the ITCZ related displacement of the North Equatorial Counter Current and a subsequent compression or dilation of a recirculation cell and the Guinea Dome. The eastern part of this recirculation cell, positioned south-east of the Cabo Verde islands is the northward flowing Mauretania Current, which is deflected westward at the Cape Verde Front and continues as the Cabo Verde Current. Upwelling and elevated surface chlorophyll concentrations in the region south of Cape Blanc, at around 20° N are observable during December to May and virtually absent during the remainder of the year. The Canary Upwelling Ecosystem as a whole is an important socio-economic, oceanographic and climatological region.

Exemplary research topics

The already existing and continuing time series harbours manifold options for the quantitative and qualitative investigation of decadal-scale changes in all kinds of parameters associated with fluxes from the surface ocean to the deep sea.

Total flux, flux composition and its variability

Amongst the first and standard analyses of the material derived from the sediment traps are the determination of total mass and the flux composition in chemical terms. The matter flux from the surface ocean to the sea floor consists to variable amounts of organic matter (the remains of planktic organisms and their excretions), carbonate and silicate shells of planktic organisms and lithogenic particles (dust or in high latitudes minerals and sediments trapped in melting sea ice). The total mass flux as well as the relative amounts of these components in the samples differ dramatically over the course of a year and between consecutive years. Any of these components can be analysed in detail for its composition. The lithogenic part for the constituent mineral grains and their most likely source of origin; the shell of planktic organisms for their taxonomic composition and the organic matter for its detailed chemical composition.

Flux attenuation

Most types of material, organic as well as inorganic, experience modification during descend through the water column. Organic material is degraded by microorganisms, calcitic shells can start to dissolve. The amount of modifications depends to a large part on the sinking speed of the particles or aggregates; low density leads to longer residence times in the water column and usually stronger degradation. High density particles or aggregates sink faster and reach zones with low temperatures, slowing down biological activity, and ultimately the sea floor faster. An interesting facet of the flux of matter in the oceans in this regard is ballasting; low density aggregates of marine snow are ballasted by high density particles increasing the overall sinking speed and consequently to total amount of mass transferred from the surface ocean to the deep sea. Lithogenic particles (dust) and the shell of planktic organisms can serve as ballast.

Monthly climatology of remotely sensed chlorophyll a concentration in the Cape Blanc region.