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The biological carbon pump (BCP) exports organic carbon from the surface to the deep ocean through which it drives the oceanic carbon storage of atmospheric CO2. The majority of this exported carbon is mediated by sinking marine particles, which are mostly formed by primary producers i.e. phytoplankton, and detrital material including dead zoo- and phytoplankton as well as fecal pellets. The efficiency at which these marine particles are exported is largely dependent on the interplay between their degradation and the duration the particles settle through the water column. Particle and aggregate degradation is largely driven by attached microbes and zooplankton grazing, which is most prominent in the upper few hundred meters of the water column. Settling velocities of particles or aggregates are largely determined by their density and thus composition. Therefore, fast settling aggregates or particles sink more rapidly through the upper water column, which increases the carbon export efficiency. The incorporation of ballasting components, such as sediments or minerals, during particle aggregation may increase the size-specific settling velocities of the aggregates and are thus an important process for carbon export via the biological pump. The manuscripts within this dissertation focus on the impact of aggregate morphology and ballasting on settling velocities and how carbon export is mediated by slow versus fast settling aggregates. We investigated (1) the horizontal relocation of slow vs. fast settling aggregates in form of subsurface particle clouds, (2) aggregate ballasting through the incorporation of minerals which are released by melting sea-ice and (3) the role of the aggregate microstructure on settling.

Settling marine aggregates, such as zooplankton fecal pellets and marine snow, transport organic matter from the surface ocean to the deep sea and are largely responsible for the ocean's sequestration of carbon. However, our understanding of the functioning of the biological pump and the distribution of particulate organic matter in the water column often hinge on limited bulk data from sediment traps, large volume filtration or instantaneous snap-shots from in situ optical systems that only see a small part of the particle and aggregate spectra. We evaluated the added value of combining different optical systems to detect a range of organic and inorganic particle types during a case-study in the Cape Blanc upwelling region. Laboratory calibrations showed that one camera system detected large organic aggregates well and in situ data showed that it correlated positively with fluorescence. The other camera was better at detecting small, mainly inorganic particles which were not seen by the first camera and correlated positively with turbidity. The combined deployments of the two optical systems together with fluorescence and turbidity sensors showed potentials for added insights into spatial (depth) and temporal (diurnal) particle dynamics. The case study exemplified the different efficiencies of two camera systems to detect particles of different types in marine waters. From this, the results highlighted the importance of discriminating between qualitative and quantitative ranges of imaging systems, in order to understand the quantitative range of sizes as well as types of particles detected by a given system. This is especially important when optical systems are used to estimate carbon fluxes and particulate organic matter distribution in the water column from vertical profiles of particle size-distribution and abundance.

Tropical oceans provide a benchmark for future primary productivity in increasingly warmer, stratified and nutrient-depleted waters. In this context, we assess the export fluxes of calcifying phytoplankton (coccolithophores) across the tropical North Atlantic, from upwelling affected NW Africa, via three ocean sites along 12 degrees N to the Caribbean. Sampling was undertaken by means of a spatial array of four time-series sediment traps collecting particle fluxes in two-week intervals, from October 2012 to February 2014, allowing to track temporal changes along the southern margin of the North Atlantic central gyre. Species composition and seasonal export fluxes of coccolithophores show steep gradients in two groups. Upper photic zone (UPZ) and placolith-bearing species dominated by Emiliania huxleyi and Gephyrocapsa oceanica are most abundant in the mesotrophic surface waters of the Cape Blanc upwelling system off NW Africa. They decline gradually towards the Caribbean, paralleled by increasing surface temperatures and decreasing surface chlorophyll-a. Meanwhile the abundance of lower photic zone (LPZ) species Florisphaera profunda and Gladiolithus flabellatus increase in the same direction, reaching fluxes up to 3-5 times higher in the western end of the transect compared to the UPZ flora in mesotrophic waters. Adapted to low light conditions, the LPZ species follow the geostrophic wind-forced deepening of the thermocline/nutricline westward in ever lower species diversity towards the Caribbean. Temporal changes were marked by weak seasonality in coccolith fluxes at all four sites, modulated by latitudinal migration of the Intertropical Convergence Zone (ITCZ) at the tropical sites Ml, M2 and M4, and by spatiotemporal variation in wind-forced upwelling at site CB. A seasonal mismatch was observed between LPZ and UPZ-oligotrophic taxa (i.e. Umbellosphaera spp., Rhabdosphaera spp.) vs. UPZ-opportunistic species (E. huxleyi, G. oceanica) at the western site M4 contrasting with the more similar seasonal patterns amongst all species towards site CB. We interpret this as reflecting the entire photic zone becoming increasingly nutrient enriched towards east whenever wind-forced mixing occurs due to the eastward shoaling of the thermocline/nutricline. Our synoptic observations of seasonally resolved export fluxes at four sites contribute to the spatiotemporal understanding of coccolithophore fluxes across the entire tropical North Atlantic, urging for considering phytoplankton- and carbonate production across the entire photic zone when projecting the effects of ocean warming on future primary production. NWONetherlands Organization for Scientific Research (NWO) [822.01.008] ERCEuropean Research Council (ERC) [311152] University of Bremen European UnionEuropean Union (EU) [600411, 796802] Portuguese Science Foundation (FCT)Portuguese Foundation for Science and Technology [UID/MAR/04292/2013] [SFRH/BPD/118760/2016] info:eu-repo/semantics/publishedVersion

Hundreds to thousands of tons of mineral dust are emitted from the eartha s deserts to the atmosphere each year. Subsequently, mineral dust can be transported over thousands of kilometres with the prevailing winds in the atmosphere until it is deposited on land or in the ocean. On the one hand, the atmospheric dust transport and deposition has an important impact on human health and the eartha s climate system. On the other hand, the dust emission, transport and deposition is controlled by parameters of climate change such as precipitation, wind speed, temperature and vegetation cover. Last but not least, the sensitivity of mineral dust properties to environmental parameters is used as a proxy to reconstruct the climate of the past which is required to test climate models. The large mineral dust emissions, ita s relation to climate and impact on human health underscores the importance to understand the dust cycle. However, research gaps still exist with respect to modern emission, transport and deposition of mineral dust. The Saharan desert is the worlda s largest source of atmospheric mineral dust and has been studied at least since the 18th century. Up to now, the major potential dust source areas are known and it has been unravelled that precipitation results in a decrease in the particle size of deposited Saharan dust on the NW African margin. Further, it is well-known that the seasonal change in insolation on the African continent results in a seasonal change in the altitude of Saharan dust transport. However, research gaps still exists with respect to the influence of the seasonal change in the dust transport on the resulting particle size, mineralogical composition and provenance of Saharan dust. Therefore, the aim of the present Ph.D. thesis was to investigate whether the seasonal change in atmospheric circulation results in a seasonal variation in particle size, mineralogical composition and provenance of Saharan dust. The strategy was to measure the particle size and mineralogy of modern Saharan dust collected during several years onshore NW Africa in continental dust traps and offshore NW Africa in marine sediment trap moorings. By comparing the particle size and mineralogy data to concurrent meteorological data and satellite images, possible connections between the dust properties and meteorological conditions were inferred. Dust which deposited offshore NW Africa was transported with the trades in winter and in the Saharan air layer (SAL) during summer. The particle size of the deposited dust varied seasonally according to dust-storm events year-round, trade-wind speed in winter and wet deposition from the SAL in summer. A seasonal change in the dust mineralogical composition and provenance was observed which could be related to the seasonal change in the dust transport. Onshore NW Africa, the particle size of deposited dust varied seasonally according to meteorological parameters depending on the location of the sampling site. In the major dust source area at 20 AdegreeN, dust was predominantly transported in the trade wind layer year-round. Therefore, the particle size varied seasonally according to the trade wind speed and the mineralogical composition and dust sources according to the direction of the trade winds. In contrast, the particle size of Saharan dust deposited outside a major dust source area at 15 AdegreeN was controlled by the dust transport distance during spring and by the wet deposition from the SAL during summer.

Mg/Ca- and oxygen isotope ratios in the shells of planktic foraminifera are widely used in paleoceanographic studies for the reconstruction of water temperatures. Standard investigations analyze multiple shells of a species at once. However, when single shells are analyzed substantial differences of the Mg/Ca- and oxygen isotope ratios are found. These are explained by seasonality and natural variability, and are increasingly used for the reconstruction of past environmental conditions. The present thesis analyzes the differences of the Mg/Ca- and oxygen isotope ratios between individual shells in the upwelling region off Northwest-Africa. The aim of this thesis is to quantify natural variability, to improve the applicability of single shells for the reconstruction of temperature seasonality and to show the potential of single specimens for recording past ocean stratification.

Abrupt climate changes during the last glacial-interglacial cycle known as Dansgaard-Oeschger (D-O) oscillations. Although, it is evident that these climate changes are most severe in the North Atlantic realm, the forcing of the D-O climate oscillations is still controversial. Feedback mechanisms between ocean heat transport, atmospheric circulation, and low latitude climate changes are thought to play an important role in the continuation, amplification, and possibly initiation of D-O climate oscillations. However, the role low-latitude climate zones and their potential to influence or initiate high-latitude climate change is still unknown. The region off Cape Blanc (Northwest Africa) is situated between the North Atlantic mid latitudes and the equatorial Atlantic, and is a key region to study climate forcing in the subtropics.Millennial-scale changes of the terrigenous sediment composition based on non-destructive XRF core scanning and grain-size variations of the terrigenous sediment fraction indicate coherent high- and low- climate variations during the last glacial-interglacial cycle. Water corrected X-Ray Fluorescence (XRF) Core Scanner measurements are used to obtaining chemical changes of the eolian dust input. End member modeling of the terrigenous fraction reveal two eolian end members, and a hemipelagic end member. Additionally, opal and organic matter accumulations are used to reconstruct upwelling intensities and surface water productivity off Cape Blanc. These data indicate increased continental aridity, trade wind intensities, and upwelling intensities accompanied by reduced surface water productivity associated with North Atlantic Heinrich events and extreme Greenland stadial events. Moreover, both the eolian contributions off Cape Blanc and simulations of the vegetation (obtained from the Atmosphere-Ocean-Vegetation model CLIMBER 2) indicate that Northwest African humidity is forced by the strength of the African Monsoon during interglacial conditions.