Description: Organic and inorganic components from terrestrial origin can be transported into the marine system by wind and rivers. In the study region wind transport forms the major transport mechanism by episodically transporting large amounts of particles into the upper water column. Recent studies have revealed that these particles are most probably not transported down through the water column immediately but that there are several layers within the water column where these particles remain in suspension. It is suggested that the residence time of these particles within these so called nepheloid layers might be up to several thousands of years. For adequate subsampling of these layers with the help of in-situ pumps and a Rosette containing 18 Niskin bottles (10 l volume), the temperature, density, chlorophyll and oxygen differences of the upper 600m of the water column was determined using a CTD (seabird 911+).

Description: A valid assessment of selective aerobic degradation on organic matter (OM) and its impact on OM-based proxies is vital to produce accurate environmental reconstructions. However, most studies investigating these effects suffer from inherent environmental heterogeneities. In this study, we used surface samples collected along two meter-scale transects and one longer transect in the northeastern Arabian Sea to constrain initial OM heterogeneity, in order to evaluate selective aerobic degradation on temperature, productivity and alteration indices at the sediment-water interface. All of the studied alteration indices, the higher plant alkane index, alcohol preservation index, and diol oxidation index, demonstrated that they are sensitive indicators for changes in the oxygen regime. Several export production indices, a cholesterol-based stanol/stenol index and dinoflagellate lipid- and cyst-based ratios, showed significant (more than 20%) change only over the lateral oxygen gradients. Therefore, these compounds do not exclusively reflect surface water productivity, but are significantly altered after deposition. Two of the proxies, glycerol dibiphytanyl glycerol tetraether-based TEX86 sea surface temperature indices and indices based on phytol, phytane and pristane, did not show any trends related to oxygen. Nevertheless, unrealistic sea surface temperatures were obtained after application of the TEX86, TEX86L, and TEX86H proxies. The phytol-based ratios were likely affected by the sedimentary production of pristane. Our results demonstrate the selective impact of aerobic organic matter degradation on the lipid and palynomorph composition of surface sediments along a short lateral oxygen gradient and suggest that some of the investigated proxies may be useful tracers of changing redox conditions at the sediment-water interface.

Description: In order to investigate a possible connection between tropical northeast (NE) Atlantic primary productivity, Atlantic meridional overturning circulation (AMOC), and drought in the Sahel region during Heinrich Stadial 1 (HS1), we used dinoflagellate cyst (dinocyst) assemblages, Mg/Ca based reconstructed temperatures, stable carbon isotopes (d13C) and geochemical parameters of a marine sediment core (GeoB 9508-5) from the continental slope offshore Senegal. Our results show a two-phase productivity pattern within HS1 that progressed from an interval of low marine productivity between ~ 19 and 16 kyr BP to a phase with an abrupt and large productivity increase from ~ 16 to 15 kyr BP. The second phase is characterized by distinct heavy planktonic d13C values and high concentrations of heterotrophic dinocysts in addition to a significant cooling signal based on reconstructions of past sea surface temperatures (SST). We conclude that productivity variations within HS1 can be attributed to a substantial shift of West African atmospheric processes. Taken together our results indicate a significant intensification of the North East (NE) trade winds over West Africa leading to more intense upwelling during the last millennium of HS1 between ~ 16 and 15 kyr BP, thus leaving a strong imprint on the dinocyst assemblages and sea surface conditions. Therefore, the two-phase productivity pattern indicates a complex hydrographic setting suggesting that HS1 cannot be regarded as uniform as previously thought.

Description: Long-term changes in vegetation and climate of southern Brazil, as well as ocean dynamics of the adjacent South Atlantic, were studied by analyses of pollen, spores and organic-walled dinoflagellate cysts (dinocysts) in marine sediment core GeoB2107-3 collected offshore southern Brazil covering the last 73.5 cal kyr BP. The pollen record indicates that grasslands were much more frequent in the landscapes of southern Brazil during the last glacial period if compared to the late Holocene, reflecting relatively colder and/or less humid climatic conditions. Patches of forest occurred in the lowlands and probably also on the exposed continental shelf that was mainly covered by salt marshes. Interestingly, drought-susceptible Araucaria trees were frequent in the highlands (with a similar abundance as during the late Holocene) until 65 cal kyr BP, but were rare during the following glacial period. Atlantic rainforest was present in the northern lowlands of southern Brazil during the recorded last glacial period, but was strongly reduced from 38.5 until 13.0 cal kyr BP. The reduction was probably controlled by colder and/or less humid climatic conditions. Atlantic rainforest expanded to the south since the Lateglacial period, while Araucaria forests advanced in the highlands only during the late Holocene. Dinocysts data indicate that the Brazil Current (BC) with its warm, salty and nutrient-poor waters influenced the study area throughout the investigated period. However, variations in the proportion of dinocyst taxa indicating an eutrophic environment reflect the input of nutrients transported mainly by the Brazilian Coastal Current (BCC) and partly discharged by the Rio Itajaí (the major river closest to the core site). This was strongly related to changes in sea level. A stronger influence of the BCC with nutrient rich waters occurred during Marine Isotope Stage (MIS) 4 and in particular during the late MIS 3 and MIS 2 under low sea level. Evidence of Nothofagus pollen grains from the southern Andes during late MIS 3 and MIS 2 suggests an efficient transport by the southern westerlies and Argentinean rivers, then by the Malvinas Current and finally by the BCC to the study site. Major changes in the pollen/spore and dinocyst assemblages occur with similar pacing, indicating strongly interlinked continental and marine environmental changes. Proxy comparisons suggest that the changes were driven by similar overarching factors, of which the most important was orbital obliquity.

Description: To extend information about dinoflagellate cysts (dinocysts) and planktonic foraminifera production, succession, excystment and transport in the upper water column, we investigated their fluxes during a 7-day survey in the active upwelling off Cape Blanc (NW Africa) in November 2018 with drifting traps at 100 m, 200 m, and 400 m water depth. The 7-day survey covered a successive change from active upwelling toward stratified conditions. Highest production of organic dinocysts and planktonic foraminifera were observed under active upwelling conditions and decreased drastically towards the end of the survey. Calcareous dinocysts appeared later during upwelling relaxation. Collected cytoplasm bearing (full) dinocysts and foraminifera were produced in the water column above the traps (〈100 m depth). Part of the organic-walled empty dinocysts were resuspended implying that sediments below the survey site are likely to contain both local and allochthonous cyst assemblages. We are the first to show that excystment in the upper water column is species-specific. Brigantedinium excysted in the upper water column before reaching deeper depths whereas no upper water column excystment was observed for the other dinoflagellate species. Dinoflagellate and planktonic foraminifera associations showed a clear succession. Echinidinium zonneveldiae, Brigantedinium spp., other peridinioids, Echinidinium spp., cysts of Pentapharsodinium dalei and other photosynthetic organic-walled dinocysts” as well as Neogloboquadrina incompta and Globigerinella calida were collected during active upwelling. Lingulodinium machaerophorum was produced during upwelling relaxation and Gymnodiniaceae cysts (G. microreticulatum, G. catenatum) as well as the foraminifera Globigerina bulloides and Orbulina universa were collected during stratified conditions. Apart from enhancing the biological knowledge of these species, our observations will allow more detailed reconstructions of upwelling history in the Cape Blanc region based on sedimentary archives using fossilised dinoflagellate and planktonic foraminifera assemblages.