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  • 1
    Language: English
    In: Marine Geology, 2010, Vol.275(1), pp.178-198
    Description: A major outcome from studying the sedimentary archives of the Timiris Canyon off Mauritania is that climate shifts in the Saharan hinterland not only influenced offshore pelagic sedimentation processes, but also controlled turbidite activity in the canyon system. Sediment supply to the slope, predominantly by eolian dust and remobilised eolian sands, was greatest during hyperarid glacial and deglacial periods and, due to overall increased Trade wind strengths, upwelling induced pelagic productivity attained highest magnitudes. Turbidite activity in the Timiris Canyon responded rapidly to these shifts in the surrounding sedimentary regime on the continental slope. During high-glacial and early deglacial times, the tributary gullies on the uppermost slope were rapidly and repetitively filled, subsequently turbidity currents were released and large turbulent flows passed through the lower Timiris Canyon, leading to overspill sequences on its levees. In addition to this scenario, partial flooding of the exposed outer shelf during successive sea-level rises in MIS 3 may have caused re-mobilization of huge aeolian dune fields, that had expanded close to the shelf edge during glacial exposure. During the Holocene, no turbidite activity is recorded in the Lower Timiris Canyon, whereas a core from a widened area in the Upper Timiris Canyon reveals an interesting climate controlled 900-year cyclicity of carbonate- and mud-turbidite events throughout the Holocene.
    Keywords: Submarine Canyons ; Slope Sedimentary Regimes ; Turbidite Frequencies ; Hyperarid Continental Margins ; Climate Archives in Canyons ; Nw-African Maritime Climate ; Oceanography ; Geology
    ISSN: 0025-3227
    E-ISSN: 1872-6151
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  • 2
    Language: English
    In: Palaeogeography, Palaeoclimatology, Palaeoecology, 2011, Vol.302(3), pp.311-322
    Description: The present study is the first study on the stable oxygen isotope composition of the photosynthetic calcareous-walled dinoflagellate species off NW Africa during the last 45,000 yr. based temperature estimates of sediment core GeoB 8507-3 were compared with those obtained from the stable oxygen isotopes of the planktic foraminifera and (pink), and the Mg/Ca ratio of (pink). We show that the isotopic composition of and the temperature estimates based on the equation for inorganically precipitated calcite provide comparable results to those obtained from (pink) isotopes and Mg/Ca ratios with exception of the Early Holocene and the Younger Dryas. The recently proposed palaeotemperature equation of Zonneveld et al. (2007), however, provides unrealistic temperature reconstructions that are about 16 °C lower than those based on planktic foraminifera. Thus, this equation needs to be revised. The difference between and isotopic and temperature reconstructions can be ascribed to differences in the ecology of both species, especially with regard to their depth habitat and/or seasonal production in the research area. All temperature proxies suggest comparable conditions during the last glacial and Holocene. Small differences between the reconstructed temperature values of and the other proxies can be explained by differences in seasonal production of the individual species. The relatively low temperatures recorded by . at about 15,000 to 8,000 yr BP are interpreted to reflect an increase in duration and/or intensity of the upwelling in the vicinity of the core site in comparison to the last glacial, with an abrupt and strong decrease of upwelling intensity and/or duration during the Early Holocene and the Younger Dryas. ► First δ O record on the calcareous-walled dinoflagellate . ► as an indicator of the upwelling duration and intensity. ► Increase in upwelling duration and/or intensity in the Early Holocene/Younger Dryas.
    Keywords: Calcareous-Walled Dinoflagellate Cyst ; Stable Oxygen Isotopes ; Upwelling ; Palaeotemperature ; Deep-Chlorophyll Maximum ; Nw Africa ; Geology
    ISSN: 0031-0182
    E-ISSN: 1872-616X
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  • 3
    Language: English
    In: Palaeogeography, Palaeoclimatology, Palaeoecology, 2010, Vol.291(3), pp.443-455
    Description: NW African climate shows orbital and millennial-scale variations, which are tightly connected to changes in marine productivity. We present an organic-walled dinoflagellate cyst (dinocyst) record from a sediment core off Cape Yubi at about 27°N in the Canary Basin covering the time period from 47 to 3 ka before present (BP). The dinocyst record reflects differences in upwelling intensity and seasonality as well as the influence of fluvial input. Sea-level changes play an important role for the upwelling pattern and productivity signals at the core site. Within the studied time interval, four main phases were distinguished. (1) From 45 to 24 ka BP, when sea-level was mostly about 75 m lower than today, high relative abundances of cysts of heterotrophic taxa point to enhanced upwelling activity, especially during Heinrich Events, while relatively low dinocyst accumulation rates indicate that filament activity at the core location was strongly reduced. (2) At sea-level lowstand during the LGM to H1, dinocyst accumulation rates suggest that local filament formation was even more inhibited. (3) From the early Holocene to about 8 ka BP, extraordinary high accumulation rates of most dinocyst species, especially of , suggest that nutrient supply via fluvial input increased and rising sea-level promoted filament formation. At the same time, the upwelling season prolongated. (4) A relative increase in cysts of photoautotrophic taxa from about 8 ka BP onwards indicates more stratified conditions while fluvial input decreased. Our study shows that productivity records can be very sensitive to regional features. From the dinocyst data we infer that marine surface productivity off Cape Yubi during glacial times was within the scale of modern times but extremely enhanced during deglaciation.
    Keywords: Northwest Africa ; Dinoflagellate Cysts ; Fluvial Input ; Sea-Level ; Productivity ; Upwelling ; Geology
    ISSN: 0031-0182
    E-ISSN: 1872-616X
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  • 4
    In: Paleoceanography, May 2014, Vol.29(5), pp.423-437
    Description: The sea surface temperature (SST) of the tropical Indian Ocean is a major component of global climate teleconnections. While the Holocene SST history is documented for regions affected by the Indian and Arabian monsoons, data from the near‐equatorial western Indian Ocean are sparse. Reconstructing past zonal and meridional SST gradients requires additional information on past temperatures from the western boundary current region. We present a unique record of Holocene SST and thermocline depth variations in the tropical western Indian Ocean as documented in foraminiferal Mg/Ca ratios and O from a sediment core off northern Tanzania. For Mg/Ca and thermocline O, most variance is concentrated in the centennial to bicentennial periodicity band. On the millennial time scale, an early to mid‐Holocene (~7.8–5.6 ka) warm phase is followed by a temperature drop by up to 2°C, leading to a mid‐Holocene cool interval (5.6–4.2 ka). The shift is accompanied by an initial reduction in the difference between surface and thermocline foraminiferal O, consistent with the thickening of the mixed layer and suggestions of a strengthened Walker circulation. However, we cannot confirm the expected enhanced zonal SST gradient, as the cooling of similar magnitude had previously been found in SSTs from the upwelling region off Sumatra and in Flores air temperatures. The SST pattern probably reflects the tropical Indian Ocean expression of a large‐scale climate anomaly rather than a positive Indian Ocean Dipole‐like mean state. Holocene tropical western Indian Ocean surface temperatures changed markedly Mid‐Holocene cooling parallels Eastern Indian Ocean temperature decline Pronounced centennial‐scale SST variability present
    Keywords: Holocene ; Indian Ocean ; Mg/Ca ; Foraminifera
    ISSN: 0883-8305
    E-ISSN: 1944-9186
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  • 5
    Language: English
    In: Geology, May, 2007, Vol.35(5), p.387(4)
    Description: We present well-dated high-resolution Holocene records of sea-surface temperature (SST) and upwelling intensity off northwest (NW) Africa. We identify long-term cooling trends over the Holocene in the subtropical North Atlantic in response to boreal summer insolation. A pronounced cooling event of ~1[degrees]C ca. 8.5 cal ka indicates a large-scale reorganization of the ocean current system possibly induced by melt-water from the northern North Atlantic. Our alkenone SST record off Cape Ghir provides strong evidence for the impact of ocean circulation changes on subtropical North Atlantic SSTs. It is likely that cold waters were propagated to the subtropics via the Canary Current in a way similar to Heinrich events and the Younger Dryas off Cape Blanc. We find 2-3 k.y. periodic variations in SST and upwelling intensity off NW Africa superimposed on the cooling trend. Such a cycle has been documented in various paleoclimate archives in phase with solar forcing. We show that these variations on millennial time scales are linked to the North Atlantic subtropical gyre circulation and the Northern Hemisphere atmospheric circulation, and in particular to changes in the pressure gradient between the Icelandic Low and the Azores High. This suggests that oceanic circulation, in response to solar forcing, played a more important role in the generation of 2-3 k.y. cyclicity than has been previously considered. Keywords: alkenone sea-surface temperature, Holocene, North Atlantic subtropical gyre circulation, upwelling intensity, trade winds.
    Keywords: Holocene Paleogeography -- Research ; Ocean Temperature -- Forecasts And Trends ; Ocean Temperature -- Research ; Thermohaline Circulation -- Environmental Aspects ; Thermohaline Circulation -- Research
    ISSN: 0091-7613
    E-ISSN: 19432682
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  • 6
    Language: English
    In: Biogeosciences, Feb 24, 2017, Vol.14(4), p.827
    Description: Planktonic foraminifera preserved in marine sediments archive the physical and chemical conditions under which they built their shells. To interpret the paleoceanographic information contained in fossil foraminifera, the recorded proxy signals have to be attributed to the habitat and life cycle characteristics of individual species. Much of our knowledge on habitat depth is based on indirect methods, which reconstruct the depth at which the largest portion of the shell has been calcified. However, habitat depth can be best studied by direct observations in stratified plankton nets. Here we present a synthesis of living planktonic foraminifera abundance data in vertically resolved plankton net hauls taken in the eastern North Atlantic during 12 oceanographic campaigns between 1995 and 2012. Live (cytoplasm-bearing) specimens were counted for each depth interval and the vertical habitat at each station was expressed as average living depth (ALD). This allows us to differentiate species showing an ALD consistently in the upper 100#xE2;#x80;#xAF;m (e.g., Globigerinoides ruber white and pink), indicating a shallow habitat; species occurring from the surface to the subsurface (e.g., Globigerina bulloides, Globorotalia inflata, Globorotalia truncatulinoides); and species inhabiting the subsurface (e.g., Globorotalia scitula and Globorotalia hirsuta). For 17 species with variable ALD, we assessed whether their depth habitat at a given station could be predicted by mixed layer (ML) depth, temperature in the ML and chlorophyll#xC2;#xA0;a concentration in the ML. The influence of seasonal and lunar cycle on the depth habitat was also tested using periodic regression. In 11 out of the 17 tested species, ALD variation appears to have a predictable component. All of the tested parameters were significant in at least one case, with both seasonal and lunar cyclicity as well as the environmental parameters explaining up to 〉#xE2;#x80;#xAF;50#xE2;#x80;#xAF;% of the variance. Thus, G. truncatulinoides, G. hirsuta and G. scitula appear to descend in the water column towards the summer, whereas populations of Trilobatus sacculifer appear to descend in the water column towards the new moon. In all other species, properties of the mixed layer explained more of the observed variance than the periodic models. Chlorophyll#xC2;#xA0;a concentration seems least important for ALD, whilst shoaling of the habitat with deepening of the ML is observed most frequently. We observe both shoaling and deepening of species habitat with increasing temperature. Further, we observe that temperature and seawater density at the depth of the ALD were not equally variable among the studied species, and their variability showed no consistent relationship with depth habitat. According to our results, depth habitat of individual species changes in response to different environmental and ontogenetic factors and consequently planktonic foraminifera exhibit not only species-specific mean habitat depths but also species-specific changes in habitat depth.
    Keywords: Chlorophyll – Protection and Preservation ; Marine Sediments – Protection and Preservation
    ISSN: 1726-4170
    E-ISSN: 17264189
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  • 7
    Language: English
    In: Biogeosciences Discussions, 09/19/2016, pp.1-48
    Description: Planktonic foraminifera preserved in marine sediments archive the physical and chemical conditions under which they built their shells. To interpret the paleoceanographic information contained in fossil foraminifera, the proxy signals have to be attributed to the habitat of individual species. Much of our knowledge on habitat depth is based on indirect methods, which reconstruct the depth at which the largest portion of the shell has been calcified. However, habitat depth can be best studied by direct observations in stratified plankton nets. Here we present a synthesis of living planktonic foraminifera abundance data in vertically resolved plankton net hauls taken in the eastern North Atlantic during twelve oceanographic campaigns between 1995 and 2012. Live (cytoplasm-bearing) specimens were counted for each depth interval and the vertical habitat at each station was expressed as average living depth (ALD). This allows us to differentiate species showing an ALD consistently above 100 m (e.g. 〈i〉Globigerinoides ruber〈/i〉 white and pink), indicating a shallow habitat; species occurring from the surface to the subsurface (e.g. 〈i〉Globigerina bulloides〈/i〉, 〈i〉Globorotalia inflata〈/i〉, 〈i〉Globorotalia truncatulinoides); and species inhabiting the subsurface (e.g. Globorotalia scitula〈/i〉 and 〈i〉Globorotalia hirsuta〈/i〉). For 17 species with variable ALD, we assessed whether their depth habitat at a given station could be predicted by mixed layer (ML) depth, temperature in the ML and chlorophyll 〈i〉a〈/i〉 concentration in the ML. The influence of seasonal and lunar cycle on the depth habitat was also tested using periodic regression. In 11 out of the 17 tested species, ALD variation appears to have a predictable component. All of the tested parameters were significant at least in one case, with both seasonal and lunar cyclicity as well as the environmental parameters being able to explain up to 〉 50 % of the variance. Whereas 〈i〉G. truncatulinoides〈/i〉, 〈i〉G. hirsuta〈/i〉 and 〈i〉G. scitula〈/i〉 appear to deepen their living depth towards the summer, populations of Trilobatus sacculifer appears to descend in the water column towards the new moon. In all other species, properties of the mixed layer explained more of the observed variance. Chlorophyll 〈i〉a〈/i〉 concentration seems least important for ALD, whilst shoaling of the habitat with deepening of the ML is observed most frequently. We observe both shoaling and deepening of species habitat with increasing temperature. Further, we observe that temperature and seawater density at the depth of the ALD were not equally variable among the studied species, and their variability showed no consistent relationship with depth habitat. According to our results, depth habitat of individual species changes in response to different environmental and ontogenetic factors and consequently planktonic foraminifera exhibit not only species-specific mean habitat depths but also species-specific changes in habitat depth.
    Keywords: Biology;
    ISSN: Biogeosciences Discussions
    E-ISSN: 1810-6285
    Source: CrossRef
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  • 8
    Language: English
    Description: This book describes the latest advances at the Helmholtz “Earth System Science Research School” where scientists from the Alfred Wegener Institute in Bremerhaven, the University of Bremen, and the Jacobs University are involved in research. One of the greatest challenges is understanding ongoing environmental...
    Keywords: Earth Sciences -- Earth System Sciences; Earth Sciences -- Atmospheric Sciences
    ISBN: 978-3-319-13864-0
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  • 9
    In: Geophysical Research Letters, November 2004, Vol.31(22), pp.n/a-n/a
    Description: The northward extent of the influence of the W African monsoon during humid periods of interglacials is subject to investigations highlighting feedback mechanisms, such as vegetation. To detect this regional variation and the climate system acting farther to the north will be the aim of this paper focussing on the Holocene. We present two very high‐resolution Holocene sediment records off NW Africa located at 31°N and 27°N. The well‐known mid‐Holocene climate change from the “African Humid Period” to present arid conditions is well reflected by the terrigenous input in the southern core. In contrast, in the northern core spectral and wavelet analyses indicate a periodic oscillation of about 900 years of the terrigenous input throughout the last 9000 years B.P. We conclude that the W African monsoonal influence characterized by the abrupt climatic change at 5000 years B.P. can be separated from the influence of the N Atlantic climate system reflected by a periodic oscillation throughout the Holocene.
    Keywords: Quaternary Geology ; Oceanography ; Africa ; Arctic Region ; Atlantic Ocean ; Cape Blanc ; Cenozoic ; Climate Change ; Continental Margin ; Cores ; Correlation ; Gisp2 ; Greenland ; Holocene ; Humid Environment ; Ice Cores ; Interglacial Environment ; Leg 108 ; Lithofacies ; Middle Holocene ; Monsoons ; North Africa ; North Atlantic ; North Atlantic Oscillation ; Northeast Atlantic ; Ocean Drilling Program ; Odp Site 658 ; Paleoclimatology ; Quaternary ; Terrestrial Environment ; Terrigenous Materials ; West Africa ; Western Sahara ; Wind Transport;
    ISSN: 0094-8276
    E-ISSN: 1944-8007
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  • 10
    Language: English
    In: Deep-Sea Research Part I, 2009, Vol.56(1), pp.89-106
    Description: Seasonal depth stratified plankton tows, sediment traps and core tops taken from the same stations along a transect at 29°N off NW Africa are used to describe the seasonal succession, the depth habitats and the oxygen isotope ratios (δ O ) of five planktic foraminiferal species. Both the δ O and shell concentration profiles show variations in seasonal depth habitats of individual species. None of the species maintain a specific habitat depth exclusively within the surface mixed layer (SML), within the thermocline, or beneath the thermocline. (white) and (pink) occur with moderate abundance throughout the year along the transect, with highest abundances in the winter and summer/fall season, respectively. The average δ O of (w) from surface sediments is similar to the δ O values measured from the sediment-trap samples during winter. However, the δ O of (w) underestimates sea surface temperature (SST) by 2 °C in winter and by 4 °C during summer/fall indicating an extension of the calcification/depth habitat into colder thermocline waters. (p) continues to calcify below the SML as well, particularly in summer/fall when the chlorophyll maximum is found within the thermocline. Its vertical distribution results in δ O values that underestimate SST by 2 °C. Shell fluxes of are highest in summer/fall, where it lives and calcifies in association with the deep chlorophyll maximum found within the thermocline. and , dwelling and calcifying a part of their lives in the winter SML, record winter thermocline (∼180 m) and deep surface water (∼350 m) temperatures, respectively. Our observations define the seasonal and vertical distribution of multiple species of foraminifera and the acquisition of their δ O .
    Keywords: Planktic Foraminifers ; Seasonality ; Depth Habitat ; Stable Oxygen Isotopes ; Disequilibrium Effects ; Ecology ; Biology ; Oceanography ; Ecology
    ISSN: 0967-0637
    E-ISSN: 1879-0119
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