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  • 1
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    Online Resource
    Late Quaternary variations of sea surface salinity and temperature in the western tropical Atlantic: Evidence from δ 18 O of Globigerinoides sacculifer
    American Geophysical Union (AGU) ; 1997
    In:  Paleoceanography Vol. 12, No. 6 ( 1997-12), p. 764-772
    Details
    In: Paleoceanography, American Geophysical Union (AGU), Vol. 12, No. 6 ( 1997-12), p. 764-772
    Abstract: In order to reconstruct hydrographic changes during glacial‐interglacial cycles for a transequatorial transect we analyzed oxygen isotopes of Globigerinoides sacculifer (without sac‐like chamber) and abundances of Globorotalia truncatulinoides (dextral) from FS Meteor cores GeoB 2204‐2 (Brazilian continental slope) and GeoB 1523‐1 (Ceara Rise). Δδ 18 O values of G. sacculifer between the two cores were calculated. Modern Δδ 18 O ( G. sacculifer ) is ∼0.2‰ between the two core positions, reflecting differences in sea surface salinity (SSS). Higher SSS at GeoB 1523‐1 (Ceara Rise) is the result of increased precipitation in the region of the Intertropical Convergence Zone. During glacials the δ 18 O records from the two cores converge to the same absolute value, resulting in Δδ 18 O values of around 0‰. Maximum abundances of G. truncatulinoides (dex) correlate with minimum Δδ 18 O, suggesting a possible increase of SSS at GeoB 1523‐1 during stages 2, 3, 4, and 6, which is related to a glacial weakening of the tropical Hadley Cell [ Gates , 1976]. Variations in tropical sea surface temperatures are assumed to be low [ Climate: Long‐Range Investigation, Mapping, and Prediction (CLIMAP), 1981].
    Type of Medium: Online Resource
    ISSN: 0883-8305 , 1944-9186
    URL: Article
    DOI: 10.1029/97PA02270
    Language: English
    Publisher: American Geophysical Union (AGU)
    Publication Date: 1997
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  • 2
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    Eolian‐transported freshwater diatoms and phytoliths across the equatorial Atlantic record: Temporal changes in Saharan dust transport patterns
    American Geophysical Union (AGU) ; 1999
    In:  Journal of Geophysical Research: Oceans Vol. 104, No. C2 ( 1999-02-15), p. 3211-3222
    Details
    In: Journal of Geophysical Research: Oceans, American Geophysical Union (AGU), Vol. 104, No. C2 ( 1999-02-15), p. 3211-3222
    Abstract: The deposition of freshwater diatoms (FD) and phytotliths (P) was determined from sediment traps for a wide region (20°N–7°S) in the tropical and equatorial Atlantic, along a N‐S transect in the eastern equatorial Atlantic around 10°W and in the western equatorial Atlantic around 25°W. These siliceous organisms are derived from the Sahara and Sahel regions of Africa, and eolian transport with direct settling over the open ocean is assumed to be the transport agent. Depositional rates of FD and P revealed strong coupling with seasonal changes in Saharan dust transport that are associated with seasonal precipitation patterns, major wind systems, and the geographical extension of the dust plume across the Atlantic. Mean daily fluxes were highest south of Cape Verde (FD = 9 × 10 4 valves m −2 d −1 ; P = 2 × 10 4 bodies m −2 d −1 ), moderately high off Cape Blanc and in the Guinea Basin north of the equator (of the order of 3 × 10 4 valves m −2 d −1 for FD, and 0.7 × 10 4 bodies m −2 d −1 for P), and consistently low south of the equator and in the western equatorial Atlantic. In traps north of the equator, seasonal changes were marked. Aulacoseira granulata and A. islandica were the most abundant FD in the traps, regardless of trap location and season. However, the number of FD species was higher in the Cape Blanc and Cape Verde areas. The morphological diversity (shape and size) of the P assemblage decreased with increasing distance from the African continent. Patterns of FD and P accumulation rates in surface sediments coincided with those in the traps. Robust freshwater diatom and phytolith records associated with seasonal eolian transport from Saharan and Sahelian regions into the Atlantic furnish clues that can help in our present understanding of the processes linking transport between land, atmosphere, and ocean.
    Type of Medium: Online Resource
    ISSN: 0148-0227
    URL: Article
    DOI: 10.1029/1998JC900070
    Language: English
    Publisher: American Geophysical Union (AGU)
    Publication Date: 1999
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  • 3
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    Late Quaternary paleoceanography in the Fram Strait
    American Geophysical Union (AGU) ; 1997
    In:  Paleoceanography Vol. 12, No. 1 ( 1997-02), p. 65-78
    Details
    In: Paleoceanography, American Geophysical Union (AGU), Vol. 12, No. 1 ( 1997-02), p. 65-78
    Abstract: Four sediment cores from the Fram Strait (78°N) have been studied to reconstruct the paleoceanography of this major connection of the Arctic Ocean to the world ocean. Back to oxygen isotope stage 5 (128 kyr ago) the stratigraphic interpretation of the cores is based on oxygen isotope and accelerator mass spectrometry data; the identification of oxygen isotope stage 6 is based on paleomagnetic and coccolith data. Parameters indicative for contrasting environmental conditions such as high input of ice‐rafted detritus (IRD) or seasonally open waters display characteristic sequences by which the paleoce anography of this region can be reconstructed for the last 180 kyr. The geographic setting of Fram Strait and the sediment data indicate a permanent meridional circulation pattern throughout the last 180 kyr, as it is typical, for example, for the present‐day circulation. Depending on the strength of advection, Atlantic waters appeared in Fram Strait as surface waters, resulting in seasonally ice‐free conditions, or as subsurface water masses, underlying polar waters perenially covered by ice. Sustained periods of seasonally ice‐free waters were largely restricted to interglacial stages 5.5, 5.1, and the Holocene. However, a few short events characterized by seasonally ice‐free conditions at 78°N are recorded during glacial stages as well, for example, stage 6, late stage 3, and stage 2 (“Nordway” events). These events provide for supply of moisture, and therefore they are closely related to the history of the European Arctic ice sheets. Strong input of IRD, reflecting severe glaciations on surrounding landmasses, occurred during most of stage 6, in early stage 3, and during stage 2. Variations in IRD input allow differentiation between three major source areas: (1) Svalbard/Barents Sea during major glacier advances in stages 2 to 5; (2) most likely Siberia during stage 6; and (3) Fennoscandia during the short events in glacial periods marked by seasonal open waters. Thus the circulation pattern in the Fram Strait remained rather stable throughout the last 180 kyr, while variations in the sediment composition are mainly due to the intensity of Atlantic water advection and to the prevailing IRD input pathway.
    Type of Medium: Online Resource
    ISSN: 0883-8305 , 1944-9186
    URL: Article
    DOI: 10.1029/96PA02753
    Language: English
    Publisher: American Geophysical Union (AGU)
    Publication Date: 1997
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  • 4
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    Offshore influence of coastal upwelling off Mauritania, NW Africa, as recorded by diatoms in sediment traps at 2195 m water depth
    Elsevier BV ; 1998
    In:  Deep Sea Research Part I: Oceanographic Research Papers Vol. 45, No. 6 ( 1998-01), p. 985-1013
    Details
    In: Deep Sea Research Part I: Oceanographic Research Papers, Elsevier BV, Vol. 45, No. 6 ( 1998-01), p. 985-1013
    Type of Medium: Online Resource
    ISSN: 0967-0637
    URL: Article
    DOI: 10.1016/S0967-0637(97)00103-9
    Language: English
    Publisher: Elsevier BV
    Publication Date: 1998
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  • 5
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    High-Resolution Marine Record of Climatic Change in Mid-latitude Chile during the Last 28,000 Years Based on Terrigenous Sediment Parameters
    Cambridge University Press (CUP) ; 1999
    In:  Quaternary Research Vol. 51, No. 1 ( 1999-01), p. 83-93
    Details
    In: Quaternary Research, Cambridge University Press (CUP), Vol. 51, No. 1 ( 1999-01), p. 83-93
    Abstract: Marine sediment cores from the continental slope off mid-latitude Chile (33°S) were studied with regard to grain-size distributions and clay mineral composition. The data provide a 28,000-yr 14 C accelerator mass spectrometry-dated record of variations in the terrigenous sediment supply reflecting modifications of weathering conditions and sediment source areas in the continental hinterland. These variations can be interpreted in terms of the paleoclimatic evolution of mid-latitude Chile and are compared to existing terrestrial records. Glacial climates (28,000–18,000 cal yr B.P.) were generally cold–humid with a cold–semiarid interval between 26,000 and 22,000 cal yr B.P. The deglaciation was characterized by a trend toward more arid conditions. During the middle Holocene (8000–4000 cal yr B.P.), comparatively stable climatic conditions prevailed with increased aridity in the Coastal Range. The late Holocene (4000–0 cal yr B.P.) was marked by more variable paleoclimates with generally more humid conditions. Variations of rainfall in mid-latitude Chile are most likely controlled by shifts of the latitudinal position of the Southern Westerlies. Compared to the Holocene, the southern westerly wind belt was located significantly farther north during the last glacial maximum. Less important variations of the latitudinal position of the Southern Westerlies also occurred on shorter time scales.
    Type of Medium: Online Resource
    ISSN: 0033-5894 , 1096-0287
    URL: Article
    DOI: 10.1006/qres.1998.2010
    RVK:
    RA 1000
    Language: English
    Publisher: Cambridge University Press (CUP)
    Publication Date: 1999
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  • 6
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    Late Quaternary Temperature Variability in the Benguela Current System Derived from Alkenones
    Cambridge University Press (CUP) ; 1999
    In:  Quaternary Research Vol. 52, No. 1 ( 1999-07), p. 92-103
    Details
    In: Quaternary Research, Cambridge University Press (CUP), Vol. 52, No. 1 ( 1999-07), p. 92-103
    Abstract: Three sediment cores on a transect across the continental slope off Namibia at about 23°S were investigated for alkenone-derived past sea-surface temperature (SST) and total organic carbon (TOC) content. These records are used to reconstruct variations of surface circulation, coastal upwelling, and paleoproductivity in the northern Benguela Current System for the last 150,000 yr. The SST record most distant from the coast resembles a SST pattern typical of the pelagic ocean, with the lowest SST at full-glacial periods and the highest SST during the Eemian and the Holocene. In contrast to the modern conditions where annual mean SST decreases toward the coast, the shelf-edge SST record has the most prominent warm anomalies of about 2°C during isotope stages 2 and 6 compared with the open ocean. The glacial SST minimum in the record close to the shelf is observed between 50,000 and 35,000 yr B.P., while the record midway along the transect shows intermediate temperature conditions between the offshore and nearshore records. The causal process for the warm anomalies under full ice-age conditions close to the coast may be similar to that of recent “Benguela Niño events” that originate from perturbations in the tradewind system over the western tropical Atlantic. During these events the Angola–Benguela Front, located at about 16°S, weakens and intensive southward protrusions of tropical water masses extend into the nearshore upwelling area as far as 25°S. Thus, the two nearshore records primarily responded to variations in the time-integrated balance between upwelling intensity and southward protrusions of anomalously warm and nutrient-poor Angolan surface waters, as indicated by the good anticorrelation of SST and TOC content. Accordingly, surface water cooling off Namibia over the last 150,000 yr was most intense during stage 3 due to strong winds that worked in favor of upwelling and a decrease of Angolan warm water influence.
    Type of Medium: Online Resource
    ISSN: 0033-5894 , 1096-0287
    URL: Article
    DOI: 10.1006/qres.1999.2040
    RVK:
    RA 1000
    Language: English
    Publisher: Cambridge University Press (CUP)
    Publication Date: 1999
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  • 7
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    Correlated Millennial-Scale Changes in Surface Hydrography and Terrigenous Sediment Yield Inferred from Last-Glacial Marine Deposits off Northeastern Brazil
    Cambridge University Press (CUP) ; 1998
    In:  Quaternary Research Vol. 50, No. 2 ( 1998-09), p. 157-166
    Details
    In: Quaternary Research, Cambridge University Press (CUP), Vol. 50, No. 2 ( 1998-09), p. 157-166
    Abstract: The stable isotope composition of planktonic foraminifera correlates with evidence for pulses of terrigenous sediment in a sediment core from the upper continental slope off northeastern Brazil. Stable oxygen isotope records of the planktonic foraminiferal species Globigerinoides sacculifer and Globigerinoides ruber (pink) reveal sub-Milankovitch changes in sea-surface hydrography during the last 85,000 yr. Warming of the surface water coincided with terrigenous sedimentation pulses that are inferred from high XRF intensities of Ti and Fe, and which suggest humid conditions in northeast Brazil. These tropical signals correlate with climatic oscillations recorded in Greenland ice cores (Dansgaard-Oeschger cycles) and in sediment cores from the North Atlantic (Heinrich events). Trade winds may have caused changes in the North Brazil Current that altered heat and salt flux into the North Atlantic, thus affecting the growth and decay of the large glacial ice sheets.
    Type of Medium: Online Resource
    ISSN: 0033-5894 , 1096-0287
    URL: Article
    DOI: 10.1006/qres.1998.1992
    RVK:
    RA 1000
    Language: English
    Publisher: Cambridge University Press (CUP)
    Publication Date: 1998
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  • 8
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    Short-term sedimentation pulses recorded with a fluorescence sensor and sediment traps at 9-m depth in the Canary basin
    Wiley ; 1996
    In:  Limnology and Oceanography Vol. 41, No. 6 ( 1996-09), p. 1354-1359
    Details
    In: Limnology and Oceanography, Wiley, Vol. 41, No. 6 ( 1996-09), p. 1354-1359
    Type of Medium: Online Resource
    ISSN: 0024-3590
    URL: Article
    DOI: 10.4319/lo.1996.41.6.1354
    Language: English
    Publisher: Wiley
    Publication Date: 1996
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  • 9
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    Reconstruction of past nutrient utilization in the Eastern Angola Basin based on sedimentary 15 N/ 14 N ratios
    American Geophysical Union (AGU) ; 1997
    In:  Paleoceanography Vol. 12, No. 4 ( 1997-08), p. 604-614
    Details
    In: Paleoceanography, American Geophysical Union (AGU), Vol. 12, No. 4 ( 1997-08), p. 604-614
    Abstract: The δ 15 N records of two sediment cores from the Angola Basin have been used to investigate past changes in nutrient utilization in the surface waters of this highly productive region. Low 15 N/ 14 N ratios in the cores (from 〉 3000 m water depth) generally correspond to low sea surface temperatures and high sedimentary organic content, demonstrating a link between upwelling, productivity, and the degree of nitrate depletion. Glacial δ 15 N was lower by 0.5–0.7‰, and paleoproductivity was elevated by around 30% relative to interglacial periods, indicating that although productivity was higher, nitrate was less depleted during glacial than interglacial periods. More pronounced than the glacial/interglacial changes in δ 15 N values of both cores are precession‐related (23 kyr) fluctuations. These cyclic δ 15 N variations, which are in phase with those of paleoproductivity and sea surface temperature, indicate that changes in trade wind driven upwelling intensity and the advection of cold, nutrient‐rich water from the south drive nutrient availability and productivity off the southwest coast of Africa. No strong evidence was found from sedimentary δ 15 N values for denitrification in the water column in the Angola Basin during the past 180 kyr.
    Type of Medium: Online Resource
    ISSN: 0883-8305 , 1944-9186
    URL: Article
    DOI: 10.1029/97PA00819
    Language: English
    Publisher: American Geophysical Union (AGU)
    Publication Date: 1997
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  • 10
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    Response of the tropical Atlantic thermocline to Late Quaternary Trade Wind changes
    American Geophysical Union (AGU) ; 1999
    In:  Paleoceanography Vol. 14, No. 3 ( 1999-06), p. 374-383
    Details
    In: Paleoceanography, American Geophysical Union (AGU), Vol. 14, No. 3 ( 1999-06), p. 374-383
    Abstract: We applied the modern analog technique to foraminiferal counts from three equatorial Atlantic gravity cores to estimate paleothermocline depths in the eastern and western tropical Atlantic during the past 250 kyr. The eastern equatorial core GeoB 1105‐4 is characterized by variations in the 23‐kyr cycle, whereas the western sediment cores show a dominant periodicity at the 41‐kyr band. The phase relationships with respect to insolation and ice volume suggest that thermocline fluctuations in the east are primarily driven by the intensity of the monsoon due to variations in low‐latitude insolation. In contrast, changes of the thermocline depth in the western tropical Atlantic, which lead global ice volume at the 41‐kyr band by 3‐4 kyr, seem to depend on the strength of the SE trade winds in response to the meridional temperature gradients in the South Atlantic.
    Type of Medium: Online Resource
    ISSN: 0883-8305 , 1944-9186
    URL: Article
    DOI: 10.1029/1999PA900011
    Language: English
    Publisher: American Geophysical Union (AGU)
    Publication Date: 1999
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