Dekkers, Mark J.

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Dekkers, Mark J.
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Dekkers, Mark J.

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    Item-typ:Veröffentlichung,
    Petrology, rock magnetism and stratigraphy of late quaternary Ice-rafted detritus at the southeast Grand Banks slope of Newfoundland, Atlantic Canada
    (2024-06-11)
    Bukar, Shettima
    ;
    Dekkers, Mark J.
    ;
    Dobeneck, Tilo von
    ;
    Dobeneck, Tilo von
    The Quaternary sediments of the Atlantic Canada margin usually contain numerous evidences of paleoclimate oscillations. The short-lived, abrupt and episodic surges of sediment-laden armadas of icebergs calved from Laurentide Ice Sheet (LIS) into the North Atlantic Ocean via ice stream conduits has deposited ice-rafted detritus (IRD) all across the northern North Atlantic oceanic basin. Such events have had considerable impact on ocean water circulation system (i.e. Atlantic meridional overturning circulation – AMOC) and deep water formation owing to the huge influx of meltwater delivered by subglacial discharge and iceberg melting. This catastrophic instability of the LIS has resulted in a stratigraphic sequence of detrital-rich Heinrich Layers (HL) during the cold phases of the Late Quaternary period. As IRD fragments are the direct evidence of the interplay between ice sheets and the North Atlantic Ocean, we still lack understanding of their hidden information on ice sheet dynamics, location of ice margins, the provenance and spatial dispersion of distinct terrigenous materials and the petromagnetic properties and contributions of different IRD species to magnetic HL records. In this PhD thesis, I investigated a sediment core (GeoB 18530-1) acquired at the foot of Southeast Grand Banks Slope of Newfoundland, that represents a continuous Late Pleistocene stratigraphic sequence of Heinrich Event layers 1 - 6 (H1 – H6). Discrete sample sets were taken at a very high resolution of 2 cm for various analyses including digital light microscopy, thin section petrography, X-ray fluorescence, rock magnetics, and grain-size sieving. In a first step of the thesis project, a total of 8243 coarse-grained (>1 mm) IRD particles were petrologically identified and their lithologies petrographically classified into 22 microscopically distinguishable categories. The abundances of these IRD lithologies were determined by counting individual particles at a high resolution of 2 cm steps throughout the continuous stratigraphic sequence of HLs (H1 – H6). This revealed the temporal and spatial change in the deposition of each IRD lithology. It was found that dolomitic IRD predominates the mid-part of all HLs whilst muscovite-biotite granite was found in greater abundance at the top and bottom of the HLs. Additionally, the statistical investigation of the count records of the 22 IRD lithologies manifested three well-defined groups based on whether their deposition is higher within HLs or interlayers or similar in both. Compositionally, it was found that H1, H2, H4, and H5 are kindred to each other while H3 and H6 are individually distinct. The second step of the thesis was an advancement of the finding from the initial part where the petrological information that was gleaned is used for magnetic analysis of particularly HLs and individual IRD lithologies. The pronounced magnetic susceptibility signal of HLs is usually intriguing compared to the interlayer units thus, it is momentous to discern the IRD lithology that influences the signal. Though, the >1 mm IRD underrepresents the total amount of IRD where finer grains are much more abundant, a lithology-specific identification can only be conducted using the coarser IRD fractions. From the magnetic properties of the coarse IRD it was found that out of the 22 different lithologies, as many as 16 possess a relatively lower magnetic susceptibility than the background sediment. Only 6 IRD lithologies are typified by a higher susceptibility among which the muscovite-biotite granite is four-fold higher magnetic than the background. With the complementary XRF data is was revealed clearly that the viii susceptibility enhancement by the (K/Fe denoted) granite is partly compensated by the (Ca/Sr represented) dolomitic IRD. Therefore, with the muscovite-biotite granite being second in abundance to dolomitic IRD, it mainly controls the pronounced magnetic susceptibility signals of HLs. In modeling magnetic susceptibility based on coarse-grained IRD count data a misfit of the model was observed proposing that IRD sedimentation does not only occur through detritus rain-out from icebergs but rather through other transport processes like advection or gravitational flow, at least for the terminal part of the sediment’s trajectory. Lastly, traditional magnetic susceptibility measurements are obtained in ways, where the inhomogeneity of sediment records is mostly played down. This is because the sensor coils of whole-core magnetic susceptometers average signals over a large volume, thereby integrating the microscale susceptibility distribution over a considerable volume. Split-core susceptibility logs produced with a spot sensor at high resolution average the magnetic signal over a much smaller volume of a material. Using a novel protocol, this spot sensor technique provides the freedom to scan across an entire split core. By logging multiple parallel tracts at a high resolution of 1 cm along and between each tract, the derived contour plot reveals coarse IRD distribution, where the color spectrum denotes lateral variability in magnetic susceptibility.
    Dissertation
      68  70
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    Item-typ:Veröffentlichung,
    Paleoenvironmental signatures of aeolian and fluvial sedimentary end-members off NW Africa
    (2012-03-22)
    Just, Janna
    ;
    Dekkers, Mark J.
    ;
    Dobeneck, Tilo von
    ;
    Dobeneck, Tilo von
    This thesis presents a new approach to characterize properties of terrigenous sediments deposited off subtropical Africa. Grain-size distributions, geochemical compositions, and magnetic parameters of core material off the Gambia and Senegal rivers were analyzed. By end-member (EM) unmixing and least-square fitting, mixing coefficients and multi-parameters properties of fluvial and aeolian fractions were obtained. The properties are influenced by different factors and the joint characterization enables a three-level interpretation: (1) Grain-sizes reflect transport energy. (2) Geochemical composition differs in fluvial and aeolian sediments. (3) Magnetic mineralogy mirrors weathering and environmental conditions in the source areas. Volumetric EM representations enable to budget fluxes of aeolian and fluvial material. The highest dust accumulation occurred during Heinrich Stadial 1 and corresponds to a 100 times higher flux with respect to today. Such high values have not been reported yet off NW Africa and emphasize that disturbances of the meridional overturning circulation cause dramatic environmental changes also in low-latitudes.
    Dissertation
      562  177
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    Rock magnetic and geochemical signals of late Quaternary climate variability over northwest Africa
    (2008-10-10)
    Itambi Cletus, Achakie
    ;
    Dekkers, Mark
    ;
    von Dobeneck, Tilo
    ;
    von Dobeneck, Tilo
    This thesis investigates the paleo- climatic and environmental changes over northwest Africa by focusing on the Senegal region and the Gulf of Guinea. Emphasis was placed on the timing of past climate changes, the frequency of occurrence, and regional impact. Possible shifts in the position of the intertropical conversion zone (ITCZ) were also investigated. A multi proxy approach integrating rock and environmental magnetic parameters diffuse reflectance spectrophotometry, scanning electron microscopy, color reflectance and climate modeling experiments was employed. The results show that a climatic boundary which certainly reflects the southward shift of the northern limit of summer position of the ITCZ existed between 13 and 12à �à °N during the last glacial. The West African monsoon varied at D O times-scales at 12à �à °N, demonstrating that drier conditions occurred further south of the Sahel than previously reported. A fresh water hosing experiment using coupled climate model experiment suggested that the North Atlantic sea surface temperature and West African climate are linked by shifts in the position of the monsoon trough and the mid tropospheric African Easterly jet. The cooling of the North Atlantic sea surface is directly linked to the weakening of the Atlantic Meridional Overturing Circulation. Further more, climate change regulated the influx of magnetic minerals and post depositional alteration in the Senegalese sediments. Interglacial showed lower concentrations of magnetic particles, with high concentration of euhedral and framboidal pyrite. Wind derived sediments dominated the glacial with hematite and goethite prominent at these intervals as a result of increase input of Saharan dust. This studies shows that climate over this region varies at different times scales and is interconnected with global climatic signals.
    Dissertation
      623  428