| 1. |
- Hällberg, Lars Petter, et al.
(författare)
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Magnetic susceptibility parameters as proxies for desert sediment provenance
- 2020
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Ingår i: Aeolian Research. - : Elsevier BV. - 1875-9637 .- 2212-1684. ; 46
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Tidskriftsartikel (refereegranskat)abstract
- Magnetic susceptibility in sediments has been thoroughly studied as a paleoenvironmental proxy over the last decades. However, it is unknown to what extent magnetic susceptibility variation is also a diagnostic of different sediment sources. Here we investigate if multiple magnetic susceptibility-based parameters can effectively be used as sediment source indicators. New magnetic property data from the Mu Us and Tengger Deserts in China are compared to previously known sediment provenance based on other well-established proxies. To assess the magnetic properties of these deserts, magnetic susceptibility and its out-of-phase component, its dependence on frequency, temperature and low-field amplitude are analyzed. Our results indicate similar sources for the western Mu Us Desert and the Tengger Desert and a distinct source for the eastern Mu Us, in-line with previously hypothesized provenance patterns. However, magnetic properties within the Tengger Desert sediments are homogenous, which may suggest a uniform provenance for the entire Tengger Desert, that the sediments are greatly reworked, or similar magnetic properties in potential multiple source regions. Magnetite is the major magnetic mineral in the study area and the dominant causes for divergence in magnetic properties are the magnetic mineral concentration and domain state. The results here, in particular from the Mu Us, suggest considerable promise for using magnetic susceptibility parameters in desert sediment provenance research.
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| 2. |
- O'Regan, Matt, et al.
(författare)
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Stratigraphic Occurrences of Sub-Polar Planktic Foraminifera in Pleistocene Sediments on the Lomonosov Ridge, Arctic Ocean
- 2019
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Ingår i: Frontiers in Earth Science. - : FRONTIERS MEDIA SA. - 2296-6463. ; 7
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Tidskriftsartikel (refereegranskat)abstract
- Turborotalita quinqueloba is a species of planktic foraminifera commonly found in the sub-polar North Atlantic along the pathway of Atlantic waters in the Nordic seas and sometimes even in the Arctic Ocean, although its occurrence there remains poorly understood. Existing data show that T. quinqueloba is scarce in Holocene sediments from the central Arctic but abundance levels increase in sediments from the last interglacial period [Marine isotope stage (MIS) 5, 71-120 ka] in cores off the northern coast of Greenland and the southern Mendeleev Ridge. Turborotalita also occurs in earlier Pleistocene interglacials in these regions, with a unique and widespread occurrence of the less known Turborotalita egelida morphotype, proposed as a biostratigraphic marker for MIS 11 (474-374 ka). Here we present results from six new sediment cores, extending from the central to western Lomonosov Ridge, that show a consistent Pleistocene stratigraphy over 575 km. Preliminary semi-quantitative assessments of planktic foraminifer abundance and assemblage composition in two of these records (LOMROG12-7PC and AO16-5PC) reveal two distinct stratigraphic horizons containing Turborotalita in MIS 5. Earlier occurrences in Pleistocene interglacials are recognized, but contain significantly fewer specimens and do not appear to be stratigraphically coeval in the studied sequences. In all instances, the Turborotalita specimens resemble the typical T. quinqueloba morphotype but are smaller (63-125 mu m), smooth-walled and lack the final thickened calcite layer common to adults of the species. These results extend the geographical range for T. quinqueloba in MIS 5 sediments of the Arctic Ocean and provide compelling evidence for recurrent invasions during Pleistocene interglacials.
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| 3. |
- Shephard, G. E., et al.
(författare)
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A North Pole thermal anomaly? Evidence from new and existing heat flow measurements from the central Arctic Ocean
- 2018
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Ingår i: Journal of Geodynamics. - : Elsevier BV. - 0264-3707 .- 1879-1670. ; 118, s. 166-181
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Tidskriftsartikel (refereegranskat)abstract
- Constraining the thermal evolution of the Arctic Ocean is hampered by notably sparse heat flow measurements and a complex tectonic history. Previous results from the Lomonosov Ridge in the vicinity of the North Pole, and the adjacent central Amundsen Basin reveal varied values, including those higher than expected considering plate cooling or simple uniform stretching models. Furthermore, in the vicinity of the North Pole an anomalously slow velocity perturbation exists in upper mantle seismic tomography models. However, whether these observations are related to a thermal anomaly in the mantle remains unknown. We present new heat flow results gathered from 17 sediment cores acquired during the Arctic Ocean 2016 and SWERUS-C3 expeditions on the Swedish icebreaker Oden. Three sites located on oceanic lithosphere in the Amundsen Basin between 7 degrees W-71E degrees reveal surface thermal conductivity of 1.07-1.26 W/mK and heat flow in the order of 71-95 mW/m(2), in line-with or slightly higher (1-21 mW/m(2)) than expected from oceanic heat flow curves. These results contrast with published results from further east in the Amundsen Basin, which indicated surface heat flow values up to 2 times higher than predicted from oceanic crustal cooling models. Heat flow of 49-61 mW/m(2) was recovered from the Amerasia Basin. Sites from the submerged continental fragments of the Lomonosov Ridge and Marvin Spur recovered heat flow in the order of 53-76 and 51-69 mW/m(2) respectively. When considering the additional potential surface heat flux from radiogenic heat production in the crust, these variable measurements are broadly in line with predictions from uniform extension models for continental crust. A seismically imaged upper mantle velocity anomaly in the central Arctic Ocean may arise from a combination of compositional and thermal variations but requires additional investigation. Disentangling surface heat flow contributions from crustal, lithospheric and mantle processes, including variable along-ridge rifting rates and timing, density and phase changes, conductive and advective dynamics, and regional tectonics, requires further analysis.
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| 4. |
- Snowball, Ian, 1963-, et al.
(författare)
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Coring induced sediment fabrics at IODP Expedition 347 Sites M0061 and M0062 identified by anisotropy of magnetic susceptibility (AMS) : criteria for accepting palaeomagnetic data
- 2019
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Ingår i: Geophysical Journal International. - : Oxford University Press (OUP). - 0956-540X .- 1365-246X. ; 217:2, s. 1089-1107
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Tidskriftsartikel (refereegranskat)abstract
- Anisotropy of magnetic susceptibility data obtained from discrete subsamples recovered from two Integrated Ocean Drilling Program sites (Expedition 347 sites M0061 and M0062 in the Baltic Sea) by an Advanced Piston Corer are compared to results obtained on subsamples recovered by replicate 6-m-long Kullenberg piston cores. Characteristic natural remanence directions were obtained from the total of 1097 subsamples using principal component analyses. The three principal anisotropy axes of subsamples taken from Advanced Piston Core liners align to the subsample axes, with the maximum axis (K1) parallel to the split core surfaces, possibly caused by outwards relaxation of the core-liners after splitting. A second anomalous anisotropy fabric is characterized by steep values of the angular difference between the inclination of the minimum anisotropy axes (K3) and that expected for horizontal bedding (90°). This fabric is confined to the upper 1–2 m of the Kullenberg cores and specific sections of the advanced piston cores, and we attribute it to conical deformation caused by either excessive penetration speeds and downwards dragging of sediment along the edge of the liner or stretching caused by undersampling. By using our data in an example, we present a protocol to accept palaeomagnetic secular variation data that uses (i) a threshold 90-K3 value of 15°, combined with a modelled, locally applicable minimum inclination of 65° and (ii) an A95 cone of confidence based on Fisher statistics applied to virtual geomagnetic pole distributions.
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| 5. |
- Wiers, Steffen, et al.
(författare)
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Late Pleistocene Chronology of Sediments From the Yermak Plateau and Uncertainty in Dating Based on Geomagnetic Excursions
- 2019
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Ingår i: Geochemistry Geophysics Geosystems. - 1525-2027. ; 20:7, s. 3289-3310
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Tidskriftsartikel (refereegranskat)abstract
- The Yermak Plateau is one of several regions in the Arctic Ocean where paleomagnetism yields controversial results. Despite low sedimentation rates, late Pleistocene paleomagnetic excursions have been reconstructed from many cores in the region, but they are characterized by considerably longer durations when compared to established ones. Self-reversal during maghemitization of (titano)magnetite has been proposed as one explanation. Rock magnetic, C-14 dating, sedimentological and stable isotope (delta O-18) methods were employed to three new sediment cores to put paleomagnetic results in the context of the regional stratigraphy and chronology. Coherence of lithological parameters and delta O-18 variations validated the ratio of anhysteretic remanent susceptibility to bulk magnetic susceptibility (kappa(ARM)/kappa) as a parameter for cross-core correlation. As established by earlier studies, we use the link between glacial/interglacial cycles and kappa(ARM)/kappa to tune our records to a global delta O-18 stack, which provides age models that are independent of radicarbon ages and paleomagnetic data. Our results show that zones of negative magnetic inclination are asynchronous across the plateau. Alternating field demagnetization data revealed that negative inclinations are contained in a medium-high-coercivity (>25-35mT) magnetic phase that may be the result of postdepositional alteration of (titano)magnetite. We note a positive relationship between water depth and excursion duration, which may be driven by changes in water mass circulation on glacial/interglacial timescales.
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| 6. |
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| 7. |
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| 8. |
- Wiers, Steffen, et al.
(författare)
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The Arctic Ocean Manganese Cycle, an Overlooked Mechanism in the Anomalous Palaeomagnetic Sedimentary Record
- 2020
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Ingår i: Frontiers in Earth Science. - : Frontiers Media SA. - 2296-6463. ; 8
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Tidskriftsartikel (refereegranskat)abstract
- Palaeomagnetic records obtained from Arctic Ocean sediments are controversial because they include numerous and anomalous geomagnetic excursions. Age models that do not rely on palaeomagnetic interpretations reveal that the majority of the changes in inclination do not concur with the established global magnetostratigraphy. Seafloor oxidation of (titano)magnetite to (titano)maghemite with self-reversal of the (titano)maghemite coatings has been proposed as an explanation. However, no existing model can explain when the self-reversed components formed and how they are linked to litho-stratigraphic changes in Arctic Ocean sediments. In this study, we present new palaeo- and rock magnetic measurements of a sediment core recovered from the Arlis Plateau, close to the East Siberian Shelf. The magnetic data set is evaluated in the context of the regional stratigraphy and downcore changes in physical and chemical properties. By cross-core correlation, we show that magnetic inclination changes in the region do not stratigraphically align, similar to results of studies of sediments from the Lomonosov Ridge and Yermak Plateau. Rock magnetic and chemical parameters indicate post-depositional diagenetic changes in the magnetic mineral assemblage that can be linked to manganese cycling in the Arctic Ocean. The potential presence of a magnetic remanence bearing manganese-iron oxide phase, which can undergo self-reversal, leads to an alternative hypothesis to primary seafloor oxidation of (titano)magnetite. This phase may form by precipitation from seawater or by changing redox conditions in the sediment column by mineral precipitation from ions dissolved in pore water. These findings highlight the need for further investigation into the magnetic mineral assemblage, its link to manganese cycling and pore water geochemistry in Arctic Ocean sediments.
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| 9. |
- Wiers, Steffen, 1985-
(författare)
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The Arctic Ocean Palaeomagnetic Record : A Stratigraphic Approach
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The Arctic Ocean is an essential component of the global climate system, yet understanding of its geological archives is hampered by difficulties in age modelling. In addition to the scarcity of dateable microfossils, the palaeomagnetic record of Arctic sediments is unusual. Palaeomagnetic inclination sequences from Arctic sediments display numerous changes from steep positive to steep negative values that do not match the established geomagnetic polarity timescale. Independent age constraints suggest that most changes in the upper few meters below the sea-floor took place within the Brunhes normal chron. It has been suggested that zones of reversed inclination contain reversed titanomaghemite, formed by sea-floor oxidation of titanomagnetite. Until now, self-reversed components of Arctic records have not been studied in the context of regional stratigraphic frameworks, which could elucidate the relative timing of significant diagenetic changes, or their synchronicity between different records.This thesis examines marine sediments from three different areas of the Arctic. All records were evaluated within their regional stratigraphic frameworks and cross-correlated with existing records. A combined approach that used magnetic and sedimentological data was employed to identify the effect of depositional and diagenetic processes on the palaeo- and rock magnetic properties. Chemical, palaeo- and rock magnetic investigations on sediments from the Arlis Plateau and the Lomonosov Ridge revealed a complex magnetic mineralogy and constrained reversed inclinations to a medium to high coercivity magnetic phase. The important role of manganese in the Arctic Ocean, its involvement in iron (oxyhydr)oxide enrichment during interglacial periods and its role in diagenesis led to the hypotheses that an unidentified magnetic ferromanganese phase is involved in the anomalous palaeomagnetic record. Elevated pore water manganese concentration a few meters below the sea-floor in central Arctic sediments is evidence of ongoing diagenesis that involves manganese-oxides. A relationship with iron-oxides is likely and can lead to alteration of existing magnetic minerals and the precipitation of new magnetic phases. This approach revealed 1) an inconsistent alignment of zones of negative inclination when different records were correlated using lithological parameters, 2) no correlation of inclination changes with stratigraphic boundaries and 3) a link between diagenesis and the palaeomagnetic record.
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