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  • 1
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    Precipitation Over Southern Africa: Moisture Sources and Isotopic Composition (2022)
    Details
    Publication Date: 2023-12-14
    Description: Southern Africa, with its vast arid to semiarid areas, is considered vulnerable to precipitation changes and amplifying weather extremes. However, during the last 100 ka, huge lakes existed in the currently dry central Kalahari. It has been suggested that these lakes could have existed due to altered atmospheric circulation pattern, leading to an increase in precipitation or to changes in the annual precipitation distribution. Past climate changes are recorded in paleo‐archives, yet, for a proper interpretation of paleo‐records, for example, from sedimentological archives or fossils, it is essential to put them in a context with recent observations. This study’s objective is, therefore, to analyze spatially differing annual precipitation distributions at multiple locations in southern Africa with respect to their stable water isotope composition, moisture transport pathways, and sources. Five different precipitation distributions are identified by end‐member modeling and respective rainfall zones are inferred, which differ significantly in their isotopic compositions. By calculating backward trajectories, different moisture source regions are identified for the rainfall zones and linked to typical circulation patterns. Our results furthermore show the importance of the seasonality, the amount effect, and the traveled distance of the moisture for the general isotopic composition over the entire southern Africa. The identified pattern and relationships can be useful in the evaluation of isotope‐enabled climate models for the region and are potentially of major importance for the interpretation of stable water isotope composition in paleo‐records in future research.
    Description: Key Points: We identified five different annual precipitation distributions in southern Africa that cluster in space and define rainfall zones. Lagrangian source diagnostic shows that the rainfall zones have notably different moisture sources. The isotopic composition differs significantly between rainfall zones.
    Description: Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659
    Description: https://nucleus.iaea.org/wiser
    Description: https://doi.pangaea.de/10.1594/PANGAEA.944811
    Description: https://doi.org/10.24381/cds.bd0915c6
    Description: http://iacweb.ethz.ch/staff/sprenger/lagranto/
    Description: https://forobs.jrc.ec.europa.eu/products/glc2000/products.php
    Description: https://doi.org/10.6084/m9.figshare.7504448.v3
    Description: https://doi.org/10.5066/F7J38R2N
    Description: https://cran.r-project.org/bin/windows/base/old/
    Description: https://cran.r-project.org/web/packages/EMMAgeo/index.html
    Description: https://cran.r-project.org/web/packages/party/index.html
    Keywords: ddc:551.5 ; Lagrangian moisture source diagnostic ; stable water isotopes ; precipitation end‐member ; random forest ; annual rainfall distribution ; moisture pathways
    Language: English
    Type: doc-type:article
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    CITATION GEO-LEO
  • 2
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    Complexity of sea-ice food webs: the role of sympagic meiofauna (2010)
    In:  [Talk] In: IGS International Symposium on sea ice in the physical and biogeochemical system, 01.06.2010, Tromso, Norway .
    Details
    Publication Date: 2012-02-23
    Type: Conference or Workshop Item , NonPeerReviewed
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    OceanRep: Talk
  • 3
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    PCO2 Variations of Equatorial Surface Water Over the Last 330,000 Years: The δ13C Record of Organic Carbon (1994)
    Springer
    In:  In: Carbon Cycling in the Glacial Ocean: Constraints on the Ocean’s Role in Global Change. , ed. by Zahn, R., Pedersen, T. F., Kaminski, M. A. and Labeyrie, L. NATO ASI Series I: Global Environmental Change, 17 . Springer, Berlin, pp. 367-382.
    Details
    Publication Date: 2019-06-24
    Description: We reconstructed past variations in CO2 partial pressure (local PCO2) in the surface waters of the East Atlantic equatorial upwelling zone over the last 330,000 years, based on the δ13C record of the (marine) organic matter in ‘Meteor’ core 16772. To deduce the initial δ13Corganic values of plankton and the CO2 solubility in surface water, the δ13C record was adjusted for i) past variations in (winter) sea surface temperature, ii) variations in the δ13C composition of inorganic carbon dissolved in the surface waters, using the δ13C values of G. ruber (white), and iii) isotopic fractionation during the degradation of settling organic matter in the water column and on the sediment surface. The calculated paleo-PCO2 variations in the surface waters show a strong signal at the obliquity frequency and are approximately parallel to the VOSTOK ice-core record of atmospheric PCO2 over the last 140,000 years. Holocene PCO2 values varied within the range of modern local PCO2, which is 350–400 ppmv compared to a pre-industrial atmospheric pCO2 level of 280 ppmv. This positive anomaly demonstrates the persistent CO2 release from upwelled subsurface water. The glacial-to-interglacial amplitudes of local PCO2 (at the core site) exceeded those of atmospheric pCO2 by 20–60%, with values of less than 250 to 300 ppmv during cold isotopic stages, which indicate a decreased net carbon outgassing from the ocean to the atmosphere. The close correlation between high paleo-PCO2 and low paleo-nutrient contents and paleoproductivity (r=0.7–0.8) suggests that the local PCO2 variations resulted mainly from CO2 transfer by phytoplankton production, especially over the last 170,000 years.
    Type: Book chapter , NonPeerReviewed
    Format: text
    DOI: 10.1007/978-3-642-78737-9_16
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    OceanRep: Book chapter
  • 4
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    Biologically mediated crystallization of buddingtonite in the Paleoproterozoic: Organic-igneous interactions from the Volyn pegmatite, Ukraine (2017)
    Mineralogical Society of America
    Details
    Publication Date: 2017-10-03
    Description: The Volyn pegmatites from Volodarsk-Volynskyi in the Zhytomyr Oblast, NW Ukraine, are associated with granites genetically related to the Paleoproterozoic Korosten pluton. Their late-stage evolution is characterized by the formation of opal-cemented breccia. A polymineralic pseudomorph after beryl within the breccia includes bertrandite (±euclase) + F-muscovite (with tobelite component) + buddingtonite + organic matter (OM) + opal (+ traces of K-feldspar, albite, columbite, FeS 2 , barite, REE-minerals). Sector-zoned and platy to fibrous buddingtonite has variable (K+Na)- vs. NH 4 -contents (electron microprobe analyses) and some H 2 O or H 3 O + , as indicated by microscope infrared spectroscopy. We suggest that ammonium was produced by decay of OM, which is partly preserved in the pseudomorph. Energy-dispersive electron microprobe data of the OM show with increasing O–decreasing C-N-content due to degassing; the OM contains the high field strength elements Zr (≤7 at%), Y (≤3 at%), Sc (≤0.8 at%), REE (≤0.3 at%), Th (≤0.2 at%), and U (≤1.25 at%), which increase with increasing O-content. Transmission electron microscopy of the OM confirms the presence of N; Zr, Si, and O (with other HFSE) are concentrated in nanometer-sized areas and at the transition from OM to opal in nanometer-sized platy Zr-Si-O crystals. C-rich areas are amorphous but show poorly developed lattice fringes. OM is present in the pseudomorph also as brown pigmentation of opal and in pegmatitic beryl from Volyn as a component in late stage fluid inclusions, identified by C-H vibrational bands in infrared spectra. Stable isotope investigations of C and N of buddingtonite, black opal and kerite (fibrous OM known from the literature to occur in the Volyn pegmatites and interpreted as microfossils) indicate a biogenic origin of the OM. We propose that OM in the pseudomorph is condensed kerite, which achieved the high concentrations of high field strength elements via fluid-pegmatite interaction. Although no age determination of minerals in the pseudomorph is available, textural arguments and phase equilibria indicate its formation in a late stage of the pegmatite evolution, at P-T conditions below ~100 MPa/150 °C. We favor a conceptual model for the formation of the Volyn buddingtonite in analogy to Phanerozoic occurrences of buddingtonite, where over and around the shallow anorthosite-granite Korosten pluton hydrothermal convection cells introduced N-bearing hydrocarbons and its precursors into the cooling igneous rocks. Due to the elevated temperature, the OM disintegrated into degassing volatile and non-volatile residual components analogous to petroleum maturation. Organic N, released as NH 4 , was then incorporated into buddingtonite.
    Print ISSN: 0003-004X
    Electronic ISSN: 1945-3027
    Topics: Geosciences
    Published by Mineralogical Society of America
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  • 5
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    Hypoxia causes preservation of labile organic matter and changes seafloor microbial community composition (Black Sea) (2017)
    American Association for the Advancement of Science (AAAS)
    Details
    Publication Date: 2017-02-12
    Description: Bottom-water oxygen supply is a key factor governing the biogeochemistry and community composition of marine sediments. Whether it also determines carbon burial rates remains controversial. We investigated the effect of varying oxygen concentrations (170 to 0 μM O 2 ) on microbial remineralization of organic matter in seafloor sediments and on community diversity of the northwestern Crimean shelf break. This study shows that 50% more organic matter is preserved in surface sediments exposed to hypoxia compared to oxic bottom waters. Hypoxic conditions inhibit bioturbation and decreased remineralization rates even within short periods of a few days. These conditions led to the accumulation of threefold more phytodetritus pigments within 40 years compared to the oxic zone. Bacterial community structure also differed between oxic, hypoxic, and anoxic zones. Functional groups relevant in the degradation of particulate organic matter, such as Flavobacteriia , Gammaproteobacteria , and Deltaproteobacteria , changed with decreasing oxygenation, and the microbial community of the hypoxic zone took longer to degrade similar amounts of deposited reactive matter. We conclude that hypoxic bottom-water conditions—even on short time scales—substantially increase the preservation potential of organic matter because of the negative effects on benthic fauna and particle mixing and by favoring anaerobic processes, including sulfurization of matter.
    Electronic ISSN: 2375-2548
    Topics: Natural Sciences in General
    Published by American Association for the Advancement of Science (AAAS)
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  • 6
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    Distribution and composition of thiotrophic mats in the hypoxic zone of the Black Sea (2016)
    In:  EPIC3Frontiers in Microbiology
    Details
    Publication Date: 2017-01-30
    Description: At the Black Sea chemocline, oxygen- and sulfide-rich waters meet and form a niche for thiotrophic pelagic bacteria. Here we investigated an area of the Northwestern Black Sea off Crimea close to the shelf break, where the chemocline reaches the seafloor at around 150–170 m water depth, to assess whether thiotrophic bacteria are favored in this zone. Seafloor video transects were carried out with the submersible JAGO covering 20 km2 on the region between 110 and 200 m depth. Around the chemocline we observed irregular seafloor depressions, covered with whitish mats of large filamentous bacteria. These comprised 25–55% of the seafloor, forming a belt of 3 km width around the chemocline. Cores from the mats obtained with JAGO showed higher accumulations of organic matter under the mats compared to mat-free sediments. The mat-forming bacteria were related to Beggiatoa-like large filamentous sulfur bacteria based on 16S rRNA sequences from the mat, and visual characteristics. The microbial community under the mats was significantly different from the surrounding sediments and enriched with taxa affiliated with polymer degrading, fermenting and sulfate reducing microorganisms. Under the mats, higher organic matter accumulation, as well as higher remineralization and radiotracer-based sulfate reduction rates were measured compared to outside the mat. Mat-covered and mat-free sediments showed similar degradability of the bulk organic matter pool, suggesting that the higher sulfide fluxes and subsequent development of the thiotrophic mats in the patches are consequences of the accumulation of organic matter rather than its qualitative composition. Our observations suggest that the key factors for the distribution of thiotrophic mat-forming communities near to the Crimean shelf break are hypoxic conditions that (i) repress grazers, (ii) enhance the accumulation and degradation of labile organic matter by sulfate-reducers, and (iii) favor thiotrophic filamentous bacteria which are adapted to exploit steep gradients in oxygen and sulfide availability; in addition to a specific seafloor topography which may relate to internal waves at the shelf break.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , peerRev
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  • 7
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    Hypoxia causes preservation of labile organic matter and changes microbial community composition (Black Sea shelf) (2017)
    In:  EPIC3Science Advances, 3(2)
    Details
    Publication Date: 2018-02-23
    Description: Bottom-water oxygen supply is a key factor governing the biogeochemistry and community composition of marine sediments. Whether it also determines carbon burial rates remains controversial. We investigated the effect of varying oxygen concentrations (170 to 0 μM O2) on microbial remineralization of organic matter in seafloor sediments and on community diversity of the northwestern Crimean shelf break. This study shows that 50% more organic matter is preserved in surface sediments exposed to hypoxia compared to oxic bottom waters. Hypoxic conditions inhibit bioturbation and decreased remineralization rates even within short periods of a few days. These conditions led to the accumulation of threefold more phytodetritus pigments within 40 years compared to the oxic zone. Bacterial community structure also differed between oxic, hypoxic, and anoxic zones. Functional groups relevant in the degradation of particulate organic matter, such as Flavobacteriia, Gammaproteobacteria, and Deltaproteobacteria, changed with decreasing oxygenation, and the microbial community of the hypoxic zone took longer to degrade similar amounts of deposited reactive matter. We conclude that hypoxic bottom-water conditions—even on short time scales—substantially increase the preservation potential of organic matter because of the negative effects on benthic fauna and particle mixing and by favoring anaerobic processes, including sulfurization of matter.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , isiRev , info:eu-repo/semantics/article
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  • 8
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    Eastern Mediterranean surface water temperatures and d18O composition during deposition of sapropels in the late Quaternary (2003)
    In:  EPIC3Paleoceanography, 18(1), 1005 p.
    Details
    Publication Date: 2019-07-16
    Description: Water column stratification increased at climatic transitions from cold to warm periods during the lateQuaternary and led to anoxic conditions and sapropel formation in the deep eastern Mediterranean basins. Highresolutiondata sets on sea-surface temperatures (SST) (estimated from U37k0 indices) and d18O of planktonicforaminifer calcite (d18Ofc) across late Pleistocene sapropel intervals show that d18Ofc decreased (between 1 and4.6%) and SST increased (between 0.7 and 6.7 C). Maximal d18Oseawater depletion of eastern Mediterraneansurface waters at the transition is between 0.5 and 3.0%, and in all but one case exceeded the depletion seen in awestern Mediterranean core. The depletion in d18Oseawater is most pronounced at sapropel bases, in agreementwith an initial sudden input of monsoon-derived freshwater. Most sapropels coincide with warming trends ofSST. The density decrease by initial freshwater input and continued warming of the sea surface pooled freshwater in the surface layer and prohibited deep convection down to ageing deep water emplaced during cold andarid glacial conditions. An exception to this pattern is glacial sapropel S6; its largest d18Oseawater depletion(3%) is almost matched by the depletion in the western Mediterranean Sea, and it is accompanied by surfacewater cooling following an initially rapid warming phase. A second period of significant isotopic depletion is inisotope stage 6 at the 150 kyr insolation maximum. While not expressed as a sapropel due to cold SST, it is inaccord with a strengthened monsoon in the southern catchment.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , isiRev
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  • 9
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    A multiproxy reconstruction of the evolution of deep and surface waters in the Nordic Seas over the last 30,000 years (2001)
    In:  EPIC3Quaternary Science Reviews, 20, pp. 659-678
    Details
    Publication Date: 2019-07-16
    Description: On the basis of various lithological, micropaleontological and isotopic proxy records covering the last 30,000 calendar years (cal kyr) the paleoenvironmental evolution of the deep and surface water circulation in the subarctic Nordic seas was reconstructed for a climate interval characterized by intensive ice-sheet growth and subsequent decay on the surrounding land masses. The data revealconsiderable temporal changes in the type of thermohaline circulation. Open-water convection prevailed in the early record,providing moisture for the Fennoscandian-Barents ice sheets to grow until they reached the shelf break at &26 cal. kyr and started to deliver high amounts of ice-rafted debris (IRD) into the ocean via melting icebergs. Low epibenthic d 18O values and small-sized subpolar foraminifera observed after 26 cal. kyr may implicate that advection of Atlantic water into the Nordic seas occurred at the subsurface until 15 cal. kyr. Although modern-like surface and deep-water conditions first developed at ca. 13.5 cal. kyr, thermohaline circulation remained unstable, switching between a subsurface and surface advection of Atlantic water until 10 cal. kyr when IRDdeposition and major input of meltwater ceased. During this time, two depletions in epibenthic d 13 C are recognized just before and after the Younger Dryas indicating a notable reduction in convectional processes. Despite an intermittent cooling at ca 8 cal. kyr,warmest surface conditions existed in the central Nordic seas between 10 and 6 cal. kyr. However, already after 7 cal. kyr the present day situation gradually evolved, verified by a strong water mass exchange with the Arctic Ocean and an intensifying deep convection as well as surface temperature decrease in the central Nordic seas. This process led to the development of the modern distribution of water masses and associated oceanographic fronts after 5 cal. kyr and, eventually, to today's steep east}west surface temperaturegradient. The time discrepancy between intensive vertical convection after 5 cal. kyr but warmest surface temperatures already between 10 and 6 cal. kyr strongly implicates that widespread postglacial surface warming in the Nordic seas was not directly linked to the rates in deep-water formation.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , isiRev
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  • 10
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    Planktic and benthic foraminifera as indicators of past ocean changes in surface and deep waters of the Nordic Seas (2001)
    In:  EPIC3In: The northern North Atlantic: A changing environment / Schäfer, P., Ritzrau, W., Schüter, M., Thiede, J. (eds). Springer-Verlag, Berlin, pp. 411-421, ISBN: 3-540-67231-1
    Details
    Publication Date: 2019-07-17
    Repository Name: EPIC Alfred Wegener Institut
    Type: Inbook , peerRev
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