English
norskBlar i forfatter "Niemann, Helge"
Viser treff 21-32 av 32
-
Multi-proxy approach to unravel methane emission history of an Arctic cold seep
(Journal article; Tidsskriftartikkel; Peer reviewed, 2020-08-13)Arctic Ocean sediments contain large amounts of methane in the form of free gas and gas hydrate. This highly dynamic methane reservoir is susceptible to be modified by bottom water warming. The warming may lead to gas hydrate destabilization releasing elevated methane fluxes to the seafloor and seawater. Reconstructing past methane dynamics can be achieved by using specific proxies left in the ... -
Physical controls of dynamics of methane venting from a shallow seep area west of Svalbard
(Journal article; Tidsskriftartikkel; Peer reviewed, 2019-12-06)We investigate methane seepage on the shallow shelf west of Svalbard during three consecutive years, using discrete sampling of the water column, echosounder-based gas flux estimates, water mass properties, and numerical dispersion modelling. The results reveal three distinct hydrographic conditions in spring and summer, showing that the methane content in the water column is controlled by a combination ... -
The Plastic Pathfinder: A macroplastic transport and fate model for terrestrial environments
(Journal article; Tidsskriftartikkel; Peer reviewed, 2022-08-24)Land-based plastic waste is the major source for freshwater and marine plastic pollution. Yet, the transport pathways over land remain highly uncertain. Here, we introduce a new conceptual model to forecast plastic transport on land: the Plastic Pathfinder; a numerical model that simulates the spatiotemporal distribution of macroplastic (>0.5 cm) at a river basin scale. The plastic transport driving ... -
Plastic photodegradation under simulated marine conditions
(Journal article; Tidsskriftartikkel; Peer reviewed, 2023-01-12)Ocean plastic pollution is a problem of increasing magnitude; yet, the amount of plastic at the sea surface is much lower than expected. Solar ultraviolet (UV) radiation can induce photodegradation, but its importance in determining the longevity of floating plastic remains unconstrained. Here, we measured photodegradation rates of different plastic types slightly larger than microplastics (virgin ... -
The Potential Role of Marine Fungi in Plastic Degradation – A Review
(Journal article; Tidsskriftartikkel; Peer reviewed, 2021-11-29)Plastic debris has been accumulating in the marine realm since the start of plastic mass production in the 1950s. Due to the adverse effects on ocean life, the fate of plastics in the marine environment is an increasingly important environmental issue. Microbial degradation, in addition to weathering, has been identified as a potentially relevant breakdown route for marine plastic debris. Although ... -
Redox-dependent niche differentiation provides evidence for multiple bacterial sources of glycerol tetraether lipids in lakes
(Journal article; Tidsskriftartikkel; Peer reviewed, 2018-10-23)Terrestrial paleoclimate archives such as lake sediments are essential for our understanding of the continental climate system and for the modeling of future climate scenarios. However, quantitative proxies for the determination of paleotemperatures are sparse. The relative abundances of certain bacterial lipids, i.e., branched glycerol dialkyl glycerol tetraethers (brGDGTs), respond to changes in ... -
Reduced methane seepage from Arctic sediments during cold bottom-water conditions
(Journal article; Tidsskriftartikkel; Peer reviewed, 2020-01-13)Large amounts of methane are trapped within gas hydrate in subseabed sediments in the Arctic Ocean, and bottom-water warming may induce the release of methane from the seafloor. Yet the effect of seasonal temperature variations on methane seepage activity remains unknown as surveys in Arctic seas are conducted mainly in summer. Here we compare the activity of cold seeps along the gas hydrate stability ... -
Seasonal shifts of microbial methane oxidation in Arctic shelf waters above gas seeps
(Journal article; Tidsskriftartikkel; Peer reviewed, 2021-04-07)The Arctic Ocean subseabed holds vast reservoirs of the potent greenhouse gas methane (CH<sub>4</sub>), often seeping into the ocean water column. In a continuously warming ocean as a result of climate change an increase of CH<sub>4</sub> seepage from the seabed is hypothesized. Today, CH<sub>4</sub> is largely retained in the water column due to the activity of methane-oxidizing bacteria (MOB) that ... -
Sources and sinks of methane in sea ice: Insights from stable isotopes
(Journal article; Tidsskriftartikkel; Peer reviewed, 2021-10-27)We report on methane (CH<sub>4)</sub> stable isotope (d<sup>13</sup>C and d<sup>2</sup> H) measurements from landfast sea ice collected near Barrow (Utqiagvik, Alaska) and Cape Evans (Antarctica) over the winter-to-spring transition. These measurements provide novel insights into pathways of CH<sub>4</sub> production and consumption in sea ice. We found substantial differences between the two ... -
A stable isotope assay with 13C-labeled polyethylene to investigate plastic mineralization mediated by Rhodococcus ruber
(Journal article; Tidsskriftartikkel; Peer reviewed, 2022-11-30)Methods that unambiguously prove microbial plastic degradation and allow for quantification of degradation rates are necessary to constrain the influence of microbial degradation on the marine plastic budget. We developed an assay based on stable isotope tracer techniques to determine microbial plastic mineralization rates in liquid medium on a lab scale. For the experiments, <sup>13</sup>C-labeled ... -
Temporal constraints on hydrate-controlled methane seepage off Svalbard
(Journal article; Tidsskriftartikkel; Peer reviewed, 2014-01-17)Methane hydrate is an icelike substance that is stable at high pressure and low temperature in continental margin sediments. Since the discovery of a large number of gas flares at the landward termination of the gas hydrate stability zone off Svalbard, there has been concern that warming bottom waters have started to dissociate large amounts of gas hydrate and that the resulting methane release may ... -
Water column methanotrophy controlled by a rapid oceanographic switch
(Journal article; Tidsskriftartikkel; Peer reviewed, 2015-04-20)Large amounts of the greenhouse gas methane are released from the seabed to the water column, where it may be consumed by aerobic methanotrophic bacteria. The size and activity of methanotrophic communities, which determine the amount of methane consumed in the water column, are thought to be mainly controlled by nutrient and redox dynamics. Here, we report repeated measurements of methanotrophic ...
