Alfred Wegener Institute for Polar and Marine Research
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Antarctic ice-shelf meltwater outflows in satellite radar imagery: ground-truthing and basal channel observations
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Ice shelves regulate the flow of the Antarctic ice sheet toward the ocean and its contribution to sea-level rise. Accurately monitoring the basal and surface melting of ice shelves is therefore essential for predicting the ice sheet's response to climatic warming. In this study, we utilize Sentinel-1A synthetic aperture radar satellite imagery combined with shipboard measurements of water temperature and salinity to investigate the presence of surficial meltwater plumes along the Antarctic coastline. Our approach reveals a strong correlation between areas of pronounced low radar backscatter extending from ice shelves and significant decreases in water temperature and salinity, suggesting meltwater-enriched ocean waters. We propose that the low radar backscatter signature of meltwater outflows is caused by stable stratification of the upper water column, driven by density contrasts from buoyant, low-salinity meltwater and surface current shear that reduce Bragg scattering waves. The resulting smooth water surfaces were observed adjacent to the surface expression of deep basal channels, documented in a helicopter survey along part of the Bellingshausen Sea ice edge. We present high-temporal resolution satellite radar as a tool for identifying meltwater release from beneath ice shelves, capable of all-weather, day-and-night imaging.</jats:p
Diurnal versus spatial variability of greenhouse gas emissions from an anthropogenically modified lowland river in Germany
Get PDFGreenhouse gas (GHG) emissions from rivers are globally relevant, but quantification of these emissions comes with considerable uncertainty. Quantification of ecosystem-scale emissions is challenged by both spatial and short-term temporal variability. We measured spatio-temporal variability of CO2 and CH4 fluxes from a 1km long reach of the lowland river Elbe in Germany over 3d to establish which factor is more relevant to be taken into consideration: small-scale spatial variability or short-term temporal variability of CO2 and CH4 fluxes. GHG emissions from the river reach studied were dominated by CO2, and 90% of total emissions were from the water surface, while 10% of emissions were from dry fallen sediment at the side of the river. Aquatic CO2 fluxes were similar at different habitats, while aquatic CH4 fluxes were higher at the side of the river. Artificial structures to improve navigability (groynes) created still water areas with elevated CH4 fluxes and lower CO2 fluxes. CO2 fluxes exhibited a clear diurnal pattern, but the exact shape and timing of this pattern differed between habitats. By contrast, CH4 fluxes did not change diurnally. Our data confirm our hypothesis that spatial variability is especially important for CH4, while diurnal variability is more relevant for CO2 emissions from our study reach of the Elbe in summer. Continuous measurements or at least sampling at different times of the day is most likely necessary for reliable quantification of river GHG emissions
Faktencheck Artenvielfalt Bestandsaufnahme und Perspektiven für den Erhalt der biologischen Vielfalt in Deutschland. ZUSAMMENFASSUNG für die gesellschaftliche Entscheidungsfindung
Get PDFSeasonal and diurnal variability of sub-ice platelet layer thickness in McMurdo Sound from electromagnetic induction sounding
Get PDFHere, we present observations of temporal variability of sub-ice platelet layer over seasonal and diurnal timescales under Ice Shelf Water-influenced fast ice in McMurdo Sound. Electromagnetic induction (EM) sounding time-series measurements of the thicknesses of fast ice and sub-ice platelet layer were made in winter and late spring of 2018. Winter objectives were to measure the seasonal growth of fast ice and sub-ice platelet layer near the McMurdo Ice Shelf in the east, while in late spring we assessed the diurnal variability of sub-ice platelet layer with coincident EM time-series and oceanographic measurements collected in the main outflow path of supercooled Ice Shelf Water in the west. During winter, we observed when the sub-ice platelet layer formed beneath consolidated ice. Episodes of rapid sub-ice platelet layer growth (∼ 0.5–1 m) coincided with strong southerly wind events and polynya activity, suggesting wind-enhanced Ice Shelf Water circulation from the McMurdo–Ross Ice Shelf cavity. In late spring, we investigated how the tides and ocean properties influenced the sub-ice platelet layer. Over a 2-week neap–spring tidal cycle, changes in sub-ice platelet layer thickness were observed to correlate with the tides, increasing more during neap than spring tide cycles, and on diurnal timescales, more on ebb than flood tides. Neap and ebb tides correspond with stronger northward circulation out of the cavity, indicating that sub-ice platelet layer growth was driven by tidally enhanced Ice Shelf Water outflow. The observed variability indicated that wind-driven circulation and the tides influence Ice Shelf Water outflow in McMurdo Sound and, consequently, sub-ice platelet layer evolution over a range of timescales
SAR deep learning sea ice retrieval trained with airborne laser scanner measurements from the MOSAiC expedition
Get PDFAutomated sea ice charting from synthetic aperture radar (SAR) has been researched for more than a decade, and we are still not close to unlocking the full potential of automated solutions in terms of resolution and accuracy. The central complications arise from ground truth data not being readily available in the polar regions. In this paper, we build a data set from 20 near-coincident x-band SAR acquisitions and as many airborne laser scanner (ALS) measurements from the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC), between October and May. This data set is then used to assess the accuracy and robustness of five machine-learning-based approaches by deriving classes from the freeboard, surface roughness (standard deviation at 0.5 m correlation length) and reflectance. It is shown that there is only a weak correlation between the radar backscatter and the sea ice topography. Accuracies between 44 % and 66 % and robustness between 71 % and 83 % give a realistic insight into the performance of modern convolutional neural network architectures across a range of ice conditions over 8 months. It also marks the first time algorithms have been trained entirely with labels from coincident measurements, allowing for a probabilistic class retrieval. The results show that segmentation models able to learn from the class distribution perform significantly better than pixel-wise classification approaches by nearly 20 % accuracy on average
The fate of fixed nitrogen in Santa Barbara Basin sediments during seasonal anoxia
Get PDFDespite long-standing interest in the biogeochemistry of the Santa Barbara Basin (SBB), there are no direct rate measurements of different nitrogen transformation processes. We investigated benthic nitrogen cycling using in situ incubations with 15NO3- addition and quantified the rates of total nitrate (NO3-) uptake, denitrification, anaerobic ammonia oxidation (anammox), N2O production, and dissimilatory nitrate reduction to ammonia (DNRA). Denitrification was the dominant NO3- reduction process, while anammox contributed 0%-27% to total NO3- reduction. DNRA accounted for less than half of NO3- reduction except at the deepest station at the center of the SBB where NO3- concentration was lowest. NO3- availability and sediment total organic carbon content appeared to be two key controls on the relative importance of DNRA. The increasing importance of fixed N retention via DNRA relative to fixed N loss as NO3- deficit intensifies suggests a negative feedback loop that potentially contributes to stabilizing the fixed N budget in the SBB. Nitrous oxide (N2O) production as a fraction of total NO3- reduction ranged from 0.2% to 1.5%, which was higher than previous reports from nearby borderland basins. A large fraction of NO3- uptake was unaccounted for by NO3- reduction processes, suggesting that intracellular storage may play an important role. Our results indicate that the SBB acts as a strong sink for fixed nitrogen and potentially a net source of N2O to the water column
A continuum mechanics perspective on the rheology of firn in the context of firn densification
Get PDFWhile the complex nonlinear rheology of ice is well known and often discussed, for example in the context of large-scale ice sheet modeling, calving, and isotropy occurring at shear margins, the rheology of firn is often considered to be rather simple. According to Truesdell’s first metaphysical principle, which states that ”all properties of a mixture must be mathematical consequences of properties of the constituents” (Truesdell, C. (1984), Rational Thermodynamics, Springer-Verlag, p. 221), the material behavior of firn should be related to that of ice, since firn is primarily a mixture of ice and air. What distinguishes firn from ice is its microstructure. The field of continuum mechanics provides methods to relate the microstructural properties of a material to its macroscopic material behavior.
Here we review a homogenization method developed for the densification of nonlinear creeping metallic powders and first applied to the simulation of firn densification by Gagliardini and Meyssonnier (1997, Annals of Glaciology, 24, pp. 242–248). The method links the rheology of ice to that of firn by describing firn as a porous medium with an ice matrix. The advantage of this approach is that it is formulated in all three spatial dimensions, allowing the inclusion of horizontal divergence due to ice flow without additional parameterization. A large database of dated firn cores allows the determination of the governing model parameters using an optimization approach. We discuss the results, advantages, and limitations of this approach, as well as validation strategies
Post-depositional modification on seasonal-to-interannual timescales alters the deuterium-excess signals in summer snow layers in Greenland
Get PDFAbstract. We document the isotopic evolution of near-surface snow at the East Greenland Ice Core Project (EastGRIP) ice core site in northeast Greenland using a time-resolved array of 1 m deep isotope (δ18O, δD) profiles. The snow profiles were taken from May–August during the 2017–2019 summer seasons. An age–depth model was developed and applied to each profile, mitigating the impacts of stratigraphic noise on isotope signals. Significant changes in deuterium excess (d) are observed in surface snow and near-surface snow as the snow ages. Decreases in d of up to 5 ‰ occur during summer seasons after deposition during two of the three summer seasons observed. The d always experiences a 3 ‰–5 ‰ increase after aging 1 year in the snow due to a broadening of the autumn d maximum. Models of idealized scenarios coupled with prior work indicate that the summertime post-depositional changes in d (Δd) can be explained by a combination of surface sublimation, forced ventilation of the near-surface snow down to 20–30 cm, and isotope-gradient-driven diffusion throughout the column. The interannual Δd is also partly explained with isotope-gradient-driven diffusion, but other mechanisms are at work that leave a bias in the d record. Thus, d does not just carry information about source-region conditions and transport history as is commonly assumed, but also integrates local conditions into summer snow layers as the snow ages through metamorphic processes. Finally, we observe a dramatic increase in the seasonal isotope-to-temperature sensitivity, which can be explained solely by isotope-gradient-driven diffusion. Our results are dependent on the site characteristics (e.g., wind, temperature, accumulation rate, snow properties) but indicate that more process-based research is necessary to understand water isotopes as climate proxies. Recommendations for monitoring and physical modeling are given, with special attention to the d parameter.
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IceBird CAN24 Campaign Report
Get PDFIceBird is a series of scientific, long-term airborne campaigns carried out by AWI to observe the variability and change of Arctic sea ice on seasonal, interannual, and decadal scales. The IceBird campaigns include a unique electromagnetic, laser, and radar instrument suite to obtain coincident measurements of sea ice and snow thickness and freeboard. IceBird CAN24 had a special focus on sea ice and terrestrial snow in the Canadian Arctic, and for the first time operated an additional imaging radar dedicated to the observation of snow on land and sea ice, the CryoSAR of the University of Waterloo. Between April 11 and May 6 thirteen survey flights were carried out between Inuvik, NWT, and Station Nord, Greenland. These covered a range of different sea ice regimes and terrestrial snow sites including over Devon Ice Cap. Flights included surveys of sites with extensive insitu measurements by numerous collaborators for validation of our airborne sea ice and snow retrievals
Aurantiochytrium sp. Meal as Feed Additive for Pacific White Shrimp Reared under Low Temperature and Challenged by WSSV in Association with Thermal Stress
Get PDFA study was conducted to test the inclusion of Aurantiochytrium sp. meal in the rearing of Penaeus vannamei grown in a clear water system and at a suboptimal temperature of 22 °C. The doses tested were 0 (control), 1, 2, 3, and 4% at a stocking density of 100 shrimp/m3. Rearing was carried out with aeration and individual heaters, and seawater temperature was controlled with a chiller. After nine weeks, shrimp were weighed and hemolymph was collected for hemato-immunological tests, and growth performance were calculated. Shrimp raised at 22 °C and fed a 1% of dietary supplementation of Aurantiochytrium sp. meal were superior in immunological parameters. After the dietary assay, a total of 42 shrimps (10.9 ± 0.06 g) per treatment were infected with White Spot Syndrome Virus (WSSV). They were orally infected with 2.6 ± 106 virus particles per g of the animal, maintained at a suboptimal temperature of 22 °C for 108 h, and acclimated to an optimal temperature of 28 °C for an additional 48 h. At 7 days post infection, surviving shrimp were collected for hemato-immunological analysis. Cumulative mortality results showed that shrimp fed diets containing 3% and 4% Aurantiochytrium sp. meal had higher survival than other treatments when challenged with WSSV