Alfred Wegener Institute for Polar and Marine Research
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Investigating firn and ice anisotropy around the EastGRIP Camp, North East Greenland Ice Stream, from ambient noise surface waves
Get PDFWe use cross-correlations of ambient seismic noise data between pairs of 9 broadband three component seismometers to investigate variations in velocity structure and anisotropy in the vicinity of the EastGRIP camp along and across flow of the Northeast Greenland Ice Stream (NEGIS).
From the 9-component correlation tensors associated with all station pairs we derive dispersion curves of Rayleigh and Love wave group velocities between station pairs at frequencies from 1 to 25 Hz. The distributions of the Rayleigh and Love group velocities exhibit anisotropy variations for the along and across flow component. To better assess those variations, we invert the dispersions curves to shear wave velocities in the horizontal (Vsh) and vertical (Vsv) direction for the top 300 m of the NEGIS using a Markov Chain Monte Carlo approach.
The reconstructed 1-D shear velocity model revels radial anisotropy in the NEGIS. Along and across flow vertical shear wave velocities (Vsv) identify comparable velocity profiles for all depths. However, horizontal shear wave velocities (Vsh) are faster by approximately 250 m/s in the along flow direction below a depth of 100 m, i.e. below the firn-ice transition.
This type of anisotropy seems to arise from the alignment of a crystallographic preferred orientation, due to deformation associated with shear zones. The role of anisotropy as e.g. created by air bubbles in the firn and ice matrix, is yet unclear.
Faster Vsh velocities in the along flow direction support that the NEGIS has crystal orientation alignment normal to the plane of shear compression (i.e. ice crystals orientated across flow) within the upper 300 m of the ice stream and are in alignment with the results from other methods. We demonstrate that simple, short duration (2-3 weeks), passive seismic deployment and environmental noise-based analysis can be used to determine the anisotropy of the upper part of ice masses
Local Seismicity and Sediment Deformation in the West Svalbard Margin: Implications of Neotectonics for Seafloor Seepage
Get PDFIn the Fram Strait, mid-ocean ridge spreading is represented by the ultra-slow system of the Molloy Ridge, the Molloy Transform Fault and the Knipovich Ridge. Sediments on oceanic and continental crust are gas charged and there are several locations with documented seafloor seepage. Sedimentary faulting shows recent stress release in the sub-surface, but the drivers of stress change and its influence on fluid flow are not entirely understood. We present here the results of an 11-month-long ocean bottom seismometer survey conducted over the highly faulted sediment drift northwards from the Knipovich Ridge to monitor seismicity and infer the regional state of stress. We obtain a detailed earthquake catalog that improves the spatial resolution of mid-ocean ridge seismicity compared with published data. Seismicity at the Molloy Transform Fault is occurring southwards from the bathymetric imprint of the fault, as supported by a seismic profile. Earthquakes in the northern termination of the Knipovich Ridge extend eastwards from the ridge valley, which together with syn-rift faulting identified in seismic reflection data, suggests that a portion of the currently active spreading center is buried under sediments away from the bathymetric expression of the rift valley. This hints at the direct link between crustal rifting processes and faulting in shallow sediments. Two earthquakes occur close to the seepage system of the Vestnesa Ridge further north from the network. We suggest that deeper rift structures, reactivated by gravity and/or post-glacial subsidence, may lead to accommodation of stress through shallow extensional faults, therefore impacting seepage dynamics
The pan-Arctic catchment database (ARCADE)
Get PDFThe Arctic is rapidly changing. Outside the Arctic, large-sample catchment databases have transformed catchment science from focusing on local case studies to more systematic studies of watershed functioning. Here we present an integrated pan-ARctic CAtchments summary DatabasE (ARCADE) of > 40 000 catchments that drain into the Arctic Ocean and range in size from 1 to 3.1 × 106 km2. These watersheds, delineated at a 90 m resolution, are provided with 103 geospatial, environmental, climatic, and physiographic catchment properties. ARCADE is the first aggregated database of pan-Arctic river catchments that also includes numerous small watersheds at a high resolution. These small catchments are experiencing the greatest climatic warming while also storing large quantities of soil carbon in landscapes that are especially prone to degradation of permafrost (i.e., ice wedge polygon terrain) and associated hydrological regime shifts. ARCADE is a key step toward monitoring the pan-Arctic across scales and is publicly available: https://doi.org/10.34894/U9HSPV (Speetjens et al., 2022)
The hydrological cycle and ocean circulation of the Maritime Continent in the Pliocene: results from PlioMIP2
Get PDFThe Maritime Continent (MC) forms the western boundary of the tropical Pacific Ocean, and relatively small changes in this region can impact the climate locally and remotely. In the mid-Piacenzian warm period of the Pliocene (mPWP; 3.264 to 3.025 Ma) atmospheric CO2 concentrations were ∼ 400 ppm, and the subaerial Sunda and Sahul shelves made the land–sea distribution of the MC different to today. Topographic changes and elevated levels of CO2, combined with other forcings, are therefore expected to have driven a substantial climate signal in the MC region at this time. By using the results from the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2), we study the mean climatic features of the MC in the mPWP and changes in Indonesian Throughflow (ITF) with respect to the preindustrial. Results show a warmer and wetter mPWP climate of the MC and lower sea surface salinity in the surrounding ocean compared with the preindustrial. Furthermore, we quantify the volume transfer through the ITF; although the ITF may be expected to be hindered by the subaerial shelves, 10 out of 15 models show an increased volume transport compared with the preindustrial. In order to avoid undue influence from closely related models that are present in the PlioMIP2 ensemble, we introduce a new metric, the multi-cluster mean (MCM), which is based on cluster analysis of the individual models. We study the effect that the choice of MCM versus the more traditional analysis of multi-model mean (MMM) and individual models has on the discrepancy between model results and data. We find that models, which reproduce modern MC climate well, are not always good at simulating the mPWP climate anomaly of the MC. By comparing with individual models, the MMM and MCM reproduce the preindustrial sea surface temperature (SST) of the reanalysis better than most individual models and produce less discrepancy with reconstructed sea surface temperature anomalies (SSTA) than most individual models in the MC. In addition, the clusters reveal spatial signals that are not captured by the MMM, so that the MCM provides us with a new way to explore the results from model ensembles that include similar models
Examination of gammarid transcriptomes reveals a widespread occurrence of key metabolic genes from epibiont bdelloid rotifers in freshwater species
Get PDFPrevious data revealed the unexpected presence of genes encoding for long-chain polyunsaturated fatty acid (LC-PUFA) biosynthetic enzymes in transcriptomes from freshwater gammarids but not in marine species, even though closely related species were compared. This study aimed to clarify the origin and occurrence of selected LC-PUFA biosynthesis gene markers across all published gammarid transcriptomes. Through systematic searches, we confirmed the widespread occurrence of sequences from seven elongases and desaturases involved in LC-PUFA biosynthesis, in transcriptomes from freshwater gammarids but not marine species, and clarified that such occurrence is independent from the gammarid species and geographical origin. The phylogenetic analysis established that the retrieved elongase and desaturase sequences were closely related to bdelloid rotifers, confirming that multiple transcriptomes from freshwater gammarids contain contaminating rotifers’ genetic material. Using the Adineta steineri genome, we investigated the genomic location and exon–intron organization of the elongase and desaturase genes, establishing they are all genome-anchored and, importantly, identifying instances of horizontal gene transfer. Finally, we provide compelling evidence demonstrating Bdelloidea desaturases and elongases enable these organisms to perform all the reactions for de novo biosynthesis of PUFA and, from them, LC-PUFA, an advantageous trait when considering the low abundance of these essential nutrients in freshwater environments
Mapping Arctic and Antarctic Seas with towed camera systems
Get PDFTowed camera sleds are regularly used as imaging platforms in the exploration of deep-sea seafloor ecosystems. A drawback of the majority of towed camera sleds is that the information received on benthic communities and habitats is limited to the field of view of the cameras mounted on the sled, and is typically restricted to images of the seafloor from directly underneath the device. In order to increase our understanding of benthic ecosystems and to identify benthic communities, their structure, biodiversity and ecosystems functioning over extended regions, combining image and acoustic data can be a useful approach to achieve this. One such system is the Ocean Floor Observation and Bathymetry System (OFOBS), developed by the Alfred-Wegener-Institute, used recently to discover and quantify the sponge grounds of the high Arctic and the world’s largest nesting fish colony in Antarctica.
In the Weddell Sea, the Antarctic Ocean, a vast breeding ground with millions of fish nests of the Antarctic Icefish was discovered with the OFOBS. The bathymetric, acoustic data collected by the system was used to infer the extended distribution of round fish nest depressions in the seafloor for swathes of 50 m on each side of the device during each deployment tow. In the Arctic, the OFOBS system has been used to assess the state and distribution of deep-sea trawl marks across the Svalbard shelf and the distribution of geological and biological assemblages across the Fram Strait.
In this presentation, we demonstrate how variations in slope and surface structure influence the inhabiting benthic community in the Antarctic as well as the Arctic Ocean, as inferred by the data collected by this combined camera and acoustic towed system
Deciphering stable water isotope records of firn cores from a strongly maritime, high-accumulation site on the Antarctic Peninsula
Get PDFStable water isotope records of six firn cores retrieved from two adjacent plateaus on the northern Antarctic Peninsula between 2014 and 2016 are presented and investigated for their connections with firn-core glacio-chemical data, meteorological records and modelling results. Average annual accumulation rates of 2500 kg m-2 a-1 largely reduce the modification of isotopic signals in the snowpack by post-depositional processes, allowing excellent signal preservation in space and time. Comparison of firn-core and ECHAM6-wiso modelled δ18O and d-excess records reveals a large agreement on annual and sub-annual scales, suggesting firn-core stable water isotopes to be representative of specific synoptic situations. The six firn cores exhibit highly similar isotopic patterns in the overlapping period (2013), which seem to be related to temporal changes in moisture sources rather than local near-surface air temperatures. Backward trajectories calculated with the HYSPLIT model suggest that prominent δ18O minima in 2013 associated with elevated sea salt concentrations are related to long-range moisture transport dominated by westerly winds during positive SAM phases. In contrast, a broad δ18O maximum in the same year accompanied by increased concentrations of black carbon and mineral dust corresponds to the advection of more locally derived moisture with northerly flow components (South America) when the SAM is negative
Iron and manganese availability drives primary production and carbon export in the Weddell Sea
Get PDFNext to iron (Fe), recent phytoplankton-enrichment experiments identified manganese (Mn) to (co-)limit Southern Ocean phytoplankton biomass and species composition. Since taxonomic diversity affects aggregation time and sinking rate, the efficiency of the biological carbon pump is directly affected by community structure. However, the impact of FeMn co-limitation on Antarctic primary production, community composition, and the subsequent export of carbon to depth requires more investigation. In situ samplings of 6 stations in the understudied southern Weddell Sea revealed that surface Fe and Mn concentrations, primary production, and carbon export rates were all low, suggesting a FeMn co-limited phytoplankton community. An Fe and Mn addition experiment examined how changes in the species composition drive the aggregation capability of a natural phytoplankton community. Primary production rates were highest when Fe and Mn were added together, due to an increased abundance of the colonial prymnesiophyte Phaeocystis antarctica. Although the community remained diatom dominated, the increase in Phaeocystis abundance led to highly carbon-enriched aggregates and a 4-fold increase in the carbon export potential compared to the control, whereas it only doubled in the Fe treatment. Based on the outcome of the FeMn-enrichment experiment, this region may suffer from FeMn co-limitation. As the Weddell Sea represents one of the most productive Antarctic marginal ice zones, our findings highlight that in response to greater Fe and Mn supply, changes in plankton community composition and primary production can have a disproportionally larger effect on the carbon export potential