Abstract: In coastal Antarctica, outlet glaciers exhibit complex dynamics materialized by intense internal deformation, enhanced basal sliding and strong thermo-mechanical interactions with the ocean. Here we aim to use seismic observations to unravel these various processes and their link with glacier and ocean dynamics. As part of the SEIS-ADELICE project (2020-2024) supported by the French Polar Institute IPEV, in January 2022 we deployed four permanent and six temporary (1 month long) broadband seismic stations on and around the Astrolabe Glacier (Terre Adélie, East Antarctica), as well as four ocean-bottom seismometers at sea near the terminus of the floating tongue. In January 2023 we will be supplementing this setup by a temporary network of 50 seismic nodes above the grounding line of the glacier.

Preliminary detection and classification of seismic events reveals a wide variety of cryo-seismic signals. The most pervasive events correspond to icequakes, are located close to the surface, and exhibit clear tidal modulation. We interpret these events as being generated by the brittle fracturing of ice associated with crevasse opening. We also observe numerous short and similar repetitive events of much lower amplitude that are located at few restricted locations near the ice-bedrock interface. These events are likely produced by basal stick-slip over punctual bedrock asperities. Finally, we observe glacial tremors which could result from hydraulic sources at the ice-bedrock interface, although further analysis is required to confirm this hypothesis.

This preliminary work provides useful grounds for deeper analysis to be done in the future on source characteristics and their more quantitative links with glacier dynamics.

Abstract: Group living is thought to have important antipredator benefits for animals, owing to the mechanisms of shared vigilance (“many-eyes” hypothesis), risk dilution (“dilution effect” hypothesis), and relative safety in the center of the group (“selfish herd” hypothesis). However, it can also incur costs since social stimuli, such as conspecific aggression, may distract individuals from anti-predator behavior (“distracted prey” hypothesis). We simultaneously evaluated how these four different hypotheses shape anti-predator behaviors of breeding king penguins (Aptenodytes patagonicus), which aggregate into large colonies, experience frequent aggressive social interactions, and are regularly exposed to predation by giant petrels (Macronectes sp.) and brown skuas (Catharacta loonbergi) when breeding on land. We approached 200 incubating penguins at four different periods of the breeding season across a range of overall increasing colony densities. We measured the distance at which focal birds detected the approaching threat (alert distance: AD), whether birds decided to flee or not, and the distance of flight initiation (flight initiation distance: FID, viz. the bird attempting to walk away with its egg on its feet). We quantified relative local neighbor density, centrality within the colony (rank), and the number of aggressions the focal bird emitted towards neighbors during the approach. We found that birds engaged in aggressive conflicts with neighbors were less likely to flee, and that increasing relative local neighbor density at low and medium overall colony density resulted in a decrease in bird AD, both supporting the “distracted prey” hypothesis. However, at maximal overall colony density, increasing relative local neighbor density resulted in longer AD, supporting the “many-eyes” hypothesis. We found no support for the “dilution effect” and “selfish herd” hypotheses, and no effects of any hypothesis on FID.

Abstract: Polar amplification of climate change has the potential to cause large-scale shifts in the dissolved oxygen (DO) dynamics of Arctic lakes, with implications for fish survival, greenhouse gas production, and drinking water quality. While DO is also a sentinel of environmental changes of physical, chemical, and biological nature (e.g., ice cover, temperature, dissolved organic carbon, photosynthesis, and respiration), no synthesis exists of current knowledge of DO dynamics across the diverse freshwater systems of the Arctic. We thus conducted a systematic review of the literature that yielded DO data from 167 sites north of the Subarctic limit (based on vegetation zones), spanning 76 years and including 40 sites with time series. The compilation revealed insufficient observations for adequate representativeness of oxygen dynamics over Arctic ecosystem gradients. We described the main processes controlling DO budgets of Arctic lakes and tested relationships of summer oxygen depletion with maximum depth and latitude. The meta-analysis showed that most sites with low O2 concentrations were shallow (<10 m) and situated toward the southern end of the latitudinal gradient. Permanently stratified lakes with deep, perennially anoxic basins were located toward the northern end of the gradient. By way of a conceptual model, we identified the direct and indirect drivers and mechanisms that lead to changes in oxygen budgets in the context of the warming Arctic. This comprehensive update on available data allowed us to suggest future research directions and recommend the use of moored instruments for continuous all-season observations, combined with modeling, remote sensing, and paleo-reconstructions.

Abstract: Seismic discontinuities in the mantle are indicators of its thermo-chemical state and offer clues to its dynamics. Ray-based seismic methods, though limited by the approximations made, have mapped mantle transition zone discontinuities in detail, but have yet to offer definitive conclusions on the presence and nature of mid-mantle discontinuities. Here, we show how to use a wave-equation-based imaging method, reverse-time migration of precursors to surface-reflected seismic body waves, to uncover both mantle transition zone and mid-mantle discontinuities, and interpret their physical nature. We observe a thinned mantle transition zone southeast of Hawaii, and a reduction in impedance contrast around 410 km depth in the same area, suggesting a hotter-than-average mantle in the region. Here, we furthermore reveal a 4000–5000 km-wide reflector in new images of the mid mantle below the central Pacific, at 950–1050 km depth. This deep discontinuity exhibits strong topography and generates reflections with polarity opposite to those originating at the 660 km discontinuity, implying an impedance reversal near 1000 km. We link this mid-mantle discontinuity to the upper reaches of deflected mantle plumes upwelling in the region. Reverse-time migration full-waveform imaging is a powerful approach to imaging Earth’s interior, capable of broadening our understanding of its structure and dynamics and shrinking modeling uncertainties.