Underwater acoustic signatures of glacier calving

Climate-driven ice-water interactions in the contact zone between marine-terminating glaciers and the ocean surface show a dynamic and complex nature. Tidewater glaciers lose volume through the poorly understood process of calving. A detailed description of the mechanisms controlling the course of calving is essential for the reliable estimation and prediction of mass loss from glaciers. Here we present the potential of hydroacoustic methods to investigate different modes of ice detachments. High-frequency underwater ambient noise recordings are combinedwith synchronized, high-resolution, time-lapse photography of the Hans Glacier cliff in Hornsund Fjord, Spitsbergen, to identify three types of calving events: typical subaerial, sliding subaerial, and submarine. A quantitative analysis of the data reveals a robust correlation between ice impact energy and acoustic emission at frequencies below 200 Hz for subaerial calving. We suggest that relatively inexpensive acoustic methods can be successfully used to provide quantitative descriptions of the various calving types

Subglacial Discharge Reflux and Buoyancy Forcing Drive Seasonality in a Silled Glacial Fjord

Fjords are conduits for heat and mass exchange between tidewater glaciers and the coastal ocean, and thus regulate near-glacier water properties and submarine melting of glaciers. Entrainment into subglacial discharge plumes is a primary driver of seasonal glacial fjord circulation; however, outflowing plumes may continue to influence circulation after reaching neutral buoyancy through the sill-driven mixing and recycling, or reflux, of glacial freshwater. Despite its importance in non-glacial fjords, no framework exists for how freshwater reflux may affect circulation in glacial fjords, where strong buoyancy forcing is also present. Here, we pair a suite of hydrographic observations measured throughout 2016–2017 in LeConte Bay, Alaska, with a three-dimensional numerical model of the fjord to quantify sill-driven reflux of glacial freshwater, and determine its influence on glacial fjord circulation. When paired with subglacial discharge plume-driven buoyancy forcing, sill-generated mixing drives distinct seasonal circulation regimes that differ greatly in their ability to transport heat to the glacier terminus. During the summer, 53%–72% of the surface outflow is refluxed at the fjord's shallow entrance sill and is subsequently re-entrained into the subglacial discharge plume at the fjord head. As a result, near-terminus water properties are heavily influenced by mixing at the entrance sill, and circulation is altered to draw warm, modified external surface water to the glacier grounding line at 200 m depth. This circulatory cell does not exist in the winter when freshwater reflux is minimal. Similar seasonal behavior may exist at other glacial fjords throughout Southeast Alaska, Patagonia, Greenland, and elsewhere.This work was supported by NSF Arctic Natural Sciences grants OPP-1503910, 1504191, 1504288, and 1504521. The authors thank Pat Dryer, Dylan Winters, Erin Pettit, and the crews of the R/V Pelican and M/V Stellar for their contributions to the fieldwork. The authors thank Petersburg High School and the U.S. Forest Service for accommodating this project, and our two anonymous reviewers for their feedback in improving the manuscript. The authors also acknowledge the Shtax'héen Kwáan Tlingits, whose ancestral lands lie in this region.Ye

Formation, flow and break-up of ephemeral ice mélange at LeConte Glacier and Bay, Alaska.

© The Author(s), 2020. Published by Cambridge University Press. This is an Open Access article, distributed under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike licence (http://creativecommons.org/licenses/by-nc-sa/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the same Creative Commons licence is included and the original work is properly cited. The written permission of Cambridge University Press must be obtained for commercial re-use. Supplementary material. The supplementary material for this article can be found at https://doi.org/10.1017/jog.2020.29Ice mélange has been postulated to impact glacier and fjord dynamics through a variety of mechanical and thermodynamic couplings. However, observations of these interactions are very limited. Here, we report on glaciological and oceanographic data that were collected from 2016 to 2017 at LeConte Glacier and Bay, Alaska, and serendipitously captured the formation, flow and break-up of ephemeral ice mélange. Sea ice formed overnight in mid-February. Over the subsequent week, the sea ice and icebergs were compacted by the advancing glacier terminus, after which the ice mélange flowed quasi-statically. The presence of ice mélange coincided with the lowest glacier velocities and frontal ablation rates in our record. In early April, increasing glacier runoff and the formation of a sub-ice-mélange plume began to melt and pull apart the ice mélange. The plume, outgoing tides and large calving events contributed to its break-up, which took place over a week and occurred in pulses. Unlike observations from elsewhere, the loss of ice mélange integrity did not coincide with the onset of seasonal glacier retreat. Our observations provide a challenge to ice mélange models aimed at quantifying the mechanical and thermodynamic couplings between ice mélange, glaciers and fjords.This work was supported by the US NSF awards OPP-1503910, OPP-1504191, OPP-1504288, OPP-1504521 and DMR-1506307. The WorldView imagery and DEM were provided by the Polar Geospatial Center under US NSF awards OPP-1043681, OPP-1559691 and OPP-1542736. The IfSAR DEM is distributed through the USGS Earth Resources Observation Center. Field logistics was provided by CH2MHill Polar Field Services and would not have been possible without the help of the crew of the MV Steller and MV Pelican, Temsco Helicopters, Petersburg High School and the US Forest Service. We also thank J.B. Mickett, D.S. Winters, W.P. Dryer, A. Stewart, M. Michels, C. Carr, T. Moon, A. Simpson and E.C. Pettit for assistance with field work and data processing and M. Truffer for loaning the GPRI radar interferometer.Ye

TESLA Technical Design Report Part III: Physics at an e+e- Linear Collider

The TESLA Technical Design Report Part III: Physics at an e+e- Linear ColliderComment: 192 pages, 131 figures. Some figures have reduced quality. Full quality figures can be obtained from http://tesla.desy.de/tdr. Editors - R.-D. Heuer, D.J. Miller, F. Richard, P.M. Zerwa

Heavy quarkonium: progress, puzzles, and opportunities

A golden age for heavy quarkonium physics dawned a decade ago, initiated by the confluence of exciting advances in quantum chromodynamics (QCD) and an explosion of related experimental activity. The early years of this period were chronicled in the Quarkonium Working Group (QWG) CERN Yellow Report (YR) in 2004, which presented a comprehensive review of the status of the field at that time and provided specific recommendations for further progress. However, the broad spectrum of subsequent breakthroughs, surprises, and continuing puzzles could only be partially anticipated. Since the release of the YR, the BESII program concluded only to give birth to BESIII; the $B$-factories and CLEO-c flourished; quarkonium production and polarization measurements at HERA and the Tevatron matured; and heavy-ion collisions at RHIC have opened a window on the deconfinement regime. All these experiments leave legacies of quality, precision, and unsolved mysteries for quarkonium physics, and therefore beg for continuing investigations. The plethora of newly-found quarkonium-like states unleashed a flood of theoretical investigations into new forms of matter such as quark-gluon hybrids, mesonic molecules, and tetraquarks. Measurements of the spectroscopy, decays, production, and in-medium behavior of c\bar{c}, b\bar{b}, and b\bar{c} bound states have been shown to validate some theoretical approaches to QCD and highlight lack of quantitative success for others. The intriguing details of quarkonium suppression in heavy-ion collisions that have emerged from RHIC have elevated the importance of separating hot- and cold-nuclear-matter effects in quark-gluon plasma studies. This review systematically addresses all these matters and concludes by prioritizing directions for ongoing and future efforts.Comment: 182 pages, 112 figures. Editors: N. Brambilla, S. Eidelman, B. K. Heltsley, R. Vogt. Section Coordinators: G. T. Bodwin, E. Eichten, A. D. Frawley, A. B. Meyer, R. E. Mitchell, V. Papadimitriou, P. Petreczky, A. A. Petrov, P. Robbe, A. Vair

Identification of a Bacterial-Like HslVU Protease in the Mitochondria of Trypanosoma brucei and Its Role in Mitochondrial DNA Replication

ATP-dependent protease complexes are present in all living organisms, including the 26S proteasome in eukaryotes, Archaea, and Actinomycetales, and the HslVU protease in eubacteria. The structure of HslVU protease resembles that of the 26S proteasome, and the simultaneous presence of both proteases in one organism was deemed unlikely. However, HslVU homologs have been identified recently in some primordial eukaryotes, though their potential function remains elusive. We characterized the HslVU homolog from Trypanosoma brucei, a eukaryotic protozoan parasite and the causative agent of human sleeping sickness. TbHslVU has ATP-dependent peptidase activity and, like its bacterial counterpart, has essential lysine and N-terminal threonines in the catalytic subunit. By epitope tagging, TbHslVU localizes to mitochondria and is associated with the mitochondrial genome, kinetoplast DNA (kDNA). RNAi of TbHslVU dramatically affects the kDNA by causing over-replication of the minicircle DNA. This leads to defects in kDNA segregation and, subsequently, to continuous network growth to an enormous size. Multiple discrete foci of nicked/gapped minicircles are formed on the periphery of kDNA disc, suggesting a failure in repairing the gaps in the minicircles for kDNA segregation. TbHslVU is a eubacterial protease identified in the mitochondria of a eukaryote. It has a novel function in regulating mitochondrial DNA replication that has never been observed in other organisms

The Short Non-Coding Transcriptome of the Protozoan Parasite Trypanosoma cruzi

The pathway for RNA interference is widespread in metazoans and participates in numerous cellular tasks, from gene silencing to chromatin remodeling and protection against retrotransposition. The unicellular eukaryote Trypanosoma cruzi is missing the canonical RNAi pathway and is unable to induce RNAi-related processes. To further understand alternative RNA pathways operating in this organism, we have performed deep sequencing and genome-wide analyses of a size-fractioned cDNA library (16–61 nt) from the epimastigote life stage. Deep sequencing generated 582,243 short sequences of which 91% could be aligned with the genome sequence. About 95–98% of the aligned data (depending on the haplotype) corresponded to small RNAs derived from tRNAs, rRNAs, snRNAs and snoRNAs. The largest class consisted of tRNA-derived small RNAs which primarily originated from the 3′ end of tRNAs, followed by small RNAs derived from rRNA. The remaining sequences revealed the presence of 92 novel transcribed loci, of which 79 did not show homology to known RNA classes

Extensive retreat of Greenland tidewater glaciers 2000-2010

Overall mass loss from the Greenland ice sheet nearly doubled during the early 2000s resulting in an increased contribution to sea-level rise, with this step-change being mainly attributed to the widespread frontal retreat and accompanying dynamic thinning of tidewater glaciers. Changes in glacier calving-front positions are easily derived from remotely sensed imagery and provide a record of dynamic change. However, ice-sheet-wide studies of calving fronts have been either spatially or temporally limited. In this study multiple calving-front positions were derived for 199 Greenland marine-terminating outlet glaciers with width greater than 1 km using Landsat imagery for the 11-year period 2000–2010 in order to identify regional seasonal and inter-annual variations. During this period, outlet glaciers were characterized by sustained and substantial retreat summing to more than 267 km, with only 11 glaciers showing overall advance. In general, the pattern of mass loss detected by GRACE (Gravity Recovery and Climate Experiment) and other measurements is reflected in the calving record of Greenland glaciers. Our results suggest several regions in the south and east of the ice sheet likely share controls on their dynamic changes, but no simple single control is apparent