Physical processes controlling the rifting of Larsen C Ice Shelf,Antarctica, prior to the calving of iceberg A68

The sudden propagation of a major preexisting rift (full-thickness crack) in late 2016 on the Larsen C Ice Shelf, Antarctica led to the calving of tabular iceberg A68 in July 2017, one of the largest icebergs on record, posing a threat for the stability of the remaining ice shelf. As with other ice shelves, the physical processes that led to the activation of the A68 rift and controlled its propagation have not been elucidated. Here, we model the response of the ice shelf stress balance to ice shelf thinning and thinning of the ice mélange encased in and around preexisting rifts. We find that ice shelf thinning does not reactivate the rifts, but heals them. In contrast, thinning of the mélange controls the opening rate of the rift, with an above-linear dependence on thinning. The simulations indicate that thinning of the ice mélange by 10 to 20 m is sufficient to reactivate the rifts and trigger a major calving event, thereby establishing a link between climate forcing and ice shelf retreat that has not been included in ice sheet models. Rift activation could initiate ice shelf retreat decades prior to hydrofracture caused by water ponding at the ice shelf surface.

The Larsen A and Larsen B ice shelves, in the Antarctic Peninsula, collapsed in spectacular fashion in 1995 and 2002, respectively, as a result of climate warming (1, 2). While the loss of the Larsen A and B ice shelves did not impact sea level directly, it affected their upstream glaciers in a major way (3). The Larsen A and Larsen B glaciers experienced a three- to eightfold acceleration in speed following the collapse of these buttressing ice shelves (4–7), which increased land ice discharge into the ocean and contributed to sea level rise from the Antarctic Peninsula. These two events demonstrated the importance of ice shelf buttressing and exemplified what could happen elsewhere in Antarctica as climate warming extends farther south. If all Antarctic glaciers with ice shelves were to accelerate eightfold, sea level would rise 4 m per century.The Larsen C Ice Shelf, immediately south of Larsen A and B (Fig. 1), is the largest ice shelf in the Antarctic Peninsula (46,465 km2). It drains a land area of 18,120 km2, with an ice flux of 14.5 Gt/y and an ice volume equivalent to a global sea level rise of 0.9 cm (8). As warming continues, Larsen C is expected to collapse (9). While Larsen C does not hold back a large volume of land ice, it stands north of the Ronne Ice Shelf, one of the largest ice shelves in Antarctica, which buttresses glaciers with a 158-cm sea level rise equivalent, or orders of magnitude larger than those in the Antarctic Peninsula. Addressing the fate of the Larsen ice shelves is therefore an issue of considerable importance for sea level rise from Antarctica.Open in a separate windowFig. 1.(A–F) Larsen Ice Shelf, Antarctic Peninsula with (A) backscatter image from Sentinel-1 EW on 27 July 2017 (62) with Inset map for location in Antarctica and the location of the compressive arch of stability of the ice shelf (purple); (B) color-coded 20- (red), 40- (brown), and 100-m (black) contour levels in surface elevation with time-tagged (blue triangles) position of the rupture tip of A68 from Sentinel-1a interferometric SAR; (C) surface elevation above mean sea level (in meters) from TanDEM-X digital elevation model in years 2013 and 2014; (D) zoom-in around GIR; (E) contour levels color coded from 5 to 80 m in 5-m increments; and (F) surface speed (in meters per year) on a logarithmic scale color coded from slow (brown) to fast (blue/red) (63) with grounding line position (red) from ERS-1/2 differential SAR interferometry (64).The prevailing view for explaining the evolution of Larsen A and B and their collapse is the hydrofracture theory (10–13). In this theory, melt water accumulates at the surface of an ice shelf with sufficient warming, collects in cracks, and refreezes at depth at the end of the melt season, which results in further cracking of the ice shelf. Melting ponds have been observed in the Eastern Antarctic Peninsula during warm summers throughout the 20th century (14–16). Using Landsat and Earth Remote Sensing (ERS)-1/2, melt ponds were identified in the northern section of Larsen B and on Larsen A in the summer of 1988 and more evidently in 1993 (10, 17), 2 y before the collapse in January 1995 (1). In the late 1990s, warmer summers and enhanced melting seasons (18, 19) spread meltwater ponds southward to reach their southernmost extension of 1999 just north of Cape Disappointment (11). As the melting season lengthened (20), melt ponds were observed through the entire Larsen B Ice Shelf until its collapse in March 2002 (21). The hydrofracture theory, however, does not explain why the ice front of Prince Gustav Channel Ice Shelf, north of Larsen A, started to retreat as early as 1957, Larsen A Ice Shelf started to retreat in 1975, and Larsen B Ice Shelf in 1986, i.e., decades before their collapse (16, 17). Similarly, the hydrofracture theory does not explain why A68 calved in the middle of the Antarctic winter, in the absence of melt water.Calving events on ice shelves dominantly originate from ice front-parallel rifts that propagate in a direction transverse to the ice flow (22–25). When the ice blocks detach from the shelf, they form tabular icebergs. Iceberg production for Larsen C averages 31 Gt/y, or twice the grounding line flux. The ice shelf also loses mass from the bottom in contact with warm, salty ocean waters at a rate of 21 Gt/y (26). The initiation and propagation (or arrest) of rifts exert a major control on iceberg production and therefore on ice shelf mass balance (27).Several studies have attempted to quantitatively couple rift growth (followed by ice breakup) to its destabilizing effect on Antarctic ice shelves, although calving processes are not well understood and modeled (see ref. 28 for a review on calving criteria). The “compressive arch” concept was introduced during an analysis of the strain rate distribution in the Larsen A Ice Shelf before its collapse in 1995 (23). In this theory, if the ice front breaks through a compressive arch, where only the least principal strain-rate component is compressive, the ice front retreat becomes irreversible. Fractures propagating seaward of the arch, where both principal strain-rate components are extensive, do not pose a risk to ice shelf stability. As pointed out elsewhere, ice shelf fractures tend to strike in a direction perpendicular to the ice flow and their propagation rates are maximized when the first principal stress and the fracture strike form a right angle (24). The distribution of angles between ice flow direction, principal stress component, and rift orientation can be used as an indicator of ice shelf stability (24, 29, 30).Ice shelves are composed of meteoric units fed by inland glaciers, glued together along suture zones. Suture zones in Larsen C form seaward of the Joerg and Churchill Peninsula and around Tonkin and Francis Islands, in places where the ice shelf rifts apart from stress singularities along the coastline (Fig. 1). These fractured areas get filled with marine ice (31, 32), which accumulates in the downflow direction and progressively heals the fractures over time on time scales of decades to centuries. A similar infill accretes in between rift flanks, which are full-thickness cracks in the shelf (33). Depending on the exposure of ice fractures to the ocean and atmosphere freezing, suture zones and rifted areas are filled by a heterogeneous mixture of accreted ice, blown snow, and iceberg debris termed ice mélange. This ice mélange builds up over time into a thick, mechanically resistant and cohesive material (34–38). Areas filled by ice mélange are softer, warmer, and less prone to favor rift propagation than cold meteoric ice (9, 39–41). Rifts often stop propagating when they reach these suture zones (32, 42, 43).Prior work on Larsen B and C has shown that melting of accreted ice in rifted areas may alter the longitudinal stress orientation on the ice shelf, facilitating rifting through suture zones and potentially destabilizing the ice shelf (24, 29, 32). As a consequence, the distribution of accreted ice (marine ice underneath shelves and ice mélange in atmosphere exposed areas), modulated by oceanic and atmospheric forcings, may be a link between climate forcing, rift propagation, and ice shelf retreat. Prior work also evaluated the impact of ice rigidity, ice fabric, and ice damage in controlling rift initiation or propagation (36, 44–48). Other studies have correlated the incidence of ocean swells or tsunamis to rifting episodes by using a combination of satellite monitoring and in situ seismometers (49–54). Ocean waves of sufficient energy and suitable period impact the ice front and induce cyclical flexural stresses to the ice shelves (25, 55–57). Excessive shelf bending in response to infragravity (period of ground swells) or longer period waves (storms or tsunamis) is speculated to cause fatigue damaging, hence controlling the onset of crack initiation and propagation (27, 51, 58). The effective impact of waves on ice shelves is substantially modulated by the presence of sea ice in the vicinity of the ice front, which forms a buffer layer that dissipates wave energy (59). Loss of this protective layer exposes ice shelves to the arrival of large swells or tsunamis, which may trigger calving and potentially cause ice shelf disintegration (60). Similar to marine ice and ice mélange in fractured areas, a warming climate curtails sea ice distribution, which exposes ice fronts to enhanced wave-induced stress.When combined together, processes such as hydrofracture and viscoelastic flexure of ice shelves can lead to runaway processes such as iceberg-capsize tsunamigenesis as described in ref. 61, potentially leading to catastrophic collapse of ice shelves. Coupling between vertical bending of an ice shelf and horizontal stresses such as lateral shearing and longitudinal expansion is, however, currently poorly described. In particular, little is known about how the tsunamigenesis type collapse of an ice shelf competes or coexists with a scenario of increasing horizontal weakening of an ice shelf such as observed prior to the Larsen A collapse.Iceberg A68 calved after the along-front propagation of a crack that had grown since 2005, stayed dormant for 10 y, opened up around 2014, stopped, and reopened up again in November 2016 (29, 30), to culminate with the release of A68 on 12 July 2017 (Fig. 1B). To put this event in context, the ice volume of A68 is equivalent to 42 y of calving history of Larsen C and brought its ice front into its farthest back position since the discovery of Larsen C by Captain Carl Arton Larsen in 1893. The ice front is now closer to the compressive arch of stability of the ice shelf than in the past century. How this event unfolded and how it relates to climate warming are essential elements to understand to model the evolution of Antarctic ice shelves in a warming climate.The first phase of rift propagation in year 2014 stopped at the suture zone downstream of Joerg Peninsula (32, 43) that includes a significant fraction of ice mélange (Fig. 1 C–E). In the second phase, starting in November 2016, the rift opened up again and progressed rapidly across the ice front (Fig. 1B), growing mostly orthogonal to maximum tensile stresses (30). As of this date, no physical process has been proposed to explain the reactivation of the rift in November 2016. At the end of the calving, the ice front was in its most retreated position since first being discovered in 1893 and closest to the compressive arch (Fig. 1A) derived from ice velocity data collected from 2007 to 2009.Here, we present a stress-balance analysis of the state of the ice shelf prior to the propagation that includes rifts. We evaluate the relationship between ice thinning and the opening rate of A68. We consider two major effects: 1) ice shelf thinning, which has been documented elsewhere (65), and 2) thinning of the ice mélange encased in the fractured sections of the ice shelf within and around the rifts, which has not been well studied. We use the modeling results to conclude on the impact of climate forcing on ice shelf rifting, ice shelf calving and retreat, and the future of Larsen C Ice Shelf and other Antarctic ice shelves.     

Direct Evidence of Ice Crystallization Inhibition by Dielectric Relaxation of Hydrated Ions

In this paper, the inhibition effect of an alternative current (AC) electric field on ice crystallization in 0.9 wt % NaCl aqueous solution was confirmed thermodynamically with characterization. An innovative experimental and analytical method, combining differential scanning calorimeter (DSC) measurement with an externally applied electric field was created by implanting microelectrodes in a sample crucible. It was found that the ice crystallization, including pure ice and salty ice, was obviously inhibited after field cooling with an external AC electric field in a frequency range of 100 k–10 MHz, and the crystallization ratio was related to frequency. Compared with non-field cooling, the crystallization ratio of ice crystals was reduced to less than 20% when E = 57.8 kV/m and f = 1 MHz. The dielectric spectrum results show that this inhibition effect of an alternating electric field on ice crystal growth is closely related to the dielectric relaxation process of hydrated ions.     

关节松动术联合冰敷治疗冻结肩的临床研究

目的：观察关节松动术及冰敷改善肩关节功能障碍的疗效和安全性。方法：冻结肩患者65例,随机分为综合组33例和对照组32例,均给予关节松动术,包括盂肱关节分离牵引,长轴牵引,肩前屈、内旋、后伸外旋和外展等活动。综合组关节松动术后即给予局部冰疗20～30min。治疗前后采用疼痛视觉模拟评分（VAS）和Con-stant-Murley肩关节功能评定法（C-M评分）对肩关节疼痛程度和活动范围评定。结果：经过15d的治疗,2组VAS评分较治疗前明显下降,C-M评分明显提高,综合组较对照组更显著（均P〈0.01）;临床疗效比较,综合组优良率明显高于对照组（P〈0.05）。结论：关节松动术联合冰敷治疗冻结肩可显著提高治愈率,改善肩关节运动功能。     

冰敷双侧颈动脉在脑出血昏迷患者转院途中的应用

目的：探讨冰敷双侧颈动脉,在脑出血昏迷患者转院途中的作用效果。方法：选取2007年9月～2009年9月我院救护车接诊的脑出血昏迷患者120例,随机分为对照组和观察组,各60例,对照组采取常规急救措施,观察组除用常规急救措施外,加用了冰敷双侧颈动脉。复查头颅CT,观察脑出血量的变化。结果：观察组转院的患者,其加重脑出血的几率明显低于对照组（P〈0.001或P〈0.05）。结论：脑出血转运途中,加用冰敷双侧颈动脉,可以避免加重脑出血,从而提高转院抢救成功率。     

超声弹性成像监测冷冻手术的仿真实验研究

从生物组织冻结前后其弹性模量会发生显著改变的角度出发,在提出了低温外科手术中监测冰球形成与消融过程的超声弹性成像方法的研究基础上,设计了3类概念性超声弹性监测试验：含玻璃体模、常温组织体模及冻结组织体模仿真试验,首次从实验上证实了超声弹性成像方法在低温外科手术中对冰球实施高对比度成像的可行性。文章分析了该方法在应用中的一些测量误差,指出了进一步改进的方向。冰球这种与水呈高弹性对比度的临床监测对象,可望成为超声弹性成像医学应用中的一个重要主题。     

Sea spray aerosol as a unique source of ice nucleating particles

Ice nucleating particles (INPs) are vital for ice initiation in, and precipitation from, mixed-phase clouds. A source of INPs from oceans within sea spray aerosol (SSA) emissions has been suggested in previous studies but remained unconfirmed. Here, we show that INPs are emitted using real wave breaking in a laboratory flume to produce SSA. The number concentrations of INPs from laboratory-generated SSA, when normalized to typical total aerosol number concentrations in the marine boundary layer, agree well with measurements from diverse regions over the oceans. Data in the present study are also in accord with previously published INP measurements made over remote ocean regions. INP number concentrations active within liquid water droplets increase exponentially in number with a decrease in temperature below 0 °C, averaging an order of magnitude increase per 5 °C interval. The plausibility of a strong increase in SSA INP emissions in association with phytoplankton blooms is also shown in laboratory simulations. Nevertheless, INP number concentrations, or active site densities approximated using “dry” geometric SSA surface areas, are a few orders of magnitude lower than corresponding concentrations or site densities in the surface boundary layer over continental regions. These findings have important implications for cloud radiative forcing and precipitation within low-level and midlevel marine clouds unaffected by continental INP sources, such as may occur over the Southern Ocean.     

Converging evidence for the under-reporting of concussions in youth ice hockey

Background

Concussions are potentially serious injuries. The few investigations of prevalence or incidence in youth ice hockey have typically relied on prospective reports from physicians or trainers and did not survey players, despite the knowledge that many athletes do not report probable concussions.

Objective

This study sought to compare concussion rates in youth ice hockey that were estimated from a variety of reporting strategies.

Methods

Rates were calculated from British Columbia Amateur Hockey Association (BCAHA) official injury reports, from direct game observation by minor hockey volunteers (such as coaches and managers), as well as from retrospective surveys of both elite and non‐elite youth players. All research was conducted within the BCAHA.

Results

Estimates from official injury reports for male players were between 0.25 and 0.61 concussions per 1000 player game hours (PGH). Concussion estimates from volunteer reports were between 4.44 and 7.94 per 1000 PGH. Player survey estimates were between 6.65 and 8.32 per 1000 PGH, and 9.72 and 24.30 per 1000 PGH for elite and non‐elite male youth hockey, respectively.

Conclusion

It was found that concussions are considerably under‐reported to the BCAHA by youth hockey players and team personnel.     

眼针治疗脑卒中后吞咽困难46例

目的:观察眼针治疗脑卒中后吞咽困难的临床疗效。方法:选取90例急性脑卒中出现吞咽障碍患者,随机分为眼针组46例和对照组44例。对照组给予体针及口腔冰刺激治疗,眼针组在对照组的基础上加用眼针治疗。观察两组患者的临床疗效和治疗前后洼田饮水试验评分。结果:眼针组有效率95.65%,对照组有效率为81.82%,两组患者临床疗效比较,差异有统计学意义(P0.05)。治疗后两组患者洼田饮水试验评分低于治疗前,且眼针组低于对照组,差异均有统计学意义(P0.05)。结论:眼针治疗脑卒中吞咽困难临床疗效显著,可显著改善患者的临床症状。     

蜂王浆含片的性功能药效研究

目的:观察蜂王浆含片益肾壮阳的药效学作用。方法:采用氢化可的松和去势手术造成阳虚模型,观察比较各组体温、体质量、自主活动;采用戊巴比妥钠制作的去势大鼠模型,观察比较各组的阴茎勃起潜伏期。结果:蜂王浆含片能提高氢化可的松肾虚模型小鼠的体质量、体温、自主次数,可缩短去势大鼠阴茎勃起潜伏期,同时明显增强雄性大鼠的交配能力。结论:蜂王浆含片有补肾壮阳作用。