Systematic experiments reveal that the flexural strength of freshwater S2 columnar-grained ice loaded normal to the columns increases upon cyclic loading. Specifically, over the range of stress ...amplitudes 0.1–2.6 MPa the flexural strength increases linearly with increasing stress amplitude. The experiments were conducted upon both reversed and non-reversed cyclic loading over ranges of frequencies from 0.03 to 2 Hz and temperatures from −25 to −3°C. Strengthening can also be imparted through bending-induced creep. The fundamental requirement for strengthening is that the surface that undergoes maximum tensile stress during failure must have been pre-stressed in tension. Flexural strength is governed by crack nucleation. We suggest that the process is resisted by an internal back-stress that opposes the applied stress and builds up through either crystal dislocations piling up or grain boundaries sliding.
The combination of thinning ice, larger waves, and damage due to diurnal thermal cycling motivate the need to better understand the impact of flexing under the action of oceanic waves on the strength ...of thermally cracked ice. To that end, new experiments were performed on freshwater, lab‐grown ice and first‐year natural sea ice. Both materials were cracked by thermal shocking and then subsequently cyclically flexed. Initially, the thermal cracks weakened both materials. When the cracked ice of either origin was cyclically flexed under fully reversed loading, its flexural strength, initially reduced by the stress‐concentrating action of the cracks, recovered to the strength of non‐cracked, non‐flexed ice. When the cracked ice was cyclically flexed non‐reversely, its strength recovered only partially. During reversed cyclic flexing, the cracked region experienced alternately compressive and tensile stresses. We suggest compression resulted in contact of opposing crack faces followed by sintering leading to strength recovery. During non‐reversed cyclic flexing, contact and sintering were reduced and ice strength did not fully recover. The tendency for cracks to heal during cyclic flexing may lessen their threat to the structural integrity of an ice cover.
Plain Language Summary
Changes in climate such as thinner ice covers, larger waves, and a decrease in ice cover extent motivate the need to better understand how ice bending by ocean waves impact ice strength. An important factor is thermal cracking which occurs in nature and may weaken the ice and potentially result in fatigue failure. With that in mind, thermal cracks were introduced into natural sea ice and into lab‐grown freshwater ice. Initially, the thermal cracks weakened the materials. However, upon cycling, the ice started to recover its strength. The recovery is attributed to sintering at points of contact across the crack faces.
Key Points
Flexural strength of cracked freshwater ice and sea ice recovers fully upon reversed cycling
Flexural strength of cracked freshwater ice and sea ice recovers partially upon non‐reversed cycling
Rapid Healing of Thermal Cracks in Ice Murdza, Andrii; Schulson, Erland M.; Renshaw, Carl E. ...
Geophysical research letters,
16 September 2022, Volume:
49, Issue:
17
Journal Article
Peer reviewed
Open access
The structural integrity of the arctic sea ice cover is under threat owing largely to the combination of thinning and larger waves. Another contributor may be thermal cracking. In concentrating ...stress, thermal cracks may weaken the cover. Of interest, therefore, is the strength of thermally damaged ice. To that end, new experiments were performed on sea ice and on lab‐grown saline and salt‐free ice that had been cracked by thermal shocking. As expected, the cracks weakened the materials in accord with fracture mechanics. However, within tens to hundreds of seconds of shocking, the strength recovered completely, for the ice had healed. Healing is attributed to thermally activated sintering related to surface diffusion, assisted possibly by the formation of a quasi‐liquid layer on crack faces. Whether behavior on the small scale is indicative of behavior on the large scale remains to be determined.
Plain Language Summary
The combination of thinning ice cover and increasing wave amplitude (owing to increasing oceanic fetch) threatens the structural integrity of the ice cover on the Arctic Ocean. Another contributor may be thermal cracking. In concentrating stress, thermal cracks may weaken the cover. Of interest, therefore, is the strength of thermally damaged ice. With that in mind, thermal cracks were introduced into sea ice and into lab‐grown saline ice and freshwater ice that was subsequently bent to failure in the laboratory. The cracks weakened the materials, in accord with fracture mechanics. However, within a short period of time, the strength completely recovered to non‐cracked levels. The cracks had healed. Healing is attributed to sintering upon warming, assisted possibly by the presence of a thin liquid layer on opposing crack faces. Whether behavior on the small scale is indicative of behavior on the large scale remains to be determined.
Key Points
Thermally induced cracks within salty and salt‐free ice heal within tens to hundreds of seconds
Healing completely restores flexural strength
Cyclic strengthening of lake ice Murdza, Andrii; Marchenko, Aleksey; Schulson, Erland M. ...
Journal of glaciology,
02/2021, Volume:
67, Issue:
261
Journal Article
Peer reviewed
Open access
Further to systematic experiments on the flexural strength of laboratory-grown, fresh water ice loaded cyclically, this paper describes results from new experiments of the same kind on lake ice ...harvested in Svalbard. The experiments were conducted at −12 °C, 0.1 Hz frequency and outer-fiber stress in the range from ~ 0.1 to ~ 0.7 MPa. The results suggest that the flexural strength increases linearly with stress amplitude, similar to the behavior of laboratory-grown ice.
The ductile-to-brittle transition was investigated in prestrained columnar ice at −10 °C. Laboratory-grown specimens of freshwater and saline ice were prestrained under uniaxial across-column ...compression (to levels from εp = 0.003 to εp = 0.20, at constant strain rates in the ductile regime) and likewise reloaded (at rates from 1 × 10−6s−1 to 3 × 10−2s−1). Prestrain caused solid-state recrystallization as well as damage in the form of non-propagating microcracks. The ductile-to-brittle transition strain rate ε˙D/B increased by a factor of 3–10 after prestrain of εp = 0.035 in both freshwater and saline ice, compared to that of initially undamaged ice of the same type. Additional prestrain had little further effect on ε˙D/B. The results are interpreted within the framework of a model (proposed by Schulson, 1990, and Renshaw and Schulson, 2001) that predicts the transition strain rate based on the micromechanical boundary between creep and fracture processes. Model parameters primarily affected by prestrain were the power-law creep coefficient B (more so than the creep exponent n), Young's modulus E and, by extension, the fracture toughness KIc.
Display omitted
mRNA vaccines have proven to be pivotal in the fight against COVID-19. A recommended booster, given 3 to 4 weeks post the initial vaccination, can substantially amplify protective antibody levels. ...Here, we show that, compared to contralateral boost, ipsilateral boost of the SARS-CoV-2 mRNA vaccine induces more germinal center B cells (GCBCs) specific to the receptor binding domain (RBD) and generates more bone marrow plasma cells. Ipsilateral boost can more rapidly generate high-affinity RBD-specific antibodies with improved cross-reactivity to the Omicron variant. Mechanistically, the ipsilateral boost promotes the positive selection and plasma cell differentiation of pre-existing GCBCs from the prior vaccination, associated with the expansion of T follicular helper cells. Furthermore, we show that ipsilateral immunization with an unrelated antigen after a prior mRNA vaccination enhances the germinal center and antibody responses to the new antigen compared to contralateral immunization. These findings propose feasible approaches to optimize vaccine effectiveness.
Display omitted
•Ipsilateral boost of mRNA vaccine induces superior GC and plasma cell responses•Ipsilateral boost more rapidly induces high-affinity antibodies with improved breadth•Ipsilateral boost directly activates the ongoing GCs from the prior vaccination•Ipsilateral new antigen immunization post mRNA vaccination improves humoral responses
COVID-19 mRNA vaccines require a prime-boost approach to stimulate robust levels of protective antibodies. Jiang et al. demonstrate that receiving the booster shot or a different vaccine on the same side following a prior mRNA vaccination may result in more optimal B cell and antibody responses.
New systematic experiments reveal that the flexural strength of saline S2 columnar-grained ice loaded normal to the columns can be increased upon cyclic loading by about a factor of 1.5. The ...experiments were conducted using reversed cyclic loading over ranges of frequencies from 0.1 to 0.6 Hz and at a temperature of −10 ∘C on saline ice of two salinities: 3.0 ± 0.9 and 5.9 ± 0.6 ‰. Acoustic emission hit rate during cycling increases with an increase in stress amplitude of cycling. Flexural strength of saline ice of 3.0 ± 0.9 ‰ salinity appears to increase linearly with increasing stress amplitude, similar to the behavior of laboratory-grown freshwater ice (Murdza et al., 2020b) and to the behavior of lake ice (Murdza et al., 2021). The flexural strength of saline ice of 5.9 ± 0.6 ‰ depends on the vertical location of the sample within the thickness of an ice puck; i.e., the strength of the upper layers, which have a lower brine content, was found to be as high as 3 times that of lower layers. The fatigue life of saline ice is erratic. Cyclic strengthening is attributed to the development of an internal back stress that opposes the applied stress and possibly originates from dislocation pileups.
Abstract Soils are a principal global reservoir of mercury (Hg), a neurotoxic pollutant that is accumulating through anthropogenic emissions to the atmosphere and subsequent deposition to terrestrial ...ecosystems. The fate of Hg in global soils remains uncertain, however, particularly to what degree Hg is re-emitted back to the atmosphere as gaseous elemental mercury (GEM). Here we use fallout radionuclide (FRN) chronometry to directly measure Hg accumulation rates in soils. By comparing these rates with measured atmospheric fluxes in a mass balance approach, we show that representative Arctic, boreal, temperate, and tropical soils are quantitatively efficient at retaining anthropogenic Hg. Potential for significant GEM re-emission appears limited to a minority of coniferous soils, calling into question global models that assume strong re-emission of legacy Hg from soils. FRN chronometry poses a powerful tool to reconstruct terrestrial Hg accumulation across larger spatial scales than previously possible, while offering insights into the susceptibility of Hg mobilization from different soil environments.
Input of organic matter into stream channels is the primary energy source for headwater ecosystems and ultimately carbon to the oceans and hence is an important component of the global carbon cycle. ...Here, we quantify organic‐rich fine sediment mobilization, transport, and storage in a Strahler fourth‐order stream during individual intermediate‐sized storm events. By combining measurements of fallout radionuclides (FRNs) 7Be and 210Pb and stable water isotopes with a conceptual model of suspended load trapping by channel margins, we find that the channel bed was consistently a source of suspended load to the channel margins. Relative to storage on the channel margins, suspended load export increased through the spring and summer, perhaps related to the in‐channel decomposition of organic debris as indicated by its FRN exposure age and changing bulk δ13C composition. Trapping of suspended load by riparian margins limits sediment transport distances, which, given sufficient discharge to fully suspend the load, is nearly independent of stream discharge for sub‐bankfull discharges. Limited data indicate that the fractional size of the channel margins where trapping occurs decreases with increasing watershed area. Increasing transport length and decreasing fractional margin area with increasing watershed area results in a systematic downstream decoupling of the channel from local terrestrial organic matter exchange. These findings provide a framework for understanding suspended load dynamics in formerly glaciated regions where sediment production and fluxes are generally low and thus the annual input of organic debris is a major component of suspended load budget.
Plain Language Summary
The decomposition of organic‐rich debris (leaves, twigs, etc.) within stream channels serves as an important organic carbon source to stream margins or banks. During moderate storm events, we observed that the channel bed was consistently a source of organic‐rich suspended load that is then trapped by the channel margins. Through spring and summer less of the suspended load is trapped by the margins, increasing the fraction of the suspended load exported. This decreased trapping and increased export may be related to changes in the character of the suspended load due to the in‐channel decomposition of organic debris. Trapping of suspended load by channel margins limits the transport distance of suspended load, systematically decoupling the channel from the channel margins with increasing watershed size. These findings provide a framework for understanding suspended load transport in formerly glaciated regions where the annual input of organic debris is a significant component of suspended load budget.
Key Points
We observe seasonal variations in the ratio of suspended load exported from the watershed versus stored in channel margins
Greater fractional area of deposition along margins of headwaters facilitates trapping of suspended load that limits suspended load export
Increasing transport length with increasing watershed area systematically decouples the channel from terrestrial organic exchange
Because of the high logistical and financial costs of direct measurements of riverine suspended sediment, remote sensing is increasingly used to supplement the direct‐observation record. The accuracy ...of this method is poorly constrained, and its potential as a tool for understanding river sediment transport is thus limited. We introduce and apply global‐scale methods for estimating depth‐integrated suspended‐sediment concentrations (SSCs) using Landsat 5 and 7 satellite imagery calibrated with 134,697 in situ SSC measurements. We account for river‐to‐river variability in the relationship between water optical properties and SSC by (a) categorizing rivers using unsupervised K‐means clustering and/or (b) correcting calibration estimates for individual rivers using local in situ measurements of SSC, suspended‐sediment grain size, and percent organic carbon (POC). In the absence of site‐specific in situ SSC measurements, clustering rivers reduces the average relative error of SSC estimates from 97% to 73%. We show that as few as five site‐specific in situ measurements combined with our algorithm further reduces average relative error to 49% and average relative at‐a‐station bias in SSC to 7%. Little additional improvement in accuracy or bias is gained by including measurements of percent sand or POC of the suspended sediment. Since only modest additional accuracy is gained after ~5–10 paired in situ SSC measurements and satellite observations, sampling campaigns should prioritize limited sampling at diverse locations rather than intensive sampling at a limited number of sites. In addition, we publish standalone calibrations for 151 rivers made solely with in situ SSC measurements local to those sites.
Plain Language Summary
We present methods for using satellite imagery from the Landsat 5 and 7 missions to detect suspended‐sediment concentrations in large rivers. We establish best practices for determining the accuracy of these methods and identify specific ways for improving that accuracy. We show that little additional accuracy in the method is gained after about five paired measurements of suspended‐sediment concentration and satellite observation, implying that it is better to collect a limited number of paired samples at a large number of sites rather than to intensively sample just a few sites.
Key Points
A Calibration set of 134,697 in situ measurements was used to estimate suspended‐sediment concentration from Landsat satellite imagery
A Base calibration model (97% uncertainty) is improved by automated river clustering (74% uncertainty) and added in situ data (50% uncertainty)
Cosmopolitan suspended‐sampling campaigns may best refine satellite imagery approaches
PDF
