ALBERT

Description: Franklin Glacier is an 18-km-long valley glacier that originates in a broad icefield below the west face of Mt. Waddington in the central British Columbia Coast Mountains, Canada. Radiocarbon-dated wood samples from the proximal faces of lateral moraines flanking Franklin Glacier show that the glacier expanded at least nine times since 13,000 cal. yr BP. A probable Younger Dryas advance of Franklin Glacier at 12,910–12,690 cal. yr BP followed the late glacial retreat and down wasting of the Cordilleran Ice Sheet from ca. 16,000 to 12,900 cal. yr BP. During the succeeding early Holocene warm period, Franklin Glacier appears to have retreated significantly, leaving no record of glacial expansion until the mid-Holocene when it repeatedly advanced at 6360–6280, 5470–5280 and 4770–4580 cal. yr BP. Down wasting of the glacier surface after ca. 4770–4580 cal. yr BP was followed by intervals of expansion at 4260–4080, 3210–3020 and 2620–2380 cal. yr BP. Following ice expansion at ca. 2620–2380 cal. yr BP into trees over 224 years in age, there is no record of the glacier activity until 1570–1480 cal. yr BP when Franklin Glacier thickened and advanced into young subalpine fir trees. During the ‘Little Ice Age’, advances at 800–680, 610–560 and 570–510 cal. yr BP preceded a mid-19th to early 20th century advance that saw Franklin Glacier attain its maximum Holocene extent. The dendroglaciological record at Franklin Glacier is among the most comprehensive recovered from the British Columbia Coast Mountains and showcases the complexity of mid- to late Holocene glacier expansion in the region.

Description: The mid-Holocene behaviour of five glaciers in the British Columbia Coast Mountains was reconstructed from radiocarbon ages and stratigraphic analysis. Subfossil wood evidence at Canoe, Fyles, Jacobsen, Tchaikazan and Icemaker glaciers suggests these glaciers were expanding into standing forests prior to 6630, 4900 and 4200 cal. yr BP. Stratigraphically constrained woody detritus at Fyles Glacier records the progradational history of a Gilbert-type delta forming in response to glacial expansion between 7020 and 5470 cal. yr BP. This research provides the first evidence for mid-Holocene glacial expansion in the central and northern British Columbia Coast Mountains. Proxy records describing mid-Holocene climates in the British Columbia Coast Mountains indicate a trend towards slightly cooler and possibly wetter conditions than present. Glacial expansion occurring between 7500 and 4000 cal. yr BP has regional correlatives, suggesting coherent broad-scale climate forcing mechanisms influenced glacial mass balance at this time.

Description: Holocene glacier activity in the northern British Columbia Coast Mountains, Canada, is described following investigations in the recently deglaciated forefields of White and South Flat glaciers spilling from the Cambria Icefield. Glacially overridden stumps and detrital wood was radiocarbon and dendrochronologically dated to describe an advance between ad 250 and 650. Expansion and thickening of White Glacier by ad 765 resulted in creation of an ice-dammed lake in South Flat valley that persisted until ad 1080. Following this the lake drained, before refilling in the early ‘Little Ice Age’ prior to ad 1300. Shortly thereafter White and South Flat glaciers became confluent and flowed over the valley side toward White Lake. The characteristics of the site and the preservation of 1500 yr old deposits indicate that the two glaciers remained confluent throughout the remainder of the ‘Little Ice Age’, only separating following terminal retreat early in the 20th century. The late-Holocene glacial history of White and South Flat glaciers appears synchronous with those of other glaciers in northern portion of Pacific Northwest.

Description: Dendroglaciological investigations near Mt. Waddington in the central British Columbia Coast Mountains provide an enhanced perspective of Holocene glacial activity. Field investigations at Confederation, Franklin, and Jambeau glaciers led to the discovery of subfossil wood mats encased in glacial deposits and glacially sheared stumps buried beneath till. Radiocarbon-dated wood collected from valley-bottom and lateral moraine sites at Confederation Glacier suggest that an early-Holocene advance occurred at c . 5665 cal. yr BP, followed by succeeding intervals of glacier expansion at c . 3700 and 3500 cal. yr BP. At Jambeau Glacier detrital wood mats buried close to the contemporary lateral moraine crests document glacier expansion at c . 3000 cal. yr BP. Detrital subfossil wood buried in lateral moraines at the confluence of Confederation and Franklin glaciers records distinct episodes of ‘Little Ice Age’ glacier expansion as early as c . 1212 cal. yr ad , and suggests the glacier surface continued to thicken until at least c . 1330–1410 cal. yr ad. An interval of downwasting and retreat followed, before late ‘Little Ice Age’ advances such as those at Jambeau Glacier were overwhelming valley-bottom forests by c . 1740 cal. yr ad. With the exception of the previously unrecognized advance of Confederation Glacier at c . 3700 cal. yr BP, our dendroglaciological findings corroborate the emerging record of Holocene glacier activity in the British Columbia Coast Mountains.

Description: Climate and glacier mass balance trends from 1780 to present within the Columbia Mountains, British Columbia, Canada, were investigated using tree-ring data. Annually resolved ring width, maximum density and maximum cell wall thickness chronologies were constructed from increment core samples of Engelmann spruce ( Picea engelmannii ), mountain hemlock ( Tsuga mertensiana ) and subalpine fir ( Abies lasiocarpa ) trees collected in Glacier and Revelstoke national parks. Proxy records of maximum June–August temperature, August precipitation and February snowpack provide insight into long-term seasonal climate changes. The intervals 1790–1810, 1850–1870 and 1930–1950 were characterized by warm summer temperatures. While overall August precipitation totals decrease over the period of record, the late 1700s, 1805–1835, the 1880s, 1960–1975 and the mid 1980s were notably wetter. February snowpacks have generally decreased in depth since 1873. The reconstructed winter, summer and net mass balance records indicate that glacier cover in the Columbia Mountains has varied primarily in response to decreased snowfall since the mid 1700s. Intervals of terminus advance and retreat were shown to reflect glaciological responses to relatively short-duration variations in summer temperature and winter snowfall. The rate of cumulative mass balance decline within the last 30 years exceeds that observed at any time over the last 250 years.

Description: Very low Reynolds number propulsion is a topic of enduring interest due to its importance in biological systems such as sperm migration in the female reproductive tract. Motivated by the fibrous nature of cervical mucus, several recent studies have considered the effect of anisotropic rheology; these studies have generally employed the classical swimming sheet model of G. I. Taylor. The models of Cupples et al. (J. Fluid Mech. vol. 812, 2017, pp. 501-524) and Shi & Powers (Phys. Rev. Fluids vol. 2, 2017, 123102) consider related problems which in a common limit (passive, slightly anisotropic) make different predictions regarding how swimming speed depends on alignment angle. In the present paper we find that this discrepancy is due to missing terms in the analysis of Cupples et al., and that when these terms are correctly included, the models agree in their common limit. We further explore the predictions of the corrected model for both passive and active cases; it is found that for certain combinations of alignment angle and activity parameter, propulsion is halted; in other cases the small amplitude asymptotic expansion is no longer valid, motivating future numerical study. © 2018 Cambridge University Press.

Description: Taylor’s swimming sheet is a classical model of microscale propulsion and pumping. Many biological fluids and substances are fibrous, having a preferred direction in their microstructure; for example, cervical mucus is formed of polymer molecules which create an oriented fibrous network. Moreover, suspensions of elongated motile cells produce a form of active oriented matter. To understand how these effects modify viscous propulsion, we extend Taylor’s classical model of small-amplitude zero-Reynolds-number propulsion of a ‘swimming sheet’ via the transversely isotropic fluid model of Ericksen, which is linear in strain rate and possesses a distinguished direction. The energetic costs of swimming are significantly altered by all rheological parameters and the initial fibre angle. Propulsion in a passive transversely isotropic fluid produces an enhanced mean rate of working, independent of the initial fibre orientation, with an approximately linear dependence of the energetic cost on the extensional and shear enhancements to the viscosity caused by fibres. In this regime, the mean swimming velocity is unchanged from the Newtonian case. The effect of the constant term in Ericksen’s model for the stress, which can be identified as a fibre tension or alternatively a stresslet characterising an active fluid, is also considered. This stress introduces an angular dependence and dramatically changes the streamlines and flow field; fibres aligned with the swimming direction increase the energetic demands of the sheet. The constant fibre stress may result in a reversal of the mean swimming velocity and a negative mean rate of working if it is sufficiently large relative to the other rheological parameters.

Description: A hybrid boundary integral/slender body algorithm for modelling flagellar cell motility is presented. The algorithm uses the boundary element method to represent the 'wedge-shaped' head of the human sperm cell and a slender body theory representation of the flagellum. The head morphology is specified carefully due to its significant effect on the force and torque balance and hence movement of the free-swimming cell. The technique is used to investigate the mechanisms for the accumulation of human spermatozoa near surfaces. Sperm swimming in an infinite fluid, and near a plane boundary, with prescribed planar and three-dimensional flagellar waveforms are simulated. Both planar and 'elliptical helicoid' beating cells are predicted to accumulate at distances of approximately 8.5 - 22 μm from surfaces, for flagellar beating with angular wavenumber of 3π to 4π. Planar beating cells with wavenumber of approximately 2.4π or greater are predicted to accumulate at a finite distance, while cells with wavenumber of approximately 2π or less are predicted to escape from the surface, likely due to the breakdown of the stable swimming configuration. In the stable swimming trajectory the cell has a small angle of inclination away from the surface, no greater than approximately 0.5°. The trapping effect need not depend on specialized non-planar components of the flagellar beat but rather is a consequence of force and torque balance and the physical effect of the image systems in a no-slip plane boundary. The effect is relatively weak, so that a cell initially one body length from the surface and inclined at an angle of 4°-6° towards the surface will not be trapped but will rather be deflected from the surface. Cells performing rolling motility, where the flagellum sweeps out a 'conical envelope', are predicted to align with the surface provided that they approach with sufficiently steep angle. However simulation of cells swimming against a surface in such a configuration is not possible in the present framework. Simulated human sperm cells performing a planar beat with inclination between the beat plane and the plane-of-flattening of the head were not predicted to glide along surfaces, as has been observed in mouse sperm. Instead, cells initially with the head approximately 1.5-3 μm from the surface were predicted to turn away and escape. The simulation model was also used to examine rolling motility due to elliptical helicoid flagellar beating. The head was found to rotate by approximately 240° over one beat cycle and due to the time-varying torques associated with the flagellar beat was found to exhibit 'looping' as has been observed in cells swimming against coverslips. © 2009 Cambridge University Press.