Oceanographic observations at the shelf break of the Amundsen Sea, Antarctica

The part of the West Antarctic Ice Sheet that drains into the Amundsen Sea is currently thinning at such a rate that it contributes nearly 10% of the observed rise in global mean sea level. Acceleration of the outlet glaciers appears to be caused by thinning at their downstream ends, where the ice goes afloat, indicating that the changes are probably being forced from the ocean. Observations made since the mid-1990s on the Amundsen Sea continental shelf have revealed that the deep troughs, carved by previous glacial advances, are flooded by almost unmodified Circumpolar Deep Water (CDW) with temperatures around 3-4°C above the freezing point, and that this water mass drives rapid melting of the floating ice. Here we report observations of water properties and currents made in the region where one of those troughs reaches the continental shelf edge. We estimate the absolute circulation within the trough from a combination of detided Acoustic Doppler Current Profiler data and geostrophic shear derived from Conductivity-Temperature-Depth sections. The shelf edge region is characterised by a landward deepening of the pycnocline separating CDW from the overlying colder and fresher surface waters. This feature, the so-called Antarctic Slope Front (ASF), is almost circumpolar in extent, and is typically a full-depth feature, the pycnocline intersecting the seabed over the upper continental slope. However, the ASF is weaker in the Amundsen Sea, where it is rarely a full-depth feature. Geostrophic shear associated with the ASF leads to a weakening of the associated westward current with depth, and an eastward undercurrent of varying strength has been reported at other locations. At the time of our Amundsen Sea observations the westward surface flow was weak, giving rise to a strong eastward undercurrent flowing along the continental shelf edge and upper slope. At the upstream (western) side of the trough the undercurrent turns south, driving a net on-shelf flow of CDW in the western part of the trough, and leaving a weakened shelf edge flow that re-establishes itself on the downstream (eastern) side of the trough. An analogous feature was captured, albeit crudely, in an earlier coarse-resolution model of the circulation on the Amundsen Sea shelf, and variability in its strength, associated with variability in the surface wind stress, was the main cause of variations in the heat content of the waters on the inner continental shelf. Our observations thus lend support to the earlier hypothesis that changes in atmospheric forcing over the continental shelf edge could be the ultimate driver of changes in the West Antarctic Ice Sheet outlet glaciers that drain into the Amundsen Sea

West Antarctic Ice Sheet retreat in the Amundsen Sea driven by decadal oceanic variability

Mass loss from the Amundsen Sea sector of the West Antarctic Ice Sheet has increased in recent decades, suggestive of sustained ocean forcing or an ongoing, possibly unstable, response to a past climate anomaly. Lengthening satellite records appear to be incompatible with either process, however, revealing both periodic hiatuses in acceleration and intermittent episodes of thinning. Here we use ocean temperature, salinity, dissolved-oxygen and current measurements taken from 2000 to 2016 near the Dotson Ice Shelf to determine temporal changes in net basal melting. A decadal cycle dominates the ocean record, with melt changing by a factor of about four between cool and warm extremes via a nonlinear relationship with ocean temperature. A warm phase that peaked around 2009 coincided with ice-shelf thinning and retreat of the grounding line, which re-advanced during a post-2011 cool phase. These observations demonstrate how discontinuous ice retreat is linked with ocean variability, and that the strength and timing of decadal extremes is more influential than changes in the longer-term mean state. The nonlinear response of melting to temperature change heightens the sensitivity of Amundsen Sea ice shelves to such variability, possibly explaining the vulnerability of the ice sheet in that sector, where subsurface ocean temperatures are relatively high

Properties of organosiloxane liquid crystals for dye guest host ferroelectric display devices

The purpose of this thesis is to investigate a new series of organosiloxane liquid crystals and the effects of dye addition by mixing and synthetic addition. The organosiloxane compounds are the result of a building block process that seeks to combine the high tilt and wide phase range properties of laterally substituted biphenyl benzoate mesogens with the polymer ruggedness of siloxane head groups. Initially the biphenyl benzoate mesogenic precursors are investigated with bromine, chlorine and fluorine substituents. The materials yield 20-45° tilt angles with 70-130 nCcm"^ spontaneous polarisation. When combined with a siloxane head group, the only liquid crystal phase seen is a 50°C wide SmC* phase in each of the three compounds. The siloxane moieties micro-segregate and form a virtual backbone. As a consequence of the new molecular arrangement, temperature independent, near 45° or 45° tilt angles are measured for all three compounds. Switching time measurements reveal sub-millisecond switching down to 40°C below the I�>SmC* transition. The attachment of a biphenyl benzoate molecule at either end of the siloxane core results in a series of bi-mesogenic compounds. These compounds yield 60°C wide antiferroelectric SmC*A phases. The antiferroelectric nature is proposed to arise due to a bent conformation of the bi-mesogenic molecules. The spontaneous polarisation is further enhanced to 100-140 nCcm'^ whilst the other electro optic properties broadly follow similar trends to the mono-siloxane compounds. The synthesis of 45° tilt ferroelectric compounds opens up the possibility of many new device applications. The feasibility of transferring the benefits offered by ferroelectrics to the field of dyed and single polariser devices is investigated. Three commercial dyes are mixed with the laterally halogen substituted mono mesogenic organosiloxanes. The phase sequence of the host is maintained along with the near 45° tilt angles of the host material. The magnitude of the spontaneous polarisation is reduced in line with the dilution effect of the non-liquid crystalline dye compounds. The switching times, although marginally increased with dye addition, still remained in the hundred microsecond regime demonstrated by the hosts. With the addition of one of the commercial dyes to an antiferroelectric host, three optically distinct states can be observed in a suitably oriented cell. This single polariser tri-state dye guest host device is only made possible by the near 45° tilt angle of the bi-mesogenic organosiloxane compounds. In order to introduce more dye into the organosiloxane hosts, a nitrostilbene dye molecule is grafted onto a siloxane core via an eleven carbon length alkyl chain. Following the pattern of the host organosiloxanes, monomeric and dimeric nitrostilbene compounds are studied, and the effect that grafting has on the dye's absorption properties is examined. This attempt to mimic the host structure proved highly successful, with the resultant organosiloxane dyes exhibiting liquid crystalline phases and showing complete miscibility with the organosiloxane hosts over the entire concentration range. Of particular note is the presence of antiferroelectricity in a mono-mesogenic ferroelectric host on addition of a dimeric non-ferroelectric dye. The results of this work show that the virtual backbone effect in low molar mass organosiloxane liquid crystals can be used to increase the solubility of a dye guest moiety and influence the stability and form of the mesophases. The ferroelectric / antiferroelectric characteristics of the liquid-crystal host can be retained and room temperature ferroelectric phases are created with broad temperature invariant properties. In this thesis, as well as the study of the physical and chemical properties of these new materials, some of the potential devices that now become possible are considered

The neodymium isotopic composition of waters masses in the eastern Pacific sector of the Southern Ocean

The Antarctic Circumpolar Current is one of the key components of ocean circulation, and a knowledge of its isotopic composition is essential to the use of neodymium (Nd) isotopes to trace circulation now and in the past. Here we present 57 new analyses of the Nd isotopic composition of the water column in the eastern Pacific sector of the Southern Ocean, documenting both the variation in three dimensions as well as the controls on that variability. Nd isotopic data for the middle of the water column demonstrate the homogeneity of Circumpolar Deep Water (CDW) at an εNd value of −8.7 ± 0.1. This homogeneity reflects the large Nd inventory in the ACC flow, and the degree to which this large inventory buffers the Nd characteristics of the ACC against extra-oceanic inputs from the continents, either via dust from the atmosphere or through dissolved and particulate material from the adjacent continents. CDW upwells onto the Amundsen Sea shelf and, even here, its Nd isotopic properties are close to conserved in the middle of the water column (εNd = −8.0 ± 0.2 at 600 m). At the top and bottom of the shelf water column, however, the Nd isotopic and concentration characteristics are strongly modified (to εNd as high as −4.5). All the shelf water column data obtained here are consistent with net addition of Nd to bottom and surface waters with a contrasting isotopic composition that is matched by local sediment (εNd = −1 to −2), followed by conservative mixing of that water into intermediate levels. Mixing with this shelf composition also leads to significant modification of open ocean surface water (Nd isotopic shift around 1 epsilon unit) in the ACC as it flows eastwards. Modification of open ocean bottom waters by interaction with sediment is more subtle, but there is marked non-conservative removal of Nd accompanied by significant changes in isotopic composition in waters within 10 m of the seabed. The new data demonstrate the conservativity of Nd in the middle water column, especially for such large volume flows as the ACC. Though boundary exchange-type processes are clearly important in this region, and their imprint on both the shelf and open-ocean surface water is significant, there is no observable impact on the main core of CDW. This finding augurs well for the use of Nd isotopes as a conservative water-mass tracer now and in the past. For example, these data suggest that the only likely control on the temporal variability of southern-component deep water exported northwards into the Atlantic through the last glacial cycle is variations in the input of North Atlantic Deep Water in the Atlantic sector. On the other hand, the data also suggest caution in the use of sedimentary archives of bottom water Nd as records of deep water Nd isotopic characteristics, given the dramatic modification of bottom waters on the shelf and the more subtly non-conservative behaviour in the open ocean bottom water that are both suggestive of modification of the Nd characteristics of bottom waters by sediment

A continuous record of Florida Current temperature transport at 27°N

As part of a newly funded international program to monitor ocean heat transport at mid‐latitudes in the North Atlantic, a continuous estimate of the temperature transport of the Florida Current is required. Since 1982, volume transports have been inferred from voltage measurements monitored by submarine telephone cables across the Straits of Florida. Electromagnetic induction theory suggests that the cable voltage should actually give a more direct measure of conductivity transport than pure volume transport. Due to the strong dependence of conductivity on temperature, this would in theory result in a direct and continuous estimate of the Florida Current temperature transport. This hypothesis is investigated using data from a large number of temperature and velocity sections (58) across the Florida Current at the cable location, leading to a new calibration of the voltage signal for the temperature transport of the Florida Current, crucial for trans‐basin heat flux estimates

Low Molar Mass Organosiloxane Liquid Crystalline Dyes for Dye Guest Host Ferroelectric Display Devices

In low molar mass organosiloxane liquid-crystal materials the siloxane moieties micro-separate and aggregate in planes that could be regarded as an effective or virtual two-dimensional polymer backbone. We show that if a siloxane moiety is attached to a dichroic dye molecule, the micro-segregation of the siloxane moieties makes it possible to include a high concentration of the guest dye (more than 50%) in a host organosiloxane solution. This effect, combined with the temperature independent tilt angles achievable with ferroelectric organosiloxane liquid crystals, provide an ideal material for high-contrast surface-stabilised ferroelectric display devices. We present dyed ferroelectric materials with a temperature independent tilt angle greater than 42 degrees, a wide (room temperature to over 100°C) mesomorphic temperature range and a response time shorter than 500μs in the dye guest host mode

Southern Weddell Sea shelf edge geomorphology: Implications for gully formation by the overflow of high-salinity water

Submarine gullies are the most common morphological features observed on Antarctic continental slopes. The processes forming these gullies, however, remain poorly constrained. In some areas, gully heads incise the continental shelf edge, and one hypothesis proposed is erosion by overflow of cold, dense water masses formed on the continental shelf. We examined new multibeam echo sounder bathymetric data from the Weddell Sea continental slope, the region that has the highest rate of cold, dense water overflow in Antarctica. Ice Shelf Water (ISW) cascades downslope with an average transport rate of 1.6 Sverdrups (Sv) in the southern Weddell Sea. Our new data show that within this region, ISW overflow does not deeply incise the shelf edge. The absence of gullies extending deeply into the glacial sediments at the shelf edge implies that cold, high salinity water overflow is unlikely to have caused the extensive shelf edge erosion observed on other parts of the Antarctic continental margin. Instead, the gullies observed in the southern Weddell Sea are relatively small and their characteristics indicative accumulation and subsequent failure of proglacial sediment during glacial maxima