Caroline Lavoie

Boundary condition of grounding lines prior to collapse, Larsen-B Ice Shelf, Antarctica

Top-down rather than bottom-up change The Larsen-B Ice Shelf in Antarctica collapsed in 2002 because of a regional increase in surface temperature. This finding, reported by Rebesco et al., will surprise many who supposed that the shelfs disintegration probably occurred because of thinning of the ice shelf and the resulting loss of support by the sea floor beneath it. The authors mapped the sea floor beneath the ice shelf before it fell apart, which revealed that the modern ice sheet grounding line was established around 12,000 years ago and has since remained unchanged. If the ice shelf did not collapse because of thinning from below, then it must have been caused by warming from above. Science, this issue p. 1354 Surface warming caused the disintegration of the Larsen Ice Shelf in 2002. Grounding zones, where ice sheets transition between resting on bedrock to full floatation, help regulate ice flow. Exposure of the sea floor by the 2002 Larsen-B Ice Shelf collapse allowed detailed morphologic mapping and sampling of the embayment sea floor. Marine geophysical data collected in 2006 reveal a large, arcuate, complex grounding zone sediment system at the front of Crane Fjord. Radiocarbon-constrained chronologies from marine sediment cores indicate loss of ice contact with the bed at this site about 12,000 years ago. Previous studies and morphologic mapping of the fjord suggest that the Crane Glacier grounding zone was well within the fjord before 2002 and did not retreat further until after the ice shelf collapse. This implies that the 2002 Larsen-B Ice Shelf collapse likely was a response to surface warming rather than to grounding zone instability, strengthening the idea that surface processes controlled the disintegration of the Larsen Ice Shelf.

Late Holocene glacial advance and ice shelf growth in Barilari Bay, Graham Land, west Antarctic Peninsula

Three marine sediment cores were collected along the length of the fjord axis of Barilari Bay, Graham Land, west Antarctic Peninsula (65°55′S, 64°43′W). Multi-proxy analytical results constrained by high-resolution geochronological methods ( 210 Pb, radiocarbon, 137 Cs) in concert with historical observations capture a record of Holocene paleoenvironmental variability. Our results suggest early and middle Holocene (>7022–2815 cal. [calibrated] yr B.P.) retreated glacial positions and seasonally open marine conditions with increased primary productivity. Climatic cooling increased sea ice coverage and decreased primary productivity during the Neoglacial (2815 to cal. 730 cal. yr B.P.). This climatic cooling culminated with glacial advance to maximum Holocene positions and expansion of a fjord-wide ice shelf during the Little Ice Age (LIA) (ca. 730–82 cal. yr B.P.). Seasonally open marine conditions were achieved and remnant ice shelves decayed within the context of recent rapid regional warming (82 cal. yr B.P. to present). Our findings agree with previously observed late Holocene cooling and glacial advance across the Antarctic Peninsula, suggesting that the LIA was a regionally significant event with few disparities in timing and magnitude. Comparison of the LIA Antarctic Peninsula record to the rest of the Southern Hemisphere demonstrates close synchronicity in the southeast Pacific and southern most Atlantic region but less coherence for the southwest Pacific and Indian Oceans. Comparisons with the Northern Hemisphere demonstrate that the LIA Antarctic Peninsula record was contemporaneous with pre-LIA cooling and sea ice expansion in the North Atlantic–Arctic, suggesting a global reach for these events.

Functional avoidance of lung in plan optimization with an aperture-based inverse planning system.

PURPOSE To implement SPECT-based optimization in an anatomy-based aperture inverse planning system for the functional avoidance of lung in thoracic irradiation. MATERIAL AND METHODS SPECT information has been introduced as a voxel-by-voxel modulation of lung importance factors proportionally to the local perfusion count. Fifteen cases of lung cancer have been retrospectively analyzed by generating angle-optimized non-coplanar plans, comparing a purely anatomical approach and our functional approach. Planning target volume coverage and lung sparing have been compared. Statistical significance was assessed by a Wilcoxon matched pairs test. RESULTS For similar target coverage, perfusion-weighted volume receiving 10 Gy was reduced by a median of 2.2% (p=0.022) and mean perfusion-weighted lung dose, by a median of 0.9 Gy (p=0.001). A separate analysis of patients with localized or non-uniform hypoperfusion could not show which would benefit more from SPECT-based treatment planning. Redirection of dose sometimes created overdosage regions in the target volume. Plans consisted of a similar number of segments and monitor units. CONCLUSIONS Angle optimization and SPECT-based modulation of importance factors allowed for functional avoidance of the lung while preserving target coverage. The technique could be also applied to implement PET-based modulation inside the target volume, leading to a safer dose escalation.

Dose escalation in the radiotherapy of non-small-cell lung cancer with aperture-based intensity modulation and photon beam energy optimization for non-preselected patients.

PURPOSE To verify the potential of aperture-based intensity-modulated radiotherapy (AB-IMRT) to realize dose escalation plans for non-preselected non-small-cell lung cancer (NSCLC) patients, using photon beam energy optimization. METHODS AND MATERIALS Seven cases of NSCLC were retrospectively studied. Clinical reference plans were made at 60 Gy by an experienced dosimetrist. Dose escalation was applied to PTV2, a subvolume within the main PTV1. Escalation plans were optimized by considering beam angles (table and gantry), energy (6 and 23 MV) and weights, for an increasing dose to the PTV2, starting from 66 Gy and keeping 30 fractions. RESULTS In five cases, doses over 78 Gy could be achieved before exceeding organs at risk (OARs) standard tolerance. Peripheral overdosages, as well as lung and spinal cord tolerance doses, limited escalation. Means+/-SD V(95%) parameters were (97.3+/-0.9)% for PTV1s and (96.7+/-2.2)% for PTV2s. Doses to OARs were also maintained at acceptable levels. Optimized plans made use of both low- and high-energy beams and had a similar number of monitor units compared to the 60 Gy clinical plans. CONCLUSIONS The AB-IMRT system can successfully realize dose escalation for a sizeable number of cases. Plans produced contained few large segments, and are applicable to a wide range of tumor volumes and locations.

Environmental drivers of benthic communities and habitat heterogeneity on an East Antarctic shelf

Abstract This study presents the first analysis of benthic megafauna and habitats from the Sabrina Coast shelf, encompassing a proposed Marine Protected Area. Sea bed imagery indicated an abundant benthic fauna compared to other parts of the Antarctic shelf, dominated by brittle stars, polychaete tubeworms, and a range of other sessile and mobile taxa. The distribution of taxa was related (ρ=0.592, P<0.001) to variations in water depth, latitude, substrate type and phytodetritus. High phytodetritus cover was associated with muddy/sandy sediments and abundant holothurians and amphipods, while harder substrates hosted abundant brachiopods, hard bryozoans, polychaete tubeworms, massive and encrusting sponges, and sea whips. Brittle stars, irregular urchins and anemones were ubiquitous. Variations in substrate largely reflected the distribution of dropstones, creating fine-scale habitat heterogeneity. Several taxa were found only on hard substrates, and their broad regional distribution indicated that the density of dropstones was sufficient for most sessile invertebrates to disperse across the region. The hexactinellid sponge Anoxycalyx joubini and branching hydrocorals exhibited a more restricted distribution, probably related to water depth and limited dispersal capability, respectively. Dropstones were associated with significant increases in taxa diversity, abundance and biological cover, enhancing the overall diversity and biomass of this ecosystem.

The seafloor imprint of the Gerlache–Boyd Ice Stream (65–62° S), northern Antarctic Peninsula

The northern Antarctic Peninsula (NAP) forms a narrow stretch of land that extends to a relatively low latitude (63° S) and is subject to a humid, maritime-influenced climate, especially on its western side. During the Last Glacial Maximum (LGM), the NAP was covered by the northern part of the Antarctic Peninsula Ice Sheet (APIS) (Lavoie et al. 2015). The APIS fed ice streams flowing on both sides of the NAP, including the Gerlache–Boyd Ice Stream (GBIS) (Canals et al. 2000). Fast-flowing ice streams are the most dynamic components of ice sheets and largely determine ice-sheet mass loss and stability (Bentley 1987; Bamber et al. 2000). They transport large volumes of sediments both subglacially and englacially, and shape a variety of landforms both on land and the seafloor (e.g. Dowdeswell & Elverhoi 2002). During the LGM and throughout deglaciation, the role of ice streams was particularly significant. We describe the landforms associated with the GBIS based on a comprehensive compilation of multibeam bathymetry. The set of landforms and deposits left by the GBIS ranges in age from LGM to present, and illustrates both the products of ice-stream dynamics and how post-glacial processes can mask this glacial imprint. Our aim is to understand the dynamics of the GBIS and its temporal evolution since the LGM. In addition, the role of underlying geological control on the overall physiography of the GBIS is assessed. The study area extends 365 km from the southern end of Gerlache Strait (GS) to the South Shetland deep-sea trench and is fed by a glacierized catchment of about 23 000 km2 (Fig. 1a) (Canals et al. 2000). The height difference from the ice divide to the deep-sea trench is c. 6700 m. The GBIS system was the main drainage pathway of this catchment (O Cofaigh et …

Morphodynamic and Slope Instability Observations at Wabush Lake, Labrador

Since 1964, Iron Ore Company of Canada (IOCC) has deposited iron tailings resulting from mining operations into Wabush Lake, Western Labrador. Bathymetric surveys were carried out between 2004 and 2008 as part of the overall environmental IOCC plans to maintain safe disposal strategies of tailings into the lake. In this paper, we present the evolution in distribution and morphological characteristics over the last 4 years of the tailings overlying lacustrine sediments and bedrock. In 2004, a high definition multibeam survey of Wabush Lake was carried out for the first time and revealed lake floor features, including a fine network of channels and a delta foreslope with well-developed knickpoints. Other features such as older subaqueous mass movement scars and an esker were also identified. The delta foreslope channels, in 2004, led into a deeper long channel connected with the deepest part of the lake where turbidite deposition took place. A second bathymet-ric survey was carried out in 2006. Many of the features seen on the 2004 map were already buried by the advancing delta front. Results indicate that the 2004 channel system was almost completely obliterated with the generation of many new others. Development of the knickpoints is also observed where some are still present and new ones are discovered. A third bathymetric survey conducted in 2008 demonstrates a different spatial arrangement of features. Well developed wider channels and new knickpoints are observed on the foreslope delta. The sequential analysis of 2004, 2006 and 2008 surveys proved to be a useful tool to evaluate: (1) the rate of infilling of the lake, where accumulation took place and by which mechanisms (2) the evolution of the foreslope delta gradient and, (3) the evolution of instability areas. Our results indicate that these recently developed techniques are useful tools for monitoring underwater tailings disposal and stability.

Grounding-zone wedges and mega-scale glacial lineations in the Mertz Trough, East Antarctica

Glacial erosion and deposition have shaped the Mertz Trough, East Antarctica, where seafloor grounding-zone wedges (GZWs) are associated with mega-scale glacial lineations (MSGLs) (McMullen et al. 2006). GZWs form along grounded glacial margins constrained by ice shelves during stillstands and consist of wedge-shaped glacially transported sediment (Powell & Domack 2002). MSGLs are parallel elongate bedforms that typically form in soft sediments beneath rapidly flowing ice streams (Clark 1993; Canals et al. 2000; Clark et al. 2003). They are found in glacial troughs, usually parallel to trough margins. MSGLs are generally 6 to >100 km long, 200–1300 m wide and spaced 0.3–5 km apart, crest-to-crest (Clark et al. 2003; McMullen et al. 2006). Mertz Trough is located perpendicular to George V coast of Wilkes Land, East Antarctica (Fig. 1a, b). Multibeam-bathymetric imagery reveals distinct features on the seafloor including parallel elongate ridge and groove pairs, two sinuous sediment mounds with streamlined bedforms on their surfaces, and depressions (Fig. 1c). Fig. 1. Multibeam-bathymetric image of the central Mertz Trough, East Antarctica. ( a ) Location of study area (red box; map from …

A Holocene volcanic knoll within a glacial trough, Antarctic Sound, northern Antarctic Peninsula

Jaegyu Knoll is located in Antarctic Sound, between Trinity Peninsula and islands of the Joinville Island Group, on the northern Antarctic Peninsula (Fig. 1a). Jaegyu Knoll is interpreted as a Holocene submarine intraplate volcano based on its morphology, in situ observations such as bottom videos and high-resolution photographs (Quinones et al. 2005), a rock dredge that recovered fresh volcanic rock (Hatfield et al. 2004) and a measured geothermal anomaly (Hatfield et al. 2004). All aspects of the knoll are consistent with recent volcanic activity, which appears to have been persistent in the northern Antarctic Peninsula region from Mesozoic times to the present (e.g. Baker et al. 1973; Gonzalez-Ferran 1991; Gracia et al. 1997). The knoll, and at least two other smaller volcanic features in Antarctic Sound (Fig. 1a), lie within an overdeepened glacial trough that was presumably sculpted by ice during the Last Glacial Maximum (LGM; 23–19 ka BP). Fig. 1. Jaegyu Knoll (63° 29′ 45″ S, 56° 26′ 45″ W) located on the seafloor of the Antarctic Sound glacial trough, northern Antarctic Peninsula. Jaegyu Knoll was discovered and mapped for the first time in 2001 and named in honour of the young Korean scientist Mr Jun Jaegyu who succumbed …

Cold-Water coral distribution in an erosional environment: The strait of Gibraltar gateway

Publisher Summary The sill of the Strait of Gibraltar is the morphological, oceanographical, and ecological gateway between the Atlantic Ocean and the Mediterranean Sea for the post-Messinian crisis period. The seabed is composed of synorogenic Betic-Rif clayey flysch overlaid by Pliocene and/or Quaternary calcareous conglomerates and coral accumulations, as well as current transported sand and mud in the deepest parts. The Strait of Gibraltar is one of the busiest maritime zones in the world and is thus affected by invasive species, but the benthic community is poorly studied. Furthermore, the study zone is affected by benthic trawl fisheries on the shelf and near-shelf areas and by the laying of submarine cables. Overall, the naturalness of the study area is considered to be largely unmodified. Based on the Benthic Terrain model scheme, a morphological classification has been made of the bathymetric MBES data, indicating several morphological habitats in the area that can cause fragmentation in the main ecosystem. This classification is based on the rugosity, slope, curvature, depth, bathymetric positioning index (BPI) with annulus neighborhood of 1,125, 300, 120, and 60 m, and the range of standard deviation of the depth over a distance of 45 m based on a bathymetric grid of 15 m. The predicted textural distribution map is based on the rugosity, slope, and objectively classified morphological zones identified in combination with textural information of the sampled stations used in a maximum-likelihood statistic algorithm provided by ArcGIS. Coral distribution is based on grab samples and the layers of the derived morphological grids.