Caytoniales in Early Jurassic floras from Antarctica

first_imgThe Caytoniales are represented in the Early Jurassic floras from Hope Bay and Botany Bay, Antarctica, as completeleaves and detached leaflets (Sagenopteris) together with microsporophylls (Caytonanthus). Systematic sampling at Botany Bay has shown that they occur only in a one metre interval within the sedimentary sequence. Such intimate association of leaves and microsporophylls has been recorded previously in only a few Northern Hemisphere floras and has been used as evidence to assign them to the same order (the Caytoniales). Caytonanthus is described here for the first time from a gondwanan flora and its close association with Sagenopteris provides further evidence that they are related biologically. The leaflets show a large range in morphology, possessing entire, undulating or lobed margins, but are all assignable to the same species. Such a range in leaflet shape has not been documented previously for a species of Sagenopteris. It can be explained by considering possible variations in the original habitat of “the Caytonia plant”.last_img read more

Climatology of the three coastal Antarctic stations Dumont d’Urville, Neumayer, and Halley

first_imgLong term air and snowfall chemistry measurements have been performed at the three coastal Antarctic stations Dumont d’Urville (66°40′S, 140°1′E), Neumayer (70°39′S, 8°15′W), and Halley (75°35′S, 26°19′W). The results have to be interpreted and compared with respect to the regional meteorological conditions. In this study the 3-hourly synoptic surface observations taken at the three stations between 1991 and 1995, as well as the daily upper air soundings from 1993, are analyzed to describe the aspects of station climatologies relevant for the air and snowfall chemistry measurements discussed in the papers of this special section. Although the three stations are comparable, being situated close to the coastline of Antarctica, the meteorological conditions differ. While at Dumont d’Urville katabatic winds cause predominant strong and relatively dry surface winds from the interior of Antarctica, Neumayer and Halley are frequently influenced by easterly winds associated mostly with eastward moving cyclones. From April through October the wind field above 5 km is governed by a circumpolar vortex with westerly winds increasing in intensity with height. Dumont d’Urville represents a station at the edge of this vortex with extreme stratospheric wind velocities up to above 50 m s−1. Neumayer and Halley are mostly situated within the vortex and isolated from air masses advecting from lower latitudes into the upper troposphere and lower stratosphere during the Austral winter.last_img read more

The impact of winter ice retreat on Antarctic winter sea-ice extent and links to the atmospheric meridional circulation

first_img15 years of weekly Antarctic ice extent and 10 m winds from numerical meteorological reanalyses have been used to test the hypothesis that the Antarctic final winter ice extent (FWE) is brought about by ice retreats as much as by advances, and that both are strongly affected by the meridional (north-south) component of the atmospheric circulation. This hypothesis is found to be correct and it is shown that extensive FWE fails to occur when total winter retreat is anomalously large. This is the case even in the coldest Antarctic regions. Retreats reduce the time available for advance, notably when they are substantial, with the ice cover taking up to several weeks to recover. Systematic changes in the meridional winds between retreats and advance are also detected in all regions; retreats are consistently associated with northerly winds supporting ice compaction and ice drift. The results agree with Antarctic case studies. Close similarities are found between several Antarctic and sub-Arctic regions in terms of the prevalence of retreat in some winters. its impact on the FWE and its relationship to the meridional atmospheric circulation. The study also reveals a more complex picture of the atmospheric circulation during sub-monthly ice retreats and advances. In particular, retreats in some Pacific regions are, on average, associated with anomalous anticyclonic circulations. This helps to explain why evidence of strong cyclone-ice extent relationships has not been found previously. A meteorological explanation is also sought for total retreat in winter being small in a winter of limited ice extent in the Bellingshausen Sea despite this being the mildest Antarctic region. In such cases, limited winter ice extent is attended by reduced advance caused by ice compaction due to northerly winds and waves and also anomalously high air temperatures. Reduced advance then limits opportunities for retreat to take place compared with winters with more advance and sea ice reaching lower latitudes. Overall. the results point to sub-monthly ice-atmospheric circulation interactions largely determining the winter ice extent throughout much of the Antarctic.last_img read more

Candida nivariensis, an emerging pathogenic fungus with multidrug resistance to antifungal agents

first_imgIn 2005, Candida nivariensis, a yeast species genetically related to Candida glabrata, was described following its isolation from three patients in a single Spanish hospital. Between 2005 and 2006, 16 fungal isolates with phenotypic similarities to C nivariensis were submitted to the United Kingdom Mycology Reference Laboratory for identification. The strains originated from various clinical specimens, including deep, usually sterile sites, from patients at 12 different hospitals in the United Kingdom. PCR amplification and sequencing of the D1D2 and internal transcribed spacer 1 (ITS1) regions of the nuclear ribosomal gene cassette confirmed that these isolates from the United Kingdom are genetically identical to C. nivariensis. Biochemically, C. glabrata and C. nivariensis are distinguished by their differential abilities to assimilate trehalose. However, in contrast to the original published findings, we found that C. glabrata isolates, but not C. nivariensis isolates, are capable of assimilating this substrate. Antifungal susceptibility tests revealed that C. nivariensis isolates are less susceptible than C. glabrata isolates to itraconazole, fluconazole, and voriconazole and to have significantly higher flucytosine MICs than C. glabrata strains. Finally, C. nivariensis could be rapidly distinguished from the other common pathogenic fungus species by pyrosequencing of the ITS2 region. In the light of these data, we believe that C. nivariensis should be regarded as a clinically important emerging pathogenic fungus.last_img read more

Long-term changes in deep-water fish populations in the northeast Atlantic: a deeper reaching effect of fisheries?

first_imgA severe scarcity of life history and population data for deep-water fishes is a major impediment to successful fisheries management. Long-term data for non-target species and those living deeper than the fishing grounds are particularly rare. We analysed a unique dataset of scientific trawls made from 1977 to 1989 and from 1997 to 2002, at depths from 800 to 4800 m. Over this time, overall fish abundance fell significantly at all depths from 800 to 2500 m, considerably deeper than the maximum depth of commercial fishing (approx. 1600 m). Changes in abundance were significantly larger in species whose ranges fell at least partly within fished depths and did not appear to be consistent with any natural factors such as changes in fluxes from the surface or the abundance of potential prey. If the observed decreases in abundance are due to fishing, then its effects now extend into the lower bathyal zone, resulting in declines in areas that have been previously thought to be unaffected. A possible mechanism is impacts on the shallow parts of the ranges of fish species, resulting in declines in abundance in the lower parts of their ranges. This unexpected phenomenon has important consequences for fisheries and marine reserve management, as this would indicate that the impacts of fisheries can be transmitted into deep offshore areas that are neither routinely monitored nor considered as part of the managed fishery areas.last_img read more

Palaeoglaciology of the Alexander Island ice cap, western Antarctic Peninsula, reconstructed from marine geophysical and core data

first_imgThe glacial history of the continental shelf northwest of Alexander Island is not well known, due mainly to a lack of targeted marine data on Antarctica’s palaeo-ice sheets in their inter-ice-stream areas. Recently it has been argued that the region was ice-free at the Last Glacial Maximum (LGM) and thus a potential site for glacial refugia. In this paper, multibeam swath bathymetry, sub-bottom profiles and sediment cores are used to map the Alexander Island sector of the Antarctic Peninsula margin, in order to reconstruct the shelf’s palaeoglaciology. Sea-floor bedforms provide evidence that an independent ice cap persisted on Alexander Island through the LGM and deglaciation. We show that this ice cap drained via two major, previously-undescribed tidewater outlets (Rothschild and Charcot Glaciers) sourced from an ice dome centred over the west of the island and near-shore areas. The glaciers grounded along deep, fjord-like cross-shelf troughs to within at least similar to 10-20 km of the shelf edge, and probably reached the shelf break. Only one small outer-shelf zone appears to have remained free of ice throughout an otherwise extensive LGM. During retreat, grounding-line geomorphology indicates periodic stabilisation of Charcot Glacier on the mid-shelf after 13,500 cal yrs BP, while Rothschild Glacier retreated across its mid-shelf by 14,450 cal yrs BP. The timing of these events is in phase with retreat in nearby Marguerite Trough, and we take this as evidence of a common history and forcing with the Antarctic Peninsula Ice Sheet. The fine details of ice flow documented by our new reconstruction highlight the importance of capturing complex ice flow patterns in models (e.g. in inter-stream areas), for understanding how region-specific parts of Antarctica may change in the future. Moreover, the reconstruction shows that glacial refugia, if present, cannot have been extensive on the Alexander Island shelf at the LGM as indicated by previous biological studies; instead, we argue that any ice-free refugia were probably restricted to isolated outer-shelf pockets, that opened, closed, or were maintained through diachronous ice-sheet advance and retreat.last_img read more

Retreat of the West Antarctic Ice Sheet from the western Amundsen Sea shelf at a pre- or early LGM stage

first_imgRecent palaeoglaciological studies on the West Antarctic shelf have mainly focused on the wide embayments of the Ross and Amundsen seas in order to reconstruct the extent and subsequent retreat of the West Antarctic Ice Sheet (WAIS) since the Last Glacial Maximum (LGM). However, the narrower shelf sectors between these two major embayments have remained largely unstudied in previous geological investigations despite them covering extensive areas of the West Antarctic shelf. Here, we present the first systematic marine geological and geophysical survey of a shelf sector offshore from the Hobbs Coast. It is dominated by a large grounding zone wedge (GZW), which fills the base of a palaeo-ice stream trough on the inner shelf and marks a phase of stabilization of the grounding line during general WAIS retreat following the last maximum ice-sheet extent in this particular area (referred to as the Local Last Glacial Maximum, ‘LLGM’). Reliable age determination on calcareous microfossils from the infill of a subglacial meltwater channel eroded into the GZW reveals that grounded ice had retreated landward of the GZW before ∼20.88 cal. ka BP, with deglaciation of the innermost shelf occurring prior to ∼12.97 cal. ka BP. Geophysical sub-bottom information from the inner-, mid- and outer shelf indicates grounded ice extended to the shelf edge prior to the formation of the GZW. Assuming the wedge was deposited during deglaciation, we infer the timing of maximum grounded ice extent occurred before ∼20.88 cal. ka BP. This could suggest that the WAIS retreat from the outer shelf was already underway during or even prior to the global LGM (∼23–19 cal. ka BP). Our new findings give insights into the regional deglacial behaviour of this understudied part of the West Antarctic shelf and at the same time support early deglaciation ages recently presented for adjacent drainage sectors of the WAIS. If correct, these findings contrast with the hypothesis that initial deglaciation of Antarctic Ice Sheets occurred synchronously at ∼19 cal. ka BP.last_img read more

Seasonal variability of the warm Atlantic Water layer in the vicinity of the Greenland shelf break

first_imgThe warmest water reaching the east and west coast of Greenland is found between 200 m and 600 m. Whilst important for melting Greenland’s outlet glaciers, limited winter observations of this layer prohibit determination of its seasonality. To address this, temperature data from Argo profiling floats, a range of sources within the World Ocean Database and unprecedented coverage from marine-mammal borne sensors have been analysed for the period 2002-2011. A significant seasonal range in temperature (~1-2 °C) is found in the warm layer, in contrast to most of the surrounding ocean. The phase of the seasonal cycle exhibits considerable spatial variability, with the warmest water found near the eastern and southwestern shelf-break towards the end of the calendar year. High-resolution ocean model trajectory analysis suggest the timing of the arrival of the year’s warmest water is a function of advection time from the subduction site in the Irminger Basin.last_img read more

Regional Antarctic snow accumulation over the past 1000 years

first_imgHere we present Antarctic snow accumulation variability at the regional scale over the past 1000 years. A total of 79 ice core snow accumulation records were gathered and assigned to seven geographical regions, separating the high-accumulation coastal zones below 2000 m of elevation from the dry central Antarctic Plateau. The regional composites of annual snow accumulation were evaluated against modelled surface mass balance (SMB) from RACMO2.3p2 and precipitation from ERA-Interim reanalysis. With the exception of the Weddell Sea coast, the low-elevation composites capture the regional precipitation and SMB variability as defined by the models. The central Antarctic sites lack coherency and either do not represent regional precipitation or indicate the model inability to capture relevant precipitation processes in the cold, dry central plateau. Our results show that SMB for the total Antarctic Ice Sheet (including ice shelves) has increased at a rate of 7 ± 0.13 Gt decade−1 since 1800 AD, representing a net reduction in sea level of ∼ 0.02 mm decade−1 since 1800 and ∼ 0.04 mm decade−1 since 1900 AD. The largest contribution is from the Antarctic Peninsula (∼ 75 %) where the annual average SMB during the most recent decade (2001–2010) is 123 ± 44 Gt yr−1 higher than the annual average during the first decade of the 19th century. Only four ice core records cover the full 1000 years, and they suggest a decrease in snow accumulation during this period. However, our study emphasizes the importance of low-elevation coastal zones, which have been under-represented in previous investigations of temporal snow accumulation.last_img read more

Relevance of detail in basal topography for basal slipperiness inversions: a case study on Pine Island Glacier, Antarctica

first_imgGiven high-resolution satellite-derived surface elevation and velocity data, ice-sheet modelsgenerally estimate mechanical basal boundary conditions using surface-to-bed inversion methods.In this work, we address the sensitivity of results from inversion methods to the accuracy of thebed elevation data on Pine Island Glacier. We show that misfit between observations and modeloutput is reduced when high-resolution bed topography is used in the inverse model. By lookingat results with a range of detail included in the bed elevation, we consider the separation of basaldrag due to the bed topography (form drag) and that due to inherent bed properties (skin drag).The mean value of basal shear stress is reduced when more detailed topography is included inthe model. This suggests that without a fully resolved bed a significant amount of the basal shearstress recovered from inversion methods may be due to the unresolved bed topography. However,the spatial structure of the retrieved fields is robust as the bed accuracy is varied; the fields areinstead sensitive to the degree of regularisation applied to the inversion. While the implications forthe future temporal evolution of PIG are not quantified here directly, our work raises the possibilitythat skin drag may be overestimated in the current generation of numerical ice-sheet modelsof this area. These shortcomings could be overcome by inverting simultaneously for both bedtopography and basal slipperiness.last_img read more