Month: May 2021

Month: May 2021

first_imgThe intra-oceanic South Sandwich subduction system is distinctive in having a narrow slab with slab edges at its northern and southern ends. We present new geochemical data to investigate magma genesis beneath the parts of the arc and back-are segments that lie close to the two slab edges: Kemp and Nelson seamounts at the southern edge of the South Sandwich arc, and segments E1 and E2 in the south, plus segments E9 and E10 in the north, of the East Scotia Sea. In the arc, Kemp and Nelson seamounts exhibit enhanced subduction fluxes compared to the remainder of the arc. The southernmost (Nelson) has the isotope (low Nd and high Sr isotope ratios) and elemental (ultra-high Th and Ba and high Hf/Nd ratios) characteristics of a sediment melt, or supercritical aqueous fluid, component. The more northerly (Kemp) has the same characteristics as the remainder of the arc (high Nd and slightly raised Sr isotope ratios, high Nd/Hf ratios, high Ba/Th ratios), indicative of a fluid component derived mainly from subducted crust, but has a greater mass fraction of that component than the rest of the arc. In the back-are basin, the slab-edge segments are generally fed by more fertile mantle (E-MOR-B in all but E I) than the segments in the centre of the basin (N-MORB). At the edges, segments furthest from the trench (E2, E9) have small subduction components while those nearer to the trench (E1, E10) have larger subduction components and slightly more depleted mantle. We argue that several processes were important at the slab edges: roll-back of the slab, forcing sideways flow of relatively enriched mantle into the mantle wedge; convergence of the arc with the back-are spreading centre, imparting a greater subduction component into the back-arc lavas; and anomalous heating of the subducting slab, increasing subduction fluxes and the contribution of sediment melts to the subduction component.last_img read more


Month: May 2021

first_imgQuestion: How does geothermal activity influence terrestrial plant colonization, species composition and community development in the Antarctic? Location: South Sandwich Islands, maritime Antarctic. Methods: Bryophytes were documented during a biological survey of the archipelago in January and February 1997. Particular attention was given to sites under current or recent influence of geothermal activity. Temperature profiles obtained across defined areas of activity on several islands were linked with the presence of specific bryophytes. Results: Greatest bryophyte richness was associated with geothermally influenced ground. Of 35 moss and nine liverwort species recorded, only four mosses were never associated with heated ground, while eight of the liverworts and 50% of the mosses were found only on actively or recently heated ground. Some species occur in unheated sites elsewhere in the maritime Antarctic, but were absent from such habitats on the South Sandwich Islands. Several species occurred in distinct zones around fumaroles. Maximum temperatures recorded within the upper 0.5 cm of the vegetation surface were 40 – 47 °C, with only Campylopus introflexus tolerating such temperatures. Maximum temperatures 2.5 or 5 cm below the vegetation surface of this moss reached 75 °C. Other bryophytes regularly present in zoned vegetation included the mosses Dicranella hookeri, Sanionia georgicouncinata, Pohlia nutans and Notoligotrichum trichodon, and the liverworts Cryptochila grandiflora and Marchantia berteroana. Surface temperatures of 25 – 35 °C and subsurface temperatures of 50 – 60 °C were recorded in these species. Conclusions: These exceptional plant communities illustrate the transport of viable propagules into the Antarctic. Individually ephemeral in nature, the longer term existence of geothermal habitats on islands along the Scotia Arc may have provided refugia during periods of glacial expansion, facilitating subsequent recolonization of Antarctic terrestrial habitats.last_img read more


Month: May 2021

first_imgStratospheric change associated with the Antarctic ozone hole is clearly implicated in changing surface climate near 65°S in late summer, in both measurements and models, via downward propagation of height anomalies following the final warming. But one of the largest changes in surface temperature in Antarctica has occurred in the Antarctic Peninsula at 60 to 65°S in winter, and most of the change at 65°S occurred before the ozone hole. Stratospheric change can cause tropospheric change in Antarctic winter by modifying the reflection and refraction of planetary waves, whereby a stronger stratospheric vortex moves the tropospheric jets polewards, which can modify the Southern Annular Mode (SAM) in surface pressure that forces the tropospheric circumpolar winds. We examine stratospheric influence on the SAM in winter by inter-annual correlation of the SAM with the solar-cycle and volcanic aerosols, which act to change forcing of the stratospheric vortex in winter. Correlations are a maximum in June (midwinter) and are significant then, but are poor averaged over winter months. Hence the potential of change in the stratosphere to change Antarctic tropospheric climate in winter by dynamical means is low. This negative result is important given the proven high potential for change in summer by dynamical means.last_img read more


Month: May 2021

first_imgRecent studies have reported widespread retreat and acceleration of glaciers on the Antarctic Peninsula, attributed to regional warming. The loss of ice is a contributor to sea-level rise, but its volume and impact on sea level is poorly known. There are few ground measurements of ice thickness change and existing satellite altimeters are ineffective over the mountainous terrain.An accurate assessment of changes in surface height, and hence ice volume, of glaciers on the Antarctic Peninsula over past decades is needed to aid better estimates of their past impact on sea-level rise and predictions of their future contribution.There is an archive of over 30,000 aerial photographs going back to the 1940s for parts of the Antarctic Peninsula and photogrammetry of time-series of these historic photographs is now the only way to reconstruct changes in glacier surface height over the last fifty years. However, the historic aerial photographs are difficult to use for detailed measurements due to inadequate ground control, unfavourable sortie characteristics, incomplete calibration data and use of paper prints.This paper describes a method to provide control for historic photos without ground fieldwork by linking them to a newly-acquired, highly-accurate photogrammetric model adjusted through direct kinematic GPS positioning of the camera. It assesses the achievable accuracy through a worked example using a glacier on the Antarctic Peninsula with typical aerial photography at five dates from 1947 to 2005.Overall measurement accuracy of better than 2 m RMSE in X, Y and Z was achieved for all the photography types, which is precise enough to allow reliable measurement of changes in ice thickness for the glacier over decadal periods. The principal constraints are image quality of the historic photographs and using paper prints.last_img read more


Month: May 2021

first_imgIt has been proposed that plants are capable of producing methane by a novel and unidentified biochemical pathway. Emission of methane with an apparently biological origin was recorded from both whole plants and detached leaves. This was the first report of methanogenesis in an aerobic setting, and was estimated to account for 10-45 per cent of the global methane source. Here, we show that plants do not contain a known biochemical pathway to synthesize methane. However, under high UV stress conditions, there may be spontaneous breakdown of plant material, which releases methane. In addition, plants take up and transpire water containing dissolved methane, leading to the observation that methane is released. Together with a new analysis of global methane levels from satellite retrievals, we conclude that plants are not a major source of the global methane production.last_img read more


Month: May 2021

first_imgThe accepted mechanism for whistler generation implicitly assumes that the causative lightning stroke occurs within reasonable proximity to the conjugate foot point of the guiding magnetic field line and that nighttime whistlers are prevalent because of low transionospheric attenuation. However, these assumptions are not necessarily valid. In this study we consider whistler observations from Rothera, a station on the Antarctic Peninsula, and contrast their occurrence with global lightning activity from the World Wide Lightning Location Network. The correlation of one-hop whistlers observed at Rothera with global lightning yields a few regions of significant positive correlation. The most probable source region was found over the Gulf Stream, displaced slightly equatorward from the conjugate point. The proximity of the source region to the conjugate point is in accord with the broadly accepted whistler production mechanism. However, there is an unexpected bias toward oceanic lightning rather than the nearby continental lightning. The relationship between the diurnal pattern of the Rothera whistlers and the conjugate lightning exhibits anomalous features which have yet to be resolved: the peak whistler rate occurs when it is daytime at both the source and the receiver and when source lightning activity is at its lowest. As a result, we propose that preferential whistler-wave amplification in the morning sector is a possible cause of the high whistler occurrence, although this does not account for the bias toward oceanic lightning.last_img read more


Month: May 2021

first_imgObserved phases and amplitudes of VLF radio signals propagating on very long paths are used to validate electron density parameters for the lowest edge of the (D region of the) Earth’s ionosphere at low latitudes and midlatitudes near solar minimum. The phases, relative to GPS 1 s pulses, and the amplitudes were measured near the transmitters (∼100–150 km away), where the direct ground wave is dominant, and also at distances of ∼8–14 Mm away, over mainly all-sea paths. Four paths were used: NWC (19.8 kHz, North West Cape, Australia) to Seattle (∼14 Mm) and Hawaii (∼10 Mm), NPM (21.4 kHz, Hawaii) and NLK (24.8 kHz, Seattle) to Dunedin, New Zealand (∼8 Mm and ∼12 Mm). The characteristics of the bottom edge of the daytime ionosphere on these long paths were found to confirm and contextualize recently measured short-path values of Wait’s traditional height and sharpness parameters, H′ and β, respectively, after adjusting appropriately for the (small) variations of H′ and β along the paths that are due to (1) changing solar zenith angles, (2) increasing cosmic ray fluxes with latitude, and (3) latitudinal and seasonal changes in neutral atmospheric densities from the (NASA) Mass Spectrometer Incoherent Scatter- (MSIS-) E-90 neutral atmosphere model. The sensitivity of this long-path (and hence near-global) phase and amplitude technique is ∼ ± 0.3 km for H′ and ∼ ± 0.01 km−1 for β, thus creating the possibility of treating the height (H′ ∼70 km) as a fiduciary mark (for a specified neutral density) in the Earth’s atmosphere for monitoring integrated long-term (climate) changes below ∼70 km altitude.last_img read more


Month: May 2021

first_imgDebris‐covered glaciers are ubiquitous in the Himalaya, and supraglacial debris significantly alters how glaciers respond to climate forcing. Estimating debris thickness at the glacier scale, however, remains a challenge. This study inverts a subdebris melt model to estimate debris thickness for three glaciers in the Everest region from digital elevation model difference‐derived elevation change. Flux divergences are estimated from ice thickness and surface velocity data. Monte Carlo simulations are used to incorporate the uncertainties associated with debris properties, flux divergence, and elevation change. On Ngozumpa Glacier, surface lowering data from 2010 to 2012 and 2012 to 2014 are used to calibrate and validate the method, respectively. The debris thickness estimates are consistent with existing in situ measurements. The method performs well over both actively flowing and stagnant parts of the glacier and is able to accurately estimate thicker debris (>0.5 m). Uncertainties associated with the thermal conductivity and elevation change contribute the most to uncertainties of the debris thickness estimates. The surface lowering associated with ice cliffs and supraglacial ponds was found to significantly reduce debris thickness, especially for thicker debris. The method is also applied to Khumbu and Imja‐Lhotse Shar Glaciers to highlight its potential for regional application.last_img read more


Month: May 2021

first_imgIncreasing ocean and air temperatures have contributed to the removal of floating ice shelves from several Greenland outlet glaciers; however, the specific contribution of these external forcings remains poorly understood. Here we use atmospheric, oceanographic and glaciological time series data from the ice shelf of Petermann Gletscher, NW Greenland to quantify the forcing of the ocean and atmosphere on the ice shelf at a site ~16 km from the grounding line within a large sub-ice-shelf channel. Basal melt rates here indicate a strong seasonality, rising from a winter mean of 2 m a−1 to a maximum of 80 m a−1 during the summer melt season. This increase in basal melt rates confirms the direct link between summer atmospheric warming around Greenland and enhanced ocean-forced melting of its remaining ice shelves. We attribute this enhanced melting to increased discharge of subglacial runoff into the ocean at the grounding line, which strengthens under-ice currents and drives a greater ocean heat flux toward the ice base.last_img read more


Month: May 2021

first_imgAccurate models of past Antarctic ice sheet behaviour require realistic reconstructions of the evolution of bedrock topography. However, other than a preliminary attempt to reconstruct Antarctic topography at the Eocene–Oligocene boundary, the long-term evolution of Antarctica’s subglacial topography throughout its glacial history has not previously been quantified. Here, we derive new reconstructions of Antarctic topography for four key time slices in Antarctica’s climate and glacial history: the Eocene–Oligocene boundary (ca. 34 Ma), the Oligocene–Miocene boundary (ca. 23 Ma), the mid-Miocene climate transition (ca. 14 Ma), and the mid-Pliocene warm period (ca. 3.5 Ma). To reconstruct past topography, we consider a series of processes including ice sheet loading, volcanism, thermal subsidence, horizontal plate motion, erosion, sedimentation and flexural isostatic adjustment, and validate our models where possible using onshore and offshore geological constraints. Our reconstructions show that the land area of Antarctica situated above sea level was ~25% larger at the Eocene–Oligocene boundary than at the present-day. Offshore sediment records and terrestrial constraints indicate that the incision of deep subglacial topographic troughs around the margin of East Antarctica occurred predominantly in the Oligocene and early Miocene, whereas in West Antarctica erosion and sedimentation rates accelerated after the mid-Miocene. Changes to the topography after the mid-Pliocene were comparatively minor. Our new palaeotopography reconstructions provide a critical boundary condition for models seeking to understand past behaviour of the Antarctic Ice Sheet, and have implications for estimating changes in global ice volume, temperature, and sea level across major Cenozoic climate transitions.last_img read more


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