Attenuation and source parameters at Deception Island (South Shetland Islands,Antarctica)

Deception Island is the most active volcano of the South Shetland Islands-Antarctic Peninsula group, experiencing eruptions in 1967, 1969 and 1970. Local attenuation derived from coda analysis and source parameters derived from Brune's model, for well located seismic events, have been studied in order to complement the available geophysical information. Results show abnormally lowQ ₀ values and an abnormally high frequency dependence, as well as large dispersion. These factors are strongly dependent on the path travelled by the seismic wave. Retrieved values of the source parameters (stress drop, seismic moment and source radius), are again abnormally low compared to world-wide average values, for example, those obtained for the Oroville, California aftershock series between June and September, 1976. These results are consistent with some aspects of the geology of Deception Island, such as the very high degree of fracturing and faulting, and the existence of a strong hydrothermal alteration affecting most of the subaerial volcanic rocks. Moreover, the pattern defined for the lateral variations ofQ ₀ shows minimum values in the inner bay of the island, close to the most recent eruption vents. A large reduction in spectral amplitudes over a particular frequency range occurs in several observations, corresponding to the path crossing the zone of highest attenuation. This observation suggests the existence of a hot magmatic intrusion produced during the most recent eruption, and coincides with the superficial low density mass distribution obtained from the gravimetric model and the long wave magnetic field component obtained from magnetic surveys. The width of this intrusion is estimated to be about 200 m, in agreement with the previous results obtained analyzing residuals of the location of seismic events.     

Subaqueous, basaltic lava dome and carapace breccia on King George Island, South Shetland Islands, Antarctica

 On King George Island during latest Oligocene/earliest Miocene time, submarine eruptions resulted in the emplacement of a small (ca. 500 m estimated original diameter) basalt lava dome at Low Head. The dome contains a central mass of columnar rock enveloped by fractured basalt and basalt breccia. The breccia is crystalline and is a joint-block deposit (lithic orthobreccia) interpreted as an unusually thick dome carapace breccia cogenetic with the columnar rock. It was formed in situ by a combination of intense dilation, fracturing and shattering caused by natural hydrofracturing during initial dome effusion and subsequent endogenous emplacement of further basalt melt, now preserved as the columnar rock. Muddy matrix with dispersed hyaloclastite and microfossils fills fractures and diffuse patches in part of the fractured basalt and breccia lithofacies. The sparse glass-rich clasts formed by cooling-contraction granulation during interaction between chilled basalt crust and surrounding water. Together with muddy sediment, they were injected into the dome by hydrofracturing, local steam fluidisation and likely explosive bulk interaction. The basalt lava was highly crystallised and degassed prior to extrusion. Together with a low effusion temperature and rapid convective heat loss in a submarine setting, these properties significantly affected the magma rheology (increased the viscosity and shear strength) and influenced the final dome-like form of the extrusion. Conversely, high heat retention was favoured by the degassed state of the magma (minimal undercooling), a thick breccia carapace and viscous shear heating, which helped to sustain magmatic (eruption) temperatures and enhanced the mobility of the flow. Received: 1 August 1996 / Accepted: 15 September 1997     

Trace metals in sediment cores from Deception and Penguin Islands (South Shetland Islands, Antarctica)

This paper presents information on the levels of trace elements in sediments collected at Deception and Penguin Islands and tracks the sources of natural and anthropogenic inputs of metals into this sub-Antarctic region. The results suggest that natural processes, such as volcanic activity, hydrothermal processes and sediment transport, are more important than anthropogenic inputs in accounting for the metal concentrations measured in sediments at Deception Island. The higher levels of trace metals recorded in sediments at Penguin Island seem to reflect the composition of the source rocks of the island, which are dominated by the olivine-basalt group. Our findings show that human activities in the study areas may contribute to negligible levels of trace metals associated with anthropogenic inputs (e.g., Cr and Zn) in sediments, and these results can be used in the future as background levels related to low anthropogenic impacts.     

K-Ar geochronology of the South Shetland Islands,Lesser Antarctica: apparent lateral migration of Jurassic to Quaternary island arc volcanism

Approximately 70 K-Ar whole-rock ages for low-K tholeiitic and andesitic volcanic and intrusive rocks from the South Shetland Islands, Antarctica, including about 50 not previously published, are reviewed. Activity mainly spanned the range 130 to 30 Ma (Jurassic/Cretaceous to Oligocene/Miocene) with a very recent (～ 2 Ma), more alkaline, renewal. Throughout the main period of activity magmatism (or, perhaps, its cessation) migrated continuously northeastwards along the length of the island chain.     

Seismicity in the vicinity of Ross Island, Antarctica

Earthquakes in the Ross Sea, Antarctica, are of two types: volcanic, and those which appear to be of tectonic origin. Volcanic events in the vicinity of Ross Island are associated exclusively with Mount Erebus, Ross Island; this volcano erupts regularly, generating several earthquakes per day whose characteristics are quite distinct from non-volcanic events. These nonvolcanic earthquakes are recognizable by their distinct P- and S-wave arrivals, and a lack of the high frequency, often monochromatic character typical of Erebus events.One hundred fifty-seven tectonic microearthquakes (M < 2.0) were recorded in 1983 and 1984 by the ten station network on Ross Island; these events were located using the least-squares routine, HYPOELLIPSE. Of these events, 106 have RMS residual traveltime errors of less than or equal to 0.6 seconds; they are clustered in the vicinity of Ross Island, but are not restricted to it. There is a linear trend of epicenters cutting across the island and continuing northward. Most activity seems to center beneath Mount Terra Nova, between Mount Erebus and Mount Terror. Mean depth for events is 8.2 km; however, depths are rather evenly distributed over a range of 0 to 25 km.Modelling based on Bouger gravity anomalies and seismic refraction studies indicates a depth to the Moho of about 40 km beneath the continent, shallowing to 27 km beneath the Ross Sea. This 27 km depth is approximately equal to the lower limit of the tectonic seismicity detected by the Erebus network; hence, events are of crustal origin. These data suggest, with the rift-type geochemistry of Erebus' magma, that the Ross Sea is a site of active crustal extension and rifting.     

Enhanced bedrock weathering in association with late-lying snowpatches: Evidence from Livingston Island,Antarctica

An indication of the extent of weathering on different aspects of rock outcrops on Livingston Island, Antarctica, was obtained by means of a Schmidt hammer, a cone indenter and measurement of weathering rind thickness. Results show that weathering, particularly chemical weathering, is enhanced on the lee side of outcrops where snow accumulates as a result of prolonged wetting by the melting snow. Rock moisture and temperature data indicate that the south-facing, snow-accumulation side of obstacles have high rock moisture levels and maintain relatively high temperatures. Whilst chemical weathering is greater on the leeward side of outcrops, mechanical processes are greater on the windward side. The presence of late-lying snow thus appears to exert a strong influence on weathering.     

High-latitude dust deposition in snow on the glaciers of James Ross Island,Antarctica

High-latitude dust (HLD) depositions on four glaciers of James Ross Island (the Ulu Peninsula) were analysed. The deposition rate on the selected glaciers varies from 11.8 to 64.0 g m⁻², which is one order of magnitude higher compared to the glaciers in Antarctica or elsewhere in the world. A strong negative relationship between the sediment amount and altitude of a sampling site was found. This is most likely caused by the higher availability of aeolian material in the atmospheric boundary layer. General southerly and south-westerly wind directions over the Ulu Peninsula – with exceptions based on local terrain configuration – help to explain the significantly lower level of sediment deposition on San Jose Glacier and the high level on Triangular Glacier. X-ray fluorescence (XRF) spectrophotometry was used to estimate the relative proportions of the main and trace (lithophile) elements in the sediment samples. Both the sediment amount and the XRF results are analysed in a depth profile at each locality and compared among the glaciers, suggesting long-range transport of fine mineral material from outside James Ross Island. The distribution of aeolian sediment among the glaciers corresponds well with the prevailing wind direction on the Ulu Peninsula. © 2020 John Wiley & Sons, Ltd.     

First recorded eruption of Mount Belinda volcano (Montagu Island), South Sandwich Islands

The MODVOLC satellite monitoring system has revealed the first recorded eruption of Mount Belinda volcano, on Montagu Island in the remote South Sandwich Islands. Here we present some initial qualitative observations gleaned from a collection of satellite imagery covering the eruption, including MODIS, Landsat 7 ETM+, ASTER, and RADARSAT-1 data. MODVOLC thermal alerts indicate that the eruption started sometime between 12 September and 20 October 2001, with low-intensity subaerial explosive activity from the islands summit peak, Mount Belinda. By January 2002 a small lava flow had been emplaced near the summit, and activity subsequently increased to some of the highest observed levels in August 2002. Observations from passing ships in February and March 2003 provided the first visual confirmation of the eruption. ASTER images obtained in August 2003 show that the eruption at Mount Belinda entered a new phase around this time, with fresh lava effusion into the surrounding icefield. MODIS radiance trends also suggest that the overall activity level increased significantly after July 2003. Thermal anomalies continued to be observed in MODIS imagery in early 2004, indicating a prolonged low-intensity eruption and the likely establishment of a persistent summit lava lake, similar to that observed on neighboring Saunders Island in 2001. Our new observations also indicate that lava lake activity continues on Saunders Island.Editorial responsibility: J. Gilbert     

Discovery of a recurrent lava lake on Saunders Island (South Sandwich Islands) using AVHRR imagery

Monitoring of the remote South Sandwich Islands volcanic arc, using advanced very high resolution radiometer (AVHRR) data, has identified a radiant pixel on channels 3 (3.55–3.93 μm) and (rarely) 4 (10.3–11.3 μm). The position of the pixel is coincident with Mount Michael, Saunders Island, the active summit crater of a snow-covered basaltic stratovolcano with a persistent steam plume. The radiant pixel was continuously present in successive AVHRR images acquired during intervals of several months in the period examined (March 1995–February 1998), although apparently disappearing for similar time intervals. More than 5000 images were examined during this study. The radiant pixel is interpreted to indicate that the crater has contained a lava lake, the first to be recorded in the South Sandwich Islands. The lake appears to persist in the crater for several months at a time, at least. It may have drained completely at times and was probably absent when the crater was viewed briefly during an overflight in January 1997. Persistent or recurring lava lakes are very uncommon world-wide and that at Mount Michael is one of only two recorded in the Antarctic region.     

Ultrabasic inclusion from the basalts of the Hut Point area,Ross Island,Antarctica

The basaltic rocks of the Hut Point area contain a diverse suite of inclusions. The xenolithic nature of many sedimentary, metamorphic and igneous inclusion types is clearly displayed, but previous studies have left considerable doubt as to whether the ultrabasic inclusions are noncognate xenoliths, and possible relicts from the mantle brought to the surface in a magma originating in this zone; or rather of cognate and/or cumulative origin. Olivine grains in the dunite and enstatite bearing peridotite inclusions commonly display undulose extinction and strain bands (translation laminae), and preferred indicatrix orientations expressed as B γ girdles. Enstatite frequently contains clinopyroxene exsolution laminae, and many grains are deformed and/or disrupted by shear zones. Weak B γ olivine fabrics have also been detected in the titanaugite peridotites and pyroxenites, but no evidence of post-crystallization deformation was detected. Titanaugite also occurs as phenocrysts (xenocrysts?) and in the groundmass of the host basalt chemical and optical data suggest that the titanaugite grains in the basalt and the inclusions are genetically related. These preliminary findings suggest that the titanaugite bearing inclusions may be of cognate origin, and that the dunite and enstatite bearing peridotite fragments are more likely noncognate xenoliths. Field and laboratory studies of this problem are continuing as supported by a grant from the National Science Foundation.     

Sediment profile characterisation at contaminated and reference locations in the Windmill Islands, East Antarctica

Sediment profiles for pH, Eh, 28 elements, water and organic content are presented here for human impacted and reference locations in the Windmill Islands, East Antarctica. Variations in element concentrations are observed with increasing depth, especially at Brown Bay where the impact of past human activities is most pronounced in the top 10 cm. Spatial differences were observed between sediment profiles at reference and impacted locations and were largely explained by Pb variability in the top 5 cm. Median element concentrations from surface, middle and bottom regions of the sediment profile were compared to composite sample medians (no depth stratigraphy) for 11 elements at O’Brien Bay (reference) and Brown Bay (impacted). Pronounced differences were observed for Brown Bay, particularly surface and middle sections, implying that composite samples dilute the near surface anthropogenic signal by mixing with deeper uncontaminated sediment.     

The P-wave velocity structure of Deception Island,Antarctica, from two-dimensional seismic tomography

Deception Island is a volcanic island with a flooded caldera that has a complex geological setting in Bransfield Strait, Antarctica. We use P-wave arrivals recorded on land and seafloor seismometers from airgun shots within the caldera and around the island to invert for the P-wave velocity structure along two orthogonal profiles. The results show that there is a sharp increase in velocity to the north of the caldera which coincides with a regional normal fault that defines the northwestern boundary of the Bransfield Strait backarc basin. There is a low-velocity region beneath the caldera extending from the seafloor to > 4 km depth with a maximum negative anomaly of 1 km/s. Refracted arrivals are consistent with a 1.2-km-thick layer of low-velocity sediments and pyroclastites infilling the caldera. Synthetic inversions show that this layer accounts for only a small portion of the velocity anomaly, implying that there is a significant region of low velocities at greater depths. Further synthetic inversions and melt fraction calculations are consistent with, but do not require, the presence of an extensive magma chamber beneath the caldera that extends downwards from ≤ 2 km depth.     

Gravity field of Ascension Island,South Atlantic

A gravity survey of Ascension Island was carried out in 1964, a total of 210 stations being occupied. The results suggest that magma has reached the surface via three distinct conduits, the easternmost of which has been associated with extrusion of trachytic lavas. A regional decrease in gravity from west to east across the island is probably due to the proximity to the east of the crest of the Mid Atlantic Ridge, underlain by low density mantle.     

PCBs and chlorinated pesticides in the atmosphere and aquatic organisms of Ross Island, Antarctica

PCBs, p,p′-DDT, p,p′-DDE and lindane (γ-hexachlorocyclohexane) were monitored in the lower atmosphere of Ross Island, in Antarctica for 2 yr. Geometrical means were 15.2 pg m⁻³ for PCBs, 2.0 pg m⁻³ for p,p′-DDT, 1.0 pg m⁻³ for p,p′-DDE and 25.8 pg m⁻³ for lindane. Atmospheric levels of lindane were positively correlated with temperature, and a significant difference was found between spring-summer and summer-winter concentrations. No season related differences were found for the other chlorinated hydrocarbons, possibly owing to their lower vapour pressure and the cold climate. Periods with increased atmospheric levels of PCBs and DDT compounds were recorded. Lindane, p,p′-DDE and PCBs were present in fish and zooplankton sampled close to Ross Island. Pollutant levels in the zooplankton (on an extractable fat basis) were highest during the Antarctic spring and autumn and were inversely correlated to their fat content.     

Seasonal copula models for the analysis of glacier discharge at King George Island,Antarctica

Modelling glacier discharge is an important issue in hydrology and climate research. Glaciers represent a fundamental water resource when melting of ice and snow contributes to runoff. Glaciers are also studied as natural global warming sensors. GLACKMA association has implemented one of their Pilot Experimental Catchment areas at the King George Island in the Antarctica which records values of the liquid discharge from Collins glacier. In this paper, we propose the use of time-varying copula models for analyzing the relationship between air temperature and glacier discharge, which is clearly non constant and non linear through time. A seasonal copula model is defined where both the marginal and copula parameters vary periodically along time following a seasonal dynamic. Full Bayesian inference is performed such that the marginal and copula parameters are estimated in a one single step, in contrast with the usual two-step approach. Bayesian prediction and model selection is also carried out for the proposed model such that Bayesian credible intervals can be obtained for the conditional glacier discharge given a value of the temperature at any given time point. The proposed methodology is illustrated using the GLACKMA real data where there is, in addition, a hydrological year of missing discharge data which were not possible to measure accurately due to problems in the sounding.     

Biostratigraphy of Famennian in Hainan Island, South China

On the basis of the faunas including conodonts Palmatolepis gracilis sigmoidalis, P.gracilis gracilis, Polygnathus germanus, which are found for the first time, and corals Cystophrentis kalaohoensis it is confirmed that the Changjiang Formation refers to the Famennian, and may correlate with the Shaodong Formation and Menggongao Formation of central Hunan. This is the only Devonian deposits known in Hainan Island so far. It means that the outstanding problem of whether the Devonian rocks exist in that island is now settled. Meanwhile, the new evidences indicate that the lectostratotype section of Nanhao Formation of Baoting area refers to the Silurian,rather than the Lower Carboniferous as considered by most geologists formerly. Furthermore, the Member 3 of Nanhao Formation at Shilu-Jinbo district is proved to be the Upper Carboniferous,rather than Tournaisian or Middle-Upper Devonian as deemed to be before.     

Mercury in the ecosystem of Admiralty Bay,King George Island,Antarctica: Occurrence and trophic distribution

Mercury (Hg) can reach the environment through natural and human-related sources, threatening ecosystems all over the planet due to its well known deleterious effects. Therefore, Antarctic trophic webs, despite being relatively isolated, are not exempt of its influence. To evaluate Hg concentrations in an Antarctic ecosystem, different tissues from 2 species of invertebrates, 2 of fish, 8 of birds, 4 of pinnipeds and at least 5 of vegetation were investigated (n = 176). For animals, values ranged from 0.018 to 48.7 μg g^? 1 dw (whole Antarctic krill and Antarctic Fur Seal liver). They were generally correlated to trophic position (assessed by δ¹⁵N and δ¹³C) but also to cephalopods and myctophids consumption. For vegetation, values ranged from 0.014 to 0.227 μg g^? 1 dw (Colobanthus quitensis and an unidentified lichen), with lichens presenting significantly higher values than mosses, likely due to year-round exposure and absorption of animal derived organic matter, as hypothesized by literature.