Underplating of an accretionary prism: An example from the LeMay group of central Alexander Island, Antarctic Peninsula

first_imgThe LeMay Group of Alexander Island, Antarctic Peninsula, is a Mesozoic accretionary prism that was constructed during subduction of Pacific and proto-Pacific oceanic crust. In central Alexander Island, several distinct lithologic associations can be identified, including two interpreted as underplated units: (i) a sandstone-mudstone association, consisting of thin- to medium-bedded non-channelled turbidite deposits, representing probable trench fill, and (ii) a basalt-chert association, representing oceanic crustal rocks, and its siliceous sedimentary cover. These two units are complexly deformed by dominantly westward-(oceanward-) directed thrusting. Structural relief introduced by later faulting reveals a wide range of structural styles and metamorphic grades representing different levels within the progressively deforming underplated units. Deformation ranges from thrust-related stratal disruption of poorly-lithified clastic sediment, achieved by independent particulate flow and cataclasis, through solution-dominated processes in clastic and siliceous rocks, to the development of pervasive cleavage fabrics at green-schist and transitional blueschist facies, with local crystal-plastic deformation. Later deformation (crenulation fabrics and isolated zones of folding) is of uncertain origin but probably resulted from further accretionary adjustments within the underplated units. The deepest levels may have been partially exhumed by syn-accretionary backthrusting or by transpression within a strike-parallel zone related to oblique convergence. Microstructural evidence reveals the importance of fluids in controlling deformation. Fluids were introduced with the underthrusting sediment and/or were generated during diageneis and metamorphism. In particular, evidence for locally elevated pore-fluid pressures is consistent with the rapid tectonic burial of a lithologically heterogeneous sequence and its subsequent evolution in a semi-closed system, with only limited fluid escape. Such microstructural criteria may be crucial to the identification of underplated units in other ancient accretionary prisms where the overall large-scale structural geometry cannot be reconstructed from fragmentary exposures.last_img read more

Polar mesospheric clouds observed by an iron Boltzmann lidar at Rothera (67.5°S, 68.0°W), Antarctica from 2002 to 2005: properties and implications

first_imgLidar observations of polar mesospheric clouds (PMC) were made at Rothera, Antarctica, from December 2002 to March 2005. Overall, 128 hours of PMC were detected among the 459 hours of observations, giving a mean occurrence frequency of 27.9%. The mean PMC centroid altitude is 84.12 ± 0.12 km, the mean PMC total backscatter coefficient is 2.34 ± 0.11 × 10−6 sr−1, and the mean layer RMS width is 0.93 ± 0.03 km. The distribution of PMC centroid altitudes over all observations is symmetric (nearly Gaussian), with the most probable altitude (∼84 km) near the center of the distribution. The distribution of PMC brightness is non-Gaussian and is dominated by weak PMC. The observed PMC altitudes at Rothera support the earlier lidar findings that Southern Hemispheric PMC are on average 1 km higher than corresponding Northern Hemispheric PMC, and higher PMC occur at higher latitudes. Significant interannual and diurnal variations are observed in PMC centroid altitude and brightness. Mean PMC altitude varies more than 1 km from one year to another. In addition, 24-hour, 12-hour, and 8-hour oscillations are clearly shown in PMC centroid altitude and brightness. The altitude distribution of PMC brightness peaks at a nearly constant altitude of 84 km, with weaker PMC found on either side of this altitude. The mean PMC altitudes averaged in brightness bins are anticorrelated with the PMC brightness, where weaker PMC occur at higher altitude and the PMC altitudes are proportional to the logarithm of the PMC brightness.last_img read more

Towards orbital dating of the EPICA Dome C ice core using deltaO2/N2

first_imgBased on a composite of several measurement series performed on ice samples stored at −25 °C or −50 °C, we present and discuss the first δO2/N2 record of trapped air from the EPICA Dome C (EDC) ice core covering the period between 300 and 800 ka (thousands of years before present). The samples stored at −25 °C show clear gas loss affecting the precision and mean level of the δO2/N2 record. Two different gas loss corrections are proposed to account for this effect, without altering the spectral properties of the original datasets. Although processes at play remain to be fully understood, previous studies have proposed a link between surface insolation, ice grain properties at close-off, and δO2/N2 in air bubbles, from which orbitally tuned chronologies of the Vostok and Dome Fuji ice core records have been derived over the last four climatic cycles. Here, we show that limitations caused by data quality and resolution, data filtering, and uncertainties in the orbital tuning target limit the precision of this tuning method for EDC. Moreover, our extended record includes two periods of low eccentricity. During these intervals (around 400 ka and 750 ka), the matching between δO2/N2 and the different insolation curves is ambiguous because some local insolation maxima cannot be identified in the δO2/N2 record (and vice versa). Recognizing these limitations, we restrict the use of our δO2/N2 record to show that the EDC3 age scale is generally correct within its published uncertainty (6 kyr) over the 300–800 ka period.last_img read more

A new bipolar ice core record of volcanism from WAIS Divide and NEEM and implications for climate forcing of the last 2000 years

first_imgVolcanism is a natural climate forcing causing short-term variations in temperatures. Histories of volcanic eruptions are needed to quantify their role in climate variability and assess human impacts. We present two new seasonally resolved, annually dated non-sea-salt sulfur records from polar ice cores—WAIS Divide (WDC06A) from West Antarctica spanning 408 B.C.E. to 2003 C.E. and NEEM (NEEM-2011-S1) from Greenland spanning 78 to 1997 C.E.—both analyzed using high-resolution continuous flow analysis coupled to two mass spectrometers. The high dating accuracy allowed placing the large bi-hemispheric deposition event ascribed to the eruption of Kuwae in Vanuatu (previously thought to be 1452/1453 C.E. and used as a tie-point in ice core dating) into the year 1458/1459 C.E. This new age is consistent with an independent ice core timescale from Law Dome and explains an apparent delayed response in tree rings to this volcanic event. A second volcanic event is detected in 1453 C.E. in both ice cores. We show for the first time ice core signals in Greenland and Antarctica from the strong eruption of Taupo in New Zealand in 232 C.E. In total, 133 volcanic events were extracted from WDC06A and 138 from NEEM-2011-S1, with 50 ice core signals—predominantly from tropical source volcanoes—identified simultaneously in both records. We assess the effect of large bipolar events on temperature-sensitive tree ring proxies. These two new volcanic records, synchronized with available ice core records to account for spatial variability in sulfate deposition, provide a basis for improving existing time series of volcanic forcing.last_img read more

Mesoproterozoic subduction under the eastern edge of the Kalahari-Grunehogna Craton preceding Rodinia assembly: the Ritscherflya detrital zircon record, Ahlmannryggen (Dronning Maud Land, Antarctica)

first_imgThe ∼2000 m thick clastic and volcaniclastic sedimentary rock pile of the Mesoproterozoic Ritscherflya Supergroup is located near the eastern margin of the Archaean Grunehogna Craton of Dronning Maud Land (East Antarctica). The sedimentary rocks were deposited proximal to an active volcanic arc formed during subduction prior to the assembly of the Rodinia supercontinent. In this study, we investigated internal zonation and U-Pb ages of detrital zircon grains from all formations of the Ahlmannryggen and Jutulstraumen groups of the Ritscherflya Supergroup. Our results show an age distribution with a dominant age peak at ∼ 1130 Ma, close to the sedimentation age of the sedimentary rocks (∼ 1130-1107 Ma), which strongly supports the model of deposition of the sediments in a convergent margin setting. Older peaks in the Ritscherflya sedimentary rock zircon spectrum with ages up to 3445± 7 Ma that were also identified in samples from the Grunehogna Craton basement reflect tectono-magmatic events in the Kalahari Craton. This provides further evidence for the Archaean and Proterozoic connection of the Grunehogna province to the African Kalahari Craton. Parts of the Mesoproterozoic volcanic arc were located on Archaean cratonic basement (∼2800-3450 Ma), whereas other parts tapped late Palaeoproterozoic crust (∼1750 Ma). This is evident from a number of inherited Archaean and Proterozoic cores in zircons with Stenian rims. The Ritscherflya zircon record, therefore, supports models of the eastern margin of the Kalahari-Grunehogna Craton that include inward subduction with an active continental margin prior to collision in Dronning Maud Land. The intercalation of the clastic sedimentary rocks with volcaniclastic materials strongly support the interpretation of a very proximal volcanic source. The sedimentary rocks were affected by regional low-grade metamorphism during the collisional orogeny related to Rodinia assembly and during the Pan-African orogeny related to the assembly of Gondwana. This is evident from metamorphic recrystallisation of zircon at 1086± 4 Ma and from discordancy of many grains pointing to late Neoproterozoic to early Phanerozoic lead loss.last_img read more

Halley Research Station, Antarctica: calving risks and monitoring strategies

first_imgThe British Antarctic Survey’s Halley Research Station is located on the Brunt Ice Shelf, Antarctica, where it is potentially vulnerable to calving events. Existing historical records show that the Brunt Ice Shelf is currently extended further into the Weddell Sea than it was before its last large calving event, so a new calving event may be overdue. We describe three different possible future scenarios for a large-scale calving event on Brunt Ice Shelf, and conclude that the currently most threatening scenario for the Halley Research Station is a calving event on the neighbouring Stancomb–Wills Glacier Tongue, with subsequent detrimental consequences for the stability of the Brunt Ice Shelf. Based on available data, we suggest an increasing likelihood of this scenario occurring after 2020. We furthermore describe ongoing monitoring efforts aimed at giving advanced warning of an imminent calving event.last_img read more

New recombinant cold-adapted and organic solvent tolerant lipase from psychrophilic Pseudomonas sp. LSK25, isolated from Signy Island Antarctica

first_imgIn recent years, studies on psychrophilic lipases have become an emerging area of research in the field of enzymology. The study described here focuses on the cold-adapted organic solvent tolerant lipase strain Pseudomonas sp. LSK25 isolated fromSigny Station, SouthOrkney Islands,maritime Antarctic. Strain LSK25 lipase was successfully cloned, sequenced, and over-expressed in an Escherichia coli system. Sequence analysis revealed that the lipase gene of Pseudomonas sp. LSK25 consists of 1432 bp, lacks an N-terminal signal peptide and encodes a mature protein consisting of 476 amino acids. The recombinantLSK25 lipase was purified by single-step purification using Ni-Sepharose affinity chromatography and had a molecular mass of approximately 65 kDa. The final recovery and purification fold were 44% and 1.3, respectively. The LSK25 lipase was optimally active at 30 °C and at pH 6. Stable lipolytic activity was reported between temperatures of 5–30 °C and at pH 6–8. A significant enhancement of lipolyticactivity was observed in the presence of Ca2+ ions, the organic lipids of rice bran oil and coconut oil, a synthetic C12 ester and a wide range of water immiscible organic solvents. Overall, lipase strain LSK25 is a potentially desirable candidate for biotechnological application, due to its stability at low temperatures, across a range of pH and in organic solvents.last_img read more

Utah’s Stockton Signs With German Team

first_imgAugust 3, 2018 /Sports News – Local Utah’s Stockton Signs With German Team FacebookTwitterLinkedInEmail(Salt Lake City, UT)  — Jazz guard David Stockton is taking his talents to Germany.Multiple reports state Stockton has agreed to a deal with a team in Germany.Stockton is the son of long time Jazz point guard John Stockton.  He appeared in three games for Utah last season. Robert Lovell Written by Tags: David Stockton/Utah Jazzlast_img

Simone Biles makes history as 1st woman to win 4 all-around world titles

first_imgNovember 1, 2018 /Sports News – National Simone Biles makes history as 1st woman to win 4 all-around world titles FacebookTwitterLinkedInEmailFrancois Nel(DOHA, Qatar) — Simone Biles made history as the first woman to win four all-around world titles when she took home her fourth world championship title today in Doha, Qatar.Biles, 21, beat out Mai Murakamai of Japan and fellow Team USA member Morgan Hurd to win the gold medal, despite falling two times during her all-around final.Biles skyrocketed to fame as a member of the “Final Five” women’s gymnastics team that took home gold medals during the 2016 Summer Olympics in Rio de Janeiro, Brazil.She may have taken two tumbles during the competition, but Biles was rewarded by judges for the level of difficulty she went for, according to reports.Despite her history-making win, the Texas native says she still believes she can do better.Biles wrote in an Instagram post that she had “so many emotions” and that it has been “less than a year since I got back into training so I try not to be too hard on myself.”She continued: “Although I still placed 1st and MADE HISTORY I’m not entirely pleased with my performance. I’m still trying to find my confidence. But I won’t give up ?? I’m proud of myself! —> now onto finals.”When not competing, Biles has become an increasingly vocal critic of USA Gymnastics leadership in the wake of the Larry Nassar sexual abuse scandal. Copyright © 2018, ABC Radio. All rights reserved. Written bycenter_img Beau Lundlast_img read more

Silva’s volley lifts RSL to 1-1 draw with Minnesota United

first_img Tags: Marcelo Silva/MLS/Real Salt Lake July 20, 2019 /Sports News – Local Silva’s volley lifts RSL to 1-1 draw with Minnesota United Associated Press FacebookTwitterLinkedInEmailSANDY, Utah (AP) — Marcelo Silva tied it in the 71st minute and Real Salt Lake held on for a 1-1 draw, ending Minnesota United’s four-game winning streak Saturday night.Jefferson Savarino gathered a clearance attempt and crossed it to Silva, who finished with a volley from the middle of the penalty area.Darwin Quintero gave Minnesota United (10-7-4) the lead in the 57th minute. Mason Toye headed Jan Gregus’ corner toward goal but right of the target and Quintero slipped behind the defense to redirect it into the net from close range.Real Salt Lake (9-9-3) had won three of four. Written bylast_img read more