Further evidence of fracture-zone induced tectonic segmentation of the Antarctic Peninsula from detailed aeromagnetic anomalies

first_imgAeromagnetic anomaly data collected between 67°S and 70°S crossing the Antarctic Peninsula and adjacent offshore areas show a prominent NW-SE trend in the magnetic fabric. Apparent lateral offsets, previously recognized in the Pacific Margin Anomaly, have been mapped in detail and are shown to be much smaller than previously suggested. A 35 km wide zone of subdued magnetic anomalies at the Western edge of the Pacific Margin Anomaly, bounded by these apparent offsets, is interpreted as a downfaulted block of the mafic-intermediate batholith thought to be responsible for the Pacific Margin Anomaly. The trends of both fracture zones and magnetic lineaments strongly support the link between faulting in the Antarctic Peninsula magmatic arc and offshore tectonics.last_img read more

Temperature variability beneath Ronne Ice Shelf, Antarctica, from thermistor cables

first_imgThermistor cables have been deployed at two sites beneath Ronne Ice Shelf, Antarctica. One site is to the east of a submarine ridge that delineates the eastern boundary of the Ronne Depression, and the other is 100 km to the north, above the eastern slope of the depression. Long records from the cables (up to 22 months) indicate a large difference in the temperature variability at the two sites, being an order of magnitude greater in the Ronne Depression (site 2). Although the records appear otherwise similar, there is no significant correlation between them. The high variability in the site 2 record has allowed the construction of a simple descriptive model of the local oceanographic regime. Winter freezing in the open water north of the ice front generates Western Shelf Water (WSW), a type of High Salinity Shelf Water, which travels southwest beneath the ice shelf, appearing at site 2 as a slope-trapped current at the bottom of the water column. Baroclinic instability in the flow manifests itself in the site 2 temperature record as oscillations on time scales of 5 to 15 days. The disturbances cause a periodic east-west advection of water masses across the Ronne Depression. Site 2 is on the eastern slope of the depression, where the wave-induced eastward motion forces Ice Shelf Water to rise, resulting in periodic ice-platelet formation in the water column, as surmised from conductivity-temperature-depth measurements at the site. The depth of the WSW layer decreases by 40 to 60 m during a 100-day period, starting some 4 months after the beginning of the summer. Assuming an absence of significant WSW production during the summer, the 4-month delay implies a minimum average speed of WSW flow of about 0.02 m s−1. The WSW flux into the Ronne Depression is estimated at 3 × 105 m3s−1.last_img read more

An assessment of the role of the marginal ice zone in the carbon cycle of the Southern Ocean

first_imgThe dense phytoplankton blooms observed in earlier studies in the Southern Ocean Marginal Ice Zone (MIZ) may not be representative of all regions, so that some previous assessments of the overall importance of this system in total primary production may have been overestimated. Recent observations have shown that increased phytoplankton production may not always be associated with the retreating ice-edge, due to the unpredictability of meltwater-induced stability. Complex interactions between the MIZ and hydrographic fronts have also been indicated. A range of simple simulations, based on biomass inventories for the major biogeochemical systems in the Southern Ocean, show that the greater part of chlorophyll biomass is located in the extensive regions between the major fronts. Consideration of the fronts and the MIZ only, which we surmize may be the principal sites of export production, indicates that the MIZ is clearly the most important single feature. Even if the occurrence of MIZ blooms in the simulations is reduced dramatically, such blooms still appear to make a substantial contribution to production and, by implication, carbon export.last_img read more

The terrestrial arthropod fauna of Cierva Point SSSI, Danco Coast, northern Antarctic Peninsula

first_imgWe report the composition of terrestrial arthropod collections made at Cierva Point Site of Special Scientific Interest, Dance Coast (northern Antarctic Peninsula, 64 degrees 10’S – 60 degrees 57’W), during the 1994/5 and 1995/6 austral summers. A total of 15 free-living micro-arthropod species were found in hand collections and heat extractions of 12 different substrate types, including one Diptera, three Collembala and 11 Acari. In addition, one vertebrate parasitic tick was recorded (Acari, Metastigmata), one probably introduced ”show scorpion fly” (Insecta, Mecoptera), and at least one species of Astigmata (Acari), which may be either parasitic or nidicolous. Greatest diversity was found in mosses-and algal mats (13 species), although within individual samples it was similar (5-6 species per sample) from most low-altitude vegetated or open habitats. Greatest population densities were recorded from mats of the alga, Prasiola crispa. With the exception of vertebrate-associated species, which were obtained from nest material or in the vicinity of bird colonies, mast micro-arthropods were generally distributed, although with widely-varying abundance. Species diversity was slightly lower than, but similar to, that obtained in recent detailed studies of other sites in the maritime Antarctic. This indicates a terrestrial arthropod fauna representative of the maritime Antarctic in general, although two species, the collembolan Isotoma octooculata and the oribatid mite Globoppia loxolineata, were present more widely than reported eleswhere.last_img read more

Illicioxylon, an element of Gondwanan polar forests? Late Cretaceous and early Tertiary woods of Antarctica

first_imgThe Late Cretaceous and early Tertiary sediments from the northern Peninsula region of Antarctica yield a rich assemblage of fossil wood with well preserved anatomy. Wood specimens of a previously recognized morphotype are described. The woods are characterized by diffuse porous wood, mainly solitary vessels with long scalariform perforation plates, scalariform and opposite vessel-ray pitting, generally uniseriate and biseriate heterogeneous rays, and tracheids with obvious uniseriate, circulate, bordered pits. These fossil specimens show greatest anatomical similarity to the organ genus Illicioxylon Gottwald and extant members of the Illiciaceae. The occurrence of illiciaceous-like wood in Gondwana suggests that the distribution of this family may have been more widespread in the geological past and that a relatively warm temperate climate prevailed over the northern Peninsula region of Antarctica during the Late Cretaceous and early Cenozoic.last_img read more

On the use of IMAGE FUV for estimating the latitude of the open/closed magnetic field line boundary in the ionosphere

first_imgA statistical comparison of the latitude of the open/closed magnetic field line boundary (OCB) as estimated from the three far ultraviolet (FUV) detectors onboard the IMAGE spacecraft (the Wideband Imaging camera, WIC, and the Spectrographic Imagers, SI-12 and SI-13) has been carried out over all magnetic local times. A total of over 400 000 OCB estimations were compared from December 2000 and January and December of 2001–2002. The modal latitude difference between the FUV OCB proxies from the three detectors is small, <1°, except in the predawn and evening sectors, where the SI-12 OCB proxy is found to be displaced from both the SI-13 and WIC OCB proxies by up to 2° poleward in the predawn sector and by up to 2° equatorward in the evening sector. Comparing the IMAGE FUV OCB proxies with that determined from particle precipitation measurements by the Defense Meteorological Satellites Program (DMSP) also shows systematic differences. The SI-12 OCB proxy is found to be at higher latitude in the predawn sector, in better agreement with the DMSP OCB proxy. The WIC and SI-13 OCB proxies are found to be in better agreement with the DMSP OCB proxy at most other magnetic local times. These systematic offsets may be used to correct FUV OCB proxies to give a more accurate estimate of the OCB latitude.last_img read more

Short-period planetary waves in the Antarctic middle atmosphere

first_imgPlanetary waves with periods between two and four days in the middle atmosphere over Antarctica are characterized using one year of data from the medium-frequency spaced antenna (MFSA) radars at Scott Base, Rothera, and Davis. In order to investigate the origin of the observed waves, the ground-based data are complemented by temperature measurements from the Earth Observing System Microwave Limb Sounder (EOS MLS) instrument on the Aura satellite as well as wind velocity data from the United Kingdom Met. Office (UKMO) stratospheric assimilation. Observed characteristics of waves with a period of approximately two days in summer are consistent with the quasi-two-day wave (QTDW) generally found after the summer solstice at low- and mid-latitudes. The Scott Base observations of the QTDW presented here are the highest-latitude ground-based observations of this wave to date. Waves with preferred periods of two and four days occur in bursts throughout the winter with maximum activity in June, July, and August. The mean of the two- and four-day wave amplitudes is relatively constant, suggesting constant wave forcing. When several waves with different periods occur at the same time, they often have similar phase velocities, supporting suggestions that they are quasi-non-dispersive. In 2005, a “warmpool” lasts from late July to late August. An alternative interpretation of this phenomenon is the presence of a structure propagating with the background wind. Consideration of the role of vertical shear (baroclinic instabilities) and horizontal shear (barotropic instabilities) of the zonal wind suggests that instabilities are likely to play a role in the forcing of the two- and four-day waves, which are near-resonant modes and thus supported by the atmosphere.last_img read more

Subduction over the Southern Indian Ocean in a high-resolution atmosphere-ocean coupled model

first_imgThis study examines the subduction of the Subantarctic Mode Water in the Indian Ocean in an ocean–atmosphere coupled model in which the ocean component is eddy permitting. The purpose is to assess how sensitive the simulated mode water is to the horizontal resolution in the ocean by comparing with a coarse-resolution ocean coupled model. Subduction of water mass is principally set by the depth of the winter mixed layer. It is found that the path of the Agulhas Current system in the model with an eddy-permitting ocean is different from that with a coarse-resolution ocean. This results in a greater surface heat loss over the Agulhas Return Current and a deeper winter mixed layer downstream in the eddy-permitting ocean coupled model. The winter mixed layer depth in the eddy-permitting ocean compares well to the observations, whereas the winter mixed layer depth in the coarse-resolution ocean coupled model is too shallow and has the wrong spatial structure. To quantify the impacts of different winter mixed depths on the subduction, a way to diagnose local subduction is proposed that includes eddy subduction. It shows that the subduction in the eddy-permitting model is closer to the observations in terms of the magnitudes and the locations. Eddies in the eddy-permitting ocean are found to 1) increase stratification and thus oppose the densification by northward Ekman flow and 2) increase subduction locally. These effects of eddies are not well reproduced by the eddy parameterization in the coarse-resolution ocean coupled model.last_img read more

Sea ice as a source of sea salt aerosol to Greenland ice cores: a model-based study

first_imgGrowing evidence suggests that the sea ice surface is an important source of sea salt aerosol and this has significant implications for polar climate and atmospheric chemistry. It also offers the opportunity to use ice core sea salt records as proxies for past sea ice extent. To explore this possibility in the Arctic region, we use a chemical transport model to track the emission, transport and deposition of sea salt from both the open ocean and the sea ice, allowing us to assess the relative importance of each. Our results confirm the importance of sea ice sea salt (SISS) to the winter Arctic aerosol burden. For the first time, we explicitly simulate the sea salt concentrations of Greenland snow and find they match high resolution Greenland ice core records to within a factor of two. Our simulations suggest that SISS contributes to the winter maxima in sea salt characteristic of ice cores across Greenland. A north-south gradient in the contribution of SISS relative to open ocean sea salt (OOSS) exists across Greenland, with 50 % of sea salt being SISS at northern sites such as NEEM, while only 10 % of sea salt is SISS at southern locations such as ACT10C. Our model shows some skill at reproducing the inter-annual variability in sea salt concentrations for 1991–1999 AD, particularly at Summit where up to 62 % of the variability is explained. Future work will involve constraining what is driving this inter-annual variability and operating the model under different paleoclimatic conditions.last_img read more

Characteristics of mesospheric gravity waves over Antarctica observed by Antarctic Gravity Wave Instrument Network imagers using 3-D spectral analyses

first_imgWe have obtained horizontal phase velocity distributions of the gravity waves around 90 km from four Antarctic airglow imagers, which belong to an international airglow imager/instrument network known as ANGWIN (Antarctic Gravity Wave Instrument Network). Results from the airglow imagers at Syowa (69°S, 40°E), Halley (76°S, 27°W), Davis (69°S, 78°E) and McMurdo (78°S, 167°E) were compared, using a new statistical analysis method based on 3-D Fourier transform [Matsuda et al., 2014] for the observation period between 7 April and 21 May 2013. Significant day-to-day and site-to-site differences were found. The averaged phase velocity spectrum during the observation period showed preferential westward direction at Syowa, McMurdo and Halley, but no preferential direction at Davis. Gravity wave energy estimated by I’/I was ~5 times larger at Davis and Syowa than at McMurdo and Halley. We also compared the phase velocity spectrum at Syowa and Davis with the background wind field and found that the directionality only over Syowa could be explained by critical level filtering of the waves. This suggests that the eastward propagating gravity waves over Davis could have been generated above the polar night jet. Comparison of nighttime variations of the phase velocity spectra with background wind measurements suggested that the effect of critical level filtering could not explain the temporal variation of gravity wave directionality well, and other reasons such as variation of wave sources should be taken into account. Directionality was determined to be dependent on the gravity wave periods.last_img read more