Weddell Sea Sector Research Articles

40Ar/39Ar geochronology of Ferrar Dolerite sills from the Transantarctic Mountains, Antarctica: Implications for the age and origin of the Ferrar magmatic province

The Ferrar Dolerite constitutes the hypabyssal phase of the tholeiitic Ferrar Group of Antarctica. Sills with compositions representing most of the range of geochemical variation of the Ferrar Dolerite, and separated by distances of as much as 1400 km, have been analyzed by the 40 Ar/ 39 Ar method on feldspar and biotite separates. The 40 Ar/ 39 Ar ages for five individual sills range from 176.2 to 177.2 Ma and show no significant difference. These ages reflect crystallization at 176.7 ± 1.8 Ma (where the uncertainty includes provision for systematic uncertainty in the age of the neutron-fluence monitor calibrated relative to MMhb-1 at 513.5 Ma). Combining data from these sills with previous determinations on coeval lavas and underlying pyroclastic units indicates an age of 176.6 ± 1.8 Ma for the Ferrar tholeiitic rocks as a whole. The duration of magmatic activity was less than approximately 1 m.y. By extension, other rocks in the Ferrar magmatic province, which occur from southeastern Australia, along the Transantarctic Mountains to the Theron Mountains, are inferred to have this age. The short duration of magmatic activity as well as the consistent pattern of geochemical variation and distinctiveness of the Ferrar rocks suggest that magmas were transported laterally by an extensive dike swarm which is inferred to have originated in the Weddell Sea sector of the province.

Evidence for winter/spring denitrification of the stratosphere in the nitrate record of Antarctic firn cores

A firn core from within the polar vortex in the Weddell Sea sector of Antarctica shows nitrate peaks that occur in the spring or early summer. They are far less prominent in two other cores from sites that are further north. Although other data are needed, circumstantial evidence suggests that the peak may be due to sedimentation of polar stratospheric clouds from the stratosphere during winter and spring. No change in the form of the peak is seen over the last three decades. It may be possible to observe past stratospheric conditions over longer time scales from ice cores, but other sources may obliterate the signal in ice from the glacial periods.

Time-trends in the pattern of ocean-atmosphere exchange in an ice core from the Weddell Sea sector of Antarctica

Time-trends in the pattern of ocean-atmosphere exchange in an ice core from the Weddell Sea sector of Antarctica

Time-trends in the pattern of ocean-atmosphere exchange in an ice core from the Weddell Sea sector of Antarctica

The east coast of the Antarctic Peninsula is strongly influenced by air masses that have traversed the Weddell Sea zone. A continuous record of annual-average values for δ18O, δD, Cl- and non sea-salt SO42- in snowfall deposited since 1795, has been obtained on an ice core drilled on Dolleman Island (70°35.2’S, 60°55.5’W). Chemical changes along the ice core seem to be linked to changes in the concentration of the ice cover in the marginal ice zone. In the period since 1956, these variations appear to be coupled to the atmospheric circulation, as indexed by the atmospheric pressure gradient across the marginal ice zone. The largest anomaly in the 200-year sequence occurs in the period 1820-1880, during the final stages of the Little Ice Age. Exceptionally high concentrations of Cl-, low concentrations of biologically-derived sulphate, and high deuterium excess suggest that at this time there was a dense, compacted marginal ice zone with cyclones tracking more frequently than normal across ocean areas to the north of the ice edge. During the past century, there has been a marked decrease in deuterium excess of about 4‰, which implies that there has been a progressively increasing contribution to precipitation from moisture sources at lower temperature, probably from within the marginal ice zone. The implication is that there may have been significant weakening of the ice cover in this zone during the past century, despite satellite evidence which reveals no significant change in the position of the ice edge, at least since 1973.

Spatial and temporal variability of global surface solar irradiance

A fast scheme for computing surface solar irradiance using data from the International Satellite Cloud Climatology Project (ISCCP) is described. Daily mean solar irradiances from the fast scheme reproduce the detailed global results from full radiative transfer model calculations to within 6 and 10 W m−2 over the ocean and land, respectively. In particular, the fast scheme reproduces the same dependence of surface irradiance on solar zenith angle which is critical for proper calculation of daily, seasonal, and latitudinal variability. Validation of both model results is limited because globally distributed data sets of high quality are lacking, particularly over the oceans. However, comparison of calculated monthly mean results using 5 months of ISCCP data (July 1983 to July 1984) with climatology from the 1970s at six temperate latitude ocean weather stations shows agreement within published estimates of interannual variability of monthly means at the individual stations. A further test against a 17‐day time series at a continental site (43°N, 90°W, October–November 1986; 13–170 W m−2 range of irradiance), where ground and satellite data were spatially and temporally coincident, showed an accuracy of better than 9 W m−2 on a daily basis and less than 4% bias in the 17‐day mean. Frequently used bulk formulae for solar irradiance were also evaluated in each of these tests. All suffered in comparison because they did not include a parameterization of the effects of the global variability in mean cloud optical thickness. Data from July (1983 and 1984) and January (1984 and 1985) were used to examine the spatial and temporal variability of surface irradiance and its potential impact on biospheric processes. Results show that the oceans and land experience fundamentally different light regimes, with continents receiving significantly greater irradiance. In summer, major interocean differences in zonally averaged irradiance are found in the northern hemisphere with the Atlantic greater than Pacific by up to 80 W m−2; in the southern hemisphere, interocean differences are small. Regional interannual variability (July 1983 versus 1984) ranged between +100 and −100 W m−2. The variability, perhaps due to the 1982–1983 El Niño event, occurred mostly in the Pacific but extended beyond the tropics over the entire north Pacific basin. The nutrient‐rich northern and southern ocean waters are almost perpetually cloud covered; however, there is a correspondence between higher than average surface irradiance and productivity in nutrient‐rich areas of the southwest Atlantic and Weddell Sea sector of the circumpolar current. This suggests that solar irradiance must be considered as an important factor governing the productivity of these waters.

The longitudinal dependence of whistler and chorus characteristics observed on the ground near L = 4

Whistler activity at L ≃ 4 is known to be a function of longitude, peaking in the Weddell Sea sector of Antarctica; a combination of source and propagation factors, the latter possibly partly associated with the South Atlantic geomagnetic anomaly, is believed to be responsible. There is evidence, for example from satellite surveys, that chorus and hiss activity may also be longitude dependent. To investigate this further, we have compared VLF data from four L ≃ 4 Antarctic stations from a 2‐day period in June 1982. Siple, Halley, and Sanae form a closely spaced (∼20°–0° geomagnetic longitude) triplet, while Kerguelen is ∼120° (geomagnetic) to the east, on the opposite side of the anomaly. To a large extent there was a repeatable diurnal variation in activity at all stations on the two days. Events observed at Siple tended to be similar to those observed ∼9 hours earlier (the same MLT) at Kerguelen on the same day. There was a very marked drop‐off in both whistler and VLF emission activity between Siple and Halley on the one hand and Sanae on the other. The reason for this is not clear; it may be either a source effect such as the lower occurrence of lightning over eastern North America compared to the adjacent Atlantic Ocean, or else a wave‐particle interaction effect whereby the conditions for wave growth or amplification are more favorable, or substorm particle injections penetrate the magnetosphere more deeply, at the longitude of Siple than further east. Comparison of the spectral forms of whistler mode activity at neighboring stations suggests that wave generation occurs simultaneously over relatively wide longitude (or local time) sectors (≳30° or 2 hours). Individual interaction regions are smaller than this, ≲5° in longitude, comparable with the previously inferred sizes of whistler ducts. Data sets with longer time coverage and better spatial resolution are required to answer some of the problems raised by this limited study.

Ice-Core Evidence For Weddell Sea Ice Extent During The Past 200 Years

A continuous, detailed, 200-years record of the anionic species, chloride, nitrate and sulphate, has been measured on an ice core from Dolleman Island (70°35.2′ S, 60°55.5′ W), Antarctic Peninsula. The site lies on the east coast of the Peninsula, and the chemistry of the core is dominated by the changing pattern of sea-ice distribution and storm activity in the Wed dell Sea. Strong annual cycles in chloride and non sea salt sulphate reflect the dominance of the seasonal cycle in sea-ice distribution in the Weddell Sea, observed in time series derived from satellite imagery since the early 1970s. However, in the case of chloride there is also an exceptionally strong interannual variability, which in many parts of the core dominates the seasonal cycle.Secular variations in the sea-ice extent appear to have a strong influence on the climate of the region and may play a major role in determining how long-term climate change in the Antarctic Peninsula relates to global climate change. The paper examines documented evidence for sea-ice extent in the Weddell Sea sector, and evaluates the usefulness of ice-core data for reconstructing this parameter in the earlier period.

Ice-Core Evidence For Weddell Sea Ice Extent During The Past 200 Years

A continuous, detailed, 200-years record of the anionic species, chloride, nitrate and sulphate, has been measured on an ice core from Dolleman Island (70°35.2′ S, 60°55.5′ W), Antarctic Peninsula. The site lies on the east coast of the Peninsula, and the chemistry of the core is dominated by the changing pattern of sea-ice distribution and storm activity in the Wed dell Sea. Strong annual cycles in chloride and non sea salt sulphate reflect the dominance of the seasonal cycle in sea-ice distribution in the Weddell Sea, observed in time series derived from satellite imagery since the early 1970s. However, in the case of chloride there is also an exceptionally strong interannual variability, which in many parts of the core dominates the seasonal cycle. Secular variations in the sea-ice extent appear to have a strong influence on the climate of the region and may play a major role in determining how long-term climate change in the Antarctic Peninsula relates to global climate change. The paper examines documented evidence for sea-ice extent in the Weddell Sea sector, and evaluates the usefulness of ice-core data for reconstructing this parameter in the earlier period.

The Scottish National Antarctic Expedition 1902–04

On 21 July 1904, just over 80 years ago, the barque-rigged, Norwegian-built auxiliary steamship Scotia sailed home up the Clyde with members of the Scottish National Antarctic Expedition (SNAE), concluding one of the most successful expeditions of the heroic period of Antarctic exploration. Contemporaneous with the more spectacular British Antarctic Expedition (1901–03) commanded by Robert Falcon Scott, the Scotia party under William Spiers Bruce had overwintered on Laurie Island (60° 44ʹ S, 44° 50ʹ W) in the South Orkney Islands, explored for the first time the oceanography of the Weddell Sea, assembled an important collection of scientific material, and discovered Coats Land, an icebound stretch of the East Antarctica coast.While Scott's Discovery expedition had emphasized geographical exploration inland from the Ross Sea sector of Antarctica, Bruce in the Scotia had concentrated more on scientific discovery in the Weddell Sea sector. On 12 November 1904 in Edinburgh, members of the Scotia and Discovery expeditions were guests at the 20th anniversary dinner of the Royal Scottish Geographical Society, Bruce and Scott together responding to a presidential toast that honoured the success of both.

Antarctic sea ice and related atmospheric circulation during FGGE

Anomalies in zonally averaged and regional Antarctic sea ice extent during the FGGE year (1978–79) are outlined. Interest is centred on the early months of 1979 when substantially less than normal ice occurred in the Ross Sea sector, and more than normal in the Weddell Sea sector. The latter anomaly was maintained and extended eastward to the African sector during the winter and spring of 1979. The ice anomalies are found to be associated with contrasting general circulation conditions described primarily by indices of the strength of the mid-latitude westerlies and regional poleward anomalies in the displacement of the subtropical high pressure belt. In general however, the track, intensity and frequency of cyclones in relation to both the ice edge location and to the prevailing thermal conditions at the ocean surface appear to be the most significant factors affecting the development and maintenance of the ice anomalies.

Analysis Of Interannual Changes In Antarctic Sea-Ice Cover Using Passive Microwave Observations (Abstract only)

A quantitative comparison of seasonal and interannual Antarctic sea-ice coverage over the four years 1973-76 has been accomplished through the use of passive microwave imagery from the Nimbus-5 satellite. For the entire Southern Ocean both the total ice extent (area with ice concentration greater than 15%) and the actual ice area (the spatially-integrated ice concentration) have decreased over this period of 4 a, but not uniformly in all regions. From 1973 to 1976 the annual-mean value of total ice extent decreased from 13.8 × 106 km2 to 12.1 × 106 km2, yielding an average decrease of 4.0% a−1. The inter-annual difference is greatest during the spring, as the ice decays, with the decrease in the December-mean averaging 8.4% a−1, the largest of any month. The decrease in the November-mean averaged 4.5% a−1. The overall decrease was principally due to the consistent yearly decrease of ice In the Weddell Sea sector (60°W to 20°E). Other sectors show less consistency. For instance, the ice in the Ross Sea sector (130°W to 160°E) increased from 1973 to 1974 and then decreased from 1974 to 1976, and no consistent trend is apparent in the ice extent between 20°E and 160°E. The total ice extent in the Bellingshausen- Amundsen seas sector (60°W to 130°W) actually increased slightly from 1973 to 1976. The area of the open water within the ice pack behaved differently from the total ice area, Increasing each year from February to November but having no clear interannual trend. A detailed analysis of the passive microwave imagery for the Antarctic region is planned for publication in an atlas.

Analysis Of Interannual Changes In Antarctic Sea-Ice Cover Using Passive Microwave Observations (Abstract only)

A quantitative comparison of seasonal and interannual Antarctic sea-ice coverage over the four years 1973-76 has been accomplished through the use of passive microwave imagery from the Nimbus-5 satellite. For the entire Southern Ocean both the total ice extent (area with ice concentration greater than 15%) and the actual ice area (the spatially-integrated ice concentration) have decreased over this period of 4 a, but not uniformly in all regions. From 1973 to 1976 the annual-mean value of total ice extent decreased from 13.8 × 106 km2 to 12.1 × 106 km2, yielding an average decrease of 4.0% a−1. The inter-annual difference is greatest during the spring, as the ice decays, with the decrease in the December-mean averaging 8.4% a−1, the largest of any month. The decrease in the November-mean averaged 4.5% a−1. The overall decrease was principally due to the consistent yearly decrease of ice In the Weddell Sea sector (60°W to 20°E). Other sectors show less consistency. For instance, the ice in the Ross Sea sector (130°W to 160°E) increased from 1973 to 1974 and then decreased from 1974 to 1976, and no consistent trend is apparent in the ice extent between 20°E and 160°E. The total ice extent in the Bellingshausen- Amundsen seas sector (60°W to 130°W) actually increased slightly from 1973 to 1976. The area of the open water within the ice pack behaved differently from the total ice area, Increasing each year from February to November but having no clear interannual trend. A detailed analysis of the passive microwave imagery for the Antarctic region is planned for publication in an atlas.

On the movement of lows in the Ross and Weddell Sea sectors in summer : Van Loon H., 1962. Notos, 11 (1/4): 47–50

On the movement of lows in the Ross and Weddell Sea sectors in summer : Van Loon H., 1962. Notos, 11 (1/4): 47–50