Amsterdam Island Research Articles

Seasonal variations of atmospheric sulfur dioxide and dimethylsulfide concentrations at Amsterdam Island in the southern Indian Ocean

Daily measurements of atmospheric sulfur dioxide (SO2) concentrations were performed from March 1989 to January 1991 at Amsterdam Island (37°50′ S–77°30′ E), a remote site located in the southern Indian Ocean. Long-range transport of continental air masses was studied using Radon (222Rn) as continental tracer. Average monthly SO2 concentrations range from less than 0.2 to 3.9 nmol m-3 (annual average = 0.7 nmol m-3) and present a seasonal cycle with a minimum in winter and a maximum in summer, similar to that described for atmospheric DMS concentrations measured during the same period. Clear diel correlation between atmospheric DMS and SO2 concentrations is also observed during summer. A photochemical box model using measured atmospheric DMS concentrations as input data reproduces the seasonal variations in the measured atmospheric SO2 concentrations within ±30%. Comparing between computed and measured SO2 concentrations allowed us to estimate a yield of SO2 from DMS oxidation of about 70%.

Covariations in oceanic dimethyl sulfide, its oxidation products and rain acidity at Amsterdam Island in the Southern Indian Ocean

Simultaneous measurements of rain acidity and dimethyl sulfide (DMS) at the ocean surface and in the atmosphere were performed at Amsterdam Island over a 4 year period. During the last 2 years, measurements of sulfur dioxide (SO2) in the atmosphere and of methane sulfonic acid (MSA) and non-sea-salt-sulfate (nss-SO4 2-) in rainwater were also performed. Covariations are observed between the oceanic and atmospheric DMS concentrations, atmospheric SO2 concentrations, wet deposition of MSA, nss-SO4 2-, and rain acidity. A comparable summer to winter ratio of DMS and SO2 in the atmosphere and MSA in precipitation were also observed. From the chemical composition of precipitation we estimate that DMS oxidation products contribute approximately 40% of the rain acidity. If we consider the acidity in excess, then DMS oxidation products contribute about 55%.

The oceanic source of carbonyl sulfide (COS)

Carbonyl sulfide (COS) concentrations both in sea water and in the marine atmospheric boundary layer have been measured for 2 years at Amsterdam Island, as well as during oceanographic cruises in regions of different productivity (coastal areas, upwelling areas and open ocean) in the Indian Ocean and Mediterranean Sea. Surface waters of the Indian Ocean and Mediterranean Sea were found to be supersaturated with respect to the atmosphere. The supersaturation of surface coastal sea water in COS displays a diurnal cycle with a midday/midnight ratio of approximately 4, and in addition a seasonal cycle with maximum values during summer and a summer/winter ratio of 2. The supersaturation of open ocean surface sea water is 3 times lower than that of the coastal areas. Taking into account the diurnal, seasonal and geographical variations of the sea water supersaturation in COS, along with the seasonal variation of the sea-air exchange coefficient and the Henry's law coefficient, a global oceanic flux of COS of 0.43 (0.30 to 0.60)Tg COS per year is estimated.

Precipitation composition and its variability in the southern Indian Ocean: Amsterdam Island, 1980–1987

Event precipitation samples have been collected on Amsterdam Island (37° 47′ S, 77° 31′ E) from May 1980 through January 1987 and analyzed for SO4=, NO3−, Cl−, Na+, Mg++, K+, Ca++, H+, HCOOt, and CH3COOt. The objective of this paper is to assess the processes which influence variability in the chemical composition of precipitation at this remote marine site. Back trajectories and gas phase 222Rn measurements were used to identify source regions and their relative contributions to precipitation composition. The technique of cluster analysis was applied to trajectory data as a method for determining to what degree different atmospheric flow patterns influence variability in the observed composition. The dominant source regions for chemical deposition were found to be Madagascar and/or S.E. Africa and the ocean region north and east of Amsterdam Island. A strong seasonal signal in the precipitation composition is illustrated. Processes which influence variability in the chemical composition include the following: seasonality in the source strength of biogenic precursors for non‐sea‐salt (nss) SO4=, NH4+ and carboxylic acids; seasonality in biomass‐burning continental sources for the concentration of NO3− and a portion of nss Cl−; and variations in source region driven by daily changes in meteorology, as well as seasonal and annual differences in transport and removal patterns. In addition, interannual differences in nss SO4= concentration appear to be related to fluctuations in large‐scale circulation patterns as defined by the Southern Oscillation.

Frontal structure and transport of the Antarctic Circumpolar Current in the south Indian Ocean sector, 40–80°E

Based on recent conductivity-temperature-depth (CTD) data at the Kerguelen-Amsterdam passage plus historical expendable bathy thermograph (XBT) and hydrographie data from the Crozet Basin, the frontal structure and volume transport of the Antarctic Circumpolar Current (ACC) in the south Indian Ocean sector are described. Recent findings on sea-level variability from GEOSAT altimeter data are reviewed as well as barotropic transport variability from bottom pressure measurements at Kerguelen and Amsterdam Islands. The ACC here is mostly concentrated in a narrow frontal zone of 2–3° of latitude, without any noticeable multi-band structure, as a result of the confluence of the Subantarctic and Subtropical Fronts. The Polar Front in this sector is not associated with any prominent current core. This regional particularity of the ACC was further supported by GEOSAT altimetry data. About 80% of the ACC transport (106Sv; 1 Sv=10 6 m 3s −1) passes north of Kerguelen, concentrated along the northern flank of the Kerguelen Plateau; only 30 Sv passes south of Kerguelen. The bottom-pressure-derived barotropic transport variability at Kerguelen-Amsterdam passage was estimated to lie between 10 and 30 Sv, consistent with findings at Drake Passage. It is suggested that the study area constitutes a prominent ACC observatory during the WOCE (World Ocean Circulation Experiment).

Annual variation of atmospheric carbonyl sulfide in the marine atmosphere in the Southern Indian Ocean

Measurements of the concentrations of carbonyl sulfide (COS) in the marine atmosphere were made over a period of two years in the southern Indian Ocean (Amsterdam Island, 37°50′ S–77°31′ E; March 1987–February 1988 and April 1989–February 1990). The mean atmospheric COS concentration for the whole period was 475±48 pptv (n=544). Atmospheric COS concentrations show no significant seasonal variation with a summer to winter ratio of 1.05. Taking into account the observed variability of the atmospheric COS concentration (10%), a value of 1.4 yr is estimated as a lower limit for the atmospheric COS lifetime. A comparison of the COS data at Amsterdam Island with those obtained in the Southern Hemisphere in the past 12 yr does not reveal any significant trend in the tropospheric background COS mixing ratio.

Non-seasonal variations of atmospheric CO2 concentrations at Amsterdam Island

Non-seasonal variations of atmospheric CO2 concentrations at Amsterdam Island

Non-seasonal variations of atmospheric CO<sub>2</sub> concentrations at Amsterdam Island

The atmospheric CO2 monitoring at Amsterdam island has shown a general increasing trend of the increase rate, which shifted from 1.1 ppm year-1 in January 1981 to 1.8 ppm year-1 in January 1989. Besides this general trend, three different kinds of variations were observed: (a) seasonal variations, whose magnitude is small at this station, that is less than ± 0.7 ppm; (b) small non-seasonal variations, as in 1985, likely related to changes in the atmospheric transport, also observed in other stations, but with different magnitudes and features; (c) important non-seasonal variations of the order of 1 ppm, negatively correlated to ENSO events of 1983 and 1987, with delays of 6 and 3 to 4 months, respectively. These variations are ascribed to a general change of the CO2 emissions to the atmosphere, resulting from global climatic perturbations, rather than to transport processes. This is particularly shown at our sampling site through 222Rn monitoring, that enables us to characterize direct air injections from South Africa into subantarctic areas.

NMHC in the marine atmosphere: Preliminary results of monitoring at Amsterdam Island

Between January 1984 and May 1987, C2 to C5 NMHC concentrations, and Radon-222 activities were measured at Amsterdam Island in the Indian Ocean. A large variability of about one order of magnitude was observed in the NMHC concentrations. Most of the samples were collected under marine influence. Using ethene as a reference compound for marine emissions, it appears that the NMHC/ethene composition of the air and its variability directly reflect the composition of dissolved gases in surface seawater. Only the ethane/ethene ratio presents a significant deviation from this typical composition and this can be attributed to a continental component. At sea level, the reation frequency of OH radicals with the NMHC is similar to that of methane and carbon monoxide. Thus, the contribution of marine NMHC should be taken into account in the modelling of oxidants in remote atmospheres.

Seasonal variation of atmospheric dimethylsulfide at Amsterdam Island in the southern Indian Ocean

Daily measurements of atmospheric concentrations of dimethylsulfide (DMS) were carried out for two years in a marine site at remote area: the Amsterdam Island (37°50′S–77°31′E) located in the southern Indian Ocean. DMS concentrations were also measured in seawater. A seasonal variation is observed for both DMS in the atmosphere and in the sea-surface. The monthly averages of DMS concentrations in the surface coastal seawater and in the atmosphere ranged, respectively, from 0.3 to 2.0 nmol l-1 and from 1.4 to 11.3 nmol m-3 (34 to 274 pptv), with the highest values in summer. The monthly variation of sea-to-air flux of DMS from the southern Indian Ocean ranges from 0.7 to 4.4 μmol m-2 d-1. A factor of 2.3 is observed between summer and winter with mean DMS fluxes of 3.0 and 1.3 μmol m-2 d-1, respectively.

Radon-calibrated emissions of CO<sub>2</sub> from South Africa

Atmospheric CO2 and 222Rn have been monitored at Amsterdam Island since 1980. Data were selected in order to eliminate any local influence. Typical CO2 concentrations of the subantarctic marine atmosphere can be determined by selecting those values for which 222Rn radioactivity was particularly low: less than 1 pCi m-3. 222Rn concentrations higher than 2 pCi m-3 are mainly due to injections into the subantarctic atmosphere from the continental source of South Africa. The passage of air masses under continental influence also shows typical CO2 variations, well correlated with 222Rn variations. From the knowledge of the global continental fluxes of 222Rn, it has been possible to estimate CO2 fluxes into the atmosphere from South Africa. The mean CO2 flux corresponding to a 6-month period from May to October is about 5 millimole m-2 h-1. Continental CO2 emissions reach a maximum in August.

Structure sociale de la population de bovins sauvages de l’île Amsterdam, sud de l’Océan Indien

A herd of about 1 600 feral cattle (Bos taurus) lives on Amsterdam Island (southern Indian Ocean) since 1871. A preliminary study of its population structure and social organization was carried out during a whole year (1985-1986), along 4 altitudinal transects of fixed width (200 m). Females may be pregnant at 11/2 year of age. Calving takes place throughout the year, with a peak at the end of November and a low from April to August. Most deaths occur in August and September, chiefly of malnutrition. The Amsterdam feral cattle population consists of 60 % of adults, and 10 % of suckling calves less than 3 months old. The sex ratio is equal at birth ; males temporarily predominate among 1 to 2-3 years old animals, but there are only 30 males left for 100 females among adults as a consequence, at least in part, of selective hunting. For most of the year, males and females live separately, the younger males remaining within the maternal groups. In mots cases males also live at higher altitudes than females, joining them only briefly during the rutting season.

Breeding biology and current status of the Amsterdam Island Albatross Diomedea amsterdamensis

A small population of the ‘Great Albatrosses’ has been discovered on Amsterdam Island (Indian Ocean) and described in 1983 as a new species: Diomedea amsterdamensis (Roux et al. 1983). We present here data on the biometry, plumage variability and preliminary results of a long‐term study on the breeding cycle and population dynamics of this relict population. The current population is estimated at 65 birds including 21 breeding pairs.

Mineralogical composition of aerosols at Amsterdam Island

In this paper, data on the mineralogical composition of insoluble particulate matter collected in May, July and September 1984 at Amsterdam Island, South Indian Ocean (34°47'S, 77°31'E) are presented for the first time. Atmospheric levels of these insoluble aerosols remain very low (from 0.25 to 0.01 μg m -3 ); the size of the particles is mainly in the submicronic range. The long-range continental origin of these particles is well confirmed by atmospheric tracers such as 222 Rn and 210 Pb and by air mass back-trajectories. Continental inputs, mainly from South Africa and Madagascar, that can be identified by “radonic storms” have a high illite/kaolinite ratio. However those from Australia, though less frequent at Amsterdam Island, have a low illite/kaolinite ratio. Thus, kaolinite can be considered a good tracer of transport of Australian aerosols into the subantarctic. DOI: 10.1111/j.1600-0889.1989.tb00313.x

Marine isopod crustaceans from the St. Paul and Amsterdam Islands, southern Indian Ocean

Marine isopod crustaceans from the St. Paul and Amsterdam Islands, southern Indian Ocean

Régime alimentaire et distribution du Chat haret (Felis catus) sur l’ Île Amsterdam

The diet of feral cats on Amsterdam Island was studied by examining 241 scats collected during one year. Introduced rodents, mice (Mus musculus) and Norway rats (Rattus norvegicus) formed the bulk of the diet. Mice were the most common prey ; they occured in 82 % of the scats and were the only cat preys during the Southern winter. Norway rats occured in 30 % of the scats ; young animals weighing less than 100 g were especially preyed. Birds remains were found in only 8 % of the scats. Cats also scavenged on dead fur seals and cows. Cats settled all over the island, yet they are most numerous in the periphery, under an altitude of 200 meters.

Genomagnetic secular variation during the Brunhes epoch inferred from the Paleomagnetism and the last 200 years geomagnetic field.

Paleomagnetic data were compiled from the previous studies on volcanic materials in the southern part of Central Japan to investigate the geomagnetic secular variation during the Brunhes epoch. The distribution of compiled 113 VGPs is examined whether or not the Fisher statistics can be applied. Chi-square test indicates that the hypothesis of uniform azimuthal distribution around a mean VGP, which is assumed in the Fisher statistics, is rejected at a 0.05 significance level. Besides, the VGPs tend to concentrate in a certain direction. Azimuthal distributions of the Brunhes epoch VGPs for other regions, France, the Aleutian Islands, the Canary Islands, Comores Island, Crozet Island, Reunion Island and Amsterdam Island, also show similar elongation and azimuthal uniformity can be rejected for these assemblages. Hence the elongation of VGP distribution appears to be a global characteristic in the Brunhes epoch. These elongations can be explained neither by the westward drift of the nondipole fields, by the wobble of the geocentric dipole, nor by the contortion due to transforming field directions into VGPs. Gauss coefficients of the last 200 years geomagnetic field indicate that several nondipole terms changed linearly with the geocentric axial dipole component of g01. Some of these relationship is possible to have held well not only in the last 200 years but also throughout the Brunhes epoch. In addition, the previous paleointensity studies for the Plio-Pleistocene infer that g01 changed approximately in a normal distribution. On the basis of these results, a new model for the secular variation is proposed, in which the intensity of dipole term g01 changes in a normal distribution and those of the correlating nondipole terms vary linearly with a deviation of g01 from its mean intensity. All combinations of nondipole components are examined and consequently the model with a quadrupole component h12 seems to predict the elongation in azimuthal distribution and the global trend of latitude dependence in angular dispersion. This model appears to have an ability to give a significant and quantitative constraint to the homogeneous dynamo process.

Seasonal variations of atmospheric CO2in the southern Indian Ocean

Atmospheric CO2 recording at Amsterdam Island shows a 0.7 ppm seasonal effect, nonattributable to local causes and significantly different from the results obtained in other southern hemisphere sta...

The 1982-1983 El Niño: a 6 billion ton CO2release

The atmospheric CO 2 concentration at Amsterdam Island showed important variations of the increase rate during the 1982-1983 El Nino. After elimination of the weak local and regional interferences, the main feature of the CO 2 record is a slower than normal increase rate at the beginning and at the end of this El Nino, i.e., from April to December 1982, and from July 1983 to April 1984. and higher than usual from December 1982 to July 1983. In the whole, the CO 2 increase curve is shifted upwards by 1 ppm, which corresponds to a supplementary input into the atmosphere of about 6 billion tons of CO 2 . DOI: 10.1111/j.1600-0889.1987.tb00283.x

Atmospheric carbon dioxide measurements in the Australian region: data from surface observatories

Observations of the atmospheric carbon dioxide (CO 2 ) concentration made at surface locations in the Australian and Antarctic regions are presented for the period 19761984. These observations complement those already reported which were based on samples collected from aircraft throughout the troposphere and lower stratosphere in the same region. Data are presented for 4 sites, Cape Grim (40°41'S; 144°41'E). Mawson (67°37'S; 62°52'E), Macquarie Island (54°29'S; 158°58'E) and Wilbinga (31°25'S; 115°33'E) and for air samples collected on board ships traversing the Southern Ocean. By far the most comprehensive monitoring program described is that for the Cape Grim Observatory, where continuous in situ observations have been made since 1976. Methods are considered for the selection of data to represent the baseline or background concentration and a description is given of the natural variability of CO 2 concentration, including within-hour and diurnal variability, seasonal cycles, interannual and secular trends. The average rate of concentration increase over the period represented by the data was 1.5 ppmv yr -1 . A comparison is made between the selected data from these programs and similar data sets collected at Amsterdam Island and in New Zealand by other international projects. DOI: 10.1111/j.1600-0889.1987.tb00269.x