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Sea-spray deposition in Antarctic coastal and plateau areas from ITASE traverses

Published online by Cambridge University Press:  14 September 2017

Silvia Benassai
Affiliation:
Department of Chemistry, University of Florence, Via della Lastruccia 3, I-50019 Sesto Fiorentino (Florence), Italy, E-mail: silvia.becagli@unifi.it
Silvia Becagli
Affiliation:
Department of Chemistry, University of Florence, Via della Lastruccia 3, I-50019 Sesto Fiorentino (Florence), Italy, E-mail: silvia.becagli@unifi.it
Roberto Gragnani
Affiliation:
ENEA, Centro Ricerche Casaccia, PO Box 2400, I-00100 Rome, Italy
Olivier Magand
Affiliation:
Laboratoire de Glaciologie et Gèophysique de l’Environnement (CNRS–UJF), 54 rue Molière, BP 96, 38402 Saint-Martin-d’Hères Cedex, France
Marco Proposito
Affiliation:
ENEA, Centro Ricerche Casaccia, PO Box 2400, I-00100 Rome, Italy
Ilaria Fattori
Affiliation:
Department of Chemistry, University of Florence, Via della Lastruccia 3, I-50019 Sesto Fiorentino (Florence), Italy, E-mail: silvia.becagli@unifi.it
Rita Traversi
Affiliation:
Department of Chemistry, University of Florence, Via della Lastruccia 3, I-50019 Sesto Fiorentino (Florence), Italy, E-mail: silvia.becagli@unifi.it
Roberto Udisti
Affiliation:
Department of Chemistry, University of Florence, Via della Lastruccia 3, I-50019 Sesto Fiorentino (Florence), Italy, E-mail: silvia.becagli@unifi.it
Corresponding
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Abstract

Sea-salt markers (Na+, Mg2+ and Cl) were analyzed in recent snow collected at more than 600 sites located in coastal and central areas of East Antarctica (northern Victoria Land–Dome C–Wilkes Land), in order to understand the effect of site remoteness, transport efficiency and depositional and post-depositional processes on the spatial distribution of the primary marine aerosol. Firn-core, snow-pit and 1m integrated superficial snow samples were collected in the framework of the International Trans-Antarctic Scientific Expeditions (ITASE) project during recent Italian Antarctic Campaigns (1992–2002). The sampling sites were mainly distributed along coast–inland traverses (northern Victoria Land– Dome C) and an east–west transect following the 2100m contour line (Wilkes Land). At each site, the snow ionic composition was determined. Here, we discuss the distribution of sea-spray components (Na+, Mg2+ and Cl) as a function of distance from the sea, altitude and accumulation rate, in order to discover the pulling-down rate, possible fractionating phenomena and alternative sources moving inland from coastal areas. Sea-spray depositional fluxes decrease as a function of distance from the sea and altitude. A two-order-of-magnitude decrease occurs in the first 200km from the sea, corresponding to about 2000ma.s.l. Correlations of Mg2+ and Cl with Na+ and trends of Mg2+/Na+ and Cl/Na+ ratios showed that chloride has other sources than sea spray (HCl) and is affected by post-depositional processes. Accumulation rate higher than 80 kgm–2 a–1 preserves the chloride record in the snow. Sea-spray atmospheric scavenging is dominated by wet deposition in coastal and inland sites.

Type
Research Article
Copyright
Copyright © The Author(s) [year] 2005

1. Introduction

Sea spray is the main contributor to the ionic budget of the atmospheric aerosol in Antarctic coastal regions (Reference DelmasDelmas, 1992; Reference UdistiWagenbach and others, 1998). Although sea spray is mainly distributed in the super-micrometric aerosol fractions, several studies report that significant sea-salt loads are also present in the sub-micrometric range, especially in particles larger than 0.13 μm (Reference MurphyO’Dowd and others 1993, Reference O’Dowd and Smith1997; Reference Mulvaney, Coulson and CorrMurphy and others, 1998). The sub-micrometric sea-salt particles have long atmospheric residence times, allowing their large-scale distribution by long-range transport. This size fraction is also involved in relevant radiative processes: sea spray in the sub-micrometric mode is responsible for the majority of aerosol-scattered sunlight and constitutes a significant fraction of the cloud condensation nuclei (Reference Mulvaney, Coulson and CorrMurphy and other, 1998).

On a global scale, sea spray is mainly produced by bubble bursting and wind blowing on the wave crests (Reference BrimblecombeBrimblecombe, 1996). However, the processes leading to sea-salt production from the marine surface in polar regions are somewhat more complex. The emission flux is directly correlated to the wind speed but also depends on seasonal changes in sea-ice coverage. Sea ice affects the primary sea-spray production (its extent increases by 15×106km2 over winter each year (Reference GjessingGloersen and Campbell, 1991)) in two opposite ways: on the one hand, the sea-ice cover reduces the open-sea surface, weakening the source intensity; on the other hand, its surface is the preferential site for the formation of fragile structures of sea-salt crystals, known as ‘frost flowers’, caused by evaporation/ condensation sea-water processes during sea-ice growth (Reference PropositoRankin and others, 2000, Reference Rankin, Auld and Wolff2002). Rankin and others (2000, 2002) suggested that frost flowers play a role as sea-salt source to the atmosphere. Sea-spray inputs from the wind ablation of frost flowers are more vigorous during sea-ice formation, and their contribution to the sea-salt aerosol load has been demonstrated for coastal areas (Reference PropositoRankin and others, 2000, Reference Rankin, Auld and Wolff2002). Frost flowers are also thought to have a significant role in sea-salt supply at inland Antarctic sites, and their higher production in glacial periods is postulated to be related to the increase in sea-salt fluxes in the Last Glacial Maximum (Rankin 2000, 2002; Reference WagenbachWolff and others, 2003), but experimental evidence is necessary to support these hypotheses. Usually, abrupt sea-salt signatures in ice cores have been attributed to increased open sea water and more efficient transport inland, probably due to stormier weather. Reference Kerminen, Teinilä and HillamoKreutz and others (2000) proposed a method to reconstruct the past sea-level pressure changes in the Amundsen Sea region from Siple Dome (West Antarctica) ice-core stratigraphy. The method was based on the spatial correlation analysis between the monthly sea-level pressure fields and the annual-averaged record of a parameter (empirical orthogonal function) linked to sea-spray components.

Due to their role in the atmospheric chemistry and their use as environmental and climatic markers, sea-spray components analyzed in ice cores allow the reconstruction of paleoatmospheric composition, assuming univocal source and preservation in snow layers.

Measurements of ionic chemical components of Antarctic aerosol, snow and ice show that the ratios among the airborne sea-salt compounds can be significantly modified with respect to the bulk sea-water composition (Reference Aristarain, Delmas and BriatAristarain and others, 1982, 2002; Reference Legrand and DelmasLegrand and Delmas, 1988; Reference FrezzottiGjessing, 1989; Reference Mouri, Nagao, Okada, Koga and TanakaMulvaney and others, 1993; Reference UdistiWagenbach and others, 1998). Such fractionating processes can occur, in the atmosphere or in the superficial snow layers, by interaction between sea-spray particles and acidic species (HNO3 and H2SO4), mainly distributed in gas phase or into sub-micrometric aerosol particles. The acid–base exchange reaction leads to the formation of gaseous HCl, which follows different transport pathways than sea-spray particles (Reference Wolff, Artaxo and RöthlisbergerWouters and others 1990; Reference Mayewski and LegrandMcInnes and others, 1994; Reference Minikin, Wagenbach, Graf and KipfstuhlMouri and others, 1996; Reference Innocenti and ColacinoKerminen and other 2000). Other possible fractionating effects, mainly involving sulfate, can result from selective precipitation of mirabilite (Na2SO410H2O) on the pack surface in particular temperature and humidity conditions (Reference UdistiWagenbach and others, 1998) and the above-discussed formation of frost flowers (Reference Rankin, Auld and WolffRankin and others, 2002). Consequently, achieving extensive and reliable datasets on the sea-spray spatial distribution in coastal and central areas of Antarctica is necessary to clarify the processes controlling sources, transport and deposition of the marine primary aerosol.

In this paper, we discuss the distribution of some sea-spray components (Na+, Mg2+ and Cl) at more than 600 sites located in a large, roughly triangular area with the vertices set at northern Victoria Land (NVL), Dome C (DC), central East Antarctica, and Dumont d’Urville (DdU), Wilkes Land. The main goals of this study were: to evaluate the contribution of sea-salt flux to the chemical composition of the snow deposition, as a function of geographical (altitude and distance from the sea) and climatic (accumulation rate) parameters; to identify enrichment or depletion processes, with respect to the sea-water composition, due to the extra contribution of other sources or caused by fractionating effects occurring during the transport or after the deposition; and to clarify the role of accumulation rate in preserving the Cl record in the snow layers.

2. Sampling and Analysis

Superficial snow and/or firn was collected from snow pits and shallow firn cores at more than 600 sites in the NVL– DC–DdU sector. The samplings were carried out during the field seasons from 1992/93 to 2001/02, in the framework of the International Trans-Antarctic Scientific Expeditions (ITASE), Station Concordia (French–Italian collaboration) and Programma Nazionale di Ricerche in Antartide (PNRA) programs.

Altogether, four shallow firn cores (5–50 m) were drilled, 27 snow pits (1.5–7 m) were hand-dug and 590 snow samples (1m integrated cores) were collected along coast– inland (Terra Nova Bay–DC and GV7–Talos Dome) and east–west (D66–Talos Dome) transects and at several sites located in NVL, within about 300km of the Italian base ‘Mario Zucchelli’. The sampling area is reported in Figure 1, where the main sampling sites (snow pits and firn core) are shown. Table 1 reports the geographical features of snow pits and shallow firn-core sampling sites. Details of sampling, depth resolution, storage and decontamination procedures have been reported previously (Reference Udisti, Bellandi and PiccardiUdisti and others 1998, Reference Udisti, Traversi, Becagli and Piccardi1999; Reference Fischer and WagenbachFrezzotti and Flora, 2002; Reference O’Dowd, Smith, Consterdine and LoweProposito and others, 2002; Reference BecagliBecagli and others, 2003, Reference Becagli2004, Reference Becagli2005). Samples were analyzed by ion chromatography for anion and cation content (Reference Gloersen and CampbellGragnani and others, 1998; Reference Udisti, Becagli, Castellano, Traversi, Vermigli and PiccardiUdisti and others, 2004a). Here, we discuss the spatial distribution of the depositional-flux mean values of sea-spray components (Na+, Mg2+ and Cl) at each site. The mean chemical composition was calculated on the most superficial firn-core and snow-pit samples, covering approximately the last 5 years of snow deposition. At Dome C, mean data were calculated on three snow pits, in order to obtain a larger significance at a site involved in deep ice-core drilling (EPICA community, 2004). Dome C mean values are related to snow layers younger than 1991 (about the first 0.5 m), as revealed by the sulfate peak of the Pinatubo (Philippines) eruption, recorded in the Antarctic ice sheet in 1991. The 1m integrated samples cover periods of about 5–20 years as a function of the site accumulation rate. The dating procedures were based on stratigraphies of seasonal markers (Reference TraversiUdisti, 1996; Reference Shaw, Oeschger and LangwayStenni and others, 2000), evaluated by the AD 1965–66 b-tritium peak or estimated from the accumulation rate/temperature and accumulation rate/latitude relationships (Reference Frezzotti and FloraFrezzotti and others, 2004; Reference Legrand and DelmasMagand and others, 2004).

Fig. 1. (a) Map of the sampled area. Squares are related to 1998/99 ITASE traverse; dots indicate snow-pit sites sampled during the 2001/02 ITASE traverse and in NVL region. Contour line interval is 250 m. (b) An enlarged scale of NVL.

Table 1. Geographical features of snow pits and firn-core sampling sites, sampling methods and year of sampling

3. Results and Discussion

3.1. Calculation of sea-salt and non-sea-salt fractions

Na+, Mg2+ and Cl have possible alternative sources with respect to sea spray. Na+ and Mg2+ also originate from crustal primary aerosol, and Cl is also supplied by adsorption processes of gaseous HCl, in turn mainly originating from acid–base chemical exchange between NaCl and HNO3 or H2SO4, into aerosol particles or directly on the snow surface (Reference Aristarain and DelmasAristarain and others, 1982; Reference Legrand and DelmasLegrand and Delmas, 1988; Reference FrezzottiGjessing, 1989; Reference Mouri, Nagao, Okada, Koga and TanakaMulvaney and others, 1993; Reference UdistiWagenbach and others, 1998; Reference Aristarain, Delmas and BriatAristarain and Delmas, 2002).

The Na+ sea-salt fraction (ssNa+) was evaluated by subtracting from the measured total Na+ concentration the crustal contribution, calculated on the basis of the Ca2+ content. Since Ca2+ also has marine and crustal sources, its non-sea-salt fraction (nssCa2+) must also be evaluated. We used a simple two-equation system to evaluate sea-spray and crustal contribution of Na+ and Ca2+:

where (Na+/Ca2+)crust is the mean ratio in the Earth’s crust and (Ca2+/Na+)seawater the mean ratio in bulk sea water. Ratios are expressed as weight on weight (w/w).

Though ssSO4 2– was also considered as a sea-spray component, we do not discuss it for two main reasons: (1) ssSO4 2– is a minor fraction of the sulfate budget at the great majority of the sampled sites (the major contribution comes from oceanic biogenic source of H2SO4; Reference BecagliBecagli and others, 2005); and (2) a large uncertainty affects the ssSO4 2– calculation because the bulk sea-water ratio usually used (SO4 2–/Na+ = 0.253 w/w) was changed in winter sea-spray aerosol by precipitation of mirabilite (Na2SO4), temperature-tuned (Reference UdistiWagenbach and others, 1998). In addition, Reference Rankin, Auld and WolffRankin and others (2002) supposed that the ice flowers formed on the sea-ice surface during its seasonal growth by volatilization– condensation processes, similarly weakened in SO4 2– with respect to sea-water bulk composition, could constitute a major source of sea-spray aerosol, at least for coastal sites and in the winter period. The contribution of mirabilite precipitation and ice-flower growth to the global budget of annual ssSO4 2– deposition in areas located at different distances from the sea and altitudes is difficult to quantify, making the uncertainties related to ssSO4 2– calculation too large.

Possible enrichments or depletions of Mg2+ and Cl, with respect to the bulk sea-water composition, are discussed in section 3.3, based on their linear regression with ssNa+ and on the trends of the Mg2+/ssNa+ and Cl/ssNa+ ratios.

3.2. Geographical variability

Figure 2 shows the ssNa+ percentage fraction as a function of distance from the sea for all the sampling sites. The sea-salt contribution to the total Na+ budget is always dominant (>80% for most sites). It is lower only at a few stations in NVL characterized by crustal contribution from ice-free areas of the Transantarctic Chain and at some sites in Wilkes Land affected by a particular atmospheric circulation pattern (Reference Legrand and DelmasMagand and others, 2004). Indeed, as a consequence of increasing remoteness, the percentage of ssNa+ shows a slight and noisy decreasing trend as distance from the sea increases.

Fig. 2. Percentage contribution of ssNa+ with respect to total Na+ reported as a function of distance from the sea.

In order to obtain a higher significance between the sea-spray snow content and the site location (altitude and distance from the sea), total depositional flux was here preferred to concentration, as a parameter less affected by the large changes in accumulation rate observed in the studied area. It is important to clarify that flux represents the total net deposition of an atmospheric component by both wet and dry removal processes. The flux is closely related to the atmospheric aerosol load only for components measured at sites where dry deposition is dominant (Mayewsky and Legrand, 1990; Reference UdistiUdisti and others, 2004b). Even so, the relative contribution of wet and dry deposition to the snow content of atmospheric components is valuable by a graphic elaboration, if a number of sites are sampled in the same area (Reference Kreutz, Mayewski, Pittalwala, Meeker, Twickler and WhitlowLegrand and Delmas, 1987; Reference FattoriFischer and Wagenbach, 1996). This approach applied to coastal and inner areas is described in section 3.5.

Total (wet and dry) depositional fluxes of Na+, Mg2+ and Cl are calculated by multiplying concentration (mgL–1) by the mean annual accumulation rate (kgm–2 a–1) at the sampling site:

Snow composition along the coast–inland transect reflects aerosol fractionating effects occurring during the transport of humid air masses from the oceanic source areas to the deposition site. In this way, the parameters mainly affecting the reconstruction of the load and chemical composition of the atmospheric aerosol are accumulation rate, altitude and temperature. In the NVL–DC–DdU area, these factors are interrelated: a positive relationship links distance from the sea and altitude, while these two parameters are inversely correlated to the temperature (Reference Frezzotti and FloraFrezzotti and others, 2004; Reference Legrand and DelmasMagand and others, 2004). We have chosen distance from the sea as the reference parameter in evaluating the sea-spray spatial trend in this Antarctic region.

The inverse correlation between sea-spray atmospheric load as revealed by the snow composition, and distance from the sea is clear in Figure 3. The sharpest decrease in depositional fluxes of Na+, Mg2+ and Cl occurs in the first 200km from the sea (see also smaller plots at lower y scale). From 500 km inland, the fluxes of the three components reach stable values about two orders of magnitude lower than those measured at the coastal sites. When plotted as a function of altitude (not shown), depositional fluxes show the most rapid decrease in the first 2000ma.s.l. This pattern is caused by the fast decrease in atmospheric load of aerosol particles mainly belonging to the super-micrometric classes, which undergo more efficient atmospheric scavenging processes by wet and dry deposition (short atmospheric residence time) (e.g. Reference Migliori, Becagli, Benassai, Fattori, Traversi, Udisti and ColacinoMinikin and others 1994; Reference Udisti, Bellandi and PiccardiUdisti and others, 1998; Reference BecagliBecagli and others, 2003; Reference Traversi, Becagli, Castellano, Largiuni, Udisti, Colacino and GiovannelliTraversi and others, 2004). Aerosol measurements at Terra Nova Bay (Reference Innocenti and ColacinoKerminen and others, 2000; Reference McInnes, Covert, Quinn and GermaniMigliori and others, 2002; Fattori and others, 2004, Reference Fattori, Bellandi, Benassai, Innocenti, Mannini, Udisti and Colacino2005) and at Dome C (Reference Udisti, Becagli, Castellano, Traversi, Vermigli and PiccardiUdisti and others, 2004a) show the sea-spray components are mainly distributed in the ‘coarse’ aerosol fraction, with a prevalent mode around 2.1–1.1 μ m (Reference Gragnani, Smiraglia, Stenni and TorciniInnocenti and others, 2004). The trends of sea-salt components shown in Figure 3 show that the first 200km from the sea, corresponding to a ~2000ma.s.l. altitude step, could be considered a critical threshold for sea-spray penetration in the central Antarctic area.

Fig. 3. Na+ (a), Mg2+ (b) and Cl (c) depositional fluxes as a function of distance from the sea. Smaller plots inside the panels show the same plot at lower y scale.

The decreasing pattern can be described by a power function. Figure 3 shows the curve-fits (correlation coefficient R = 0.68–0.71, n = 621). The constant flux values from 500km inland for Na+ and Mg2+ could support the hypothesis that background sea-spray aerosol reaches the central areas of Antarctica due to the long residence time in the atmosphere of sea-spray particles in the accumulation mode (~0.3–1 μm) (Reference RöthlisbergerShaw, 1989; Reference Mulvaney, Coulson and CorrMurphy and others, 1998).

The different pattern of chloride flux (Fig. 3c), which shows a continuous decreasing trend inland (although slightly less so with respect to the first 200 km step), can be explained by the increasing effects of post-depositional re-emission of HCl into the atmosphere as accumulation rate decreases, i.e. the distance from the sea increases (see section 3.4).

3.3. Correlations between Mg2+ or Cl– and ssNa+

To identify possible other sources of Mg2+ and Cl or fractionating effects of sea spray during the atmospheric transport, we plotted the Mg2+ and Cl concentrations vs the ssNa+ concentration and the Mg2+/ssNa+ and Cl/ssNa+ ratios as functions of ssNa+ flux and distance from the sea.

Figures 4a and 5a show good correlations between Mg2+ or Cl and ssNa+ at all sites (R = 0.966 and 0.984, respectively). The correlation is also seen at sites where sea-spray input is low. The linear regression slopes (0.120 for Mg2+, 1.77 for Cl) are very close to the bulk sea-water ratio (0.129 and 1.81 w/w, respectively), confirming that sea spray mainly drives the relationships.

Fig. 4. (a) Correlation plot between Mg2+ and ssNa+. (b, c) Mg2+/ Na+ ratio reported as a function of ssNa+ depositional flux (b) and distance from the sea (c). Lines in (b) and (c) represent the Mg2+/ Na+ ratio in sea water.

Fig. 5. Same as Figure 4, but for Cl instead of Mg2+.

While the Mg2+ vs ssNa+ regression line shows an insignificant y intercept (about 1 ppb), the Cl vs ssNa+ intercept value (about 23 ppb) shows that Cl in snow precipitation is supplied by other sources than sea spray, which becomes more and more evident as the sea-spray load decreases (low ssNa+ flux, i.e. when the distance from the sea increases; Fig 5b). Figure 4b and c show the pattern of the Mg2+/ssNa+ ratio as a function of the ssNa+ flux and distance from the sea. When ssNa+ decreases, i.e. at low sea-spray input, a large increase in data scattering is visible, but no particular trend is shown. This scattering is mainly attributed to the natural variability of the atmospheric transport of sea spray because the analytical reproducibility at ppb levels is better than 3% for both components (total reproducibility better than 10%). By plotting the Mg2+/ssNa+ ratio vs distance from the sea (Fig. 4c), the values stay near the bulk sea-water ratio (shown by the line at 0.129 w/w) without sharp shifts. We conclude that sea spray is the main source of Mg2+ at all stations and no significant fractionating effects occur during the transport or deposition processes. By contrast, Cl exhibits a clear, large enrichment, with respect to the bulk sea-water composition, as the sea-spray load decreases (Fig. 5b). The Cl/ssNa+ ratio reaches values up to five times higher than sea-water composition when the ssNa+ flux is lower than 2×104 μ g m–2 a–1. Figure 5c shows this enrichment is not abrupt, but progressively increases with distance from the sea. Therefore, other sources than sea spray for Cl deposition at inland Antarctic sites must be considered. As discussed in section 3.1, the major non-sea-salt source for Cl is the deposition of HCl, mainly formed by acid–base exchange between NaCl and atmospheric acidic species (H2SO4, HNO3). The contribution of this additional input becomes more evident when the main source (sea spray) is low.

3.4. Post-depositional re-emission of HCl

The increase of the Cl/ssNa+ ratio by the extra contribution of HCl deposition is evident only because we considered mean values calculated on the uppermost snow layers, related to the first 0.5–1.0m at sites with low accumulation rate. Actually, post-depositional processes leading to reemission into the atmosphere of HCl directly deposited on the snow surface, absorbed into cloud droplets or adsorbed on the snowflake and ice-crystal surface heavily affect chloride depth profiles. Snow acidity is the key parameter for preservation of chloride in the snow layers. Glacial snow, enriched in crustal particles, showed much higher chloride concentrations by transformation of volatile HCl in nonvolatile salts (Reference Rankin, Wolff and MartinRöthlisberger and others, 2002; Reference UdistiUdisti and others, 2004b). In the present condition, when Antarctic snow is acidic, especially at inland sites where H2SO4 and HNO3 dominate the ionic budget (Reference McInnes, Covert, Quinn and GermaniMigliori and others, 2002; Reference Udisti, Becagli, Castellano, Traversi, Vermigli and PiccardiUdisti and others, 2004a; Reference Fattori, Bellandi, Benassai, Innocenti, Mannini, Udisti and ColacinoFattori and others 2005), chloride is preserved only when the site accumulation rate is relatively high (Reference StenniTraversi and others, 2000). Here, we aim to evaluate the snow accumulation rate threshold able to preserve the original chloride deposition.

Figure 6 shows the depth profiles of the Cl/Na+ ratio in sites where high-resolution (2.5–5.0 cm) chemical analysis was carried out on snow-pit and firn-core samples. Accumulation rates range from 213 kgm–2 a–1 at D66 to 15 kgm–2 a–1 at M2. At the first three stations (D66, Evans Névé and Talos Dome), the Cl/Na+ ratio does not show a decreasing trend with depth. Their heavily smoothed trends (line in the plots) show constant values (1.5–2) close to the bulk sea-water composition (1.81 w/w). By contrast, the stations located far from the sea and/or characterized by low accumulation rates show a sharp decrease in the Cl/Na+ ratio from high superficial values (higher than sea water) to values equal to or lower than sea-water composition on the snow-pit bottom. In such stations, where sea-spray contribution is low, the HCl extra source to chloride is clearly seen in the uppermost layers (Cl/Na+ higher than sea water), but accumulation rate is not sufficient to preserve the HCl original deposition, leading to progressive loss of chloride. In addition, the high snow acidity, as revealed by ionic balances (Reference Udisti, Becagli, Castellano, Traversi, Vermigli and PiccardiUdisti and others, 2004a), causes chemical exchange between NaCl particles and acidic species (mainly H2SO4), leading to further loss of chloride (Cl/Na+ values lower than sea water).

Fig. 6. Cl/Na+ depth profiles from snow pits and firn core related to selected sites with different accumulation rates.

To evaluate the accumulation rate threshold for the chloride preservation, we plotted a Cl/Na+ decreasing index (D1) as a function of accumulation rate (Fig. 7). The Cl/Na+ is the decrease in Cl/Na+ ratio for each depth meter evaluated in the uppermost 2.0m snow layer. Cl/Na+ ratios were measured on the smoothed curves shown in Figure 6, in order to minimize the high background noise. Two simple linear regressions were drawn for high accumulation rates (where no re-emission occurs: Cl/Na+ DI = 0) and for low accumulation rates. The intercept between the regression lines roughly indicates the accumulation rate from which chloride is preserved in the snow in the present acidic conditions. This threshold is about 80 kgm–2 a–1 for the NVL–DC–DdU sector.

Fig. 7. Cl/Na+ decreasing index as a function of accumulation rate. See text for calculation of decreasing index.

3.5. Wet and dry contributions to net depositional fluxes

The net depositional flux of sea spray is the result of three contributions (Reference FattoriFischer and Wagenbach, 1996):

(1)

where F d and F w indicate the dry and wet deposition fluxes and DF accounts for post-depositional processes (such as reemission into the atmosphere of chloride). The term ΔF can be disregarded for Na+ and Mg2+ because they are irreversibly deposited in the snow. It also plays a minor role for chloride if only very superficial layers are considered. Therefore, for the uppermost layers we have:

(2)

where C p is the concentration associated with the wet deposition and A is the accumulation rate. By assuming C p constant for precipitation occurring along coast–inland pathways (e.g. Reference FattoriFischer and Wagenbach, 1996), a linear regression should be obtained when plotting F tot as a function of accumulation rate; the y intercept (flux axis) of such a regression line represents the contribution of the dry deposition.By plotting Na+, Mg2+ and Cl total fluxes as a function of accumulation rate (Fig. 8a–c), an exponential regression seems better able to represent the trends of NVL–DC and sectors (see Fig. 8a–c).

Fig. 8. Na+ (a), Mg2+ (b) and Cl (c) depositional fluxes as a function of accumulation rate: grey dots represent Wilkes Land sites and dark dots represent NVL sites. Smaller plots inside show the same plot at lower y scale.

(3)

where F is the values of F tot when A = 0 and can be considered as the dry deposition flux and K is a term related to the wet deposition which is different as a function of airmass transport pathway.

The highest fluxes measured at the NVL coastal sites indicate a larger sea-spray input than that occurring in WL at similar accumulation rate. This is due to the different atmospheric circulation pattern in the two sectors: NVL is affected by air masses coming directly from the Ross Sea, whereas Wilkes Land presents more continental conditions and low intrusion of atmospheric moisture correlated with the persistence of higher atmospheric pressure in the central areas of the Wilkes Land transect (Reference Legrand and DelmasMagand and others, 2004). The exponential regressions drawn in Figure 8a–c improve the estimation of the y-intercept values, allowing evaluation of the dry contribution (when accumulation rate tends to zero). Unlike that observed for H2SO4 and MSA (Reference BecagliBecagli and others, 2005), dry deposition seems to play a minor role in sea-spray scavenging processes. This means that, in the Ross Sea and Pacific Ocean sectors, wet removal accounts for the majority of sea-spray deposition both in coastal and in inland areas.

4. Conclusions

The spatial distribution of sea-spray components (Na+, Mg2+ and Cl) in superficial snow was evaluated by site-mean values calculated at 621 stations located in the NVL–DC– DdU sector. ssNa+ is the dominant fraction of the Na+ budget, contributing more than 80% at the majority of sites. Sea-spray depositional fluxes are heavily controlled by distance from the sea and altitude. The first 200km from the sea and the initial altitude step of 2000ma.s.l. constitute threshold values in pulling down the sea spray by wet deposition. Areas beyond these limits show depositional fluxes of Na+, Mg2+ and Cl about 100 times lower. Sea spray is also the dominant source for Mg2+ for inland sites, as shown by the Mg2+ vs ssNa+ linear regression and by the trends of the Mg2+/Na+ ratio as a function of ssNa+ flux or distance from the sea. Chloride shows other sources than sea spray. In spite of a good linear correlation with ssNa+, the high intercept of the linear regression and the trend of the Cl/ssNa+ ratio as ssNa+ flux decreases show that HCl is deposited in the snow layers at the same time as NaCl. The HCl contribution becomes progressively more evident as distance from the sea increases (especially from 200km inland). The Cl/ssNa+ ratio reaches values more than five times higher than sea-water composition at the inland Antarctic sites, but chloride preservation is heavily controlled by the accumulation rate in the present snow acidity conditions. Sites where accumulation rate is higher than 80 kgm–2 a–1 do not show post-depositional losses of chloride. Re-emission of HCl into the atmosphere from the superficial snow layers is negatively correlated to accumulation rates lower than 80 kgm–2 a–1, as revealed by the Cl/ssNa+ decreasing index.

Wet deposition dominates sea-spray atmospheric scavenging both in coastal and in inland Antarctic areas. A different pattern in the scavenging rate as a function of accumulation rate is shown in NVL and Wilkes Land sectors due to the different atmospheric circulation pathways.

More knowledge of present transport, depositional and post-depositional sea-spray processes in central Antarctic regions is fundamental to the correct interpretation of paleoenvironmental and paleoclimatic records from deep ice cores.

Acknowledgements

This research was financially supported by the Ministero Istruzione, Università, Ricerca (MIUR)–PNRA program, through a cooperation agreement between the PNRA consortium and Milano-Bicocca and Venice Universities in the framework of the ‘Glaciology’ and ‘Environmental Contamination’ projects. This work is an Italian contribution to the ITASE project. It is an associate program of the ‘European Project for Ice Coring in Antarctica’ (EPICA), a joint European Science Foundation/European Commission (EC) scientific program, funded by the EC and by national contributions from Belgium, Denmark, France, Germany, Italy, the Netherlands, Norway, Sweden, Switzerland and the United Kingdom. This is EPICA publication No. 113.

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