- Email: [email protected]
Antarctic sea-ice zone research during the International Polar Year, 2007–2009
Deep-Sea Research II 58 (2011) 993–998
Contents lists available at ScienceDirect
Deep-Sea Research II journal homepage: www.elsevier.com/locate/dsr2
Introduction
Antarctic sea-ice zone research during the International Polar Year, 2007–2009
1. Introduction The International Polar Year 2007-09 provided a unique opportunity for Antarctic sea-ice research, with near-coincident research voyages by the US and Australian programs to the Bellingshausen Sea and East Antarctica, respectively, during Spring 2007. The research activities of the two programs were broadly similar insomuch as they focused on characterising the physical, biological and biogeochemical characteristics of the pack-ice zone, and their interactions. Both programs employed a range of measurement techniques for in situ and underway observations, and also focused on a calibration/validation program for NASA’s Ice Cloud and land Elevation Satellite (ICESat), which rescheduled its Geoscience Laser Altimeter System (GLAS) operations to coincide with the timing of the field campaigns. The US program was conducted aboard the research vessel Nathaniel B. Palmer, which operated in the Bellingshausen Sea (80-1201W) between 24 September and 27 October (Fig. 1), while the Australian program operated from the research and supply vessel Aurora Australis in the region 115-1301E between 9 September and 11 October, 2007 (Fig. 2). Hence the voyages overlapped in the sea-ice zone for a period of several weeks, and provided a unique opportunity to examine regional differences in sea-ice conditions. The ice conditions and experimental design of the two programs varied in a number of important ways. The Bellingshausen Sea ice zone is considerably farther south and comprised of thicker, more compact, less mobile, first-year sea ice than the East Antarctic sea-ice zone, which is characterized by thinner, although often highly deformed ice with less snow cover but higher drift rates. The contrast in the two ice regimes has led to some valuable assessments of the relative importance of different processes. In particular, the relationship between ice and snow thickness varies between the two study regions. Negative ice freeboards were common in both east and west Antarctica, as was the formation of flooded layers and snow ice; however an empirical relationship equating mean freeboard to mean snow thickness appears to hold generally for west Antarctica, but not for the heavily ridged areas in east Antarctica. The regional differences in sea-ice and snow thickness distributions yield different empirical relationships for converting satellite-derived snow freeboard to ice thickness, and these results have been used to ground-truth satellite laser altimeter data from NASA’s Ice Cloud and land Elevation Satellite (ICESat). Differences in the seaice physical parameters were also reflected in the biomass distribution of ice algae. Ice algal biomass was moderate to high 0967-0645/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2011.01.001
in the Bellingshausen Sea, with maximum concentrations occurring in the sea-ice interior (Fritsen et al., 2011). In contrast, ice algal biomass off East Antarctic was much lower and elevated algal concentrations were restricted to the bottom layers of the sea-ice floes (Meiners et al. 2011; van der Merwe et al. 2011). The other major difference between the two field campaigns is that the SIMBA program focused the bulk of its time on a single ice floe, effectively conducting a long-term, Lagrangian drift experiment observing temporal changes in ice conditions, whereas the SIPEX program conducted a series of 15 shorterterm ice stations covering a wider range of ice conditions. The scientific goals and summary of achievements of each program are described in the sections below. In keeping with the goals of IPY to promote science education and outreach, both the SIMBA and SIPEX programs had teachers onboard to learn about the science and report on their experiences. Websites updated from both ships were enthusiastically followed by a broad cross-section of school students, staff and the general public.
2. Sea Ice Mass Balance in the Antarctic (SIMBA) After a four-day transit through the sea-ice cover, a drifting station, Ice Station Belgica, was established and occupied for 25 days (Lewis et al., 2011). The Nathaniel B. Palmer berthed in a ‘hot mooring’, with no physical attachment to the ice floe, depending on the surrounding ice cover, wind direction, and some use of thrusters to maintain position on the floe during operations on the sea ice. Typically the vessel stood off the floe overnight conducting CTD operations, then reconfigured in the mooring position in the early morning to support on-ice work. The first three days of the ice station were confined to the vessel due to blizzard white-out conditions. When the weather cleared the floe was surveyed and two sites, Brussels and Liege, were designated for the ice biogeochemical sampling. To maintain uncontaminated conditions, the sites were placed, on appropriate level ice, at distances of 0.8 and 1.2 km from the ship. Protocols were established so that quadrants would be sampled in a particular order during the period of the ice station. The sites were flagged as clean areas so that they would only be sampled or visited by the designated personnel in the appropriate order for sampling. A five-day cycle of sampling and lab measurements was established and successfully executed over five cycles. Three tower sites were also established on the floe and three ice mass balance buoys installed adjacent to the biogeochemical sites (Lewis et al.,
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Introduction / Deep-Sea Research II 58 (2011) 993–998
Fig. 1. Ship’s track for SIMBA . (from Lewis et al., 2011, Fig. 1)
114°
116°
118°
120°
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Southbound ice edge
−62°
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ASPeCt ice thickness [m] 5.0 Leg
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rg
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Fig. 2. Ship’s track for SIPEX . (from Worby et al., 2011, Fig. 1a)
1.5 1.0
& 116°
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)
be ce ni e lto ngu to
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Introduction / Deep-Sea Research II 58 (2011) 993–998
2011). Two of the tower sites measured snow particle fluxes at various heights. One tower measured turbulent atmospheric fluxes and additional measurements were made of albedo during the five cycles at each site. The three ice mass balance buoys described above were intended to bridge the gaps in physical properties of the ice (temperature, ice thickness change, snow depth change, light penetration, heat flux to the base) between the sampling cycles. The mass balance program was complemented by an investigation of the bio-optical properties of the sea ice including the measurements of the transmission of photosynthetically active radiation and absorption by sea-ice particulates (microalgae and detritus) (Fritsen et al., 2011). At three other locations, the geophysics program established traverse lines and conducted between one and three cycles of measurements on snow depths, ice thickness by drilling, ice thickness gauges, snow properties, and concurrent measurements of thickness using an EMI instrument in a hand-held mode (Weissling et al., 2011; Xie et al., 2011). For the time series sampling at an ice drifting station for the biochemistry and biology, all of the original objectives were satisfied. With a prior arrangement, NASA agreed to reschedule its GLAS campaign to coincide (Oct 3 to Nov 5 2008) with the surface truth measurements of sea ice elevation and thickness that we were to obtain from the cruise (Xie et al., 2011). Comparison of the drift and deformation characteristics from buoys to high-resolution radar satellite measurements were also planned, with acquisition of 13 RadarSat images planned over the region of measurements. An acquisition of European Space Agency radar data from EnviSat was also planned with project participants as co-investigators on an ESA proposal to obtain and archive that imagery for post processing. These imagery acquisition activities were all carried out successfully (Ozsoy-Cicek et al., 2011). Underway measurements were also an important element of the voyage. We reached the sea-ice zone of the Bellingshausen Sea near Peter I Island after transiting for four days (Sept 24 2007). During the open-water transit we deployed 15 Argo drifters (accompanied by an XBT cast) and ten SOLO drifters. Three short ice stations were conducted for geophysical and biogeochemical sampling, and CTD casts during the three-day transit to the main Ice Station location. ASPeCt visual ice observations were conducted hourly in the sea-ice zone (Ozsoy-Cicek et al., 2011), supported by downward and side-looking cameras (continuous imagery) and electromagnetic induction measurements for ice thickness. Marine mammal and seabird observations were also maintained around the clock during both the open-water and sea-ice transits. With the shortened time for the cruise, due to a fire in the biology laboratory that forced a return to port early in the voyage, it was only possible to conduct CTD and trace-metal casts on the transit back to Punta Arenas, without ice station work. The shortened voyage also impacted on the longevity of the drifting buoys deployed on the sea ice, since it was originally proposed they would be deployed in the Amundsen Sea, where ice conditions are more suitable for long-term deployments (Lewis et al., 2011). For decades sea ice was assumed to be an impermeable and inert barrier for air-sea exchange of CO2 so that global climate models do not include CO2 exchange between the oceans and the atmosphere in the polar region. However, uptake of atmospheric CO2 over sea-ice cover has been recently reported raising the need to further investigate pCO2 dynamics in the marine cryosphere realm and related air-ice CO2 fluxes. In the course of the biogeochemistry cycles experiment on SIMBA, we experienced a full cycle of cooling and warming of the air temperature with large changes in the snow-cover. Temperature and snow cover changes affected brine salinities, drastically increasing the instability of the brine column in the initial isothermal and
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porous sea ice. Cooling of the surface layer significantly increased the surface layer brine salinity, triggering a downwards transfer of brines into the underlying porous ice (Lewis et al., 2011).This downward transfer was likely counterbalanced by upwards (or lateral) transfer of sea water into the ice, a flooding-like process. While the sea-ice cover was under-saturated in CO2 with respect to the atmosphere, convective processes significantly affected the partial pressure of CO2 (pCO2) of the brines, promoting the increase of pCO2 and reducing the magnitude of related air-ice CO2 transfer. Dissolved iron profiles were obtained from the Trace Metal casts on the expedition along the continental slope (water depths 1680-3050 m) and close to Peter I Island (water depth 1000 m). We observed smooth nutrient type DFe profiles, which increased from 0.6-0.6 nM to 1.1-1.2 nM in deep water as a function of distance to the continental shelf and water depth. Near Peter I Island there was 10 nM DFe in the upper 200 m and 2-3 nM below to the sea floor. We conclude that not only the numerous icebergs and melting sea ice are significant pelagic iron sources in the region to support primary production, but also continental and island shelves can contribute significantly.
3. Sea-Ice Physics and Ecosystem eXperiment (SIPEX) The Sea Ice Physics and Ecosystem eXperiment (SIPEX) was a multi-disciplinary science program focused on sea-ice research in the region 115-1301E. The objectives of the voyage were to investigate the relationships between the physical sea-ice environment and the sea ice biology, in particular the presence of algae and krill under the ice. A range of novel instruments and techniques were used, including for the first time, radar and laser altimetry from helicopter for measuring the snow thickness and freeboard height above sea level. An instrumented Remotely Operated Vehicle (ROV) was used for under-ice observations, a custom-build trawl was deployed for catching krill under the ice (O’Brien et al., 2011), and trace-metal clean sampling equipment was developed to determine the concentration and distribution of iron in sea ice (van der Merwe et al., 2011). An overarching goal of the project was to understand the links between sea ice physical, chemical and biological parameters and their importance for seaice zone productivity in the Australian Antarctic Territory off East Antarctica. The ship departed Hobart on 4 September 2007 and steamed directly to the ice edge at approximately 621 300 S, 1281 000 E. We then steamed south through the pack ice to the fast ice adjacent to the Antarctic coast, then west through the persistent flaw lead past the Dalton Iceberg tongue. At approximately 1191E we began heading north again, encountering particularly heavy ice and poor weather. Our track was generally in a NW direction towards the ice edge at 1161E. We then steamed east to 1201E while remaining about 60-100 miles south of the ice edge, before finally heading NE to Hobart. We departed the ice edge at 13:00 local time on 11 October at 621 030 S, 1241 250 E, returning to Hobart on 17 October. The ice conditions encountered during the voyage were particularly difficult at times, not only for navigation but for conducting scientific work on the ice. A strong shear zone at the southern end of the first transect, adjacent to the Dalton Iceberg Tongue, was associated with particularly deformed ice that prevented us making any further progress south to the fast ice. In general, we had difficulty finding floes that were suitable to work on, and helicopter reconnaissance flights were particularly important for navigating through the heaviest ice, which in localized areas was up to 10 m thick and some of the thickest reported in the East Antarctic region. The weather conditions were unfavourable at times, with three blizzards hitting the ship
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Introduction / Deep-Sea Research II 58 (2011) 993–998
drilling. These included detailed assessments of snow stratigraphy, snow type classification, and measurement of snow vertical temperature profile, grain size, density and wetness (using a dielectric probe). Snow samples were melted for analysis of salinity (range 0 to 41.3 ppt) and stable isotope (d18O) signature (the latter using samples returned to Australia). Snow density ranged from 180 kg m 3 for newly-fallen snow to 638 kg m 3 for wetted basal snow. Snow average grain size (estimated as a function of snow type) ranged from 0.2 mm for newly fallen snow to 5 mm for depth hoar. Overall, the snow covers sampled were relatively homogeneous, being largely composed of wind packed snow and depth hoar (with relatively few internal icy layers and well developed depth hoar). Full-thickness ice cores were collected with a 9-cm diameter coring auger and ice temperatures were measured in these at 5- or 10-cm intervals as soon as they were extracted. The cores were analysed onboard the ship for visual stratigraphy and salinity, and samples were collected for stable isotope analysis in Australia. Salinity and isotope samples were taken approximately every 5 cm, cut according to the structural observations. A comprehensive data set of oceanographic as well as chemical and biological sea-ice parameters was collected. Under-ice water mass properties and current profiles were measured during major ice stations and used for determination of winter mixed-layer depths and the location of major oceanographic features (Williams et al., 2011). Ice coring on a trace-metal-clean sampling site was carried out to determine sea-ice dissolved and particulate iron fractions. Comparison of pack ice and one fast-ice station revealed strong differences in the iron fractionation with possible implications for iron release into the water column during ice melt (van der Merwe et al., 2011). Sampling during SIPEX showed that sea ice also contained high concentrations of coloured dissolved organic carbon that may affect optical properties in the pelagic realm during the sea-ice melt season (Norman et al., 2011). The results highlight the role of sea ice as a temporal reservoir for iron and dissolved organic carbon during the Antarctic winter and early spring. Sea ice algal biomass was primarily controlled by sea ice porosity affecting nutrient availability of the ice algal communities
during the voyage. The third of these resulted in four consecutive days of white-out conditions that prevented flying operations at a time when they were needed most for the ICESat satellite validation program, and thus precluded making coincident airborne and satellite altimetry measurements. In total, 15 ice stations were conducted in addition to underway sampling and aircraft-based observations from helicopters. Some of the ice stations were completed within one day, while at some locations we stayed for two days to allow measurements to be collected over a full diurnal cycle. A number of ice stations were also just a couple of hours, with the primary aim of collecting ice cores with high algal biomass for the various biology groups onboard. Table 1 provides a summary of icestation locations. Ice and snow thickness and properties measurements were made along transects at eleven ice stations. These measurements provide the basic data for assessing ice characteristics, and for validating and ‘‘calibrating’’ other techniques including airborne and surface snow radar, inductive electromagnetic, ROV sonar and ASPeCt ship-based observations. At each ice stations, a straight transect was laid out across the floe with a survey tape. This was usually 200 m in length, although some shorter lines were measured and additional cross lines were also measured at two sites. Snow thickness and ice/ snow interface temperatures were measured using arrow shafts with temperature sensors in the head inserted into the undisturbed snow at 1-m intervals along the line. Ice thickness, ice freeboard height, snow thickness and snow freeboard height were measured at the same 1-m intervals through holes drilled through the ice, and from these data Worby et al. (2011) calculated the relationship between surface elevation and snow thickness. This was also calculated using the SIMBA data by Xie et al. (2011) and the two results show how regionally variable the ice conditions are, and the importance of taking account of such variability when calculating sea ice thickness from altimetry data, whether from aircraft or satellite. Snow pit and ice core measurements were made at several locations along the transect line. Snow pit measurements (at 50 snow pits in total) were made before the ice was penetrated by
Table 1 Summary of SIPEX ice station locations. Stn #
Lat dd mm
Lon ddd mm
Date 2007
Transect length (m)
Snow pits (no.)
Ice cores (no.)
Ice thick av/stdev (m) a
Snow av/stdev (m) a
Note
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
64 64 64 65 65 65 65 65 65 64 65 64 64 64 64
127 128 127 125 124 122 121 118 118 119 117 116 116 116 120
11-Sep 12-Sep 14-Sep 17-Sep 18-Sep 21-Sep 22-Sep 25-Sep 28-Sep 30-Sep 3-Oct 5-Oct 6-Oct 7-Oct 10-Oct
100 200 100 – 200 200 – 100 50 200 100 – 200 200 –
3 5 3 1 5 5 1 5 1 5 3 – 6 7 –
1 3 3 – 3 2 – 4 – 3 5 1 3 3 1
0.81/0.54 1.03/0.37 0.99/0.44 – 0.71/0.13 1.15/0.67 – 0.59/0.43 2.22/0.82 0.70/0.46 1.06/0.32 – 1.04/0.45 1.14/0.76 –
0.18/0.10 0.20/0.13 0.17/0.14 – 0.09/0.06 0.02/0.04 – 0.08/0.04 0.44/0.22 0.09/0.07 0.22/0.14 – 0.15/0.11 0.22/0.13 –
b
13 29 24 06 31 35 34 33 20 56 01 53 44 19 41
56 03 08 40 45 35 30 52 34 09 32 58 49 49 37
Notes: (a) Average and standard deviation ice and snow thickness are for main transect only. (b) Ice thickness was measured to 100 m only, while snow thickness and ice surface temperature was measured to 200 m. (c) 2 100-m cross-section transects also measured for snow thickness/temperature only.
c
Introduction / Deep-Sea Research II 58 (2011) 993–998
(van der Merwe et al., 2011). Generally cold temperatures, high brine salinities and low brine volumes limited elevated ice-algal concentrations to the warmer and more porous sea-ice layers at the ice-water interface (Meiners et al., 2011). Ice algal biomass was positively correlated with sampling date demonstrating that SIPEX captured the onset of the spring ice-algal bloom when biomass accumulates with time. Kramer et al. (2011) sampled seaice meiofauna communities and report generally low metazoan biomass and low biodiversity in East Antarctic sea ice when compared to late winter-early spring samples from other sectors of the Southern Ocean. Sea-ice communities can serve as an important food source for Antarctic krill (Euphausia suberba), and the distribution of krill is closely linked to sea-ice extent in many regions of the Southern Ocean (e.g., Atkinson et al., 2004; Nicol et al., 2000). O’Brien et al. (2011) report major differences in growth, diet and condition of larval and postlarval krill sampled from open water and the under-ice environments, indicating that different over-wintering strategies are used by different life-cycle stages, and highlighting the role of sea-ice biota as food source for krill larvae off East Antarctica. Their findings were supported by under-ice observations with an ROV showing juvenile krill feeding at the subsurface of the sea ice and in cracks in areas of rafted ice at many of the SIPEX stations. Combined the multi-disciplinary sampling program highlighted the structuring role of sea ice in East Antarctic marine ecosystem function and biogeochemical cycling. Two meso-scale buoy arrays were also deployed as part of SIPEX to improve our understanding of the drift and dynamics of the sea ice. Deployment locations were at the southern end of the SIPEX meridional transects at 1281 and 1161E. Both arrays were deployed just to the north of the narrow (but strong) shear zone between the fast ice adjacent to the Antarctic continent, and the bulk of the westward current. The arrays comprised five buoys, four of which formed a square with 30-km sides, with the fifth in the middle. These deployment configurations were chosen to be comparable with the grid resolution used in current modelling studies. Hourly GPS positions were received from all buoys, plus atmospheric pressure and air temperature from one in each array. Finally, airborne laser altimetry was conducted from a helicopter using a Riegl 2D laser scanner LMS-Q240i. The data collected on this voyage enabled aircraft measurements to be validated over drilled thickness lines on the ice, as described in Worby et al. (2011), while at the same time improving our knowledge of the issues surrounding long-base-line GPS positioning that complicate the data analysis. Subsequent measurements in the Antarctic sea-ice zone using the same airborne system have significantly improved positioning and surface elevation accuracies and provide the necessary data for satellite altimetry cal/val.
4. Summary It is clear from recent studies that the distribution of sea ice around Antarctica has changed in recent decades. While a net increase of 1.2% per decade in extent has been reported between 1979 and 2008 (Comiso and Nishio, 2008), far greater regional changes have occurred. In the Bellingshausen Sea some regions now experience an annual sea-ice season that is 3 months shorter than in 1979, whereas parts of the Ross Sea have an annual sea ice season that is 2 months longer (Parkinson, 2004). Critical to understanding behavior in sea-ice thickness, and the potential impacts of this on both the physical and biological Southern Ocean environment, are field studies that measure and monitor critical sea-ice processes, and provide the necessary ground validation data for hemispheric to global-scale satellite monitoring.
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The SIMBA and SIPEX field campaigns had this as a key goal, encompassing a broad range of experiments that have yielded valuable insights to the sea-ice environment in western and eastern Antarctica respectively. The papers presented in this volume present the results of a suite of highly multi-disciplinary and highly collaborative field experiments conducted as part of the International Polar Year 2007-09.
Acknowledgments These two major ship-based field experiments to the Antarctic sea-ice zone could not have been achieved without the support of the US National Science Foundation (SIMBA) and the Australian Antarctic Division and Antarctic Climate and Ecosystems Cooperative Research Centre (SIPEX). Our thanks go to the Captains, crew and support staff of both the Nathaniel B. Palmer and the Aurora Australis, and to the many land-based support staff involved in logistics planning. This work was supported in part by the Australian Government through the Antarctic Climate and Ecosystems Cooperative Research Centre.
References Atkinson, A., Siegel, V., Pakhomov, E., Rothery, P., 2004. Long-term decline in krill stock and increase in salps within the Southern Ocean. Nature 432, 100–103. Comiso, J.C., Nishio, F., 2008. Trends in the sea ice cover using enhanced and compatible AMSR-E, SSM/I and SMMR data. Journal of Geophysical Research 113, C02S07. doi:10.1029/2007JC004257. Fritsen, C.H., Wirthlin, E.D., Momberg, D.K., Lewis, M.J., Ackley, S.F., 2011. Biooptical properties of Antarctic pack in the early spring. Deep-Sea Research II 58 (9–10), 1052–1061. Kramer, M., Swadling, K.M., Meiners, K.M., Kiko, R., Scheltz, A., Nicolaus, M., Werner, I., 2011. Antarctic sympagic meiofauna in winter: comparing diversity, abundance and biomass between perennially and seasonally ice-covered regions. Deep-Sea Research II 58 (9–10), 1062–1074. Lewis, M.J., Tison, J.L., Weissling, B., Delille, B., Ackley, S.F., Brabant, F., Xie, H., 2011. Sea ice and snow cover characteristics during the winter–spring transition in the Bellingshausen Sea: an overview of SIMBA 2007. Meiners, K.M., Norman, L., Granskog, M.A., Krell, A., Heil, P., Thomas, D.N., 2011. Physico-ecobiogeochemistry of East Antarctic pack ice during the winterspring transition. Deep-Sea Research II 58 (9–10), 1172–1181. Nicol, S., Pauly, T., Bindoff, N.L., Wright, S., Thiele, D., Woehler, E., Hosie, G., Strutton, P., 2000. Ocean circulation off East Antarctica affects ecosystem structure and sea-ice extent. Nature 406, 504–507. Norman, L., Thomas, D.N., Stedmon, C.A., Granskog, M.A., Papadimitriou, S., Krapp, R.H., Meiners, K.M., Lannuzel, D., vanderMerwe, P., Dieckmann, G.S., 2011. The characteristics of dissolved organic matter (DOM) and chromophoric dissolved organic matter (CDOM) in Antarctic sea ice. Deep-Sea Research II 58 (9–10), 1075–1091. O’Brien, C., Virtue, P., Kawaguchi, S., Nichols, P.D., 2011. Aspects of krill growth and condition during late winter-early spring off East Antarctica (110–130E). Deep-Sea Research II 58 (9–10), 1121–1221. Ozsoy-Cicek, Burcu, Kern, Stefan, Ackley, Stephen, F., Xie, Hongjie, Tekeli, Ahmet, E., 2011. Intercomparisons of Antarctic sea ice types from visual ship, RADARSAT-1 SAR, Envisat ASAR, QuikSCAT, and AMSR-E satellite observations in the Bellingshausen Sea. Deep-Sea Research II 58 (9–10), 1092–1111. Parkinson, C., 2004. Southern Ocean sea ice and its wider linkages: insights revealed from models and observations. Antarctic Science 16 (4), 387–400. doi:10.1017/S0954102004002214. van der Merwe, P., Lannuzel, D., Bowie, A.R., Mancuso Nichols, C.A., Meiners, K.M., 2011. Iron fractionation in pack and fast ice in East Antarctica: temporal decoupling between the release of dissolved and particulate iron during spring melt. Deep-Sea Research II 58 (9–10), 1222–1236. Weissling, B.P., Lewis, M.J., Ackley, S.F., 2011. Sea ice thickness and mass at Ice Station Belgica, Bellingshausen Sea, Antarctica. Deep-Sea Research II 58 (9–10), 1112–1124. Williams, G.D., Meijers, A.J.S., Poole, A., Mathiot, P., Tamura, T., Klocker, A., 2011. Late winter oceanography off the Sabrina and BANZARE coast (117–1281E), East Antarctica. Deep-Sea Research II 58 (9–10), 1194–1210. Worby, A.P., Steer, A., Lieser, J.L., Heil, P., Yi, D., Markus, T., Allison, I., Massom, R.A., Galin, N., Zwally, H.J., 2011. Regional-scale sea ice and snow thickness distributions from in situ and satellite measurements over East Antarctica during SIPEX 2007. Deep-Sea Research II 58 (9–10), 1125–1136. Xie, H., Ackley, S.F., Yi, D., Zwally, H.J., Wagner, P., Weissling, B., Lewis, M., Ye, K., 2011. Sea ice thickness distribution of the Bellingshausen Sea from surface measurements and ICESat altimetry. Deep-Sea Research II 58 (9–10), 1039–1051.
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Introduction / Deep-Sea Research II 58 (2011) 993–998
A.P. Worby n Australian Antarctic Division, Channel Highway, Kingston 7050, Tasmania, Australia
E-mail address: [email protected]
K.M. Meiners Australian Antarctic Division, Channel Highway, Kingston 7050, Tasmania, Australia
n
Corresponding author.
S.F. Ackley Department of Geological Sciences, University of Texas at San Antonio, One UTSA Circle, San Antonio, TX 78249, USA
Contents lists available at ScienceDirect
Deep-Sea Research II journal homepage: www.elsevier.com/locate/dsr2
Introduction
Antarctic sea-ice zone research during the International Polar Year, 2007–2009
1. Introduction The International Polar Year 2007-09 provided a unique opportunity for Antarctic sea-ice research, with near-coincident research voyages by the US and Australian programs to the Bellingshausen Sea and East Antarctica, respectively, during Spring 2007. The research activities of the two programs were broadly similar insomuch as they focused on characterising the physical, biological and biogeochemical characteristics of the pack-ice zone, and their interactions. Both programs employed a range of measurement techniques for in situ and underway observations, and also focused on a calibration/validation program for NASA’s Ice Cloud and land Elevation Satellite (ICESat), which rescheduled its Geoscience Laser Altimeter System (GLAS) operations to coincide with the timing of the field campaigns. The US program was conducted aboard the research vessel Nathaniel B. Palmer, which operated in the Bellingshausen Sea (80-1201W) between 24 September and 27 October (Fig. 1), while the Australian program operated from the research and supply vessel Aurora Australis in the region 115-1301E between 9 September and 11 October, 2007 (Fig. 2). Hence the voyages overlapped in the sea-ice zone for a period of several weeks, and provided a unique opportunity to examine regional differences in sea-ice conditions. The ice conditions and experimental design of the two programs varied in a number of important ways. The Bellingshausen Sea ice zone is considerably farther south and comprised of thicker, more compact, less mobile, first-year sea ice than the East Antarctic sea-ice zone, which is characterized by thinner, although often highly deformed ice with less snow cover but higher drift rates. The contrast in the two ice regimes has led to some valuable assessments of the relative importance of different processes. In particular, the relationship between ice and snow thickness varies between the two study regions. Negative ice freeboards were common in both east and west Antarctica, as was the formation of flooded layers and snow ice; however an empirical relationship equating mean freeboard to mean snow thickness appears to hold generally for west Antarctica, but not for the heavily ridged areas in east Antarctica. The regional differences in sea-ice and snow thickness distributions yield different empirical relationships for converting satellite-derived snow freeboard to ice thickness, and these results have been used to ground-truth satellite laser altimeter data from NASA’s Ice Cloud and land Elevation Satellite (ICESat). Differences in the seaice physical parameters were also reflected in the biomass distribution of ice algae. Ice algal biomass was moderate to high 0967-0645/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2011.01.001
in the Bellingshausen Sea, with maximum concentrations occurring in the sea-ice interior (Fritsen et al., 2011). In contrast, ice algal biomass off East Antarctic was much lower and elevated algal concentrations were restricted to the bottom layers of the sea-ice floes (Meiners et al. 2011; van der Merwe et al. 2011). The other major difference between the two field campaigns is that the SIMBA program focused the bulk of its time on a single ice floe, effectively conducting a long-term, Lagrangian drift experiment observing temporal changes in ice conditions, whereas the SIPEX program conducted a series of 15 shorterterm ice stations covering a wider range of ice conditions. The scientific goals and summary of achievements of each program are described in the sections below. In keeping with the goals of IPY to promote science education and outreach, both the SIMBA and SIPEX programs had teachers onboard to learn about the science and report on their experiences. Websites updated from both ships were enthusiastically followed by a broad cross-section of school students, staff and the general public.
2. Sea Ice Mass Balance in the Antarctic (SIMBA) After a four-day transit through the sea-ice cover, a drifting station, Ice Station Belgica, was established and occupied for 25 days (Lewis et al., 2011). The Nathaniel B. Palmer berthed in a ‘hot mooring’, with no physical attachment to the ice floe, depending on the surrounding ice cover, wind direction, and some use of thrusters to maintain position on the floe during operations on the sea ice. Typically the vessel stood off the floe overnight conducting CTD operations, then reconfigured in the mooring position in the early morning to support on-ice work. The first three days of the ice station were confined to the vessel due to blizzard white-out conditions. When the weather cleared the floe was surveyed and two sites, Brussels and Liege, were designated for the ice biogeochemical sampling. To maintain uncontaminated conditions, the sites were placed, on appropriate level ice, at distances of 0.8 and 1.2 km from the ship. Protocols were established so that quadrants would be sampled in a particular order during the period of the ice station. The sites were flagged as clean areas so that they would only be sampled or visited by the designated personnel in the appropriate order for sampling. A five-day cycle of sampling and lab measurements was established and successfully executed over five cycles. Three tower sites were also established on the floe and three ice mass balance buoys installed adjacent to the biogeochemical sites (Lewis et al.,
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Fig. 1. Ship’s track for SIMBA . (from Lewis et al., 2011, Fig. 1)
114°
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Fig. 2. Ship’s track for SIPEX . (from Worby et al., 2011, Fig. 1a)
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Introduction / Deep-Sea Research II 58 (2011) 993–998
2011). Two of the tower sites measured snow particle fluxes at various heights. One tower measured turbulent atmospheric fluxes and additional measurements were made of albedo during the five cycles at each site. The three ice mass balance buoys described above were intended to bridge the gaps in physical properties of the ice (temperature, ice thickness change, snow depth change, light penetration, heat flux to the base) between the sampling cycles. The mass balance program was complemented by an investigation of the bio-optical properties of the sea ice including the measurements of the transmission of photosynthetically active radiation and absorption by sea-ice particulates (microalgae and detritus) (Fritsen et al., 2011). At three other locations, the geophysics program established traverse lines and conducted between one and three cycles of measurements on snow depths, ice thickness by drilling, ice thickness gauges, snow properties, and concurrent measurements of thickness using an EMI instrument in a hand-held mode (Weissling et al., 2011; Xie et al., 2011). For the time series sampling at an ice drifting station for the biochemistry and biology, all of the original objectives were satisfied. With a prior arrangement, NASA agreed to reschedule its GLAS campaign to coincide (Oct 3 to Nov 5 2008) with the surface truth measurements of sea ice elevation and thickness that we were to obtain from the cruise (Xie et al., 2011). Comparison of the drift and deformation characteristics from buoys to high-resolution radar satellite measurements were also planned, with acquisition of 13 RadarSat images planned over the region of measurements. An acquisition of European Space Agency radar data from EnviSat was also planned with project participants as co-investigators on an ESA proposal to obtain and archive that imagery for post processing. These imagery acquisition activities were all carried out successfully (Ozsoy-Cicek et al., 2011). Underway measurements were also an important element of the voyage. We reached the sea-ice zone of the Bellingshausen Sea near Peter I Island after transiting for four days (Sept 24 2007). During the open-water transit we deployed 15 Argo drifters (accompanied by an XBT cast) and ten SOLO drifters. Three short ice stations were conducted for geophysical and biogeochemical sampling, and CTD casts during the three-day transit to the main Ice Station location. ASPeCt visual ice observations were conducted hourly in the sea-ice zone (Ozsoy-Cicek et al., 2011), supported by downward and side-looking cameras (continuous imagery) and electromagnetic induction measurements for ice thickness. Marine mammal and seabird observations were also maintained around the clock during both the open-water and sea-ice transits. With the shortened time for the cruise, due to a fire in the biology laboratory that forced a return to port early in the voyage, it was only possible to conduct CTD and trace-metal casts on the transit back to Punta Arenas, without ice station work. The shortened voyage also impacted on the longevity of the drifting buoys deployed on the sea ice, since it was originally proposed they would be deployed in the Amundsen Sea, where ice conditions are more suitable for long-term deployments (Lewis et al., 2011). For decades sea ice was assumed to be an impermeable and inert barrier for air-sea exchange of CO2 so that global climate models do not include CO2 exchange between the oceans and the atmosphere in the polar region. However, uptake of atmospheric CO2 over sea-ice cover has been recently reported raising the need to further investigate pCO2 dynamics in the marine cryosphere realm and related air-ice CO2 fluxes. In the course of the biogeochemistry cycles experiment on SIMBA, we experienced a full cycle of cooling and warming of the air temperature with large changes in the snow-cover. Temperature and snow cover changes affected brine salinities, drastically increasing the instability of the brine column in the initial isothermal and
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porous sea ice. Cooling of the surface layer significantly increased the surface layer brine salinity, triggering a downwards transfer of brines into the underlying porous ice (Lewis et al., 2011).This downward transfer was likely counterbalanced by upwards (or lateral) transfer of sea water into the ice, a flooding-like process. While the sea-ice cover was under-saturated in CO2 with respect to the atmosphere, convective processes significantly affected the partial pressure of CO2 (pCO2) of the brines, promoting the increase of pCO2 and reducing the magnitude of related air-ice CO2 transfer. Dissolved iron profiles were obtained from the Trace Metal casts on the expedition along the continental slope (water depths 1680-3050 m) and close to Peter I Island (water depth 1000 m). We observed smooth nutrient type DFe profiles, which increased from 0.6-0.6 nM to 1.1-1.2 nM in deep water as a function of distance to the continental shelf and water depth. Near Peter I Island there was 10 nM DFe in the upper 200 m and 2-3 nM below to the sea floor. We conclude that not only the numerous icebergs and melting sea ice are significant pelagic iron sources in the region to support primary production, but also continental and island shelves can contribute significantly.
3. Sea-Ice Physics and Ecosystem eXperiment (SIPEX) The Sea Ice Physics and Ecosystem eXperiment (SIPEX) was a multi-disciplinary science program focused on sea-ice research in the region 115-1301E. The objectives of the voyage were to investigate the relationships between the physical sea-ice environment and the sea ice biology, in particular the presence of algae and krill under the ice. A range of novel instruments and techniques were used, including for the first time, radar and laser altimetry from helicopter for measuring the snow thickness and freeboard height above sea level. An instrumented Remotely Operated Vehicle (ROV) was used for under-ice observations, a custom-build trawl was deployed for catching krill under the ice (O’Brien et al., 2011), and trace-metal clean sampling equipment was developed to determine the concentration and distribution of iron in sea ice (van der Merwe et al., 2011). An overarching goal of the project was to understand the links between sea ice physical, chemical and biological parameters and their importance for seaice zone productivity in the Australian Antarctic Territory off East Antarctica. The ship departed Hobart on 4 September 2007 and steamed directly to the ice edge at approximately 621 300 S, 1281 000 E. We then steamed south through the pack ice to the fast ice adjacent to the Antarctic coast, then west through the persistent flaw lead past the Dalton Iceberg tongue. At approximately 1191E we began heading north again, encountering particularly heavy ice and poor weather. Our track was generally in a NW direction towards the ice edge at 1161E. We then steamed east to 1201E while remaining about 60-100 miles south of the ice edge, before finally heading NE to Hobart. We departed the ice edge at 13:00 local time on 11 October at 621 030 S, 1241 250 E, returning to Hobart on 17 October. The ice conditions encountered during the voyage were particularly difficult at times, not only for navigation but for conducting scientific work on the ice. A strong shear zone at the southern end of the first transect, adjacent to the Dalton Iceberg Tongue, was associated with particularly deformed ice that prevented us making any further progress south to the fast ice. In general, we had difficulty finding floes that were suitable to work on, and helicopter reconnaissance flights were particularly important for navigating through the heaviest ice, which in localized areas was up to 10 m thick and some of the thickest reported in the East Antarctic region. The weather conditions were unfavourable at times, with three blizzards hitting the ship
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drilling. These included detailed assessments of snow stratigraphy, snow type classification, and measurement of snow vertical temperature profile, grain size, density and wetness (using a dielectric probe). Snow samples were melted for analysis of salinity (range 0 to 41.3 ppt) and stable isotope (d18O) signature (the latter using samples returned to Australia). Snow density ranged from 180 kg m 3 for newly-fallen snow to 638 kg m 3 for wetted basal snow. Snow average grain size (estimated as a function of snow type) ranged from 0.2 mm for newly fallen snow to 5 mm for depth hoar. Overall, the snow covers sampled were relatively homogeneous, being largely composed of wind packed snow and depth hoar (with relatively few internal icy layers and well developed depth hoar). Full-thickness ice cores were collected with a 9-cm diameter coring auger and ice temperatures were measured in these at 5- or 10-cm intervals as soon as they were extracted. The cores were analysed onboard the ship for visual stratigraphy and salinity, and samples were collected for stable isotope analysis in Australia. Salinity and isotope samples were taken approximately every 5 cm, cut according to the structural observations. A comprehensive data set of oceanographic as well as chemical and biological sea-ice parameters was collected. Under-ice water mass properties and current profiles were measured during major ice stations and used for determination of winter mixed-layer depths and the location of major oceanographic features (Williams et al., 2011). Ice coring on a trace-metal-clean sampling site was carried out to determine sea-ice dissolved and particulate iron fractions. Comparison of pack ice and one fast-ice station revealed strong differences in the iron fractionation with possible implications for iron release into the water column during ice melt (van der Merwe et al., 2011). Sampling during SIPEX showed that sea ice also contained high concentrations of coloured dissolved organic carbon that may affect optical properties in the pelagic realm during the sea-ice melt season (Norman et al., 2011). The results highlight the role of sea ice as a temporal reservoir for iron and dissolved organic carbon during the Antarctic winter and early spring. Sea ice algal biomass was primarily controlled by sea ice porosity affecting nutrient availability of the ice algal communities
during the voyage. The third of these resulted in four consecutive days of white-out conditions that prevented flying operations at a time when they were needed most for the ICESat satellite validation program, and thus precluded making coincident airborne and satellite altimetry measurements. In total, 15 ice stations were conducted in addition to underway sampling and aircraft-based observations from helicopters. Some of the ice stations were completed within one day, while at some locations we stayed for two days to allow measurements to be collected over a full diurnal cycle. A number of ice stations were also just a couple of hours, with the primary aim of collecting ice cores with high algal biomass for the various biology groups onboard. Table 1 provides a summary of icestation locations. Ice and snow thickness and properties measurements were made along transects at eleven ice stations. These measurements provide the basic data for assessing ice characteristics, and for validating and ‘‘calibrating’’ other techniques including airborne and surface snow radar, inductive electromagnetic, ROV sonar and ASPeCt ship-based observations. At each ice stations, a straight transect was laid out across the floe with a survey tape. This was usually 200 m in length, although some shorter lines were measured and additional cross lines were also measured at two sites. Snow thickness and ice/ snow interface temperatures were measured using arrow shafts with temperature sensors in the head inserted into the undisturbed snow at 1-m intervals along the line. Ice thickness, ice freeboard height, snow thickness and snow freeboard height were measured at the same 1-m intervals through holes drilled through the ice, and from these data Worby et al. (2011) calculated the relationship between surface elevation and snow thickness. This was also calculated using the SIMBA data by Xie et al. (2011) and the two results show how regionally variable the ice conditions are, and the importance of taking account of such variability when calculating sea ice thickness from altimetry data, whether from aircraft or satellite. Snow pit and ice core measurements were made at several locations along the transect line. Snow pit measurements (at 50 snow pits in total) were made before the ice was penetrated by
Table 1 Summary of SIPEX ice station locations. Stn #
Lat dd mm
Lon ddd mm
Date 2007
Transect length (m)
Snow pits (no.)
Ice cores (no.)
Ice thick av/stdev (m) a
Snow av/stdev (m) a
Note
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
64 64 64 65 65 65 65 65 65 64 65 64 64 64 64
127 128 127 125 124 122 121 118 118 119 117 116 116 116 120
11-Sep 12-Sep 14-Sep 17-Sep 18-Sep 21-Sep 22-Sep 25-Sep 28-Sep 30-Sep 3-Oct 5-Oct 6-Oct 7-Oct 10-Oct
100 200 100 – 200 200 – 100 50 200 100 – 200 200 –
3 5 3 1 5 5 1 5 1 5 3 – 6 7 –
1 3 3 – 3 2 – 4 – 3 5 1 3 3 1
0.81/0.54 1.03/0.37 0.99/0.44 – 0.71/0.13 1.15/0.67 – 0.59/0.43 2.22/0.82 0.70/0.46 1.06/0.32 – 1.04/0.45 1.14/0.76 –
0.18/0.10 0.20/0.13 0.17/0.14 – 0.09/0.06 0.02/0.04 – 0.08/0.04 0.44/0.22 0.09/0.07 0.22/0.14 – 0.15/0.11 0.22/0.13 –
b
13 29 24 06 31 35 34 33 20 56 01 53 44 19 41
56 03 08 40 45 35 30 52 34 09 32 58 49 49 37
Notes: (a) Average and standard deviation ice and snow thickness are for main transect only. (b) Ice thickness was measured to 100 m only, while snow thickness and ice surface temperature was measured to 200 m. (c) 2 100-m cross-section transects also measured for snow thickness/temperature only.
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Introduction / Deep-Sea Research II 58 (2011) 993–998
(van der Merwe et al., 2011). Generally cold temperatures, high brine salinities and low brine volumes limited elevated ice-algal concentrations to the warmer and more porous sea-ice layers at the ice-water interface (Meiners et al., 2011). Ice algal biomass was positively correlated with sampling date demonstrating that SIPEX captured the onset of the spring ice-algal bloom when biomass accumulates with time. Kramer et al. (2011) sampled seaice meiofauna communities and report generally low metazoan biomass and low biodiversity in East Antarctic sea ice when compared to late winter-early spring samples from other sectors of the Southern Ocean. Sea-ice communities can serve as an important food source for Antarctic krill (Euphausia suberba), and the distribution of krill is closely linked to sea-ice extent in many regions of the Southern Ocean (e.g., Atkinson et al., 2004; Nicol et al., 2000). O’Brien et al. (2011) report major differences in growth, diet and condition of larval and postlarval krill sampled from open water and the under-ice environments, indicating that different over-wintering strategies are used by different life-cycle stages, and highlighting the role of sea-ice biota as food source for krill larvae off East Antarctica. Their findings were supported by under-ice observations with an ROV showing juvenile krill feeding at the subsurface of the sea ice and in cracks in areas of rafted ice at many of the SIPEX stations. Combined the multi-disciplinary sampling program highlighted the structuring role of sea ice in East Antarctic marine ecosystem function and biogeochemical cycling. Two meso-scale buoy arrays were also deployed as part of SIPEX to improve our understanding of the drift and dynamics of the sea ice. Deployment locations were at the southern end of the SIPEX meridional transects at 1281 and 1161E. Both arrays were deployed just to the north of the narrow (but strong) shear zone between the fast ice adjacent to the Antarctic continent, and the bulk of the westward current. The arrays comprised five buoys, four of which formed a square with 30-km sides, with the fifth in the middle. These deployment configurations were chosen to be comparable with the grid resolution used in current modelling studies. Hourly GPS positions were received from all buoys, plus atmospheric pressure and air temperature from one in each array. Finally, airborne laser altimetry was conducted from a helicopter using a Riegl 2D laser scanner LMS-Q240i. The data collected on this voyage enabled aircraft measurements to be validated over drilled thickness lines on the ice, as described in Worby et al. (2011), while at the same time improving our knowledge of the issues surrounding long-base-line GPS positioning that complicate the data analysis. Subsequent measurements in the Antarctic sea-ice zone using the same airborne system have significantly improved positioning and surface elevation accuracies and provide the necessary data for satellite altimetry cal/val.
4. Summary It is clear from recent studies that the distribution of sea ice around Antarctica has changed in recent decades. While a net increase of 1.2% per decade in extent has been reported between 1979 and 2008 (Comiso and Nishio, 2008), far greater regional changes have occurred. In the Bellingshausen Sea some regions now experience an annual sea-ice season that is 3 months shorter than in 1979, whereas parts of the Ross Sea have an annual sea ice season that is 2 months longer (Parkinson, 2004). Critical to understanding behavior in sea-ice thickness, and the potential impacts of this on both the physical and biological Southern Ocean environment, are field studies that measure and monitor critical sea-ice processes, and provide the necessary ground validation data for hemispheric to global-scale satellite monitoring.
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The SIMBA and SIPEX field campaigns had this as a key goal, encompassing a broad range of experiments that have yielded valuable insights to the sea-ice environment in western and eastern Antarctica respectively. The papers presented in this volume present the results of a suite of highly multi-disciplinary and highly collaborative field experiments conducted as part of the International Polar Year 2007-09.
Acknowledgments These two major ship-based field experiments to the Antarctic sea-ice zone could not have been achieved without the support of the US National Science Foundation (SIMBA) and the Australian Antarctic Division and Antarctic Climate and Ecosystems Cooperative Research Centre (SIPEX). Our thanks go to the Captains, crew and support staff of both the Nathaniel B. Palmer and the Aurora Australis, and to the many land-based support staff involved in logistics planning. This work was supported in part by the Australian Government through the Antarctic Climate and Ecosystems Cooperative Research Centre.
References Atkinson, A., Siegel, V., Pakhomov, E., Rothery, P., 2004. Long-term decline in krill stock and increase in salps within the Southern Ocean. Nature 432, 100–103. Comiso, J.C., Nishio, F., 2008. Trends in the sea ice cover using enhanced and compatible AMSR-E, SSM/I and SMMR data. Journal of Geophysical Research 113, C02S07. doi:10.1029/2007JC004257. Fritsen, C.H., Wirthlin, E.D., Momberg, D.K., Lewis, M.J., Ackley, S.F., 2011. Biooptical properties of Antarctic pack in the early spring. Deep-Sea Research II 58 (9–10), 1052–1061. Kramer, M., Swadling, K.M., Meiners, K.M., Kiko, R., Scheltz, A., Nicolaus, M., Werner, I., 2011. Antarctic sympagic meiofauna in winter: comparing diversity, abundance and biomass between perennially and seasonally ice-covered regions. Deep-Sea Research II 58 (9–10), 1062–1074. Lewis, M.J., Tison, J.L., Weissling, B., Delille, B., Ackley, S.F., Brabant, F., Xie, H., 2011. Sea ice and snow cover characteristics during the winter–spring transition in the Bellingshausen Sea: an overview of SIMBA 2007. Meiners, K.M., Norman, L., Granskog, M.A., Krell, A., Heil, P., Thomas, D.N., 2011. Physico-ecobiogeochemistry of East Antarctic pack ice during the winterspring transition. Deep-Sea Research II 58 (9–10), 1172–1181. Nicol, S., Pauly, T., Bindoff, N.L., Wright, S., Thiele, D., Woehler, E., Hosie, G., Strutton, P., 2000. Ocean circulation off East Antarctica affects ecosystem structure and sea-ice extent. Nature 406, 504–507. Norman, L., Thomas, D.N., Stedmon, C.A., Granskog, M.A., Papadimitriou, S., Krapp, R.H., Meiners, K.M., Lannuzel, D., vanderMerwe, P., Dieckmann, G.S., 2011. The characteristics of dissolved organic matter (DOM) and chromophoric dissolved organic matter (CDOM) in Antarctic sea ice. Deep-Sea Research II 58 (9–10), 1075–1091. O’Brien, C., Virtue, P., Kawaguchi, S., Nichols, P.D., 2011. Aspects of krill growth and condition during late winter-early spring off East Antarctica (110–130E). Deep-Sea Research II 58 (9–10), 1121–1221. Ozsoy-Cicek, Burcu, Kern, Stefan, Ackley, Stephen, F., Xie, Hongjie, Tekeli, Ahmet, E., 2011. Intercomparisons of Antarctic sea ice types from visual ship, RADARSAT-1 SAR, Envisat ASAR, QuikSCAT, and AMSR-E satellite observations in the Bellingshausen Sea. Deep-Sea Research II 58 (9–10), 1092–1111. Parkinson, C., 2004. Southern Ocean sea ice and its wider linkages: insights revealed from models and observations. Antarctic Science 16 (4), 387–400. doi:10.1017/S0954102004002214. van der Merwe, P., Lannuzel, D., Bowie, A.R., Mancuso Nichols, C.A., Meiners, K.M., 2011. Iron fractionation in pack and fast ice in East Antarctica: temporal decoupling between the release of dissolved and particulate iron during spring melt. Deep-Sea Research II 58 (9–10), 1222–1236. Weissling, B.P., Lewis, M.J., Ackley, S.F., 2011. Sea ice thickness and mass at Ice Station Belgica, Bellingshausen Sea, Antarctica. Deep-Sea Research II 58 (9–10), 1112–1124. Williams, G.D., Meijers, A.J.S., Poole, A., Mathiot, P., Tamura, T., Klocker, A., 2011. Late winter oceanography off the Sabrina and BANZARE coast (117–1281E), East Antarctica. Deep-Sea Research II 58 (9–10), 1194–1210. Worby, A.P., Steer, A., Lieser, J.L., Heil, P., Yi, D., Markus, T., Allison, I., Massom, R.A., Galin, N., Zwally, H.J., 2011. Regional-scale sea ice and snow thickness distributions from in situ and satellite measurements over East Antarctica during SIPEX 2007. Deep-Sea Research II 58 (9–10), 1125–1136. Xie, H., Ackley, S.F., Yi, D., Zwally, H.J., Wagner, P., Weissling, B., Lewis, M., Ye, K., 2011. Sea ice thickness distribution of the Bellingshausen Sea from surface measurements and ICESat altimetry. Deep-Sea Research II 58 (9–10), 1039–1051.
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A.P. Worby n Australian Antarctic Division, Channel Highway, Kingston 7050, Tasmania, Australia
E-mail address: [email protected]
K.M. Meiners Australian Antarctic Division, Channel Highway, Kingston 7050, Tasmania, Australia
n
Corresponding author.
S.F. Ackley Department of Geological Sciences, University of Texas at San Antonio, One UTSA Circle, San Antonio, TX 78249, USA