Mertz Glacier Research Articles

Identifying potential sources of iron-binding ligands in coastal Antarctic environments and the wider Southern Ocean

The availability of iron (Fe) to marine microbial communities is enhanced through complexation by ligands. In Fe limited environments, measuring the distribution and identifying the likely sources of ligands is therefore central to understanding the drivers of marine productivity. Antarctic coastal marine environments support highly productive ecosystems and are influenced by numerous sources of ligands, the magnitude of which varies both spatially and seasonally. Using competitive ligand exchange adsorptive cathodic stripping voltammetry (CLE-AdCSV) with 2-(2-thiazolylazo)-p-cresol (TAC) as a competing artificial ligand, this study investigates Fe-binding ligands (FeL) across the continental shelf break in the Mertz Glacier Region, East Antarctica (64 - 67°S; 138 - 154°E) during austral summer of 2019. The average FeL concentration was 0.86 ± 0.5 nM Eq Fe, with strong conditional stability constants (Log KFeL) averaging 23.1 ± 1.0. The strongest binding ligands were observed in modified circumpolar deep water (CDW), thought to be linked to bacterial Fe remineralisation and potential siderophore release. High proportions of excess unbound ligands (L’) were observed in surface waters, as a result of phytoplankton Fe uptake in the mixed layer and euphotic zone. However, FeL and L’ concentrations were greater at depth, suggesting ligands were supplied with dissolved Fe from upwelled CDW and particle remineralisation in benthic nepheloid layers over the shelf. Recent sea-ice melt appeared to support bacterial production in areas where Fe and ligands were exhausted. This study is included within our newly compiled Southern Ocean Ligand (SOLt) Collection, a database of publicly available Fe-binding ligand surveys performed south of 50°S. A review of the SOLt Collection brings attention to the paucity of ligand data collected along the East Antarctic coast and the difficulties in pinpointing sources of Fe and ligands in coastal environments. Elucidating poorly understood ligand sources is essential to predicting future Fe availability for microbial populations under rapid environmental change.

Circumpolar Deep Water and Shelf Sediments Support Late Summer Microbial Iron Remineralization

AbstractDespite widespread iron (Fe) limitation in the Southern Ocean, intense phytoplankton blooms are observed around productive coastal regions such as the Mertz Polynya (off George V Land and Adelie Land, East Antarctica; 140–155°E). Sources of Fe across coastal East Antarctica vary, with limited data available for late summer months. We investigated the sources of dissolved Fe (dFe; <0.2 μm) at 19 oceanographic stations in the Mertz Glacier Region (64–67°S; 138–154°E), between January and March of 2019. Concentrations of dFe ranged from below detection limit (0.03 nM) at the surface, to 0.34 nM above the base of the mixed layer (35 m), reaching 0.59 nM at depth (520 m). Using oceanographic features and trace element ratios (manganese and titanium), we identified Circumpolar Deep Water (CDW) and shelf sediment resuspension in modified CDW as contributors of dFe to the region over this period. Microbial Fe remineralization was evident where nutrient‐rich water met highly oxygenated waters over the continental shelf. Reduced Fe concentrations in the mixed layer and euphotic zones suggested rapid biological uptake prior to sampling. Despite proposals for pelagic Fe recycling by marine animals, preliminary investigations reveal no significant spatial relationship between animal presence and surface ocean Fe concentrations over the study area. Further research is required to identify seasonal changes to Fe supply in coastal areas which will strengthen our understanding of the Fe cycle and its influence on microbial and primary productivity in this globally significant region.

Calving Event Led to Changes in Phytoplankton Bloom Phenology in the Mertz Polynya, Antarctica

AbstractPolynyas are subject to variability in winds and ocean circulation and are important sites of ecological productivity. In February 2010, the B09B iceberg collided with the Mertz Glacier Tongue (MGT), calving a 78 × 40‐km giant iceberg which modified the icescape and primary productivity of the Mertz polynya. In this study, we use satellite ocean color and sea ice concentration to investigate the variability, trends, and drivers of phytoplankton blooms in the Mertz polynya since 1997. During the bloom, over 21 years, we found (i) a later ice retreat time, (ii) an increase in sea ice concentration, (iii) a decrease in open‐water period, (iv) a later bloom start, and (v) a decrease in bloom duration. Our results suggest that major postcalving changes in the physical characteristics of the polynya, mainly its icescape, are the primary drivers of phytoplankton phenology. More specifically, the MGT calving event resulted in significant seasonal and regional changes, with higher eastern chl‐a and mean summer chl‐a postcalving. While satellite data are useful to study long‐term variability in these inhospitable areas, they only focus on the ocean surface and are obscured by ice and clouds. Additional subsurface parameters from seal tags, gliders and moorings in the southernmost polar regions would strengthen our comprehension of phytoplankton and physical changes in ocean dynamics that may have far‐reaching consequences, from global circulation to carbon export.

Trace metal and nutrient dynamics across broad biogeochemical gradients in the Indian and Pacific sectors of the Southern Ocean

The Southern Ocean is the largest high-nutrient low-chlorophyll environment in the global ocean, and represents an important source of intermediate and deep waters to lower latitudes. Constraining Southern Ocean trace metal biogeochemical cycling is therefore important not just for understanding biological productivity and carbon cycling regionally, but also for understanding trace metal distributions throughout the lower latitude oceans. We present dissolved Fe, Ni, Cu, Zn, Cd, Pb and macronutrient concentrations in the Indian and Pacific sectors of the Southern Ocean from the Antarctic Circumnavigation Expedition (austral summer 2016-17), which included the first opportunities to study trace metal cycling at the Mertz Glacier Polynya and the Balleny Islands, as well as two meridional cross-frontal transects. Dissolved Ni, Cu, Zn, Cd and macronutrient concentrations show similar or greater variability latitudinally within surface waters than vertically through the water column, reflecting the combined influence of circulation and biological drawdown in shaping the distributions of nutrient-type elements in the Southern Ocean. Slopes of Cu-Si(OH)4 and Cd-PO4 increase from the Polar Frontal Zone to south of the Southern ACC Boundary (Cu-Si(OH)4) and from the Subantarctic Zone to the Antarctic Zone (Cd-PO4). Latitudinal differences are also observed for Ni-Si(OH)4 and Zn-PO4, with distinct Subantarctic Zone trends relative to those south of the Polar Front. Similarities between our Zn-Si(OH)4 and Cd-PO4 correlations and global compilations reflect the importance of exported Southern Ocean waters in setting these metal-macronutrient couples globally. Distinct Ni-macronutrient correlations are observed in this dataset relative to the global ocean, which supports a distinct cycling of Ni in the Southern Ocean compared to other basins. Concentrations of Pb are among the lowest observed in the global ocean; however, a local maximum is seen along the density level corresponding with Antarctic Intermediate Water. Concentrations within this isopycnal decrease with increasing latitude, which can be explained by decreasing atmospheric Pb input to more recently subducted waters.Substantial biological uptake of metals and macronutrients is observed at the Mertz Glacier Polynya. Here, inferred metal:macronutrient uptake ratios are comparable to those found in the Amundsen Sea Polynya, in Southern Ocean phytoplankton, and to metal-macronutrient correlations in our data set as a whole, highlighting the potential of Southern Ocean polynyas as natural systems for trace metal uptake and export studies. The Balleny Islands are a source of Fe to surface waters and the islands also appear to influence distributions of Zn, Cu and macronutrients, which may reflect the combined impact of Fe supply on biological uptake, mixing, and scavenging in deeper waters. The Kerguelen Plateau is also a source of Fe, as previously identified. Throughout our dataset, the ferricline is found deeper than the nitricline, in agreement with existing data and indicating that Fe is less easily entrained into the surface ocean than NO3. Additionally, Fe:NO3 ratios in most samples throughout the water column are Fe-limiting (<0.01 mmol mol−1). Therefore deep mixing, identified previously as the main Fe source to much of the Southern Ocean, would ultimately act to maintain Fe limitation.

Cycling of zinc and its isotopes across multiple zones of the Southern Ocean: Insights from the Antarctic Circumnavigation Expedition

Zinc (Zn) is an essential micronutrient, playing an important role in several key processes in marine phytoplankton. Here, we present the first high-resolution depth profiles for dissolved Zn and δ66Zn from all major zones of the Southern Ocean, collected during the Antarctic Circumnavigation Expedition in Austral Summer 2016/2017. The dataset reveals that Zn cycling changes between different regions of the Southern Ocean. Within the Antarctic Circumpolar Current (ACC), Zn cycling is closely linked to phosphate (PO4), governed by uptake and regeneration, seasonal mixing, and upwelling. Here, uptake by phytoplankton is associated with a very small fractionation (α = 0.99995), resulting in slightly elevated surface δ66Zn (up to +0.67‰), overlying a shallow subsurface δ66Zn minimum (+0.36‰ at ∼200 m). South of the ACC, a partial coupling of Zn and silicate (Si) results in a shift of the Zn isotope systematics and a deep enrichment of Zn and Si. Two possible mechanisms could potentially cause this change: 1) reversible scavenging onto sinking particulates, or, 2) association of isotopically heavy Zn with diatom frustules. We also observe the effects of regional processes on Zn in the surface Southern Ocean: firstly, natural Fe fertilization near the Balleny Islands appears to lead to reduced Zn uptake (relative to phosphate) by phytoplankton, that is associated with a greater apparent Zn isotope fractionation than elsewhere in the Southern Ocean (α = 0.99987); secondly, meltwater inputs from the Mertz Glacier add small amounts of isotopically light Zn to surface waters near the Antarctic shelf. Overall, we propose that the lack of distinct δ66Zn signatures transported in intermediate waters of the lower latitude global oceans is due to near-complete uptake of Zn by phytoplankton in the surface Southern Ocean with only a small isotope fractionation, in contrast to elements like cadmium and silicon.

High-resolution Cd isotope systematics in multiple zones of the Southern Ocean from the Antarctic Circumnavigation Expedition

The Southern Ocean plays a major role in determining the global distribution of trace metals such as cadmium (Cd). Here, we present 17 high-depth-resolution profiles of dissolved Cd and its stable isotope composition (δ114Cd) over the top 1000 m of the Pacific and Atlantic sectors of the Southern Ocean, collected during the Antarctic Circumnavigation Expedition. Our dataset reinforces the view that Cd and its isotopes are dominated by shallow biological cycling in this region. A close examination of variations in Cd cycling across the different zones of the Southern Ocean reveals how the interplay between uptake and regeneration, seasonal mixing, and upwelling controls both Cd and δ114Cd in this region. The only deviations from these systematics are due to the influence of local processes such as continental influence or Fe-fertilization, close to the Mertz Glacier and the Balleny Islands, respectively. Deep convection during winter incorporates the Southern Ocean Cd isotope signatures into Subantarctic Mode Water and Antarctic Intermediate Water during water mass formation. Incorporating published data, we present the first complete picture of how Cd is cycled through the entire Pacific Ocean, revealing the manner in which the Southern Ocean controls the global cycling of Cd and δ114Cd; analogous to Si or Zn, we propose that Southern Ocean processes, in combination with global ocean circulation, cause a division into two separate Cd regimes, a Cd-depleted surface ocean above a Cd-rich deep ocean loop. Therefore, the relationship between Cd and PO4 on a global scale is largely a result of these processes in the Southern Ocean, rather than a local correlation between the two elements.

Changes in Antarctic coastline between 1997 and 2016 using RADARSAT and MODIS data

ABSTRACTStudies have already indicated more disintegrations and calving of ice margins and a significant negative ice balance at certain parts of Antarctica in recent times. Changes in extent of ice shelves as well as glacier fronts are indeed good signatures of climate change. This paper essentially discusses the changes in Antarctic ice margins between 1997 and 2016. Change-detection technique has been employed using the Radar Satellite (RADARSAT) mosaic of 1997 and the mosaic of Antarctic ice margins generated using MODerate resolution Imaging Satellite (MODIS) images of 2016, with the help of Earth Resources Development Assessment system (ERDAS) Imagine software. Hotspots, where significant changes occurred, have been identified. Most part of the Antarctic coast between 75° E and 45° W, covering all West Antarctica and about half of East Antarctica, has shown definite retreat during this period. Major retreats are observed over the both sides of Ross ice shelf, Ronne ice shelf, Thwaites glacier, and Mertz glacier. Major advancements are observed over Filchner ice shelf, Amery ice shelf, middle portion of Ross ice shelf, and Stancomwills glacier tongue. Over the East Antarctica, areas of advancement and retreat are nearly same. But, over the West Antarctica, the area of retreat is significantly higher than area of advancement.

Landfast ice controls on sea-ice production in the Cape Darnley Polynya: A case study

Antarctic coastal polynyas serve as crucially-important sea-ice “factories” and are (in certain cases) of global significance as sites of Antarctic Bottom Water (AABW) formation e.g., the Cape Darnley Polynya (CDP) in East Antarctica. As such, understanding change and variability in their behaviour, and the factors responsible, is a high priority in climate (change) science. One such important factor that is both poorly understood and quantified is their linkage with adjacent landfast sea ice (fast ice). Here, we present a first detailed analysis that identifies and quantifies key linkages between the dynamics/behaviour of the CDP and fast ice. By forming a barrier to the incursion of pack ice at the eastern boundary, fast ice immediately upstream of the CDP (associated with the distribution of grounded icebergs) plays a crucially-important role in the polynya formation, maintenance and variability. This satellite data-based study builds upon earlier conceptual work on polynya-fast ice feedback mechanisms in the Mertz Glacier region by showing that statistical estimation of the rates of sea ice production (and hence AABW formation) in the CDP is significantly improved through inclusion of two key fast-ice metrics, compared to use of atmospheric covariates alone. These two metrics are the meridional extent of fast ice upstream of the polynya and total fast ice area within the region of high sea ice production. The work highlights the importance of accurate representation of the coupled Antarctic coastal polynya-fast ice system in global climate models.

Chromium biogeochemistry and stable isotope distribution in the Southern Ocean

Abstract Despite the potential of stable chromium (Cr) isotope compositions as a proxy for past changes in oceanic redox conditions, a detailed understanding of the processes that govern their spatial distribution in the modern ocean is still lacking. Here, we report seawater Cr isotope compositions and concentrations from the uppermost 1000 m of the water column in the Southern Ocean. The survey includes a cross-frontal transect from Tasmania to Antarctica, sites near the Antarctic ice-edge and in the vicinity of the Balleny Islands, as well as sites in the Drake Passage. Although a coastal influence is clearly visible in the silicon-nitrate relationship at the stations neighbouring the Balleny Islands, close to the Mertz Glacier and adjacent to the western Antarctic Peninsula, seawater δ53Cr and Cr concentrations remain largely unaffected. As for the coastal sites, Cr depletion and isotopic shifts are also virtually absent in Antarctic and Subantarctic surface waters of the open ocean. Biological uptake of Cr and/or scavenging of Cr onto sinking particles are apparently not strong enough to induce water column variability. Contrasting with the small variations in δ53Cr and Cr concentrations at each site, there are, however, systematic meridional changes. The seawater samples show an increase in Cr concentrations and a parallel decrease in δ53Cr southwards from the Subantarctic across the Polar Frontal into the Antarctic Zone. Chromium concentrations and δ53Cr are, however, uniform at all stations south of the Polar Front. The spatial pattern is consistent with the mixing of Southern Ocean sourced Cr with an isotopically heavier Cr pool from northward of the studied area, as evidenced by strong correlations of Cr and δ53Cr with salinity at the level of Subantarctic Mode Water and at shallower levels. The heavy δ53Cr signature of the northerly Cr pool could either result from Cr cycling in the subtropical gyre or originate in oxygen minimum zones. On a regional scale, δ53Cr is strongly correlated with phosphate concentrations, which may hint at a time-integrated effect of major nutrient drawdown on δ53Cr. Southern Ocean data support previous work demonstrating a strong relationship between seawater Cr concentrations and δ53Cr. At the studied sites, this relationship reflects mixing of isotopically distinct Cr pools. The end-members observed in the studied area are, however, consistent with the previously described closed-system Rayleigh type fractionation of Cr in seawater characterized by a single fractionation factor. The underlying processes, which produce the observed fractionation, are not fully constrained to date.

Two Recent Massive Breeding Failures in an Adélie Penguin Colony Call for the Creation of a Marine Protected Area in D'Urville Sea/Mertz

In the d’Urville Sea in East Antarctica, a population of roughly 20,000 pairs of Adelie penguins of Iles des Petrels (Terre Adelie) has experienced two massive breeding failures, with no chick surviving the 2013–14 and 2016–17 breeding seasons. In both seasons the extent of sea ice in front of the colony persisted throughout the breeding cycle of the birds. The timing of sea-ice recession differed greatly between seasons and the absence of polynya in a crucial phase of the cycle were paramount in driving these failures. The change in the icescape in front of Ile des Petrels following the calving of the Mertz glacier in 2010, together with increase in precipitations and changes in sea-ice firmness explain this situation and are discussed in the present manuscript. To prevent additional future impacts on this colony, like competition with fisheries for instance, we strongly support a scientific research zone in the d’Urville Sea—Mertz area, one of the three zones of proposed Marine Protected Area in East Antarctica to the Commission for the Conservation of Antarctic Marine Living Resources.

Rapid volume reduction in Antarctic Bottom Water off the Adélie/George V Land coast observed by deep floats

Recent changes in the Antarctic Bottom Water (AABW) off the Adélie/George V Land coast, East Antarctica, were examined using 20-months observation with deep floats and historical hydrographic surveys. The salinity of the AABW along isopycnals was largely reduced by about 0.005 during the winter of 2011; since then, it had changed little until the end of the float observation in August 2014. The thickness of the AABW would have decreased since around 2011 by about 50 m yr−1, which is about four to five times the average rate since the 1990s. The change of density structure in the deep ocean, including the rapid contraction of the AABW, is believed to have raised the local sea level by 5.0 mm yr−1 due to steric changes between depths of 1900 and 4000 dbar. This could explain the altimetric average increase of 5.8 (standard error; ± 1.8) mm yr−1 for 2011–2014, coincident with a steric height change of 0.5 ( ± 1.5) mm yr−1 for the upper ocean of 0–1900 dbar and mass change of about 2.0 mm yr−1. The most likely reason for the large change in the AABW is the collapse of the Mertz Glacier Tongue in February 2010. The rapid contraction of the AABW could be due to the reduced supply of the Adélie Land Bottom Water (ALBW) after the calving and the associated decrease in sea ice production. The rapid contraction may continue for a long time because the drastic change of the icescape could prevent the ALBW supply from recovering to its pre-calving volume. The glacier collapse might prevent the AABW from freshening for a long time, even though the collapse would have initially resulted in the large isopycnal freshening of the AABW. Because it may take a decade or so for the supply of the ALBW to recover to pre-calving levels, the moderate freshening of recent decades could yield within 10 years an AABW that is fresher than the freshened AABW observed by the deep floats.

Mapping Antarctic Suspension Feeder Abundances and Seafloor Food-Availability, and Modeling Their Change After a Major Glacier Calving

Seafloor communities are a critical part of the unique and diverse Antarctic marine life. Processes at the ocean-surface can strongly influence the diversity and abundance of these communities, even when they live at hundreds of meters water depth. However, even though we understand the importance of this link, there are so far no quantitative spatial predictions on how seafloor communities will respond to changing conditions at the ocean surface. Here, we map patterns in abundance of important habitat-forming suspension feeders on the seafloor in East Antarctica, and predict how these patterns change after a major disturbance in the icescape, caused by the calving of the Mertz Glacier Tongue. We use a purpose-built ocean model for the time-period before and after the calving of the Mertz-Glacier Tongue in 2010, data from satellites and a validated food-availability model to estimate changes in horizontal flux of food since the glacier calving. We then predict the post-calving distribution of suspension feeder abundances using the established relationships with the environmental variables, and changes in horizontal flux of food. Our results indicate strong increases in suspension feeder abundances close to the glacier calving site, fueled by increased food supply, while the remainder of the region maintains similar suspension feeder abundances despite a slight decrease in total food supply. The oceanographic setting of the entire region changes, with a shorter ice-free season, altered seafloor currents and changes in food-availability. Our study provides important insight into the flow-on effects of a changing icescape on seafloor habitat and fauna in polar environments. Understanding these connections is important in the context of current and future effects of climate change, and the mapped predictions of the seafloor fauna as presented for the study region can be used as a decision-tool for planning potential marine protected areas, and for focusing future sampling and monitoring initiatives.

Accurate coastal DEM generation by merging ASTER GDEM and ICESat/GLAS data over Mertz Glacier, Antarctica

Mertz Glacier (MG) calved in February 2010 and a 70-years' calving cycle of MG was reported recently because of the shallow Mertz Bank. To better investigate the calving process, a high accurate surface Digital Elevation Model (DEM) with accurate date marker over MG is urgently required for numerical modeling, especially over the ice tongue. However it is quite challenging to generate an accurate DEM over high relief and steep slope regions. To solve this problem, a new method for accurate coastal DEM production by merging ASTER GDEM and ICESat/GLAS data is designed, which can effectively discriminate accurate elevation data from ASTER GDEM. Then, a DEM corresponding to the end of 2002 over MG is generated, which has an accuracy of 0.99±17.50m. This DEM is several times better in accuracy than the original ASTER GDEM (accuracy: −8.17±54.31m) and two new characteristics of ASTER GDEM have been found through analysis. First, ASTER GDEM has elevation bias (varying from −23m to 28m), spatially correlated to ASTER ground tracks, which was most probably caused by uncertainty of attitude measurements of ASTER. Second, ASTER GDEM grids with stacking number ≥4 can be effectively adjusted by ground control points so as to improve elevation accuracy. Additionally, this DEM has the best accuracy by comparing with other DEMs (RAMP DEM: 47.71±91.61m, ICESat/GLAS DEM: −1.01±30.33, Bamber DEM: −1.07±33.04m, Bedmap-2 DEM: 9.19±48.34m, and Cryosat-2 DEM: −5.42±32.02m). The high performance of our DEM in accuracy indicates that our method is effective and has a potential to be widely used to improve existing ASTER GDEM along Antarctic coast.

Regional Changes in Icescape Impact Shelf Circulation and Basal Melting

AbstractIce shelf basal melt is the dominant contribution to mass loss from Antarctic ice shelves. However, the sensitivity of basal melt to changes in icescape (grounded icebergs, ice shelves, and sea ice) and related ocean circulation is poorly understood. Here we simulate the impact of the major 2010 calving event of the Mertz Glacier Tongue (MGT), East Antarctica, and related redistribution of sea ice and icebergs on the basal melt rate of the local ice shelves. We find that the position of the grounded tabular iceberg B9B controls the water masses that reach the nearby ice shelf cavities. After the calving of the MGT and the removal of B9B, warmer water is present both within the MGT cavity and on the continental shelf driving a 57% increase of the deep MGT basal melting. Major changes in icescape influence the oceanic heat flux responsible for basal ice shelf melting.

Late‐summer biogeochemistry in the Mertz Polynya: East Antarctica

AbstractA marked reconfiguration of the Mertz Polynya following the 2010 calving of the Mertz Glacier Tongue has been associated with a decrease in the size and activity of the polynya. We report observations of the oceanic carbonate (CO2) system in late‐summer 2013, the third post‐calving summer season. Estimates of seasonal net community production (NCP) based on inorganic carbon deficits and the oxygen‐argon ratio indicate that the waters on the shelf to the east of Commonwealth Bay (adjacent to the Mertz Glacier) remain productive compared to pre‐calving conditions. The input of residual or excess alkalinity from melting sea ice is found to contribute to the seasonal enhancement of carbonate saturation state and pH in shelf waters. Mean rates of NCP in 2012–2013 are more than twice as large as those observed in the pre‐calving summers of 2001 and 2008 and suggest that the new (post‐calving) configuration of the polynya favors enhanced net community production and a stronger surface ocean sink for atmospheric CO2 due at least in part to the redistribution of sea ice and associated changes in summer surface stratification.

Sea-Ice Production in Antarctic Coastal Polynyas Estimated From AMSR2 Data and Its Validation Using AMSR-E and SSM/I-SSMIS Data

Antarctic coastal polynyas are very high sea-ice production areas. The resultant large amount of brine rejection leads to the formation of dense water. The dense water forms Antarctic bottom water, which is the densest water in the global overturning circulation and a key player in climate change as a significant sink for heat and carbon dioxide. In this study, an algorithm was developed that uses Advanced Microwave Scanning Radiometer 2 (AMSR2) data (2012–present) to detect polynya area and estimates thin ice thickness by a method similar to that used to develop the algorithm for Advanced Microwave Scanning Radiometer for EOS (AMSR-E) data. Landfast sea-ice areas were also detected using AMSR2 data. Ice production in the polynyas was estimated by a heat flux calculation using AMSR2 sea-ice data. In four major polynyas, AMSR2 ice production was compared with AMSR-E (2003–2011) ice production through comparison of them with Special Sensor Microwave Imager (SSM/I) and Special Sensor Microwave Imager/Sounder (SSMIS) ice production. The comparison confirmed that the ice production from AMSR-E/2 data, which have higher spatial resolution than SSM/I-SSMIS data, can be used to analyze time series covering more than 10 years. For example, maps of annual ice production based on AMSR-E/2 data revealed detailed changes of the Mertz Polynya, where the ice production decreased significantly after the Mertz Glacier Tongue calving in 2010. Continuous monitoring of the coastal polynyas by the AMSR series sensors is essential for climate-change-related analyses in the Antarctic Ocean.

Changes in water properties and flow regime on the continental shelf off the Adélie/George VLand coast, East Antarctica, after glacier tongue calving

AbstractOceanic changes before and after the relocation of iceberg B9B and calving of the Mertz Glacier Tongue (MGT) in February 2010 are examined on the continental shelf off the Adélie Land/George V Land coast, East Antarctica. Summer hydrographic observations, including stable oxygen isotope ratio (δ18O), in 2001/2008 and 2011/2015 and results of a numerical model are used. Along the western flank of the MGT, temperature decreased between 2001 and 2015 for most of the water column in the Adélie Depression. δ18O generally decreased, especially at the MGT draft depths on the northern side. West of the MGT, temperature, salinity, and δ18O decreased in the intermediate layer. East of the MGT, in contrast, temperature increased between 2001 and 2011 at intermediate depths, salinity increased in the intermediate and deep layers, and δ18O slightly decreased in the deep layer but did not change much around 300 dbar. The numerical experiment exhibits a change in ocean circulation, revealing an increase in modified Circumpolar Deep Water (mCDW) inflow in the east and a decrease in the west. The contrasting changes in mCDW intrusion are consistent between the observations and numerical model, and are indicative of the effect of removal of the ice barriers. The contrast is overlain by overall decreases in salinity and δ18O, which suggests an increase in the continental meltwater fraction of 5–20% and might reveal a wide‐ranging influence from West Antarctica. The oxygen isotope ratio is, hence, effective in monitoring the increase in continental melt over the Antarctic shelf.

22-year surface salinity changes in the Seasonal Ice Zone near 140°E off Antarctica

Abstract Seasonal and interannual variations in sea surface salinity (SSS) are analyzed in the Sea Ice Zone south of 60°S, from a 22-year time series of observations near 140°E. In the northern sea-ice zone during the warming, melting cycle from October to March, waters warm by an average of 3.5 °C and become fresher by 0.1 to 0.25. In the southern sea-ice zone, the surface temperatures vary from − 1 to 1 °C over summer, and the maximal SSS range occurs in December, with a minimum SSS of 33.65 near the Southern Boundary of the ACC, reaching 34.4 in the shelf waters close to the coast. The main fronts, normally defined at subsurface, are shown to have more distinct seasonal characteristics in SSS than in SST. The interannual variations in SSS are more closely linked to variations in upstream sea-ice cover than surface forcing. SSS and sea-ice variations show distinct phases, with large biannual variations in the early 1990s, weaker variations in the 2000s and larger variations again from 2009 onwards. The calving of the Mertz Glacier Tongue in February 2010 leads to increased sea-ice cover and widespread freshening of the surface layers from 2011 onwards. Summer freshening in the northern sea-ice zone is ~ 0.05–0.07 per decade, increasing to 0.08 per decade in the southern sea-ice zone, largely influenced by the Mertz Glacier calving event at the end of our time series. The summer time series of SSS on the shelf at 140°E is in phase but less variable than the SSS observed upstream in the Adelie Depression, and thus represents a spatially integrated index of the wider SSS variations.

Circulation and water mass transports on the East Antarctic shelf in the Mertz Glacier region

The East Antarctic shelf off Adélie-George V Land is known to be an important region for Dense Shelf Water (DSW) formation as a result of intense sea ice production in the Mertz Glacier Polynya during the winter season. It is also a region where the warm modified Circumpolar Deep Water (mCDW) penetrates onto the shelf during the summer. Using hydrographic observations from a summer survey in 2008 we implement a box inverse model to propose a comprehensive view of the steady state circulation on this shelf in summer. Additional information from mooring observations collected on the depression slope is used to provide context to the retrieved circulation scheme. Over the depression slope, the summer baroclinic structure of the currents is found to contrast with the almost barotropic structure in winter.The summer circulation is strongly constrained by the DSW distribution and forms a clockwise circulation primarily transporting the fresh surface waters and the warm mCDW around the dome of DSW. Over the upper flank of the Mertz Bank, the inflow branch transports the mCDW towards the Mertz Glacier, while, over the lower part of the slope, the outflow branch returns to the sill a diluted mode of the same water mass. A total of 0.19Sv of mCDW inflows at the sill and two-third reach the Mertz Glacier and recirculate in front of it, allowing the mCDW to penetrate into the deeper part of the depression. Possible scenarios of interaction between the mCDW and the DSW with the glacier are examined. It is shown that, despite the water mass pathways and transports suggest possible ice-ocean interaction, both lateral and basal melting were likely small in summer 2008. Finally, our results suggest that, in addition to bathymetric features, the distribution of the residual DSW which is left from the preceding winter sets up regional pressure gradients which provide a seasonal control on the shelf circulation. In particular, the spring collapse of the convective patch would contribute to setting up a deep pycnocline which strongly impacts the shelf circulation in the following summer, with possible feedback of the mCDW transports on the polynya activity and water mass formation.

The Mertz Glacier Tongue, East Antarctica. Changes in the past 100 years and its cyclic nature - Past, present and future

The Mertz Glacier Tongue (MGT) in East Antarctica has been studied since 1911. Early expeditions produced maps using ground or ship-based observations followed later by vertical and/or oblique aerial photography from aircraft. In the modern era, extensive digital satellite imagery is available which has also been supplemented by the resurrection and scanning of some historic U.S., now ‘declassified’, film-based satellite images. Much of the MGT became detached and drifted away following the collision by the B-9B iceberg in February 2010 and a similar sequence, or extension-detachment cycle, must have occurred some-time after Mawson's 1911–1914 observations. All the available information on the position, shape or appearance of the MGT has been re-examined in an attempt to comprehensively study its past, present and possible future motion. Feature tracking cross correlation methods have been applied for suitably detailed image pairs to accurately measure the MGT advance velocity. The derived mean rate for 1947–2010 is 1180±14my−1 with an accompanying ice-front loss rate of ~190my−1. A simple model for the MGT exhibits a ~73year quasi-periodic cycle of rebirth, growth and demise which will affect the volume of bottom water produced in the nearby polynya. Some evidence for this cyclic oceanographic change has recently been reported from regional sediment data. Somewhat speculatively, the model suggests a date of ~1937 for the previous MGT break-off and around or before ~2083 for the next. With the MGT being primed to break-off every ~73years, the precise date being governed by unpredictable external events such as collisions by large icebergs, such cycles have probably occurred for many thousands of years.