Mertz Polynya Research Articles

Revisiting circulation and water masses over the East Antarctic margin (80–150°E)

Full-depth hydrographic sections of the BROKE experiment in 1996 (across the Antarctic margin from 80 to 150°E; Bindoff et al., 2000) were revisited for the first time during the 2018/2019 austral summer. We describe the subsurface physical oceanography in 2019 and the hydrographic changes between 1996 and 2019 not documented in earlier studies. The survey captured decadal changes in ocean structure from the southern flank of the Antarctic Circumpolar Current (ACC) to the continental shelves. In five cross-slope meridional sections, where 1996 and 2019 measurements are comparable (112, 120, 128, 140, and 150°E), the poleward shift of the southern boundary of the ACC (50–120 km) prevailed near the continental rise. The simultaneous displacement of barotropic ACC fronts and poleward migration of deep water contributed to full-depth warming (0.1–1.6 °C) and potentially to a reduction in the bottom water volume. Freshening was widely observed from the deep to bottom layers (∼0.02 g/kg), with the signal extending from the upper continental slope. Bottom-intensified freshening was accompanied by an oxygenation of 10–20 μmol/kg, indicating that freshening-driven oxygenation of bottom layers counteracted the deoxygenation effect of the poleward barotropic frontal shift. Westward transport of the Antarctic Slope Current decreased by more than 10 Sv from 1996 to 2019 in the five cross-slope sections; its frontal features and current axis shifted offshore by more than 20 km in 112–140°E. Additionally, subsurface warming along modified Circumpolar Deep Water by up to 0.4 °C was commonly detected across the upper continental slope. For the 2019 hydrography, shelf water sufficiently dense to form bottom water (>28.35 kg/m3) was found to the east of Mertz Polynya (142–148°E), implying a pathway for dense shelf water export from the eastern margin of Mertz Polynya. Our findings underscore the importance of sustained efforts for in-situ observations that widely cover the East Antarctic margin.

Distribution and export of particulate organic carbon in East Antarctic coastal polynyas

Polynyas represent regions of enhanced primary production because of the low, or absent, sea-ice cover coupled with the proximity of nutrient sources. However, studies throughout the Southern Ocean suggest elevated primary production does not necessarily result in increased carbon export. Three coastal polynyas in East Antarctica and an off-shelf region were visited during the austral summer from December 2016 to January 2017 to examine the vertical distribution and concentration of particulate organic carbon (POC). Carbon export was also examined using thorium-234 (234Th) as a proxy at two of the polynyas. Our results show that concentrations and integrated POC stocks were higher within the polynyas compared to the off-shelf sites. Within the polynyas, vertical POC concentrations were higher in the Mertz and Ninnis polynyas compared to the Dalton polynya. Similarly, higher carbon export was measured in the diatom-dominated Mertz polynya, where large particles (>53 μm) represented a significant fraction of the particulate 234Th and POC (average 50% and 39%, respectively), compared to the small flagellate-dominated Dalton polynya, where almost all the particulate 234Th and POC were found in the smaller size fraction (1–53 μm). The POC to Chlorophyll-a ratios suggest that organic matter below the mixed layer in the polynyas consisted largely of fresh phytoplankton at this time of the year. In combination with a parallel study on phytoplankton production at these sites, we find that increased primary production at these polynyas does lead to greater concentrations and export of POC and a higher POC export efficiency.

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.

A Continental Shelf Pump for CO2 on the Adélie Land Coast, East Antarctica

AbstractWe quantify the transport of inorganic carbon from the continental shelf to the deep ocean in Dense Shelf Water (DSW) from the Mertz and Ninnis Polynyas along the Adélie Land coast in East Antarctica. For this purpose, observations of total dissolved inorganic carbon (TCO2) from two summer hydrographic surveys in 2015 and 2017 were paired with DSW volume transport estimates derived from a coupled ocean‐sea ice‐ice shelf model to examine the fate of inorganic carbon in DSW from Adélie Land. Transports indicate a net outflow of 227 ± 115 Tg C yr−1 with DSW in the postglacial calving configuration of the Mertz Polynya. The greatest outflow of inorganic carbon from the shelf region was delivered through the northern boundary across the Adélie and Mertz Sills, with an additional transport westward from the Mertz Polynya. Inorganic carbon in DSW is derived primarily from inflowing TCO2‐rich modified Circumpolar Deep Water; local processes (biological productivity, air‐sea exchange of CO2, and the addition of brine during sea ice formation) make much smaller contributions. This study proposes that DSW export serves as a continental shelf pump for CO2 and is a pathway to sequester inorganic carbon from the shallow Antarctic continental shelf to the abyssal ocean, removing CO2 from atmospheric exchange on the time scale of centuries.

Change in Dense Shelf Water and Adélie Land Bottom Water Precipitated by Iceberg Calving

AbstractAntarctic Bottom Water supplies the deep limb of the global overturning circulation and ventilates the abyssal ocean. Antarctic Bottom Water has warmed, freshened, and contracted in recent decades, but the causes remain poorly understood. We use unique multiyear observations from the continental shelf and deep ocean near the Mertz Polynya to examine the sensitivity of this bottom water formation region to changes on the continental shelf, including the calving of a large iceberg. Postcalving, the seasonal cycle of Dense Shelf Water (DSW) density almost halved in amplitude and the volume of DSW available for export reduced. In the deep ocean, the density and volume of Adélie Land Bottom Water decreased sharply after calving, while oxygen concentrations remained high, indicating continued ventilation by DSW. This natural experiment illustrates how local changes in forcing over the Antarctic continental shelf can drive large and rapid changes in the abyssal ocean.

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.

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.

Brief communication: Impacts of a developing polynya off Commonwealth Bay, East Antarctica, triggered by grounding of iceberg B09B

Abstract. The dramatic calving of the Mertz Glacier tongue in 2010, precipitated by the movement of iceberg B09B, reshaped the oceanographic regime across the Mertz Polynya and Commonwealth Bay, regions where high-salinity shelf water (HSSW) – the precursor to Antarctic bottom water (AABW) – is formed. Here we present post-calving observations that suggest that this reconfiguration and subsequent grounding of B09B have driven the development of a new polynya and associated HSSW production off Commonwealth Bay. Supported by satellite observations and modelling, our findings demonstrate how local icescape changes may impact the formation of HSSW, with potential implications for large-scale ocean circulation.

Carbonate chemistry in the Mertz Polynya (East Antarctica): Biological and physical modification of dense water outflows and the export of anthropogenic CO2

Dense shelf water (DSW) is formed in East Antarctica from enhanced sea‐ice production driven by air‐sea interaction in coastal polynyas. Cross‐shelf export and downslope mixing of this DSW produces Antarctic Bottom Water, contributing to the lower limb of the global overturning circulation. We present biogeochemical observations from the Mertz Polynya region in summer 2007/2008, with additional observations from spring 2001 and winter 1996. The seasonal changes in mixed‐layer carbonate chemistry are driven by a combination of air‐sea CO2 exchange, biological activity and the formation and melt of sea‐ice. The air‐sea fluxes in 2008 were ∼15 mmol C m−2 d−1, and net community production, estimated from the summertime surface dissolved inorganic carbon (DIC) deficit, ranged from 0.9 to 1.2 mol C m−2 yr−1. We show that biological modification of carbonate chemistry over the shelf in summer preconditions the DSW outflows from the Adélie Depression. This process appears to supply both organic material, and water depleted in CO2, and with enhanced carbonate saturation state (relative to inflowing water), to coral communities on the slope. We combined model‐based transports of exported DSW with the natural and anthropogenic (Cant) carbon concentrations and estimate that the annual outflows of DIC and Cant from the Mertz Polynya range from 320 to 560 Tg C yr−1, and from 3 to 6 Tg Cant yr−1, respectively. The formation and export of dense water from this region, and by extension all similar polynyas around Antarctica, is an effective mechanism for the transfer of anthropogenic carbon into the deep ocean.

Glacier tongue calving reduced dense water formation and enhanced carbon uptake

AbstractDense shelf water formed in the Mertz Polynya supplies the lower limb of the global overturning circulation, ventilating the abyssal Indian and Pacific Oceans. Calving of the Mertz Glacier Tongue (MGT) in February 2010 altered the regional distribution of ice and reduced the size and activity of the polynya. The salinity and density of dense shelf water declined abruptly after calving, consistent with a reduction of sea ice formation in the polynya. Breakout and melt of thick multiyear sea ice released by the movement of iceberg B9B and the MGT freshened near‐surface waters. The input of meltwater likely enhanced the availability of light and iron, supporting a diatom bloom that doubled carbon uptake relative to precalving conditions. The enhanced biological carbon drawdown increased the carbonate saturation state, outweighing dilution by meltwater input. These observations highlight the sensitivity of dense water formation, biological productivity, and carbon export to changes in the Antarctic icescape.

Interannual variability of zooplankton in the Dumont d’Urville sea (139°E − 146°E), east Antarctica, 2004–2008

Spatial and temporal variability of zooplankton was studied during five summers (2004–2008) in the Dumont d’Urville Sea, east Antarctica. The species recorded, based on the catch of a 500 μm-mesh Bongo net, were typical of southern continental shelf communities in Antarctica, including Euphausia crystallorophias, polychaetes, pteropods and biomass-dominant copepods. There was a strong degree of temporal variation in abundance, possibly related to the thickness and extent of the sea ice cover during each spring prior to the surveys. Total mean abundance was highly variable between years, with a minimum of 961 ind. 1000 m −3 in 2004 (range 65–3407 ind. 1000 m −3) and a maximum of 15,627 ind. 1000 m −3 in 2005 (range 5109–33,869 ind. 1000 m −3). Spatially, within each year, abundances were also variable, and there were no uniform patterns in abundance from year to year. Water column physical characteristics (temperature and salinity) were relatively constant and did not contribute substantially to variation between the years. It is likely that variation in zooplankton distribution was largely related to a combination of localised features, such as the thickness and extent of sea ice cover, the position and extent of the Mertz Polynya, local wind conditions and bathymetric features.

Antarctic Bottom Water from the Adélie and George V Land coast, East Antarctica (140–149°E)

We report on observations of dense shelf water overflows and Antarctic Bottom Water (AABW) formation along the continental margin of the Adélie and George V Land coast between 140°E and 149°E. Vertical sections and bottom layer water mass properties sampled during two RVIB Nathaniel B Palmer hydrographic surveys (NBP00–08, December 2000/January 2001 and NBP04–08, October 2004) describe the spreading of cold, dense shelf water on the continental slope and rise from two independent source regions. The primary source region is the Adélie Depression, exporting high‐salinity dense shelf water through the Adélie Sill at 143°E. An additional eastern source region of lower‐salinity dense shelf water from the Mertz Depression is identified for the first time from bottom layer properties northwest of the Mertz Sill and Mertz Bank (146°E–148°E) that extend as far as the Buffon Channel (144.75°E) in summer. Regional analysis of satellite‐derived ice production estimates over the entire region from 1992 to 2005 suggests that up to 40% of the total ice production for the region occurs over the Mertz Depression and therefore this area is likely to make a significant contribution to the total dense shelf water export. Concurrent time series from bottom‐mounted Microcats and ADCP instruments from the Mertz Polynya Experiment (April 1998 to May 1999) near the Adélie Sill and on the upper continental slope (1150 m) and lower continental rise (3250 m) to the north describe the seasonal variability in downslope events and their interaction with the ambient water masses. The critical density for shelf water to produce AABW is examined and found to be 27.85 kg m−3 from the Adélie Depression and as low as 27.80 kg m−3 from the Mertz Depression. This study suggests previous dense shelf water export estimates based on the flow through the Adélie Sill alone are conservative and that other regions around East Antarctica with similar ice production to the Mertz Depression could be contributing to the total AABW in the Australian‐Antarctic Basin.

Cenozoic deformation in the George V Land continental margin (East Antarctica)

This investigation is based on the analysis of multichannel seismic data collected in the continental shelf of the George V Land, between 140°E and 155°E on the East Antarctic margin. Most of the East Antarctic continental shelf is covered by permanent, thick sea and terrestrial ice and it is therefore still unexplored. This is the reason why the tectonic deformation affecting the Antarctic margin during the Mesozoic rift from the Australian plate and the Cenozoic post-rift phase is poorly known. The few coastal polynyas (such as the Mertz polynya, in the George V Land continental shelf) are the only places where the oldest sedimentary section can be studied with the existing technology. The data presented were not collected to address tectonic questions, however the relevance of this study is to document for the first time the occurrence of rift and post-rift tectonic structures in the sedimentary section near the coast, where the oldest sediment section is shallowest. These considerations are particularly relevant as Mertz-Ninnis trough, also known as George V Basin is located near the area of transition (around the Spencer Fracture Zone) between the extensionally-dominated Wilkes Land–Great Australian Bight Basin conjugate segment of the Australian–Antarctic Rift and the transtensional, strike-slip kinematics of the Otway Basin–South Tasman Rise–Oates Land segment. The tectonic structure in the George V Land sector presented in this study is two-fold with two rift phases: one being connected to the breakup process and the later one associated to a change in plate rotation. A former extensional phase opened structural grabens, with axis oriented WNW–ESE and possibly NE–SW. A latter transpressional phase reactivated previous structures and tilted, faulted and folded sedimentary strata, located in the inner continental shelf. The first tectonic phase is likely related with the Cretaceous rifting between the Antarctic and Australian plates. The second tectonic phase might be related to the onset of the fast spreading phase of Pacific–Indian Ocean, that caused uplift, inversion and folding of post-rift strata in a narrow east–west oriented region, near coastal basement outcrop, in Paleocene–Eocene times.

Recent investigations of the Mertz Polynya and George Vth Land continental margin, East Antarctica

Recent investigations of the Mertz Polynya and George Vth Land continental margin, East Antarctica

Wintertime oceanography of the Adélie Depression

Abstract The formation of high-salinity shelf waters beneath coastal polynyas from enhanced sea-ice production and brine rejection during wintertime is critical to the production of Antarctic Bottom Waters. We report on the first wintertime measurements of high-salinity shelf water formation over the Adelie Depression off the East Antarctic coast between 143° and 146°E, during the Mertz Glacier Polynya Experiment in July–August 1999. The general circulation and evolution of water masses in the Adelie Depression during winter are described, and in particular we quantify the rates of brine rejection (sea-ice growth), ocean heat flux, and latent heat flux, in the Mertz polynya central to the formation of high-salinity shelf waters. We find shelf waters above the minimum sill depth with sufficient density to become Adelie Land Bottom Water and suggest the highest production rates occur near Commonwealth Bay. Sea-ice growth and heat transfer rates are calculated from the divergence of heat and freshwater fields around a closed volume beneath the Mertz polynya. The sea-ice growth estimates and associated Monte Carlo errors are found to range from 4.8±1.7, 4.1±1.5, 8.4±1.7 cm d−1, respectively, over a 3-week period. The average growth rate is 5.8 cm d−1. Near the Mertz polynya the sensible heat from ocean transport is between 15±6 and 43±11 W m−2, with an average of 30 W m−2, and the latent heat budget is between 125±45 and 254±52 W m−2, with an average of 174 W m−2. We conclude that during winter the Mertz polynya is primarily a latent heat polynya.