Australian-Antarctic Basin Research Articles

Neodymium isotope composition and rare earth element distribution of East Antarctic continental shelf and deep waters

Circumpolar Deep Water (CDW) and Antarctic Bottom Water (AABW) play key roles in the Earth's climate system. Both water masses form critical components of the Antarctic Circumpolar Current and Meridional Overturning Circulation and therefore directly influence the large-scale redistribution of heat, nutrients and carbon. Reconstruction of past CDW transport and AABW production and export has been a key target in palaeoceanography. One promising proxy to achieve this has been the neodymium (Nd) isotope composition of seawater. The biogeochemical processes controlling Nd in the ocean, however, remain underconstrained, and modern observations of Nd isotopes in the Southern Ocean are still geographically limited.To overcome this limitation, 61 seawater samples were collected for Nd isotope and rare earth element (REE) analysis at nine stations along the Wilkes Land continental margin and in the Australian-Antarctic Basin (65°S 125°E) near East Antarctica. The results show that the different water masses have the following Nd isotope characteristics: Antarctic Surface Water (AASW), εNd = −9.0 ± 1.0 (2SD; n = 22); Modified CDW (MCDW), εNd = −8.8 ± 0.8 (2SD; n = 22); AABW, εNd = −8.3 ± 0.5 (2SD; n = 17).There is no evidence of continental REE inputs to surface waters on the Wilkes Land margin. Observed zonal variability of Nd isotope composition in AASW can be attributed to seasonal competition between the poleward flow of warm AASW from the AAG and the westward export of cold surface shelf waters by the Antarctic Slope Current.In terms of deep and bottom waters, mixing of upwelled CDW with AASW and AABW exclusively controls the Nd isotope composition of MCDW, with no indication of boundary processes modifying the Nd isotope composition of MCDW as it encroaches the shelf or slope. The regional Nd isotope signature for AABW is intermediate between published data for the Atlantic sector AABW (εNd = −9.1 ± 0.7) and Pacific sector AABW (εNd = −7.4 ± 0.9). In the absence of active convection, this Nd isotope signature can be explained by a mixture of advected AABW from the Adélie Land coast with a more dominant component of local MCDW and CDW.The data presented in this study constitute the first Nd isotope data set in this part of the Southern Ocean and suggest conservative behaviour of Nd in proximity to the old East Antarctic continent.

Distribution of stable oxygen isotope in seawater and implication on freshwater cycle off the coast from Wilkes to George V Land, East Antarctica

To investigate the cross-slope freshwater exchange in the Australian-Antarctic Basin between 80 and 150°E, the stable oxygen isotopic ratio (δ18O) was evaluated based on top-to-bottom hydrographic sections by R/V Kaiyo-Maru in 2018/2019 summer. The southern flank of the Antarctic Circumpolar Current (ACC) flows eastward in the north and the Antarctic Slope Current (ASC) flows westward along the continental slope in the south of the analysis domain. Continental shelf regions including the area off Sabrina Coast were also studied. We obtained the spatial δ18O structure characteristics of water masses that were transformed by freshwater input from the highest Circumpolar Deep-Water value. δ18O-salinity relationship in the near-surface layer revealed a regional difference between the upper and lower continental slopes. The fractions of sea ice melt and meteoric water were estimated using mass-balance calculations assuming constant endmembers with no spatial and temporal variabilities. The fraction of sea ice meltwater was usually highest at the surface and rapidly decreased to the temperature minimum layer, while the meteoric water fraction was also highest at the surface, gradually decreasing to the temperature maximum layer. The meteoric water fraction is usually high on the upper slope and continental shelf. The area-average of sea ice meltwater integrated above the temperature minimum is about 0.7 m water-equivalent, with large accumulation off and downstream of the coastal polynyas. Given the projected future glacial ice discharge and sea ice cover changes, repeated δ18O measurement is crucial to implicate the future response of the freshwater cycle closer to the Antarctic continental shelf.

Recent reduced abyssal overturning and ventilation in the Australian Antarctic Basin

Dense water formed near Antarctica, known as Antarctic bottom water (AABW), drives deep ocean circulation and supplies oxygen to the abyssal ocean. Observations show that AABW has freshened and contracted since the 1960s, yet the drivers of these changes and their impact remain uncertain. Here, using observations from the Australian Antarctic Basin, we show that AABW transport reduced by 4.0 Sv between 1994 and 2009, during a period of strong freshening on the continental shelf. An increase in shelf water salinity between 2009 and 2018, previously linked to transient climate variability, drove a partial recovery (2.2 Sv) of AABW transport. Over the full period (1994 to 2017), the net slowdown of −0.8 ± 0.5 Sv decade−1 thinned well-oxygenated layers, driving deoxygenation of −3 ± 2 μmol kg−1 decade−1. These findings demonstrate that freshening of Antarctic shelf waters weakens the lower limb of the abyssal overturning circulation and reduces deep ocean oxygen content.

Changes in Antarctic bottom water off the Wilkes Land coast in the Australian-Antarctic Basin

Long-term changes in Antarctic Bottom Water (AABW) off the Wilkes Land coast were examined using historical hydrographic surveys and data from Deep Argo floats. Since the 1980s at the latest, AABW has contracted by approximately 11–14 m yr−1 and undergone freshening by approximately −0.6 × 10−3 yr−1. The contraction and freshening of AABW has accelerated slightly over the past few decades, with the former being led by the latter. The contraction of AABW was mainly derived from thinning of the denser layers of AABW, which was compensated for by thickening of the upper AABW. This contrast between thinning and compensative thickening has strengthened gradually over time, suggesting that the meridional overturning circulation driven by AABW formation has weakened in the Australian-Antarctic Basin. Dissolved oxygen on isotherms in AABW appears to have remained constant since 1970, except for the possibility of some oxygenation after 2000, which was not statistically significant. After 2010, changes in AABW deviated from the long-term trends, with contraction and freshening of AABW accelerating significantly from 2011 to 2015 and then slowing down from 2015 to 2019. Meanwhile, the bottommost AABW, which is defined as the bottommost 300 m of the water column, increased in salinity significantly in the latter 2010s, mainly in response to contraction of AABW. These processes have resulted in the recent salinification of the bottommost AABW in the eastern Australian-Antarctic Basin, even in areas where the saltier Ross Sea Bottom Water has not arrived yet. In 2018, marked freshening and oxygenation of AABW was observed in the 110°E section, probably due to newly formed bottom water in the Vincennes Bay Polynya.

Diapycnal and isopycnal mixing along the continental rise in the Australian–Antarctic Basin

Shipboard measurements of temperature, salinity and horizontal current velocity were used to estimate diapycnal and isopycnal diffusivities in the Australian sector of the Southern Ocean between 120∘E and 150∘E near 64∘S, along the climatological positions of the Southern Antarctic Circumpolar Current Front and the Southern Boundary. The diapycnal diffusivities estimated by the finescale parameterization on the neutral density surfaces of 1028.05, 1028.2, and 1028.3 kg m−3 are (9.5±6.8)×10−5, (10.7±2.6)×10−5, and (13.8±3.7)×10−5 m2s−1, respectively (mean ± one standard deviation). At these densities, salinity and temperature decreased with depth but salt fingering was unlikely. Using the water mass transformation framework and a box inverse model, the isopycnal diffusivities were estimated from the diapycnal diffusivities. Climatological estimates of the isopycnal diffusivities in the density ranges of Upper Circumpolar Deep Water, Lower Circumpolar Deep Water, and Antarctic Bottom Water were −30±116, 16±319, and −30±83 m2s−1, respectively. These diffusivities were likely underestimates by 10 to 20 m2s−1 because eddy transport could not be considered in the climatological data. The weak isopycnal diffusivities were not inconsistent with diffusivity suppressed by the frontal jets. The large uncertainties were caused by the uncertainty in the box inverse model and the fact that the size of the box was comparable to the advection speed times the diffusion time scale.

Ocean models of the paleooceanographic development of the Australian Antarctic Basin through the Oligocene

Ocean models of the paleooceanographic development of the Australian Antarctic Basin through the Oligocene

Shoaling of abyssal ventilation in the Eastern Indian Sector of the Southern Ocean

Antarctic Bottom Water formation, which ventilates the abyssal layers of the Southern Ocean, is an integral component of the global ocean meridional overturning circulation. Considering evident freshening and density decreases in the source waters, widespread warming in the Southern Ocean suggests a weakening in the Antarctic Bottom Water supply. We demonstrate that the weakening is robust based on water mass warming in the deep and abyssal layers of the Australian-Antarctic Basin, which remained after removing the southward shift effect of the Southern Antarctic Circumpolar Current Front. Moreover, a decrease in apparent oxygen utilisation and reduced warming in the intermediate density layer below Circumpolar Deep Water extended further from the Australian-Antarctic Basin to the South Australian Basin. We suggest that a concurrent weakening in the densest portion and strengthening in the less dense portion shape the multi-basin change in the meridional overturning circulation that originates from the Southern Ocean.

Probing the Mysteries of Deep, Dense Antarctic Seawater

Twelve freely drifting Deep Argo floats reveal year-round dynamics of bottom water flow in the Australian-Antarctic Basin.

Spatial Variability of Antarctic Bottom Water in the Australian Antarctic Basin From 2018–2020 Captured by Deep Argo

AbstractThere are two varieties of Antarctic Bottom Water present in the Australian Antarctic Basin (AAB): locally produced Adélie Land Bottom Water (ALBW) and distantly produced Ross Sea Bottom Water (RSBW). Between 2014 and 2018, RSBW has rebounded from a multidecade freshening trend. The return of the salty RSBW to the AAB is revealed by six Deep Argo floats that have occupied the region from January of 2018 to March of 2020. The floats depict a zonal variation in temperature and salinity in the bottom waters of the AAB, driven by the inflow of RSBW. A simple Optimum Multiparameter Analysis based on potential temperature and salinity gives a sense of scale to the composition of the bottom waters, which are nearly 80% of the new, salty RSBW in the south‐east corner of the basin by 2019 and generally less than 40% to the west closer to the ALBW outflow region and the abyssal plain.

Reversal of freshening trend of Antarctic Bottom Water in the Australian-Antarctic Basin during 2010s

The Antarctic continental margin supplies the densest bottom water to the global abyss. From the late twentieth century, an acceleration in the long-term freshening of Antarctic Bottom Waters (AABW) has been detected in the Australian-Antarctic Basin. Our latest hydrographic observations reveal that, in the late 2010s, the freshening trend has reversed broadly over the continental slope. Near-bottom salinities in 2018–2019 were higher than during 2011–2015. Along 170° E, the salinity increase between 2011 and 2018 was greater than that observed in the west. The layer thickness of the densest AABW increased during the 2010s, suggesting that the Ross Sea Bottom Water intensification was a major source of the salinity increase. Freshwater content on the continental slope decreased at a rate of 58 ± 37 Gt/a in the near-bottom layer. The decadal change is very likely due to changes in Ross Sea shelf water attributable to a decrease in meltwater from West Antarctic ice shelves for the corresponding period.

An Integrated History of the Australian-Antarctic Basin

The first basin-wide compilation of seismic and geologic data shows that both margins experienced similar sedimentation patterns prior to the onset of Antarctic glaciation.

Structure formation peculiarities at early stage of Antarctic–Australia separation based on physical modeling

The paper addresses crustal formation in the Australian–Antarctic basin at the early period of separation of Australia and Antarctica. The study covers long rifting (~160–80 Ma), ultraslow spreading (~80–45 Ma) with the formation of proto-oceanic, mainly ultrabasic crust, spreading (~45-40 Ma), and stationary spreading at medium velocities (after 40 Ma). The different stages of oceanic opening are clearly expressed in the changes of basement morphology (the top of the second oceanic layer) on seismic profiles. Physical modeling is used to reveal the peculiarities in the surface morphology of the oceanic (magmatic) crust which developed in the transitional conditions from ultraslow to slow and medium spreading. Our experiments established that (1) the presence of a stronger block in the pre-breakup model lithosphere in the pathway of the propagating rift faults can significantly affect the geometry of the spreading axis in its vicinity and lead to the development of transversal structures and a highly rugged relief; (2) under the conditions of ultraslow ocean accretion, numerous jumps of the spreading axes occur; (3) the temporary cessation of spreading leads to the development of linear high-amplitude uplifts corresponding to amagmatic ridges in the natural conditions.

Structure-Forming Peculiarities at the Early Stage of Antarctic–Australia Separation Based on Physical Modeling

The paper addresses crustal formation in the Australian–Antarctic basin at the early period of separation of Australia and Antarctica. The study covers long rifting (~160–80 Ma), ultraslow spreading (~80–45 Ma) with the formation of proto-oceanic, mainly ultrabasic crust, spreading (~45-40 Ma), and stationary spreading at medium velocities (after 40 Ma). The different stages of oceanic opening are clearly expressed in the changes of basement morphology (the top of the second oceanic layer) on seismic profiles. Physical modeling is used to reveal the peculiarities in the surface morphology of the oceanic (magmatic) crust which developed in the transitional conditions from ultraslow to slow and medium spreading. Our experiments established that (1) the presence of a stronger block in the pre-breakup model lithosphere in the pathway of the propagating rift faults can significantly affect the geometry of the spreading axis in its vicinity and lead to the development of transversal structures and a highly rugged relief; (2) under the conditions of ultraslow ocean accretion, numerous jumps of the spreading axes occur; (3) the temporary cessation of spreading leads to the development of linear high-amplitude uplifts corresponding to amagmatic ridges in the natural conditions.

Barrier layer characteristics of the Indian Ocean sector of the Southern Ocean during austral summer and autumn

Barrier layer in the Enderby Basin (EB) and the Australian Antarctic Basin (AAB) during late summer (December & January) and early autumn (February & March) are studied using temperature-salinity profiles collected between 1975 and 2012. A distinct difference in mixed layer depth is observed over the eastern (i.e. the EB) compared to western (i.e. the AAB) side of the Kerguelen Plateau (KP), with shallower mixed layer depths on the eastern side. Mixed layers show an increase from less than 50 m–∼150 m from south to north in the EB. During autumn, the wind strengthens and the upwelling over the eastern side of the KP (i.e. in the AAB) weakens, resulting in deeper mixed layers (∼80 m–100 m) compared to summer. During summer, deep barrier layer (BL) values (∼50 m or more) with porosity less than 0.3 was seen over the Chun Spur region. The fresher melt water from the EB brought by the Fawn trough current (FTC) across the KP may be responsible for the occurrence of BL over the region. During autumn, BL is spread over a much larger area around the Chun Spur, which could be attributed to the increase in the strength of FTC due to the intensification of wind over the region. A thorough study of BL condition over this region is required to understand the processes behind the discrepancies in sea ice conditions.

Accelerated freshening of Antarctic Bottom Water over the last decade in the Southern Indian Ocean

Southern Ocean abyssal waters, in contact with the atmosphere at their formation sites around Antarctica, not only bring signals of a changing climate with them as they move around the globe but also contribute to that change through heat uptake and sea level rise. A repeat hydrographic line in the Indian sector of the Southern Ocean, occupied three times in the last two decades (1994, 2007, and, most recently, 2016), reveals that Antarctic Bottom Water (AABW) continues to become fresher (0.004 ± 0.001 kg/g decade-1), warmer (0.06° ± 0.01°C decade-1), and less dense (0.011 ± 0.002 kg/m3 decade-1). The most recent observations in the Australian-Antarctic Basin show a particularly striking acceleration in AABW freshening between 2007 and 2016 (0.008 ± 0.001 kg/g decade-1) compared to the 0.002 ± 0.001 kg/g decade-1 seen between 1994 and 2007. Freshening is, in part, responsible for an overall shift of the mean temperature-salinity curve toward lower densities. The marked freshening may be linked to an abrupt iceberg-glacier collision and calving event that occurred in 2010 on the George V/Adélie Land Coast, the main source region of bottom waters for the Australian-Antarctic Basin. Because AABW is a key component of the global overturning circulation, the persistent decrease in bottom water density and the associated increase in steric height that result from continued warming and freshening have important consequences beyond the Southern Indian Ocean.

Dissolved oxygen change and freshening of Antarctic Bottom water along 62°S in the Australian-Antarctic Basin between 1995/1996 and 2012/2013

Significant warming and freshening signals observed in the world׳s deep oceans are generally stronger southward. In the Australian-Antarctic Basin along 62°S – near the southern edge of the western Pacific and Indian oceans – we compared water properties of Antarctic Bottom Water observed in 1995/1996 and 2012/2013. The basin collects newly ventilated dense water from the Western Pacific sector of the Antarctic coast and the Ross Sea. Compared on pressure surfaces, freshening of more than 0.003g/kg in Absolute Salinity (0.003 in Practical Salinity Scale 1978 or PSS-78) was found below 3000dbar. The freshening was stronger (0.02g/kg or 0.02 in PSS-78) eastward and towards the bottom, suggesting that it originated from the regions of new bottom water production. We also detected warming (>0.2°C) on pressure surfaces below the surface layer. Freshening of similar magnitudes were observed on density surfaces (neutral densities >1028.1kg/m3). Adiabatic southward migration of water masses, observed by satellite altimetry, can explain the warming but not the freshening. A notable increase in dissolved oxygen on density surfaces was found near the bottom. The increase appears due to density decrease of near-bottom water, which can be explained by a change in the mixing ratio of bottom water from near the Adélie Depression (Adélie Land Bottom Water (ALBW), higher oxygen) with that from the more distant Ross Sea (Ross Sea Bottom Water (RSBW), lower oxygen): an increased contribution of ALBW and a decreased contribution of RSBW. This change might be associated with a decrease in the RSBW supply following an earlier ice-calving event in the polynya region. Oxygen rich (>242μmol/kg) and dense (>1028.35kg/m3) bottom water observed in 1995/1996 in troughs near 130°E disappeared in 2012/2013, which also might reflect changes in bottom water production along the coast, possibly in Mertz and Dibble Polynyas.

Interannual Bottom Pressure Signals in the Australian–Antarctic and Bellingshausen Basins

Abstract Analyses of large-scale (>750 km) ocean bottom pressure pb fields, derived from the Gravity Recovery and Climate Experiment (GRACE) and from an Estimating the Circulation & Climate of the Ocean (ECCO) state estimate, reveal enhanced interannual variability, partially connected to the Antarctic Oscillation, in regions of the Australian–Antarctic Basin and the Bellingshausen Basin, with pb magnitudes comparable to those of sea level and good correlation between the GRACE and ECCO pb series. Consistent with the theory of Gill and Niiler, the patterns of stronger pb variability are partly related to enhanced local wind curl forcing and weakened gradients in H/f, where H is ocean depth and f is the Coriolis parameter. Despite weaker H/f gradients, motions against them are sufficiently strong to play a role in balancing the local wind input. Topographic effects are as or more important than changes in f. Additionally, and contrary to the dominance of barotropic processes at subannual time scales, baroclinic effects are not negligible when balancing wind input at periods of a few years. Results highlight the emerging capability to accurately observe and estimate interannual changes in large-scale pb over the Southern Ocean, with implications for the interpretation of low-frequency variability in sea level in terms of steric height and heat content.

Freshening drives contraction of Antarctic Bottom Water in the Australian Antarctic Basin

AbstractAntarctic Bottom Water (AABW) in the Australian Antarctic Basin has become fresher and lighter since the late 1960s, with largest changes observed near the sources of AABW. The volume of AABW with neutral density (γn) > 28.30 kg m−3 decreased by ≥50% between 1969–1971 and 2008–2012, thinning at a rate of >100 m per decade. Contraction of dense AABW was balanced primarily by expansion of water lighter than γn = 28.15 kg m−3 prior to 1995 and by inflation of less dense classes of AABW after 1995. Oxygen concentrations remained unchanged in the densest AABW and increased on neutral surfaces for AABW with γn > 28.31 kg m−3, between 1995 and 2011–2012, consistent with continued ventilation over this time period. The observed changes in potential temperature, salinity, density, volume, and oxygen of AABW can be accounted for by freshening of the source waters but cannot be explained by changes in formation rate alone.

Modeling interannual dense shelf water export in the region of the Mertz Glacier Tongue (1992-2007)

[1] Ocean observations around the Australian-Antarctic basin show the importance of coastal latent heat polynyas near the Mertz Glacier Tongue (MGT) to the formation of Dense Shelf Water (DSW) and associated Antarctic Bottom Water (AABW). Here, we use a regional ocean/ice shelf model to investigate the interannual variability of the export of DSW from the Adélie (west of the MGT) and the Mertz (east of the MGT) depressions from 1992 to 2007. The variability in the model is driven by changes in observed surface heat and salt fluxes. The model simulates an annual mean export of DSW through the Adélie sill of about 0.07 ± 0.06 Sv. From 1992 to 1998, the export of DSW through the Adélie (Mertz) sills peaked at 0.14 Sv (0.29 Sv) during July to November. During periods of mean to strong polynya activity (defined by the surface ocean heat loss), DSW formed in the Adélie depression can spread into the Mertz depression via the cavity under the MGT. An additional simulation, where ocean/ice shelf thermodynamics have been disabled, highlights the fact that models without ocean/ice shelf interaction processes will significantly overestimate rates of DSW export. The melt rates of the MGT are 1.2 ± 0.4 m yr−1 during periods of average to strong polynya activity and can increase to 3.8 ± 1.5 m/yr during periods of sustained weak polynya activity, due to the increased presence of relatively warmer water interacting with the base of the ice shelf. The increased melting of the MGT during a weak polynya state can cause further freshening of the DSW and ultimately limits the production of AABW.

Influence of Ross Sea Bottom Water changes on the warming and freshening of the Antarctic Bottom Water in the Australian-Antarctic Basin

Abstract. Changes to the properties of Antarctic Bottom Water in the Australian-Antarctic Basin (AA-AABW) between the 1990s and 2000s are documented using data from the WOCE Hydrographic Program (WHP) and repeated hydrographic surveys. Strong cooling and freshening are observed on isopycnal layers denser than γn = 28.30 kg m−3. Changes in the average salinity and potential temperature below this isopycnal correspond to a basin-wide warming of 1300 ± 200 GW and freshening of 24 ± 3 Gt year−1. Recent changes to dense shelf water in the source regions in the Ross Sea and George V Land can explain the freshening of AA-AABW but not its extensive warming. An alternative mechanism for this warming is a decrease in the supply of AABW from the Ross Sea (RSBW). Hydrographic profiles between the western Ross Sea and George V Land (171–158° E) were analyzed with a simple advective-diffusive model to assess the causes of the observed changes. The model suggests that the warming of RSBW observed between the 1970s and 2000s can be explained by a 21 ± 23% reduction in RSBW transport and the enhancement of the vertical diffusion of heat resulting from a 30 ± 7% weakening of the abyssal stratification. The documented freshening of Ross Sea dense shelf water leads to a reduction in both salinity and density stratification. Therefore the direct freshening of RSBW at its source also produces an indirect warming of the RSBW. A simple box model suggests that the changes in RSBW properties and volume transport (a decrease of 6.7% is assumed between the year 1995 and 2005) can explain 51 ± 6% of the warming and 84 ± 10% of the freshening observed in AA-AABW.