Astrid Ridge Research Articles

Absence of photophysiological response to iron addition in autumn phytoplankton in the Antarctic sea-ice zone

Abstract. The high nutrient–low chlorophyll condition of the Southern Ocean is generally thought to be caused by the low bioavailability of micronutrients, particularly iron, which plays an integral role in phytoplankton photosynthesis. Nevertheless, the Southern Ocean experiences seasonal blooms that generally initiate in austral spring, peak in summer, and extend into autumn. This seasonal increase in primary productivity is typically linked to the seasonal characteristics of nutrient and light supply. To better understand the potential limitations on productivity in the Antarctic sea-ice zone (SIZ), the photophysiological response of phytoplankton to iron addition (2.0 nM FeCl3) was investigated during autumn along the Antarctic coast off Dronning Maud Land. Five short-term (24 h) incubation experiments were conducted around Astrid Ridge (68∘ S) and along a 6∘ E transect, where an autumn bloom was identified in the region of the western SIZ. Surface iron concentrations ranged from 0.27 to 1.39 nM around Astrid Ridge, and 0.56 to 0.63 nM along the 6∘ E transect. Contrary to expectation, the photophysiological response of phytoplankton to iron addition, measured through the photosynthetic efficiency and the absorption cross-section for photosystem II, showed no significant responses. It is thus proposed that since the autumn phytoplankton in the SIZ exhibited a lack of an iron limitation at the time of sampling, the ambient iron concentrations may have been sufficient to fulfil the cellular requirements. This provides new insights into extended iron replete post-bloom conditions in the typically assumed iron deficient high nutrient–low chlorophyll Southern Ocean.

First phytoplankton community assessment of the Kong Håkon VII Hav, Southern Ocean, during austral autumn

Abstract. We studied phytoplankton and protozooplankton community composition based on light microscopy, flow cytometry, and photosynthetic pigment data in the Atlantic sector of the Southern Ocean during March 2019 (early austral autumn). Sampling was focused on the area east of the prime meridian in the Kong Håkon VII Hav, including Astrid Ridge, Maud Rise, and a south–north transect at 6∘ E. Phytoplankton community composition throughout the studied area was characterized by oceanic diatoms typical of the iron-depleted high-nutrient, low-chlorophyll (HNLC) Southern Ocean. Topography and wind-driven iron supply likely sustained blooms dominated by the centric diatom Chaetoceros dichaeta at Maud Rise and at a station north of the 6∘ E transect. For the remainder of the 6∘ E transect, diatom composition was similar to the previously mentioned bloom stations, but flagellates dominated in abundance, suggesting a post-bloom situation and likely top-down control by krill on the bloom-forming diatoms. Among flagellates, species with haptophyte-type pigments were the dominating group. At Astrid Ridge, overall abundances were lower and pennate diatoms were more numerous than centric diatoms, but the community composition was nevertheless typical of HNLC areas. The observations described here show that C. dichaeta can form blooms beyond the background biomass level and also fuels both carbon export and upper trophic levels within HNLC areas. This study is the first thorough assessment of phytoplankton communities in this region and can be compared to other seasons in future studies.

Variability of Sea-Air Carbon Dioxide Flux in Autumn Across the Weddell Gyre and Offshore Dronning Maud Land in the Southern Ocean

Sea surface fugacity of carbon dioxide (fCO2ssw) was measured across the Weddell gyre and the eastern sector in the Atlantic Southern Ocean in autumn. During the occupation between February and April 2019, the region of the study transect was a potential ocean CO2 sink. A net CO2 flux (FCO2) of −6.2 (± 8; sink) mmol m–2 d–1 was estimated for the entire study region, with the largest average CO2 sink of −10.0 (± 8) mmol m–2 d–1 in the partly ice-covered Astrid Ridge (AR) region near the coast at 68°S and −6.1 (± 8) mmol m–2d–1 was observed in the Maud Rise (MR) region. A CO2 sink was also observed south of 66°S in the Weddell Sea (WS). To assess the main drivers describing the variability of fCO2ssw, a correlation model using fCO2 and oxygen saturation was considered. Spatial distributions of the fCO2 saturation/O2 saturation correlations, described relative to the surface water properties of the controlling variables (chlorophyll a, apparent oxygen utilization (AOU), sea surface temperature, and sea surface salinity) further constrained the interplay of the processes driving the fCO2ssw distributions. Photosynthetic CO2 drawdown significantly offsets the influence of the upwelling of CO2-rich waters in the central Weddell gyre and enhanced the CO2 sink in the region. FCO2 of −6.9 mmol m–2 d–1 estimated for the Weddell gyre in this study was different from FCO2 of −2.5 mmol m–2 d–1 in autumn estimated in a previous study. Due to low CO2 data coverage during autumn, limited sea-air CO2 flux estimates from direct sea-surface CO2 observations particularly for the Weddell gyre region are available with which to compare the values estimated in this study. This highlights the importance of increasing seasonal CO2 observations especially during autumn/winter to improving the seasonal coverage of flux estimates in the seasonal sea ice-covered regions of the Southern Ocean.

The Jurassic history of the Africa–Antarctica corridor — new constraints from magnetic data on the conjugate continental margins

Finding the best fit for East- and West-Gondwana requires a detailed knowledge of the initial Jurassic movements between Africa and Antarctica. This study presents results of systematic and densely spaced aeromagnetic measurements, which have been conducted in 2009/2010 across the Astrid Ridge (Antarctica) and in the western Riiser-Larsen Sea to provide constraints for the early seafloor spreading history between both continents. The data reveal different magnetic signatures of the northern and southern parts of the Astrid Ridge, which are separated by the Astrid Fracture Zone. The southern part is weakly magnetised, corresponding to the low amplitude anomaly field of the southwestern Riiser-Larsen Sea. In contrast, the northern Astrid Ridge bears strong anomalies of positive value. Furthermore, several sets of trends are visible in the data.In the Mozambique Channel, we extended the existing magnetic spreading anomaly identifications. Combined with the spreading anomalies in the conjugate Riiser-Larsen Sea, they were used to establish a new model of the relative movements of Africa and Antarctica after the breakup of Gondwana in Jurassic times. A detailed model for the emplacement of the Mozambique Ridge is now incorporated.The model postulates a tight fit between Africa and Antarctica and two stages of breakup, the first of which lasting until ~159Ma (M33n). During this stage, Antarctica rotated anticlockwise with respect to Africa. The Grunehogna Craton cleared the Coastal Plains of Mozambique to a position east of the Mozambique Fracture Zone. The southern Astrid Ridge is interpreted to consist of oceanic crust, also formed during this first stage, prior to the Riiser-Larsen Sea. During the second stage, Antarctica moved southward with respect to Africa forming the Mozambique Basin and the conjugate Riiser-Larsen Sea. The Mozambique Ridge and the Northern Natal Valley were formed at different spreading centres being active subsequently.

Crustal structure and tectonic provinces of the Riiser-Larsen Sea area (East Antarctica): results of geophysical studies

About 16,000 km of multichannel seismic (MCS), gravity and magnetic data and 28 sonobuoys were acquired in the Riiser-Larsen Sea Basin and across the Gunnerus and Astrid Ridges, to study their crustal structure. The study area has contrasting basement morphologies and crustal thicknesses. The crust ranges in thickness from about 35 km under the Riiser-Larsen Sea shelf, 26–28 km under the Gunnerus Ridge, 12–17 km under the Astrid Ridge, and 9.5–10 km under the deep-water basin. A 50-km-wide block with increased density and magnetization is modeled from potential field data in the upper crust of the inshore zone and is interpreted as associated with emplacement of mafic intrusions into the continental margin of the southern Riiser-Larsen Sea. In addition to previously mapped seafloor spreading magnetic anomalies in the western Riiser-Larsen Sea, a linear succession from M2 to M16 is identified in the eastern Riiser-Larsen Sea. In the southwestern Riiser-Larsen Sea, a symmetric succession from M24B to 24n with the central anomaly M23 is recognized. This succession is obliquely truncated by younger lineation M22–M22n. It is proposed that seafloor spreading stopped at about M23 time and reoriented to the M22 opening direction. The seismic stratigraphy model of the Riiser-Larsen Sea includes five reflecting horizons that bound six seismic units. Ages of seismic units are determined from onlap geometry to magnetically dated oceanic basement and from tracing horizons to other parts of the southern Indian Ocean. The seaward edge of stretched and attenuated continental crust in the southern Riiser-Larsen Sea and the landward edge of unequivocal oceanic crust are mapped based on structural and geophysical characteristics. In the eastern Riiser-Larsen Sea the boundary between oceanic and stretched continental crust is better defined and is interpreted as a strike-slip fault lying along a sheared margin.

Crustal analysis of maud rise from combined satellite and near-surface magnetic survey data

Abstract We produced a crustal magnetization model for the Maud Rise in the southwest Indian Ocean off the coast of East Antarctica using magnetic observations from the Ørsted satellite and near-surface surveys complied by the Antarctic Digital Magnetic Anomaly Project (ADMAP). Joint inversion of the two anomaly fields suggests that the magnetic effects due to crustal thickness variations and remanence involving the normal polarity Cretaceous Quiet Zone (KQZ) dominate at satellite altitude (~700 km). The crustal thickness effects were modeled in the Ørsted data using crustal thickness variations derived from satellite altitude gravity data. Modeling of the residual Ørsted and near-surface magnetic anomalies supports extending the KQZ eastwards to the Astrid Ridge. The remaining near-surface anomalies involve crustal features with relatively high frequency effects that are strongly attenuated at satellite altitudes. The crustal modeling can be extended by the satellite magnetic anomalies across the Indian Ocean Ridge for insight on the crustal properties of the conjugate Agulhas Plateau. The modeling supports the Jurassic reconstruction of Gondwana when the African Limpopo-Zambezi and East Antarctic Princess Astrid coasts were connected as part of a relatively demagnetized crustal block.

Deep Crustal Structure of the Continental Margin off the Explora Escarpment and in the Lazarev Sea, East Antarctica

This study presents the results of a seismic refraction experiment that was carried out off Dronning Maud Land (East Antarctica) along the Explora Escarpment (14° W–12° W) and close to Astrid Ridge (6°E). Oceanic crust of about 10 km thickness is observed northwest of the Explora Escarpment. Stretched continental crust, observed southeast of the escarpment, is most likely intruded by volcanic material at all crustal levels. Seismic velocities of 7.0–7.4 km/s are modelled for the lower crust. The northern boundary of this high velocity body coincides approximately with the Explora Escarpment. The upper crystalline crust is overlain by a 4-km thick and 70-km wide wedge of volcanic material: the Explora Wedge. Seismic velocities for the oceanic crust north of the Explora Escarpment are in good agreement with global studies. The oceanic crust in the region of the Lazarev Sea is also up to 10-km thick. The lower crystalline crust shows seismic velocities of up to 7.4 km/s. This, together with the larger crustal thickness might point to higher mantle temperatures during the formation of the oceanic crust. The more southerly rifted continental crust is up to 25-km thick, and also has seismic velocities of 7.4 km/s in the lower crystalline crust. This section is interpreted to consist of stretched continental crust, which is heavily intruded by volcanic material up to approximately 8-km depth. Multichannel seismic data indicate that, in this region, two volcanic wedges are present. The wedges are interpreted to have evolved during different time/rift periods. The wedges have a total width of at least 180 km in the Lazarev Sea. Our results support previous findings that the continental margin off Dronning Maud Land between ≈2°E and ≈13°E had a complex and long-lived rift history. Both continental margins can be classified as rifted volcanic continental margins that were formed during break-up of Gondwana.

A Compilation of Geophysical Data from the Lazarev Sea and the Riiser-Larsen Sea, Antarctica

On the basis of new geophysical data acquired by the Federal Institute of Geosciences and Natural Resources (BGR) and the Polar Marine Geological Research Expedition (PMGRE) as well as existing data new geophysical maps were compiled for the Lazarev Sea and the Riiser-Larsen Sea between 10°W and 25°E. The new results are: – The drastic change in the strike direction of the volcanic Explora Wedge between longitudes 10°W and 5°W is accompanied with a gradual change from one major wedge, i.e. the Explora Wedge, into at least two wedge-shaped volcanic constructions, each manifested by a sequence of seaward-dipping reflectors in the seismic records. – The southern Lazarev Sea is best described as a continental margin affected by multiple rifting episodes accompanied with transient volcanism. – A distinct N80°E striking basement depression separates the volcanic-prone continental margin of the southern Lazarev Sea from oceanic crust upon which the Maud Rise rests. The southern scarp of the narrow depression was presumably aligned with the eastern scarp of the Mozambique Ridge during the Early Cretaceous. – The Astrid Ridge proper occupies the transition from the volcanic-prone continental margin of the Lazarev Sea to old oceanic crust of the Riiser -Larsen Sea, and it rests upon a large volcanic apron which covers the basement of the southwestern Riiser-Larsen Sea. – No evidence was found that prolific volcanism has affected the early opening of the Riiser-Larsen Sea. – The Lazarev Sea is a sediment-starved region.

Magnetic susceptibility and ice-rafted debris in surface sediments of the Atlantic sector of the Southern Ocean

Spatial distribution of magnetic susceptibility and the gravel fraction in surface sediments in the Atlantic sector of the Southern Ocean were investigated to reconstruct source areas and recent transport pathways of magnetominerals and ice-rafted debris. Maxima of magnetic susceptibility were observed offshore from areas where mafic source rocks occur, e.g. Queen Maud Land and the northern Antarctic Peninsula. The glacigenic input of debris and subsequent redeposition of fine material by bottom and turbidity currents on the continental margins result in regional variations of the gravel and susceptibility values. In the deep sea, however, the mixing of ice-rafted debris and turbidites from distal source areas causes a homogenous distribution of the susceptibility signal. On submarine elevations such as Maud Rise and Astrid Ridge, dust input may be an additional source for magnetominerals.

Miocene diatom biostratigraphy at ODP Sites 689, 690, 1088, 1092 (Atlantic sector of the Southern Ocean)

Four ODP sites located between 64°S and 41°S in the eastern Atlantic sector of the Southern Ocean were investigated to refine the Miocene diatom biostratigraphic zonation tied to the geomagnetic chronology. The Miocene diatom stratigraphy from two sites located on Maud Rise (ODP Leg 113) is revised considering the progress in diatom biostratigraphic research, diatom taxonomy and magnetostratigraphic age assignment during the past 10 years. A new diatom zonation was erected for Site 1092 (ODP Leg 177) located on Meteor Rise integrating a magnetostratigraphic interpretation of the shipboard data. This zonation was also applied to Site 1088 (ODP Leg 177) located on Astrid Ridge. The study is focused to Middle and Upper Miocene sequences. It reveals latitudinal differentiations in stratigraphic species ranges and species occurrence pattern that are related to latitudinal differences in surface water masses reflecting the climatic development of the Antarctic cryosphere. Considering the latitudinal differences two stratigraphic zonations are proposed that are applicable to the northern and southern zone of the Southern Ocean, respectively. The southern Southern Ocean Miocene diatom biostratigraphic zonation consists of 16 zones in which 11 represent new or modified zones. The northern biostratigraphic zonation contains 10 diatom zones allowing a stratigraphic resolution in the range of 0.2–2 Myr. This paper also includes the taxonomic transfer of seven Miocene diatom taxa from genus Nitzschia Hassal to Fragilariopsis Hustedt.

Cretaceous reconstructions of East Antarctica, Africa and Madagascar

Abstract Some Cretaceous reconstructions of East Antarctica, Africa, and Madagascar overlap the Mozambique ridge onto the margin of Dronning Maud Land, and others place Madagascar west of the Gunnerus ridge, into the Riiser–Larsen Sea. Our M10Ny (131.9 Ma), M0 (120.4 Ma), 96 Ma, and 34y (83 Ma) marine free-air gravity and topography reconstructions illustrate a solution to the overlap problem and also demonstrate that Madagascar separated from Antarctica east of the Gunnerus ridge. Our reconstructions chronicle the growth of the conjugate Mozambique and Riiser–Larsen Basins created at the Southwest Indian spreading ridge. Well defined spreading corridors, identified by conjugate fracture zones and Mesozoic magnetic anomaly identifications, extend from one basin into the other. We find that the eastern scarp of the Mozambique ridge aligns with the Astrid ridge, and there is no room for Madagascar west of the Gunnerus ridge. The overlap of the Mozambique ridge is solved by taking into account the motion of an independent Mozambique ridge microplate that existed from Chron M11 to circa M2.

The Cretaceous-Tertiary sea floor off Dronning Maud Land, Antarctica

A study of the bathymetric and linear magnetic anomalies between Dronning Maud Land, Antarctica and the South West Indian Ridge (SWIR) revealed a Mesozoic sequence of linear magnetic anomalies, M0 to M12 (108–126 Ma), a fracture zone offset (∼- 130 km), and the Cretaceous magnetic quiet zone between isochrons M0 (108 Ma) and 34 (80 Ma). The study sheds light on: 1. (1) the evolution of an anomalous “stretched” pattern of seafloor anomalies during magnetic isochrons M0 and 34, and the associated offset (> 10°) in some segments of the northeastern part of the SWIR; 2. (2) continuation of a proposed buried fracture zone to the south, where it abuts the Astrid Ridge off Dronning Maud Land. The history of spreading is related to declining dynamic pressures at the plate margin due to the Kergeulen-Heard mantle plume during Cretaceous time (∼-110 Ma).

Isostatic response of the lithosphere beneath the Mozambique Ridge (SW Indian Ocean) and geodynamic implications

SUMMARY The SW Indian Ocean is characterized by the presence of several aseismic features. The Mozambique Ridge, an elongated feature lying roughly parallel to the SE coast of Africa, is by far the least known of those structures, mainly due to the scarcity of marine data. To date, the crustal nature and the origin of the ridge are still controversial points. Since knowledge of the origin of the Mozambique Ridge is important for a better understanding of the evolution of the SW Indian Ocean, the isostatic response of the lithosphere beneath the ridge is analysed in order to characterize its effective elastic thickness and the emplacement process of the feature. Two different approaches are applied, the direct computation of the geoid anomaly over the ridge, by means of a 2.5-dimensional method, and the computation of the admittances between the bathymetry and both the geoid and gravity anomalies. Both approaches point to a local isostatic response of the lithosphere. The crustal thickness beneath the Mozambique Ridge ranges from 17 to 30 km and the average density, from 2.78 to 2.7 × 103 kg m−3, varying with locality, in good agreement with the few existing refraction profiles. Based on our results, on the geochemical similarity between the basalts cored at the DSDP site 249 and the MOR basalts (Erlank & Reid 1974; Thompson et al. 1982) and on the present knowledge of the SW Indian Ocean kinematics (Martin & Hartnady 1986), we propose an on-ridge origin for the Mozambique Ridge. The ridge would have been formed by the anomalous activity of a spreading axis linking the northern Mozambique and the Transkei basins accretion centres, between M10 and M2 times. At the M2 epoch, a ridge jump event would have caused the spreading to cease. The Astrid Ridge, a poorly known aseismic structure lying close to the coast of Antarctica near 15°E, may be the antarctic counterpart of the Mozambique Ridge, formed by the same accretionary phenomenon, the two ridges breaking apart at M2 time due to the ridge jump event. However, any admittance computation at such high latitudes is impossible at present, due to the very poor quality of both bathymetry and altimetry, and so this hypothesis cannot be verified.

Geology and structure of the Astrid Ridge, Dronning Maud Land, Antarctica

Bathymetric (458 line km), Seismic reflection, higher energy sparker source (405 line km) and magnetic (452 line km) studies of the Astrid ridge, a prominent broad bathymetric feature in water depths of about 1520–2900 m on the continental margin of Dronning Maud Land, are carried out. The crest of the ridge in the area surveyed trends N‐S in the south but veers to NNW‐SSE at about 68° S Latitude and is found to change to NNE‐SSW at about 67° S Latitude. The western flanks of the ridge in the south are gently sloping while the eastern flanks are marked by NNE‐SSW trending scarps of about 80–280 m. The northern area is marked by a NW to NNW‐SE to SSE trending valley, about 5–15 km wide and 240–380 m deep with steep gradients. The ridge is mantled by layered sediments of a top transparent layer (50 to 0.4 s) underlain by a prominent reflector (50 ms to 100 ms) and the acoustic basement. The basement occur at 0.4 to 0.6 s depth. The sediments dip away from the crest of the ridge to the flanks. The seismic records further indicated that the NNE‐SSW trending scarps on the eastern flanks represent faults which extend only to the younger sediments. They are not reflected in the magnetics indicating that they are not deeper structures. The NW‐NNW to SE‐SSE trending narrow valleys with steep gradient walls occurring on the ridge crest in the study area are grabens where the basement and associated sediments have been faulted down. The graben in the north is marked by a series of magnetic anomalies of about 50 to 200 nT superposed over the peak of a broad high amplitude anomaly of a mafic material buildup and appears to be a deep crustal feature. The positive bathymetric feature of the ridge part in the south of the study area is not reflected in magnetic anomaly signatures, implying heterogeneity in composition. The magnetic basement is interpreted at depths to top of 1.8 km to 4.0 km. The graben appears to be a major tectonic feature of the area and marks a change in the trend of the ridge at about 68°S Latitude.

Underlying fracture zone nature of Astrid Ridge off Antarctica's Queen Maud Land

New geophysical data are used to refine the extent and setting of Astrid Ridge, a volcanic edifice lying off the Princess Astrid Coast section (5°E to 20°E) of Queen Maud Land in Antarctica. New findings presented here include a well‐defined fracture zone, occurring as a narrow basement ridge which extends northeast from Astrid Ridge, and an adjacent set of Mesozoic magnetic anomalies M0 to M9, which are offset by 120 km from an identical, previously described set. These two sets of anomalies are combined with the conjugate Mozambique Basin anomalies to define an Africa‐Antarctica rotation pole at 10°S, 29°W for anomaly M2 time. The northern edge of Astrid Ridge and the sharp southern margin of the Mozambique Ridge on the African plate are probably related to the same cessation of excess volcanic activity near the end of the Cretaceous normal polarity interval. Both features had their eastern and western limits confined by a contiguous pair of fracture zones. A possible major swing in fracture zone trend prior to anomaly M9 is also suggested.

Plate tectonic development of the south west Indian Ocean: A revised reconstruction of East Antarctica and Africa

A revised sequence of reconstructions for East Antarctica relative to Africa is generated from published seafloor spreading and fracture zone anomalies. These date from the Cretaceous/Tertiary boundary to the Late Jurassic and lead to a revised predrift reconstruction for east and west Gondwanaland: Euler pole 1.67°S, 35.99°E, rotation angle 53.43°. This reconstruction derived wholly from marine data places East Antarctica adjacent to the Lebombo Mountains and south of and subparallel to the Sabi acid volcanic suite, supporting the postulate that these rocks represent Mesozoic plate boundaries. This refit implies that part of the coastal plain of Mozambique is underlain by oceanic or very highly extended continental crust. The reconstruction is supported by the alignment of (1) the Cape Fold Belt with its Antarctic equivalents and (2) the western limit of Pan‐African reactivation of the Mozambique Mobile Belt with the western limit of the Sverdrupfjella metamorphic suite. Antarctica initially moved along a transform fault lying immediately east of and subparallel to the Lebombo Mountains, the eastern face of the Tugela Cone, the southeastern face of the Falkland Plateau in its reconstructed position, and the eastern flank of the Agulhas Plateau in its reconstructed position. At M10 time a triple junction formed near the tip of the Falkland Plateau implying that the oldest lineated magnetic anomalies in the eastern Weddell Sea associated with this triple junction cannot predate M10. At M2 time the Mozambique Ridge rifted from the Astrid Ridge in a ridge jump episode. Such an event may also explain the presence of magnetic anomalies in the southern Mozambique Basin. Gunnerus Ridge, Conrad Rise, and Del Cano Rise are all tectonically associated with the Madagascar Ridge.

Post-miocene disconformities and paleoceanography in the Atlantic sector of the Southern Ocean

The magnetostratigraphy and biostratigraphy of 166 Islas Orcadas piston cores in the Atlantic sector of the Southern Ocean reveal disconformities in the sedimentary record which are attributed to bottom-current erosion. The hiatus frequency as a function of water depth reveals both shallow erosion by Circumpolar Deep Water (CDW) and deep erosion by Antarctic Bottom Water (AABW). The post-Miocene history of erosion by both bottom-currents is delineated by analysis of areal and temporal fluctuations of hiatus frequency. Erosion by CDW caused disconformities on the Maud Rise, Astrid Ridge, Indian—Antarctic Ridge, Scotia Sea, and Islas Orcadas Rise. The shallow disconformities increased in frequency and size from ∼ 4.0 to 1.0 Ma with two periods of distinctly widespread disconformities from 5.5 to 4.5 Ma and 2.5 to 1.5 Ma; after 1.0 Ma the frequency and extent of the hiatuses decreased. Erosion by AABW caused disconformities in the Weddell Abyssal Plain, Argentine Basin, and through a narrow conduit in the South Sandwich Basin downstream from the Bullard Fracture Zone. The deep disconformities increased in frequency and size from 4.5 to 5.0 Ma, ∼ 3.5 to 4.0 Ma and from ∼ 1.0 to 2.5 Ma; after ∼ 1.5 Ma the frequency and extent of hiatuses decreased. Fluctuations in hiatus frequency are consistent with results of a similar study in the southeast Indian Ocean and may be correlated with paleoceanographic events. Southern Ocean cooling and possible re-establishment of the West Antarctic ice sheet and fringing ice-shelves at ∼ 4.3-3.5 Ma was accompanied by increased AABW and CDW activity. The formation (expansion?) of the Northern Hemisphere ice sheets at ∼ 2.5–3.0 Ma resulted in a decrease in AABW velocity and was accompanied by increased CDW activity. Both deep and shallow bottom-currents increased in velocity during the late Matuyama Chron (∼ 2.0–1.5 Ma) as a result of Antarctic cooling. After ∼ 1.0 Ma, however, the frequency and extent of shallow and deep hiatuses was reduced, and the modern extent of hiatuses is the lowest within the post-Miocene.

Sedimentology of the seabed sediments from the Astrid Ridge, Princess Astrid coast, Queen Maud Land, Antarctica : Siddiquie, H.N. and N.H. Hashimi, 1983. In: Scientific Report of First Indian Expedition to Antarctica. Technical Publication No. 1; Department of Ocean Development, New Delhi, India; pp. 114–122

Sedimentology of the seabed sediments from the Astrid Ridge, Princess Astrid coast, Queen Maud Land, Antarctica : Siddiquie, H.N. and N.H. Hashimi, 1983. In: Scientific Report of First Indian Expedition to Antarctica. Technical Publication No. 1; Department of Ocean Development, New Delhi, India; pp. 114–122

A geomorphologic, seismic and magnetic study of the Astrid Ridge, Dronning Maud Land, Antarctica : Siddiquie, H.N., D.G. Rao and M.V. Ramana, 1983. In: Scientific Report of First Indian Expedition to Antarctica. Technical Publication No. 1; Department of Ocean Development, New Delhi, India; pp. 166–181

A geomorphologic, seismic and magnetic study of the Astrid Ridge, Dronning Maud Land, Antarctica : Siddiquie, H.N., D.G. Rao and M.V. Ramana, 1983. In: Scientific Report of First Indian Expedition to Antarctica. Technical Publication No. 1; Department of Ocean Development, New Delhi, India; pp. 166–181