Antarctic Continental Margin Research Articles

Sea level changes forced by Southern Ocean winds

On regional scales, changes in sea level are significantly affected by local dynamical changes. Westerly winds over the Southern Ocean have been strengthening and shifting southward in recent decades, and this change is projected to continue in the future. This study applies wind forcing anomalies to an eddy‐permitting ocean model to study the dynamical response to a Southern Hemisphere westerly wind increase and/or southward shift. It is shown that the applied wind anomalies result in a change in sea surface slope across the Antarctic Circumpolar Current such that a fall in sea level occurs around the Antarctic continental margin. The Antarctic Circumpolar Current transport and regional sea level are particularly sensitive to latitudinal shifts in the wind, with a much more muted response found when only wind strengthening is applied. In addition to the local sea level changes, Southern Ocean winds also have a global effect through changing ocean heat content and the global overturning circulation.

Global sediment thickness data set updated for the Australian‐Antarctic Southern Ocean

We present a new, 5 min sediment thickness grid for the Australian‐Antarctic region (60°E–155°E, 30°S–70°S). New seismic reflection and refraction data have been used to add detail to the conjugate Australian and Antarctic margins and intervening ocean floor where regional sediment thickness patterns were poorly known previously. On the margins, sediment thickness estimates were computed from velocity‐depth functions from sonobuoy/refraction velocity solutions ground‐truthed against seismic reflection data. For the Southeast Indian Ridge abyssal plains, sediment thickness contours from Geli et al. (2007) were used. The new regional minimum sediment thickness grid was combined with the global National Geophysical Data Center (NGDC) sediment grid to create an updated global grid. Even using the minimum estimates, sediment accumulations on the extended Australian and Antarctic continental margins are 2 km thicker across large regions and up to 9 km thicker in the Ceduna Basin compared to the global NGDC compilation of sediment thickness data.

Seismic stratigraphic record of the Amundsen Sea Embayment shelf from pre-glacial to recent times: Evidence for a dynamic West Antarctic ice sheet

Studies of the sedimentary architecture and characteristics of the Antarctic continental margin provide clues about past ice sheet advance-retreat cycles and help improve constraints for paleo-ice dynamic models since early glacial periods. A first seismostratigraphic analysis of the Amundsen Sea Embayment shelf and slope of West Antarctica reveals insights into the structural architecture of the continental margin and shows stages of sediment deposition, erosion and transport reflecting the history from pre-glacial times to early glaciation and to the late Pleistocene glacial-interglacial cycles. The shelf geometry consists of a large pre- and syn-rift basin in the middle shelf region between basement cropping out on the inner shelf and buried basement ridge and highs on the outer shelf. A subordinate basin within the large basin on the mid-shelf may be associated with motion along an early West Antarctic Rift System branch. At least 4 km of pre-glacial strata have been eroded from the present inner shelf and coastal hinterland by glacial processes. Six major sedimentary units (ASS-1 to ASS-6) separated by five major erosional unconformities (ASS-u1 to ASS-u5) are distinguished from bottom to top. Unconformity ASS-u4 results from a major truncational event by glacial advance to the middle and outer shelf, which was followed by several episodes of glacial advance and retreat as observed from smaller-scale truncational unconformities within the units above ASS-u4. Some of the eroded sediments were deposited as a progradional wedge that extends the outer shelf by 25 to 65 km oceanward of the pre-glacial shelf-break. We compare the observed seismic characteristics with those of other Antarctic shelf sequences and assign an Early Cretaceous age to bottom sedimentary unit ASS-1, a Late Cretaceous to Oligocene age to unit ASS-2, an Early to Mid-Miocene age to unit ASS-3, a Mid-Miocene age to unit ASS-4, a Late Miocene to Early Pliocene age to unit ASS-5, and a Pliocene to Pleistocene age to the top unit ASS-6. Buried grounding zone wedges in the upper part of unit ASS-5 on the outer shelf suggest pronounced warming phases and ice sheet retreats during the early Pliocene as observed for the Ross Sea shelf and predicted by paleo-ice sheet models. Our data also reveal that on the middle and outer shelf the flow-path of the Pine Island-Thwaites paleo-ice stream system has remained stationary in the central Pine Island Trough since the earliest glacial advances, which is different from the Ross Sea shelf where glacial troughs shifted more dynamically. This study and its stratigraphic constraints will serve as a basis for future drilling operations required for an improved understanding of processes and mechanisms leading to change in the West Antarctic Ice Sheet, such as the contemporary thinning and grounding line retreat in the Amundsen Sea drainage sector.

Fishing down the food web of the Antarctic continental shelf and slope

ABSTRACTThe history of biotic exploitation for the continental margin (shelf and slope) of the Antarctic Large Marine Ecosystem (LME) is reviewed, with emphasis on the period from 1970 to 2010. In the Antarctic Peninsula portion, marine mammals were decimated by the 1970s and groundfish by the early 1980s. Fishing for Antarctic krill Euphausia superba began upon the demise of groundfish and now is the only fishing that remains in this region. Surveys show that cetacean and most groundfish stocks remain severely depressed, harvest of which is now prohibited by the International Whaling Commission and the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR). On the other hand, krill fishing in this region is underway and in recent years has contributed up to 72% of the Southern Ocean catch, depending on fishing conditions and the CCAMLR conservation measures in force. Elsewhere along the Antarctic continental margin, marine mammals were also severely depleted by the 1970s, followed directly by relatively low-level fisheries for krill that continued until the early 1990s. Recently in these areas, where fin-fishing is still allowed, fisheries for Antarctic toothfish Dissostichus mawsoni have been initiated, with one of this fish's main prey, grenadiers Macrourus spp., being taken significantly as by-catch. Continental margin fishing currently accounts for ~25% of the total toothfish catch of the Southern Ocean. Fishing along the Antarctic continental margin, especially the Antarctic Peninsula region, is a clear case of both the tragedy of the commons and ‘fishing down the food web’.

Geomorphic signature of Antarctic submarine gullies: Implications for continental slope processes

Five quantitatively distinct gully types are identified on the Antarctic continental margin from swath bathymetric data of over 1100 individual features. The gullies differ in terms of length, width, depth, width/depth ratio, cross-sectional shape, branching order, sinuosity and spatial density. Quantitative analysis suggests that Antarctic gully morphology varies with local slope character (i.e. slope geometry, gradient), regional factors (i.e. location of cross-shelf troughs, trough mouth fans, subglacial meltwater production rates, drainage basin size), sediment yield and ice-sheet history. In keeping with interpretations of previous researchers, most gullies are probably formed by hyperpycnal flows of sediment-laden subglacial meltwater released from beneath ice-sheets grounded at the continental shelf edge during glacial maxima. The limited down-slope extent of gullies on the western Antarctic Peninsula is explained by the steep gradient and slope geometry at the mouth of Marguerite Trough, which cause flows to accelerate and entrain seawater more quickly, resulting in a reduction of the negative buoyancy effect of the sediment load. Due to pressure gradients at the ice-sheet bed caused by variations in ice thickness inside and outside palaeo-ice stream troughs, subglacial meltwater flow was generally focussed towards trough margins. This has resulted in gullies with larger cross-sectional areas and higher sinuosities at the trough margins. A unique style of gullying is observed off one part of the western Antarctic Peninsula, corresponding to an area in which the ice-sheet grounding line is not thought to have reached the shelf edge during the Last Glacial Maximum. We interpret the features in this area as the cumulative result of slope processes that operated over a long period of time in the absence of hyperpycnal meltwater flows.

Southern Weddell Sea shelf edge geomorphology: Implications for gully formation by the overflow of high‐salinity water

Submarine gullies are the most common morphological features observed on Antarctic continental slopes. The processes forming these gullies, however, remain poorly constrained. In some areas, gully heads incise the continental shelf edge, and one hypothesis proposed is erosion by overflow of cold, dense water masses formed on the continental shelf. We examined new multibeam echo sounder bathymetric data from the Weddell Sea continental slope, the region that has the highest rate of cold, dense water overflow in Antarctica. Ice Shelf Water (ISW) cascades downslope with an average transport rate of 1.6 Sverdrups (Sv) in the southern Weddell Sea. Our new data show that within this region, ISW overflow does not deeply incise the shelf edge. The absence of gullies extending deeply into the glacial sediments at the shelf edge implies that cold, high salinity water overflow is unlikely to have caused the extensive shelf edge erosion observed on other parts of the Antarctic continental margin. Instead, the gullies observed in the southern Weddell Sea are relatively small and their characteristics indicative of small‐scale slides, probably resulting from the rapid accumulation and subsequent failure of proglacial sediment during glacial maxima.

The first Late Quaternary diatom δ18O record from the west Antarctic Peninsula continental margin

The first Late Quaternary diatom δ18O record from the west Antarctic Peninsula continental margin

New continental margin magnetic anomalies of East Antarctica

Over the past decade, Australian, Norwegian and Russian marine surveys have collected integrated seismic, gravity and magnetic data in the southern Indian Ocean. The more than 350,000 line-km of new airborne and marine magnetic observations for the East Antarctic continental margin have been compiled into an improved definition of crustal magnetic anomaly patterns. This compilation provides important new constraints on the breakup processes and igneous activity related to the formation of the passive margin of East Antarctica. The eastern sector of the map from Bruce Rise in the west to the D'Urville Sea in the east is largely dominated by seafloor spreading magnetic anomalies. The ‘Adélie Rift Block’ of highly stretched and extensively faulted continental crust is associated with a smooth anomaly fabric. Abrupt magnetic anomaly changes along the oceanic-continent transition in the Cooperation Sea including the Enderby Basin Anomaly extend for more than 1680km from the Kerguelen Plateau towards the Cosmonaut Sea. Three sectors of the East Antarctic continental margin exhibit pronounced disparities in the anomaly patterns that strongly suggest different modes of seafloor formation. Strong positive seafloor magnetic anomalies mark the southern margin of the Kerguelen Plateau, the Maud Rise and adjacent areas in the Riiser-Larsen Sea. The new compilation suggests that at least 300km of the Enderby Basin and Shackleton Basin may be part of the Cretaceous Kerguelen Volcanic Province and possibly maps an abandoned ‘fossil’ spreading center in the central Enderby Basin. The majority of the published age models for the Enderby Basin and “Australian sector” of the East Antarctic margin are not in agreement with the structural grain of magnetic anomalies in the newly compiled map.

Holocene subsurface temperature variability in the eastern Antarctic continental margin

We reconstructed subsurface (∼45–200 m water depth) temperature variability in the eastern Antarctic continental margin during the late Holocene, using an archaeal lipid‐based temperature proxy (TEX86L). Our results reveal that subsurface temperature changes were probably positively coupled to the variability of warmer, nutrient‐rich Modified Circumpolar Deep Water (MCDW, deep water of the Antarctic circumpolar current) intrusion onto the continental shelf. The TEX86Lrecord, in combination with previously published climatic records, indicates that this coupling was probably related to the thermohaline circulation, seasonal variability in sea ice extent, sea temperature, and wind associated with high frequency climate dynamics at low‐latitudes such as internal El Niño Southern Oscillation (ENSO). This in turn suggests a linkage between centennial ENSO‐like variability at low‐latitudes and intrusion variability of MCDW into the eastern Antarctic continental shelf, which might have further impact on ice sheet evolution.

Air and shipborne magnetic surveys of the Antarctic into the 21st century

The Antarctic geomagnetics' community remains very active in crustal anomaly mapping. More than 1.5million line-km of new air- and shipborne data have been acquired over the past decade by the international community in Antarctica. These new data together with surveys that previously were not in the public domain significantly upgrade the ADMAP compilation. Aeromagnetic flights over East Antarctica have been concentrated in the Transantarctic Mountains, the Prince Charles Mountains – Lambert Glacier area, and western Dronning Maud Land (DML) — Coats Land. Additionally, surveys were conducted over Lake Vostok and the western part of Marie Byrd Land by the US Support Office for Aerogeophysical Research projects and over the Amundsen Sea Embayment during the austral summer of 2004/2005 by a collaborative US/UK aerogeophysical campaign. New aeromagnetic data over the Gamburtsev Subglacial Mountains (120,000line-km), acquired within the IPY Antarctica's Gamburtsev Province project reveal fundamental geologic features beneath the East Antarctic Ice sheet critical to understanding Precambrian continental growth processes. Roughly 100,000line-km of magnetic data obtained within the International Collaboration for Exploration of the Cryosphere through Aerogeophysical Profiling promises to shed light on subglacial lithology and identify crustal boundaries for the central Antarctic Plate. Since the 1996/97 season, the Alfred Wegener Institute has collected 90,000km of aeromagnetic data along a 1200km long segment of the East Antarctic coast over western DML. Recent cruises by Australian, German, Japanese, Russian, British, and American researchers have contributed to long-standing studies of the Antarctic continental margin. Along the continental margin of East Antarctica west of Maud Rise to the George V Coast of Victoria Land, the Russian Polar Marine Geological Research Expedition and Geoscience Australia obtained 80,000 and 20,000line-km, respectively, of integrated seismic, gravity and magnetic data. Additionally, US expeditions collected 128,000line-km of shipborne magnetic data in the Ross Sea sector.

Advances in on-sea-ice seismic reflection methods using an air gun: McMurdo Sound, Antarctica

During the austral spring of 2008, 48 km of multichannel seismic reflection data were collected on a sea-ice platform east of New Harbor, Southern McMurdo Sound, Antarctica. These seismic data were collected to support the Antarctic Geological Drilling Program whose goal is to recover sedimentary rock cores from the continental margin of Antarctica and to better understand the climatic, cryospheric, and tectonic history of Antarctica. A Generator-Injector air gun, deployed through holes drilled in the sea ice, was used for a seismic source, and reflected energy was recorded on a snow streamer consisting of 60 gimbaled geophones. The Offshore New Harbor seismic system has increased the rate of data acquisition by 53% compared to previous Antarctic air-gun/snow-streamer seismic systems. In addition to conventional on-sea-ice seismic data processing, new processing steps were used to correct problems associated with the use of a Generator-Injector air gun in a sea-ice environment. Source timing errors, caused by miscommunications between the Generator-Injector air gun and air-gun controller, were corrected. Newly discovered bubble-plume static errors and bubble-plume wavelet distortions degraded seismic data quality and are attributed to bubble plumes created by the Generator-Injector air-gun blast. Our investigation shows that these bubble plumes were preserved for at least 40 hours beneath the sea ice and created isolated low-velocity zones. Using finite-element methods, we reproduced static errors and wavelet distortions by incorporating the effects of bubble plumes into the finite-element model. The results of finite-element modeling confirmed our hypothesis that bubble-plume static errors and bubble-plume wavelet distortions were caused by near-surface low-velocity features. Therefore, we incorporated new processing steps which remove bubble-plume effects and improve the quality of final seismic sections.

Linkages between East Antarctic Ice Sheet extent and Southern Ocean temperatures based on a Pliocene high‐resolution record of ice‐rafted debris off Prydz Bay, East Antarctica

Ice‐rafted debris mass accumulation rates (IRD MAR) at a drill site on the Antarctic continental margin are investigated to evaluate the linkages between East Antarctic Ice Sheet extent and Southern Ocean temperatures in the early to mid‐Pliocene. ODP Site 1165 is within 400 km of the Antarctic coastline and in the direct pathway of icebergs released by the Amery Ice Shelf. The Amery Ice Shelf is the largest ice shelf in East Antarctica and it buttresses the Lambert Glacier drainage system, which accounts for 14% of the outflow from the East Antarctic Ice Sheet. IRD MAR were low during peak Southern Ocean warming in the early Pliocene. After a brief precursor, a tenfold increase in IRD MAR at 3.3 Ma marks the termination of the early Pliocene ice sheet minimum, coincident with the M2 glacial. For the mid‐Pliocene, a strong correlation exists between the high‐amplitude signal in the LR04 benthic stack and IRD MAR, suggesting linkages between East Antarctic ice extent, global ice volume and deep‐water temperatures. The IRD record at Site 1165 provides evidence of greater sensitivity of the Lambert Glacier‐Amery Ice Shelf system to Southern Ocean warming than is currently predicted by ice sheet models, which may relate to uncertainties in the understanding of ocean heat uptake, poleward heat transport and ice sheet‐ocean interactions.

Automatic extraction of floating ice at Antarctic continental margin from remotely sensed imagery using object-based segmentation

Information of Antarctic iceberg and sea ice are valuable to Antarctic ice melting patterns studies which are helpful to understand climate conditions and general trends of our planet. This paper presents an automatic floating ice extraction method based on image segmentation technology using region growing. It effectively solves the over-segmentation and under-segmentation problems by merging the gray, contour, location and other information of each ice-object. A pixel-based extraction method is proposed to extract the small ices within 5 pixels. LANDSAT TM data, Chinese environment disaster satellite HJ1B data, and MODIS 1B data used to detect Floating ice at Antarctic continental margin respectively. The results showed that the extraction accuracies of the three kinds of data were all higher than 81 percent, while the accuracies of both TM data and HJ1B data were higher than 90%. Object-based information extraction methods can not only obtain the total area and number of floating ice objects in the whole region, but also provide precise details of single objects, including area, perimeter, contour, average brightness.

Cyclochronology of the Eocene–Oligocene transition from the Cape Roberts Project-3 core, Victoria Land basin, Antarctica

About 34 million years ago, at the Eocene–Oligocene (E–O) transition, Earth's climate underwent a substantial change from relatively ice-free “green house” conditions to a glacial state marked by the establishment of a permanent ice sheet on Antarctica. Our understanding of the Antarctic cryospheric evolution across the E–O climate transition relies on indirect marine geochemical proxies and, hitherto, it has not been possible to reconcile the pattern of inferred ice-sheet growth from these “far-field” proxy records with direct physical evidence of ice sheet behaviour from the proximal Antarctic continental margin. Here we present a correlation of cyclical changes recorded in the CRP-3 drill hole sediment core from the western Ross Sea, that are related to oscillations in the volume of a growing East Antarctic Ice Sheet, with well dated lower latitude records of orbital forcing and climate change across the E–O transition. We evaluate the results in the light of the age model available for the CRP-3A succession. Our cyclostratigraphy developed on the basis of repetitive vertical facies changes and clast peak abundances within sequences matches the floating cyclochronology developed in deep-sea successions for major glacial events. The astrochronological calibration of the CRP-3 succession represents the first high-resolution correlation of direct physical evidence of orbitally controlled glaciation from the Antarctic margin to geochemical records of paleoclimate changes across the E–O climate transition.

Large Multidecadal Salinity Trends near the Pacific–Antarctic Continental Margin

Abstract Ocean temperature and salinity measurements on and near the Antarctic continental shelf in the southwest Pacific sector are evaluated for evidence of temporal change. Shelf water in the southwest Ross Sea has declined in salinity by 0.03 decade−1 from 1958 to 2008, while its temperatures have increased in proportion to the influence of salinity on the sea surface freezing point. Modified deep-water intrusions that reach the central Ross Ice Shelf have freshened at a similar rate and cooled by ∼0.5°C since the late 1970s. Salinity has decreased by 0.08 decade−1 in the westward coastal and slope front currents, consistent with increased melting of continental ice upstream in the Amundsen Sea. Overturning of those near-surface waters during winter sea ice formation and mixing across the slope front is sufficient to account for the 5-decade shelf water salinity change. A strong correlation between the freshening and change in the southern annular mode index suggests a link with the large-scale atmospheric circulation. Salinity has decreased by ∼0.01 decade−1 in bottom and lower deep waters north of the continental slope between 140E° and 180°. Accompanying abyssal temperature changes are minor and variability is high, but density has declined along with salinity. Continued increases in water column stratification will modify the mode and formation rate as well as the properties of bottom and deep waters produced in this region.

Observations of the Antarctic Slope Undercurrent in the southeastern Weddell Sea

The Antarctic Slope Front presents a dynamical barrier between the cold Antarctic shelf waters in contact with ice shelves and the warmer subsurface waters offshore. Two hydrographic sections with full‐depth current measurements were undertaken in January and February 2009 across the slope and shelf in the southeastern Weddell Sea. Southwestward surface‐intensified currents of ∼30 cm s−1, and northeastward undercurrents of 6–9 cm s−1, were in thermal‐wind balance with the sloping isopycnals across the front, which migrated offshore by 30 km in the time interval between the two sections. A mid‐depth undercurrent on February 23 was associated with a 130‐m uplift of the main pycnocline, bringing Warm Deep Water closer to the shelf break. This vertical displacement, comparable to that caused by seasonal variations in wind speed, implies that undercurrents may affect the exchanges between coastal and deep waters near the Antarctic continental margins.

The presence of polynyas in the Weddell Sea during the Last Glacial Period with implications for the reconstruction of sea-ice limits and ice sheet history

Cores from a contourite drift on the continental slope in the NW Weddell Sea, Antarctica, contain up to four sandy foraminifer-rich layers (WA1–4), with dispersed ice-rafted debris (IRD), interbedded with silty–clayey sands and sandy muds. Accelerator Mass Spectrometry (AMS) 14C dating on Neogloboquadrina pachyderma from the foraminifer-rich layers WA2 and WA3 yielded ages spanning the Last Glacial Maximum (LGM) and Marine Isotope Stage (MIS) 2 (20,319–28,543 cal yr BP) and the middle part of MIS 3 (41,349–43,242 cal yr BP), respectively. We attribute the high abundances of planktonic and benthic foraminifera to deposition in a seasonally open-marine environment. Given that the NW Weddell Sea shelf was covered by grounded ice at the LGM (and probably during MIS 2 and MIS 3) and that diatom-based reconstructions place the northern limit of winter and summer sea-ice coverage to the north of the core sites during the Last Glacial Period, our results document the presence of a seasonal or perennial polynya in the NW Weddell Sea. The polynya may have been formed by katabatic winds blowing off the Antarctic Peninsula Ice Sheet (APIS) when it was grounded at the continental shelf break, although polynya formation through upwelling of deep water cannot be ruled out. In addition we argue that previously published data from the southern, southeastern and southwestern Weddell Sea as well as the Ross Sea may indicate the widespread occurrence of polynyas along the Antarctic continental margin during the Last Glacial Period. The widespread occurrence of polynyas could help explain how (i) AABW formation (through brine rejection) continued throughout the LGM, and (ii) Antarctic shelf benthos survived glacial periods. The presence of polynyas through glacial periods also has implications for sea-ice reconstructions from ice core records, possibly biasing the signal towards a reduced sea-ice cover. Finally we propose that the fringes of the APIS, which probably influence the presence or absence of polynyas in the NW Weddell Sea, are sensitive to changes in Antarctic temperature that may be related to a reduction in oceanic heat transport from the North Atlantic.

BMPix and PEAK tools: New methods for automated laminae recognition and counting—Application to glacial varves from Antarctic marine sediment

We present tools for rapid and quantitative detection of sediment lamination. The BMPix tool extracts color and gray scale curves from images at pixel resolution. The PEAK tool uses the gray scale curve and performs, for the first time, fully automated counting of laminae based on three methods. The maximum count algorithm counts every bright peak of a couplet of two laminae (annual resolution) in a smoothed curve. The zero‐crossing algorithm counts every positive and negative halfway passage of the curve through a wide moving average, separating the record into bright and dark intervals (seasonal resolution). The same is true for the frequency truncation method, which uses Fourier transformation to decompose the curve into its frequency components before counting positive and negative passages. The algorithms are available at doi:10.1594/PANGAEA.729700. We applied the new methods successfully to tree rings, to well‐dated and already manually counted marine varves from Saanich Inlet, and to marine laminae from the Antarctic continental margin. In combination with AMS14C dating, we found convincing evidence that laminations in Weddell Sea sites represent varves, deposited continuously over several millennia during the last glacial maximum. The new tools offer several advantages over previous methods. The counting procedures are based on a moving average generated from gray scale curves instead of manual counting. Hence, results are highly objective and rely on reproducible mathematical criteria. Also, the PEAK tool measures the thickness of each year or season. Since all information required is displayed graphically, interactive optimization of the counting algorithms can be achieved quickly and conveniently.

40Ar–39Ar dating of volcanogenic products from the AND-2A core (ANDRILL Southern McMurdo Sound Project, Antarctica): correlations with the Erebus Volcanic Province and implications for the age model of the core

The AND-2A drillcore (Antarctic Drilling Program—ANDRILL) was successfully completed in late 2007 on the Antarctic continental margin (Southern McMurdo Sound, Ross Sea) with the aim of tracking ice proximal to shallow marine environmental fluctuations and to document the 20-Ma evolution of the Erebus Volcanic Province. Lava clasts and tephra layers from the AND-2A drillcore were investigated from a petrographic and stratigraphic point of view and analyzed by the 40Ar–39Ar laser technique in order to constrain the age model of the core and to gain information on the style and nature of sediment deposition in the Victoria Land Basin since Early Miocene. Ten out of 17 samples yielded statistically robust 40Ar–39Ar ages, indicating that the AND-2A drillcore recovered ≤230 m of Middle Miocene (∼128–358 m below sea floor, ∼11.5–16.0 Ma) and >780 m of Early Miocene (∼358–1093 m below sea floor, ∼16.0–20.1 Ma). Results also highlight a nearly continuous stratigraphic record from at least 358 m below sea floor down hole, characterized by a mean sedimentation rate of ∼19 cm/ka, possible oscillations of no more than a few hundreds of ka and a break within ∼17.5–18.1 Ma. Comparison with available data from volcanic deposits on land, suggests that volcanic rocks within the AND-2A core were supplied from the south, possibly with source areas closer to the drill site for the upper core levels, and from 358 m below sea floor down hole, with the “proto-Mount Morning” as the main source.

Impacts of bottom corrugations on a dense Antarctic outflow: NW Ross Sea

Prominent seabed corrugations, axially oriented roughly down‐slope, are present along the Antarctic continental slope. We use analytical and numerical model results to assess the potential impact of these corrugations on outflows of dense shelf water that contribute to Antarctic Bottom Water. Down‐slope flow increases with increasing corrugation height and varies with along‐slope wavelength. For parameters appropriate to the northwest Ross Sea, where heights and wavelengths are ∼10–20 m and ∼1.5 km, respectively, we estimate that the corrugations increase the down‐slope transport of dense water, relative to the smooth bottom case, by ∼13%. Corrugations enhance entrainment and reduce along‐slope speed of the dense outflow. Larger amplitude corrugations (∼100 m) observed in other regions may impact outflows elsewhere around the poorly mapped Antarctic continental margin. Our results emphasize the need to consider small‐scale local topography when modeling dense outflows.