Eastern Ross Sea Research Articles

Cenozoic prograding sequences of the Antarctic continental margin: a record of glacio-eustatic and tectonic events

Sedimentary sections up to 6–14 km thick lie beneath many areas of the Antarctic continental margin. The upper parts of the sections contain up to 6 km of Cenozoic glacial and possibly non-glacial sequences that have prograded the continental shelf up to 85 km. We describe the Cenozoic sequences using two general categories based on their acoustic geometries. Type IA sequences, which account for most prograding of the Antarctic continental shelf, have complex sigmoidal geometries and some acoustic characteristics atypical of low-latitude margins, such as troughs and mounds lying parallel and normal to the shelf edge and high velocities (2.0–2.6 km/s) for flat layers within 150 m of the seafloor. Type IIA sequences, which principally aggrade the paleoshelf, lie beneath type IA sequences and have mostly simple geometries and gently dipping reflections. The prograding sequences are commonly located near the seaward edges of major Mesozoic and older margin structures. Relatively rapid Cenozoic subsidence has occured due to the probable rifting in the Ross Sea, thermal subsidence in the Antarctic Peninsula, and isostatic crustal flexure in Wilkes Land. In Prydz Bay and the Weddell Sea, prograding sequences cover Mesozoic basins that have undergone little apparent Cenozoic tectonism. Grounded ice sheets are viewed by us, and others, as the principal mechanism for depositing the Antarctic prograding sequences. During the initial advance of grounded ice the continental shelf is flexurally overdeepened, the inner shelf is heavily eroded, and gently dipping glacial strata are deposited on the shelf (i.e type IIA sequences). The overdeepened shelf profile is preserved (a) during glacial times, by grounded ice sheets episodically crossing the shelf, eroding sediments from onshore and inner shelf areas, and depositing sediments at the front of the ice sheet as outer shelf topset-banks and continental slope foreset-aprons (i.e. type IA sequences), and (b) during interglacial times, like today, by little or no clastic sedimentation on the continental shelf other than beneath retreated ice shelves lying far from the continental sheld edge. Ice streams carve broad depressions across the shelf and carry abundant basal sediments directly to the continental shelf edge, thereby creating troughmouth fans and sheet-like prograding sequences (i.e. type IA sequences). Numerous acoustic unconformities and multiple overcompacted layers within the prograding sequences suggest major fluctuations of the Antarctic Ice Sheet. The available drilling and seismic interpretations provide the following history: (1) Cenozoic ice sheets have existed in places near the continental shelf since middle to late Eocene time. (2) A grounded Antarctic ice sheet first expanded to the continental shelf edge, with probable overdeepening of the outer shelf, in late Eucene to early Oligocene time in Prydz Bay, possibly in early Miocene time in the Ross Sea, and at least by middle Miocene time in the Weddell Sea. (3) The relative amounts of shelf prograding and inferred ice-volume variations (and related sea-level changes) have increased since middle to late Miocene time in the eastern Ross Sea, Prydz Bay, and possibly Weddell Sea. Our analysis is preliminary. Further acoustic surveys and scientific drilling are needed to resolve the proximal Antarctic record of glacio-eustatic, climatic, and tectonic events recorded by the prograding sequences.

Chlorofluorocarbon evidence for rapid ventilation of the Ross Sea

Chlorofluorocarbon (CFC)-12 and -11 (CF 2Cl 2 and CFCl 3) measurements were made in seawater on the Ross Sea continental shelf and adjacent slope region in 1984. Concentrations of CFC-12 and CFC-11 in Ross Sea continental shelf water averaged nearly half that of saturated surface water. Circumpolar Deep Water within 50 km of the Ross Sea continental shelf also contained measureable CFC-12 and CFC-11, but an order of magnitude less than shelf waters. CFC-12 and CFC-11 concentrations in the deep water overlying the continental slope increased with depth, indicating recent ventilation and bottom water formation near the continental shelf. Several water masses on the continental shelf that are commonly distinguished on the basis of temperature and salinity characteristics also varied in CFC content, and thus in their modes and rates of ventilation and renewal. A time-dependent model reproducing the 1984 sub-surface shelf water CFC concentrations demonstrates the relative importance of mixed layer entrainment, gas exchange through leads in the winter sea ice field, and mixing with source waters derived from the Circumpolar Deep Water. Model results show that mixed layer entrainment at the beginning of winter is the dominant process ventilating sub-surface shelf water in the eastern Ross Sea, while western shelf waters also require significant gas exchange during winter. Shelf water residence time can only be constrained by the model and available data to 10 years, but most probable values are ∼2.5 years in the eastern Ross Sea and ∼4 years in the western Ross Sea. CFC concentrations in water which has circulated beneath and has been modified by the Ross Ice Shelf indicate that the transformation time from High Salinity Shelf Water to Ice Shelf Water can be as short as 3.5 years. With reasonable oxygen consumption and nutrient regeneration rates, the balance of ventilation and mixing processes that reproduced observed shelf water CFC-12 concentrations also can account for observed oxygen and nitrate levels, if the western surface waters are undersaturated by ∼10% at entrainment.

Cenozoic Prograding Sequences of the Antarctic Continental Margin--What Balance Between Structural and Eustatic Control?: ABSTRACT

Multichannel seismic reflection profiles across the Antarctic continental margin commonly reveal prograding sedimentary sequences that are bounded by unconformities. These sequences are as much as 5 km thick and, where sampled, are composed entirely of late Eocene( )-early Oligocene and younger glacial rocks. On nonpolar margins, prograding sequences generally are attributed to relative changes in sea level, sediment supply, and tectonism. Around Antarctica, ice sheets have also been important in controlling the geometry and location of prograding sequences. The Antarctic sequences may provide a proximal record of major Cenozoic ice volume changes and related sea level changes not obtainable from low-latitude continental shelves. Presently, the Antarctic record is poorly known because of limited core data. Two categories of prograding (P) and aggrading (A) sigmoidal sequences are observed around Antarctica: (1) P sequences that build principally outward (common) and (2) AP sequences that build largely upward and outward (less common). P sequences may result principally from grounded ice sheets, and AP sequences from open-marine basinal processes. Major rift embayments of Antarctica (e.g., eastern Ross Sea eastern Weddell Sea Lambert graben Wilkes basin) are also pathways for major ice movement. In general, most areas with P sequences lie within or adjacent tomore » Mesozoic or older rift embayment, whereas the primary area with AP sequences (eastern Ross Sea) lies within a likely Cenozoic rift embayment. The Pacific side of the Antarctic Peninsula where Cenozoic ice sheets and Cenozoic tectonism have been active, is also marked by a P sequence. Scientific drilling on the Antarctic continental shelf has recovered openwater glacial deposits (Ross Sea) as well as glacial diamicts that were deposited beneath and in front of grounded glacier ice (Ross Sea and Prydz Bay).« less

Biogenic-silica accumulation in the Ross Sea and the importance of Antarctic continental-shelf deposits in the marine silica budget

Thirty-five box cores were collected from the continental shelf in the Ross Sea during cruises in January and February, 1983. Pb-210 and Pu-239, 240 geochronologies coupled with biogenic-silica measurements were used to calculate accumulation rates of biogenic silica. Sediment in the southern Ross Sea accumulates at rates ranging from ≤0.6 to 2.7 mm/y, with the highest values occurring in the southwestern Ross Sea. Biogenic-silica content in surface sediments ranges from 2% (by weight) in Sulzberger Bay and the eastern Ross Sea to 41% in the southwestern Ross Sea. Biogenic-silica accumulation in the southwestern Ross Sea averages 2.7 × 10 −2 g/ cm 2/ y and is comparable to accumulation rates in high-productivity, upwelling environments from low-latitude continental margins ( e.g., Gulf of California, coast of Peru). The total rate of biogenic-silica accumulation in the southern Ross Sea is approximately 0.2 × 10 14 g/ y, with most of the accumulation occurring in basins (500–1000 m water depth). If biogenic-silica accumulation in the southern Ross Sea continental shelf is typical of other basins on the Antarctic continental shelf, as much as 1.2 × 10 14 g/ y of silica could be accumulating in these deposits. Biogenic-silica accumulation on the Antarctic continental shelf may account for as much as a fourth of the dissolved silica supplied to the world ocean by rivers and hydrothermal vents.

A geophysical transect of the Adélie Margin, East Antarctica

In 1982, a geophysical survey of the Antarctic margin, including multichannel seismic, gravity, magnetic and bathymetric surveying, was carried out off Adélie Coast-Wilkes Land and in the eastern Ross Sea. Of the 5000 km of lines recorded, 3000 km were in the Adélie Coast area. Lines ATC 101–102, approximately following meridian 138°E, show the first complete transect of the Adélie Coast margin from the Southeast Indian abyssal plain at DSDP Site 269 to the continental shelf. These lines reveal a thick sedimentary series divided into three main acoustic units by two major unconformities considered to be Upper Eocene (42 Ma) and late Oligocene (25 Ma). Oceanic or continental basement can be traced under the whole area, and the ocean-continent boundary clearly lies beneath the lower continental slope. A deep, high-amplitude, low-frequency horizon, extending under the oceanic basement for over 300 km of line, is considered to be the Moho. Our interpretation supports a recent revision be Cande and Mutter proposing an early Upper Cretaceous opening between Australia and East Antarctica.

History of the Ross Sea region during the deposition of the Beacon Supergroup 400 - 180 million years ago

Abstract The Beacon Supergroup is the generally flat-lying sedimentary sequence about 2½km thick that rests unconformably on Ordovician and older rocks of the East Antarctic craton. In the Ross Sea region exposure is almost entirely limited to the Transantarctic Mountains, though Devonian rocks are now known from Marie Byrd Land. The history of the region is summarised on five palaeogeographic maps using a chronology based on palynomorphs but with help from several other fossil groups and the simple lithostratigraphy. However, the time-transgressive nature of the stratigraphy is recognised from the appearance of volcanic debris in the Permian at successively higher levels down the palaeoslope. The persistent feature of sedimentation during Beacon times is the development of elongate sedimentary basins sited along the present Transantarctic Mountains. Then the eastern Ross Sea and Marie Byrd Land were similar in elevation to the East Antarctic craton on the other side of the Beacon basins, from which it is...

The Campbell Plateau and its relationship with the Ross Sea, Antarctica

Reconstructions of Gondwanaland place the Campbell Plateau adjacent to the eastern Ross Sea continental shelf and western Marie Byrd Land prior to approximately Late Cretaceous times. Seismic reflection data show that many basement structures on the Campbell Plateau trend in an approximately northeast—southwest direction, parallel to the axis of rifting of New Zealand from Antarctica. The only structures which may have been continuous in pre-rifting times are the Campbell Basin, underlying the southwest part of the Campbell Plateau, and a deep sedimentary trough underlying the eastern Ross Sea. A linear belt of large-amplitude magnetic anomalies trends east—west across the northern Campbell Plateau, apparently crossing the eastern shelf edge obliquely. They are probably caused by magnetic rocks of pre-Late Cretaceous age.

Sea ice motions off Antarctica in the vicinity of the eastern Ross Sea as observed by satellite

Satellite imagery provided by the NOAA 2 satellite was used to make observations of the motions of sea ice near the Ross Sea off Antarctica. The sea ice studied displayed a highly variable motion when it was viewed over short time intervals. The generalized flow suggested a westward movement near shore as an effect of the antarctic polar current.

Ross sea oceanography and antarctic bottom water formation

The oceanographic features of the Ross Sea during the austral summer are described, based primarily upon observations taken aboard the U.S.N.S. Eltanin in 1968. Surface-actuated multiple samplers (SAMS) provided calibration data for electronically recorded in situ salinity-temperature-depth ( STD) measurements. The eastern portion of Pennell Bank differs considerably from its outline on earlier charts. The westerly current of Circumpolar Deep Water (CDW) over the Ross Sea continental slope provides an dynamic barrier to northwanrd thermohaline flow of dense Ross Sea Shelf Water (RSSW). Insufficient brine may be releasd by the freezing of sea ice to produce the major portion of RSSW. Ice Shelf Water (ISW), identified by a deep temperature minimum, has temperatures as low as — 2·13°C near its source at the base of the Ross Ice Shelf. Low-salinity Antarctic Bottom Water (AABW) is formed during sumer over the continental slope in the eastern Ross Sea from a mixture of CDW and ISW. High salinity AABW is produced in summer in the western Ross Sea from a combination of RSSW and CDW. On the continental slope both AABW types are characterized by a sharp decrease in temperature and increase in oxygen near bottom. Gradient changes in continuously-recorded vertical profiles of temperature, salinity and light scattering were observed between CDW and AABW on several stations. Bottom current measurements revealed strong westerly and northerly flows down and along the continental slope, with magnitudes exceeding 15 cm/sec at eight locations.

A CASE STUDY OF THE WINTER CIRCULATION AT 700 AND 500 MB IN MIDDLE AND HIGH SOUTHERN LATITUDES

Abstract Synoptic chart data for the Southern Hemisphere south of 30°S (excluding the Pacific sector) are examined for the winter of 1962 in relation to the frequency of cyclones and anticyclones at the 700- and 500-mb levels. Regions of high cyclonic frequency are noted and aspects of those in the eastern Ross Sea and in Prydz Bay are discussed. Cyclonic frequency over east Antarctica is very low, but clearly defined anticyclones at 500 mb are apparent on 1day in 6 over the highlands close to 80°S, 60°E. The longitudinal frequency of prominent troughs and ridges and some aspects of the pattern of cyclogenesis for this period are described.

Submarine geomorphology of Eastern Ross Sea and Sulzberger Bay, Antarctica : Lepley L. K., 1964. Marine Geol., 2 (4): 253–261.

Abstract The submarine morphological features of the eastern Ross Sea and Sulzberger Bay are classified into four main categories: (1) outer shelf— the deep-lying terrace of the continental shelf; (2) large, enclosed depressions within the continental shelf — the transverse depression perpendicular to the coast and the longitudinal depressions parallel to the coast; (3) inner shelf— the narrow, relatively shallow portions of the shelf lying between shore and the longitudinal depressions; and (4) moraine-like ridges referred to as transverse ridges or longitudinal ridges, depending on whether they are respectively perpendicular or parallel to the shoreline. The outer shelf break averages about 255 fathoms; the great depth of the outer shelf is attributed to remnant isostatic depression by the continental ice sheet of glacial maxima when the ice extended to the shelf break. The great (600 fathoms) transverse depression of Sulzberger Bay may be the product of erosion by a locally accelerated “ice stream” (during glacial maxima) whose position was controlled by bed rock structure. The origin of the longitudinal depressions can be attributed to erosion by continental ice along zones of weakness due to lithologic changes or faulting parallel to the shore line. The longitudinal ridges are interpreted as end moraines formed during pauses in the retreat of the continental ice sheet. The transverse ridges are interpreted as lateral and end moraines of a former extension of the Ross Ice Sheet.

Submarine geomorphology of eastern Ross Sea and Sulzberger Bay, Antarctica

The submarine morphological features of the eastern Ross Sea and Sulzberger Bay are classified into four main categories: (1) outer shelf— the deep-lying terrace of the continental shelf; ( 2) large, enclosed depressions within the continental shelf — the transverse depression perpendicular to the coast and the longitudinal depressions parallel to the coast; (3) inner shelf— the narrow, relatively shallow portions of the shelf lying between shore and the longitudinal depressions; and ( 4) moraine-like ridges referred to as transverse ridges or longitudinal ridges, depending on whether they are respectively perpendicular or parallel to the shoreline. The outer shelf break averages about 255 fathoms; the great depth of the outer shelf is attributed to remnant isostatic depression by the continental ice sheet of glacial maxima when the ice extended to the shelf break. The great (600 fathoms) transverse depression of Sulzberger Bay may be the product of erosion by a locally accelerated “ice stream” (during glacial maxima) whose position was controlled by bed rock structure. The origin of the longitudinal depressions can be attributed to erosion by continental ice along zones of weakness due to lithologic changes or faulting parallel to the shore line. The longitudinal ridges are interpreted as end moraines formed during pauses in the retreat of the continental ice sheet. The transverse ridges are interpreted as lateral and end moraines of a former extension of the Ross Ice Sheet.

MARINE-SEDIMENT THICKNESS IN THE EASTERN ROSS SEA AREA, ANTARCTICA

Thickness of ocean sediments was obtained at two locations in the eastern Ross Sea area, Antarctica, during the International Geophysical Year. Sediment thickness in the vicinity of the Little America Station is about 1325 m. Cores from the upper meter of these sediments are composed mainly of a mixture of coarse and fine glacial till.