Sedimentological signatures of sub-ice-shelf circulation: An example from Vincennes Bay, East Antarctica (original) (raw)
Related papers
Sedimentological signatures of the sub-Amery Ice Shelf circulation
Antarctic Science, 2007
Two sediment cores collected from beneath the Amery Ice Shelf, East Antarctica describe the physical sedimentation patterns beneath an existing major embayed ice shelf. Core AM01b was collected from a site of basal freezing, contrasting with core AM02, collected from a site of basal melting. Both cores comprise Holocene siliceous muddy ooze (SMO), however, AM01b also recovered interbedded siliciclastic mud, sand and gravel with inclined bedding in its lower 27 cm. This interval indicates an episode of variable but strong current activity before SMO sedimentation became dominant. 14 C ages corrected for old surface ages are consistent with previous dating of marine sediments in Prydz Bay. However, the basal age of AM01b of 28250 AE 230 14 C yr BP probably results from greater contamination by recycled organic matter. Lithology, 14 C surface ages, absolute diatom abundance, and the diatom assemblage are used as indicators of sediment transport pathways beneath the ice shelf. The transport pathways suggested from these indicators do not correspond to previous models of the basal melt/freeze pattern. This indicates that the overturning baroclinic circulation beneath the Amery Ice Shelf (near-bed inflow -surface outflow) is a more important influence on basal melt/freeze and sediment distributions than the barotropic circulation that produces inflow in the east and outflow in the west of the ice front. Localized topographic (ice draft and bed elevation) variations are likely to play a dominant role in the resulting sub-ice shelf melt and sediment distribution.
Subglacial sediments: A regional geological template for ice flow in West Antarctica
Geophysical Research Letters, 2001
We use aerogeophysical data to estimate the distribution of marine subglacial sediments and fault-bounded sedimentary basins beneath the West Antarctic Ice Sheet (WAIS). We find that significant ice flow occurs exclusively in regions covered by subglacial sediments. The onsets and lateral margins of ice streams coincide with the limit of marine sediments. Lateral margins are also consistently linked with fault-bounded basins. We predict that the inland migration of ice streams B and Cx towards the ice divide outside the region covered by marine or rift sediments is unlikely. The subglacial geology has the potential to modulate the dynamic evolution of the ice streams and the WAIS.
Late Quaternary glacial marine to marine sedimentation in the Pennell Trough (Ross Sea, Antarctica)
Palaeogeography, Palaeoclimatology, Palaeoecology, 2006
Late Pleistocene and Holocene palaeoenvironmental changes were studied in four gravity cores up to 7.8 m long from the Pennell Trough, a NW-SE-trending basin 160 km long and 60 km wide in the central Ross Sea, Antarctica, with depths occasionally greater than 600 m. Differences in environments and depositional processes during the last glacial and postglacial epochs were investigated using X-rays and volume magnetic susceptibility (VMS). Further analyses included bulk and clay mineral composition, micropalaeontological studies (both benthic and planktic foraminifera) and radiometric dating. We compare our sedimentological, geochemical (organic carbon and nitrogen content), and geotechnical (shear strength and water content) results to those on cores previously taken from the region. These analyses suggest that prior to the Last Glacial Maximum (LGM), a glacial marine diamicton (b 37,000 yr BP uncorrected age) was deposited across the basin from beneath an expanded Ross ice shelf that was grounded on the basin flanks. Sediment gravity flow deposits (27,000-21,000 yr BP uncorrected ages) that overlie the diamicton in the deepest part of the southernmost area of the basin are interpreted to have been deposited during the Last Glacial Maximum (~18,000 yr BP) as remobilized subglacial diamicton from the flanks of the basin, initiated by the movement of grounded ice. These sediments are followed by a period of non-deposition caused by basin starvation after retreat of the grounding line of Ross Sea ice far to the south. As a consequence, terrigenous supply was limited, and the persistence of floating shelf ice followed by multi-year sea-ice coverage inhibited the biogenic activity. During the Holocene, as climate became warmer, summer open-sea conditions began to dominate, leading to the deposition of a thin diatomaceous mud/ooze draping the basin. D
The stability of floating ice shelves is an important indicator of ocean circulation and ice shelf mass balance. A sub ice-shelf sediment core collected during the Austral summer of 2000-2001 from site AM02 (69°42.8’S, 72°38.4’E) on the Amery Ice Shelf, East Antarctica, contains a full and continuous record of glacial retreat. The AM02 core site is ~80 km south of the floating ice shelf edge and contains a 0.5 m thick Holocene surface layer of siliceous mud and diatom ooze of marine origin. Core data are supportive of sub ice-shelf circulation models which predict the landward flow of oceanic water, and prove that the landward transport of hemipelagic sediments occurs beneath floating ice shelves over distances of at least ~80 km. An increase in sea-ice associated diatom deposition in the upper part of the Holocene suggests that a major retreat of the Amery Ice Shelf to at least 80 km landward of its present location may have occurred during the mid-Holocene climatic optimum.
Journal of the Geological Society, 2015
Collapse of Antarctic ice shelves in response to a warming climate is well documented, but its legacy in terms of depositional landforms is little known. This paper uses remote-sensing, structural glaciological and sedimentological data to evaluate the evolution of the c. 25000 km 2 George VI Ice Shelf, SW Antarctic Peninsula. The ice shelf occupies a north-south-trending tectonic rift between Alexander Island and Palmer Land, and is nourished mainly by ice streams from the latter region. The structure of the ice shelf is dominated by inherited foliation and fractures, and with velocity data indicates a largely compressive flow regime. The formation of a moraine complex at the margin of the ice shelf is controlled by debris entrained within foliation and folds. This englacial debris is of basal origin, and includes both local Mesozoic sedimentary and volcanic lithologies, and exotic crystalline rocks from Palmer Land. Folding of basal ice to a high level in the source glaciers on Palmer Land is required to bring the debris to the surface. These results have implications for understanding flow dynamics of ice shelves under compressive flow, and debris entrainment and moraine formation associated with palaeo-ice shelves. Supplementary material: Photographs of ice-shelf morphology, ice facies and ice structure; detailed descriptions of ice facies, including foliation logs, supporting evidence for interpreting sedimentary facies; complete dataset of sedimentological data, including triangular plots of clast shape, clast roundness histograms, particle-size distribution of sand-size and lower, and pie chart of local clasts versus exotic clasts from Palmer Land; and a table summarizing the characteristics of representative clasts in the ice-shelf moraine, based on thin-section analysis, with indication of their provenance are available at http://www.geolsoc.org.uk/SUP18831
Paleoceanography and Paleoclimatology, 2020
The rapidly thinning Totten Glacier on the Sabrina Coast, East Antarctica, is the primary drainage outlet for ice within the Aurora Subglacial Basin, which could destabilize under the current atmospheric warming trend. There is growing need for direct geological evidence from the Sabrina Coast to frame late twentieth century Totten melting in the context of past warm climate analogs. Addressing this need, sediment archives were recovered from two sites on the Sabrina Coast slope and rise that record changes in terrigenous sedimentation and primary productivity in the region over glacial cycles since the mid-Pleistocene transition (MPT). This research presents physical properties, grain size, diatom abundance and assemblages, and geochemical analysis from the two sites to determine how the processes that control sedimentation change between glacial and interglacial phases. The stratigraphic sequences in both cores record cyclic variations in physical properties and diatom abundances, which radiocarbon and biostratigraphic chronologies reveal as 100 Kyr glacial-interglacial cyclicity. During glacials, terrigenous sediment deposition is enhanced by advanced grounded ice on the shelf, while primary productivity is restricted due to permanent summer sea ice extending past the continental slope. During interglacials, pelagic sedimentation suggests high surface productivity associated with contractions of regional sea ice cover. Comparison with post-MPT slope records from Wilkes Land and the Amundsen Sea shows that this pattern is consistent in slope sediments around the margin. The higher-amplitude variations in Antarctic ice volume and sea ice extent post-MPT ensure that these signals are pervasive around the Antarctic margin. Plain Language Summary To improve predictions of future Antarctic ice sheet behavior, knowledge of how Antarctica responded in the past, particularly when temperatures were similar to or higher than today, is required. Geological records recovered from ice proximal sediments can provide this information. The sediments record variations in physical, chemical, and biological properties and therefore act as indicators of paleoenvironmental change. Two sediment cores recovered from the Sabrina Coast continental slope and rise, East Antarctica, are used to study past changes in sediment deposition, as influenced by glacial and oceanographic processes. The two archives show clear variations in sediment composition and microfossil assemblage between glacial and interglacial conditions on 100 Kyr timescales over the last 350,000 years, driven by the movement of the ice sheet across the continental shelf and the extent of sea ice cover. This research suggests that the drivers of sedimentation and the patterns revealed in slope sediments are consistent around the Antarctic margin due to the larger variations in climate since the mid-Pleistocene transition.
Marine Geology, 2021
The repeated proximity of West Antarctic Ice Sheet (WAIS) ice to the eastern Ross Sea continental shelf break during past ice age cycles has been inferred to directly influence sedimentary processes occurring on the continental slope, such as turbidity current and debris flow activity; thus, the records of these processes can be used to study the past history of the WAIS. Ross Sea slope sediments may additionally provide an archive on the history and interplay of density-driven or geostrophic oceanic bottom currents with ice-sheet-driven depositional mechanisms. We investigate the upper 121 m of Hole U1525A, collected during International Ocean Discovery Program (IODP) Expedition 374 in 2018. Hole U1525A is located on the southwestern external levee of the Hillary Canyon (Ross Sea, Antarctica) and the depositional lobe of the nearby trough-mouth fan. Using core descriptions, grain size analysis, and physical properties datasets, we develop a lithofacies scheme that allows construction of a detailed depositional model and environmental history of past ice sheet-ocean interactions at the eastern Ross Sea continental shelf break/slope since ~2.4 Ma. The earliest Pleistocene interval (~2.4- ~ 1.4 Ma) represents a hemipelagic environment dominated by ice-rafting and reworking/deposition by relatively persistent bottom current activity. Finely interlaminated silty muds with ice-rafted debris (IRD) layers are interpreted as contourites. Between ~1.4 and ~0.8 Ma, geostrophic bottom current activity was weaker and turbiditic processes more common, likely related to the increased proximity of grounded ice at the shelf edge. Silty, normally-graded laminations with sharp bases may be the result of flow-stripped turbidity currents overbanking the canyon levee during periods when ice was grounded at or proximal to the shelf edge. A sandy, IRD- and foraminifera-bearing interval dated to ~1.18 Ma potentially reflects warmer oceanographic conditions and a period of stronger Antarctic Slope Current flow. This may have enhanced upwelling of warm Circumpolar Deep Water onto the shelf, leading to large-scale glacial retreat at that time. The thickest interval of turbidite interlamination was deposited after ~1 Ma, following the onset of the Mid-Pleistocene Transition, interpreted as a time when most ice sheets grew and glacial periods were longer and more extreme. Sedimentation after ~0.8 Ma was dominated by glacigenic debris flow deposition, as the trough mouth fan that dominates the eastern Ross Sea continental slope prograded and expanded over the site. These findings will help to improve estimations of WAIS ice extent in future Ross Sea shelf-based modelling studies, and provide a basis for more detailed analysis of the inception and growth of the WAIS under distinct oceanographic conditions.
Antarctic icebergs reorganize ocean circulation during Pleistocene glacials
Nature, 2021
The dominant feature of large-scale mass transfer in the modern ocean is the Atlantic meridional overturning circulation (AMOC). The geometry and vigour of this circulation influences global climate on various timescales. Palaeoceanographic evidence suggests that during glacial periods of the past 1.5 million years the AMOC had markedly different features from today 1 ; in the Atlantic basin, deep waters of Southern Ocean origin increased in volume while above them the core of the North Atlantic Deep Water (NADW) shoaled 2. An absence of evidence on the origin of this phenomenon means that the sequence of events leading to global glacial conditions remains unclear. Here we present multi-proxy evidence showing that northward shifts in Antarctic iceberg melt in the Indian-Atlantic Southern Ocean (0-50° E) systematically preceded deep-water mass reorganizations by one to two thousand years during Pleistocene-era glaciations. With the aid of iceberg-trajectory model experiments, we demonstrate that such a shift in iceberg trajectories during glacial periods can result in a considerable redistribution of freshwater in the Southern Ocean. We suggest that this, in concert with increased sea-ice cover, enabled positive buoyancy anomalies to 'escape' into the upper limb of the AMOC, providing a teleconnection between surface Southern Ocean conditions and the formation of NADW. The magnitude and pacing of this mechanism evolved substantially across the mid-Pleistocene transition, and the coeval increase in magnitude of the 'southern escape' and deep circulation perturbations implicate this mechanism as a key feedback in the transition to the '100-kyr world', in which glacial-interglacial cycles occur at roughly 100,000-year periods. In the modern ocean, the AMOC is characterized by the deep, southward spread of NADW towards the Southern Ocean, balanced by the northward return of surface, mode, intermediate and bottom waters 3,4. Although the widely documented shoaling of NADW during Pleistocene glacial periods has previously been explained through changes in North Atlantic Ocean processes 5 , this paradigm has been challenged by studies invoking 'upstream' disruptions (for example, variable Agulhas Leakage) 6,7. Such disruptions would modify the shallow return of waters to the North Atlantic required to complete this 'upper cell' of the overturning circulation. Furthermore, the southward-and northward-spreading components of the upper cell are connected via wind-and buoyancy-related processes in the Southern Ocean, with much of the outcropping NADW first taking an indirect route, via the so-called 'lower cell' of overturning circulation, before joining the AMOC return limb via the upper cell 4 (Extended Data Fig. 1). Southern Ocean conditions have therefore been increasingly invoked by modelling studies 7-9 as key in setting NADW dynamics; however, palaeoceanographic evidence supporting such a causal link remains scarce 10. Here we show a tight surface water-deep water coupling between the presence of far-travelled Antarctic icebergs and deep-water mass structure at the northern limit of the modern Subantarctic Zone (SAZ) during the past 1.65 million years (Myr). We examine new and previously published records of ice-rafted debris (IRD) mass-accumulation
Geosphere, 2012
The AND-2A drill hole (ANDRILL [Antarctic Geological Drilling Program] Southern McMurdo Sound Project), ~10 km from the East Antarctica coastline, records nearly 6 m.y. of sedimentation across the Miocene climatic optimum at a high-latitude site. Sedimentological studies of bedforms and particle size distributions indicate that the paleoenvironment was strongly affected by waves and currents, consistent with deposition in a glacially infl uenced neritic environment. We document abrupt shifts in mud percent within glacial-interglacial cycles ca. 17.8 Ma and between ca. 16.7 and 15.7 Ma that we attribute to the hydrodynamic effects of wave stirring tied to episodes of ice growth and decay. Although wave climate and geodynamic forcing of the paleobathymetry simultaneously affect wave stirring on a highlatitude shelf, both are ultimately controlled by the size of the ice sheet. The mud percent record displays cyclicity at short-eccentricity time scales (94-99 k.y.) and, unexpectedly, ice retreat phases interpreted from the particle size record coincide with eccentricity minima. We attribute the eccentricity-paced ice retreat phases during the late Early Miocene polythermal glacial conditions and the cool orbital parameters to marine ice sheet instability in response to changes in ocean circulation and heat transport. The particle size record of the AND-2A core provides unique near-field evidence for orbitally paced changes in high-latitude climate and ice volume during the Miocene climatic optimum and important insights into the mechanisms of ice sheet growth and decay in a period of global warmth.