Understanding ice sheet evolution through the geologic past can help constrain ice sheet models that predict future ice dynamics. Existing geological records of grounding line retreat in the Ross ...Sea, Antarctica, have been confined to ice‐free and terrestrial archives, which reflect dynamics from periods of more extensive ice cover. Therefore, our perspective of grounding line retreat since the Last Glacial Maximum remains incomplete. Sediments beneath Ross Ice Shelf and grounded ice offer complementary insight into the southernmost extent of grounding line retreat, yielding a more complete view of ice dynamics during deglaciation. Here we thermochemically separate the youngest organic carbon to estimate ages from sediments extracted near the Whillans Ice Stream grounding line to provide direct evidence of mid‐Holocene (7.5–4.8 kyr B.P.) grounding line retreat in that region. Our study demonstrates the utility of accurately dated, grounding‐line‐proximal sediment deposits for reconstructing past interactions between marine and subglacial environments.
Plain Language Summary
Ice sheet grounding lines, where ice loses contact with the underlying bed and transitions to an ice shelf floating on the ocean, are critical areas that govern the stability of marine‐based ice sheets. However, the short period (years to decades) that we have been able to map grounding lines from ground, airborne, and satellite observations compared to the long periods over which ice sheets change (centuries to millenia) limits our knowledge of grounding line stability. We focus our geologic perspective of grounding line retreat in the Ross Sea, a region that has experienced a high degree of change throughout the last glacial‐interglacial cycle. Prior work reconstructing the timeline of ice sheet change in this region is based heavily on ice‐free marine sedimentary records, where dates of the first open marine sedimentation after ice retreat can be estimated. Dating sediments from beneath floating ice shelves and near grounding lines has proven difficult for both logistical and methodological reasons, limiting our understanding of grounding line evolution. We used novel radiocarbon dating methods on sediments collected from beneath the floating Ross Ice Shelf to find that the grounding line retreated inland of the current position during the mid‐Holocene and subsequently readvanced.
Key Points
Direct evidence is found for mid‐Holocene grounding line retreat on the Siple Coast, West Antarctica
Chronological constraints at Whillans Ice Stream imply western Ross Sea forcing on the southern Siple Coast, in contrast to other Siple Coast ice streams
Ramped PyrOx allows for accurate 14C dating of low carbon, grounding‐line‐proximal sediments
Our understanding of the response of the Antarctic ice sheet to climate and ocean changes requires the improvement of current ice-atmosphere-ocean models and the accurate determination of boundary ...conditions such as ice thickness and extent at key time intervals, so that satellite gravity observations and isostatic models can be adjusted. However, large portions of the Antarctic margin remain understudied or lack suitable data. One key area where data are lacking, is the Sabrina Coast portion of the East Antarctic Ice Sheet (EAIS) margin where the Totten Glacier, which has the largest ice discharge in East Antarctica, is accelerating, thinning and loosing mass at high rates. In this work, we present the results of the first geological and geophysical marine survey to the continental shelf offshore of the Dalton Ice Tongue and Moscow University ice shelf, east of the Totten Glacier. The data presented include multibeam swath bathymetry and multichannel seismic, focusing on the sea floor morphology and sedimentary section above a regional angular unconformity separating pre- and post-Miocene glacial strata. Sea floor scouring and iceberg keel marks on the outer shelf, associated with gullies on the upper slope indicate that ice expanded in the past and grounded ~5 km from the shelf edge at ~450–500 mbsl, extending ~155 km north of the current Moscow University Ice Shelf. A nearly 1000 m deep area in the inner-middle shelf, oriented NW with paleo-ice flow direction indicated by mega scale glacial lineations (MSGL) and drumlins, is interpreted as a cross shelf glacial trough. A series of geomorphic associations on the north-eastern side of the glacial trough includes glacial lobes, grounding zone wedges (GZW), glacial lineations and transverse ridges, which indicates slower ice, grounding line stabilization and collapse. These geomorphic associations are organized in 4 four sets representing different past ice-flow configurations reflecting changes in ice flow direction, grounding line position, location of fast and slow ice areas, and retreat pattern. Some of the geomorphic features identified are compatible with the presence of an organized subglacial drainage, and others are with rapid grounding line collapse. A well-preserved series of GZWs occurring at different water depths implies they were formed during different glacial stages or cycles. The inferred diminishing ice thickness for consecutives GZWs indicates that the margin of the Antarctic ice sheet evolved to a less extensive coverage of the continental shelf through successive glacial stages or cycles.
The identification of different ice flow configurations, evidence of subglacial water and past ice margin collapse indicates a dynamic ice sheet margin with varying glacial conditions and retreat modes. We observe that some of the described morphological associations are similar to those found in the Amundsen sea sector of the West Antarctic Ice Sheet (WAIS) where they are associated with ice sheet and ice stream collapse. Although further studies are needed to assess the precise timing and rates of the glacial processes involved, we conclude that there is enough evidence to support the hypothesis that the EAIS margin can behave as dynamically as the WAIS margin, especially during glacial retreat and icesheet margin collapse.
•The East Antarctic Ice Sheet in the Sabrina Coast area expanded in the past to near the continental shelf break.•We identified several different past ice flow configurations and retreat modes.•A past subglacial hydrological system produced erosion of bedforms and bedrock and deposition of glacial sediments.•The East Antarctic Ice Sheet behaved in the past as dynamically as the West Antarctic Ice Sheet.
Sub‐ice shelf sediments near Larsen C ice shelf (LIS‐C) show fine‐scale rhythmic laminations that could provide a near‐continuous seasonal‐resolution record of regional ice mass changes. Despite the ...great potential of these sediments, a dependable Late Quaternary chronology is difficult to generate, rendering the record incomplete. As with many marginal Antarctic sediments, in the absence of preserved carbonate microfossils, the reliability of radiocarbon chronologies depends on presence of high proportions of autochthonous organic carbon with minimized detrital organic carbon. Consequently, acid insoluble organic (AIO) 14C dating works best where high productivity drives high sediment accumulation rates, but can be problematic in condensed sequences with high proportions of detrital organic carbon. Ramped PyrOx 14C dating has progressively been shown to improve upon AIO 14C dates, to the point of matching foraminiferal carbonate 14C dates, through differential thermochemical degradation of organic components within samples. But in highly detrital sediments, proportions of contemporaneously deposited material are too low to fully separate autochthonous organic carbon from detrital carbon in samples large enough to 14C date. We introduce two modifications of the Ramped PyrOx 14C approach applied to highly detrital sediments near LIS‐C to maximize accuracy by utilizing ultra‐small fractions of the highly detrital AIO material. With minimization of the uncertainty cost, these techniques allow us to generate chronologies for cores that would otherwise go undated, pushing the limits of radiocarbon dating to regions and facies with high proportions of pre‐aged detritus. Wider use of these techniques will enable more coordinated a priori coring efforts to constrain regional glacial responses to rapid warming where sediments had previously been thought too difficult to date.
Key Points
We 14C date highly detrital sediment from the Antarctic margin containing very little autochthonous carbon
We employ composite and isotope dilution Ramped PyrOx 14C dating, considering blank contamination for each
Both approaches sacrifice analytical precision for gains in accuracy; application to other detrital systems depends on uncertainty limits
The Antarctic Peninsula is one of the three fastest warming regions on Earth. Here we review Holocene proxy records of marine and terrestrial palaeoclimate in the region, and discuss possible forcing ...mechanisms underlying past change, with a specific focus on past warm periods. Our aim is to critically evaluate the mechanisms by which palaeoclimate changes might have occurred, in order to provide a longer-term context for assessing the drivers of recent warming. Two warm events are well recorded in the Holocene palaeoclimate record, namely the early Holocene warm period, and the `Mid Holocene Hypsithermal' (MHH), whereas there are fewer proxy data for the `Mediaeval Warm Period' (MWP) and the `Recent Rapid Regional' (RRR) warming. We show that the early Holocene warm period and MHH might be explained by relatively abrupt shifts in position of the Southern Westerlies, superimposed on slower solar insolation changes. A key finding of our synthesis is that the marine and terrestrial records in the AP appear to show markedly different behaviour during the MHH. This might be partly explained by contrasts in the seasonal insolation forcing between these records. Circumpolar Deep Water (CDW) has been implicated in several of the prominent changes through the Holocene but there are still differences in interpretation of the proxy record that make its influence difficult to assess. Further work is required to investigate contrasts between marine and terrestrial proxy records, east—west contrasts in palaeoclimate, the history of CDW, to retrieve a long onshore high resolution record of the Holocene, and determine the role of sea ice in driving or modulating palaeoclimate change, along with further efforts to study the proxy record of the RRR and the MWP.
The continental slope and rise seaward of the Totten Glacier and the Sabrina Coast, East Antarctica features continental margin depositional systems with high sediment input and consistent ...along-slope current activity. Understanding their genesis is a necessary step in interpreting the paleoenvironmental records they contain. Geomorphic mapping using a systematic multibeam survey shows variations in the roles of downslope and along slope sediment transport influenced by broad-scale topography and oceanography. The study area contains two areas with distinct geomorphology. Canyons in the eastern part of the area have concave thalwegs, are linked to the shelf edge and upper slope and show signs of erosion and deposition along their beds suggesting cycles of activity controlled by climate cycles. Ridges between these canyons are asymmetric with crests close to the west bank of adjacent canyons and are mostly formed by westward advection of fine sediment lofted from turbidity currents and deposition of hemipelagic sediment. They can be thought of as giant levee deposits. The ridges in the western part of the area have more gently sloping eastern flanks and rise to shallower depths than those in the east. The major canyon in the western part of the area is unusual in having a convex thalweg; it is likely fed predominantly by mass movement from the flanks of the adjacent ridges with less sediment input from the shelf edge. The western ridges formed by accretion of suspended sediment moving along the margin as a broad plume in response to local oceanography supplemented with detritus originating from the Totten Glacier. This contrasts with interpretations of similar ridges described from other parts of Antarctica which emphasise sediment input from canyons immediately up-current. The overall geomorphology of the Sabrina Coast slope is part of a continuum of mixed contourite-turbidite systems identified on glaciated margins.
•The slope and rise off the Totten Glacier feature sediment ridges between submarine canyons.•Sediment ridges in the east are formed from turbidity currents flowing in adjacent canyons and a pelagic component.•In the west, larger ridges are formed from widespread hemipelagic and contouritic sedimentation.•The western ridges are separated by a submarine canyon with a convex thalweg mostly sourced from the ridge flanks.
Grounding zones, where ice sheets transition between resting on bedrock to full floatation, help regulate ice flow. Exposure of the sea floor by the 2002 Larsen-B Ice Shelf collapse allowed detailed ...morphologic mapping and sampling of the embayment sea floor. Marine geophysical data collected in 2006 reveal a large, arcuate, complex grounding zone sediment system at the front of Crane Fjord. Radiocarbon-constrained chronologies from marine sediment cores indicate loss of ice contact with the bed at this site about 12,000 years ago. Previous studies and morphologic mapping of the fjord suggest that the Crane Glacier grounding zone was well within the fjord before 2002 and did not retreat further until after the ice shelf collapse. This implies that the 2002 Larsen-B Ice Shelf collapse likely was a response to surface warming rather than to grounding zone instability, strengthening the idea that surface processes controlled the disintegration of the Larsen Ice Shelf.
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.
Key Points
Sedimentary archives from the Sabrina Coast continental slope document high‐amplitude glacial cycles since the mid‐Pleistocene transition
Sedimentation and primary productivity are primarily controlled by sea ice extent and ice sheet position on the continental shelf
These controls are amplified since the MPT and as a consequence influence sedimentation around the entire Antarctic Margin concurrently
The Southern Ocean is very important for the potential sequestration of
carbon dioxide in the oceans and is expected to be vulnerable
to changes in carbon export forced by anthropogenic climate ...warming.
Annual phytoplankton blooms in seasonal ice zones are highly productive and
are thought to contribute significantly to pCO2 drawdown in the
Southern Ocean. Diatoms are assumed to be the most important phytoplankton
class with respect to export production in the Southern Ocean; however, the
colonial prymnesiophyte Phaeocystis antarctica regularly forms huge
blooms in seasonal ice zones and coastal Antarctic waters.
There is little evidence regarding the fate of carbon produced by P. antarctica
in the Southern Ocean, although remineralization in the upper water column
has been proposed to be the main pathway in polar waters.
Here we present evidence for early and rapid carbon export from P. antarctica
blooms to deep water and sediments in the Ross Sea. Carbon sequestration
from P. antarctica blooms may influence the carbon cycle in the Southern
Ocean, especially if projected climatic changes lead to an alteration in the
structure of the phytoplankton community.
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