C44A-01
Insolation driven ice sheet changes in the early Pleistocene
Precisely dated Antarctic nearshore and offshore proxy (not direct) records of ocean surface changes imply ice sheet changes that appear to be directly and profoundly influenced by precession paced high latitude insolation. Stratigraphic records from the southwestern Ross Sea (Cape Roberts Project and ANDRILL) can be directly correlated with offshore records (ODP) of Marine Isotope Stage 31 (MIS-31) by the position of the base of the Jaramillo subchron (C1r.1n), which is identified based on biostratigraphic and isotopic dating. Microfossil data show that the southern Ross Sea was ca. 4 degrees warmer than today, and that the polar front shifted well south of its current position. Coastal sea ice was largely absent during the summer and open ocean surface waters were stratified with a shallow, low salinity mixed layer. MIS-31 was characterized by some of the highest high-latitude insolation values of the last 5 million years, with high obliquity, eccentricity, and precession, and perihelion in the austral summer early in the interglacial, at 1.08 Ma, followed by a northern hemisphere peak at 1.07Ma. These parameters drove a model-predicted +20 m eustatic sea-level rise during MIS-31 [Raymo et al., 2006], the highest of the late Pliocene/early Pleistocene and reflecting significant retreat or collapse of the Greenland Ice Sheet plus some combination of the West Antarctic Ice Sheet and marginal East Antarctic ice. Our findings appear to corroborate the hypothesis of Raymo et al. [2006] that the interhemispheric antiphased relationship of the precession cycle typically attenuates a potentially significant ice volume signal in the deep sea oxygen isotope record. MIS-31 is the "exception that proves the rule" in that the unusually strong insolation in the Antarctic (at 1.08 Ma) triggered stronger than usual melting, which was quickly followed by intense melting in Greenland (at 1.07 Ma). In this case, Greenland melting occurred prior to full regrowth of Antarctic ice. The implication of these findings and interpretations is that the ice sheets may be more susceptible to high insolation driven retreat than has been widely recognized.
C44A-02 INVITED
Seismic Facies and Stratigraphy of the Cenozoic Succession in McMurdo Sound, Antarctica: Implications for Tectonic, Climatic and Glacial History.
Integration of data from fully cored stratigraphic holes, including the new AND-1B drillhhole, with an extensive grid of seismic reflection lines in McMurdo Sound, Antarctica, has allowed the formulation of a new model for the evolution of the Cenozoic Victoria Land Basin of the West Antarctic Rift. The Early Rift phase (Eocene to Early Oligocene) is recorded by wedges of strata confined by early extensional faults, and which contain seismic facies consistent with drainage via coarse-grained fans and deltas into discrete, actively subsiding grabens and half- grabens. The Main Rift phase (Early Oligocene to Early Miocene) is represented by a lens of strata that thickens symmetrically from the basin margins into a central depocenter, and in which stratal events pass continuously over the top of the Early Rift extensional topography. The Passive Thermal Subsidence phase (Early Miocene to ~14 Ma) is recorded by an evenly distributed sheet of strata that thickens somewhat into the depocentre but is continuous across and over the earlier rift strata to the margins of the basin. The Renewed Rifting phase (~13 Ma to Recent is represented by intervals that thicken significantly into the basin depocentre and that are complicated by evidence of magmatic activity (McMurdo Volcanic Group). The Cenozoic stratigraphy of the southern Victoria Land Basin preserves archives of both climate change and the complex rift history of the basin, and coincidences between key stratal surfaces in seismic data and evidence for environmental change in drillcores suggest that tectonic and climatic drivers may be linked.
C44A-03 INVITED
Expanding Ice Sheets on the Antarctic Peninsula during the Plio/Pleistocene Recorded in Continental Rise Sediment Drifts
Sediment drifts on the continental rise west of the Antarctic Peninsula are located within 125 km from the continental shelf edge, the main contributor of terrigenous sediment during both glacial and interglacial periods. The composition of drift deposits continuously recorded changes in ice sheet volume and thermal regime as well as sea surface temperatures (SSTs) and sea ice extent. The coarse-grained terrigenous sediment (pebbles and coarse sand), a proxy for iceberg-rafted debris (IRD), was analyzed in sediments spanning the last 3.1 m.y. at Ocean Drilling Program (ODP) Sites 1101 and 1096. IRD is deposited in both glacial intervals, dominated by fine-grained laminated mud and interglacial units consisting of bioturbated muds enriched in biogenic components. Contents of biogenic opal, which reflect diatom abundance, are relatively high from 3.1-2.2 Ma. Calcareous nannofossils are present within interglacial sediments from 2.2-0.76 Ma. Both findings suggest warm SSTs and limited sea ice over the drifts during interglacial periods before the Late Pleistocene. Quartz grains picked from the IRD fraction and imaged with a scanning electron microscope (SEM) show an abrupt change in surface microtextures at 1.35 Ma. During the Late Pliocene to Early Pleistocene, many quartz grains are completely weathered and only a few show signs of crushing and abrasion, indicating that glaciers were too small to inundate the Antarctic Peninsula topography. Debris shed from mountain peaks was transported supraglacially or englacially allowing weathered grains to pass unmodified through the glaciers. Glaciers expanded in size during glacial periods from 1.35-0.76 Ma. The IRD accumulation during those periods was very high and diverse dropstone lithologies document supply from sources throughout the Antarctica Peninsula. Conditions that spawned the large polar ice sheet identified at the Last Glacial Maximum have been present on the Antarctic Peninsula during glacial periods since approximately 0.76 Ma. Since then, IRD supply has been relatively low and maxima in IRD content occurred during interglacials when sedimentation rates were low. Pebble shapes indicate the dominance of basal glacial transport paths. Quartz sand grains show high relief, fracture and abrasion common under thick ice and dropstone lithologies are more restricted.
C44A-04
Early Pliocene diatom record from the ANDRILL Mc Murdo Ice Shelf Core AND-1 with correlation to other sites within the Mc Murdo Sound and Ross Sea region
The ANDRILL AND-1 Core recovered from beneath the Ross Ice Shelf between Ross and White islands in Mc Murdo Sound, Antarctica contains a spectacular continental shelf marine record of the Pliocene. Alternating diamictite and diatom-rich sediments were deposited at this site throughout the Pliocene; this punctuated style of deposition and preservation reflecting the highly glacially influenced nature of deposition in this area. One time interval specifically, the early Pliocene, is represented by an extended diatomaceous unit almost 100m in thickness. This contiguous unit has frequent IRD within as well as varying amounts of volcanogenic material. The age of this early Pliocene unit has been constrained through the diatom assemblage present and magnetostratigraphy. Other sites from the Mc Murdo Sound area and Ross Sea have also recovered early Pliocene material but not in such breadth and completeness. The exact amount of time represented in this interval in the AND-1 core is uncertain but the species present in the assemblage would suggest an estimated time span from 4.2-4.9 Ma. The unit is bounded at its base by an erosional surface and there are three possible erosional surfaces in the upper 15 m but apart from this it appears this unit represents a period of uninterrupted sedimentation dominated by biogenic material. Across the sound in Taylor Valley, several land-based drill cores have been recovered with parts of this interval in each. The Dry Valleys Drilling Project Holes 10 and 11 contain thin intervals (2-20m) of diatomaceous-rich sediment from the early Pliocene. The very base of the CIROS-2 core, which was drilled from the sea ice in from of Ferrar Valley, also contains material from this time interval. Further out, in the Eastern Ross Sea, Deep Sea Drilling Project Hole 271 penetrated early Pliocene sediments, though core recovery was poor. The diatom data from the AND-1 material as well as the other sites mentioned suggest that for what now appears to be an extended period of time in the early Pliocene, the Ross Sea experienced minimal sea ice cover and was a highly productive region for diatoms. This record provides an important constraint on Antarctic climate and ice sheet history during a critical interval in global climate.
C44A-05
Early Pliocene evolution of an overdeepened Antarctic Peninsula continental shelf: onset of modern glacial dynamics
Seismic stratigraphic information and high abundances of Paralia sulcata, a neritic diatom, show that grounding events in the late Miocene occurred on a relatively shallow continental shelf. The absence of P. sulcata and the presence of Stephanopyxis spp., a deeper-water indicator, within the overlying lower Pliocene strata show that the shelf underwent a transition to a deeper continental shelf at ~5.2 Ma. This transition was probably coincident with the evolution of entrenched glacial troughs on the inner shelf. The development of trough mouth fans (TMFs) show that robust ice streams were a major feature of APIS grounding events during the early Pliocene. The cessation of TMF outbuilding corresponds to an upsection percentage increase in the deeper-water species (Stephanopyxis spp.). Diatom biozones suggests that this feature of the modern foredeepened shelf with overdeepened troughs and banks may have only been in existence since ~4.25 Ma. The modern physiography evolved during a relatively brief (~1-Ma) interval of enhanced sub-glacial erosion of the continental shelf. Thus, despite a much earlier onset of glacial conditions on the Antarctic Peninsula, some of the ways that the APIS and the Antarctic continental shelf participates and responds to the current climate system are geologically recent.
C44A-06
Overview of Late Pleistocene Diatom Biostratigraphic Records From Eastern, Central and Northern Basins in the Ross Sea Compared to Seismic Architecture of Sediment Strata
Models of ice grounding during LGM indicate that grounding occurred in the vicinity of the continental shelf break in the Eastern and Central Ross Sea, whereas in the Western Ross Sea the grounding line was in the vicinity of Coulman Island. Numerous piston cores have been collected from the Eastern, Central, and Western Ross Sea continental shelf, with the aim of obtaining a chronological and stratigraphical constraint on the LGM. However, only a few cores have been recovered from the Northern Basin. In Eastern Basin, grounding line features (till deltas) are built of six distinct seismic units deposited by the West Antarctic Ice Sheet. The Central Basin and Pennell Bank are characterized by troughs and banks with ice recessional ridges. During the 2003 austral summer, cruise NBP03-01A collected 10 piston and jumbo piston cores from the Northern Basin in a depth transect across Drygalski Trough – Mawson Bank, at the upper continental slope. The lithologies of the cores include diatom mud, bioclastic sediment, silt, sand, clay, and diamicton. This is in contrast to piston cores from Central and Eastern Basins, where sediment is dominated by diamicton associated with the advance of grounded ice. Here we compare diatom assemblages from the three different basins with these questions in mind: Are the different seismic units of Eastern Basin of the same age and origin? Is there a difference in the composition of Central and Eastern Basin diamictons? How does the late Pleistocene sediment from Northern Basin deposited by the East Antarctic Ice Sheet relate to the Eastern and Central Ross Sea?
C44A-07
Terrestrial and Marine Evidence for a Middle Miocene Onset of Polar Conditions in the Ross Sea Region, Antarctica
The climate transition from a dominantly wet-based temperate to a dominantly dry-based polar ice sheet represents a major step in the evolution of the Antarctic cryosphere. Terrestrial glacial deposits from the Olympus Range in the Dry Valleys sector of the Transantarctic Mountains register an abrupt shift in glacier thermal regimes, from wet to cold based, between 14.1 and 13.9 Ma. On the Ross Sea outer continental shelf, seismic stratigraphic results suggest that mega scale prograding wedges with sub-aqueous out wash channels existed during the early and middle Miocene, at least from ~18-14Ma. Similar morphological features are notably absent within younger middle Miocene strata on the outer continental shelf. The absence of prograding wedge strata and channels suggests that the marine terminous of the ice sheet experienced a shift from dominantly wet to dry basal conditions at ~14Ma, similar to that registered in the Olympus Range at about the same time. Despite the inability to make physical correlation from the Tranantarctic Mountains to the outer continental shelf at the scale of individual events, the available chrono-stratigraphic control indicates there was a regional transition in the Antarctic Cryosphere from a wet to a dry base affecting proximal terrestrial and distal marine settings during the middle Miocene.
C44A-08 INVITED
Sedimentary characteristics of till from the Transantarctic Mountains, Antarctica
Particle size distributions were measured from till samples collected at seventeen moraines along the Byrd and Nimrod Glaciers. Approximately 2 g of material was separated from till samples, treated with 35% H2O2, and measured 3 – 5 times on a Malvern Mastersizer 2000. All sites are dominated by sand- and gravel-sized material with distinct spatial patterns in the silt and clay content. Till from the north side of Byrd Glacier and along the trunk of Nimrod glacier have variable modes in the sand fraction and <5% clay. The tills are primarily derived from locally eroded bedrock. Moraines found at the head of both glaciers contain 15% -70% fines (silt plus clay) and are interpreted to represent the presence of subglacially-derived sediment. Till deposited in the Ross Sea from outlet glaciers likely contains a mixture basal debris derived from the East Antarctic craton and material eroded as glaciers