PP53C-01 13:40h
Antarctic ice sheet collapse and growth in Pliocene times - an ice sheet modeling approach
Pliocene marine diatoms found in high level glacial deposits in the Transantarctic Mountains have been ascribed to inland seas and ice-free conditions in the Antarcic interior at that time followed by a major expansion of the ice sheet that over-rode the mountains. However, circumstantial evidence has led to an alternative view that the deposits are much older, and that the diatoms have become incorporated into the deposits through post-depositional atmospheric contamination. This study tests the feasibility of the East Antarctic interior as a possible source through ice sheet modeling. We address two questions. i) under what conditions could East Antarctic ice become thick enough and warm enough at the base to have deposited glacial debris in their present high locations in the Transantarctic Mountains, and ii) under these conditions is it likely that past ice sheets could have delivered diatomaceous marine sediments to the those locations from the Antarctic interior? We will model two situations i) Taking the present topography of East Antarctica, and varying regional temperature and likely related precipitation regimes. ii) Taking the present temperature (and representative past warmer temperatures) and lowering the Transantarctic Mountains barrier to a level where the basal glacial debris now found on benches up to 2500 m above sea level could have been deposited under temperate glacial conditions. We will focus on two regions in particular, the McMurdo Sound region, where both climate and uplift history are best known, and the Beardmore Glacier area, where the interpretation of the glacial deposits is most in contention.
PP53C-02 13:55h
The Case for a Late Oligocene-Early Miocene Ice Sheet in Marie Byrd Land
A rough estimate of Neogene climate history can be inferred for coastal Marie Byrd Land (MBL) from the record contained in volcanic deposits, and from anomalies in the erosion characteristics of Cenozoic volcanic and plutonic features. As a prelude to the Neogene, a 34 Ma gabbroic pluton in eastern MBL was exhumed by removal of at least 4 km of overburden, apparently during a period of rapid uplift and erosion between 34 and 25 Ma. The depth of erosion represented by the exposure of this pluton contrasts sharply with the generally minimal erosion that seems to have characterized the MBL environment throughout much of the Neogene. Thus, the Neogene seems to have been ushered in by a pronounced decrease in erosion rate that has climatic implications. Late Oligocene hydrovolcanic deposits at the summit of Mt. Petras, on the crest of the MBL dome, seem to record the presence of glacial ice at the time they were erupted (25-29 Ma). However, uplift of the dome did not begin until the early Neogene, and the landscape 25 m.y. ago was probably one of very low relief, represented by the West Antarctic erosion surface. The inferred glacial ice was therefore most likely an ice sheet, rather than a mountain glacier. Although tentative, and contrary to interpretations of the deep sea proxy record, this suggestion of mid-Tertiary glaciation in West Antarctica is supported by two other lines of evidence. (1) The deep dissection of Mt. Petras (Oligocene and older rocks) by glacial cirques is anomalous when compared with nearby mid-Miocene (12-14 Ma) Mt. Hampton volcano, which is virtually untouched by erosion. Our analysis of this anomaly suggests that the Petras cirques were cut mainly before Mt. Hampton formed, i.e. between 14 Ma and 25 Ma, as the Petras fault block was rising. (2) Further support can be found in recently reported seismic-stratigraphic evidence for grounding of an expanded West Antarctic ice sheet in the Ross Sea around 15-17 Ma. Although the MBL data are, individually, quite fragmentary and inconclusive, the consistency among disparate sources is considerably more compelling than the story from any single source. Offshore drilling in the Amundsen Sea seems the most likely way to resolve the questions raised by these field observations.
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PP53C-03 INVITED 14:10h
Bracketing the Warm Peak Phases of the Middle Pliocene Climate
Estimates of sea surface temperature (SST) from ocean cores reveal a warm phase of the Pliocene between about 3.3 and 3.0 Ma. Pollen records from land-based cores and sections, although not as well-dated, also show evidence for a warmer climate at about the same time. Increased greenhouse forcing and altered ocean heat transports are the leading candidates for the underlying cause of Pliocene global warmth. Despite being a general period of global warmth, considerable SST variability exists within this interval. Two new SST reconstructions have been created to provide a climatological error bar for warm peak phases of the Pliocene. GISS GCM simulations that use the new "bracketing" SST reconstructions show the significant potential variability that existed during the relatively warm middle Pliocene, even when we only consider the difference between the various warm peak phases of the original PRISM time slab. While even the smallest warm peak of this period yields a significantly warmer climate than modern (+2.1C in January) the difference between the maximum and minimum warm phases (delta 1.1C in January) is a significant portion of the overall Pliocene-Modern anomaly. This implies that middle Pliocene climates could have cycled from climates that were much nearer the modern to those that were more like the warm middle Pliocene exhibited by numerous simulations conducted using the "peak-average" PRISM SST data during the past decade. The key question is what could have led to the cycling (and more importantly, is it something that could occur in the future)? Obviously, primary suspects include greenhouse gas changes and orbital cycling. However, positive feedbacks in the moisture balance over the North Atlantic, which show up in the results from GISS GCM simulations with the new SST reconstructions, suggest the importance of continuing to pursue coupled ocean-atmosphere experiments for this time period.
PP53C-04 14:25h
Basaltic Glacivolcanic Sequences: Under-utilised Proxies Of Global Change
Basaltic eruptions beneath glaciers have unique characteristics, and the resulting deposits and landforms can be used to document the presence, distribution and properties of glaciers over geological time. Many glacier characteristics are uniquely preserved in glacivolcanic sequences for pre-Quaternary periods. They are therefore important proxies for palaeoenvironmental conditions, yet they are generally neglected, probably through a lack of awareness by the geological community. At least five different types of basaltic volcanic edifices have been identified, each with differing usefulness for deciphering past ice sheets. They comprise sheet-flow sequences (2 types), pillow volcanoes, tephra mounds and tuyas. The distinctive characteristics of each glacivolcanic sequence are largely determined by the simple presence of surface ice and its characteristics (thermal regime, rheology, hydraulics). Significant advances in our understanding of products of subglacial eruptions have been made particularly over the past decade, but few glacivolcanic investigations have addressed global change issues. Neogene volcanic sequences are common in Antarctica, the region that contains the world's most important and longest-lived ice sheet. Interpretations of two, situated on east and west flanks of northern Antarctic Peninsula, illustrate the potential power of glacivolcanic research. Sequences on James Ross Island ($<$ 7 Ma) are tuya-like lava-fed deltas and tephra mounds formed mainly in association with wet-based ice sheets, none of which exceeded 600 m in thickness, alternating with ice-poor periods of marine inundation. By contrast, Quaternary ($<$ 300 ka) volcanism on Brabant Island is represented by multiple sheet-flow sequences indicative of an even thinner wet-based ice cover (not exceeding 100-150 m). The evidence for wet-based ice sheets throughout Late Neogene and Quaternary time is novel, particularly considering the cold polar regime prevailing today, with glaciers frozen to their beds. The new information also provides the only direct evidence that Neogene Antarctic Peninsula ice sheets had very low profiles during periods of extension to the shelf edge.
PP53C-05 14:40h
Miocene Flood Deposits in the Dry Valleys, Antarctica Dated Using Cosmogenic $^{3}$He Isotopes.
{\it In situ} produced cosmogenic $^{3}$He measurements on deposits in the Dry Valleys, East Antarctica, provides information on Neogene climatic variation and East Antarctic ice sheet evolution. Ferrar dolerite cobble-size boulders located in the Coombs Hills form a series of mega-ripples (wavelength approximately 50 meters) associated with scabland features and stripped, corrugated bedrock surfaces. These features, together with topographic position, indicate the boulders were deposited by subglacial floodwaters. Such outburst flooding occurred during over-riding of the northern Dry Valleys by a greatly expanded East Antarctic ice sheet. Timing of the over-riding episode has been previously assigned to between 13.6 and 14.8 Ma by correlation with volcanic ash deposits dated in the Asgard Range of the Dry Valleys. Cosmogenic $^{3}$He concentrations in clinopyroxene from Ferrar dolerite boulder samples imply a minimum of 8.6 to 10.4 Ma exposure, calculated using scaling factors appropriate to Antarctica and assuming zero erosion. These are among the oldest surface exposure dates yet measured on Earth, but are however younger than the $^{40}$Ar/$^{39}$Ar defined chronology. Erosion is an important influence on measured concentrations of cosmogenic isotopes and unconstrained erosion of samples can significantly influence the accuracy of stable cosmogenic isotope dating techniques in East Antarctica. Nearby Ferrar dolerite bedrock surfaces are used to yield an average cosmogenic $^{3}$He derived steady-state erosion rate of 0.24 m Ma$^{-1}$. Applying a conservative erosion correction, (based on $<$ 25% of this rate) to the oldest flood deposit causes apparent exposure ages to rise to over 14 Ma. These results provide independent support for the model of a stable, hyper-arid polar climate persisting in East Antarctica throughout the Neogene period and provide quantitative constraints on long term rates of erosion within the Dry Valleys.
PP53C-06 14:55h
Evidence of Active Warm Based Neogene Glaciation in the Transantarctic Mountains
Interbedded diamictites, conglomerates, sandstones, and siltstones in the Meyer Desert Formation of the Transantarctic Mountains record a dynamic history of warm-based glacier advances and retreats during Neogene time. The Oliver Bluffs (85\deg 80'S, 166\deg 83'E) consist of near-vertical bluffs up to 85 m high that extend for 2 km along the flank of the Beardmore Glacier and reveal at least four diamictites separated by stratified sands and gravels. Bedding dips gently to the northwest ($<$ 5\deg ) and appears to continue under the Meyer Desert Plateau to the southeast. The lower three diamictites consist of light gray to bluish gray fine matrix and boulder clasts up to 5 m diameter. The uppermost diamictites are distinctly bluish-gray in color, contain smaller boulder clasts (0.5 m diameter), and contain previously identified soil horizons. The diamictite beds, which are up to 30 m thick, are interpreted as lodgment tills based on degree of compaction, internal glaciotectonic deformation, and medium to strong pebble fabrics, although a few thin diamictite beds may be of melt-out till or debris flow origin. Diamictites are laterally continuous over tens to hundreds of meters and the lower contacts often truncate underlying beds. Water-worked sediments include laminated silt and sand beds, cross-bedded sands and gravels, and boulder lag deposits. Several finely laminated lacustrine deposits testify to the existence of small lakes or ponds between glacial advances. Wood and leaf imprint fossils, including {\it in situ} rooting systems of {\it Nothofagus} shrubs and cushion plants, further demonstrate the presence of a wet landscape during interglacial intervals. Taken together, this facies sequence is interpreted to represent at least four warm-based glacial advances with a spatially-variable vegetated terrestrial outwash environment superimposed on a landscape with relief inherited from each previous glacial advance. This contrasts markedly with the polar desert landscape of the present day. Research supported by NSF grant OPP 0230696.
PP53C-07 15:10h
The Neogene Environment of the Beardmore Glacier, Transantarctic Mountains
Discontinuous sequences of Neogene marine and non-marine glacigenic sequences, including the Meyer Desert Formation (MDF), occur throughout the Transantarctic Mountains. The upper 85m of the MDF, consisting of interbedded diamictites, conglomerates, sandstones and siltstones, outcrops in the Oliver Bluffs on the Beardmore Glacier at $85\deg$07'S, $166\deg$35'E. The location is about 170 km south of the confluence of the Beardmore Glacier with the Ross Ice Shelf and about 500 km north of the South Pole The glacial, fluvioglacial and glaciolacustrine facies of the MDF represent a dynamic glacial margin which advanced and retreated on at least four occasions. On at least one occasion, the retreat was sufficiently long for plants and animals to colonize the head of a major fjord which existed in the place of the existing Beardmore Glacier. From the fossils we have identified at least 18 species of plants, 3 species of insects, 2 species of freshwater mollusks, and a species of fish. The plant fossils consist of pollen, seeds, fruits, flowers, leaves, wood, and in situ plants. The plants include a cryptogamic flora of mosses and liverworts, conifers, and angiosperms in the families Gramineae, Cyperaceae, Nothofagaceae, Ranunculaceae, Hippuridaceae, ?Caryophyllaceae, and ?Chenopodiaceae or ?Myrtaceae. The plants grew in a weakly developed soil developed on a complex periglacial environment that included moraines, glacial outwash streams, well-drained gravel ridges, and poorly drained depressions in which peat and marl were being deposited. The fossil assemblage represents a mosaic tundra environment of well- and poorly-drained micro-sites, in which nutrient availability would have been patchily distributed. Antarctica has been essentially in a polar position since the Early Cretaceous and at $85\deg$S receives no sunlight from the middle of March until the end of September. Today, the annual radiation received is about 42% that of Tierra del Fuego at $55\deg$S. During the Neogene, the absence of solar heating for about 5.5 months, even though there would have been additional heat from open water in the fjord, would have resulted in a low mean annual temperature. Mean summer temperatures, based on the minimal thermal requirements for Nothofagus, listroderine weevils, and freshwater molluscs, are estimated to have been between 4 and 5°C for at least two summer months. The research is supported by NSF grant OPP-0230696
PP53C-08 INVITED 15:25h
Pliocene age of Meyer Desert Fm. (Sirius Group) Terrestrial Biota at Oliver Bluffs, Dominion Range: a Comparison of Diatom Floras from Glacial, Wind and Ejecta Pathways
The age of the Meyer Desert Formation (Sirius Group) at Oliver Bluffs in the Transantarctic Mountains (TAM), and the terrestrial biota enclosed within these glacigene strata, has been a topic of discussion and disagreement. The Pliocene age derived from the occurrence of reworked late Miocene and early Pliocene marine diatoms within the enclosing sediments has been challenged by the assertion that the diatoms are surface contaminants. Reports of diatoms within Antarctic ice cores and on Antarctic surfaces in other areas of the TAM has provided an alternate explanation for the occurrence of the marine diatoms in the in the Meyer Desert Formation and other Sirius Group deposits. However, the diatom assemblage characteristics of the marine diatoms in the Meyer Desert Fm. and that of the eolian floras in ice cores and surface deposits are very different. These assemblages cannot be derived from the same source or delivered to the Meyer Desert Formation by the same processes. This paper will contrast these different diatom assemblages by comparing their (1) ecology, (2) size, (3) age, (4) taxonomic composition, and (5) potential source areas as criteria to establish the unique features of the glacial-sourced and the eolian-sourced diatom assemblages. Erosion of the face of Oliver Bluffs by a late Pleistocene advance of Beardmore Glacier, as well as ongoing erosion by wind deflation and snow-melt dissection of the Bluffs, produced fresh exposures of the Meyer Desert Formation, which were sampled for diatom analyses. These strata that yielded the marine diatom assemblages were not exposed at the time of the Eltanin asteroid impact (2.5 Ma, late Pliocene). The sampled strata have been exposed only recently to surface processes. Thus, the suggestion that marine diatoms were incorporated onto the surface of the Meyer Desert Formation by fallout of impact ejecta at this location is untenable. However, if ejecta-sourced marine diatoms did blanket the ice sheet and TAM from the 2.5 Ma event, and these diatoms were subsequently picked-up by the ice that deposited the Meyer Desert Formation, they would indicate that the Meyer Desert Formation and enclosed biota was less than 2.5 million years old. Establishing the age of this important paleontological site is critical to the correct assessment of Late Neogene climate evolution of the Antarctic region. These results affirm the Pliocene age of the Meyer Desert Formation paleoflora and associated fauna.