C33A-01 INVITED
The Importance of History for Predicting the Future of the West Antarctic Ice Sheet
The West Antarctic Ice Sheet (WAIS) initiative began in 1990, following on earlier studies of the 'Siple Coast' ice streams and the Ross Ice Shelf. The past nearly two decades of field and satellite research of the West Antarctic ice sheet have produced an astounding number of discoveries, not the least of which is the variability of the West Antarctic ice sheet on time scales from seconds (yes, seconds!) to many millennia. The shorter-time-scale variations, such as the recent acceleration and thinning of glaciers draining into the Amundsen Sea, have illustrated serious weaknesses in what were once regarded as excellent models of ice sheet dynamics. Repairing this modeling capability requires understanding and incorporating external and internal processes previously regarded as less important. Ice-sheet history remains the best means to test, tune and validate numerical models of ice sheets. Cenozoic-age behavior may seem too ancient to matter to a centennial-time-scale focus on the future, but it is precisely through a long history, that the variety of more extreme ice sheet configurations can be extracted. Such upper or lower bound estimates have served the WAIS community well over the years to help justify research needed to assess the probability of dramatic behavior. Now, with the necessity of model revisions central to the WAIS effort, time histories of ice sheet behavior over both short and long time scales will return to a position of extreme importance.
C33A-02
ANDRILL-Southern McMurdo Sound (SMS) Project: Early Miocene to Recent paleoclimate and Geological History of the Victoria Land Basin, Antarctica
During the past austral summer season, the ANtarctic geological DRILLing Program (ANDRILL) successfully completed the drilling phase of its second project: the Southern McMurdo Sound Project (SMS). Completed in early December 2007, the AND-2A drill-hole (77° 45.488 S; 165° 16.613 E) was successfully recovered from a floating sea-ice platform (8.5 meters thick), over 380 meters of water, reaching a total depth of 1138.54 mbsf (98% core-recovery). A chief objective was to recover sediment from the middle Miocene, which has long been held as one of the fundamental time intervals in development of the modern Antarctic ice sheets. The AND-2A drillcore recovered several distinct stratigraphic intervals separated by disconformities: (1) a lower Miocene section (1138.54 up to c. 800 mbsf) correlative with an interval previously recovered during Cape Roberts Project drilling; (2) a 600 m-thick early and middle Miocene interval (800-223mbsf), including an expanded section through two Miocene climatic optima and truncated by a c. 7 m.y. disconformity; and (3) an upper Miocene-Recent interval (uppermost 223 meters) that is thinner but correlative with parts of the upper Neogene section recovered by the ANDRILL MIS Project in drill core AND-1B. Shallow marine deposits dominate the lower AND-2A section until c. 1.5 Ma when the basin deepened rapidly in response to volcanic loading by Mt Erebus. Lower and middle Miocene strata record periodic lithological changes that reflect variation in sea level, glacial proximity, and climate in the SW Ross Sea between c. 20- 14.5 Ma. Sediment deposited close to or beneath grounded glaciers alternate with fine-grained marine sediments, providing clear evidence for cycles of ice advance, followed by substantial retreat during climate transitions to warmer conditions. Fossil assemblages preserved in these strata suggest non-polar climate conditions, similar to southern Patagonia today, influenced by high sediment discharge from river run-off and high coastal turbidity. An excellent chronostratigraphy for the AND-2A drillcore developed from combined biostratigraphy, magnetostratigraphy and radiometric dating of abundant tephra and volcanic materials, provides age control for the drillhole and the network of seismic lines in the western Ross Sea.
C33A-03
Evidence of long- and short-term responses of the West Antarctic Ice Sheet during the Plio-Pleistocene and their implications: ANDRILL-McMurdo Ice Shelf Project
Response of ice sheets, especially the West Antarctic Ice Sheet (WAIS), is recognized as a significant unknown in predicting future consequences of global warming. Deeper-time data from 1284m of a sediment core drilled on the NW corner of the Ross Ice Shelf (RIS) are used to assess prior WAIS dynamics and its responses to past climate changes. The core shows WAIS changed from a cold ice sheet less than 13Ma, to being warmer with significant channelized subglacial meltwater prior to ca.7.5Ma, when during interglacials, local rivers flowed to McMurdo Sound. Between 5-3Ma meltwater decreased and interglacial periods were cooler with diatoms dominating rather than local meltwater. WAIS was dynamic ca.7.5-3Ma with its grounding and calving lines retreating past Ross Island in interglacials; occasionally iceberg calving was absent when termini were mostly terrestrial. Subglacial sediment deformation occurred to ca.10m depth, and glacial advance facies are locally preserved, indicating little erosion occurred during some advances and are used to indicate the ice sheet advance was probably rapid and definitely short-lived. Glacial to interglacial facies transitions are also locally condensed, often lacking retreat packages, and suggest ice sheet retreat was rapid as a response to some past warming events. Volumes of sub-ice-sheet meltwater during glacials and local interglacial meltwater declined through to ca.0.8Ma indicating WAIS was cooling. However, it remained more dynamic than after ca.0.8Ma when it reached its present cold state, inferred from thicker diamictite packages and thin to absent interglacial mudstones.
C33A-04
High-Resolution Physical Properties Logging of the AND-1B Sediment Core – Opportunity for Detecting High-Frequency Signals of Paleoenvironmental Changes
Within the ANDRILL-MIS Project, a more than 1200 m long sediment core, dating back to about 13 Ma, was drilled beneath McMurdo Ice Shelf near Ross Island (Antarctica) in austral summer 2006/07 with the purpose of contributing to a better understanding of the Late Cenozoic history of the Antarctic Ice Sheet. One way to approach past ice dynamics and changes in the paleoenvironment quantitatively, is the analysis of high- resolution physical properties obtained from whole-core multi-sensor core logger measurements in which lithologic changes are expressed numerically. This is especially applicable for the repeating sequences of diatomites and diamictites in the upper half of the core with a prominent cyclicity between 140-300 mbsf. Rather abrupt high-amplitude variations in wet-bulk density (WBD) and magnetic susceptibility (MS) reflect a highly dynamic depositional system, oscillating between two main end-member types: a grounded ice sheet and open marine conditions. For the whole core, the WBD signal, ranging from 1.4 kg/cu.m in the diatomites to 2.3 kg/cu.m in diamictites from the lower part of the core, represents the influence of three variables: (i) the degree of compaction seen as reduction of porosities with depth of about 30 % from top to bottom, (ii) the clast content with clasts being almost absent in diatomite deposits and (iii) the individual grain density (GD). GD itself strongly reflects the variety of lithologies as well as the influence of cement (mainly pyrite and carbonate) on the matrix grain density. The calculation of residual porosities demonstrates the strong imprint of glacial loading for especially diamictites from the upper 150 m, pointing to a significant thickness of the overriding Pleistocene ice sheet. MS on the other hand mainly documents a marine vs. terrestrial source of sediments where the latter can be divided into younger local material from the McMurdo Volcanic Province and basement clasts from the Transantarctic Mountains. Values range over several orders of magnitude from <10 (10-5 SI) in the diatomites to 8000 (10-5 SI) in single clasts (mainly dolerite). Synchronous minima and maxima in both WBD and MS support dramatic changes in the depositional environment, driven by oscillations in ice extent in response to global climate fluctuations on orbital timescales. Superimposed on this, small-amplitude variations of high frequency are found within diatomite units. A rhythmic pattern of probably millennial to centennial pacing proposes an additional non-orbital forcing as control on system dynamics, at least during interglacials.
C33A-05
Brittle fractures in AND-1B core, McMurdo Ice Shelf, Antarctica: A record of Neogene rifting or glaciotectonic deformation?
The ANDRILL geological drilling program retrieved a 1285-m-long core (AND1B) from beneath the McMurdo Ice Shelf, Antarctica, in 2006-07. The drillsite is inferred to lie within the Terror Rift, a regional Neogene rift basin in the western Ross Sea, hence normal faulting and related tectonic deformation is expected to be present in the core. Sequence stratigraphic analysis has identified ~60 unconformity-bounded cycles in the core, with each base interpreted to mark erosion and subglacial deposition by an advancing Ross Ice Sheet, hence glaciotectonic deformation is also expected in the core. Systematic fracture logging of the AND-1B core identified 1,475 'natural fractures', i.e. pre-existing fractures in the rock intersected by coring. The most abundant natural fractures are normal faults and calcite veins; reverse faults, brecciated zones, and sedimentary intrusions are also present. Here we compare fracture distribution, density, type and orientation (where known) to the positions of glacial erosion surfaces in the core, together with initial information on the conditions of deformation from microstructural analysis, to discriminate rift-related from glaciotectonic formation of natural fractures in AND-1B core.
C33A-06
Plio-Pleistocene evolution and variability and the West Antarctic Ice Sheet from the ANDRILL, AND-1B geological record
While the West Antarctic Ice Sheet (WAIS) is considered to be vulnerable to future anthropogenic warming, projections of its likely behavior are hampered by a limited understanding of past variations and the main forcing mechanisms. Here we present a new proximal Plio-Pleistocene climate record constructed from the upper 600m of a sediment core (AND-1B) recovered from beneath the northwest corner of the Ross Ice Shelf by the ANDRILL program. More than forty, well-dated, sedimentary cycles in the core link ice extent to orbital-scale climate cycles dominated by obliquity during the Pliocene and Early Pleistocene - a 100,000 year-duration cycle dominates the Middle and Late Pleistocene. Our data provide the first direct evidence for an oscillating marine-based ice sheet in Ross Embayment, which periodically contracted onto terrestrial Antarctica when planetary temperatures were up to ~3°C warmer than today and atmospheric pCO2 likely no higher than 400 ppm. We describe significant change in thermal regime of the ice sheet coincident with a global cooling trend between 3 and 2.5 million years ago, evident in oxygen isotope records and associated with the onset of northern hemisphere glaciations. During this time the ephemeral WAIS and the coastal margins of the East Antarctic Ice Sheet (EAIS) cooled towards their present polar state, expanding and developing more permanent marine termini and ice shelves. Further expansion occurred across the Mid- Pleistocene Climate Transition. Thereafter, extensive ice shelves rather than open marine conditions, dominated the Ross Embayment during interglacial periods.
C33A-07
West Antarctic Ice Sheet dynamics recorded in Plio-Pleistocene strata of the Ross Sea, Antarctica
Within the 100,000 square kilometer Eastern Basin of the Ross Sea, a 290 km section, oriented parallel to depostional dip along with 10 intersecting seismic sections that are oriented parallel to depositional strike were analyzed. Using Single-Channel Seismic (SCS) data from three different seismic surveys (NBP 0306, PD9022, and NBP 9308) 36 Plio-Pleistocene sequences were correlated across the basin from the modern ice shelf edge to the contemporary shelf break. Few of the sequences are continuous across the shelf, the majority of the sequences are of limited lateral extent. The facies within the sequences were analyzed to determine ice sheet behavior at the time of deposition. Three distinct depositional environments were interpreted based upon variations in the reflection attributes within the seismic data. Subglacial facies have a spectrum of reflection attributes from reflection-free to parallel, low-amplitude, discontinuous facies. The Grounding Line Zone facies are characterized by high amplitude, mildly discontinuous reflections. Proglacial environments are distinguished by parallel, high amplitude, continuous reflection packages. The facies distribution within many of the sequences consists of Subglacial facies in updip locales, Grounding Line Zone facies widely distributed across the shelf, and Proglacial facies present at downdip sites. The facies distribution within the sequences provides a record of the variation of the extent of the West Antarctic Ice Sheet (WAIS) throughout the Plio-Pleistocene. Not all sequences have a consecutive facies relationship, which may have resulted from several causes: 1) changes in the flow of the WAIS, 2) interplay between the East Antarctic Ice Sheet (EAIS) with the WAIS, or 3) additional grounding of the WAIS on paleobasin highs. Understanding the short-lived glacial events, whether they are a function of non-deposition or cannibalization of previous deposits, provides insight into the dynamics of marine based ice sheets.
C33A-08
Late Pliocene Changes in Abyssal Southern Ocean Ventilation: New Insights from the Subantarctic Pacific
New stable isotope records from subantarctic southeast Pacific cores MV0502-4JC (50°20'S, 148°08'W, 4286m) and ELT 25-11 (50°02'S, 127°31'W, 3969m) suggest that the late Pliocene climate transition (LPCT) at ~2.75 Ma marked a dramatic increase in cold, poorly ventilated water in the deepest portion of the Southern Ocean. This conclusion is based on a striking vertical stable isotopic gradient that developed between MV0502-4JC and shallower South Atlantic ODP Site 704 (46°52'S, 7°5'E, 2532 m) in the late Pliocene, a sharp contrast to the comparable benthic δ13C and δ18O values recorded at the two sites during the late Miocene. Similar to existing shallower records from the South Atlantic, benthic stable isotope records from MV0502-4JC and ELT 25-11 record a significant decrease in δ13C accompanied by an increase in δ18O in the Late Pliocene. While the δ13C decrease and δ18O increase recorded at the South Atlantic sites appears to represent the onset of a glacial pattern of oxygen isotope enrichment and reduced deep water ventilation in the Southern Ocean, the transition in MV0502-4JC and ELT 25-11 signals a far more dramatic shift in conditions at abyssal depths. First, the benthic δ13C shift appears to be more extensive and prolonged in the deep subantarctic Pacific than in the South Atlantic, and second, the δ13C decrease was accompanied by an equally dramatic increase in benthic δ18O that stands out amongst other deep sea records prior to the late Pleistocene. In MV0502-4JC these δ13C and δ18O shifts amount to -1.05‰ and +1.1‰, respectively, over the period ~2.7 to 1.9 Ma (9.6-7.3 MBSF) and are followed by a dramatic return of the benthic δ13C values to pre-LPCT values after ~1.7 Ma that is also recorded in the planktonic record. These benthic δ13C and δ18O shifts likely indicate the accumulation of an ever-larger percentage of cold, poorly-ventilated AABW in the abyssal Southern Ocean during the late Pliocene, probably due to sea ice formation, and perhaps accompanied by reduced upwelling of abyssal Southern Ocean waters. We posit low planktonic δ13C values in these subantarctic Pacific records also support the existence of significant sea ice in the Southern Ocean during the late Pliocene, as sea ice cover reduces gas exchange and nutrient utilization south of the Polar Front leading to low δ13C surface waters in the subantarctic. Overall we infer that the pool of cold, poorly-ventilated water was at least sizeable enough to fill the Pacific sector of the Southern Ocean below a depth of 4000 m. Such a large, isolated water mass would have been capable of sequestering large amounts of CO2 within the abyssal Southern Ocean, and thus could potentially have played an important role in the amplification of Northern Hemisphere Glaciation during the late Pliocene.