V11F-01
From IGY to IPY: Volcanism Associated With the West Antarctic Rift System Interpreted From Geophysical Observations, and Possible Effects on the Stability of the West Antarctic Ice Sheet (WAIS).
Observations from a few oversnow and airborne magnetic profiles acquired over the West Antarctic Ice Sheet (WAIS) during the International Geophysical Year (1957-58) indicated numerous high amplitude, shallow source, magnetic anomalies over a very extensive area of the presently known West Antarctic rift system. Aeromagnetic surveys over the WAIS in the early 1960s and later combined with radar ice sounding in 1978- 79 defined an area >500,000 km2; these anomalies range from 100->1000 nT as observed ~1 km over the 2-3 km thick moving ice. Behrendt et al, (1962, 1964, 1994, and 2005) and Jankowski et al. (1983) interpreted these anomalies as indicating "volcanic centers." Detailed aeromagnetic and radar ice sounding surveys since 1993 have shown that >80% of these anomaly sources have been modified by the moving ice into which they were injected requiring a younger age than the WAIS (~25 Ma). Behrendt et al., (1994; 2007) conservatively estimated >1 x 106 km3 volume of volcanic sources to account for the area of the "volcanic center" anomalies and suggested the presence of a large igneous province (LIP) if this volume was intruded within a time interval of 1-10 Ma. Active volcanism at a few widely spaced exposures of alkaline volcanic rocks associated with the West Antarctic rift, which extend in age to ~34 Ma in the WAIS area, and interpreted active subglacial volcanism revealed by aerogeophysical data (Blankenship et al., 1993; and Corr and Vaughan, 2008) have raised the question of possible volcanic effects on the regime of the WAIS. Vogel and Tulaczyk (2006) argued that subglacial volcanism may play a "crucial roll" in WAIS stability, but LeMasurier (2008) has discounted this as unlikely. In my presentation I will review the geophysical evidence acquired from the IGY to the IPY, and conclude that whether unlikely or not, future effects on the stability of the WAIS should not be ignored.
V11F-02 INVITED
POLENET Seismic and GPS Network in West Antarctica
The Polar Earth Observing Network (POLENET) multinational consortium has begun deployment of new
networks of in situ sensors in the Arctic and across Antarctica for the International Polar Year (IPY). The
West Antarctic component of POLENET, led by U.S. investigators, will consist of 16 co-located continuous
GPS stations and broadband seismic sensors, which may remain in place long-term as infrastructure. Each of
these stations has Iridium communications to transfer data or state-of-health daily to USA archives.
Additional arrays of CGPS and seismic stations are being deployed for 2-3 years. After the 2007-08 field
season, 18 CGPS and 10 seismic stations are in place; up to 4-6 new stations will be deployed in 2008-09; a
major deployment is planned for 2009-10. The data from this extensive new network will be integrated with
measurements from POLENET partners. The integrated network will provide synoptic measurements across
the interior of West Antarctica, as well as much of the perimeter of East Antarctica, allowing refinement of
estimates of recent ice mass change of the Antarctic ice sheets. We are measuring the steady vertical
velocity field due to isostatic rebound with GPS and will constrain earth rheology (elasticity, viscosity) through
seismic studies. The new in situ measurements will provide an accurate 'PGR correction' for GRACE,
improving GRACE-based estimates of recent ice mass change. New understanding of ice dynamics,
tectonics, global seismic tomography and inner-core anisotropy will also be attained.
http://www.polenet.org
V11F-03 INVITED
Cenozoic Motion Between East and West Antarctica and the Global Plate Circuit
A critical element in kinematic models linking plate motions in the Indo-Atlantic basins to the Pacific basin is quantifying the motion within the Antarctic plate. A major change in the resolution of the reconstructions occurs around Chron 20 (50 Ma). Motion between East and West Antarctica since Chron 20 is constrained both by seafloor spreading magnetic anomalies in the Adare Basin and by summing the regional plate circuits linking East Antarctica-Australia-West Antarctica and East Antarctica-Australia-Pacific-West Antarctica. Both the plate circuit solutions and the direct observations of magnetic anomalies are in good agreement and show that there was roughly 160 km of extension in the Northern Ross Sea between chrons 20 and 8 (26 Ma). Prior to Chron 20 the motion is not nearly as well constrained. Although there are marine geophysical observations in the northern Ross Sea that constrain the rotation of the Iselin Bank between chrons 27 (61 Ma) and 24 (53 Ma), seafloor spreading anomalies equivalent to the Adare Basin anomalies have not been found. In addition, the regional plate circuits are more poorly constrained. Perhaps the most troublesome link is the one between Australia and East Antarctica. Seafloor spreading between Australia and East Antarctica was very slow prior to Chron 20 and the fracture zones formed during this period are not clearly defined in satellite radar altimetry data. Consequently there are three different models using slightly different constraints leading to three different sets of rotations for Australia-East Antarctica motion. When inserted into the regional plate circuits these give widely variable estimates of the amount and direction of motion between East and West Antarctica in the early Cenozoic. Uncertainties in the portion of the plate circuit between Australia-Lord Howe Rise and the Pacific plate prior to Chron 24 introduce additional errors in the estimates of East-West Antarctic motion. However, despite the large uncertainties in the regional plate circuits through Antarctica in the early Cenozoic, all of the solutions share a common characteristic: they fail to generate the “pre-Bend” track of the Hawaiian-Emperor seamount chain when used in conjunction with absolute plate motion models based on hotspots from the Indo-Atlantic basins.
V11F-04
Evidence for the Involvement of Medium- to High- Pressure Fractionation Processes in the Origin of Marie Byrd Land Pantellerites, West Antarctic Rift System.
Pantellerites, trachytes, and phonolites, of mid-Miocene (~14Ma) to Holocene age, occur in close proximity to each other in 6 large felsic shield volcanoes along the coast of western Marie Byrd Land (MBL). Work on pantellerites in the Afar region of Ethiopia, a good analog for MBL, showed that low-pressure fractional crystallization of transitional basalt provided a reasonable mechanism for the development of Afar pantellerites (Barberi, et al., 1974). In MBL, however, the predominant basaltic rocks are ne-normative basanites, which could be expected to produce phonolites via low-pressure fractionation. The problem here is to determine what mechanisms, and what sort of plumbing system can produce pantellerite, phonolite, and trachyte within the same time interval, and in close proximity. There are suggestions from cumulate nodules, trace element chemistry, and modeling, that fractionation of kaersutite, and perhaps orthopyroxene, at depths between 25 and 50 km, was a factor in the origin of MBL pantellerites, though as many previous studies have shown, it is difficult to unambiguously demonstrate the involvement of high- pressure phases when these processes have been overprinted by low- pressure crystallization. Further fractionation, at higher levels in the crust, under conditions of low PH2O and low fO2, appears to have produced the high peralkalinity and high FeO content of MBL pantellerites. Isotopic and trace element data provide no evidence for crustal contamination as a factor in the origin of these rocks.
V11F-05
Petrology and source of lavas from seamounts in the Adare Basin, Western Ross Sea: Implications for the origin of Cenozoic magmatism in Antarctica
Hundreds of volcanic seamounts are randomly distributed in the Adare Basin, northern Ross Sea, and on the adjacent continental shelf of north Victoria Land. The cluster of volcanic seamounts directly east of Cape Adare, which we designate as southern Adare Basin Seamounts (ABS), were intruded through a thick (~2000 m) pile of sediments that have been accumulating in the Adare Basin since its opening ~43 Ma and have youthful morphology suggesting that they were formed contemporaneously with the other Cenozoic volcanoes in West Antarctica. Lavas dredged from the southern ABS on the continental shelf (~600 to 400 mbsl) and within the ocean basin (~2000 to 1400 mbsl) range from basanite and phonotephrite to trachyte and rhyolite (MgO >10 to <1 wt.%; Ni>300 to <10 ppm), are alkaline (K2O >1 to <5 wt.%; Ne-norm 2-18 wt.%, Ba >600 to <100 ppm), are light rare earth element enriched (La/YbN 15-27), and seven samples have low 87Sr/86Sr (<0.7029) and high 143Nd/144Nd (>0.51295) ratios. Lavas are fine-grained to glassy, porphyritic and vesicular with phenocrysts of olivine, pyroxene, feldspars, magnetite and rare amphibole. The incompatible trace element ratios of the most differentiated ABS lava are similar to those of more mafic ABS lavas, but have higher 87Sr/86Sr (0.7077). This is reminiscent of the compositional signature of the differentiated alkaline lavas produced by melting of previously erupted mafic lavas in a few oceanic islands. Overall, results show that volcanism within the Adare Basin is coeval and petrogenetically akin to continental volcanism in West Antarctica and thus expand the known extent of Cenozoic alkaline magmatism associated with the West Antarctic rift. The similarities between ABS and West Antarctic volcanism, as well as with other continental intraplate lavas from the southwest Pacific offer compelling support for an inherent connection between their mantle sources. Furthermore, the coupled extensional history of the oceanic and continental sectors in the northern Ross Sea, along with the broadly coincident age of the volcanic activity, strongly suggests that both continental and oceanic volcanism were triggered by the same mechanism.
V11F-06
Volcanic and Glacial Geology of the Miocene Minna Bluff Volcanic Complex, Antarctica
Minna Bluff is a 45-km long, 5-km wide Miocene alkaline volcanic peninsula that extends SE into the Ross Ice Shelf from the Mt. Discovery stratovolcano. Minna Bluff is a significant topographic barrier that has effectively blocked the Ross Ice Shelf and former grounded marine ice sheets from flowing southward into McMurdo Sound. In the late Miocene, Minna Bluff likely was a terminal pinning point for the Ross Ice Shelf. The peninsula is composed of many overlapping volcanic centers and is intensely eroded along south facing McIntosh Cliffs and east-facing Minna Hook cliffs, providing well-exposed 1000-m-thick stratigraphic sections. Mapping and sampling during 2006-7 and 2007-8 field seasons provide records of pulsating volcanism, glacial erosion, and glacial deposition in the Ross Embayment during the construction of Minna Bluff between 12 and 8 Ma. The volcanic stratigraphy along the Minna Hook cliffs contrasts markedly with the stratigraphy of McIntosh Cliffs. Stratigraphic alternations between subaerial lava and breccia and thin subaqueous pillow lava and hyaloclastite are concentrated in the lower parts of the Minna Hook sections. These alternations are interpreted as representing syneruptive interactions between lava flows and ephemeral local ice. The lower parts of the Minna Hook sequences also include widespread, undulating unconformities mantled by glacial and fluvial sediments. These unconformities and associated sediment record at least two broader scale grounded ice sheet events which are tightly constrained by 40Ar/39Ar ages (see Fargo et al, this volume). Upper parts of the Minna Hook sections resemble the McIntosh Cliff sequences, in being dominated by subaerial lava, breccia, and vent complexes and lacking subaqueous volcanic lithofacies, sedimentary rocks, and unconformities. More than 50 partially eroded, subaerially erupted domes and cinder cones were mapped and are preserved on the bluff top. An explosively erupted hydrovolcanic deposit forms a small nunatak in the saddle between Mt. Discovery and Minna Bluff. Minna Bluff rocks form a basanite to phonolite alkaline volcanic lineage. Phonolite and tephriphonolite domes are common in the lower third of the Minna Hook cliff sections and along the top of the Minna Hook, basanite to phonotephrite lavas are the dominant lithologies overall. Ongoing geochronology, geochemistry, and lithofacies analysis of more than 500 samples will provide a detailed glacial and volcanic record of this important region.
V11F-07 INVITED
Neogene ice sheet, paleoclimatic and geological history of the McMurdo Sound region, Victoria Land Basin, Antarctica: overview of ANDRILL's first two drilling projects
The ANtarctic geological DRILLing Program (ANDRILL), an international collaboration within IPY, has recovered 2 cores: from under the Ross Ice Shelf (McMurdo Ice Shelf Project (MIS) - AND-1B), and from the land-fast sea-ice of McMurdo Sound (Southern McMurdo Sound Project (SMS) - AND-2A). Drill cores reached respective total depths of 1285mbsf in c. 850m of water (MIS) and 1138.54mbsf in c. 380m of water (SMS). Repetitive facies successions in AND-1B core imply at least 60 fluctuations, of probable Milankovitch- duration, between subglacial, ice proximal and ice distal open marine environments. These are grouped into 3 types of facies cycles corresponding to glacial-interglacial variability during climatically distinct periods of Late Neogene: (1) cold-polar climate and ice (late Miocene and Pleistocene); (2) relatively warmer climate, polythermal ice and interglacials dominated by pelagic diatomite (Pliocene); (3) warmer climate, polythermal ice with interglacials dominated by hemipelagites (early late Miocene). A c. 80m-thick interval of diatomite of mid to late Pliocene age shows no apparent glacial cyclicity and represents an extended period of ice-free conditions indicating reduced or absent WAIS. Late Pliocene glacial-interglacial cycles characterized by abrupt alternations between subglacial/ice-proximal facies and open marine diatomite units imply significant WAIS dynamism, and contribution to global ice volume changes coeval with the initiation of Northern Hemisphere glaciations. A c. 4m-thick interval of diatomaceous mudstone in the mid-Pleistocene also represents warm-interglacial ice-free conditions. Intriguingly, glacial deposits interrupted by periodic, small- scale grounding-line retreats dominate the last 1m.y. Inter-hemispheric ice sheet coupling was probably controlled by Northern Hemispheric insolation and consequent glacial eustasy to account for much of the orbital-scale WAIS variability since 2.5Ma. A further expansion of WAIS occurred across the Mid-Pleistocene Climate Transition to establish the present WAIS mode. The AND-2A drillcore recovered several distinct intervals separated by disconformities: (1) a lower Miocene interval (1138.54-c. 800mbsf); (2) a 600m-thick early and middle Miocene interval (800-223mbsf), including an expanded section through two Miocene climatic optima, is truncated by a disconformity that spans c. 7m.y.; and (3) an upper Miocene-Recent interval (223-0mbsf) that is thinner but correlative to parts of the AND-1B drillcore. Shallow marine deposits dominate the lower AND-2A section until c.1.5Ma when the basin deepened rapidly from volcanic loading by Mt Erebus. Lower and middle Miocene strata record repeating lithological changes reflecting variation in sea level, glacial proximity, and climate fluctuations on the shallow marine coast of the Transantarctic Mountains. Sediments deposited close to or beneath grounded glaciers (likely flowing from East Antarctica) alternate with fine-grained marine sediments, providing clear evidence for cycles of ice advance then substantial retreat during climate transitions to warmer conditions. Fossils suggest non-polar climate conditions similar to southern New Zealand today, influenced by high sediment discharge from river run-off, and high coastal turbidity.
V11F-08
Antarctic Paleotopography Estimates at the Eocene-Oligocene Climate Transition: Implications for ClimateIce Models
Paleoclimate models for the rapid growth of Antarctic ice in the latest Eocene have advanced substantially in recent years. However, one generally recognized limitation of these models is that they are based on present topography, corrected only for removal of modern ice. For West Antarctica this results in large areas below sea level that would not host ice. In the Ross Embayment, part of the recently active West Antarctic rift system, there are reasons to suspect that other factors may have contributed to significant vertical motions since the Eocene. The clearest example is the sampling of ~26 Ma Oligocene shallow-marine sediments slightly above Paleozoic basement in DSDP Site 270 at a depth of about 1 km below present sea level. We present a model for 34-Ma paleotopography that, in addition to accounting for the load of the modern ice, also accounts for thermal contraction that results from tectonic extension and accounts for sediment redistribution and loading since 34 Ma. With support from plate-motion data, geologic observations, and limited crustal-thickness data, we assume that the West Antarctic Rift was an orogenic highland in the Early Cretaceous, with a crustal thickness of about 50 km. Three phases of extension transformed this highland to the present lowlands and basins. The first, at 100-80 Ma, affected the western and southern margins of Marie Byrd Land and the adjacent Eastern Basin. The second occurred about 70-55 Ma, as suggested Cande and Stock [2004], affecting the central Ross Embayment, including Iselin Bank, Central High, and possibly reaching as far as Siple Dome. The third, synchronous with spreading at Adare Trough about 45-25 Ma, primarily affected basins adjacent to the Transantarctic Mountains. Stretching factors are everywhere at least 2.0, and higher in sedimentary basins. Sediment thickness is fairly well mapped in the Ross Sea, but must be estimated from extremely limited data under ice. Vertical responses to changes in ice thickness, sediment thickness, and resulting changes in water load are calculated using a flexure model with effective elastic thickness tentatively set at 50 km, using 10-km grid cells. Predicted subsidence for thinly sedimented structural highs is reasonably well constrained in the range of 400-1200 m, which restores many marine areas to 100-300 m above sea level. Predicted subsidence in basins is larger but sensitive to details of the model. Structural highs in the western Ross Sea and potentially more than half of the area under the Ross Ice Shelf and adjacent areas with the modern bed below sea level restore to subaerial lowlands in late Eocene. This study shows that paleotopography models with more regions above sea level should be considered in coupled climate-ice modeling. These models could result in more rapid or earlier accumulation of ice on West Antarctica.