McMurdo Sound Research Articles

Classification and behavior of free-ranging Weddell seal dives based on three-dimensional movements and video-recorded observations

The goal of this study was to classify free-ranging (FR) dives of Weddell seals (Leptonychotes weddellii) and to compare them to isolated hole (IH) dives. Classification and comparisons were based on 58 descriptors for three-dimensional dive paths computed from data obtained by attaching video and data recorders to the backs of 12 adult Weddell seals that were free-ranging in eastern McMurdo Sound, Antarctica. We then inferred behavioral functions for the dive classes based on video-recorded observations. Three of the four dive types previously identified from seals diving at an IH occurred in FR seals. Although there were differences associated with location, Types 1, 2 and 3 dives clustered in a similar pattern in the discriminant analysis for FR and IH dives. Most prey (79%) captures occurred during Type 1 dives, and the primary (99%) prey was Antarctic silverfish (Pleuragramma antarcticum). Type 1 dives were the deepest (mean maximum depth 324–378m), longest in duration (15.0–27.0min), covered the greatest total distance (1470–2197m), and had the steepest dive angles (descents: −30°; ascents: +27°). Types 2 and 3 dives formed a continuum from short duration (3.6–7.5min), shallow (mean maximum depth 30–66m) dives that were close to the ice hole (farpoint distance 75–130m) and often involved aggressive interactions with other seals for breathing opportunities (Type 2) to progressively longer (7.9–17.2min), deeper (mean maximum depth 81–143m) dives that covered greater total distances (878–1194m) and were associated with transiting between holes, exploring and occasionally foraging (Type 3). Very long distance Type 4 exploratory dives that were identified in the IH study were completely absent in FR seals.

The physiological response to increased temperature in over-wintering sea ice algae and phytoplankton in McMurdo Sound, Antarctica and Tromsø Sound, Norway

The physiological response to increased temperature during dark exposure was examined in phytoplankton and sea ice algae that had overwintered in McMurdo Sound, Antarctica and Tromsø Sound, Norway. Under ice phytoplankton and sea ice algae from McMurdo Sound were incubated in the dark for 22days and 23days respectively at −2, 4 and 10°C, while phytoplankton from Tromsø Sound were incubated for 35days at 4, 10 and 20°C. A fluorescence approach was used to examine algal photophysiology (Fv/Fm, rETRmax and α) and changes in the concentration of chlorophyll a, while the spectrophotometric 2,4,6-tripyridyl-s-triazine (TPTZ) assay was used to quantify water-extractable carbohydrates. Prior to incubation, the photosynthetic parameters documented relatively healthy overwintering communities for both polar regions. Elevated temperature had a considerable impact on the dark survival of Arctic phytoplankton, and, to a lesser extent, Antarctic sea ice algae: photosynthetic health and stored monosaccharides declined during the incubation period, particularly at the warmest temperature regimes. In contrast, the concentration of chlorophyll a and polysaccharides remained relatively constant. When Antarctic sea ice algae were subsequently exposed to low light (~20μmol photons m−2s−1), significant photosynthetic recovery was only observed in cultures maintained at −2°C. A more robust response to increased temperature was observed in Antarctic phytoplankton and in general, variability between the −2°C and 4°C (Antarctic) and 4°C and 10°C (Arctic) temperature regimes was minimal, which suggests that increasing temperature will not limit the ability of phytoplankton to survive the polar winter and provide the inocula for bloom events.

Ice-flow switching and East/West Antarctic Ice Sheet roles in glaciation of the western Ross Sea

The long-term behavior of the East and West Antarctic Ice Sheets, and their respective responses to forcing provide essential context for assessment of modern dynamic changes in ice-flow regimes and ice-sheet and shelf margins. The western Ross Sea discharges ice from both the East and West Antarctic Ice Sheets, and the paleoglacial record from this region is therefore valuable in unraveling their long-term behavior. New, high-resolution multibeam bathymetric data reveal snapshots of well-preserved glacial landforms on the seafloor around Ross Island and McMurdo Sound. Glacial lineations, grounding zone wedges, draped recessional moraines, and meltwater channels record a series of different ice-flow events in the region, contradictions between which require major phases of ice-flow reorganization. From the glacial geomorphology, we reconstruct a four-stage model of ice-flow evolution for the last glacial cycle, consisting of: (1) northeastward flow into the Ross Sea from McMurdo Sound; (2) westward flow from the Ross Sea, around Ross Island, and onto the Victoria Land coast and coastal seafloor trough; (3) a deglacial phase of ice-sheet thinning, minor shifts in flow, and grounding line retreat into McMurdo Sound; and (4) grounding line pinning on Ross Island during regional retreat, uncoupling of a remnant Ross Island ice cap, and local oscillation of Victoria Land outlet glaciers. We find that East Antarctic Ice Sheet ice discharge had a strong influence on ice-flow geometry in this part of the Ross Sea during the last glacial stage, but that it was not necessarily in phase with the behavior of the West Antarctic Ice Sheet. It is similarly evident that the ice streams that drained the Ross Sea over the continental shelf at the Last Glacial Maximum did not all operate synchronously, and exerted different drawdown power at different times. Finally, we conclude that Ross Island acts as an important pinning point in the Ross Sea ice-sheet-shelf system, stabilizing grounding line retreat and encouraging lasting ice-shelf development.

Sea ice growth rates near ice shelves

Abstract Sea ice growth rates near ice shelves are influenced by ocean-ice shelf interactions. Sea ice growth rates and ocean observations from McMurdo Sound, Antarctica in 1999 and 2000 are presented in this paper. Growth rate measurements were made for an individual platelet crystal through video camera observations. It was found that the crystal grew in discontinuous, episodic bursts at rates of the order of 10 − 6 m s − 1 . Sea water 0.15 m beneath the lower ice surface was measured to be supercooled by 0.01 K. Indications are that supercooling was continuous over the period of episodic platelet ice crystal growth and the growth bursts are attributed to the influence of variable currents. Growth rates for bulk sea ice (i.e., columnar and incorporated platelet ice) and heat fluxes were derived from ice temperature measurements. The growth rates for bulk sea ice were found to be of the order of 10 − 7 m s − 1 , an order of magnitude less than the rates for the individual platelet ice crystal. The residual of the energy balance suggested that a negative oceanic heat flux (i.e., heat transport down into the ocean) occurred, in addition to conduction of heat up into the atmosphere. Both salinity-based growth rate models and an oxygen isotope-based growth rate model (Eicken, 1998) were found to under-predict growth rates compared to those derived from ice temperature measurements. In addition, inverting the growth rates predicted by the models and integrating over the depth of the core failed to accurately predict the date of initial sea ice formation. Modifications are proposed to the models for sea ice formation occurring near ice shelves, where platelet ice formation is likely. Differences between bulk and individual platelet ice crystal growth rates are discussed with reference to heat fluxes, oceanic flows and the Eicken (1998) model.

Decadal trends in abundance, size and condition of Antarctic toothfish in McMurdo Sound, Antarctica, 1972–2011

AbstractWe report the analyses of a dataset spanning 39 years of near‐annual fishing for Dissostichus mawsoni in McMurdo Sound, Antarctica, 1972–2011. Data on total length, condition and catch per unit effort (CPUE) were derived from the > 5500 fish caught, the large majority of which were measured, tagged and released. Contrary to expectation, the length frequency of the McMurdo Sound catch was dominated by fish in the upper two‐thirds of the overall distribution exhibited in the industrial catch for the Ross Sea shelf. Fish length and condition increased from the early 1970s to the early 1990s and then decreased. Fish length positively correlated with Ross Sea ice extent in early spring, a relationship possibly caused by more ice encouraging larger fish to move farther south over the shelf and into the study area. Fish condition positively correlated with the amount of open water in the Ross Sea during the previous summer (Feb), perhaps reflecting greater availability of prey with the higher productivity that more open water brings. Decreasing fish size corresponds to the onset of the fishery, which targets the large individuals. CPUE was constant through 2001 and then decreased dramatically. We hypothesize that this decrease is related to the industrial fishery, which began in the 1996–97 austral summer, and concentrates effort over the ice‐free Ross Sea continental slope. As a result of limited prey choices and close coupling among mesopredators of the region, Antarctic toothfish included, the fishery appears to be dramatically altering the trophic structure of the Ross Sea.

Iceberg-induced changes to polynya operation and regional oceanography in the southern Ross Sea, Antarctica, from in situ observations

AbstractTwo massive tabular icebergs calved from the Ross Ice Shelf in 2000 (B-15) and 2002 (C-19) and perturbed regional ocean processes for several years. Here we document the ocean's response in McMurdo Sound to the icebergs using in situ data collected before, during and after the icebergs’ residence in the Ross Sea. Departures from typical McMurdo Sound seasonal oceanography included the non-appearance of Antarctic Surface Water in summer, a cooler and more homogeneous water column during winter and ‘super-fresh’ High Salinity Shelf Water that gradually recovered its salinity. We found that each iceberg triggered a distinct response to regional ocean processes. B-15a, the largest piece of iceberg B-15, restricted surface circulation, cooled and freshened the upper water column and reduced melting near the ice shelf front for four years. Iceberg C-19 interrupted the operation of the Ross Sea polynya, from which McMurdo Sound took three to four years to recover, and was responsible for a geographic shift in the dense water formation region for the south-western Ross Sea. These results differ from earlier modelling studies and highlight the challenges of modelling the polar ocean. We also show that one pathway previously thought to supply dense water to the Ross Ice Shelf cavity was not operating at that time.

Development of a precise and accurate age–depth model based on 40Ar/39Ar dating of volcanic material in the ANDRILL (1B) drill core, Southern McMurdo Sound, Antarctica

High precision 40Ar/39Ar dates on a variety of volcanic materials from the AND-1B drill core provide pinning points for defining the chronostratigraphy for the core. The volcanic materials dated include 1) felsic and basaltic tephra, 2) interior of a ~3m thick intermediate submarine lava flow, and 3) felsic and basaltic volcanic clasts. In the upper 600m of the core, two felsic tephra, two basaltic tephra and the intermediate laval flow yield precise and depositional ages, with further maximum age constraints from volcanic clasts. Below 600m in the core, tephric intervals are significantly altered and maximum age constraints only are available from volcanic clasts. The ages for eight stratigraphic intervals are 1) 17.17–17.18mbsf, basaltic clast (maximum depositional age 0.310±0.039Ma, all errors quoted at 2σ), 2) 52.80–52.82mbsf, three basaltic clasts (maximum depositional age 0.726±0.052Ma), 3) 85.27–85.87mbsf felsic tephra (1.014±0.008Ma), 4) ~112–145mbsf sequence of basaltic tephra (1.633±0.057 to 1.683±0.055Ma), 5) 480.97–481.96mbsf pumice-rich mudstone (4.800±0.076Ma), 6) 646.30–649.34mbsf intermediate lava flow (6.48±0.13Ma), 7) 822.78mbsf kaersutite phenocrysts from volcanic clasts (maximum depositional age 8.53±0.53Ma) and 8) ~1280mbsf, three volcanic clasts (maximum depositional age 13.57±0.13Ma). Minimum average sediment accumulation rates or 102 and 87m/Ma for the upper and lower 650m of core, respectively were calculated using the 40Ar/39Ar analyses. The volcanic material recovered from AND-1B also reveals a general northward progression of volcanism in Southern McMurdo Sound.

Spatial and temporal variation in the heat tolerance limits of two abundant Southern Ocean invertebrates

While, in lower latitudes, population-level differences in heat tolerance are linked to temperature variability, in the Southern Ocean remarkably stable year-round temperatures prevail. Temporal variation in the physiology of Antarctic ectotherms is therefore thought to be driven by the intense seasonality in primary productivity. Here we tested for differences in the acute upper temperature limits (lethal and activity) of 2 Antarctic marine invertebrates (the omnivorous starfish Odontaster validus and the filter-feeding clam Laternula elliptica) across latitude, seasons and years. Acute thermal responses in the starfish (righting and feeding) and clam (burrowing) differed between populations collected at 77°S (McMurdo Sound) and 67°S (Marguerite Bay). Both species displayed significantly higher temperature performance at 67°S, where seawater can reach a maximum of +1.8°C in summer versus ?0.5°C at 77°S, showing that even the narrow spatial and temporal variation in environmental temperature in Antarctica is biologically meaningful to these stenothermal invertebrates. Temporal comparisons of heat tolerance also demonstrated seasonal differences in acute upper limits for survival that were consistent with physiological acclimatisation: lethal limits were lower in winter than summer and higher in warm years than cool years. However, clams had greater inter-annual variation of temperature limits than was observed for starfish, suggesting that variation in food availability is also an important factor, particularly for primary consumers. Teasing out the interaction of multiple factors on thermal tolerance will be important for refining species-specific predictions of climate change impacts.

Geometric properties of platelet ice crystals

Abstract Sea ice that forms in McMurdo Sound, Antarctica, which is adjacent to the McMurdo and Ross Ice Shelves, exhibits the uncommon ice fabric, incorporated platelet ice. This ice fabric is the subject of a high resolution thin sectioning study, consisting of ten closely spaced horizontal sections bounded above and below by vertical sections. From these we investigate the crystallographic and geometric properties of the ice grains and find a relationship between c-axis orientation and the minor axis of an ellipse fitted to a crystal's grain boundary. Although accuracy is dependent upon the size and shape of the crystal, the technique provides a means of extracting crystallographic information from historical datasets for which only thin section photographs remain. Identification of common grains in multiple, 2-D sections allows us to reconstruct an approximation of the original 3-D crystal structures. These structures exhibit a variety of growth interactions, including upward directed crystal growth and intertwining crystals. We also construct hypothetical crystal grain boundaries that might be observed for a variety of vertical thin section orientations. These indicate that grain shape can depend on thin section orientation and illustrates some of the limitations of 2-D representations of 3-D crystals. Serrated grain boundaries are identified between neighboring crystals in several vertical sections. Geometric arguments and crystallographic observations lead us to propose a mechanism of layer nucleation within grain boundary grooves that occurs as supercooling exceeds a threshold value.

Sea ice salinity and structure: A winter time series of salinity and its distribution

We present a winter time series of Antarctic sea ice salinity from eastern McMurdo Sound, an area close to an ice shelf where a subice platelet layer forms below the sea ice late in winter. This dramatically changes the sea ice structure as the sea ice grows into the subice platelet layer. Every 2 weeks during the 5 months of sea ice formation, salinity profiles were measured, along with detailed measurements of ice structure and growth rates. Once the influence of growth rate on sea ice bulk salinity is removed, the data from 69 cores and the results of a basic parameterization demonstrate that bulk salinity for platelet ice is higher than that for columnar sea ice. We also present measurements of the salinity profile close to the ice‐water interface and use these to investigate the expected regime of fluid flow within the permeable portions of the sea ice, with particular reference to mushy layer and percolation theory. Finally, we provide a new distribution of sea ice salinity from 740 measurements, which can be interpreted as the sum of two spatial fields that we attribute to sea ice samples with and without brine channels and which should be reproduced by any realistic sea ice models. This distribution suggests that two measurements of quantities linearly linked to sea ice salinity must differ by 29% if they are to be considered different with 90% confidence.

Reprint of: Late Neogene climate and glacial history of the Southern Victoria Land coast from integrated drill core, seismic and outcrop data

Late Neogene stratigraphy of southern Victoria Land Basin is revealed in coastal and offshore drill cores and a network of seismic data in McMurdo Sound, Antarctica. These data preserve a record of ice sheet response to global climate variability and progressive cooling through the past 5millionyears. Application of a composite standard age model for diatom event stratigraphy to the McMurdo Sound drill cores provides an internally precise mechanism to correlate stratigraphic data and derive an event history for the basin. These marine records are indirectly compared to data obtained from geological outcrop in the Transantarctic Mountains to produce an integrated history of Antarctic Ice Sheet response to climate variability from the early Pliocene to Recent. Four distinct chronostratigraphic intervals reflect stages and steps in a transition from a relatively warm early Pliocene Antarctic coastal climate to modern cold polar conditions. Several of these stages and steps correlate with global events identified via geochemical proxy data recovered from deep ocean cores in mid to low latitudes. These correlations allow us to consider linkages between the high southern latitudes and tropical regions and establish a temporal framework to examine leads and lags in the climate system through the late Neogene and Quaternary. The relative influence of climate–tectonic feedbacks is discussed in light of glacial erosion and isostatic rebound that also influence the history along the Southern Victoria Land coastal margin.

Reprint of: Flexural controls on late Neogene basin evolution in southern McMurdo Sound, Antarctica

The basins of southern McMurdo Sound have evolved under the influence of lithospheric flexure induced by the loads of the Erebus Volcanic Province. To characterise these basins, it is important to investigate the lithosphere's flexural properties, and estimate their influence on basin architecture and evolution. Seismic and gravity data are used to constrain 3D forward modelling of the progressive development of accommodation space within the flexural basins. Elastic plate flexure was calculated for a range of effective elastic thicknesses (Te) from 0.5 to 25km using a spectral method. Models with low, but nonzero, Te values (2km<Te<5km) produce the best fit to the gravity data, although uncertainty is high due to inaccuracies in the Digital Elevation Model. The slopes of flexural horizons revealed in seismic reflection lines are consistent with this, indicating a Te of 2km to 5km, although the depths to these horizons are not consistent, perhaps due to a northwards slope, or step, in the pre-flexural surface. These results indicate that the lithospheric strength of southern McMurdo Sound is significantly less than estimates of the regional average (Te ~20km). This low strength may reflect the weakening effects of the Terror Rift, and perhaps also the Discovery Accommodation Zone, a region of major transverse faulting. A low Te model (Te=3) for southern McMurdo Sound predicts the development of two discrete flexural depressions, each 2–2.5km deep. The predicted stratigraphy of the northern basin reflects flexure due to Ross Island, predominantly erupted since ca. 1.8Ma. The predicted stratigraphy of the southern basin reflects more gradual flexure from ca. 10Ma to ca. 2Ma, due to the more dispersed volcanoes of the Discovery subprovince. Collectively, these two basins have the potential to preserve a remarkable stratigraphic record of Antarctic climate change through the late Neogene.

Cyanobacterial diversity across landscape units in a polar desert: Taylor Valley, Antarctica

Life in the Taylor Valley, Antarctica, is dominated by microorganisms, with cyanobacteria being key primary producers in the region. Despite their abundance and ecological importance, the factors controlling biogeography, diversity, dispersal of cyanobacteria in Taylor Valley and other polar environments are poorly understood. Owing to persistent high winds, we hypothesize that the cyanobacterial diversity across this polar landscape is influenced by aeolian processes. Using molecular and pigment analysis, we describe the cyanobacterial diversity present in several prominent habitats across the Taylor Valley. Our data show that the diversity of cyanobacteria increases from the upper portion of the valley towards the McMurdo Sound. This trend is likely due to the net transport of organisms in a down-valley direction, consistent with the prevailing orientation of high-energy, episodic föhn winds. Genomic analysis of cyanobacteria present in aeolian material also suggests that wind mixes the cyanobacterial phylotypes among the landscape units. Our 16S rRNA gene sequence data revealed that (1) many of the cyanobacterial phylotypes present in our study site are common in polar or alpine environments, (2) many operational taxonomic units (OTUs) (22) were endemic to Antarctica and (3) four OTUs were potentially endemic to the McMurdo Dry Valleys.

Phenotypic plasticity in the Antarctic nototheniid fish Trematomus newnesi: a guide to the identification of typical, large mouth and intermediate morphs

Trematomus newnesi is a common inshore species with a circum-Antarctic distribution. It provides the only known example of phenotypic plasticity in Antarctic notothenioid fish, existing as populations of typical, large mouth and intermediate morphs that can be difficult to identify. Using specimens from both Potter Cove, King George/25 de Mayo Island, and from McMurdo Sound, we found that the morphometric measurements gape width/head length (HL), upper jaw length/HL and, to a lesser extent, orbit diameter/HL reliably separated the morphs. For use in a key, we converted the ratios into the qualitative characters head shape, head width and upper jaw length relative to middle of the eye. To increase the reliability of the key, we also assessed intra-morph variability in these characters. The key is supplemented with colour photographs illustrating the distinctive features for separation of the morphs. We discovered that, in the case of the specimens from Potter Cove, each morph had a distinct pattern of colouration: typical—trunk blotched, with yellow or orange-brown predominating especially on pectoral and caudal fins; large mouth—trunk blotched, with green predominating especially in pectoral and opercular regions; and intermediate—trunk less blotched, with homogeneous dark brown-grey on trunk, pectoral and caudal fins. We also discuss the ecological implications of colour in the morphs.

Fragilariopsis diatom evolution in Pliocene and Pleistocene Antarctic shelf sediments

The late Pliocene – early Pleistocene sediment record in the AND-1B core from the McMurdo Sound, Ross Sea, Antarctica, displays a rich diversity and high abundance of diatoms, including several new morphologies within the genus Fragilariopsis. These new morphologies exhibit similarities to the extinct late Miocene/early Pliocene species Fragilariopsis aurica Gersonde and Fragilariopsis praecurta Gersonde, as well as to the modern sea ice-associated species Fragilariopsis ritscheri Hustedt and Fragilariopsis obliquecostata van Heurck. From the diverse morphologies present, we use light microscopy and scanning electron microscopy to identify and describe the characteristics of three new taxa, Fragilariopsis laqueata Riesselman, Fragilariopsis bohatyi Sjunneskog et Riesselman, and Fragilariopsis robusta Sjunneskog, which are common in the diatom-bearing intervals from ~3.2 to 1.95 Ma. Comparisons with extant and extinct species are made to assess possible environmental affinities, evolutionary relationships, and potential for future biostratigraphic utility. This complex of newmorphologies diversified as conditions cooled during the Pliocene, then went into decline as heavy sea ice conditions of the Pleistocene were established. Only the lineage of F. robusta appears to continue into the late Pleistocene, where it is interpreted to have evolved into F. obliquecostata.

Reservoir potential of sands formed in glaciomarine environments: an analogue study based on Cenozoic examples from McMurdo Sound, Antarctica

Abstract This paper provides documentation of unexpectedly high-reservoir-quality glaciomarine sands found in the Cenozoic succession beneath McMurdo Sound, Antarctica, as an analogue study for evaluations of hydrocarbon prospectivity in basins elsewhere. The Oligocene to Lower Miocene succession of the Victoria Land Basin, an extant portion of the West Antarctic Rift System, comprises diamictites, mudrocks and sandstones with minor conglomerates. These lithologies are arranged in repetitive stacking patterns (cycles), interpreted to record repeated advance and retreat of glaciers into and out of the basin, with attendant eustatic and isostatic effects. Phases of ice retreat within the cycles comprise an array of mudrocks, sandy mudrocks and sandstones, deposited mainly during relative sea-level highstands. Clean, well-sorted, unconsolidated and porous sands &lt;25 m thick from such intervals, which are interpreted to be mainly deltaic in origin, were encountered. Some of these sands, which have visible porosity as high as 41%, flowed into the well bore together with significant volumes of cold formation water. Diagenetic modification of sands in these intervals is minimal, which can be attributed to the low-temperature nature of the subsurface environment. Accordingly, glaciomarine sands in near-field glaciogenic successions should be considered as potential reservoir facies in prospectivity assessments.

Neogene benthic foraminiferal assemblages and paleoenvironmental record for McMurdo Sound, Antarctica

The Antarctic Geological Drilling (ANDRILL) program recovered a 1,138-m-long core, AND-2A, during the austral spring 2007–2008 from the Victoria Land Basin for the Southern McMurdo Sound (SMS) Project. The main objective of the SMS Project was to establish a Neogene (ca. 24–2.5 Ma) history recording ice volume changes from an ice-proximal site, in particular the environmental response of the Antarctic margin during the middle Miocene climate transition. This study focuses on the foraminifera taxa recovered from AND-2A, providing a paleoenvironmental analysis including paleobathymetry and ice-proximity estimates. Three assemblages, Cibicides-Cassidulinoides , Ammoelphidiella , and Ehrenbergina , as well as two subassemblages within the Ammoelphidiella assemblage, Globocassidulina and Nonionella , were identified in this study using cluster analysis. Calcareous eurytopic taxa dominate every assemblage and display similarities between Fresh Shelf Water assemblages and Ice Edge Biofacies that were previously defined in other studies focused in the Southern Ocean, as well as assemblages recovered from previous drilling projects in the Ross Sea region. Paleoenvironmental interpretations include an early Miocene record of fluctuating periods of glacial influence from ice-distal, abrupt cooling followed by a continued warming and transition into a more ice-proximal setting in the uppermost middle Miocene. The well-preserved middle Miocene record reflects more productive stable environmental conditions with significant freshwater input from outlet glaciers and rivers at the East Antarctic margin, and coincides with previously published palynomorph data indicating a short abrupt warming. Furthermore, the persistent appearance of the planktonic species Neogloboquadrina pachyderma and glacial/sea ice assemblages can be related to the onset of progressive cooling at the Antarctic margin, including periods of fluctuating sea ice extent or glacial proximity. Finally, the Pliocene record contains taxa similar to that of Fresh Shelf Water assemblages and Ice Edge Biofacies recovered in the Southern Ocean today. Paleobathymetric estimates range from inner shelf depths (∼50–150 m) in the early Miocene reaching periods of outer-shelf depths (∼400 m) in the uppermost early–middle Miocene and Pliocene.

Early Miocene volcanic activity and paleoenvironment conditions recorded in tephra layers of the AND-2A core (southern McMurdo Sound, Antarctica)

The ANtarctic geological DRILLing Program (ANDRILL) successfully recovered 1138.54 m of core from drill hole AND-2A in the Ross Sea sediments (Antarctica). The core is composed of terrigenous claystones, siltstones, sandstones, conglomerates, breccias, and diamictites with abundant volcanic material. We present sedimentological, morphoscopic, petrographic, and geochemical data on pyroclasts recovered from core AND-2A that provide insights on eruption styles, volcanic sources, and environments of deposition. One pyroclastic fall deposit, 12 resedimented volcaniclastic deposits and 14 volcanogenic sedimentary deposits record a history of intense explosive volcanic activity in southern Victoria Land during the Early Miocene. Tephra were ejected during Subplinian and Plinian eruptions fed by trachytic to rhyolitic magmas and during Strombolian to Hawaiian eruptions fed by basaltic to mugearitic magmas in submarine and/or subglacial to subaerial environments. The long-lived Mount Morning eruptive center, located ∼80 km south of the drill site, was recognized as the probable volcanic source for these products on the basis of volcanological, geochemical, and age constraints. The study of tephra in the AND-2A core provides important paleoenvironment information by revealing that the deposition of primary and moderately reworked tephra occurred in a proglacial setting under generally open-water marine conditions.

Advances in on-sea-ice seismic reflection methods using an air gun: McMurdo Sound, Antarctica

During the austral spring of 2008, 48 km of multichannel seismic reflection data were collected on a sea-ice platform east of New Harbor, Southern McMurdo Sound, Antarctica. These seismic data were collected to support the Antarctic Geological Drilling Program whose goal is to recover sedimentary rock cores from the continental margin of Antarctica and to better understand the climatic, cryospheric, and tectonic history of Antarctica. A Generator-Injector air gun, deployed through holes drilled in the sea ice, was used for a seismic source, and reflected energy was recorded on a snow streamer consisting of 60 gimbaled geophones. The Offshore New Harbor seismic system has increased the rate of data acquisition by 53% compared to previous Antarctic air-gun/snow-streamer seismic systems. In addition to conventional on-sea-ice seismic data processing, new processing steps were used to correct problems associated with the use of a Generator-Injector air gun in a sea-ice environment. Source timing errors, caused by miscommunications between the Generator-Injector air gun and air-gun controller, were corrected. Newly discovered bubble-plume static errors and bubble-plume wavelet distortions degraded seismic data quality and are attributed to bubble plumes created by the Generator-Injector air-gun blast. Our investigation shows that these bubble plumes were preserved for at least 40 hours beneath the sea ice and created isolated low-velocity zones. Using finite-element methods, we reproduced static errors and wavelet distortions by incorporating the effects of bubble plumes into the finite-element model. The results of finite-element modeling confirmed our hypothesis that bubble-plume static errors and bubble-plume wavelet distortions were caused by near-surface low-velocity features. Therefore, we incorporated new processing steps which remove bubble-plume effects and improve the quality of final seismic sections.

Aeolian flux of biotic and abiotic material in Taylor Valley, Antarctica

We studied patterns and mechanisms controlling wind-driven flux of soils and associated organic matter in Taylor Valley, Antarctica over a 10-year period using passive aeolian traps and dynamic mass erosion particle counters. Deployment of the particle counters near meteorological stations allowed us to compare the magnitude of soil flux with data on prevailing wind. Particulate organic C, N and P measurements on transported sediment allowed us to examine connectivity of wind dispersed organic matter among landscape units.Most sediment entrainment occurred within 20cm of the soil surface during “saltation bursts” that occupied <3% of the total time within a year. These bursts corresponded to periods of strong föhn winds where wind velocities were≥20ms−1. Sediment movement was highest in the up-valley reaches of Taylor Valley and transport was down-valley towards McMurdo Sound. The general paucity of biological organic matter production throughout the McMurdo Dry Valleys, in concert with low fluvial transport, makes aeolian distribution or organic C, N and P an important factor in the distribution of organic matter throughout this polar desert ecosystem and increases connectivity among the ecosystem components.