McMurdo Ice Shelf Research Articles

Microbial population responses in three stratified Antarctic meltwater ponds during the autumn freeze

AbstractThe planktonic microbial communities of three meltwater ponds, located on the McMurdo Ice Shelf, were investigated from the end of January 2008 to early April, during which almost the entire pond volumes froze. The ponds were comprised of an upper mixed layer overlying a salt-stabilized density gradient in which planktonic communities were primarily embedded. Plankton comprised all components of the “microbial loop”, though carnivorous protists were rare. As the ponds froze and light became increasingly limited, it was expected conditions would induce physiological changes altering the functional role of autotrophic and heterotrophic microplankton within the ponds. The results showed that microbial groups responded to the onset of winter by declining in abundance, though an exception was the appearance of filamentous cyanobacteria in the water column in March. As freezing progressed, autotrophs declined more rapidly than heterotrophs and grazing rates and abundances of mixotrophic and heterotrophic organisms increased. Grazing pressure on bacteria and picophytoplankton also increased, in part explaining their decline over time. The results indicate that stressors imposed during freezing select for increasing heterotrophy within the remaining microbial communities, although all components of the food web eventually decline as the final freeze approaches.

The influence of siliciclastic input on Chaetoceros abundance in an early Pliocene segment of the ANDRILL AND-1B drill core

In the austral summer of 2006/7 the ANDRILL MIS (ANtarctic geological DRILLing- McMurdo Ice Shelf) project recovered a 1285m sediment core from beneath the Ross Ice Shelf near Ross Island, Antarctica. Contained within the upper ~600m of this core are sediments recording 38 glacial/interglacial cycles of Early Pliocene to Pleistocene time, including 13 discrete diatomite units (DU). The thickest of these, DU XI, is ~76m thick, has been assigned an Early Pliocene age (5–3Ma), and represents several extended periods of uninterrupted sediment accumulation. A combination of geochemical and biological data was used to examine the role of changes in sediment source areas on diatom community composition. Peak-area count-ratios from the data collected with the XRF core scanner include K:Fe and Ti:K. K is lower and Ti higher in the volcanic series sampled. K:Fe is an indicator of the source area of fine-grained sediment delivered to the site by terrestrial and marine sedimentary processes (glacial, fluvial, eolian, gravitational, marine currents), while Ti:K is primarily associated with Ross Sea volcanic sources. A comparison of these two elemental ratios shows an anti-correlation, indicating changes in source rocks with time. Furthermore, relative abundance of the diatom genus, Chaetoceros, varies in the same sense and magnitude as K:Fe, suggesting that climatically-driven changes in sediment source regions are related to variations in abundance. It is suggested that increases in TAM-sourced sediment at the ANDRILL AND-1B site caused increased nutrient levels in the water column, leading to blooms of this genus, which is consistent with current known ecological preferences.

Neogene tectonic and climatic evolution of the Western Ross Sea, Antarctica — Chronology of events from the AND-1B drill hole

Stratigraphic drilling from the McMurdo Ice Shelf in the 2006/2007 austral summer recovered a 1284.87m sedimentary succession from beneath the sea floor. Key age data for the core include magnetic polarity stratigraphy for the entire succession, diatom biostratigraphy for the upper 600m and 40Ar/39Ar ages for in-situ volcanic deposits as well as reworked volcanic clasts. A vertical seismic profile for the drill hole allows correlation between the drill hole and a regional seismic network and inference of age constraint by correlation with well‐dated regional volcanic events through direct recognition of interlayered volcanic deposits as well as by inference from flexural loading of pre‐existing strata. The combined age model implies relatively rapid (1m/2–5ky) accumulation of sediment punctuated by hiatuses, which account for approximately 50% of the record. Three of the longer hiatuses coincide with basin‐wide seismic reflectors and, along with two thick volcanic intervals, they subdivide the succession into seven chronostratigraphic intervals with characteristic facies:1.The base of the cored succession (1275–1220mbsf) comprises middle Miocene volcaniclastic sandstone dated at approx 13.5Ma by several reworked volcanic clasts;2.A late-Miocene sub-polar orbitally controlled glacial–interglacial succession (1220–760mbsf) bounded by two unconformities correlated with basin‐wide reflectors associated with early development of the terror rift;3.A late Miocene volcanigenic succession (760–596mbsf) terminating with a ~1my hiatus at 596.35mbsf which spans the Miocene–Pliocene boundary and is not recognised in regional seismic data;4.An early Pliocene obliquity-controlled alternating diamictite and diatomite glacial–interglacial succession (590–440mbsf), separated from;5.A late Pliocene obliquity-controlled alternating diamictite and diatomite glacial–interglacial succession (440–150mbsf) by a 750ky unconformity interpreted to represent a major sequence boundary at other locations;6.An early Pleistocene interbedded volcanic, diamictite and diatomite succession (150–80mbsf), and;7.A late Pleistocene glacigene succession (80–0mbsf) comprising diamictite dominated sedimentary cycles deposited in a polar environment.

Pleistocene variability of Antarctic Ice Sheet extent in the Ross Embayment

Cores acquired by the ANDRILL McMurdo Ice Shelf Project (AND-1B) provide the basis for a new sedimentation model for glacimarine depositional sequences that reflect cyclic glacial–interglacial fluctuations of a marine-based ice sheet in the western Ross Embayment over the past 2.0 Ma. Notwithstanding periodic erosion during advances of the ice sheet, uncertainties inherent to the sedimentological interpretation, and a limited number of chronological datums, it is clear that subglacial to grounding-zone sedimentation was dominant at the AND-1B site during the Late Pleistocene with interglacials being represented only by thin intervals of ice-shelf sediment. Each sequence is characterised by subglacial, massive diamictite that pass upwards into glacimarine diamictites and mudstones. This provides the first direct evidence that the marine-based Antarctic Ice Sheet has oscillated between a grounded and floating state at least 7 times in the Ross Embayment over the last 780ka, implying a Milankovitch orbital influence. An unconformity in AND-1B, that spans most (∼200 kyr) of the Mid-Pleistocene Transition is inferred to represent widespread expansion of a marine-based ice sheet in the Ross Embayment at 0.8 Ma. Prior to 1.0 Ma, interglacial periods are characterised by open-water conditions at the drill site with high abundances of volcanoclastic deposits and occasional diatomaceous sediments. These may have responded to precession (∼20-kyr) or obliquity (∼40-kyr) orbital control. The occurrence of 6.7 m of phonolitic glass reworked from Mt Erebus in interglacial deposits beneath Last Glacial Maximum till requires open ocean or ice shelf conditions in the western Ross Sea around the drill site within the past 250 ka and implies a Ross Ice Shelf similar to or less extensive than today during Marine Isotope Stage 7 or 5.

Lithostratigraphy from downhole logs in Hole AND-1B, Antarctica

The ANDRILL (Antarctic Drilling Project) McMurdo Ice Shelf (MIS) project drilled 1285 m of sediment in Hole AND-1B, representing the past 12 m.y. of glacial history. Downhole geophysical logs were acquired to a depth of 1018 mbsf (meters below seafloor), and are complementary to data acquired from the core. The natural gamma radiation (NGR) and magnetic susceptibility logs are particularly useful for understanding lithological and paleoenvironmental change at ANDRILL McMurdo Ice Shelf Hole AND-1B. NGR logs cover the entire interval from the seafloor to 1018 mbsf, and magnetic susceptibility and other logs covered the open hole intervals between 692 and 1018 and 237–342 mbsf. In the upper part of AND-1B, clear alternations between low and high NGR values distinguish between diatomite (lacking minerals containing naturally radioactive K, U, and Th) and diamictite (containing K-bearing clays, K-feldspar, mica, and heavy minerals). In the lower open hole logged section, NGR and magnetic susceptibility can also distinguish claystones (rich in K-bearing clay minerals, relatively low in magnetite) and diamictites (relatively high in magnetite). Sandstones can be distinguished by their high resistivity values in AND-1B. On the basis of these three downhole logs, diamictite, claystones, and sandstones can be predicted correctly for 74% of the 692–1018 mbsf interval. The logs were then used to predict facies for the 6% of this interval that was unrecovered by coring. Given the understanding of the physical property characteristics of different facies, it is also possible to identify subtle changes in lithology from the physical properties and help refine parts of the lithostratigraphy, for example, the varying terrigenous content of diatomites and the transitions from subice diamictite to open-water diatomite.

Formation of ice-shelf moraines by accretion of sea water and marine sediment at the southern margin of the McMurdo Ice Shelf, Antarctica

AbstractA combination of ground-penetrating radar surveys, physical sedimentology and ice composition measurements has been used to characterize ice and sediment accreted to the southern margin of the McMurdo Ice Shelf, Antarctica. The radar data and surface observations show that the ice-shelf margin consists of strongly layered debris-rich ice that contains marine sediment and fossils. A modified Rayleigh-based distillation system has been used to model the isotopic fractionation from sea water to ice in a closed system. The model of ice formation is consistent with formation during almost complete freezing of a sea-water reservoir. By contrast, ice on the upstream side of the grounding line has formed during the early stages of freezing in which a small fraction of the sea-water reservoir has frozen. The model results and the presence of delicate, well-preserved marine fossils are interpreted as evidence of anchor ice formation on the seabed, and rafting of glaciomarine sediment into the bottom of the ice shelf. We argue that repeated accretion of sea water and marine sediment has produced a stacked sequence of ice and glaciomarine debris that forms shore-parallel ice-cored moraines.

Fragilariopsis tigris sp. nov., a new late Pliocene Antarctic continental shelf diatom with biostratigraphic promise

A new species within the genus Fragilariopsis, F. tigris, is described and illustrated using light microscopy and scanning electron microscopy. This species is restricted to a single 8-meter-thick diatom unit within the 585-meter-long section of alternating diatomites and diamictites recovered in the upper portion of the ANtarctic geological DRILLing (ANDRILL) McMurdo Ice Shelf Project (MIS) AND-1B marine sediment core. This new taxon from a diverse, well-preserved diatom assemblage is inferred to be the youngest member of the well-documented, biostratigraphically useful F. praeinterfrigidaria – F. interfrigidaria – F. weaveri lineage and may represent a near-shore corollary to the open-ocean species F. weaveri. Based on available chronostratigraphic data from AND-1B, F. tigris appears to be restricted to the earliest late Pliocene (first occurrence datum ~3.2 Ma) and is extinct before 3.0 Ma.

Summer–winter transitions in Antarctic ponds: III. Chemical changes

AbstractObservations were made of water column chemistry in four Na-Cl dominated ponds on the McMurdo Ice Shelf from the end of January to early April in 2008. During that time the ponds went from ice-free to predominantly frozen, with only a small volume of residual hypoxic, saline liquid trapped at the base of each pond. Changes in the concentrations of inorganic solutes with time distinguished Na, Cl, Mg, K, SO4, As, U and Mn as ions and trace elements that behave mainly conservatively during freezing, from those which are affected by biological processes (removing HCO3) and the precipitation of mineral phases such as calcite (removing Ca and more HCO3). Dissolved Fe, Mo, Cu and Zn also show evidence of precipitation from the water column during freezing; geochemical speciation modelling predicts the formation of stable insoluble mineral phases such as Fe oxides and oxyhydroxides while conditions are oxic, and Fe-, Cu-, Mo- and Zn-sulphide minerals in the presence of H2S. Consequently, under winter conditions, residual liquid beneath the ice in such ponds is anticipated to be an anoxic Na-Cl brine with the capacity to develop high concentrations of toxic trace elements such as As and U.

Short Note: Antarctic toothfish heads found along tide cracks of the McMurdo Ice Shelf

Norman) heads along a 3km stretchof the 10–15km long tide crack that ran east–west in theMcMurdo Ice Shelf just north of Bratina Island (Fig. 1).From 77859.1'S, 165836.2'E, we walked along the crack to aseal breathing hole at 78800.1', 165831.3'. We walked thesame stretch on 7 December, finding also two pieces of fishskin, each 15–20cm long; and again in November 2010,when we found no new heads.There were 17 heads within a 16–25m

Snow accumulation and compaction derived from GPR data near Ross Island, Antarctica

Abstract. We present an improved method for estimating accumulation and compaction rates of dry snow in Antarctica with ground penetrating radar (GPR). Using an estimate of the emitted waveform from direct measurements, we apply deterministic deconvolution via the Fourier domain to GPR data with a nominal frequency of 500 MHz. This reveals unambiguous reflection horizons which can be observed in repeat measurements made one year apart. At two measurement sites near Scott Base, Antarctica, we extrapolate point measurements of average accumulation from snow pits and firn cores to a larger area by identifying a dateable dust layer horizon in the radargrams. Over an 800 m × 800 m area on the McMurdo Ice Shelf (77°45´ S, 167°17´ E) the average accumulation is found to be 269 ± 9 kg m−2 a−1. The accumulation over an area of 400 m × 400 m on Ross Island (77°40´ S, 167°11´ E, 350 m a.s.l.) is found to be higher (404 ± 22 kg m−2 a−1) and shows increased variability related to undulating terrain. Compaction of snow between 2 m and 13 m depth is estimated at both sites by tracking several internal reflection horizons along the radar profiles and calculating the average change in separation of horizon pairs from one year to the next. The derived compaction rates range from 7 cm m−1 at a depth of 2 m, down to no measurable compaction at 13 m depth, and are similar to published values from point measurements.

Evolution of supercooling under coastal Antarctic sea ice during winter

AbstractHere we describe the evolution through winter of a layer of in situ supercooled water beneath the sea ice at a site close to the McMurdo Ice Shelf. From early winter (May), the temperature of the upper water column was below its surface freezing point, implying contact with an ice shelf at depth. By late winter the supercooled layer was c. 40 m deep with a maximum supercooling of c. 25 mK located 1–2 m below the sea ice-water interface. Transitory in situ supercooling events were also observed, one lasting c. 17 hours and reaching a depth of 70 m. In spite of these very low temperatures the isotopic composition of the water was relatively heavy, suggesting little glacial melt. Further, the water's temperature-salinity signature indicates contributions to water mass properties from High Salinity Shelf Water produced in areas of high sea ice production to the north of McMurdo Sound. Our measurements imply the existence of a heat sink beneath the supercooled layer that extracts heat from the ocean to thicken and cool this layer and contributes to the thickness of the sea ice cover. This sink is linked to the circulation pattern of the McMurdo Sound.

Summer–winter transitions in Antarctic ponds I: The physical environment

AbstractMeltwater ponds are one of the most widespread aquatic habitats in ice-free areas of continental Antarctica. While most studies of such systems occur during the Antarctic summer, here we report on ice formation and water column attributes in four meltwater ponds on the McMurdo Ice Shelf during autumn, when they went from ice-free to > 80 cm thickness of ice. Ice thickness grew at an average rate of 1.5 cm d-1in all ponds and as ice formed, salts and gases were excluded. This resulted in conductivity rising from 3–5 to > 60 mS cm-1and contributed to the ebullition of gases. Incorporation of gas bubbles in the ice resulted in a high albedo and under-ice irradiance declined faster than incident, the former falling below 1 W m-2(daily average) by early April. After two months of ice formation, only 0–15% of the volume of each pond was still liquid, although this represented 5–35% of the pond sediment area, where much of the biological activity was concentrated. We suggest that the stresses that the freezing process imposes may be as important to structuring the biotic communities as those during the more benign summer growth period.

Eolian input into the Late Ordovician postglacial Soom Shale, South Africa

The Soom Shale Member of the Cedarberg Formation (South Africa) is a key early Paleozoic Lagerstatte that directly overlies glacigenic deposits of the Late Ordovician glacial maximum; it was deposited on the Gondwanan craton during deglacial transgression. We show that it contains a substantial coarse silt to fine sand component that occurs in discrete laminae intimately associated with plankton-derived organic material. We interpret this component as loess, sourced from glacially derived debris and blown into the sea either directly or across seasonal sea ice, as happens today on the McMurdo Ice Shelf, Antarctica. Falling through the water column, this material likely stimulated production of phytoplankton, which then sank as sediment-loaded aggregates. In marked sedimentary partitioning, these silt and/or organic laminae alternate with fine, organic-poor mud laminae that likely represent (river-derived?) nepheloid plumes and that sporadically thicken into centimeter-scale mud turbidites. Sustained eolian input directly into the surface water may have been key to maintaining the high productivity of the Soom sea and then, via eutrophication and anoxia, to the exceptional preservation of its biota.

Phylotype diversity in a benthic cyanobacterial mat community on King George Island, maritime Antarctica.

Cyanobacterial 16S ribosomal RNA gene diversity was examined in a benthic mat on Fildes Peninsula of King George Island (62º09'54.4''S, 58º57'20.9''W), maritime Antarctica. Environmental DNA was isolated from the mat, a clone library of PCR-amplified 16S rRNA gene fragments was prepared, and amplified ribosomal DNA restriction analysis (ARDRA) was done to assign clones to seven groups. Low cyanobacterial diversity in the mat was suggested in that 83% of the clones were represented by one ARDRA group. DNA sequences from this group had high similarity with 16S rRNA genes of Tychonema bourrellyi and T. bornetii isolates, whose geographic origins were southern Norway and Northern Ireland. Cyanobacterial morphotypes corresponding to Tychonema have not been reported in Antarctica, however, this morphotype was previously found at Ward Hunt Lake (83ºN), and in western Europe (52ºN). DNA sequences of three of the ARDRA groups had highest similarity with 16S rDNA sequences of the Tychonema group accounting for 9.4% of the clones. Sequences of the remaining three groups (7.6%) had highest similarity with 16S rRNA genes of uncultured cyanobacteria clones from benthic mats of Lake Fryxell and fresh meltwater on the McMurdo Ice Shelf.

Reconstructing Holocene conditions under the McMurdo Ice Shelf using Antarctic barnacle shells

This study evaluates the potential of barnacles for paleoceanographic reconstruction and, in particular, of the Antarctic species Bathylasma corolliforme to reconstruct past conditions under ice shelves. Like other barnacle species, this Antarctic barnacle secretes a robust low-Mg calcite shell with distinct growth increments on the external surface indicating growth over a number of years (30–50 in samples studied here). The Bathylasma samples used in this study grew in the Ross Sea and became entrained at the grounding line of a coastal ice shelf in McMurdo Sound, offering potential as an archive of changing conditions in this difficult to access environment under the McMurdo ice shelf. Nine barnacle shells were subsampled at high resolution (60 μm) for δ 18O and δ 13C analysis. These samples were dated with 14C and U–Th techniques, although the later did not yield useful age information due to open-system behaviour of barnacle calcite. Measured δ 18O values indicate that Bathylasma calcifies close to equilibrium with ambient seawater. One older sample (≈ 200 ka) has similar δ 18O and δ 13C values as the eight Holocene samples, suggesting that barnacle calcite is not prone to significant diagenesis in this setting. Apparent isotope equilibrium and lack of diagenesis make barnacles a promising archive for reconstruction of past ocean conditions. Cycles of δ 18O observed within each sample sometimes correlate with the external growth ridges, but not always, and have amplitudes of typically 0.6‰. This magnitude of variation suggests shell growth during a significant portion of the year, although it remains unclear whether growth continues throughout the year. However, the prominent growth bands suggest at least a signficant seasonal slowing in growth rate. Variations in barnacle δ 18O within each plate and between plates are too large to be caused by temperature, and instead reflect changes in δ 18O of the seawater, allowing some assessment of seawater salinity under the ice shelf. Salinities are lowered by addition of meltwaters, but the barnacle δ 18O data suggest values do not go below 33‰. These data indicate that Bathylasma has potential as an archive of paleoclimate information for shallow/intermediate water depths and, in particular, for difficult to reach environments such as below ice-shelves of Antarctica.

Heat Flow and Hydrologic Characteristics at the AND-1B borehole, ANDRILL McMurdo Ice Shelf Project, Antarctica

The Antarctic Drilling Program (ANDRILL) successfully drilled and cored a borehole, AND-1B, beneath the McMurdo Ice Shelf and into a fl exural moat basin that surrounds Ross Island. Total drilling depth reached 1285 m below seafl oor (mbsf) with 98 percent core recovery for the detailed study of glacier dynamics. With the goal of obtaining complementary information regarding heat fl ow and permeability, which is vital to understanding the nature of marine hydrogeologic systems, a succession of three temperature logs was recorded over a fi veday span to monitor the gradual thermal recovery toward equilibrium conditions. These data were extrapolated to true, undisturbed temperatures, and they defi ne a linear geothermal gradient of 76.7 K/km from the seafl oor to 647 mbsf. Bulk thermal conductivities of the sedimentary rocks were derived from empirical mixing models and density measurements performed on core, and an average value of 1.5 W/mK ± 10 percent was determined. The corresponding estimate of heat fl ow at this site is 115 mW/m 2 . This value is relatively high but is consistent with other elevated heat-fl ow data associated with the Erebus Volcanic Province. Information regarding the origin and frequency of pathways for subsurface fl uid fl ow is gleaned from drillers’ records, complementary geophysical logs, and core descriptions. Only two prominent permeable zones are identifi ed and these correspond to two markedly different features within the rift basin; one is a distinct lithostratigraphic subunit consisting of a thin lava fl ow and the other is a heavily fractured interval within a single thick subunit.

Geochemical provenance analysis of fine-grained sediment revealing Late Miocene to recent Paleo-Environmental changes in the Western Ross Sea, Antarctica

Bulk chemical fine-grained sediment compositions from southern Victoria Land glacimarine sediments provide significant constraints on the reconstruction of sediment provenance models in the McMurdo Sound during Late Cenozoic time. High-resolution (∼1ka) geochemical data were obtained with a non-destructive AVAATECH XRF Core Scanner (XRF-CS) on the 1285m long ANDRILL McMurdo Ice Shelf Project (MIS) sediment core AND-1B. This data set is complemented by high-precision chemical analyses (XRF and ICP-OES) on discrete samples.Statistical analyses reveal three geochemical facies which are interpreted to represent the following sources for the sediments recovered in the AND-1B core: 1) local McMurdo Volcanic Group (MVG) rocks, 2) Transantarctic Mountain rocks west of Ross Island (W TAM), and 3) Transantarctic Mountain rocks from more southerly areas (S TAM). Data indicate in combination with other sediment facies analyses (McKay et al., 2009) and provenance scenarios (Talarico and Sandroni, 2009) that diamictites at the drill site are largely dominated by local sources (MVG) and are interpreted to indicate cold polar conditions with dry-based ice. MVG is interpreted to indicate cold polar condition with dry-based ice. A mixture of MVG and W TAM is interpreted to represent polar conditions and the S TAM facies is interpreted to represent open-marine conditions.Down-core variations in geochemical facies in the AND-1B core are interpreted to represent five major paleoclimate phases over the past 14Ma. Cold polar conditions with major MVG influence occur below 1045mbsf and above 120mbsf. A section of warmer climate conditions with extensive peaks of S TAM influence characterizes the rest of the core, which is interrupted by a section from 525 to 855mbsf of alternating influences of MVG and W TAM.

Constant Holocene Southern-Ocean 14C reservoir ages and ice-shelf flow rates

Southern Ocean radiocarbon reservoir ages (i.e. non-zero radiocarbon ages in seawater) are the highest in the world's surface ocean. Constraining these reservoir ages at present and in the past is important not only because unknown reservoir ages limit the interpretation of Antarctic radiocarbon chronologies, but also because reservoir ages provide information about ocean circulation (as a recorder of past circulation and as an end member for major deep-water masses in today's ocean). In this study, we use paired U/Th and 14C ages of an unusual set of solitary coral samples trapped by fringing ice shelves in the Ross Sea to provide the first detailed study of Holocene reservoir ages for the Southern Ocean. Our results indicate a relatively constant marine radiocarbon reservoir age of 1144 ± 120 years for the past 6000 years. These results are consistent with extrapolation of the relationship between 14C and alkalinity seen elsewhere, supporting the use of this empirical relationship in high latitudes. The results also suggest constant deep-ocean circulation and air–sea exchange during the Holocene and provide a good target for tuning ocean models of modern circulation. Combining the new ages for corals with their distance from the modern-day ice-shelf grounding line provides some of the first long-term records of ice-shelf velocities for any region and indicates constant flow of the McMurdo Ice Shelf during the Holocene, at a rate similar to that observed today.

Pliocene–Pleistocene diatom biostratigraphy of nearshore Antarctica from the AND-1B drillcore, McMurdo Sound

The near-shore open-marine diatom record recovered in the ANtarctic geological DRILLing (ANDRILL) McMurdo Ice Shelf Project (MIS) AND-1B drillcore, McMurdo Sound, Antarctica, advances our understanding of the marine conditions present in the southern Ross Sea during the Pliocene and early Pleistocene. This diatom history is recorded within alternating diamictite and diatomite that reflect alternating glacial activity and high marine primary productivity. The diatomite units were deposited in a continental shelf open-marine setting during periods of reduced ice cover in West Antarctica. A new diatom biostratigraphic scheme spanning the last ca. 5Ma is proposed for the Antarctic near-shore area, based on prior work from high latitude drillcores. Four new zones are proposed for the Pliocene/Pleistocene, with eight in total for the new zonal scheme, utilizing Actinocylus fasciculatus, Actinocyclus maccollumii, Fragilariopsis bohatyii, Rouxia antarctica, and Thalassiosira fasciculata as new zonal markers. The early Pliocene shares the most assemblage commonality with that of the Southern Ocean with greater numbers of endemic species observed in the late Pliocene and early Pleistocene; a group of related Fragilaripsis species characterizes much of this later part of the time column. Two new species are proposed, Fragilariopsis tigris sp. nov. Riesselman and Thalassiosira teres sp. nov. Winter; a formal name is also proposed for another species, Rhizosolenia harwoodii sp. nov. Winter. The new zonation is tied to a robust chronology utilizing diatom biostratigraphy, volcanic 40Ar/39Ar ages and magnetostratigraphy.

Observations of flow and ice‐ocean interaction beneath the McMurdo Ice Shelf, Antarctica

The first oceanographic data from beneath the McMurdo Ice Shelf, Antarctica, are presented here with synchronous observations from southern McMurdo Sound. Multilevel current profiles at three sites over spring tide revealed primarily K1 diurnal tides with net transport from McMurdo Sound into the basin beneath the McMurdo Ice Shelf. The time series in southern McMurdo Sound captured the 14 day tidal phase shift and showed that transport into the subice shelf cavity was even greater over neap tide, with average transport ∼1.8 Sv. McMurdo Sound sea ice cover during this period was the heaviest in 25 years of satellite record and was associated with the proximity of two massive icebergs, B‐15a and C‐19. The warmest waters observed beneath the ice shelf were set at the surface freezing temperature, probably through recent interaction with this extensive sea ice cover. Observed ranges of temperature and salinity were narrowed to the extent that neither Antarctic Surface Water nor High Salinity Shelf Water were observed beneath the ice shelf. We anticipate that in years with more typical summer sea ice cover surface waters swept beneath the ice shelf will be significantly warmer, having even greater potential for basal melt near the ice shelf front. Our observations also exhibit some of the interactions particular to the subice shelf ocean environment. These include occurrence of platelet ice in ice shelf meltwater, boundary layer friction effects from the ice cover on the ocean, and tidal currents driving enhanced melting near the ice shelf front.