McMurdo Sound Research Articles

Glacial landform assemblage reveals complex retreat of grounded ice in the Ross Sea, Antarctica

The Ross Sea Embayment is the largest drainage basin of the Antarctic Ice Sheet. Multiple ice streams sourced from both the East Antarctic and West Antarctic ice sheets merged in the Ross Sea during the Last Glacial Maximum (LGM). The continental shelf record is important for understanding processes that influence retreat, constraining deglacial patterns and assessing Antarctica's contribution to eustatic sea-level rise since the LGM. Newly acquired, high-resolution multibeam bathymetry provides a detailed record of the complex deglacial history of a sector in the western Ross Sea (Fig. 1a). Retreat of grounded ice is marked by small, ice-marginal retreat landforms that were poorly resolved prior to the use of an upgraded multibeam system. Dynamic grounding zone behaviour is influenced by physiography and the presence of subglacial meltwater. Fig. 1. ( a ) Multibeam survey south of Crary Bank between Franklin Island and the Central Basin, western Ross Sea shelf (basemap modified by L. Prothro from GeoMapApp). Acquisition system Kongsberg EM122. Frequency 12 kHz. Grid-cell size 20 m. ( b ) Ross Sea showing the location of the survey area ((a) in red box; map from IBCSO v. 1.0). LAB, Little American Basin; GCB, Glomar-Challenger Basin; RB, Ross Bank; PT, Pennell Trough; PB, Pennell Bank; JT, JOIDES Trough; MB, Mawson Bank; CB, Central Bank; CrB Crary Bank; FrIS, Franklin Island; DT, Drygalski Ice Tongue; VLB, Victoria Land Basin; McM, McMurdo Sound. ( c ) Two sets of linear features interpreted as glacial lineations separated by an abrupt boundary (grey dashed line). Inset shows seismic profile across glacial lineations. Acquisition system Knudsen CHIRP. Frequency 3.5 kHz. ( d ) Asymmetrical ridges, interpreted as GZWs, deposited along the banks of a channel and on top of a volcanic landform with lineated topsets. ( e ) Profile of 4 m GZW deposited on volcanic landform showing underlying surface reflection. ( f ) Example of channel cross-sectional profile. …

Biogeochemistry and microbial diversity in the marine cavity beneath the McMurdo Ice Shelf, Antarctica

Ice shelves surround ∼ 75% of Antarctica's coastline and are highly sensitive to climate change; several have recently collapsed and others are predicted to in the near future. Marine waters beneath ice shelves harbor active ecosystems, while adjacent seas can be important areas of bottom water formation. Despite their oceanographic significance, logistical constraints have resulted in few opportunities to directly sample sub-ice shelf cavities. Here, we present the first data on microbial diversity and biogeochemistry beneath the McMurdo Ice Shelf (MIS) near Ross Island, Antarctica. Physicochemical profiles obtained via a 56 m deep borehole through the MIS revealed three vertically layered water masses (Antarctic Surface Water [AASW], Ice Shelf Water [ISW], and modified High Salinity Shelf Water [mHSSW]). Metabolically active, moderately diverse (Shannon diversity from 2.06 to 5.74) microbial communities were detected in the AASW and mHSSW. Heterotrophic bacterial production and dissolved organic matter concentrations were higher (12–37% and 24%, respectively) in mHSSW relative to AASW. Chemoautotrophic production was 5.3 nmol C L−1 d−1 and 6.0 nmol C L−1 d−1 in the AASW and mHSSW, respectively. Phytoplankton cells were more abundant and larger in the mHSSW sample relative to the AASW, which indicates sinking of phytoplankton produced in surface waters and, together with southerly flowing currents (0.09–0.16 m s−1), horizontal advection of phytoplankton from McMurdo Sound. Advected phytoplankton carbon together with in situ chemoautotrophic production provide important sources of organic matter and other reduced compounds to support ecosystem processes in the dark waters in the ice shelf cavity.

Antarctic marine ice-sheet retreat in the Ross Sea during the early Holocene

Geological constraints on the timing of retreat of the Last Glacial Maximum (LGM) Antarctic Ice Sheets provide critical insights into the processes controlling marine-based ice-sheet retreat. The over-deepened, landward-sloping bathymetry of Antarctica's continental shelves is an ideal configuration for marine ice-sheet instability, with the potential for past and future ice-sheet collapse and accelerated sea-level rise. However, the chronology of retreat of the LGM ice sheet in the Ross Sea is largely constrained by imprecise radiocarbon chronology of bulk marine sediments or by coastal records that offer more reliable dating techniques but which may be influenced by local piedmont glaciers derived from East Antarctic outlet glaciers. Consequently, these coastal records may be ambiguous in the broader context of retreat in the central regions of the Ross Sea. Here, we present a sedimentary facies succession and foraminifera-based radiocarbon chronology from within the Ross Sea embayment that indicates glacial retreat and open-marine conditions to the east of Ross Island before 8.6 cal. (calibrated) kyr B.P., at least 1 k.y. earlier than indicated by terrestrial records in McMurdo Sound. Comparing these data to new modeling experiments, we hypothesize that marine-based ice-sheet retreat was triggered by oceanic forcings along most of the Pacific Ocean coastline of Antarctica, but continued Holocene retreat into the inner shelf region of the Ross Sea occurred primarily as a consequence of bathymetric controls on marine ice-sheet instability.

Salt precipitation in sea ice and its effect on albedo, with application to Snowball Earth

AbstractDuring the initial freezing of the tropical ocean on Snowball Earth, the first ice to form would be sea ice, which contains salt within inclusions of liquid brine. At temperatures below −23°C, significant amounts of the salt begin to crystallize, with the most abundant salt being hydrohalite (NaCl·2H2O). These crystals scatter light, increasing the ice albedo. In this paper, we present field measurements of the albedo of cold sea ice and laboratory measurements of hydrohalite precipitation. Precipitation of salt within brine inclusions was observed on windswept bare ice of McMurdo Sound at the coast of Antarctica (78°S) in early austral spring. Salinity and temperature were measured in ice cores. Spectral albedo was measured on several occasions during September and October. The albedo showed a gradual increase with decreasing temperature, consistent with salt precipitation. Laboratory examination of thin sections from the ice cores showed that the precipitation process exhibits hysteresis, with hydrohalite precipitating over a range of temperatures between −28°C and −35°C but dissolving at about −23°C. The causes of the hysteresis were investigated in experiments on laboratory‐grown sea ice with different solute mixtures. All mixtures showed hysteresis, suggesting that it may be an inherent property of hydrohalite precipitation within brine inclusions rather than being due to biological macromolecules or interactions between various salts in seawater.

Have Antarctic toothfish returned to McMurdo Sound?

AbstractA dramatic reduction in catch rates of Antarctic toothfish in McMurdo Sound, Antarctica, has led to conclusions that the commercial bottom longline fishery for toothfish in the Ross Sea has drastically altered the toothfish population with cascading effects on the McMurdo Sound ecosystem. However, results from a new monitoring programme for Antarctic toothfish and other top predators carried out in McMurdo Sound in 2014 have shown toothfish catch rate, fish size and fish age similar to those observed prior to 2002. These results suggest that either large and old fish have returned to McMurdo Sound following a temporary environmentally driven absence or that they remained locally present but were not detected in the areas sampled. These findings highlight the importance of continued standardized monitoring for detecting the potential effects of fishing on the Ross Sea ecosystem.

Widespread intra-specific genetic homogeneity of coastal Antarctic benthic foraminifera

Abstract Benthic foraminifera are a major component of the Antarctic biota. Coastal foraminiferal morphospecies are widely distributed in Antarctic waters. The question is whether these morphotypes are genetically identical or, rather, they represent a cohort of cryptic species. Here, we compared genetically nine benthic foraminiferal morphospecies from Admiralty Bay (South Shetlands) and the western Ross Sea (McMurdo Sound, Terra Nova Bay), separated by a distance of ~4500 km. Additionally, for three of these morphospecies, we included specimens from Rothera (Marguerite Bay), which is located between the two main areas of interest. Our study, based on SSU and ITS rDNA sequence data, shows that all examined morphospecies share the same genotypes despite the presence of considerable intra-individual genetic variability.

Phytoplankton–bacterial interactions mediate micronutrient colimitation at the coastal Antarctic sea ice edge

Southern Ocean primary productivity plays a key role in global ocean biogeochemistry and climate. At the Southern Ocean sea ice edge in coastal McMurdo Sound, we observed simultaneous cobalamin and iron limitation of surface water phytoplankton communities in late Austral summer. Cobalamin is produced only by bacteria and archaea, suggesting phytoplankton-bacterial interactions must play a role in this limitation. To characterize these interactions and investigate the molecular basis of multiple nutrient limitation, we examined transitions in global gene expression over short time scales, induced by shifts in micronutrient availability. Diatoms, the dominant primary producers, exhibited transcriptional patterns indicative of co-occurring iron and cobalamin deprivation. The major contributor to cobalamin biosynthesis gene expression was a gammaproteobacterial population, Oceanospirillaceae ASP10-02a. This group also contributed significantly to metagenomic cobalamin biosynthesis gene abundance throughout Southern Ocean surface waters. Oceanospirillaceae ASP10-02a displayed elevated expression of organic matter acquisition and cell surface attachment-related genes, consistent with a mutualistic relationship in which they are dependent on phytoplankton growth to fuel cobalamin production. Separate bacterial groups, including Methylophaga, appeared to rely on phytoplankton for carbon and energy sources, but displayed gene expression patterns consistent with iron and cobalamin deprivation. This suggests they also compete with phytoplankton and are important cobalamin consumers. Expression patterns of siderophore- related genes offer evidence for bacterial influences on iron availability as well. The nature and degree of this episodic colimitation appear to be mediated by a series of phytoplankton-bacterial interactions in both positive and negative feedback loops.

Population Dynamics and Parasite Load of a Foraminifer on Its Antarctic Scallop Host with Their Carbonate Biomass Contributions.

We studied the population dynamics and parasite load of the foraminifer Cibicides antarcticus on its host the Antarctic scallop Adamussium colbecki from three localities differing by sea ice cover within western McMurdo Sound, Ross Sea, Antarctica: Explorers Cove, Bay of Sails and Herbertson Glacier. We also estimated CaCO3 biomass and annual production for both species. Cibicides populations varied by locality, valve type, and depth. Explorers Cove with multiannual sea ice had larger populations than the two annual sea ice localities, likely related to differences in nutrients. Populations were higher on Adamussium top valves, a surface that is elevated above the sediment. Depth did not affect Cibicides distributions except at Bay of Sails. Cibicides parasite load (the number of complete boreholes in Adamussium valves) varied by locality between 2% and 50%. For most localities the parasite load was < 20%, contrary to a previous report that ~50% of Cibicides were parasitic. The highest and lowest parasite load occurred at annual sea ice localities, suggesting that sea ice condition is not important. Rather, the number of adults that are parasitic could account for these differences. Cibicides bioerosion traces were categorized into four ontogenetic stages, ranging from newly attached recruits to parasitic adults. These traces provide an excellent proxy for population structure, revealing that Explorers Cove had a younger population than Bay of Sails. Both species are important producers of CaCO3. Cibicides CaCO3 biomass averaged 47-73 kg ha-1 and Adamussium averaged 4987-6806 kg ha-1 by locality. Annual production rates were much higher. Moreover, Cibicides represents 1.0-2.3% of the total host-parasite CaCO3 biomass. Despite living in the coldest waters on Earth, these species can contribute a substantial amount of CaCO3 to the Ross Sea and need to be incorporated into food webs, ecosystem models, and carbonate budgets for Antarctica.

Accumulation and marine forcing of ice dynamics in the western Ross Sea during the last deglaciation

Ice-shelf grounding lines off the coast of Antarctica have retreated over the past 20,000 years. Precise dating of moraines suggests the timing of retreat in the Ross Sea was controlled by the interplay between accumulation and ocean forcing. The grounding line of the ice sheet in the Ross Sea, Antarctica, retreated between the Last Glacial Maximum and the present. However, the timing of the retreat and the interplay of factors controlling ice stability in this region1 remain uncertain. Here we use 180 radiocarbon dates to reconstruct the chronology of moraine construction on the headlands adjacent to western McMurdo Sound. On the basis of these dates we then assess the timing of ice expansion and retreat in the Ross drainage system that is fed from both the East and West Antarctic ice sheets. We find that grounded ice in the western Ross Sea achieved its greatest thickness and extent during the last termination, between 12,800 and 18,700 years ago. Maximum ice thickness at our site coincides with a period of high accumulation as recorded by the West Antarctic Ice Sheet Divide ice core2. Recession of the ice sheet from the headland moraines began about 12,800 years ago, despite continued high accumulation and the expansion of land-based glaciers at this time. We therefore suggest that the grounding-line retreat reflects an increased marine influence as sea levels rose and the ocean warmed. We suggest that future instability in the ice sheet grounding line may occur whenever the ocean forcing is stronger than forcing from accumulation.

Scaling matters: incorporating body composition into Weddell seal seasonal oxygen store comparisons reveals maintenance of aerobic capacities

Adult Weddell seals (Leptonychotes weddellii) haul-out on the ice in October/November (austral spring) for the breeding season and reduce foraging activities for ~4 months until their molt in the austral fall (January/February). After these periods, animals are at their leanest and resume actively foraging for the austral winter. In mammals, decreased exercise and hypoxia exposure typically lead to decreased production of O2-carrying proteins and muscle wasting, while endurance training increases aerobic potential. To test whether similar effects were present in marine mammals, this study compared the physiology of 53 post-molt female Weddell seals in the austral fall to 47 pre-breeding females during the spring in McMurdo Sound, Antarctica. Once body mass and condition (lipid) were controlled for, there were no seasonal changes in total body oxygen (TBO2) stores. Within each season, hematocrit and hemoglobin values were negatively correlated with animal size, and larger animals had lower mass-specific TBO2 stores. But because larger seals had lower mass-specific metabolic rates, their calculated aerobic dive limit was similar to smaller seals. Indicators of muscular efficiency, myosin heavy chain composition, myoglobin concentrations, and aerobic enzyme activities (citrate synthase and β-hydroxyacyl CoA dehydrogenase) were likewise maintained across the year. The preservation of aerobic capacity is likely critical to foraging capabilities, so that following the molt Weddell seals can rapidly regain body mass at the start of winter foraging. In contrast, muscle lactate dehydrogenase activity, a marker of anaerobic metabolism, exhibited seasonal plasticity in this diving top predator and was lowest after the summer period of reduced activity.

Total Mercury in Six Antarctic Notothenioid Fishes.

We analyzed white muscle samples from six species of Antarctic fish (suborder Notothenioidei) collected in 2011 from McMurdo Sound, Ross Sea, Antarctica, to assess levels of total mercury (THg). Gymnodraco acuticeps and Trematomus bernacchii exhibited the highest concentrations of THg followed by Trematomus pennellii, Trematomus nicolai, Trematomus newnesi and Pagothenia borchgrevinki, (71.3, 53.9±32.1, 45.8±27.3, 37.2±18.6, 35.7±23.6, and 21.9±2.8 ng/g wet weight, respectively). The results from this study suggest that THg has the potential to bioaccumulate from various marine Antarctic ecosystems into biota.

Oxidative damage and antioxidant defence parameters in the Antarctic bivalve Laternula elliptica as biomarkers for pollution impacts

The Antarctic continent is increasingly vulnerable to anthropogenic pollution; however, assessments of the impacts that chemical pollutants have on cold-adapted marine organisms are limited. Oxidative stress (OS) occurs when the rate of reactive oxygen species generation exceeds the scavenging capacity of an organism’s antioxidant (AO) system and is an important unifying feature underlying the toxicity of many chemical contaminants in aquatic organisms. The Antarctic bivalve Laternula elliptica is a widely distributed, infaunal filter-feeding organism. We analysed AO enzyme activities, levels of the molecular antioxidant glutathione, protein carbonylation and lipid peroxidation as OS biomarkers in L. elliptica from contaminant-impacted sites near McMurdo Station and the relatively pristine Cape Evans. The objective was to evaluate the effectiveness of these biomarkers for detecting contaminant stress in this cold-adapted marine invertebrate. The concentrations of total polycyclic aromatic hydrocarbons were quantified as a proxy for contamination and found to be elevated in gonad and muscle tissues from L. elliptica dwelling in contaminated sites. These individuals exhibited a greater degree of OS than those from the reference site, evidenced by increases in oxidative lipid and protein damage, as well as an upregulation of AO defences. Coincidentally, L. elliptica from the contaminated sites were significantly smaller in shell length (1.3-fold) than those from the reference site. Oxidative biomarkers proved to be useful indicators of contamination exposure in the present study and were used to document ongoing biological impacts from historic and current pollution in McMurdo Sound.

Distribution of two species of the genus Nototanais spp. (Tanaidacea) in Winter Quarters Bay and waters adjoining McMurdo Station, McMurdo Sound, Antarctica

Nototanais is the most commonly found peracarid crustacean genus in Antarctic waters. The only two species in the genus, Nototanais dimorphus and Nototanais antarcticus, have been found to have overlapping circumpolar distributions in depths from 7 m to 585 m around Antarctica. However, only one species, N. dimorphus, has been recorded during yearly sampling from 2000 to 2010 at nine shallow (12–36 m) benthic pollution monitoring sites in Winter Quarters Bay and waters adjoining McMurdo Station, McMurdo Sound, Antarctica. These stations varied by depth, anchor ice formation and anthropogenic contamination. In 2011 and 2012, 1071 specimens of Nototanais spp. were collected and separated by species, gender and life stage. N. antarcticus was confirmed to co-occur with N. dimorphus at four of nine long-term monitoring sites. One station had no occurrences of either species. N. antarcticus was found in very low abundances in relation to N. dimorphus, indicating that individual species distributions may be modified by pollution and anchor ice, among other biological factors.

Diagenetic incorporation of Sr into aragonitic bivalve shells: implications for chronostratigraphic and palaeoenvironmental interpretations

AbstractAragonite is easily altered during diagenesis, therefore presumed pristine when present. In effect, beyond polymorphic transformation to calcite, alteration paths of aragonite remain poorly understood despite heavy reliance on such material to produce palaeoenvironmental and chronostratigraphic interpretations. Previous work on core material from Southern McMurdo Sound, Antarctica, showed that unlike their calcitic counterparts, seemingly unaltered aragonite shell fragments invariably produced older than expected 87Sr/86Sr ages. In this study, we pursued additional analyses of these aragonite shells and of the porewater of the core to understand this discrepancy. Aragonite mineralogy was reconfirmed and elemental mapping of shell fragments revealed growth lines within the middle layer suggestive of good preservation. The outer layer, however, showed anomalously high Sr concentrations (average 4·5 ± 0·6 mole% SrCO3; ca 25 mmol mol−1 Sr/Ca) and was depleted in 18O and 13C compared to the middle layer, both features inconsistent with pristine material. The δ18O values and Sr concentrations of the porewater were used to model outer layer compositions reasonably well. Coincidentally, porewater Sr isotope composition was in general agreement with the age model of the core only at the aragonite‐bearing interval suggesting that Sr‐isotopic disequilibrium between porewater and the carbonates was the rule rather than the exception in the core. The Sr isotope compositions of the aragonite shells are most likely the result of early diagenesis as suggested by the inconsistent O and C isotope compositions between shell layers and the anomalously high Sr concentrations. We conclude that knowledge of Sr concentration and distribution in shells is critical to determine the viability of Sr stratigraphy and the scale at which it may be applied. Reliance on traditional indicators of lack of alteration, such as cathodoluminescence, Mn‐Fe concentration, and the presence of labile mineralogies to assert chronostratigraphic and palaeoenvironmental questions may produce erroneous conclusions due to obscurely altered material.

Benthic microbial communities of coastal terrestrial and ice shelf Antarctic meltwater ponds.

The numerous perennial meltwater ponds distributed throughout Antarctica represent diverse and productive ecosystems central to the ecological functioning of the surrounding ultra oligotrophic environment. The dominant taxa in the pond benthic communities have been well described however, little is known regarding their regional dispersal and local drivers to community structure. The benthic microbial communities of 12 meltwater ponds in the McMurdo Sound of Antarctica were investigated to examine variation between pond microbial communities and their biogeography. Geochemically comparable but geomorphologically distinct ponds were selected from Bratina Island (ice shelf) and Miers Valley (terrestrial) (<40 km between study sites), and community structure within ponds was compared using DNA fingerprinting and pyrosequencing of 16S rRNA gene amplicons. More than 85% of total sequence reads were shared between pooled benthic communities at different locations (OTU0.05), which in combination with favorable prevailing winds suggests aeolian regional distribution. Consistent with previous findings Proteobacteria and Bacteroidetes were the dominant phyla representing over 50% of total sequences; however, a large number of other phyla (21) were also detected in this ecosystem. Although dominant Bacteria were ubiquitous between ponds, site and local selection resulted in heterogeneous community structures and with more than 45% of diversity being pond specific. Potassium was identified as the most significant contributing factor to the cosmopolitan community structure and aluminum to the location unique community based on a BEST analysis (Spearman's correlation coefficient of 0.632 and 0.806, respectively). These results indicate that the microbial communities in meltwater ponds are easily dispersed regionally and that the local geochemical environment drives the ponds community structure.

The genus Luticola D.G.Mann (Bacillariophyta) from the McMurdo Sound Region, Antarctica, with the description of four new species

A revision of the freshwater diatom genus Luticola from the McMurdo Sound Region, including the McMurdo Dry Valleys and Cape Royds, Antarctica, was made to contribute to a consistent flora for the entire Antarctic Region. Detailed light and scanning electron microscopic observations, review of pertinent literature, and examination of historical and type material lead to the identification of 12 Luticola species. Four new species and one new combination are proposed, including L. bradyi sp. nov., L. spainiae, sp. nov., L. macknightiae, sp. nov., L. transantarctica, sp. nov., and L. elegans, comb. nov. stat. nov. Several of these taxa were previously identified as part of the L. muticopsis (Van Heurck) D.G.Mann complex; Navicula muticopsis f. evoluta W. &amp; G.S. West, L. muticopsis f. reducta (W. &amp; G.S. West) Spaulding, and N. muticopsis f. capitata Carlson, or mistaken for the similar L. mutica (Kützing) D.G.Mann and L. cohnii (Hilse) D.G.Mann. Morphological features of all new species were compared to the closest morphologically similar taxa, and their ecology and biogeography are discussed. All Luticola species considered here show restricted Antarctic distributions, and 8 of the 12 reported species are known only from the Antarctic continent.

An apparent population decrease, or change in distribution, of Weddell seals along the Victoria Land coast

AbstractGround counts during 1959–1968 compared with counts using high resolution (0.6 m2) satellite imagery during 2008–2012 indicated many fewer Weddell seals (Leptonychotes weddellii) at two major molting areas in the western Ross Sea: Edisto Inlet‐Moubray Bay, northern Victoria Land, and McMurdo Sound, southern Victoria Land. Breeding seals have largely disappeared from Edisto‐Moubray, though the breeding population in McMurdo Sound appears to have recovered from harvest in the 1960s. The timing of decline, or perhaps spreading (lower numbers of seals in more places), is unknown but appears unrelated to changes in sea ice conditions. We analyzed both historic and satellite‐derived ice data confirming a large expansion of pack ice mostly offshore of the Ross Sea, and not over the continental shelf (main Weddell seal habitat), and a thinning of fast ice along Victoria Land (conceivably beneficial to seals). Timing of fast ice presence and extent in coves and bays along Victoria Land, remains the same. The reduction in numbers is consistent with an altered food web, the reasons for which are complex. In the context of a recent industrial fishery targeting a seal prey species, a large‐scale seal monitoring program is required to increase understanding of seal population changes.

Antarctica's Dry Valleys: A potential source of soluble iron to the Southern Ocean?

AbstractThe soluble iron content and dust emission potential of sediment samples collected from the Taylor Valley in the McMurdo Dry Valleys (MDVs) and sea ice in the McMurdo Sound were evaluated to determine whether inputs to the Southern Ocean may be sufficient to affect ocean productivity. Our results show that the dust‐generating potential from the MDVs soils are comparable to those of sediments from other major dust sources in the Southern Hemisphere. Sediments from the MDVs and sea ice are one order of magnitude richer in soluble iron than those in other dust sources in the Southern Hemisphere. Forward trajectory analyses show that the dust from the MDVs is likely to be deposited in the Southern Ocean. These results provide evidence of the possible supply of soluble iron to the Southern Ocean associated with dust transport from the MDVs, should climate change expand the exposed areas of the continent.

First-year land-fast Antarctic sea ice as an archive of ice shelf meltwater fluxes

Sampling beneath Antarctic ice shelves is sparse; therefore, tracking changes in ocean δ18O composition adjacent to ice shelves holds promise as an indicator of ice shelf basal melting. Sea ice archives of ice shelf–ocean interaction in particular could be important tools for future observational climate studies. Ocean δ18O values near the McMurdo Ice Shelf were reconstructed using observational data (sea ice δ18O, snow depth, and ice formation dates) from McMurdo Sound, Antarctica, by combining a recently revised version of an isotope fractionation model with an established thermodynamic sea ice model, resulting in improvements compared to previous approaches. Growth rates from the thermodynamic sea ice model were validated using direct growth rate measurements. That validation and supporting analysis indicated that a change was needed in ocean heat flux assumption from 0 W m−2 to around −13 W m−2 part way through the sea ice growth season. A well-constrained range (+1.84‰ to +2.21‰) of effective fractionation coefficients for sea ice was derived, along with a mean of 1.99‰. For the first time, reconstructed ocean δ18O values were validated using winter-long measurements of Antarctic near-surface water δ18O. Taking uncertainties into account, the reconstructed ocean δ18O values generally agreed to within ±0.2‰ with the measured ocean δ18O mean values. Results indicated an overall decrease in measured ocean δ18O during the winter, but this was not statistically significant given the uncertainties in the measurements. Although the method works, it currently has limited utility for determining the presence and scale of any step-changes in ocean δ18O composition associated with present day ice shelf basal melting. This is because the uncertainty of the reconstructed values (±0.2‰) is of the same magnitude as the expected change. Also, the requirement to parameterise the ocean heat flux is a barrier to the method being an entirely retrospective method (i.e., one requiring only data from the end of the sea ice growth season). In a future Antarctic scenario of increased basal melting of the ice shelves, the method may become more valuable in an Antarctic context. The method developed in this paper will be useful currently in the Arctic, because Arctic waters exhibit much larger fresh water fluxes.

Characterisation of magnetic minerals from southern Victoria Land, Antarctica

Magnetic properties of sedimentary strata recovered in Antarctic drill-cores from southern McMurdo Sound have been used to deduce climatic processes, but interpretation is limited by lack of information regarding the magnetic mineralogy of potential source rocks. Here we assess the magnetic mineralogy, concentration and grain size of magnetic remanence carriers in exposed rocks of southern Victoria Land and evaluate their potential contribution to Holocene–modern sediments. Modern sediments contain high concentrations of Ti-magnetite likely derived from the McMurdo Volcanic Group and minor concentrations of high-coercivity minerals, possibly derived from the Skelton Group and Beacon Supergroup. Significant quantities of superparamagnetic grains are present in all cover sediments, which were probably generated through glacial (grinding) processes or during sediment transport. We suggest that modern, hyper-arid conditions in southern Victoria Land prevent alteration of superparamagnetic grains and that they may be a useful proxy for the modern climate state.