Phylogenetically and functionally diverse microorganisms reside under the Ross Ice Shelf

Throughout coastal Antarctica, ice shelves separate oceanic waters from sunlight by hundreds of meters of ice. Historical studies have detected activity of nitrifying microorganisms in oceanic cavities below permanent ice shelves. However, little is known about the microbial composition and pathways that mediate these activities. In this study, we profiled the microbial communities beneath the Ross Ice Shelf using a multi-omics approach. Overall, beneath-shelf microorganisms are of comparable abundance and diversity, though distinct composition, relative to those in the open meso- and bathypelagic ocean. Production of new organic carbon is likely driven by aerobic lithoautotrophic archaea and bacteria that can use ammonium, nitrite, and sulfur compounds as electron donors. Also enriched were aerobic organoheterotrophic bacteria capable of degrading complex organic carbon substrates, likely derived from in situ fixed carbon and potentially refractory organic matter laterally advected by the below-shelf waters. Altogether, these findings uncover a taxonomically distinct microbial community potentially adapted to a highly oligotrophic marine environment and suggest that ocean cavity waters are primarily chemosynthetically-driven systems.

Ground-penetrating radar profiles of the McMurdo shear zone, Antarctica, acquired with an unmanned rover : interpretation of crevasses, fractures, and folds within firn and marine ice

The crevassed firn of the McMurdo shear zone (SZ) within the Ross Ice Shelf may also contain crevasses deep within its meteoric and marine ice, but the surface crevassing prevents ordinary vehicle access to investigate its structure geophysically. We used a lightweight robotic vehicle to tow 200- and 40 MHz ground-penetrating radar antennas simultaneously along 10 parallel transects over a 28 km² grid spanning the SZ width. Transects were generally orthogonal to the ice flow. Total firn and meteoric ice thickness was approximately 160 m. Firn crevasses profiled at 400 MHz were up to 16 m wide, under snow bridges up to 10 m thick, and with strikes near 35°–40° to the transect direction. From the top down, 200- MHz profiles revealed firn diffractions originating to a depth of approximately 40 m, no discernible structure within the meteoric ice, a discontinuous transitional horizon, and at least 20 m of stratified marine ice; 28–31 m of freeboard found more marine ice exists. Based on 10 consecutive transects covering approximately 2.5 km², we preliminarily interpreted the transitional horizon to be a thin saline layer, and marine ice hyperbolic diffractions and reflections to be responses to localized fractures, and crevasses filled with unstratified marine ice, all at strikes from 27° to 50°. We preliminarily interpreted off nadir, marine ice horizons to be responses to linear and folded faults, similar to some in firn. The coinciding and synchronously folded areas of fractured firn and marine ice suggested that the visibly unstructured meteoric ice beneath our grid was also fractured, but either never crevassed, crevassed and sutured without marine ice inclusions, or that any ice containing crevasses might have eroded before marine ice accretion. We will test these interpretations with analysis of all transects and by extending our grid and increasing our depth ranges.

Remote and autonomous measurements of precipitation for the northwestern Ross Ice Shelf, Antarctica

The Antarctic Precipitation System project deployed and maintained four sites across the northwestern Ross Ice Shelf in Antarctica from November 2017 to November 2019. The goals for the project included the collection of in situ observations of precipitation in Antarctica spanning a duration of 2 years, an improvement in the understanding of precipitation events across the Ross Ice Shelf, and the ability to validate precipitation data from atmospheric numerical models. At each of the four sites the precipitation was measured with an OTT Pluvio2 precipitation gauge. Additionally, snow accumulation at the site was measured with a sonic ranging sensor and using GPS interferometric reflectivity. Supplemental observations of temperature, wind speed, particle count, particle size and speed, and images and video from a camera were collected to provide context to the precipitation measurements. The collected dataset represents some of the first year-round observations of precipitation in Antarctica at remote locations using an autonomous measurement system. The acquired observations have been quality-controlled and post-processed, and they are available for retrieval through the United States Antarctic Program Data Center (https://doi.org/10.15784/601441, Seefeldt, 2021).

Subglacial Conditions of the Kamb Ice Stream and its Response to Environmental Change

<p>The Siple Coast ice streams, which drain the West Antarctic Ice Sheet into the Ross Ice Shelf, are susceptible to temporal changes in flow dynamics. The Kamb Ice Stream on the Siple Coast, stagnated approximately 160 years ago, thought to partially be the result of basal water diversion. The character of its subglacial environment can exert an important control on long- and short-term ice sheet and ice stream fluctuations. Were the Kamb Ice Stream to reactivate in response to subglacial or future climate change, it would have the potential to contribute more substantially to ice discharge into the Ross Ice Shelf. Therefore, it is important to characterise the present-day subglacial environment and climatic conditions that may reactivate this flow. This study investigates the present-day subglacial conditions of the Kamb Ice Stream and how these conditions may be affected by environmental perturbations. Due to the difficult nature of making direct observations of ice sheet basal conditions, other methods are employed to investigate the response of the Kamb Ice Stream to environmental change. Active source seismic surveying data obtained during the 2015/16 and 2018/19 austral summer seasons provides an instantaneous snapshot of the present-day basal conditions. Flowline and whole-continent numerical ice sheet modelling is used to investigate the longer-term response of the Kamb Ice Stream and the West Antarctic Ice Sheet. Amplitude analysis of seismic lines indicate saturated till beneath the Ross Ice Shelf in the vicinity of the grounding zone, which is supported by retreat rates of the Kamb Ice Stream grounding zone post-stagnation. Seismic reflection imaging suggests potential dewatered till conditions beneath the grounded Kamb Ice Stream. Flowline modelling of the Kamb Ice Stream indicates that changes to the water content of the subglacial sediments appear to be self regulating, with high reversibility over centennial timescales. Oceanic temperature forcings are the key driver of change of the Kamb Ice Stream, and the ice stream is susceptible to topographic pinning points in 2D and lateral drag. Future glaciological change is more likely to occur in response to oceanic than to atmospheric temperature perturbations. Results from 3D continent-wide modelling experiments also find that precipitation increases offset the effect of air temperature perturbations and influence subglacial conditions, indicating more dynamic ice stream behaviour on the Siple Coast. This study has worked to re-enforce and strengthen our existing understanding of the Kamb Ice Stream and its sensitivity to environmental change. Future work using higher-resolution simulations and a higher density of observational data may help refine these results.</p>

Subglacial Conditions of the Kamb Ice Stream and its Response to Environmental Change

<p>The Siple Coast ice streams, which drain the West Antarctic Ice Sheet into the Ross Ice Shelf, are susceptible to temporal changes in flow dynamics. The Kamb Ice Stream on the Siple Coast, stagnated approximately 160 years ago, thought to partially be the result of basal water diversion. The character of its subglacial environment can exert an important control on long- and short-term ice sheet and ice stream fluctuations. Were the Kamb Ice Stream to reactivate in response to subglacial or future climate change, it would have the potential to contribute more substantially to ice discharge into the Ross Ice Shelf. Therefore, it is important to characterise the present-day subglacial environment and climatic conditions that may reactivate this flow. This study investigates the present-day subglacial conditions of the Kamb Ice Stream and how these conditions may be affected by environmental perturbations. Due to the difficult nature of making direct observations of ice sheet basal conditions, other methods are employed to investigate the response of the Kamb Ice Stream to environmental change. Active source seismic surveying data obtained during the 2015/16 and 2018/19 austral summer seasons provides an instantaneous snapshot of the present-day basal conditions. Flowline and whole-continent numerical ice sheet modelling is used to investigate the longer-term response of the Kamb Ice Stream and the West Antarctic Ice Sheet. Amplitude analysis of seismic lines indicate saturated till beneath the Ross Ice Shelf in the vicinity of the grounding zone, which is supported by retreat rates of the Kamb Ice Stream grounding zone post-stagnation. Seismic reflection imaging suggests potential dewatered till conditions beneath the grounded Kamb Ice Stream. Flowline modelling of the Kamb Ice Stream indicates that changes to the water content of the subglacial sediments appear to be self regulating, with high reversibility over centennial timescales. Oceanic temperature forcings are the key driver of change of the Kamb Ice Stream, and the ice stream is susceptible to topographic pinning points in 2D and lateral drag. Future glaciological change is more likely to occur in response to oceanic than to atmospheric temperature perturbations. Results from 3D continent-wide modelling experiments also find that precipitation increases offset the effect of air temperature perturbations and influence subglacial conditions, indicating more dynamic ice stream behaviour on the Siple Coast. This study has worked to re-enforce and strengthen our existing understanding of the Kamb Ice Stream and its sensitivity to environmental change. Future work using higher-resolution simulations and a higher density of observational data may help refine these results.</p>

Atmospheric variability and precipitation in the Ross Sea region, Antarctica

<p>Understanding how atmospheric variability in the Pacific sector of Antarctica drives precipitation is essential for understanding current and past climate changes on the West Antarctic Ice Sheet and the Ross Ice Shelf. Precipitation plays a key role in the Antarctic climate system (via mass balance of ice sheets) and is necessary for understanding past climates (via snow and ice proxies). However precipitation is difficult to measure and model and its variability in these regions is still not well understood. This thesis compiles three separate but inter-related studies which provide further understanding of the atmospheric variability of the Ross Sea region and its role in driving precipitation. Synoptic classifications over the Southern Ocean in the Pacific sector of Antarctica (50°S–Antarctic coast, 150°E–90°W) are derived from NCEP reanalysis data (1979–2011), producing a set of six synoptic types for the region. These six types describe the atmospheric variability of the Ross and Amundsen Seas region for the past 33 years and show how hemispheric scale circulation patterns such as the El Niño-Southern Oscillation and the Southern Annular Mode are reflected in local precipitation and temperature on the Ross Ice Shelf. The synoptic types also provide understanding of how different source regions and transport pathways can influence precipitation on the Ross Ice Shelf, which is important for the interpretation of climate proxies. Because of the sparseness of in-situ meteorological measurements in Antarctica, many studies (including the two described above) rely on atmospheric reanalyses data. However, assessments of reanalyses precipitation have only been done on annual and longer timescales. An assessment of the ERA-Interim and NCEP-2 reanalyses precipitation data on synoptic timescales is developed using statistical, event-based analysis of snow accumulation data from automatic weather stations around the Ross Ice Shelf. The results show that there are important differences between the two reanalyses products and that ERA-Interim represents precipitation better than NCEP-2 for this region. Stable isotopes in snow (δ¹⁸O and δD) are widely used as temperature proxies, but are also influenced by moisture history, source region conditions, and cloud micro-physical processes. Further understanding of the relative importance of these other factors is provided by modeling the isotopic composition of snow at Roosevelt Island, an ice core site on the Ross Ice Shelf. A Rayleigh fractionation model is used to determine isotope composition on sub-storm (hourly) timescales, and the results are compared to measured isotope composition. The model is able to reproduce the significant variability of measured isotopes and shows the importance of air-mass mixing and moisture trajectories on the isotopic composition of snow at Roosevelt Island. Together, these studies show how synoptic variability influences precipitation on the Ross Ice Shelf and at Roosevelt Island in particular, and they provide a basis for interpreting stable isotopes and other precipitation-based climate proxies in ice cores from the Roosevelt Island site.</p>

Atmospheric variability and precipitation in the Ross Sea region, Antarctica

<p>Understanding how atmospheric variability in the Pacific sector of Antarctica drives precipitation is essential for understanding current and past climate changes on the West Antarctic Ice Sheet and the Ross Ice Shelf. Precipitation plays a key role in the Antarctic climate system (via mass balance of ice sheets) and is necessary for understanding past climates (via snow and ice proxies). However precipitation is difficult to measure and model and its variability in these regions is still not well understood. This thesis compiles three separate but inter-related studies which provide further understanding of the atmospheric variability of the Ross Sea region and its role in driving precipitation. Synoptic classifications over the Southern Ocean in the Pacific sector of Antarctica (50°S–Antarctic coast, 150°E–90°W) are derived from NCEP reanalysis data (1979–2011), producing a set of six synoptic types for the region. These six types describe the atmospheric variability of the Ross and Amundsen Seas region for the past 33 years and show how hemispheric scale circulation patterns such as the El Niño-Southern Oscillation and the Southern Annular Mode are reflected in local precipitation and temperature on the Ross Ice Shelf. The synoptic types also provide understanding of how different source regions and transport pathways can influence precipitation on the Ross Ice Shelf, which is important for the interpretation of climate proxies. Because of the sparseness of in-situ meteorological measurements in Antarctica, many studies (including the two described above) rely on atmospheric reanalyses data. However, assessments of reanalyses precipitation have only been done on annual and longer timescales. An assessment of the ERA-Interim and NCEP-2 reanalyses precipitation data on synoptic timescales is developed using statistical, event-based analysis of snow accumulation data from automatic weather stations around the Ross Ice Shelf. The results show that there are important differences between the two reanalyses products and that ERA-Interim represents precipitation better than NCEP-2 for this region. Stable isotopes in snow (δ¹⁸O and δD) are widely used as temperature proxies, but are also influenced by moisture history, source region conditions, and cloud micro-physical processes. Further understanding of the relative importance of these other factors is provided by modeling the isotopic composition of snow at Roosevelt Island, an ice core site on the Ross Ice Shelf. A Rayleigh fractionation model is used to determine isotope composition on sub-storm (hourly) timescales, and the results are compared to measured isotope composition. The model is able to reproduce the significant variability of measured isotopes and shows the importance of air-mass mixing and moisture trajectories on the isotopic composition of snow at Roosevelt Island. Together, these studies show how synoptic variability influences precipitation on the Ross Ice Shelf and at Roosevelt Island in particular, and they provide a basis for interpreting stable isotopes and other precipitation-based climate proxies in ice cores from the Roosevelt Island site.</p>

Last Glacial Maximum-Holocene Glacial and Depositional History From Sediment Cores at Coulman High Beneath the Ross Ice Shelf, Antarctica

<p>Sediment Cores collected from the shallow sub-sea floor beneath the Ross Ice Shelf at Coulman High have been analysed using sedimentological techniques to constrain the retreat history of the Last Glacial Maximum (LGM) ice sheet in the Ross Embayment, and to determine when the modern-day calving line location of the Ross Ice Shelf was established. A characteristic vertical succession of facies was identified in these cores, that can be linked to ice sheet and ice shelf extent in the Ross Embayment. The base of this succession consists of unconsolidated, clast rich muddy diamicts, and is interpreted to be deposited subglacially or in a grounding line proximal environment on account of a distinct provenance in the clast content which can only be attributed to subglacial transport from the Byrd Glacier 400 km to the south of the drill site. This is overlain by a mud with abundant clasts, similar in character to a granulated facies that has been documented previously in the Ross Sea, and is interpreted as being a characteristic grounding line lift-o facies in a sub-ice shelf setting. These glacial proximal facies pass upward into a mud, which comprises three distinctive units. i) Muds with sub-mm scale laminae resulting from traction currents occurring near the grounding line in a sub-ice shelf environment overlain by, ii) muds with sub-mm scale laminae and elevated biogenic content (diatoms and foraminifera) and sand/gravel clasts, interpreted as being deposited in open water conditions, passing up into a iii) bioturbated mud, interpreted as being deposited in sub-ice shelf environment, proximal to the calving line. The uppermost facies consists of a 20 cm thick diatom ooze with abundant clasts and pervasive bioturbation, indicative of a condensed section deposited during periodically open marine conditions. During post-LGM retreat of the ice sheet margin in western Ross Sea, and prior to the first open marine conditions at Coulman High, it is hypothesized that the grounding and calving line were in relative close proximity to each other. As the calving line became "pinned" in the Ross Island region, the grounding line likely continued its retreat toward its present day location. New corrected radiocarbon ages on the foraminifera shells in the interval of laminated muds with clasts, provide some of the first inorganic ages from the Ross Sea, and strengthen inferences from previous studies, that the first open marine conditions in the vicinity of Ross Island were 7,600 14C yr BP. While retreat of the calving line south of its present day position is implied during this period of mid-Holocene warmth prior to its re-advance, at present it is not possible to constrain the magnitude of retreat or attribute this to climate change rather than normal calving dynamics.</p>