The geology, chemistry and age of the Straumsvola Alkaline Complex, western Dronning Maud Land, Antarctica. The role of liquid immiscibility

Ice surface changes during recent glacial cycles along the Jutulstraumen and Penck Trough ice streams in western Dronning Maud Land, East Antarctica

<p><span>Reconstructing past ice surface changes is key to test and improve ice-sheet models. Yet, data constraining the past behaviour of the East Antarctic Ice Sheet are sparse, limiting our understanding of its response to past and future climate changes. Here, we attempt to test whether the ice-sheet margin in western Dronning Maud Land has thinned since the last glacial maximum or whether it perhaps thickened in places due to increased precipitation associated with warmer climates. We report cosmogenic multi-nuclide (<sup>10</sup>Be, <sup>26</sup>Al, <sup>36</sup>Cl,<sup> 21</sup>Ne) data from bedrock and erratics on nunataks along Jutulstraumen ice stream and the Penck Trough in western Dronning Maud Land, East Antarctica. Spanning elevations between 751-2387 m above sea level, and between 5 and 450 m above the contemporaneous local ice sheet surface, the samples record apparent exposure ages between 2 ka and 5 Ma. The highest bedrock sample indicates (near-) continuous exposure since at least the Pliocene, with a very low apparent erosion rate of 15</span><span>±</span><span>3 cm Ma<sup>-1</sup>. However, there are also clear indications of a thicker-than-present ice sheet within the last glacial cycle, thinning ~35-120 m at several nunataks during the Holocene (~2-11 ka). Owing to difficulties in retrieving suitable sample material from the often rugged and quartz-poor mountain summits, and due to the presence of inherited nuclides in many of our samples, we are unable to present robust thinning estimates from elevational profiles. Nevertheless, the results clearly indicate ice-surface fluctuations of several hundred meters within the last glacial cycle in this sector of the EAIS, between the current grounding line and the edge of the polar plateau. </span><span>Finally, inverse modelling of the cosmogenic multi-nuclide inventories in bedrock yields estimates of total erosion and ice cover across multiple glacial cycles. Our results show that the EAIS in western Dronning Maud Land was thicker than present during most of the Quaternary, covering sample sites up to 200 m above the present-day ice sheet for ~80 % of this period. Thinning of the ice since the last glacial maximum, combined with a long-term record of thicker-than-present ice, indicate that the ice sheet below the polar plateau in western Dronning Maud Land generally expands and thickens during climate cooling, despite decreasing precipitation associated with a cooler Southern Ocean.</span></p>

Extent, thickness and erosion of the Jurassic continental flood basalts of Dronning Maud Land, East Antarctica: A low-T thermochronological approach

Spatio-temporal variations of the recently determined accumulation rate are investigated using ground-penetrating radar (GPR) measurements and firn-core studies. The study area is located on Ritscherflya in western Dronning Maud Land, Antarctica, at an elevation range 1400–1560 m. Accumulation rates are derived from internal reflection horizons (IRHs), tracked with GPR, which are connected to a dated firn core. GPR-derived internal layer depths show small relief along a 22 km profile on an ice flowline. Average accumulation rates are about 190 kg m−2 a−1 (1980–2005) with spatial variability (1σ) of 5% along the GPR profile. The interannual variability obtained from four dated firn cores is one order of magnitude higher, showing 1σ standard deviations around 30%. Mean temporal variations of GPRderived accumulation rates are of the same magnitude or even higher than spatial variations. Temporal differences between 1980–90 and 1990–2005, obtained from two dated IRHs along the GPR profile, indicate temporally non-stationary processes, linked to spatial variations. Comparison with similarly obtained accumulation data from another coastal area in central Dronning Maud Land confirms this observation. Our results contribute to understanding spatio-temporal variations of the accumulation processes, necessary for the validation of satellite data (e.g. altimetry studies and gravity missions such as Gravity Recovery and Climate Experiment (GRACE)).

The study focuses on whole-rock major and trace element chemistry, as well as radiogenic isotope data from the Straumsnutane Formation lavas in western Dronning Maud Land, Antarctica. The data are compared with those from the Espungabera Formation lavas of central Mozambique, published data from the Borgmassivet intrusions in Dronning Maud Land, Antarctica and other intrusions in southern Africa which are correlated with the approximately 1100 Ma Umkondo Igneous Province. Petrographical studies indicate that the Straumsnutane lavas are dominated by plagioclase, clinopyroxene, amphibole and Fe–Ti oxides. Secondary mineral assemblages include chlorite, pumpellyite, white mica and epidote, indicating that the Straumsnutane lavas have been metamorphosed under low-grade greenschist-facies conditions followed by retrograde prehnite–pumpellyite-facies conditions. The chemical data for the Straumsnutane Formation lavas are dominantly tholeiitic and basaltic andesitic in composition, and indicate that they are of continental origin. Trace element ratio values for the Straumsnutane lavas suggest that fractional crystallization and/or crustal contamination have been significant processes in the magma evolution. Low to high 87 Sr/ 86 Sr isotopic ratios (0.682–0.720) are evident from the Straumsnutane lavas suggesting varying degrees of hydrothermal alteration/low-grade metamorphism. The calculated 87 Sr/ 86 Sr values and the negative ε Nd values at 1100 Ma suggest contamination by older continental crust during the genesis of the Straumsnutane Formation lavas. Isotopic modelling shows that the Straumsnutane lavas may have been formed from the mixing of a mid-ocean ridge basalt (MORB)-like source with approximately 4% of older crust similar to the Messica Granite Gneiss of central Mozambique. Comparison of the geochemical data and petrography of the Straumsnutane lavas with those of the Espungabera Formation lavas of central Mozambique shows that they are similar. Additional comparisons show that the Straumsnutane lavas are geochemically similar to rock units of the Umkondo Igneous Province in southern Africa. It is therefore concluded that the Straumsnutane Formation lavas also formed part of the Umkondo Igneous Province.

Abstract. Radio-echo sounding provides the opportunity to study the internal architecture of ice sheets through imaging stratified englacial reflections, known as internal reflection horizons (IRHs). They represent consistent time horizons formed at the former ice-sheet surface and buried over time, thus reflecting the ice sheet's age–depth architecture. Their analysis allows crucial insights into past and present glaciological conditions, e.g. bed topography, surface and basal mass balance, and physical properties and ice dynamics. This study presents a comprehensive data set of IRHs and insight into the age–depth distribution in western Dronning Maud Land (DML), East Antarctica, spanning the Holocene to the Last Glacial Period (4.8–91.0 ka). Using data from various radar systems deployed by the Alfred Wegener Institute between 1996 and 2023, we traced and dated nine IRHs over an area of 450 000 km2. A precise age could be assigned to the IRHs by two-way travel time to depth conversion and employing radar forward modelling based on conductivity peaks of the EPICA DML ice core. Six IRHs correlate with the timing of past volcanic eruptions, and our findings suggest that most IRHs correspond to IRHs of similar age in other regions of East and West Antarctica, thus likely originating from the same physical reflectors at depth, although some could not be physically connected. This work enhances understanding of the englacial architecture and relationships with snow accumulation and ice-dynamic processes of this sector of the Antarctic ice sheet and provides boundary conditions for numerical ice flow models and paleoclimatic studies.

Epiglacial and supraglacial lakes are characteristic lake types in Antarctica, and regardless of their mostly seasonal existence and ultraoligotrophy, some lakes have a relatively diverse microbial community. The results of water chemistry and phytoplankton, based on basic limnological methods, from five epiglacial and two supraglacial seasonal lakes are presented from western Dronning Maud Land, an area where only physical studies have been previously carried out. Electric conductivity varied mostly between 0.1 and 10 mS m−1 (25 °C), phosphorus concentration was <5 mg m−3, and nitrogen concentration was <300 mg m−3 except in some shore areas, and water pH ranged from 6 to 11. Low phytoplankton biomasses (in most cases <10 mg m−3) supported the ultraoligotrophic status of the lakes. Phytoplankton was found from both types of lakes, but less was found from supraglacial lakes. The charophyte Mesotaenium cf. berggrenii dominated the supraglacial lakes, while cyanoprokaryotes such as Gloeocapsopsis cf. magma, Planktothrix prolifica/rubescens, Nostoc cf. sphaericum, Cyanothece sp. and Phormidium sp. dominated the biomass in some epiglacial lakes. Chrysophytes (e.g. Pseudopedinella-type flagellates) were observed in both types of lakes, and they were occasionally dominant. The green alga Botryococcus braunii, some diatoms (Cyclotella sp., Diatoma tenuis, Luticola muticopsis), and non-planktonic microalgal colonies visible to the eye (incl. the cyanoprokaryote Nostoc commune) were also found. Signs of a living ecosystem with a food web were observed in one epiglacial lake, but not elsewhere, which indicates extreme circumstances in the Antarctic seasonal lakes. Altogether, only some 25 taxa were discovered.

Two automatic snow stations were deployed for one year (from December 2009–January 2011) in western Dronning Maud Land. The purposes of the experiment were: 1) to build a working snow station to measure the snow surface layer temperature, and 2) to use the data for snow heat and mass balance investigations. The data collection was successful and lasted about 400 days (9 December 2009–21 January 2011). The annual net snow accumulation at snow station 2 (continental ice sheet) was 86 cm (345 mm water equivalent) and at snow station 1 (ice shelf) more than 150 cm. The power spectra revealed daily cycle, synoptic scale variability, and variability in a low-frequency band of 60–120 days at a depth of 54 cm. The snow-air heat flux was estimated from the data, resulting in negative values (from snow to air) during autumn and winter and positive values (from air to snow) in spring and summer. The physical characterization of snow stratigraphy was done during installation and retrieval of the snow stations, including density, hardness (hand test), stratigraphy, and grain size and shape.

During the austral summer 1993/1994, the spatial distribution of snow was mapped by a ground‐based snow radar (800–2300 MHz) in western Dronning Maud Land, East Antarctica. Snow radar soundings were performed along continuous profiles extending from the ice shelf up to the polar plateau, a total distance of 1040 km. The high‐resolution radar registrations revealed subsurface layering in the uppermost 12 m of the snowpack. The travel time record was translated into snow accumulation expressed in water equivalents, based on an empirical relationship between wave speed and firn density. A good knowledge on snow density variations with depth is essential for the variability studies. Generally, the snow layering was well developed in the coastal area and less well developed on the polar plateau. High spatial variability in snow accumulation was observed on a regional as well as on a local scale. The variability was very high in areas with large surface slopes, such as the grounding zone and around nunataks. The highest variability was recorded in the nunatak area, where the standard deviation reached 59% of the spatial average accumulation. On the smooth high‐altitude plateau, variations in accumulation were less pronounced. However, here the standard deviation exceeded 22% of the average accumulation rate. Provided that the snow radar soundings are supported by dating of reference horizons along the travel route, this is a good method to obtain the accumulation rate and pattern for large areas with a high spatial resolution.

The Annandagstoppane Granite is exposed at three nunataks in Western Dronning Maud Land, Antarctica. It comprises medium- to coarse-grained granite crosscut by veins of pegmatite and graphic granite and has many S-type characteristics such as containing normative corundum greater than 1.1%, molecular Al2O3/(CaO+K2O+Na2O) greater than 1.1 and very little zircon. Hydrothermal alteration in the Granite is variably developed and has affected only certain minerals in any phase. R-Sr and Pb whole rock and mineral isotopic data suggest: 1) that Sr isotopes within it were nearly homogenized on a whole rock scale about 2823 Ma ago by this hydrothermal alteration; 2) that the Pb isotopic system was also disturbed at that time, and 3) that the Granite may have been was emplaced sometime during the interval 3115 Ma to 2945 Ma ago. The Granite was probably intruded by the Annandagstoppane Gabbro about 1200 Ma ago, resetting the Rb-Sr system in biotite. The Annandagstoppane Granite may form part of a basement complex to the Proterozoic sedimentary, volcanic and mafic igneous rocks exposed to the east in the Ahlmannryggen and the Borgmassivet. Its chemical composition and geologic history appears to be unique in Antarctica and in the Kaapvaal Craton of Southern Africa, consistent with the possibility that the Annandagstoppane Granite is part of a crustal fragment that joined Antarctica relatively late in the history of that continent.

Summer observations of the morphology and the debris accumulation processes at an actively forming pronival rampart at Grunehogna Peaks, Western Dronning Maud Land, Antarctica demonstrate that rockfall debris accumulation is causing downslope (outward) rampart extension even though the distal slope is not at the angle of repose. Field experiments show that the vast majority of rocks can traverse a stable firn surface to reach the proximal slope of the rampart and more than half end up on the distal slope or beyond. The formation processes indicate that the morphological characteristics and environmental conditions under which such features develop may be more varied than conceived in current models. Consequently, caution must be employed when fossil ramparts are used to infer palaeo‐environmental conditions. Copyright © 2010 John Wiley &amp; Sons, Ltd.

The ground temperature down to 60 cm depth in western Dronning Maud Land (WDML), has been recorded since 2009. The study area is situated in a blockfield that comprises a shallow active layer above permafrost. Using ground thermal regimes and regional climate data, the temporal (seasonal and annual) variability of the active layer was characterized. Active layer depth was calculated for each site for five consecutive summers from 2009/10–2013/14, showing interannual variability with no overall trends of decreasing or increasing active layer depth. Particular attention was paid to 2010 as it matched the average for the ground thermal regimes over the six year study period, as well as the interpolation period used by Meteonorm®. Analysis showed significant synchronous relationships of ground thermal regimes with air temperature and incoming radiation. Moreover, a correlation between pressure and measured ground temperature during the transitional season of the Southern Annual Oscillation in May and September was identified.

Time-dependent ice-sheet modelling of a 176 000 km2 area in western Dronning Maud Land, East Antarctica, provided information on the ice sheet’s response to six climate-change scenarios. Another experiment was done to study changes in ice thickness, flow and basal temperature conditions between the present ice configuration and a simulated maximum palaeo-ice sheet. The input to the model included new datasets of bed and surface topography compiled for this study. The results of the six climate-change experiments, including a 0.5°C per century global-warming scenario, show that the ice sheet has a robust behaviour with respect to the different climate changes. The maximum change in ice volume was &lt;5% of the initial volume in all climate runs. This is for only relatively short-term climate changes without major changes in global sea level, and also a simulated ice sheet without an ice shelf. The modelled long-term response time of the ice sheet, 20 kyr or more, indicates that the ice sheet may still be adjusting to the climate change that ended the Last Glacial Maximum. In the maximum palaeo-ice-sheet simulation, with a 5°C climate cooling and the grounding line located at the continental-shelf margin, ice thickness increased drastically downstream from the Heimefrontfjella mountain range but remained basically unaffected on the upstream polar plateau. Compared to present conditions, complex changes in basal temperatures were observed. The extent of areas with basal melting increased, for example in the deep trough of the Veststraumen ice stream. Areas at intermediate elevations in the landscape also experienced increased basal temperatures, with significant areas reaching the melting point. In contrast, high-altitude areas that today are clearly cold-based, such as around Heimefrontfjella and Vestfjella and the Högisen dome, experienced a 5–10°C decrease in basal temperatures in the palaeo-ice-sheet reconstruction. The results suggest that the alpine landscape within these mountain regions was formed by wet-based local glaciers and ice sheets prior to the late Cenozoic.

We recovered sedimentary and plutonic rocks from the off-ridge portion of the Southwest Indian Ridge (37°00.02ʹ E, 44°49.73ʹ S, 2165 m deep). The petrography, geochemistry, and geochronology of these plutonic rocks were analyzed. Ar–Ar dating of biotite in the plutonic rocks yielded early Paleozoic ages of approximately 475 and 490 Ma, indicating that these rocks are likely related to Antarctic orogenies during the late Neoproterozoic to early Paleozoic. In addition, the samples exhibited trace element compositions and low initial εNd values (−8 and −12) similar to those of rocks from western Dronning Maud Land and the Transantarctic Mountains, Antarctica. Such similarities suggest that these plutonic rocks from the off-ridge portion of the Southwest Indian ridge were derived from western Dronning Maud Land or the Transantarctic Mountains and deposited by melting icebergs as dropstones.

The Sverdrupfjella Group in western Dronning Maud Land is a 1200 to 900 Ma orogenic belt that experienced a thermal overprint in the early Cambrian. Evidence for distinct episodes of fluid-rock interaction is found in calc-silicate rocks, veins and retrograde mineral assemblages. A sequence of altered but undeformed basalts exhibit extreme 18 O depletion ( δ 18 O as low as −1.8%) apparently due to interaction with meteoric water during regional (possibly early Cambrian) metamorphism. In the central Kirwanveggen, metasomatic calc-silicates, and retrograde mineral assemblages are associated with late high-strain zones of probable Cambrian age. The former have δ 13 C values which overlap those of massive metacarbonate units 150 km to the NE, and imply regional scale movement of fluids. The latter record 18 O depletion relative to unretrogressed equivalent rocks and suggest interaction with externally derived fluids.