Episodic erosion in West Antarctica inferred from cosmogenic 3He and 10Be in olivine from Mount Hampton

The polar climate of Antarctica results in the lowest erosion rates on Earth. The low long-term erosion history of high elevation mountain tops that are exposed above the ice preserve a record of climate change that can be accessed using cosmogenic nuclides. However, unravelling the complexity of the long-term denudation histories of Antarctic summits is frequently hampered by intermittent ice cover. The aim of this work is to identify denudation rate changes in a surface that has been continuously exposed since the middle Miocene. We have measured stable (3He) and radioactive (10Be) cosmogenic nuclides in olivine from lherzolite xenoliths from the summit of the Mount Hampton shield volcano within the West Antarctic Ice Sheet. The peak (3200 m) has never been covered by the current ice sheet and local ice caps, consequently the data record the subaerial erosion history of a mountain top within the Antarctic interior. The 10Be concentrations in the olivines yield minimum exposure ages (33 to 501 ka) that are significantly younger than those derived from the cosmogenic 3He (90 to 1101 ka). The data reveal a complex exposure history that provide an integrated long-term erosion rate of between 0.2 and 0.7 m/My that is most likely caused by mechanical weathering. Inverse modelling shows that the data are readily explained by episodic erosion, consisting of one to five erosion pulses that may record major regional climatic changes

History of Oligocene erosion, uplift and unroofing of the Transantarctic Mountains deduced from sandstone detrital modes in CRP-3 drillcore, Victoria Land basin, Antarctica

Detrital modes determined on 68 sandstone samples from CRP-3 drillcore indicate a continuation of the dynamic history of uplift-related erosion and unroofing previously documented in CRP-1 and CRP-2/2A. The source area is identified very strongly with the Transantarctic Mountains (TAM) Dry Valleys block in southern Victoria Land. Initial unroofing of the TAM comprised removal of much of a former capping sequence of Jurassic Kirkpatrick basalts, which preceded the formation of the Victoria Land Basin. Erosion of Beacon Supergroup outcrops took place during progressive uplift of the TAM in the Oligocene. Earliest CRP-3 Oligocene samples above 788 metres below the sea floor (mbsf) were sourced overwhelmingly in Beacon Supergroup strata, including a recognisable contribution from Triassic volcanogenic Lashly Formation sandstones (uppermost Victoria Group). Moving up-section, by 500 mbsf, the CRP-3 samples are depauperate quartz arenites dominantly derived from the quartzose Devonian Taylor Group. Between c. 500 and 450 mbsf, the modal parameters show a distinctive change indicating that small outcrops of basement granitoids and metamorphic rocks were also being eroded along with the remaining Beacon (mainly Taylor Group) sequence. Apart from enigmatic fluctuations in modal indices above 450 mbsf, similar to those displayed by samples in CRP-2/2A, the CRP-3 modes are essentially constant (within a broad data scatter) to the top of CRP-3. The proportion of exposed basement outcrop remained at < 20 %, indicating negligible uplift (i.e. relative stability) throughout that period

Lithostratigraphy and volcanic evolution of Deception Island, South Shetland Islands

Deception Island is the most active volcano in the Antarctic Peninsula region. It is a large basalt–andesite shield volcano with a 10 km-wide restless caldera (Port Foster) and a complicated history of pre- and post-caldera eruptions. There has been no modern volcanological investigation of the entire island and it remains a largely unknown volcanic hazard. The pre-caldera period on the island began with the low-energy eruption of tephras from multiple centres (Fumarole Bay Formation), possibly by subaqueous fire fountaining during shoaling and likely initial emergence of the volcano. It was followed by subaerial effusive to weakly pyroclastic (Strombolian/Hawaiian) activity that constructed a small basaltic shield (Basaltic Shield Formation), and a large eruption that vented about 30 km3 of magma (Outer Coast Tuff Formation). The latter eruption may have been triggered by an influx of compositionally different magma into the main chamber feeding the volcano, and the evidence suggests that it was associated with a significant involvement with water (seawater?). The eruption was followed by caldera collapse, and there have been several small incremental caldera “collapses” subsequently. Post-caldera eruptions were all small-volume and predominantly phreatomagmatic (Baily Head and Pendulum Cove formations), but magmatic eruptions constructed several small lava deltas around the coast and also produced a local carapace of scoria and thin lavas, particularly around the caldera rim (Stonethrow Ridge Formation). Although the caldera is presently resurging, interpretation of the eruptive history of the island suggests that future eruptions are likely to be small in volume and will have only a limited regional impact

The upper Cenozoic tephra record in the south polar region: a review

Tephrochronology studies in the south polar region are reviewed and evaluated. There have been numerous investigations of tephra layers in ice cores, reflecting the continuing importance of ice cores as a principal source of palaeoenvironmental information. By contrast, tephra in marine sediment cores have been largely neglected. Chemical analyses of glass shards are not uniformly available across the region. In particular, they are currently unavailable for the northern Antarctic Peninsula. Few tephras have been dated directly, although potassic glass and minerals are commonly present and should be readily amenable to isotopic dating. Chemical ‘fingerprinting’ seems to have a high potential for successfully correlating layers and identifying source areas, but only a few studies have considered trace elements as well as major oxides. The effects of within-ash compositional variations and analytical imprecision limit the general utility of ‘fingerprinting’. The tephra record is locally much more complete than is preserved in the source volcanoes themselves. However, the effects of frequent eruptions on local depocentres may swamp other environmentally significant indicators and make the environmental record harder to interpret than in tephra-free successions. Linked studies of tephra and volcanically-derived aerosols in ice in the south polar region could be of critical importance for quantitative calculations of the volcanic contribution to atmospheric fluxes and attempts to assess the possible effects of volcanism on global climate

Lithostratigraphy of Miocene–Recent, alkaline volcanic fields in the Antarctic Peninsula and eastern Ellsworth Land

Miocene–Recent alkaline volcanic rocks form numerous outcrops scattered widely throughout the Antarctic Peninsula and eastern Ellsworth Land. They occur mainly as short-lived (typically 1–2 million years) monogenetic volcanic fields but include a large outcrop area in northern Antarctic Peninsula which includes several substantial polygenetic shield volcanoes that were erupted over a 10 million year period (the James Ross Island Volcanic Group (JRIVG)). As a whole, the outcrops are of considerable importance for our understanding of the kinematic, petrological and palaeoenvironmental evolution of the region during the late Cenozoic. Until now, there has been no formal stratigraphical framework for the volcanism. Knowledge of the polygenetic JRIVG is still relatively poor, whereas a unifying lithostratigraphy is now possible for the monogenetic volcanic fields. For the latter, two new volcanic groups and twelve formations are defined, together with descriptions of the type sections. The volcanic fields (both polygenetic and monogenetic) vary in area from c. 1 to 4500 km2, and aeromagnetic data suggest that one may exceed 7 000 km2. The rocks are divisible into two contrasting petrological ‘series’, comprising basanites–phonotephrites and alkali basalts–tholeiites. The JRIVG is dominated by alkali basalts–tholeiites but also contains rare basanites, and phonotephrite–tephriphonolite compositions occur in minor pegmatitic segregations in sills. By contrast, in the monogenetic volcanic fields, basanites–phonotephrites generally form the older outcrops (mainly 15–5.4 Ma) and alkali basalts–tholeiites the younger outcrops (4(?)–<1 Ma). Throughout the region, erupted volumes of alkali basalts–tholeiites were an order of magnitude greater, at least, than those of basanite–phonotephrite compositions. Interpretation of the lithofacies indicates varied Miocene–Recent palaeoenvironments, including eruption and deposition in a marine setting, and beneath Alpine valley glaciers and ice sheets. Former ice sheets several hundred metres thick, and fluctuating ice surface elevations, which were generally higher during the eruptive periods than at present, can also be demonstrate

The relative importance of supraglacial versus subglacial meltwater escape in basaltic subglacial tuya eruptions: an important unresolved conundrum

The hydraulic behaviour of meltwater during subglacial basaltic eruptions in temperate ice is of paramount importance in understanding the eruptive processes, lithofacies and architecture of the edifices formed. Hydraulics also determines the timing, location and volume of meltwater discharge, which may be sudden and catastrophic and via subglacial and/or supraglacial routes. Increasing our knowledge of meltwater hydraulics is therefore important for understanding, predicting and mitigating the impact of meltwater release on vulnerable human communities. New observations about eruption-related meltwater hydraulics are presented for well-exposed glaciovolcanic lava-fed deltas on James Ross Island, Antarctica, and accounts of historical eruptions are also re-examined to identify the major meltwater discharge routes. The study provides the first conceptual model for how meltwater escapes supraglacially. In the absence of a crevassed layer (which will dominate any meltwater flow), overflowing may be initiated by enhanced rates of seepage, despite the intrinsically low hydraulic conductivities of snow and firn. Once overflowing is established, the rate of spillway incision is a likely overriding control on the evolution of the system and whether the discharge is unstable (fast) or stable (slower). The James Ross Island sequences demonstrate that meltwater discharge is highly dynamic and may have involved both subglacial and supraglacial escape. Subglacial discharge probably occurs throughout basaltic tuya eruptions but some periods may be dominated by concurrent overflowing. It is still unclear if overflowing systems are sufficiently stable to enable the growth of laterally extensive glaciovolcanic lava-fed deltas