Rapid resurgence of the subglacial Bárdarbunga caldera following collapse in 2014-2015, quantified with repeated Bouguer gravity surveys

<p>The 65 km<sup>2</sup> Bárdarbunga caldera is located in the NW part of the Vatnajökull glacier in central Iceland.  The caldera floor lies under 500-800 m thick ice and the rims are fully subglacial as well.  The caldera subsided by 65 m during the Bárdarbunga-Holuhraun eruption in 2014-2015, when about 2 km<sup>3</sup> of magma drained out from a magma reservoir at ~10 km depth leading to the largest eruption in Iceland since Laki in 1783.  Deformation surveys outside the caldera have indicated inflation since soon after the end of the eruption in February 2015 and seismicity has been elevated.  The extensive ice cover precludes conventional microgravity surveys or detailed surveys of caldera floor elevation.  However, we have studied gravity changes by comparing results of repeated Bouguer anomaly surveys.  We perform a full Bouguer correction using detailed DEMs of both the ice surface and the ice-radar-derived bedrock.  Ice surface changes are also mapped, allowing the removal of effects on gravity by ice mass changes.  Possible sources of significant anomalies are either changes in bedrock elevation between surveys, other more deep-seated mass changes beneath the volcano, or changes in the water table and pore pressure.  Surveys were carried out using a Scintrex CG-5 in 2015, 2016, 2018 and 2019, with measurements done at 25-50 locations each time.  As no benchmarks exist on the ice the spatial difference in station location of 10-20 m exists between survey years.   However, post-processing provides kinematic GPS position and elevation accuracy better than 0.1 m. Analysis of the data and error sources indicate an accuracy in estimates of changes of 50-100 µGal. The results obtained indicate change with an amplitude of a few hundred µGals; over the four years between 2015-2019 a clear Bouguer anomaly increase is recorded over the caldera relative to the surrounding area. Sharp gradients in the gravity difference near the caldera boundary point to a shallow source, consistent with the gravity signal arising from or near the ice-bedrock boundary.  This indicates fast resurgence at Bárdarbunga since 2015. The elevation of bed reflections delineated from radio echo sounding profiles (~2 MHz), measured within the caldera in June 2015 and accurately repeated in June 2019, further supports this.  The suggested deformation mechanisms can be compared to geodetic observations outside the caldera for further evaluation. If all the signal is interpreted in terms of magma movements, a rise of the caldera floor by several meters and the inflow of 0.2-0.3 km<sup>3</sup> of new magma is inferred.</p>

Methods of parameters estimate of ice formations

The ice cover of the Arctic Seas is an important component of the natural conditions. It is impossible the construction and safe exploitation of the hydrotechnical structures and lines of communications on the shelf, reasonable planning and conducting of cargo and transport operations, organization of environmental protection measures without taking into account an ice cover.The information on morphometric and dynamic characteristics of ice formations, physical and mechanical ice properties, presence of icebergs and its bergy bits with various mortphometric and dynamic characteristics in the water area are necessary for an organization of successful activity on the shelf (design of hydrotechnical structures, planning of the work etc).The present article is concerned with the issues of estimation of ice formations morphometric parameters. The different remote observations methods on ice floes and icebergs are considered in the article: aerial survey, radar survey, observations using ice radar and geodetic instruments, visual observations, sonar survey of ice cover.The goal of the work is the description of peculiarities of various remote methods of observations. For each of the considered methods, the conditions of its application and peculiarities of data obtainment are considered; the list of morphometric parameters, that can be estimated, using results of corresponding observations is indicated.The mentioned algorithms and formulas are actively used during ice surveying works at the present time. The knowledge of various methods peculiarities allows to plan the composition of research works depending on their goals and tasks, determine the terms of their performance.The main merits and demerits of the considered methods of data obtainment and estimations of ice formations morphometrical parameters are phrased in the conclusion.

Initial reconnaissance for a South Georgia ice core

ABSTRACTWe present the first snow/ice chemistry and ice radar results ever collected from South Georgia as part of an initial reconnaissance with the ultimate goal of assessing the feasibility of a South Georgia ice core to reconstruct past climate in the South Atlantic. South Georgia is well situated to capture a record of past atmospheric chemical composition over the South Atlantic and of past variability in the position and intensity of the austral westerlies. The question is how well preserved an ice core record can be recovered from a region experiencing accelerated melting? The results presented in this paper offer only a preliminary step in determining the feasibility of future deep ice coring on South Georgia. However, this initial reconnaissance does provide some basic information including: the chemistry of the atmosphere over South Georgia relative to other Southern Hemisphere ice coring sites; the potential for preservation of ‘annual layers’ in old ice on the island; a possible age for deep ice in the region; and an estimate of glacier health in the lower elevation regions of the island.

Glacier dynamics near the calving front of Bowdoin Glacier, northwestern Greenland

To better understand recent rapid recession of marine-terminating glaciers in Greenland, we performed satellite and field observations near the calving front of Bowdoin Glacier, a 3 km wide outlet glacier in northwestern Greenland. Satellite data revealed a clear transition to a rapidly retreating phase in 2008 from a relatively stable glacier condition that lasted for >20 years. Ice radar measurements showed that the glacier front is grounded, but very close to the floating condition. These results, in combination with the results of ocean depth soundings, suggest bed geometry in front of the glacier is the primary control on the rate and pattern of recent rapid retreat. Presumably, glacier thinning due to atmospheric and/or ocean warming triggered the initial retreat. In situ measurements showed complex short-term ice speed variations, which were correlated with air temperature, precipitation and ocean tides. Ice speed quickly responded to temperature rise and a heavy rain event, indicating rapid drainage of surface water to the bed. Semi-diurnal speed peaks coincided with low tides, suggesting the major role of the hydrostatic pressure acting on the calving face in the force balance. These observations demonstrate that the dynamics of Bowdoin Glacier are sensitive to small perturbations occurring near the calving front.