Ice Cap Volume Research Articles

The subglacial topography of Drangajökull ice cap, NW-Iceland, deduced from dense RES-profiling

Systematic mapping of the bedrock beneath the Icelandic ice caps began in 1980. We continue this effort by presenting a detailed bedrock digital elevation model (DEM) of Drangajökull ice cap in NW-Iceland, surveyed using radio echo sounding (RES) in March 2014. About 590 km of RES profiles, generally with 200–300 m spacing, were surveyed. This is to date the densest set of RES profiles measured on an Icelandic ice cap. We describe the processing of the radar profiles and construction of the bedrock DEM. We also analyse possible errors of surveyed glacier thickness along the profiles and discuss the accuracy of the final bedrock DEM. The bedrock DEM reveals topography with the same general character as other areas of NW-Iceland (Vestfirðir). The new bedrock DEM and a series of surface DEMs allow estimates of total ice cap volume and relative volume change since the mid-20th century. The bedrock data combined with the most accurate available surface DEM of Drangajökull, from July 2011, reveals a total ice cap volume of 15.4±0.4 km³, of which ∼71\% was stored on the west side of the ice cap. At that time the average thickness of Drangajökull was 107±3 m and the maximum observed thickness, found on Kaldalónsjökull outlet glacier, was 284±14 m. In 1946–2014, the total ice cap volume (in autumn) was reduced from ∼18 km³ to ∼15 km³. Approximately half of this volume was lost in 1994–2011. We describe and discuss several interesting phenomena revealed by the RES data and the derived bedrock DEM. The presented bedrock data also helps to explain curious surface features in the accumulation area of NE-Drangajökull.

Control of the Antarctic ice sheet by ocean–ice interaction

The Antarctic ice cap is the largest ice sheet of modern times. It is of considerable importance to predict the sea level variability due to the associated changes in ice volume. We present the results of a simple grounded ice sheet model, developed from Oerlemans [Oerlemans, J., 2002. Global dynamics of the Antarctic Ice Sheet, Climate Dynamics 19, 85–93.], in which the net oceanic evaporation influences the ice cap volume in two ways, through changes in: (i) the accumulation rate, and (ii) the mean sea level. The net evaporation changes are driven by the sea surface temperature (SST) anomaly time series of Howard [Howard, W.R., 1997. A warm future in the past, Nature, 388, 418–419.] for the subantarctic Southern Ocean over the period 220 kyr to the present. The effect of the waxing and waning of the northern hemisphere ice sheets is integrated into the model using an independent model, in which ice melting depends on the SST anomaly and ice calving depends on the sea level anomaly. A series of analytical expressions are derived for the related properties of the coupled ocean–ice system applicable over time scales of 100 kyr, which show, in particular, that the Antarctic ice cap volume changes are due mainly to the effects of the northern hemisphere ice sheets on sea level (which influences ice calving), rather than directly to changes in SST, and hence the ice cap volume is greatest during interglacial periods. This conclusion, which is independent of the specification of the ice melting regime for the northern hemisphere ice sheets, strongly suggests that the changes in accumulation flux estimated from the Vostok proxy temperature data and used in other studies of the Antarctic mass balance have been overestimated. A simple expression is also presented for the lag of ice cap volume to SST, and it is found that the predictions for the mean sea level variability are similar to observations for a melting flux of the northern hemisphere ice sheets about twice their accumulation flux due to the net oceanic evaporation, except during major deglaciations when these two fluxes appear to be of similar magnitude.

Form and flow of the Devon Island Ice Cap, Canadian Arctic

In this study, 3370 km of 100 MHz ice‐penetrating radar data were acquired from Devon Ice Cap, Arctic Canada. Bed returns were obtained from >90% of flight tracks. Mean crossing point errors in ice surface elevation and ice thickness were 7–8 m. Digital elevation models of ice cap surface and bed elevation, and ice thickness, were produced and can be used as boundary conditions in numerical modeling. Devon Ice Cap, including 1960 km2 of contiguous stagnant ice to its west, is 14,010 km2. The ice cap proper is 12,050 km2. Its largest drainage basin is 2630 km2. The ice cap crest has a maximum measured elevation of 1921 m. Maximum recorded ice thickness is 880 m. Ice cap volume is 3980 km3 (about 10 mm sea level equivalent). The bed, 8% of which lies below sea level, is an upland plateau dissected by steep‐sided valleys that control the locations of the major outlet glaciers which dominate ice cap drainage. About 73 km, 4%, of the ice cap margin ends in tidewater. The margin is not floating. Icebergs of <100 m were observed offshore. Only a single outlet glacier showed signs of past surge activity. All major outlet glaciers along the eastern ice cap margin have retreated 1–3 km since 1960. Synthetic aperture radar‐interferometic velocity structure shows slow, undifferentiated flow predominating in the west and center, with fast‐flowing outlet glaciers and intervening slow‐flowing ridges typical elsewhere. Outlet glacier velocities are 7–10 times higher than in areas of undifferentiated flow. This velocity structure, of fast‐flowing units within slower‐flowing ice, appears typical of many large (103 km2) Arctic ice caps.

Form and flow of the Academy of Sciences Ice Cap, Severnaya Zemlya, Russian High Arctic

The 5,575‐km2 Academy of Sciences Ice Cap is the largest in the Russian Arctic. A 100‐MHz airborne radar, digital Landsat imagery, and satellite synthetic aperture radar (SAR) interferometry are used to investigate its form and flow, including the proportion of mass lost through iceberg calving. The ice cap was covered by a 10‐km‐spaced grid of radar flight paths, and the central portion was covered by a grid at 5‐km intervals: a total of 1,657 km of radar data. Digital elevation models (DEMs) of ice surface elevation, ice thickness, and bed elevation data sets were produced (cell size 500 m). The DEMs were used in the selection of a deep ice core drill site. Total ice cap volume is 2,184 km3 (∼5.5 mm sea level equivalent). The ice cap has a single dome reaching 749 m. Maximum ice thickness is 819 m. About 200 km, or 42%, of the ice margin is marine. About 50% of the ice cap bed is below sea level. The central divide of the ice cap and several major drainage basins, in the south and east of the ice cap and of up to 975 km2, are delimited from satellite imagery. There is no evidence of past surge activity on the ice cap. SAR interferometric fringes and phase‐unwrapped velocities for the whole ice cap indicate slow flow in the interior and much of the margin, punctuated by four fast flowing features with lateral shear zones and maximum velocity of 140 m yr−1. These ice streams extend back into the slower moving ice to within 5–10 km of the ice cap crest. They have lengths of 17–37 km and widths of 4–8 km. Mass flux from these ice streams is ∼0.54 km3 yr−1. Tabular icebergs up to ∼1.7 km long are produced. Total iceberg flux from the ice cap is ∼0.65 km3 yr−1 and probably represents ∼40% of the overall mass loss, with the remainder coming from surface melting. Driving stresses are generally lowest (<40 kPa) close to the ice cap divides and in several of the ice streams. Ice stream motion is likely to include a significant basal component and may involve deformable marine sediments.

Late Miocene paleoclimatology: Subantarctic water mass, Southwest Pacific

The Late Miocene—Early Pliocene paleoclimatic history has been evaluated for a deep drilled sediment sequence at Deep Sea Drilling Project Site 281 and a shallow water marine sediment sequence at Blind River, New ealand, both of which lay within the Subantarctic water mass during the Late Miocene. A major, faunally determined, cooling event within the latest Miocene at Site 281 and Blind River coincides with oxygen isotopic changes in benthonic foraminiferal composition at DSDP Site 284 considered by Shackleton and Kennett (1975) to indicate a significant increase in Antarctic ice sheet volume. However, at Site 281 benthonic foraminiferal oxygen isotopic changes do not record such a large increase in Antarctic ice volume. It is possible that the critical interval is within an unsampled section (no recovery) in the latest Miocene. Two benthonic oxygen isotopic events in the Late Miocene (0.5‰ and 1‰ in the light direction) may be useful as time-stratigraphic markers. A permanent, negative, carbon isotopic shift at both Site 281 and Blind River allows precise correlations to be made between the two sections and to other sites in the Pacific region. Close interval sampling below the carbon shift at Site 281 revealed dramatic fluctuations in surface-water temperatures prior to a latest Miocene interval of refrigeration (Kapitean) and a strong pulse of dissolution between 6.6 and 6.2 ± 0.1 m.y. which may be related to a fundamental geochemical change in the oceans at the time of the carbon shift (6.3−6.2 m.y.). No similar close interval sampling at Blind River was possible because of a lack of outcrop over the critical interval. Paleoclimatic histories from the two sections are very similar. Surface water temperatures and Antarctic ice-cap volume appear to have been relatively stable during the late Middle—early Late Miocene (early—late Tongaporutuan). By 6.4 m.y. cooler conditions prevailed at Site 281. Between 6.3 and 6.2 ± 0.1 m.y. the carbon isotopic shift occurred followed, within 100,000 yr, by a distinct shallowing of water depths at Blind River. The earliest Pliocene (Opoitian) is marked by increasing surface-water temperatures.