U31B-0299
Radar detection of the meteoric-accreted ice boundary over Lake Vostok
Airborne radar data collected over Lake Vostok, East Antarctica, show a reflector that approximately intersects the Vostok ice core near the depth of the meteoric--accreted ice boundary. Here we evaluate the hypothesis that this reflector is due to the meteoric--accreted ice boundary. Using ice-core-chemistry and borehole-temperature data, we seek possible causes of reflection that replicate the observed radar reflectivity (about -60 dB). Ice-core data show large variations in the number density of inclusions, chloride-ion concentration ([Cl-]), and ice- crystal size near the meteoric--accreted ice boundary. We estimate the reflectivity due to insoluble mineral inclusions to be -120 dB, based on a likely range of permittivity and inclusion-diameter values. An impurity- and temperature-dependent ice-conductivity model predicts a reflectivity of -68 ± 4 dB due to the large contrast in the concentration of soluble Cl- ions incorporated into the ice lattice near the top of the accreted ice. These results suggest that mineral inclusions do not contribute the reflection found in the radar data and that the [Cl-] contrast is likely the primary cause. The discrepancy (about 4--12 dB) between the radar-derived and [Cl-]-induced reflectivities suggests that a crystal-fabric change may also be present at the meteoric--accreted ice boundary. We argue that the observed reflection is caused by the [Cl-] contrast associated with the meteoric--accreted ice boundary, and we interpret this result together with the spatial patterns of the detection of the boundary, accreted-ice thickness, and echo intensities.
U31B-0300
Evidence for Recent Melting at the Base of the GISP2 Ice Core From Uranium-Thorium Disequilibrium Measurements
We measured 238U-234U-230Th disequilibria by mass spectrometric methods for a set of dusty ice samples from the base of the GISP2 Greenland ice core, at a depth of 3040-3052 m. The goal of this work was to further test the Fireman (1986) recoil-based model for producing uranium-series disequilibria in dusty ice on samples thought to be > 150 ka in age based on layer counting. However, the base of the GISP2 core is greatly disturbed in chemistry and dustiness relative to upper portions of the core. Samples consisted of 11 cm sections of ice core with sample weights of 340-430 g. We separated the samples into several fractions by filtration and analyzed the < 0.05 um fraction. This fraction had exceedingly high U and Th concentrations (2.5- 5 ppb U; 1.4-2.7 ppb Th). These U and Th concentrations are a factor of 1000 higher than measured for ice at Allan Hills, Antarctica. Low Th/U ratios of 0.51-0.65 indicate that a large portion of the uranium present in the samples is dissolved and not associated with particles, which are expected to have Th/U ratios around 3. However, 234U/238U activity ratios range from 0.972-0.992 (+/- 0.001), indicating a depletion of 234U relative to secular equilibrium of 1-3%. In addition, 230Th/234U activity ratios are quite low (0.18-0.24), suggesting either recent Th loss and/or U addition to the samples. This recent Th/U fractionation is not consistent with an age > 150 ka. Since liquid water would be characterized by 230Th/234U activity ratios ≪1, the low 230Th/234U activity ratios likely indicate that recent melting/freezing event(s) have occurred at the base of the GISP2 core. We can model these results with a two component mass balance calculation, with dissolved and particulate pools for each radionuclide. Although several assumptions are required to calculate ages, preliminary results of these calculations suggest that the melting events may be as young as <10 ka.
U31B-0301
Different Behavior Of Deuterium And Oxygen 18 In The Lake Vostok Ice
New isotopic (D and 18O) data from the deep Vostok ice core below 3611 m is presented. The methodological problems of data accuracy are discussed, taking into account a very low natural variability of isotopic content in the lake ice. In order to interpret the lake ice core data, a simple isotope model for lake water and ice is developed. The analyzed data demonstrates clear signature of all three processes contributing to the variability of lake ice isotopic composition: physical conditions of the ice formation, and lake water isotope variability due to glacier melt and hydrothermal water influence. Different behavior of D and 18O for certain lake ice intervals is observed, which might be related to the isotope fractionation of the both isotopes in hydrothermal circulation beneath the lake. At the same time, mean isotopic content remains nearly constant for the whole lake ice thickness yet studied, which implies overall same conditions in terms of melt-lake-hydrothermal water mixing and physical conditions of lake formation. A complete scheme of isotope water budget in lake Vostok is suggested, including an estimation of melt-water isotope content, the presence of additional (hydrothermal) water source and possibly not complete mixing of melt water with deeper lake water. The most likely value of the isotopic content of the water freezing under the glacier base in the southern part of lake Vostok is 450 ‰ for D and 58 ‰ for 18O. Other issues, including preservation of isotopic signal after diffusive smoothing, short-term isotope variability in lake ice, not-steady-state lake budget scenario and future study perspectives, are discussed.
U31B-0302
New Constraints on Maar Geometry and Sedimentary Fill of Laguna Potrok Aike, Southern Patagonia
Laguna Potrok Aike is located in Southern Patagonia, Argentina, at 52°S and 70°W. The maar lake has a diameter of 3.5 km and is almost circular and bowl-shaped with a deep, flat plain (100 m water depth) in its central part. Steep flanks separate the lake shoulders at 15 to 35 m water depth from the central plain. The lake is situated in the Pali Aike Volcanic Field at the present boundary between the Southern Hemispheric Westerlies and the Antarctic Polar Front. Its lake level is highly susceptible to changes in the Antarctic Circumpolar Current that controls the regional precipitation patterns. Its sedimentary infill possibly contains a long and continuous record of several glacial and interglacial cycles, which is unique in the southern South American realm. Two major stratigraphic units (I and II) were distinguished in the seismic sections. Unit I consists of the lacustrine infill and was further subdivided into Sub-units I-a and I-b on the lake shoulders and I-ab, I-c, and I-d in the central basin. Sub-units I-a and I-b on the lake shoulders are separated by a major unconformity and contain several paleoshoreline structures formed during a step-wise transgression after a lake level lowstand of approx. 35 m below the present lake level. In the central basin, Sub-units I-a and I-b are merged into Sub-unit I-ab, not being separated by any unconformity. Pelagic sedimentation dominates in the northern and central parts, whereas mass movement deposits were found in the southern, western and eastern parts close to the steep diatreme flanks. The boundary between I-ab and I-c is non-erosive with I-ab forming downlaps onto I-c from the eastern and western parts of the lake, pointing at a significantly lower lake level during its accumulation. Sub-unit I-d shows similar characteristics as I-ab. The bedrock (Unit II) that forms the steep diatreme flanks consists of the well-layered sandstones found in the lake surroundings.
U31B-0303
Mass Balance of the ice Sheet Above the Southern Part of Subglacial Lake Vostok Based on Field Observations
In the Antarctic field season 2001/2002, geodetic GPS markers were installed in and around Vostok station and observed for the first time. In the following field season, these observations were repeated. During the field season 2006/2007, a third observation campaign was carried out. These repeated in-situ measurements provide the basis for the precise determination of the local flow direction and velocity of the ice sheet, as well as height change rates for the GPS markers. The changes of the internal geometry between the markers were used for a strain analysis in order to quantify the convergence/divergence and acceleration of the ice flow around Vostok station. We present our results on mass balance state of both the floating ice sheet and the subglacial water body. For this purpose, we combined the marker displacements determined by geodetic means with representative surface accumulation rates and precise ice thickness data from ground-based radar-echo sounding. Due to the hydrostatic equilibrium of the floating ice some conclusions are valid for the entire lake area.