Cryosphere General Contributions I Posters
Presiding: C Shuman, NASA Goddard Space Flight Center; J Minster, Scripps Institution of Oceanography
C43A-01 1330h
Effect of Dispersed Particles on Creep and Activation Energy of Granular Ice
The effects of silt-sized particles (1 wt. %) on the compressive creep of polycrystalline ice have been studied at temperatures from -20oC to -2oC. The particles were present both within the grains and along the grain boundaries. The results indicate that particles do not affect the power-law behavior. Both types of ice show power-law creep behavior with an exponent of 3 at temperatures from -20oC to -2oC, and the particles were found to increase the creep rates at temperatures lower than -5oC. The effect of particles on creep rates decreases as temperature increases. At -2oC, creep rates of both types of ice are compatible. Calculations indicate that activation energies of both types of ice are around 70 KJ mole-1 when the temperature is between -20oC to -10oC. The activation energy shows a similar value for ice with particles but increases to around 120 KJ mole-1 for particle-free ice when the temperature is above -10oC. The increase in activation energy for particle-free ice can be explained by the increase in grain boundary sliding at high temperatures. Apparently, particles distributed along the grain boundaries inhibit the grain boundary sliding and thereby decrease the activation energy for ice with particles when the temperature is above -10oC. This research was supported by NSF Office of Polar Programs, Arctic Natural Sciences Program (OPP 011737).
C43A-02 1330h
Grain shape relaxation in elongated grained polycrystalline sample
The evolution of fabric (c-axis distributions) and texture (grain size and shape) in an ice core depends on a variety of physical conditions including, but not limited to, temperature, impurity content, strain rate and strain symmetry. It is interesting to consider the possibility of using grain elongation as an in-situ measure of the strain rate history in a core. In the absence of any processes leading to new grain formation (e.g. polygonization or recrystallization) it may be possible to relate measured grain elongation to the strain history in a simple way. A computer grain growth simulation was used to investigate the shape relaxation of elongated grains in a polycrystalline sample. The program employs only geometrical arguments and uses basic equations for grain boundary migration. The simulation was carried out in a two-dimensional sample with 1000 grains. The initial elongation of each grain was arbitrarily horizontal and the mean elongation was around 30%. The initial grain size of the crystals was normally distributed around of a mean grain size. The evolution of the mean elongation and average grain size of the sample were computed. The average elongation decreases "exponentially" with time as the elongated grains grow and transform into rounded grains. The elongation relaxation process did not affect the linear relationship between the grain size and the time. A simple (empirical) theoretical model, which includes grain growth and strain rate, was compared with the results of the simulation and the free parameters of this model were determined. The empirical model can then be used to study grain growth and elongation under arbitrary conditions of imposed strain.
http://www.agu.org
C43A-03 1330h
Differentiation of Land-Based Snow and Ice by the Moderate Resolution Imaging Spectroradiometer (MODIS)
The extent of snow and ice over land is a key factor in the global radiation budget and the hydrologic cycle, and is essential information for regional- and global-scale climate modeling. Remote sensing of land cover via satellite provides an opportunity for automated measurement of snow and ice over expanded spatial areas and is amenable to study using moderate-resolution sensors such as the Moderate Resolution Imaging Spectroradiometer (MODIS). MODIS consists of 36 discrete spectral bands from the visible through the thermal infrared parts of the electromagnetic spectrum and three spatial resolutions of 250m (bands 1-2), 500m (bands 3-7), and 1km (bands 8-36), and daily or near-daily temporal coverage. The first MODIS sensor is aboard NASA's Earth Observing System (EOS) Terra spacecraft, which was launched in December 1999. The EOS Aqua satellite was launched in May 2002, and carries the second MODIS sensor. Five years of MODIS data are now available and are useful for the creation of time series data sets. The spectral bands from MODIS of greatest potential in discrimination between snow and bare (non-snow covered) ice on land surfaces are shown to be useful for development of automated algorithms to distinguish land-based snow and bare ice based on their optical properties. An advantage of automated algorithms is the consistency of results within the time series and therefore the reduction of subjectivity in identifying climatologically-significant trends. Preliminary results of automated algorithms to distinguish snow and bare ice show that it is very difficult to separate snow and ice, for example on Greenland, using classification techniques. Clouds also present a challenge in discrimination of snow and bare ice over land. However, progress has been made and results showing a time series of data for an area in southwestern Greenland will be presented.
http://modis-snow-ice.gsfc.nasa.gov
C43A-04 1330h
Classification of first-year sea ice deformation using ENVISAT ASAR AP data
Sea ice deformation is an important factor in sea ice thickness distribution and mass balance studies. Deformation identification and characterization is also crucial for safe winter ship navigation in Arctic regions. This research evaluates the utility of individual and multiple ENVISAT ASAR system parameters and texture parameters to discriminate between smooth, rough and deformed first year Arctic sea ice. Smooth, rough and deformed classes correspond to unique insitu measured sea ice topography values and SAR backscatter properties. System parameters; single polarizations (HH, VV, HV and VH), dual polarization combinations (HH + VV, HH + HV and VV + VH) and incidence angle (15.0 to 45.2 degrees) and texture measures appropriate for SAR imagery (contrast, entropy and correlation) are investigated. Univariate and multivariate analyses are conducted using a minimum distance classifier to determine which individual or set of multiple parameters maximize the discrimination of the three sea ice topography classes. Our results indicate that increasing incidence angle results in an increased ability to discriminate sea ice deformation classes regardless of polarization combination or texture measure. For single polarizations, co-polarizations (HH & VV) outperformed cross-polarizations (HV & HH) by 6.48 % to 33.93 %. For dual polarizations, HH + VV polarizations outperformed VV + VH and HH + HV by 3.75 % to 14.93 %.
C43A-05 1330h
Multiple Reflections Inside the GLAS Footprint: Estimation of the Altimetry Bias and a Roadmap towards a Correction Scheme
ICESat's primary mission statement is to use laser ranging to estimate the altitude (hence thickness) of the Earth's ice sheets at better than 10 cm per shot, leading to a mapping of the trend at about 1 cm per year accuracy upon temporal averaging. This ambitious goal is dictated by global warming science considerations. Laser altimetry and/or ranging can be done either by detection of the earliest photon returned from a short but intense pulse or by tracking the shift of the maximum of a longer pulse. In altimetry mode, GLAS uses the latter algorithm which makes it vulnerable to spurious shifts caused by multiple reflections in the unresolved terrain. We show that, for GLAS's instrumental characteristics and typical reflection and roughness properties of ice, the resulting negative altimetry bias can be commensurate with the mission's precision goal. The bias will furthermore have a seasonal cycle which may become aliased into the annual mean. Finally, we will argue that, based on our modeling results, GLAS has within its product portfolio much of what is needed to correct the bias.
http://nis-www.lanl.gov/~adavis
C43A-06 1330h
Changes of Arctic Sea Ice and Their Attribution: Multi-model Investigations in the IPCC AR4 Framework
Sea ice is an essential component of cryosphere, which is active on relatively short time scales compared to some other cryospheric components and obviously interacts with other parts of climate system. Sea ice has been recognized as an integrative indicator of global climate change. Recent dramatic shrinking of Arctic sea ice cover has drawn much attention scientifically and socially. Amplified global warming in the polar region may result in a further rapid decrease of sea ice cover, leading to an ice-free summer in the Arctic Ocean in future, as projected in global warming scenarios. The changes of sea ice may naturally cause significant climatic consequences by changing the energy budgets and hydrological cycle and altering atmospheric circulation regimes. Moreover, changes of sea ice exports may also affect the North Atlantic deep convections and Meridional Overturning Circulation, which is associated with multidecadal climate variability. In this study, we analyze changes of sea ice cover and volume based on the multiple model outputs for IPCC AR4 (Intergovernmental Panel on Climate Change, the 4th Assessment Report), archived by PCMDI (Program for Climate Model Diagnosis and Intercomparison). We first examined the Arctic sea ice simulations for the climate of 20th century (20C3M) and validated model's performance. Considering availability of accurate sea ice observations by satellites, we selected a period of 1979-99 from ensemble means of each model's experiments and compared model results with observational data. Then, we investigated changes of sea ice extend, area and volume in the 20th century and in the 21st century under global warming scenarios (SRES A1b, SRES A2, and SRES B1) among various participating climate models and explored their attributions.
C43A-07 1330h
Are Eastern Basin (Ross Sea, Antarctica) Bathymetric Ridges Associated With the Last Glacial Maximum?
Ross Sea (Antarctica) Eastern Basin bathymetric ridges have been interpreted to be ice stream divides created during the Last Glacial Maximum (LGM) advance of the Antarctic Ice Sheet based on radiocarbon dating of organic matter from near-seafloor sediments recovered in piston cores (Domack et al., 1999). Detailed seismic correlations and contour mapping show that there are at least five thick units outcropping in Eastern Basin. Four of these seismically-defined units can be correlated to age control at DSDP sites 270 and 272. In contrast to the near-seafloor sampling, the interiors of these units were initially assigned a Pliocene age based on a variety of microfossil biozones (Hayes and Frakes, 1975). Savage and Ciesielski (1983) determined that the youngest unit was deposited during the Coscinodiscus lentiginosus (since renamed Thalassiosira lentiginosa) diatom biozone (i.e., the unit formed sometime between 0.65 Ma to Recent timeframe). Thus, seafloor units in the area probably are of Quaternary age, but not necessarily LGM age. More recently, diatom biozonations for the Southern Ocean have been revised to provide more detailed biochronostratigraphic resolution (Zielinski and Gersonde, 2002; Zielinski et al., 2002). We are using the most recently-revised Southern Ocean diatom-zonation schemes to systematically evaluate ages of samples taken from the base of piston cores penetrating the five individual seismically-defined units in Eastern Basin. Using this sampling strategy, we increase the chances of penetrating through the Recent hemipelagic drape to sample the underlying seismically-defined units.
C43A-08 1330h
Seismic Reflectivity Analysis of Regional Unconformities on Ross Sea Continental Shelf: A Geophysical Fingerprint for Antarctic Ice Sheet Grounding Surfaces
Past seismic stratigraphic studies along the continental shelf of Antarctic passive margins have interpreted Cenozoic regional unconformities as ice-sheet grounding surfaces. These unconformities are manifested seismically as high-amplitude topset reflectors. In the Ross Sea, Antarctica, we model the offset-dependent seismic reflectivity patterns of these unconformities. Six multichannel seismic lines totaling ~206 km in length were run in the North-Basin sector of Ross Sea outer continental shelf in 2003. Fold coverage was 48 and recording time extended to 5 seconds. Seismic data are stacked to highlight differences in the reflector continuity and amplitudes between zero-offset-gradient and -intercept attribute stacked seismic sections. From the variation of amplitude with offset, individual common midpoint gathers are modeled for P-wave velocity, S-wave velocity, and density changes across unconformities. Beneath regional unconformity reflectors, we look for possible zones of overcompaction in anomalous changes of P-wave, density and Poisson ratio values. Unique offset-dependent reflectivity attributes may aid in the glacial unconformity interpretation of seismic reflectors.
C43A-09 1330h
Morphological Evolution of Marguerite Bay, Antarctic Peninsula Continental Shelf
The Antarctic Peninsula Pacific margin continental shelf has a distinct trough and bank morphology which was formed in association with discrete zones of fast flowing ice, or ice streams . Rebesco et al. (1998) propose that sediment by-passed the continental shelf at the mouth of ice streams during major advances of grounded ice to the shelf edge during the Quaternary. They argue that prograding upper slopes were constructed on those portions of the shelf margin between major ice streams. The subsurface stratigraphy in the Marguerite Bay area suggests that the continental shelf may have evolved differently than the Rebesco et al. (1998) conceptual model suggests. Seismic data acquired along strike of the Antarctic Peninsula outer shelf shows that deeply buried trough-mouth fans are offset with respect to the modern positions of seafloor troughs. In contrast, trough-mouth fans are absent from the overlying aggradational section. Data from ODP Site 1097 at Marguerite Trough samples massive diamictite within that part of the hole corresponding to the trough-mouth fan and overlying aggradational section. Eyles et al. (2001) interpret these as sub-glacial deposits. Thus, the modern shelf-edge/upper slope morphology is inherited from an earlier time.
C43A-10 1330h
Seafloor Soundings in Polar and Remote Regions -- A new instrument for unattended bathymetric observations
The Seafloor Sounding in Polar and Remote Regions (SSPARR) project, under sponsorship of the National Science Foundation, is developing the capability to acquire unattended bathymetric observations in remote regions, by means of an inexpensive (expendable) depth sounder supported by a GPS navigation receiver and satellite telemetry capability. The SSPARR depth sounder will be a single beam, 12 kHz depth sounder employing a transducer with a hemispherical beam pattern. The program is developing a battery-powered buoy which houses the sounder, navigation receiver, data acquisition system, radiotelemetry system, and battery power sufficient for about three years of operation. Alternatively the depth sounder, by itself, can be incorporated into other multi-instrument moorings or buoys. During 1994, prototypes of the instruments comprising the SSPARR buoy were fabricated and tested independently, confirming the predicted performance of the depth sounder. Second generation sounder electronics have been designed and fabricated, and are planned for further testing in early 2005. Prototype buoys are planned for completion and deployment in the Arctic Ocean in the latter half of 2005. Results of testing to date will be presented. Following evaluation of the laboratory-built prototype buoys, in 2006 we plan commercial acquisition of buoys, to confirm that our specifications are adequate for production. During the International Polar Year, 2007-2008, we expect SSPARR to be mature enough for transition to an operational capability, comprising a large number of buoys deployed in the Arctic, the Southern Ocean, and the south Pacific and Indian Oceans; and a shore site where data will be checked for quality, archived, and posted to a publicly available web site. SSPARR is expected to provide significant new observational data for incorporation into such international mapping efforts as the International Bathymetric Chart of the Arctic Ocean (IBCAO) and the International Bathymetric Chart of the Southern Ocean (IBCSO). SSPARR is a collaboration between Robert Anderson of Science Applications International Corporation; Mark Rognstad of the Hawaii Mapping Research Group, University of Hawaii; Dale Chayes and Val Schmidt of Lamont-Doherty Earth Observatory of Columbia University; and Larry Mayer of the Center for Coastal and Ocean Mapping, University of New Hampshire.
C43A-11 1330h
Radiative Energy Disposition in the Coupled Atmosphere-Snow-Ice-Ocean System
The radiative interaction between the atmosphere and the surface is a factor of paramount importance in the Arctic where the net radiation is the largest component of the surface energy budget. The effect of clouds on the surface energy budget has important implications for the extension of the polar ice cover, which is highly sensitive to the surface irradiance. Also, the cloud cover is very important for the primary production, since it regulates the amount of light available for photosynthesis. Solar radiation affects snow metamorphism, which reduces the surface albedo, leading to further metamorphism (grain growth), increased snow temperature and reduction in snow and ice thicknesses (surface albedo feedback mechanism). Changes in the thickness of snow and sea ice will change the optical properties of the snow and ice system and hence cause a change in the amount of UV radiation and Photosynthetically Available Radiation (PAR) reaching the aquatic environment. Furthermore, a potential increase in ice temperature will change its transparency due to melting and enlargement of brine pockets. Accurate modelling of irradiances in snow and ice requires sophisticated methods. It is very important to take into account the tight radiative coupling between the atmosphere and the snow-sea ice-ocean system, and also the change in refractive index that occurs at the interface between the atmosphere/snow and sea ice/ocean. Such coupling effects may cause the downward irradiance to increase by 75% just beneath the air-ice interface, and the enhancement is much larger under clear sky than under cloudy conditions. A combination of field measurements, state-of-the-art radiative transfer modelling, and satellite data is required to enhance our understanding of the disposition of solar energy in this coupled system. The purpose of this presentation is to outline studies including field measurements and modeling required to quantify cloud-radiation-surface interactions and feedbacks, so as to enhance our understanding of the solar energy disposition in this coupled system, and how it affects climate evolution, and primary production in the polar regions, and the Arctic in particular.
C43A-12 1330h
Multifractal Analysis of Greenland Ice Core Data
Recent deep drilling of Greenland ice sheet (European GRIP and American GISP2 projects) yielded new rich information about climate of past 140.000 years. The most detailed data were provided by the electrical conductivity measurements (ECM) and dielectric profiling (DEP). It is known that during warm (postglacial) period both direct current conductivity, measured in ECM, and high frequency conductivity, measured in DEP, responds only to the acidity of atmospheric precipitation forming ice cover. Available data rows of electrical properties measurements exceed 100.000 points. Besides obvious large-scale variations, there is a high-frequency component in the rows, usually considered just as a noise. Meanwhile, the amplitude of this component is sufficiently smaller than the instrumental noise level and it contains useful information we lose when smoothing and averaging data rows. It can be, however, extracted with the use of statistical methods. One of the ways to explore the "thin structure" of climatic rows is treating them in the frameworks of multifractal approach. There have been considered the rows of ECM from the hole GISP2 (down to the depth of 1370 m) and DEP from the hole GRIP (down to the depth of 1320 m), corresponding to the time interval of 8000 years -- approximately from the moment of ice cover melting in Europe to the present time. There was no any prominent climatic event in this period; the mean values and dispersions of the rows (after trend removal) were constant. The considered rows were first transformed by the interpolation to the sets of equidistant values with the time step of one month. The analysis has shown that they can be considered as functions close to fractional Brownian noise. The investigation of scaling properties allowed us to choose time interval from 10 to 80 years for the calculation of multifractal spectra f(H). The "running window" of 160 years width with the step of 8 years was applied to the rows. The changes of the position of the ultimate points (Hmin and Hmax) of f(H) spectra were followed. Obtained sequences of Hmin and Hmax were smoothed by the running average for the presentation of the results. The investigation has shown that the rightmost limit of multifractal spectra (Hmax) is changing independently for two considered data rows and apparently reflects the statistics of instrumental errors of the measurements. At the same time the changes of the leftmost limit of multifractal spectrum (Hmin) for the ECM row correlate with the other ones for the DEP row. The comparison with the results of reconstruction of Holocene temperatures from the pollen spectra [Andreev, Klimanov, 2000] has demonstrated that low values of Hmin correspond to the cold periods, when the variability of the curves of electrical properties rises. During warm periods, on the contrary, the values of Hmin increase and the rows become smoother.
C43A-13 1330h
Determination of Delta 34S and S Concentration in Environmental Samples by Multi-Detector-Thermal Ionization Mass Spectrometry (MC-TIMS) Using a 33S-36S Double Spike
The variability of sulfur isotopes, caused by mass fractionation during biogeochemical processing, is commonly used for tracing the various sources of sulfur and for understanding the sulfur cycle. Snow and ice preserve a continuous chronological record of the sources, sinks, and geochemical processing of sulfur through time. The ability to decipher this record has been limited by the analytical capability, which requires > 1 Îœmole of S for precise isotopic measurements by gas source mass spectrometers. A new analytical technique that is both highly accurate and precise has been developed for the simultaneous determination of sulfur isotope composition and concentration of low concentration samples based on production of AsS+ ions by thermal ionization using silica gel as an emitter. The technique combines multi-collector thermal ionization mass spectrometry (TIMS) with a 33S/36S internal standard and has been applied to three international sulfur standards (IAEA-S-1, IAEA-S-2, and IAEA-S-3) and snow and firn samples from the Inilchek Glacier, Kyrgyzstan and Summit, Greenland. Using a well characterized 33S/36S double spike, calibrated relative to the internationally accepted consensus absolute value for the standard IAEA-S-1 (Δ34S = -0.3 ‰), a fractionation factor (α) can be determined that corrects for the instrumental fractionation (changing ratio) inherent to the TIMS technique. This correction yields the absolute true 32S/34S ratio and the Δ34S values relative to the VCDT scale. The 33S/36S double spike also allows small concentration samples to be measured for both concentration and isotope composition, because it acts as a chemical carrier by adding to the total sulfur mass in the sample. The Δ34S values determined (reported relative to VCDT (Δ34S = -0.3 ‰) were -0.32 ‰ ± 0.04 ‰ (1s, n = 4), 22.65 ‰ ± 0.04 ‰ (1s, n = 7), and -32.47 ‰ ± 0.07 ‰ (1s, n = 8) for IAEA-S-1, IAEA-S-2, and IAEA-S-3, respectively. The amount of natural sample used for these analyses ranged from 0.39 to 1.98 Îœmoles, with precisions on the S concentration measurements being typically better then 0.2 % (rsd). The uncertainties reported for sulfur isotopic composition of these standards are comparable to or better then those obtained by isotope ratio mass spectrometers (IRMS). The Δ34S values for the Inilchek Glacier samples ranged from 2.3 to 7.6 ‰ with uncertainties of 0.10 to 0.35 ‰ (1s) on sample sizes ranging from 0.1 to 1.8 Îœmoles. The uncertainties on the S concentration measurements ranged from 0.35 to 2.24% at the 95% confidence level. The Δ34S values for the Greenland samples ranged from 3.5 to 11.4 ‰ with uncertainties of 0.62 to 0.70 ‰ (1s) on sample sizes ranging from 0.04 to 0.29 Îœ moles. The uncertainties on the S concentration averaged 13% at the 95% confidence level. For both the Inilchek and Summit samples the uncertainties are dominated by blank corrections and not by measurement uncertainty. The smaller sample requirements for this technique may make it possible to access to the higher-resolution sulfur isotope record of snow and ice.