Anthony J. Gow

The Greenland Ice Sheet Project 2 Depth-age Scale: Methods and Results

The Greenland Ice Sheet Project 2 (GISP2) depth-age scale is presented based on a multiparameter continuous count approach, to a depth of 2800 m, using a systematic combination of parameters that have never been used to this extent before. The ice at 2800 m is dated at 110,000 years B.P. with an estimated error ranging from 1 to 10% in the top 2500 m of the core and averaging 20% between 2500 and 2800 m. Parameters used to date the core include visual stratigraphy, oxygen isotopic ratios of the ice, electrical conductivity measurements, laser-light scattering from dust, volcanic signals, and major ion chemistry. GISP2 ages for major climatic events agree with independent ages based on varve chronologies, calibrated radiocarbon dates, and other techniques within the combined uncertainties. Good agreement also is obtained with Greenland Ice Core Project ice core dates and with the SPECMAP marine timescale after correlation through the δ 18 O of O 2 . Although the core is deformed below 2800 m and the continuity of the record is unclear, we attempted to date this section of the core on the basis of the laser-light scattering of dust in the ice.

Record of Volcanism Since 7000 B.C. from the GISP2 Greenland Ice Core and Implications for the Volcano-Climate System.

Sulfate concentrations from continuous biyearly sampling of the GISP2 Greenland ice core provide a record of potential climate-forcing volcanism since 7000 B.C. Although 85 percent of the events recorded over the last 2000 years were matched to documented volcanic eruptions, only about 30 percent of the events from 1 to 7000 B.C. were matched to such events. Several historic eruptions may have been greater sulfur producers than previously thought. There are three times as many events from 5000 to 7000 B.C. as over the last two millennia with sulfate deposition equal to or up to five times that of the largest known historical eruptions. This increased volcanism in the early Holocene may have contributed to climatic cooling.

Changes in Atmospheric Circulation and Ocean Ice Cover over the North Atlantic During the Last 41,000 Years

High-resolution, continuous multivariate chemical records from a central Greenland ice core provide a sensitive measure of climate change and chemical composition of the atmosphere over the last 41,000 years. These chemical series reveal a record of change in the relative size and intensity of the circulation system that transported air masses to Greenland [defined here as the polar circulation index (PCI)] and in the extent of ocean ice cover. Massive iceberg discharge events previously defined from the marine record are correlated with notable expansions of ocean ice cover and increases in PCI. During stadials without discharge events, ocean ice cover appears to reach some common maximum level. The massive aerosol loadings and dramatic variations in ocean ice cover documented in ice cores should be included in climate modeling.

Effect of Freezing and Thawing on the Permeability and Structure of Soils

Chamberlain, E.J. and Gow, A.J., 1979. Effect of freezing and thawing on the permeability and structure of soils. Eng. Geol., 13: 73–92. The permeability and structure of four fine-grained soils were observed to be changed by freezing and thawing. In all cases freezing and thawing caused a reduction in void ratio and an increase in vertical permeability. The increase in permeability is attributed to the formation of polygonal shrinkage cracks and/or to the reduction of the volume of fines in the pores of the coarse fraction, the mechanism controlling the process depending on material type. No definite relationships are established; however, it appears that the largest increase in permeability occurs for the soil of highest plasticity.

Visual‐stratigraphic dating of the GISP2 ice core: Basis, reproducibility, and application

Annual layers are visible in the Greenland Ice Sheet Project 2 ice core from central Greenland, allowing rapid dating of the core. Changes in bubble and grain structure caused by near-surface, primarily summertime formation of hoar complexes provide the main visible annual marker in the Holocene, and changes in “cloudiness” of the ice correlated with dustiness mark Wisconsinan annual cycles; both markers are evident and have been intercalibrated in early Holocene ice. Layer counts are reproducible between different workers and for one worker at different times, with 1% error over century-length times in the Holocene. Reproducibility is typically 5% in Wisconsinan ice-age ice and decreases with increasing age and depth. Cumulative ages from visible stratigraphy are not significantly different from independent ages of prominent events for ice older than the historical record and younger than approximately 50,000 years. Visible observations are not greatly degraded by “brittle ice” or many other core-quality problems, allowing construction of long, consistently sampled time series. High accuracy requires careful study of the core by dedicated observers.

The in-situ dielectric constant of polar firn revisited

Abstract The success in using VHF and UHF frequency systems for sounding polar ice sheets has been tempered by an uncertainty in the in-situ dielectric constant which controls the effective velocity of an electromagnetic wave propagating in an air-ice mixture. An empirical equation for determining the relative real dielectric constant ϵ′r vs. density (specific gravity ϱ) of firn or ice was proposed in 1969 by Robin et al. where ϵ′r = (1 + 0.851 ϱ)2. However, this expression has met with uncertainty because wide-angle radar refraction sounding techniques have produced dielectric constant values that are lower than Robins equation predicts. This paper discusses radar soundings made on the McMurdo Ice Shelf, Antarctica, and compares the resulting dielectric constant determinations with Robins equation, laboratory measurements on firn and ice and other expressions given in the literature for determining ϵ′r vs. the specific gravity of dry firn and ice. Our findings indicate that the form of Robins equation is valid. Our analysis also indicates the expression could be slightly improved to read ϵ′r = (1+0.845ϱ)2. Reasons are suggested as to why previous wide-angle radar sounding studies did not reproduce Robins findings.

Antarctic Ice Sheet: Stable Isotope Analyses of Byrd Station Cores and Interhemispheric Climatic Implications

Oxygen- and hydrogen-isotope analyses from the core hole through the Antarctic Ice Sheet at Byrd Station define temperature variations over more than 75,000 years. Synchronism between major climatic changes in Antarctica and the Northern Hemisphere is strongly indicated. The Wisconsin cold interval extended from 75,000 to 11,000 years ago. Three intra-Wisconsin warmer phases were all colder than pre- or post-Wisconsin times, which suggests that North American and Eurasian continental ice sheets did not disappear at any time during the Wisconsin.

Physical and structural properties of the Greenland Ice Sheet Project 2 ice core: A review

Substantial data sets have been collected on the relaxation characteristics, density, grain size, c axis fabrics, and ultrasonic velocities of the Greenland Ice Sheet Project 2 (GISP2) core to its contact with bedrock at 3053.4 m. Changes in all these properties paralleled closely those found in cores from Byrd Station, Antarctica, and Dye 3, Greenland. Density increased progressively with depth to a maximum of 0.921 Mg/m3 at about 1400 m, at which depth the ice became bubble free. Below about 2000 m, in situ densities began to decrease in response to increasing ice sheet temperatures. Since drilling, much of the ice core has undergone significant volume expansion (relaxation) due to microcracking and the exsolving of enclathratized gases, especially in the brittle ice zone between 650 and 1400 m. Grain size increased linearly to about 1000 m, thereafter remaining fairly constant until the Younger Dryas event at 1678 m where a twofold to threefold decrease in grain size occurred. These grain size changes were accompanied by a progressive clustering of crystal c axes toward the vertical, including a small increase in c axis concentration across the Younger Dryas/Holocene boundary. Increased dust levels in the Wisconsin ice have contributed to the maintenance of a fine-grained texture which, with its strong vertical c axis fabric, persisted to nearly 3000 m. However, beginning at about 2800 m, layers of coarse-grained ice intermixed with the much finer-grained matrix ice are observed. Below 3000 m the ice became very coarse grained. This change, attributed to annealing recrystallization at elevated temperatures in the ice sheet, was accompanied by a dispersed or ring-like redistribution of the c axes about the vertical. Ultrasonic measurements of vertical and horizontal P wave velocities made at 10-m intervals along the entire length of the GISP2 core fully confirmed the results of the crystallo-optical observations. A return to fine-grained ice coincided with the first appearance of brown, silty ice 13 m above bedrock. Bedrock material consisted of 48 cm of till, including boulders and cobbles, overlying gray biotite granite comprising the true bedrock. There is evidence that disturbed structure in the GISP2 cores begins little more than 70% of the way through the ice sheet. This disturbance increases with depth until it becomes large enough to cast suspicion on features lasting centuries or more in the bottom 10% of the ice sheet.

Volcanic ash in the Antarctic ice sheet and its possible climatic implications

Abstract Approximately 2000 individual ash falls are preserved in deep cores from Antarctica. The bulk of the debris is composed of dust-sized particles of glass that can probably be attributed to volcanic sources in Antarctica, though sources outside Antarctica cannot be entirely discounted. A period of sustained infall of ash occurred during the interval 30 000 to 16 000 years ago, and isotopic (paleotemperature) data from the same cores indicate that a significant cooling of the atmosphere over Antarctica occurred at the same time. This cooling trend did not terminate until deposition of ash had virtually ceased, suggesting possibly a cause and effect relationship involving the solar-depleting effect of volcanic dust in the Antarctic stratosphere. It is conceivable that widespread eruption of volcanic ash in Antarctica during the latter part of the Wisconsin may also have triggered world-wide cooling during this period — effectively intensifying the existing glacial regime.

A quantitative description of sea ice inclusions

Structurally, sea ice consists of an ice matrix with inclusions of brine and air. A quantitative description of the size of these inclusions is critical for both interpreting and modeling the electromagnetic properties of sea ice. Photomicrographs of ice thin sections were analyzed using a personal computer-based image-processing system to determine the number of inclusions, the inclusion size distributions, and statistics for brine pockets in young ice and first-year ice and for air bubbles in a multiyear hummock. Inclusions ranging in size from thousandths of square millimeters to a few square millimeters were measured. In all cases a two-parameter lognormal distribution fits the cumulative inclusion size distributions well (correlation coefficient greater than 0.99). This includes brine pockets in both granular and columnar ice and a range of brine volumes from 2% to 40%. As ice warms, and its brine volume increases, the size distribution shifts toward larger brine pockets. This increase in brine pocket size is particularly pronounced for brine volumes greater than 10% as individual brine pockets coalesce. Air bubbles are much larger than brine pockets, with mean major axis lengths of the order of millimeters for air bubbles and tenths of a millimeter for brine pockets. Observations of inclusion shape factors indicate that, in general, brine pockets are more elongated than air bubbles.