John F. Bolzan

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.

Grain-scale processes, folding, and stratigraphic disturbance in the GISP2 ice core

Flow disturbances have been shown to alter stratigraphic order in the lower part of the ice sheet in central Greenland. Vertical thin sections of the Greenland Ice Sheet Project 2 ice core show that in the lower 30%, the expected c axis-vertical fabric is interrupted by planes of grains (“stripes”) with c axes oriented approximately in the dip directions of the planes. Stripe-parallel shear produces small-scale folds. The stripes can be explained qualitatively by a simple nucleation-and-growth model based on the strong anisotropy of ice-crystal deformation. Nucleation probably is sensitive to spatially variable rates of polygonization, producing spatially variable stripe densities. Stripes are modeled to affect the ice viscosity, so variations in stripe density may contribute to viscosity contrasts that might produce larger deformational features and loss of stratigraphic order.

Modeled Variations of Precipitation over the Greenland Ice Sheet

Abstract A parameterization of the synoptic activity at 500 hPa and a simple orographic scheme are used to model the spatial and temporal variations of precipitation over the Greenland Ice Sheet for 1963–88 from analyzed geopotential height fields produced by the National Meteorological Center (NMC). Model coefficients are fitted to observed accumulation data, primarily from the summit area of the ice sheet. All major spatial characteristics of the observed accumulation distribution are reproduced apart from the orographic accumulation maximum over the northwestern coastal slopes. The modeled time-averaged total precipitation amount over Greenland is within the range of values determined by other investigators from surface-based observations. A realistic degree of interannual variability in precipitation is also simulated. A downward trend in simulated ice sheet precipitation over the 26 years is found. This is supported by a number of lines of evidence. It matches the accumulation trends during this peri...

Polar Firn Densification and Grain Growth

A 50 m firn core from Dome C, East Antarctica, was found to consist of coarse firn, which comprised 90 to 95% of the core, and fine firn. Coarse firn was characterized by large crystals with a vertical shape orientation near the surface, connected to nearest neighbors by relatively large necks in a structure different from closest packing. Fine firn was of higher density and consisted of smaller, more spherical crystals connected by relatively narrow necks in a more nearly closest-packed configuration. Higher surface free energy in fine firn causes crystals and necks to grow more rapidly than in coarse firn. However, we find that coarse firn densifies more rapidly with time, contrary to the predictions of unconfined sintering models. Load-driven densification due to a power-law creep mechanism is found to account for the larger coarse-firn densification rate. However, if the exponent in the power law exceeds one, then densification rates are predicted to increase with depth due to increasing load, contrary to observed behavior. We speculate that different mechanisms may control the densification process in fine and coarse firn.

Potential for stratigraphic folding near ice-sheet centers

Lack of agreement between the deep portions of the Greenland Icecore Project (GRIP) and Greenland Ice Sheet Project II (GISP2) ice cores from central Greenland suggests that folds may disrupt annual layering, even near ice divides. We use a simple kinematic flow model to delineate regions where slope disturbances (wrinkles) introduced into the layering could overturn into recumbent folds, and where they would flatten, leaving the stratigraphic record intact. Wrinkles are likely to originate from flow disturbances caused internally by inhomogeneities and anisotropy in the ice rheological properties, rather than from residual surface structures (sastrugi), or from open folds associated with transient flow over bed topography. If wrinkles are preferentially created in anisotropic ice under divides, where the resolved shear stress in the easy-glide direction can be weak and variable, then the deep intact climate record at Dye 3 may result from its greater distance from the divide. Alternatively, the larger simple shear at Dye 3 may rapidly overturn wrinkles, so that they are not recognizable as folds. The ice-core record from Siple Dome may be intact over a greater fraction of its depth compared to the central Greenland records if its flat bedrock precludes fluctuations in the stress orientation near the divide.

A METHOD FOR COMPUTING SHALLOW ICE-CORE DEPTHS

SIPRE-style hand augers usually recover core sections whose bottom is somewhat shallower than the drill depth. A procedure for calculating the range of possible core depths is presented. In the field , the drill barrel is lowered down the hole by means of a series of rigid extension rods. The apparatus is then rotated to drill through debris left behind by the prior run and into fresh firn or ice. (Care is taken to ensure that the total penetration of debris and firn does not exceed about half the length of the drill barrel, otherwise the barrels capacity for core and chips will be exceeded and the drill likely jammed.) The entire apparatus is then recovered. The workers record the run number, the total drill length (barrel and extensions), the exposed length of extensions above the hole top at the end of drilling, and the length of recovered core. The cycle is then repeated. An example is shown in Table I. These data are then used to calculate drill depth, which is drill length minus the exposed length, plus an adjustment if drilling is from the base of a pit. Distance drilled is then obtained by comparison with the drill depth of the prior run. The apparent core loss is the distallce drilled minus the length of recovered core. The actual core loss is not readily calculated; it is the sum of true core loss (ground-up core and the collapse of friable strata) and the amount of core

Zweig's rule violation, SU(3) character of the decay of the new particles, and pole dominance picture☆

Abstract The decays ψ ′ → ψππ and ψ → ωϵ are studied in the framework of Zweig Rule violation via SU(4) pole dominance. The SU(3) character of the decay of the ψ is discussed in the same model. Using results of the purely hadronic decays, the cascade transition rates ψ ′ → ϵ c + γ and ϵ c ar ψ + γ are calculated and compared to recent experimental data.