Abstract If it were possible to properly extract seasonal information from ice-core isotopic records, paleoclimate researchers could retrieve a wealth of new information concerning the nature of climate changes and the meaning of trends observed in ice-core proxy records. It is widely recognized, however, that the diffusional smoothing of the seasonal record makes a “proper extraction" very difficult. In this paper, we examine the extent to which seasonal information (specifically the amplitude and shape of the seasonal cycle) is irrecoverably destroyed by diffusion in the firn. First, we show that isotopic diffusion firn is reasonably well understood. We do this by showing that a slightly modified version of the Whillans and Grootes (1985) theory makes a tenable a priori prediction of the decay of seasonal isotopic amplitudes with depth at the GISP2 site, though a small adjustment to one parameter significantly improves the prediction. Further, we calculate the amplitude decay at various other ice-core sites and show that these predictions compare favorably with published data from South Pole and locations in southern and central Greenland and the Antarctic Peninsula. We then present numerical experiments wherein synthetic ice-core records are created, diffused, sampled, reconstituted and compared to the original. These show that, alter diffusive mixing in the entire fini column, seasonal amplitudes can be reconstructed to within about 20% error in central Greenland but that all information about sub-annual signals is permanently lost there. Furthermore, most of the error in the amplitude reconstructions is due to the unknowable variations in the sub-annual signal. Finally, we explore how these results can be applied to other locations and suggest that Dye 3 has a high potential for meaningful seasonal reconstructions, while Siple Dome has no potential at all. An optimal ice-core site for seasonal reconstructions has a high accumulation rate and a low temperature.
Abstract If it were possible to properly extract seasonal information from ice-core isotopic records, paleoclimate researchers could retrieve a wealth of new information concerning the nature of climate changes and the meaning of trends observed in ice-core proxy records. It is widely recognized, however, that the diffusional smoothing of the seasonal record makes a “proper extraction" very difficult. In this paper, we examine the extent to which seasonal information (specifically the amplitude and shape of the seasonal cycle) is irrecoverably destroyed by diffusion in the firn. First, we show that isotopic diffusion firn is reasonably well understood. We do this by showing that a slightly modified version of the Whillans and Grootes (1985) theory makes a tenable a priori prediction of the decay of seasonal isotopic amplitudes with depth at the GISP2 site, though a small adjustment to one parameter significantly improves the prediction. Further, we calculate the amplitude decay at various other ice-core sites and show that these predictions compare favorably with published data from South Pole and locations in southern and central Greenland and the Antarctic Peninsula. We then present numerical experiments wherein synthetic ice-core records are created, diffused, sampled, reconstituted and compared to the original. These show that, alter diffusive mixing in the entire fini column, seasonal amplitudes can be reconstructed to within about 20% error in central Greenland but that all information about sub-annual signals is permanently lost there. Furthermore, most of the error in the amplitude reconstructions is due to the unknowable variations in the sub-annual signal. Finally, we explore how these results can be applied to other locations and suggest that Dye 3 has a high potential for meaningful seasonal reconstructions, while Siple Dome has no potential at all. An optimal ice-core site for seasonal reconstructions has a high accumulation rate and a low temperature.
Analysis of borehole temperature and Greenland Ice Sheet Project II ice-core isotopic composition reveals that the warming from average glacial conditions to the Holocene in central Greenland was large, approximately 15°C. This is at least three times the coincident temperature change in the tropics and mid-latitudes. The coldest periods of the last glacial were probably 21°C colder than at present over the Greenland ice sheet.
Paleoclimatologists face a dilemma. No sedimentary proxy is a pure recorder of quantitative climate information. Yet climate modelers and policy-makers increasingly seek quantitative comparisons between instrumentally documented, possibly anthropogenic, climate changes and those produced naturally in the past.
In the second edition of his influential book, Bradley (1999, p. 6) discussed the calibration of proxy records to learn past climate changes: “Calibration involves using modern climatic records and proxy materials to understand how, and to what extent, proxy materials are climate-dependent. It is assumed that the modern relationships observed have operated, unchanged, throughout the period of interest (the principle of uniformitarianism).” In other words, one relates the characteristics of sediment to climate at different places for one time, or at a place for short times, and then uses that relation plus characteristics of older sediments to estimate the climatic conditions that produced those sedimentary characteristics. Bradley (1999) then extensively discussed the difficulties in applying this methodology in a complex world with imperfect recorders; nonetheless, the goal of using calibrated proxies for quantitative as well as qualitative paleoclimatic reconstruction is clear.
A prominent recent use of calibrated paleoclimatic data is the assessment of whether the probably-anthropogenic warming of the latter 20th century moved beyond the band of natural variability of the prevailing climate. Bradley (2000) combined recent instrumental records with several longer proxy-based reconstructions of surface temperature including that of Mann et al. (1999). Based on this composite data set, Bradley (2000) argued that “temperatures in the late 20th century were unique in the context of the entire millennium”. The proxy records were primarily based on tree-ring data, but included isotopic and major-element geochemistry of corals, and occurrence of melt layers and isotopic ratios of water in ice cores. However, Broecker (2001) questioned the basis for the reconstruction of …
Alpine cirques are excavated by glacial erosion, a process that depends in turn on the movement of ice by basal sliding. Cirque glacier flow is usually depicted as rotational sliding of a rigid block, but this model is based on little evidence and implies unorthodox glacier behavior given typical cirque dimensions. The small (∼1 km^2^), temperate West Washmawapta Glacier occupies an archetypal overdeepened and "armchair-shaped" cirque in the Canadian Rockies. We measured (1) the annual surface velocity field, (2) ice thickness, (3) sliding and internal deformation at one borehole, and (4) sliding in a marginal cavity. The glacier moves slowly, with surface velocities of 3 to 10 m/yr. The maximum ice thickness (∼185 m) occurs in the center of the cirque basin and roughly coincides with the position of greatest ice flux. Using our field measurements, a standard constitutive relation for ice, and simplifying assumptions related to the depth distribution of strain rates, we approximated the driving and resisting forces acting on sections of the glacier, and inferred the general pattern of basal sliding. Sliding is minimum in the center of the cirque and increases toward the margins, especially up the stoss side of the riegel. Internal deformation accounts for all motion in the cirque center, even if an unusually low viscosity for temperate ice is assumed. Basal shear stresses tend toward 10^5^ Pa everywhere, a typical value for mountain glaciers. Transverse and longitudinal straining are significant in some parts of the glacier. Although a component of rotational flow must occur internally, the glacier does not conform to the rotational sliding model in any essential respect.
Abstract. Diffusive smoothing of signals on the water stable isotopes (18O and D) in ice sheets fundamentally limits the climatic information retrievable from these ice-core proxies. Past theories explained how, in polycrystalline ice below the firn, fast diffusion in the network of intergranular water veins “short-circuits” the slow diffusion within crystal grains to cause “excess diffusion”, enhancing the rate of signal smoothing above that implied by self-diffusion in ice monocrystals. But the controls of excess diffusion are far from fully understood. Here, modelling shows that water flow in the veins amplifies excess diffusion, by altering the three-dimensional field of isotope concentration and isotope transfer between veins and grains. The rate of signal smoothing depends not only on temperature, vein and grain sizes, and signal wavelength, but also on vein-water flow velocity, which can increase the rate by 1 to 2 orders of magnitude. This modulation can significantly impact signal smoothing at ice-core sites in Greenland and Antarctica, as shown by simulations for the GRIP and EPICA Dome C sites, which reveal sensitive modulation of their diffusion-length profiles when vein-flow velocities reach ~ 101–102 m yr–1. Velocities of this magnitude also produce the levels of excess diffusion inferred by previous studies for the Holocene ice at GRIP and ice of Marine Isotope Stage 19 at EPICA Dome C. Thus, vein-flow mediated excess diffusion may help explain the mismatch between modelled and spectrally-derived diffusion lengths in other ice cores. We also show that excess diffusion biases the spectral estimation of diffusion lengths from isotopic signals (by making them dependent on signal wavelength) and the reconstruction of surface temperature from diffusion-length profiles (by increasing the ice contribution to diffusion length below the firn). Our findings caution against using the monocrystal isotopic diffusivity to represent the bulk-ice diffusivity. The need to predict the pattern of excess diffusion in ice cores calls for systematic study of isotope records for its occurrence and improved understanding of vein-scale hydrology in ice sheets.
Radiocarbon measurements at ice margin sites and blue ice areas can potentially be used for ice dating, ablation rate estimates and paleoclimatic reconstructions. Part of the measured signal comes from in situ cosmogenic 14 C production in ice, and this component must be well understood before useful information can be extracted from 14 C data. We combine cosmic ray scaling and production estimates with a two‐dimensional ice flow line model to study cosmogenic 14 C production at Taylor Glacier, Antarctica. We find (1) that 14 C production through thermal neutron capture by nitrogen in air bubbles is negligible; (2) that including ice flow patterns caused by basal topography can lead to a surface 14 C activity that differs by up to 25% from the activity calculated using an ablation‐only approximation, which is used in all prior work; and (3) that at high ablation margin sites, solar modulation of the cosmic ray flux may change the strength of the dominant spallogenic production by up to 10%. As part of this effort we model two‐dimensional ice flow along the central flow line of Taylor Glacier. We present two methods for parameterizing vertical strain rates, and assess which method is more reliable for Taylor Glacier. Finally, we present a sensitivity study from which we conclude that uncertainties in published cosmogenic production rates are the largest source of potential error. The results presented here can inform ongoing and future 14 C and ice flow studies at ice margin sites, including important paleoclimatic applications such as the reconstruction of paleoatmospheric 14 C content of methane.
