CO(2) Diffusion in Polar Ice: Observations from Naturally Formed CO(2) Spikes in the Siple Dome (Antarctica) Ice Core

One common assumption in interpreting ice-core CO(2) records is that diffusion in the ice does not affect the concentration profile. However, this assumption remains untested because the extremely small CO(2) diffusion coefficient in ice has not been accurately determined in the laboratory. In this study we take advantage of high levels of CO(2) associated with refrozen layers in an ice core from Siple Dome, Antarctica, to study CO(2) diffusion rates. We use noble gases (Xe/Ar and Kr/Ar), electrical conductivity and Ca(2+) ion concentrations to show that substantial CO(2) diffusion may occur in ice on timescales of thousands of years. We estimate the permeation coefficient for CO(2) in ice is similar to 4 x 10(-21) mol m(-1) s(-1) Pa(-1) at -23 degrees C in the top 287 m (corresponding to 2.74 kyr). Smoothing of the CO(2) record by diffusion at this depth/age is one or two orders of magnitude smaller than the smoothing in the firn. However, simulations for depths of similar to 930-950m (similar to 60-70 kyr) indicate that smoothing of the CO(2) record by diffusion in deep ice is comparable to smoothing in the firn. Other types of diffusion (e.g. via liquid in ice grain boundaries or veins) may also be important but their influence has not been quantified.

Catastrophic Ice Shelf Breakup as the Source of Heinrich Event Icebergs

Heinrich layers of the glacial North Atlantic record abrupt widespread iceberg rafting of detrital carbonate and other lithic material at the extreme-cold culminations of Bond climate cycles. Both internal (glaciologic) and external ( climate) forcings have been proposed. Here we suggest an explanation for the iceberg release that encompasses external climate forcing on the basis of a new glaciological process recently witnessed along the Antarctic Peninsula: rapid disintegrations of fringing ice shelves induced by climate-controlled meltwater infilling of surface crevasses. We postulate that peripheral ice shelves, formed along the eastern Canadian seaboard during extreme cold conditions, would be vulnerable to sudden climate-driven disintegration during any climate amelioration. Ice shelf disintegration then would be the source of Heinrich event icebergs.

A Numerical Investigation of the Gulf Stream and Its Meanders in Response to Cold Air Outbreaks

The three-dimensional Princeton Ocean Model is used to examine the modification of the Gulf Stream and its meanders by cold air outbreaks. Two types of Gulf Stream meanders are found in the model. Meanders on the shoreward side of the Gulf Stream are baroclinically unstable. They are affected little by the atmospheric forcing because their energy source is stored at the permanent thermocline, well below the influence of the surface forcing. Meanders on the seaward side of the stream are both barotropically and baroclinically unstable. The energy feeding these meanders is stored at the surface front separating the Gulf Stream and the Sargasso Seal which is greatly reduced in case of cold air outbreaks. Thus, meanders there reduce strength and also seem to slow their downstream propagation due to the southward Ekman flow. Heat budget calculations suggest two almost separable processes. The oceanic heal released to the atmosphere during these severe cooling episodes comes almost exclusively from the upper water column. Transport of heat by meanders from the Gulf Stream to the shelf, though it is large, does not disrupt the principal balance. It is balanced nicely with the net heat transport in the downstream direction.