Ice cores provide highly valuable paleoenvironmental data, but records can be degraded by the occurrence of fractured "brittle ice". Following bubble closure in ice, internal deformation squeezes bubbles, raising their internal air pressure to near the overburden level. Subsequent release of external pressure during core recovery can allow the internal stresses to exceed the ice strength, causing fracturing in what is known as the "brittle ice zone" (BIZ). The depth range and degree of brittleness varies greatly between ice cores. Site characteristics including accumulation rate and temperature likely affect brittle ice behavior, but strong correlations are not apparent. Despite prior research, the mechanisms controlling BIZs are still not well-understood.

Various ice-core observations, together with physical understanding, suggest that BIZ fractures preferentially follow subgrain boundaries, linking bubbles over long distances by way of an interconnected "chessboard" pattern of subgrains anchored to the bubbles. This study focused on thin section analyses from the highly brittle Siple Dome ice core, with comparisons to the minimally brittle South Pole ice core. Using targeted image processing techniques and analyses, we identified noteworthy relationships between fractures, subgrain boundaries, and bubbles.

This study has provided initial insights into brittle-ice behavior and its physical properties and establishes a connection between subgrain prevalence and mean grain/bubble sizes in ice cores, revealing that larger grains/bubbles correlate with more severe brittle behavior. In addition, estimates for the Hercules Dome site suggest a brittle-ice zone between 625-1200 m depth with a brittleness factor of ~2 out of 5 (information that will undoubtedly be of high value for the upcoming drilling campaign). Several new algorithms were also developed as part of this project in RStudio for calculating the relevant chessboard statistics that will be publicly shared. Lastly, a catalog of new physical properties data related to brittle-ice behavior and onset were assimilated into a singular (and searchable) database. This will allow for many additional future analyses.

More specifically, we found that that subgrain chessboards and identified "Saturn bubbles" correlate with mean grain size in ice and that these two features were also found to more likely exist together within parent grains, when also found in greater numbers. As such, for Siple Dome--the sample with the largest mean grain area and incidence of both "Saturn bubbles" and subgrain chessboards--58.5% of all grains contained either subgrain chessboards or "Saturn bubbles," but with both occurring together 94.3% of the time, and in 55.1% of the total observed grains.

The majority of observed bubble "Saturn cracks" in the highly-brittle Siple Dome(SDMA) thin section, were also revealed to preferentially aligned in a manner consistent with a bulk vertical c-axis fabric within the measured section. Maximum brittle-ice behavior is likely influenced greatly by a vertical alignment of grains, at depths favorable for subgrain chessboards formation where vertical compression dominates. And while c-axis measurements were unable to be directly measured in this study, the uniformity of individual grain colors observed through cross-polarizers, along with published fabrics for similar depths by others (e.g. Gow and Meese; 2007), suggest that the overwhelming majority of crystallographic basal planes in the SDMA sample do align horizontally.

One of the most useful scientific takeaways from this project was the development of a first-generation "ice brittleness" model that will allow future ice core project teams to estimate the expected brittleness for proposed coring sites (see Figure 1).

As a result of this project, first generation graduate student Samantha Barnett completed her MS thesis in Geology at Northern Arizona University (NAU). This thesis will be soon be submitted for publication in an appropriate peer-reviewed journal. As part of her research, Samantha spent several weeks working at the NSF Ice Core Facility in Lakewood, Colorado preparing multiple thin sections for analysis. During this time, she also was mentored by USGS glaciologist and ice fabric pioneer, Dr. Joan Fitzpatrick. While in the program, Samantha also presented her research at the American Geophysical Union Conference in Fall of 22 and 23, as well as the Open Ice Core Science Meeting hosted by University of Washington. While a student at NAU, Samantha also won 1st place poster presentation at the annual graduate seminar series. Lastly, Samantha also participated in the Ice Core Young Scientists group and was a contributing author to the PAGES article, "Putting the Time in Time Machine: Methods to Date Ice Cores." (https://doi.org/10.22498/pages.30.2.100). 

Last Modified: 03/08/2024
Modified by: John M Fegyveresi