Developing water-isotope records from stratigraphically disturbed blue ice: observations, limitations, and insights from the Allan Hills of Antarctica

Abstract

Polar ice cores contain direct archives of past atmosphere and past precipitation, and water-isotope measurements of polar ice-core samples are used to reconstruct Earth's past climate. Ice cores from stable domes or ice divides -- where post-depositional ice flow results primarily from compression of more recently accumulated snowfall -- have produced continuous climate records as old as 800 ka. Climate records from older ice cores are of interest due to a notable change in Earth's climatic fluctuations around 1 Ma. Extending the ice-core paleoclimate record beyond 1 Ma likely requires measurements from more complicated depositional environments, such as from blue ice areas at the margins at the Antarctic ice sheet. The Allan Hills blue-ice area contains ice as old as 6 Ma at relatively shallow depths (<200 m below the surface), but cores from this region are typically discontinuous and stratigraphically disturbed. In addition, they have likely experienced orders of magnitude more thinning than more typical ice cores, severely limiting their temporal resolution. The development and interpretation of ice-core climate records from blue-ice areas is still in nascent stages, and is only possible due to the development of an absolute dating technique by the analysis of atmospheric 40-Ar (Bender et al., 2008). Age constraints at discrete depths along the core provide information about the average age of the ice, but significant uncertainty remains about the amount of time represented by each measurement. Here, I present new water-isotope records from discontinuous blue-ice cores from the Allan Hills blue ice area of Antarctica, and explore the implications and limitations of those data. This thesis uses high-resolution water-isotope data to explore the integrated history of ice recovered from the Allan Hills. It (1) details advancements in high-resolution water-isotope measurement methodology by continuous-flow analysis, which is the preferred method for analyzing many Allan Hills cores, (2) advances our understanding of water-isotope signal preservation and alteration in blue ice by examining the reproducibility of the water-isotope record at lateral distances of 0.22 to 140 m, (3) demonstrates that glacial-interglacial signals are preserved in some disturbed Allan Hills ice and provides insights into ice accumulation and climate history in this area, and (4) explores differences in water-isotope distillation pathways caused by atmosphere and ocean changes between the present-day and a warmer (i.e. 6-Ma) world.