Climate Snapshots and Gas Records from Antarctic Blue Ice Records: Implications for the Mid-Pleistocene Transition and the Last Interglacial

Abstract

This dissertation explores ice archives from Allan Hills Blue Ice Areas (BIAs), East Antarctica, where ice as old as 1 million years (BIT-58) was found and another ice core (S27) covering the time interval between 115 and 255 ka continuously was retrieved. Here, ice movement along the rising bedrock and surface ablation cause the ancient ice to move towards the surface.

First, we focus on two new Allan Hills ice cores (ALHIC1503 and ALHIC1502) drilled in 2015. By measuring the triple argon isotopes (40Aratm) in the trapped air, we established that the age the ice is substantially older than 1 Ma, with a single sample dating back to 2.7±0.3 Ma. These samples represent the oldest, debris free ice ever retrieved and provide the first direct observations of the concentration of carbon dioxide (CO2) as old as 2.0 Ma. CO2 samples older than 2.0 Ma are altered by respiration.

We evaluated the theories regarding the “Mid-Pleistocene Transition” (MPT) between 1.2 and 0.8 Ma. No long-term decline in atmospheric CO2 was found, thereby arguing against the hypothesis that the MPT results from a global cooling due to lowering CO2. The statistically significant correlation between CO2 and δDice (indicative of Antarctic temperature) confirm the CO2-climate coupling beyond the MPT.

Based on the elemental and isotopic analyses of the major gases (N2, O2, and Ar) in Allan Hills ice cores predating the MPT, we found no decline in O2 concentrations between 1.5 and 1.0 Ma. Furthermore, we discovered that in ice dating back to 1.5 Ma, δDice is negatively correlated with δO2/N2, a property that is inversely correlated with local summertime insolation. We argue that in the 40k world Antarctic temperature was directly forced by local insolation.

Next, a refined gas chronology of the shallow ice core S27 was constructed based on the isotopic composition of the trapped O2. A spike in the accumulation rate was found in ice dating around 128 ka, coincident with the peak Antarctic warming during MIS5e. This is interpreted as reflecting the rapid changes in glaciological conditions due to the retreat of Ross Ice Shelf grounding lines.