Polar ice structure and the integrity of ice-core paleoclimate records

Abstract

Polar ice is a unique archive of the climatic conditions in the past. However, ice sheets flow, and this flow may affect the integrity of paleoclimate records. A useful method to analyze the effect of ice flow upon climate records is the combination of microstructure mapping with ice-core line-scanning. Microstructure and stratigraphy have been mapped along the entire EPICA-DML ice core with this combined method. On the macroscale the stratigraphy seems perfectly preserved down to ca 1700 m depth (MIS4), below which minor undulations start to develop. Layers inclined up to 15° and millimeter-scale z-folds are observed below 2050 m depth. Notwithstanding, the EPICA-DML climate record appears not seriously disturbed down to ca 2400 m depth, which marks the climatic transition from the last interglacial (MIS5e) to the MIS6 glacial period. Below this depth the synchronization with the EPICA-Dome C record is lost, and stratigraphic disturbances appear up to the meter scale. On the microscopic scale, we observe dynamic recrystallization already in deep firn, leading to substantial microstructural changes prior to bubble close-off. The concentration of visible micro-inclusions in the Holocene part of the core seems to increase with depth, which could possibly indicate post-depositional formation of salts. In glacial period ice the concentration of visible micro-inclusions in certain layers is so high that these strata appear as light-scattering bands, often called “cloudy bands”. Another interesting stratigraphic feature is what we call “bubble-free bands”, viz. millimeter-thick strata deprived of bubbles and rich in clathrate hydrates, which are found within the bubble–hydrate transition zone (800–1200 m depth) and are probably caused by anomalously fast clathration. While there is hardly any interaction between visible micro-inclusions and grain boundaries down to 2500 m depth, in the deeper ice (warmer than −10 °C) many grain boundaries seem to harvest micro-inclusions. Here we discuss these observations and their relevance for the interpretation of climate records.