Quaternary stratigraphy and ice cores

摘要

Ice cores contain highly detailed continuous stratigraphic records extending from the present to 800 ka BP. A large variety of climate proxies, related to atmospheric and climate changes can be analysed and dated with an accuracy ranging from a few years, for recent centuries, to a few millennia for older records. Over the past two decades, the contribution of ice core studies to the reconstruction and interpretation of past climate changes became fundamental: they allowed the direct measurement of greenhouse gas changes in the past and the detection of rapid climatic changes. This paper presents a short summary of ice core results, restricted only to those relevant for the stratigraphy of the Quaternary.rnAt the base of the EPICA Dome C (DC) ice core the Matuyama-Bruhnes (M-B) magnetic reversal has been indirectly identified, at the end of Marine Isotopic Stage (MIS) 19, and dated 764-776 ± 6 ka BP, allowing the correlation with the M-B boundary observed in the deep-sea sediments. The details of the M-B transition observed in the EPICA DC ice core are of interest for an improved characterization of this time marker, which is crucial for the identification of the Lower-Middle Pleistocene boundary.rnA clear increase in the amplitude of the glacial/interglacial cycles is observed at 430 ka BP, at the MIS 12/11 transition in the EPICA DC climatic curves: this Mid-Bruhnes Event marks a change in the pattern of climatic changes. The Middle-Upper Pleistocene boundary at the base of the Eemian Stage is clearly detectable in the Antarctic ice cores as a sharp increase in the methane signal coinciding with the initial peak of temperature derived from stable isotope profile around 128 ka BP, in good agreement with vegetation changes on land in the Northern hemisphere.rnIn Greenland ice cores, the last climatic cycle is marked by high amplitude, millennial scale, climatic variations, bounded by abrupt limits that appear to correlate with stadial/interstadial stages. These climatic events are also present in the Antarctic ice cores, but smoothed, smaller in amplitude, and out-of-phase with respect to their Northern counterparts. Therefore, the stratigraphy of the deglaciation and of the transition from the Upper Pleistocene to the Holocene differs in the two opposite hemispheres. The Holocene in ice cores is different from previous interglacials and it deserves a specific identification in the geological history, despite its short duration compared to other epochs. The formal definition of the Pleistocene-Holocene boundary has been recently proposed and ratified on the NGRIP ice core, and fixed at 11.7 ka before AD 2000. About 200 years before present, the sharp increase of CO_2 and methane above the natural interglacial level, as well as the sudden increase of atmospheric pollutants are a clear stratigraphic evidence for a recent global event caused by the human impact.