OSSERVAZIONI SUL SIGNIFICATO DEI TERMINI LGM (UMG), TARDOGLACIALE E POSTGLACIALE IN AMBITO GLOBALE, ITALIANO ED ALPINO

Abstract

G. Orombelli; C. Ravazzi, M.B. Cita, On the significance of the terms “LGM”, “Lateglacial” and “Postglacial” at a global
and local scale. (IT ISSN 0394-3356, 2005).
The significance of the terms Last Glacial Maximum (LGM), Lateglacial, and postglacial, used for Late Quaternary time intervals with a
strong climatic signature, is critically analyzed. The term LGM originated thirty years ago after a synthesis of global evidences for a time interval of maximum integrated ice volume, reached during the last glaciation. The relevant proxies are records of the last ice-volume equivalent minimum sea level, of the isotopic oxygen composition (last maximum δ18 O values) of benthic Foraminifera and of the sea surface temperature (SST) estimated on planktonic foraminiferal populations. The global signature of the LGM cannot be reliably recorded by local glacial maxima or temperature minima, and the local expressions of the LGM event in the different parts of the world can be well diverse and asynchronous. The most recent datings of the LGM sea-level low stand constrain this interval between 19 and 30 ka cal BP. Several glacial geologists employed the term LGM in a broad, informal sense to identify the most external drift limit reached by glaciers during the last maximum advance within the Late Wisconsin / Late Weichselian / Late Würm. The term LGM is used, internationally, to refer to the maximum phase of the last global event of maximum ice volume, and therefore a definition based on local rock units is questionable. The use of the term LGM to refer to local stratigraphic units or climatic events is misleading and should be abandoned.
The chronostratigraphic regional definition of the Würm Stage in the Alps, and of its Substages is internationally agreed. The Würm
Stage is the alpine chronostratigraphic expression of the Last Glaciation, with first events recently dated to 105 ka BP. The Late Würm
indicates the last event of major glacial advance within the Last Glaciation, including the growth, the maximal phase and the deglaciation, occurred between 30 and 11.5 ka cal BP.
The term Lateglacial is currently used with different significances, including: (i) the time interval of local deglaciation; (ii) the interval
between the onset of Greenland Interstadial 1 and the Holocene beginning (i.e. 14.7 to 11.55 ka cal BP); (iii) the transition between the
LGM and the Holocene, considered at a global scale, i.e. starting at the LGM end and terminating at the beginning of the Holocene.
We are in favor of a globally valid and accepted definition. Finally, the term postglacial has been used informally with different meanings, and has no precise stratigraphic and/or chronological position.