Series, and thereby systems, are formally-defined based on Global Stratotype Section and Points (GSSP) of which two have been ratified within the last 2.7 million years. The Subcommission on Quaternary Stratigraphy has working groups dedicated to the GSSP definition and subdivision of the Pleistocene and Holocene. An overview of the ratified and proposed GSSPs can be found on the IUGS ICS website www.stratigraphy.org .
It is important to recognise the distinction between Chronostratigraphy and Climatostratigraphy. The former is the division of time as recorded in rock sequences, whereas the latter is the division of time based on past climatic oscillations. Both have their own histories with dedicated pages found here and here. Also, the chart page offers further insight into these two parallel division systems. Definitions of stratigraphical terms can be found in the Stratigraphic guide for Quaternary scientists.
The Subcommission of Quaternary Stratigraphy has published their proposed definition in the Journal of Quaternary Science (2009, Vol 25, p. 96-102):
Formal ratification of the Quaternary System/Period and the Pleistocene Series/Epoch
Additional articles reporting the definition and ratification of the base Pleistocene and Quaternary.
Gibbard, Ph. & Head, M J. Quaternaire, 20, (2), 2009, 125-133
ABSTRACT - The Quaternary is characterised by the development of widespread glaciations in temperate northern regions, and by associated physical and biotic readjustments. As a chronostratigraphical term it has attracted considerable debate. There has long been acceptance that the Quaternary should begin at the first indication of refrigeration in the Mediterranean region, which for spurious reasons was placed at 1.8 Ma. Following protracted discussions, led by the International Subcommission on Quaternary Stratigraphy (ISQS) and supported by the International Union for Quaternary Research (INQUA), the International Commission on Stratigraphy has now officially agreed that the Quaternary will henceforth begin at 2.6 Ma within a 2.8-2.4- Ma interval of profound change in EarthÕs climate system. The base of the Gelasian Stage (previously Pliocene) at 2.588 Ma offered an existing global stratotype section and point (GSSP) to define the base of the Quaternary. However, adopting this measure also necessitated lowering the base of the Pleistocene from its previously defined 1.8 Ma to that of the Quaternary to maintain hierarchical order. Acceptance of this proposal recognises the distinctive qualities of the Quaternary, complies with the hierarchical requirements of the geological time scale, and respects the historical and widespread current usage of both terms Quaternary and Pleistocene.
Gibbard, Ph. & Head, M J. Quaternaire, 20, (4), 2009, 411-412
ABSTRACT - The International Union for Geological Sciences (IUGS) on 29 June, 2009 ratified a proposal by the International Commission on Stratigraphy that the base of the Quaternary System/Period and the base of the Pleistocene Series/Epoch be lowered to that of the Gelasian Stage/Age. The Gelasian is transferred accordingly from the Pliocene to the Pleistocene. The Global Stratotype Section and Point at Monte San Nicola, Sicily, Italy, with an estimated age of 2.58 Ma, defines the lower boundary of the Gelasian, Pleistocene and Quaternary. Details of the ratification are given, and implications discussed.
The working group on the Holocene GSSP published their proposed definition in the NGRIP Greenland ice core, in the Journal of Quaternary Science (2009, Vol 24, p. 3-17):
Mike Walker, Sigfus Johnsen, Sune Olander Rasmussen, Trevor Popp, Jørgen-Peder Steffensen, Phil Gibbard, Wim Hoek, John Lowe, John Andrews, Svante Björck, Les C. Cwynar, Konrad Hughen, Peter Kershaw, Bernd Kromer, Thomas Litt, David J. Lowe, Takeshi Nakagawa, Rewi Newnham, Jakob Schwander
ABSTRACT - The Greenland ice core from NorthGRIP (NGRIP) contains a proxy climate record across the Pleistocene-Holocene boundary of unprecedented clarity and resolution. Analysis of an array of physical and chemical parameters within the ice enables the base of the Holocene, as reflected in the first signs of climatic warming at the end of the Younger Dryas/Greenland Stadial 1 cold phase, to be located with a high degree of precision. This climatic event is most clearly reflected in an abrupt shift in deuterium excess values, accompanied by more gradual changes in 18O, dust concentration, a range of chemical species, and annual layer thickness. A timescale based on multi-parameter annual layer counting provides an age of 11 700 calendar yr b2 k (before AD 2000) for the base of the Holocene, with a maximum counting error of 99 yr. A proposal that an archived core from this unique sequence should constitute the Global Stratotype Section and Point (GSSP) for the base of the Holocene Series/Epoch (Quaternary System/Period) has been ratified by the International Union of Geological Sciences. Five auxiliary stratotypes for the Pleistocene-Holocene boundary have also been recognised.
(c) Walker et al. 2009, Journal of Quaternary Science, 24, p. 3-17, John Wiley & Sons, Ltd. Digital Object Identifier (DOI) 10.1002/jqs.1227