HOLOCENE TIMESCALE

The Holocene is the second series or epoch of the quaternary System/Period following the pleistocene. It is more commonly viewed as merely the most recent interglacial or marine isotopic stage (MIS) 1. The opening of the holocene was conventionally drawn at 10,000 radiocarbon years BP, approximating to 11,500–11,700 years ago (calendar years BP) as defined in ice cores and varves, but has recently been formally defined as starting at 11,700 calendar years b2k based on a Global Stratotype Section and Point (GSSP) in the Greenland ice core from North GRIP (NGRIP).

The traditional subdivision of the Holocene was based on the blytt-sernander timescale, originating in the latter part of the nineteenth century. Using evidence from macrofossil remains in peat bogs, they defined an alternating series of zones reflecting climatic changes through the period: preboreal (increasing warmth but no trees), boreal (warm and dry), atlantic (warm and wet), subboreal (warm and dry) and subatlantic (cool and wet). This pattern was later equated with the divisions of Iversen’s interglacial cycle, a pattern assumed to have been characteristic of all recent interglacials. Furthermore, early studies of vegetation history in the British Isles by Godwin and Iversen also utilised these periods as a basis for a scheme of pollen zonation applicable across the whole area as shown in the Figure.

In the absence of radiocarbon dating, the ages of the boundaries between zones were approximate, based in the later Holocene on archaeological correlations. As radiocarbon dating developed, ages were derived from key sites and adopted over wide areas, hence the timescale was based on a very limited number of age determinations. As the number of horizons dated increased, it was realised that the vegetational changes on which the boundaries were founded were highly diachronous. As the environmental characteristics and changes described by the zones were, however, still considered largely valid the terms were retained, and are still used [Page 524][Page 525]to describe the broad subdivisions of the Holocene as chronozones. A widely used scheme in northwestern Europe is that of Mangerud et al. (1974). Defining ages within the Holocene is still very largely based on radiocarbon, although dendrochronology and ice-core dating now provide much more precise dating techniques, and thermoluminescence (TL) dating, optically stimulated luminescence (OSL dating), uranium-series dating, tephrochronology and even some forms of cosmogenic-nuclide dating have been used, albeit usually with far less precision. In the later Holocene, dating associated with archaeological evidence has also been used, and eventually human observation and documentary records can be used to improve or provide a timescale. In the absence of such evidence, and as radiocarbon dating is influenced by the suess effect and bomb effect, dating events over the past 200 years has often proved relatively imprecise relying on techniques such as lead-210 and lichenometric dating. Over the latter half of the past century, nuclear weapons testing and nuclear accidents such as Chernobyl have allowed use of caesium-137 for dating.

Definition of regional terms for periods is not such a problem in the Holocene as it is over the quaternary timescale, although a number of ill-defined terms are widely used. The period of maximum warmth experienced in the Holocene is defined as the climatic optimum or holocene thermal optimum, although the timing and length of such a period varies considerably. The Neoglacial (see neoglaciation), a term originally coined in North America, is sometimes used for the period after the thermal optimum when glaciers redeveloped in mountain areas, culminating in the little ice age when glaciers in many areas reached their maximum Holocene limits. The Little Ice Age is difficult to define and is very variable in timing and length globally, although tends to centre of the middle and later part of the last millennium. On the continent of Europe, the term Neuzeitlich is also used in a chronological sense to identify renewed glaciation.

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