4.4 Geochronometrical units
4.4.1 General properties and rules
A geochronometrical unit is a geological time unit established and defined by direct division of time expressed by a specific number of years. Unlike geochronological units, geochronometrical units are not based on the physical boundaries of specific chronostratigraphical units. The boundaries can be convenient, arbitrarily chosen ages in years (such as, for example, some of the proposed divisions of the Precambrian). A geochronometrical boundary may also be decided upon on the basis of many radiometric age determinations within a succession in which changes in lithology, fossil content, climatic indicators, etc. have been recorded, making it natural to distinguish between two contiguous time units.
The same names, rank terms and hierarchy are used for geochronometrical units as for geochronological units (Table 2, Section 4.3).
Geochronometrical units can often be linked to chronostratigraphical units that fully or partially correspond in time to the chronometrical unit. A basal boundary stratotype (typesnitt for undergrensen) should, nevertheless, not be defined in a succession of this sort. If this is done, the time unit must be redefined as a geochronological unit. Determination of a relevant time boundary in other successions must then take place by chronostratigraphical correlation (Section 4.2.1.4) to the erected boundary stratotype.
Geochronometrical definition of a time boundary has the advantage of not being dependent upon whether the succession contains fossils that can be correlated with fossils in a chronostratigraphical type section. Geochronometrical time boundaries should not be defined more closely than they can practicably be determined in a succession, given the ability for time resolution that numerical age determination methods have at any given time.
Example: As part of the process of defining the Pleistocene- Holocene boundary, the Holocene Commission, during the INQUA Congress in Paris in 1969, decided that the boundary should be placed at 10 000 14C years before present (Olausson 1982). 14C determinations are to be calculated using the Libby half-life (5568 years) with 1950 as the reference year for the present. When defined in this way, the Quaternary series, Pleistocene and Holocene, are geochronometrically defined and are therefore not dependent on a type section. The Holocene has now been geochronometrically subdivided, too.
Key reference: NACSN (1983).
