4.7.6 Climatostratigraphy and diachronous units
In Quaternary stratigraphy it has been common practice to define stratigraphical units and derived time units on the basis of palaeoclimatical criteria. Classification systems constructed on this basis have been proposed by Mangerud (1973) and Mangerud et al. (1974). The recommendations made by NSK in this Code concerning the use of climatostratigraphy are to be looked upon as an updated extension of these classification systems.
"A climatostratigraphical unit is a stratigraphical unit with the boundaries defined by geological indications of climatic changes" (Mangerud et al. 1974, p. 113). The palaeoclimatical criteria are interpreted on the basis of lithological and biostratigraphical data and isotope analyses. The boundaries of a climatostratigraphical unit are placed at marked steps in the palaeoclimatical data basis.
Climatostratigraphical units have formed the physical reference basis for "chronostratigraphical" and "geochronological" units in Quaternary stratigraphy. The use of chronostratigraphical and geochronological terminology presupposes that the units in question are isochronous (cf. Section 4.1). Climatic changes, as they can be traced in a stratal succession, will usually not be synchronous except over fairly limited areas. An ice age, for example, can begin and end at different times in different parts of the world. It is therefore possible for the sediments, or fossil fauna or flora, selected as criteria for an ice age in a stratal succession, to differ in age in different areas. The immigration of a temperature-indicative flora or fauna into an area may also be delayed relative to the climatic change itself. Those time units which can be derived from physical, climatostratigraphical reference units are therefore diachronous in character.
Climatostratigraphical time units can be termed (a) informal climatic events, e.g. "glacials" (glasialer), "interglacials" (interglasialer), "stadials" (stadialer), "interstadials" (interstadialer), "cryomers" (kryomerer), "thermomers" (termomerer), etc.; (b) formal diachronous units; (c) formal geochronometrical units. In (b) and (c) the degree to which time is divided must be adjusted to how well it can be resolved by numerical age determination methods. Geochronometrical classification implies that the time boundary in question is defined as being synchronous, even if the climatic change that originally formed the basis for the definition is time-transgressive.