In earth science, chronostratigraphy defines rock strata by their temporal relations, reconciling stratigraphy with relative and chronometric dating in the historical discipline of geology. Relative dating by stratigraphic correlations, employing bio-, magneto-, or isotope-stratigraphy, provides a relative time scale between specific events in the geologic record. Thus, chronostratigraphic unit definitions are based on age relations, which are referred to on a relative linear time scale and preferably fixed in time by chronometric (often wrongly termed absolute) dating. With some differences in scale and evidence, chronostratigraphy is also applied to cultural material left by humans.
In either case, the scale in time and/or space determines the validity (and construction) of any given chronostratigraphy, because geological and archaeological phenomena operate at different scales (Dean, 1993). The scales are mainly dictated by the nature of the record; they vary with time but are also different between disciplines (Stein, 1993), especially in archaeology, where change over time can be rapid (Blackwell and Schwarcz, 1993). Chronostratigraphies should be based on several independent lines of evidence for time sequences, such as the use of various relative stratigraphies (e.g., lithostratigraphic, magnetic, or biological) combined with chronometric age estimates developed through multiple dating techniques. Lithostratigraphic and biostratigraphic units can have diachronous or time-transgressive boundaries – i.e., variable lower starting points and upper ending points – but chronostratigraphic units always (by definition) have synchronous boundaries – i.e., start and end points are globally coeval (Holliday, 1993).
Geological chronostratigraphy is based on lithostratigraphies (Figure 1), and in contrast to archaeology, they are formalized and aimed toward global applicability (Miall, 2010). Global Boundary Stratotype Sections and Points (GSSP) are defined through internationally agreed upon stratigraphic sections or marker horizons, which are governed by international bodies (see http://www.stratigraphy.org and Remane, 1997, 2003). Such boundaries on the geologic time scale are based on paleontological changes (biostratigraphy as in the relative sequences of bone –tooth – antler in Figure 1), isotopic marker horizons (as in climatostratigraphy), or magnetic events (as in paleomagnetic dating), which can be traced over large geographical areas (Rey and Galeotti, 2008). The base of a chronostratigraphic unit is identified in a certain stratigraphic section, while the end of that unit is defined as the base of the following chronostratigraphic unit, which can be defined from evidence in a different section at some distance from the first. Such units are isochronous bodies that are bound by synchronous surfaces and formed during a single span of time (Cremeens and Hart, 1995).
Specific chronostratigraphies, like isotope chronostratigraphy (climostratigraphy; Baskaran, 2011; Bowen, 2011), biostratigraphy (Palombo and Sardella, 2007), or magneto-chronostratigraphy/paleomagnetic dating (Opdyke and Channell, 1996), refer to the construction of a time series using one specific method (i.e., analysis of isotopic chemistry, biota, or magnetism) to provide relative age information.
Chronostratigraphy in archaeology follows the same general principles as in geology (Lyman and O’Brien, 2000), but it is less strict and less formalized, given the impossibility of a worldwide application because human culture is so diverse, and there is great potential for large time gaps and sparseness of available data. Archaeological stratigraphies are constructed from the time series provided by their own lithostratigraphies, in which specific features related to cultural remains (like stone tools, ceramics, etc.), or the typological/technological compositions of the cultural remains, are repeatedly observed in strata within stratigraphies. Such entities are defined and assumed to represent meaningful units of human cultural history in space and time (cultural traditions, cultural facies, industries, horizon style, etc.), reflecting normative ideas and mental templates of extinct people (Clark, 1991). An unspecified number of archaeological assemblages showing more or less identical patterns are necessary to define such a cultural unit, which is understood to demonstrate the shared behavior reflected by a human society and, thus, a socioeconomically meaningful unit (as represented by the hand axe, pot, and wheel in Figure 1). However, any chronostratigraphic marker or event concept in geology is widespread, whereas the definitions of specific features/items of cultural remains are often based on typology (chronotypology after Clark, 1991), which can be quite localized and of limited regional extent.
The resulting cultural units provide the basic relative stratigraphic framework (e.g., the relative sequence of hand axe–pot–wheel in Figure 1), similar to lithostratigraphy and biostratigraphy (e.g., the relative sequence of bone–tooth–antler in Figure 1), sometimes sharing the type fossil (O’Brien and Lyman, 1999) or the assemblage (Clarke and Chapman, 1978) approach with paleontology. By incorporation of other relative (paleomagnetic event “PM” in Figure 1), as well as chronometric data (Y in Figure 1), chronostratigraphies are constructed on variable scales in time and geography. Linking the archaeological remains of formerly occupied sites to the paleoclimatic record (δ18O curve in Figure 1) allows more precise age estimates than would be obtainable by many chronometric dating methods, subsequently increasing the precision of the chronostratigraphies.
The resulting regional archaeological chronostratigraphies can overlap and do not necessarily correspond in time, number, or definition of units. Furthermore, archaeological chronostratigraphic units are sometimes blurred by imprecise definitions and by circular logic, e.g., when the supposed age, not the actual cultural remains themselves, is used as an argument for the attribution of an archaeological assemblage to a cultural unit. Archaeological chronostratigraphic units (sometime called phases) can have boundaries coincident with the bases of geological ones, or they can overlap. Obviously, geological and archaeological chronostratigraphic units do not have to correspond, especially as the relationship of cultural remains and geological/sedimentological processes are not always clearly defined. Such specificities like pedostratigraphy and related chronostratigraphies, especially with reference to archaeology, are discussed in detail in Cremeens and Hart (1995).
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