D 2036                                           Dating Methods (Absolute and Relative) in Archaeology of Art
                                                                     is of a phenomenon related to but not the actual
              Dating Methods (Absolute                               timeofmanufactureoftheart.Iftheseindirectages
              andRelative) in Archaeology of Art                     are in a stratigraphic relation to rock art (older or
                                                                     younger), then they produce minimum/maximum
              Juan Francisco Ruiz1 and Marvin W. Rowe2               direct dates for related imagery (Bednarik 2007;
              1                                      ´               Morwoodetal. 2010;Ruizetal.2012).
               Facultad de Ciencias de la Educacion
              y Humanidades, Universidad de Castilla La                  Rockartresearchhasbeentreatedforyearsas
              Mancha, Cuenca, Spain                                  a minor aspect of archaeology. Lack of reliable
              2Texas A&MUniversity at Qatar, Museum of               methods     to   date    ancient    imagery,    both
              NewMexico,Santa Fe, NM, USA                            pictographs and petroglyphs on open-air sites or
                                                                     inside of deep caves, kept it outside of main-
                                                                     stream archaeology. This began to change with
              Introduction                                           the introduction of scientific dating approaches,
                                                                     and there are reasons to feel optimistic about
              Chronology of rock art, ranging from Paleolithic       dating rock art at this time. Several dating groups
              to present times, is a key aspect of the archaeol-     are currently working on this around the world,
              ogy of art and one of the most controversial. It       and it is now possible to hope for interlaboratory
              was based for decades in nonscientific methods          comparison tests to help evaluate the reliability
              that used stylistic analysis of imagery to establish   and accuracy of the techniques.
              one-way evolutionary schemes. Application of
              scientific methods, also called absolute dating,
              started to be used in the 1980s and since then         KeyIssues/Current Debates/Future
              has increased more and more its significance, as        Directions/Examples
              judgedbythelargenumberofpaperspublishedin
              the last two decades on this subject (Rowe 2012).      Archaeological studies of rock art demand
                                                                     a temporal framework in which a particular
                                                                     imagery was produced, as it is the only way to
              Definition                                             relate decontextualized imagery to archaeological
                                                                     cultures. The earlier traditional methods to estab-
              Absolute and relative dating methods have been         lish chronologies of rock art sites and imagery
              used to establish tentative chronologies for rock      werebasedonassumptionsmadeoniconography,
              art. Relative dating refers to non-chronometric        style, and comparison with excavation evidence
              methodologiesthatproduceseriationbasedonsty-           and technical analysis. For example, Paleolithic
              listic comparison and stratigraphic assumptions.       mobiliary art from excavated sites in Europe was
              On the other hand, absolute dating methods are         used as a base for stylistic comparison with cave
              based    on   scientific    techniques   that   yield   imagery. These evidences, supported by superim-
              a chronometric age for a phenomenon in direct or       position analyses, produced the great stylistic
              indirect physical relation to rock art (same age,      schemes for Paleolithic art in Western Europe,
              older, or younger). Dating of some binders in pic-     which defended a one-way evolution from simple
              tographs or the alterations of surfaces by petro-      to complex figures, expanding from Aurignacian
              glyphs are examples of direct ages related to rock     to Magdaleniantimes(Pettitt&Pike2007).These
              art production. However, it is controversial to con-   systems lack enough resolution to produce an
              sider these dates as “absolute” as they merely         accurate temporal frame for rock art, above all
              reflect experimental propositions, which often          for styles without consensus on their mobiliary
              lack independent verification (Bednarik 2007;           parallels.
              Pettitt & Pike 2007). Most scientific dating                Weaknesses of these stylistic paradigms were
              methods are indirect because they produce              pointed out (see (Bednarik 2007; Pettitt & Pike
              constrainingagesforimagery,andtheageobtained           2007) for recent reviews of them), but it is
              Dating Methods (Absolute and Relative) in Archaeology of Art                                    2037       D
              important to recognize that they are still useful           Dating of charcoal pictographs has been
              for rock art chronology because it is obviously          broadly used in French and Spanish Paleolithic
                                                                                               ´
              impossible to date every figure in a site and every       caves(Alcolea&Balbın2007;Steelman&Rowe
              site all over the world. A well-defined proxy with        2012), but also in North America and Australia,
              stylistic, technical, and chemical composition           andotherregionsoftheworld.Thelargestpartof
              data would be very helpful as a complement to            the charcoal pigment dates is considered reliable
              scientific    dating.    A date archaeologically          but,  for   example, in Chauvet Cave, dates
              decontextualized is of little value, so it must be       are controversial because they are unusually                 D
              stressed that any dating should be included in           old and conflict with stylistic paradigm. Several
              archaeological hypotheses.                               authors claimed for a likely contamination of
                  Thefirst radiocarbon dating on rock paintings         datings of Chauvet, as the dates from different
                                                                                                            ˜
              wascarriedoutonacharcoalpictographinSouth                samplesofonesinglefigureinPenadeCandamo
              Africa in the late 1980s quickly followed by             (Asturias,   Spain)    showed that results         by
              others in 1990 in Australia, the USA, and Europe,        GeochronLab(USA)were15,000yearsyounger
              which added to pioneer research on engravings            than those produced by LSCE (France), respon-
              dating. A few years later, a broader conscience          sible of all Chauvet dates (Pettitt & Pike 2007).
              about these new possibilities dictated that              This situation reflects pitfalls of the method that
              scientific dating of the passage of time became           could be accompanied by contamination of
              analternative to stylistic paradigms (Lorblanchet        unknown origin, possible repainting for younger
              & Bahn 1993). However, debates on very old               dates, mistakes in laboratory treatment of sam-
              AMS 14C dates from Chauvet Cave (France)                 ples, and the presence of carbon of different ori-
              and very young ones on open-air engravings               gin, for example, incomplete dissolution of
                                                      ˆ                calcium oxalates. An improved specific protocol
              dated by several methods in Foz Coa (Portugal)
              showed that style and scientific dates were still         to remove contamination produced by calcium
              necessary for archaeology of art.                        oxalates from charcoal paintings has recently
                  The most common technique for dating rock            been developed (Bonneau et al. 2011).
              paintings worldwide is the radiocarbon dating of            It is indispensable to follow a strict protocol to
              the charcoal pigments often used to construct the        collect samples during fieldwork. The protocol
              drawings. A large number of publications have            described in literature tries to avoid contamina-
              been collected in the bibliography composed by           tions using sterile latex gloves and surgical
              Rowe (2012). Dating charcoal has been well               masks. Samples are removed from walls with
              honed by the radiocarbon community, and the              a sterile surgical blade, which is changed and
              results can be considered to be generally reliable.      discarded after each sample. They are put inside
              The main disadvantage to radiocarbon dating              of a sheet of folded sterile aluminum foil and
              charcoal pigments is that the date measured is           placed inside of a labeled plastic bag. The exact
              NOTthatofthetimeofexecutionofthepainting.                position where samples were removed should be
              Rather it dates the pigment and there are two            recorded    with    photographs.     Extreme     care
              caveats that accompany any date of charcoal:             ought to be observed on the selection of the
              old wood and old charcoal (Steelman & Rowe               sampling points to avoid major visual impact or
              2012). Old wood phenomena are situations, usu-           harm to the pictographs, for example, selecting
              ally encountered in desert areas where wood              flakes that appear likely to spall from the walls
              decays slowly, in which the wood burned to               naturally. The size of samples required is uncer-
              make charcoal may be up to centuries old. Old            tain but around 2 cm2 is generally used for
              charcoal may occur when freshly hewn wood is             pictographs with inorganic pigments and much
              burned, but not used to construct a painting until       less for charcoal-pigmented paintings, as for
              much later. Both these caveats should be kept in                14
                                                                       AMS C dating only 50–100 mg of carbon is
              mindatall times when interpreting charcoal pig-          needed for an accurate date. For pictographs
              ment radiocarbon dates.                                  with inorganic pigments, e.g., iron ochre or
              D 2038                                          Dating Methods (Absolute and Relative) in Archaeology of Art
              manganese oxides, it is essential to take an          It has been shown to be useful to get minimum
              unpainted rock sample as near to the sample           ages for petroglyphs and minimum/maximum
              takenasisfeasible.Thatbackgroundrocksample            ages for pictographs. On certain locations,
              should be processed identically to the pictograph     researchers has bracketed dates for rock paintings
              sample. A more detailed report on sampling            between two oxalate skins, producing a temporal
              protocol, reporting of radiocarbon results and        frame for pictographs in agreement with archaeo-
              laboratory pretreatment of samples, has been          logical expectations (Ruiz et al. 2012).
              just published (Steelman & Rowe 2012).                   Sample removal procedure is similar to
                 AMS14Chasbeenusedtodateanyotherkind                that described for radiocarbon dating (Cole &
              of carbon-bearing substances related to picto-        Watchman 2005). Sample sizes range from
                                                                           2         2
              graphs or petroglyphs (Aubert 2012). The              25mm to1cm,dependingonoxalatecontent.
              presence of binders has been used to produce          Oxalate dating demands microstratigraphic
              direct radiocarbon dates of beeswax paintings in      analysis and micro-excavation techniques to
              Australia (Morwood et al. 2010). Vegetal resins       avoid contamination between upper and lower
              and wax are binders of these paints. It is consid-    layers of calcium oxalate. Mechanical proce-
              ered that wax would have been fresh when              duresandlaserablationhavebeenusedsofar
              applied on walls to construct the drawings, so it     for this purpose (Watchman 2000). There are
              should be ambient source of carbon to date rock       twomaindrawbacksforoxalatedates:(1)radio-
              art. In the Australian Kimberley area, a range of     carbon age of any calcium oxalate crust is
              dates from 3,780 
 60 BP to present times were        a weighted “average” of oxalate deposited for
              obtained (Morwood et al. 2010). Rowe and              longperiodsoftime,evenintomoderntimes,so
              coworkers have dated non-charcoal paintings in        (2) they always yield minimum ages, and in
              several sites in North and South America (Rowe        consequence the archaeological significance of
              & Steelman 2003; Steelman & Rowe 2012).               them is limited by our ignorance of time lapse
              These pictographs were made with inorganic            among rock art creation and formation of the
              pigments, mainly iron oxides, so it is assumed        oxalate crust (Fig. 1).
              that someorganicbindermustbepresentinthem.               A similar approach can be used with silica
              Replicate measurements on samples of the same         skins. These accretionary crusts are formed
              pictograph yield an uncertainty of 
250 years         during evaporation of runoff water solutions
              suggesting results are reliable (Steelman &           containing monomeric silicic acid that after
              Rowe2012).                                            dehydration forms a hard noncrystalline film on
                 Indirect dating by AMS 14                          the surface of rocks (Watchman 2000). Organic
                                             C has been widely
              used to date carbon-bearing accretionary crusts       matter like diatoms and other algal has been
              (like calcium oxalate skins) or organic matter        found inside of finely laminated silica crusts
              inclusions in mineral coatings (amorphous silica      overlying pictographs in Australia, for example,
              skins). Calciumoxalatedatingisaproceduretoset         givingaminimumageforrelatedBradshaw-style
              a temporal frame on the age of a pictograph or a      figures. The same procedure has been described
              petroglyph. Calcium oxalate coatings appear           to date a silicate accreted paint layer.
              naturally  on walls in two mineral forms:                Calcium carbonate coatings interstratified
              whewellite and weddellite. The exact process of       with pictographs or engravings can be used to
              formation of these accretionary crusts is still       obtain constrains on their time of manufacture.
              unknown,but there is a broad consensus that they      U-Th series disequilibrium method is applied to
              form from ambient carbon dioxide and that they        date the formation time of calcite coatings. They
              are deposited on external faces of rocks after met-   are formed from the redeposition of dissolved
              abolic activity of lichens, microbes, and bacteria.   calcium carbonate from saturated solutions of
              ThismethodwasfirstusedinAustraliaintheearly            water that flow across the surfaces of rocks and
              1990s, and since then it has been used in sites all   eventually deposit over rock art. In these flow-
              over the world (see review by Ruiz et al. 2012).      stone crusts are contained small quantities of
             Dating Methods (Absolute and Relative) in Archaeology of Art                             2039     D
             Dating Methods
             (Absolute and Relative)
             in Archaeology of Art,
             Fig. 1 Group of zigzags
             and Levantine zoomorphs
                            ´
             from Cueva del Tıo
             Modesto (Henarejos,
             Cuenca, Spain).
             Amicrosample was
             collected from point                                                                                        D
             indicated in upper picture.
             Cross section shows two
             painting events
             interstratified with calcium
             oxalate layers (down). Two
             14C AMSdates were
             obtained related to this
             microstratigraphic packet
             (q quartz, h hematite, g
             gypsum, c-o calcium
             oxalate)
             uranium (U), an element soluble in water, while     activity of another isotope, 232Th, which can be
             they are relatively free of thorium (Th), an        detected in elevated levels in detritus.
             insoluble chemical element. The method is              Samples could be extracted by scraping with
             based on the radioactive decay chain from parent    a surgical blade or with an electric drill. They
             238U into the intermediary isotope 234U and         could be very small (10–150 mg) and very
             finally to daughter 230Th. Relative measurement      thin   (0.5–2   mm), depending on uranium
             between these three isotopes in calcium carbon-     content. Submillimeter-thick laminations can be
             ate crusts allows calculation of the age of the     accurately dated by this method. Two lab
             carbonate host as the decay rate is known.          treatments are described in literature, an acid
             Detrital materials, such as aeolian dust or silts   wash (Tac¸on et al. 2012) and a micromill
             and clays dissolved into water, incorporated into   preparation (Hoffmann et al. 2009), after which
             calcite layers are a potential contaminant that     laser ablation inductively coupled plasma mass
             could distort the results producing overestimated   spectrometry (LA-ICP-MS) is used to isotopic
             ages (Tac¸on et al. 2012). Low 230Th/232Th ratios   ratio measurements.
             are indicative of detrital contamination. This         U-Th dating was first used in South Australia
             concern can be corrected by measuring the           to give an estimation of the age of two