Developments in radiocarbon calibration for archaeology.

Developments in radiocarbon calibration for archaeology. Introduction Radiocarbon dating underpins most of the chronologies used inarchaeology for the last 50 000 years. However, it is universallyacknowledged that the radiocarbon 'ages' themselves (usuallyexpressed in terms of 14C years BP--because they are measured relativeto the standard which corresponds to AD 1950) are not an accuratereflection of the true age (in calendar years) of samples, because theproportion of radiocarbon in the atmosphere has fluctuated in the pastand because the half-life used for the calculation of radiocarbon agesis not correct. For this reason, where possible, radiocarbon dates arecalibrated cal��i��brate?tr.v. cal��i��brat��ed, cal��i��brat��ing, cal��i��brates1. To check, adjust, or determine by comparison with a standard (the graduations of a quantitative measuring instrument): against material of known age (giving ages expressed in termsof cal AD, cal BC or cal BP--which is absolute relative to AD 1950). Forrecent periods (in practice, the Holocene) this is now standard practiceamongst archaeologists. However, as we seek to extend the timescale overwhich calibration calibration/cal��i��bra��tion/ (kal?i-bra��shun) determination of the accuracy of an instrument, usually by measurement of its variation from a standard, to ascertain necessary correction factors. is possible, it is important to be aware of thediverse nature of calibration datasets and the limits to theirreliability. It is also worth considering some of the reasons behind thecontroversy over the term 'calibration' (van Andel 2005). Data for radiocarbon calibration Until recently the main data that have been employed to generatethe estimates of the radiocarbon calibration curve In analytical chemistry, a calibration curve is a general method for determining the concentration of a substance in an unknown sample by comparing the unknown to a set of standard samples of known concentration. have beenmeasurements of the radiocarbon concentration of wood which has beendendro-chronologically dated to the nearest year. This is ideal from thepoint of view of archaeologists since the wood in trees is laid downwith carbon taken from the atmosphere. The same can be said for mostplant fragments, and, through the food chain, for terrestrial animals.So, for the vast majority of archaeological material, the carbon in thesamples should have a radiocarbon concentration very close to that ofthe tree rings used to generate the calibration curve. Only when thereare samples from marine or fluvial flu��vi��al?adj.1. Of, relating to, or inhabiting a river or stream.2. Produced by the action of a river or stream.[Middle English, from Latin environments, or other unusualsituations (for example depleted de��plete?tr.v. de��plet��ed, de��plet��ing, de��pletesTo decrease the fullness of; use up or empty out.[Latin d [sup.14]C[O.sub.2] from volcanicsources, or significant oceanic upwelling up��well��ing?n.1. The act or an instance of rising up from or as if from a lower source: an upwelling of emotion.2. in some coastal situations) dowe have to worry about reservoirs of carbon with radiocarbonconcentrations that are substantially different to those in theatmosphere. Over the last couple of decades the extent of tree ring recordsavailable has been greatly expanded. In 1986 the firmly dated sectionsof the calibration curve extended back to about 7300 cal BP (Stuiver1986), although floating sections could be used to infer its form backover the full extent of the Holocene. When the IntCal04 calibrationcurve (Reimer et al. 2004) was constructed the tree ring data extendedback to about 12 400 cal BP. This record is in most places duplicatedmany times over, both in terms of the dendro-chronology and with datesmeasured at a number of different high-precision laboratories. Thislends great strength to our conviction that, within the uncertaintyquoted on IntCal04, the tree ring section of the IntCal04 curve closelyrepresents a true record for the atmosphere of the mid-latitude NorthernHemisphere (see Figures 1 & 2). The 2004 estimate of the calibrationcurve for the past 1000 years from the Southern Hemisphere, which has aslightly different radiocarbon concentration (this difference equates tono more than c. 100 [sup.14]C years in this time period), is alsoavailable in the form of the SHCal04 curve (McCormac et al. 2004) (seealso Figure 2). Furthermore, more data are always being added to thiscorpus and floating sections of wood from Germany now extend well backinto the late glacial gla��cial?adj.1. a. Of, relating to, or derived from a glacier.b. Suggesting the extreme slowness of a glacier: Work proceeded at a glacial pace.2. a. . This will almost certainly allow us to extend theterrestrial calibration curve back further in time. Equally interestingis the fact that kauri trees from New Zealand New Zealand(zē`lənd), island country (2005 est. pop. 4,035,000), 104,454 sq mi (270,534 sq km), in the S Pacific Ocean, over 1,000 mi (1,600 km) SE of Australia. The capital is Wellington; the largest city and leading port is Auckland. are found with ages thatextend right out beyond the range of radiocarbon and are currently beingdated in the age range 25-55 000 BP (oral presentation by Chris Turneyat the nineteenth International Radiocarbon Conference, Oxford). Thesedo not, and perhaps never will, provide a continuous chronology chronology,n the arrangement of events in a time sequence, usually from the beginning to the end of an event. that canbe linked together to provide a chronology like the one we have for theHolocene. However, it is likely to give us insight into the way in whichthe radiocarbon concentration in the atmosphere fluctuated in the past. [FIGURES 1-2 OMITTED] In order to calibrate To adjust or bring into balance. Scanners, CRTs and similar peripherals may require periodic adjustment. Unlike digital devices, the electronic components within these analog devices may change from their original specification. See color calibration and tweak. samples older than the extant ex��tant?adj.1. Still in existence; not destroyed, lost, or extinct: extant manuscripts.2. Archaic Standing out; projecting. tree-ring-basedcalibration curve, we need to make use of different kinds of records,and this is where things become more complicated (see Table 1). Thereasons for these complications are obvious. Ideal calibration relatesmeasurements of atmospheric radiocarbon ([sup.14]C years BP) to theabsolute calendar timescale, and according to according toprep.1. As stated or indicated by; on the authority of: according to historians.2. In keeping with: according to instructions.3. the strict definition,only the dendro-chronological record qualifies for this. Beyond the treering data, most radiocarbon samples in 'known-age' records arederived from non-terrestrial reservoirs, such as marine deposits andspeleothems (mineral cave deposits), and are therefore subject toreservoir effects. The 'known-ages' in these records alsodepend on deposition models and measurement errors. All of these issueslead to varying degrees of uncertainty, depending on the nature of thedataset, as discussed below (see also the list of 'pros andcons' given by van der Plicht et al. 2004). First of all we have the different kinds of sample that can be usedfor measurements. The main samples that have been used for this kind ofstudy are: wood, plant remains, foraminifera, corals and speleothems.The first two of these reflect atmospheric radiocarbon concentration andso are potentially ideal for calibration purposes. However foraminiferaand corals are marine organisms, and so reflect the radiocarbonconcentration in particular regions of the ocean. We know theradiocarbon concentration of the surface oceans today, but there isincreasing evidence that the difference between the oceans and theatmosphere has varied (and perhaps very considerably if we look at thelate glacial and earlier periods). This should not surprise us since one of the main phenomena of theglacial fluctuations in climate is major change in ocean circulation(Dansgaard et al. 1993). Speleothem A speleothem (from the Greek for "cave deposit") is a secondary mineral deposit formed in caves. It is the formal term for what is also known as a cave formation. In limestone and dolostone cavesOverview records are even more complex: theycontain a mixture of carbon from the atmosphere and from ground water,which is likely to have a component of carbon from geological depositsthat are essentially free of radiocarbon. Secondly, we have different methods of estimating the true age ofthe samples that are to be used for calibration. In the Holocene we havethe luxury of tree ring dates that are accurate usually to the exactyear. We do not have this in earlier periods and so we must use othermethods, the main ones being varve varve,in geology, pair of thin sedimentary layers formed annually by seasonal climatic changes. Usually found in glacial lake deposits, varves consist of a coarse-grained, light-colored summer deposit and a finer-grained, dark-colored winter deposit formed when fine counting, ice-core timescales anduranium series dating. Varve counting of lakes (such as Lake Suigetsu,Japan) is susceptible to error for a number of reasons--although suchsequences do usually provide a fairly good relative chronology. Ice-coretimescales are either based on direct counting of ice layers (as in thecase of the GISP GISP Global Invasive Species ProgrammeGISP Gonococcal Isolate Surveillance ProjectGISP Greenland Ice Sheet ProjectGISP Geographic Information Systems ProfessionalGISP Group Independent Study ProjectGISP Global Information Society Project 2 chronology and the new NGRIP NGRIP North Greenland Ice Core Project chronology back to c. 40000 BP) or based on age/depth models (as in the case of GRIP and GISP2beyond 40 000 BP). In principle, these records suffer some of the sameproblems as varved lakes (for one discussion of problems in thechronology of the well-known GISP2 ice core, see Southon 2004) but dueto the concentration of effort in these records and the degree ofduplication they are, at their best, considerably better than varves(presentation of J.P. Steffensen at the Oxford Radiocarbon Conference).They also have the benefit of being the timescale against which muchpalaeoclimate data are generated, and so, even if the absolute ages arenot correct, the relationships to these data will be. However, onefurther complication is that in order to use these timescales it isnecessary to make assumptions about the synchronicity synchronicity (singˈ·kr of global climatesignals that may not be fully justified. Finally, we have uranium seriesdating, in this case either of corals or speleothems. This is a veryprecise and accurate technique if correctly applied. However, it doesrequire very careful analysis to ensure that the samples dated have notsuffered from detrital de��tri��tus?n. pl. detritus1. Loose fragments or grains that have been worn away from rock.2. a. Disintegrated or eroded matter: the detritus of past civilizations. contamination or post-depositionalre-crystallisation. These caveats aside, the timescale derived isindependent and so provides a very useful method for radiocarboncalibration, when proven absolute (Chiu et al. 2006). So we can see that all of the records we might use for calibrationof earlier timescales do have their problems--often complicated andoften interwoven in��ter��weave?v. in��ter��wove , in��ter��wo��ven , inter��weav��ing, inter��weavesv.tr.1. To weave together.2. To blend together; intermix.v.intr. . There is some strength in the diversity of the methodsemployed and this is why for the IntCal04 calibration curve some ofthese records were used to extend the calibration curve back to 26 000cal BP on the basis that there was sufficiently good agreement betweenthe different datasets (see Figure 3). However, it should be stressedthat beyond the tree ring data this curve is essentially based on marinedata and therefore relies on assumptions about the relationship betweenthe radiocarbon concentration of the oceans and the atmosphere. Thus,this part of the atmospheric calibration curve is 'marinederived'. Further back in time the records, in part because of thevarious problems outlined above, showed poor agreement when IntCal04 wascompiled (see Figure 4). Research in this area is, however, very activeand the situation is changing rapidly. Research programmes andinvestigations in different areas are bringing the marine calibrationdatasets into much closer agreement. For example, the Cariaco basin The Cariaco Basin lies off the north central coast of Venezuela and forms the Gulf of Cariaco. It is bounded on the east by Margarita Island, Cubagua Island, and the Araya Peninsula; on the north by Tortuga and the Tortuga Banks; on the west by Cape Codera and the rocks data(Hughen et al. 2004a; Hughen et al. 1998; Hughen et al. 2004c), forwhich the initial calendar ages were based on the GISP2 timescale,agrees much better with the coral data if either the new NGRIPchronology is used or the chronology from Hulu Cave (Wang et al. 2001).Other records are also being revised as new data and methods becomeavailable (such as that of Beck et al. 2001) and it looks as if it willnot be long before a marine calibration curve can be constructed for thelast 40 000 (or even 50 000-55 000) years--as evident in presentationsby both Konrad Hughen and Richard Fairbanks at the Oxford RadiocarbonConference. [FIGURES 3-4 OMITTED] However, other discrepancies remain. These probably arise fromthree major factors: * Increasing uncertainty in the calendar age estimates for thesamples undergoing radiocarbon dating. Ice-core timescales becomeincreasingly uncertain with increasing age because of thinning of theannual layers and concatenation of errors through the record.Correlation with the oxygen isotope isotope(ī`sətōp), in chemistry and physics, one of two or more atoms having the same atomic number but differing in atomic weight and mass number. The concept of isotope was introduced by F. records also becomes morecomplicated in some periods. Uranium series dates are in principle stillvery precise over this time range but there is increasing chance ofpost-depositional change and complications of changing (or unknown)environmental conditions. * Increasing difficulty in measuring the radiocarbon concentrationof the samples accurately, especially as the records get back before 30000 [sup.14]C years BP, where the corrections for modern contaminationin processing and more recent environmental contamination in the samplesare issues which can be difficult to resolve fully (at this age onlyabout 2 per cent of the radiocarbon remains in the sample and even lowlevels of contamination become significant). * Increasing difficulty in assessing the state of the global carboncycle, including particularly the ocean circulation, deep oceanventilation and the radiocarbon production rate in these periods. Of greatest significance are indications in some of the terrestrial(but not atmospheric) records (such as the Bahamas speleothem; Beck etal. 2001) that there may be some considerable offsets between theatmosphere and the oceans at particular periods and possibly major ageinversions at or just before 40 000 cal BP which may be related to majorgeomagnetic excursions such as the Laschamp event. If this is the case,then caution will still be needed in using marine records for thecalibration of terrestrial samples. What is calibration Much debate centres on the use of the word calibration. There areof course many uses of the word 'calibrate' in the Englishlanguage English language,member of the West Germanic group of the Germanic subfamily of the Indo-European family of languages (see Germanic languages). Spoken by about 470 million people throughout the world, English is the official language of about 45 nations. , but the sense in which it is most often used in science is'to set an instrument so that readings taken from it are absoluterather than relative' (Simpson & Weiner 1989). The mathematicalmethods employed by radiocarbon calibration programs such as BCal (Bucket al. 1999), CALIB (Stuiver & Reimer 1993), CalPal (Joris &Weninger 1998), the Groningen radiocarbon calibration program(WinCal25/Cal25; van der Plicht 1993), or OxCal (Bronk Ramsey 2001) areessentially methods for mapping radiocarbon ages and their associatedlaboratory uncertainties through a mathematical function with its ownuncertainty (often known as a calibration curve) onto the calendarscale. It is the view of many in the radiocarbon community that thismapping process should really only be called 'calibration' ifthe mathematical function or calibration curve we use is derived in sucha way that we can be fairly sure that by using it we are putting oursamples (with a known degree of accuracy) onto an absolute timescale. The reason for this caution is essentially in order to prevent toomuch confusion in the disciplines served by radiocarbon dating.Archaeology has suffered too much over the last five decades from'radiocarbon revolutions' without having to experience furtherones every time a new 'calibration' record emerges. For thisreason it seems sensible to base our estimates of calibration curvessolely on data that are well corroborated cor��rob��o��rate?tr.v. cor��rob��o��rat��ed, cor��rob��o��rat��ing, cor��rob��o��ratesTo strengthen or support with other evidence; make more certain. See Synonyms at confirm. and to avoid data which(although potentially useful for other purposes) are currently seen asprovisional for calibration purposes. In this respect it would also seemsensible to draw a semantic distinction between 'calibration'as such and 'comparison' of radiocarbon dates to particularrecords. The same kinds of mathematical method can be used to undertakeboth 'calibration' and 'comparison' and the data arealmost always made freely available by the scientific community, sothere is no question of curtailing freedom as suggested by van Andel(2005). We simply urge everyone to make it clear whether they areundertaking true calibration or a comparison and draw theirreaders' attention to the difference between the two. There is an argument that 'calibration' need not be veryprecise and that even a rough calibration may be useful. This iscertainly true. However, if the calibration is to be useful it must havea statement of uncertainty attached to it and this must accuratelyreflect the true uncertainty in the absolute age estimate generated.Herein lies a problem. Each group of researchers who provide data withpotential utility for radiocarbon calibration curve estimation do theirbest to quantify their own internal sources of error and uncertainty andto report these in a standard form. What they do not and cannot do is toallow for sources of error or uncertainty that they are completelyunaware of. If we look at the currently available data for thepre-tree-ring timescale we find that there are substantial uncertaintiesthat have simply not been quantified. Buck and Blackwell (2004) providea statistical method to estimate the scale of unquantified uncertaintiesthat must be present if all of the records they considered relate to thesame underlying radiocarbon calibration record and found offsets aslarge as 2500 years (van der Plicht et al. 2004). Given this (and otherobservations about the data), the IntCal group felt that they could notprovide a reliable estimate of the radiocarbon calibration curve beyond26 000 cal BP in 2004. In the absence of an internationally agreed calibration curvebeyond 26 000 cal BP, it is natural for researchers to compare onerecord to another (exactly as the IntCal team did). In doing this,however, it is wise to avoid use of the term 'calibration'since this does suggest an absolute scale, and instead use alternatives,for example 'comparison' as proposed previously (Richards& Beck 2001; van der Plicht 2000; van der Plicht et al. 2004). Implications for archaeologists So how should archaeologists treat the data that are currentlyavailable? The data are there to be used and studied and no-one wishesto stifle speculation about what those data mean for very importantarchaeological issues. Indeed, the calendar timescale created byradiocarbon largely shapes a number of questions and debates in thelater Palaeolithic period. It is thus not realistic to assume that thoseworking in the area will wait until the research is complete beforestarting to look at such issues (as for example in Mellars 2006 and thediscussion with Turney et al. 2006). However, it is important that thearchaeological community is aware of the different nature of theradiocarbon records. Back to around 12 400 cal BP, the period for which we have multiplerecords that are in good agreement, including tree rings, it seems verylikely that the calibration curve will not change significantly as newdata come to light and calibration can in most cases be used as a toolin studying archaeological chronology even in a fairly fine-grainedmanner (see Figure 1). This period of relative certainty is likely toreach back to about 18 000 cal BP once the new work extending the treering record reported by Mike Friedrich at the Oxford RadiocarbonConference is (eventually) completed. In this time period there are someminor issues that are still to be sorted out for very high precisionwork. These centre on how the different calibration sets are compiledinto a single curve. Such a compilation is undoubtedly the best policysince it ensures that no one dataset, with its inevitable possiblefaults, is given too much weight. All of the indications are that withinany one hemisphere there are no significant regional effects althoughsome very minor differences between records have been attributed todifferences in growth seasons (Kromer et al. 2001) or proximity to oceanupwelling regions (Stuiver & Braziunas 1998). Probably moresignificant is the fact that most of the calibration data are measuredon ten- or twenty-year sections of wood and therefore average outshorter-term to annual variations (see Figure 2--this visible noise willusually in effect cancel itself out over even a few years and especiallywithin the range of many typical radiocarbon measurements and theirassociated errors--minor exceptions may occur at times of major peaks ortroughs in the radiocarbon record--e.g. AD 1788-92--but it should alsobe remembered that this single-year record is not replicated and clearlycontains substantial noise as well as signal). There are also questionsover what the best statistical methods are for combining the datasets;the IntCal04 curve (Buck & Blackwell 2004; Reimer et al. 2004) usesa statistical model which introduces a small amount of smoothing to thedata (though no more than is apparently justified by the expected randomnoise - and indeed this model better reflects underlying data when wehave annual scale input when compared to IntCa198--see Figure 2). Sincesuch methods cannot distinguish between random outliers and real extremevalues there are some real short-term fluctuations that may beattenuated AttenuatedAlive but weakened; an attenuated microorganism can no longer produce disease.Mentioned in: Tuberculin Skin Testattenuatedhaving undergone a process of attenuation. in this compilation (especially when the underlying data areonly decadal or bidecadal). There is scope for further work to refinethese statistical methods. However, from the point of view of a user ofcalibration, IntCal04 provides the most comprehensive and up-to-dateestimate of the Northern Hemisphere calibration curve and should alwaysbe the first choice for calibration. Comparison of the results withthose obtained against the IntCa198 (Stuiver, Reimer, Bard bard,in Wales, term originally used to refer to the order of minstrel-poets who composed and recited the poems that celebrated the feats of Celtic chieftains and warriors. et al. 1998)calibration curve, which used a simple binning and weighted average ofthe data then available, can be valuable as can comparison againstindividual datasets. Such a degree of complexity is however only reallywarranted in large-scale Bayesian models (when the results are usuallyinsensitive in��sen��si��tive?adj.1. Not physically sensitive; numb.2. a. Lacking in sensitivity to the feelings or circumstances of others; unfeeling.b. to such changes) or wiggle-matching of tree ring sequences(where differences are occasionally significant if the match reliespredominantly on one or two fluctuations in radiocarbon levels). Fornormal calibration the IntCal04 curve is all that is required. Between 12 400 and 26 000 cal BP, the current situation is slightlydifferent. Here the calibration curve is based on multiple records ingood agreement, but these are all marine records and therefore representour best estimate of the atmospheric concentration. There may however bechanging marine reservoir offsets that could mean the curve in somesections of this time period is out by as much as 250 [sup.14]C years BP(Bondevik et al. 2006; Kromer et al. 2004). It is very unlikely to beworse than this given the agreement of IntCal04 with other records notused in the calibration curve, such as the terrestrial macrofossil mac��ro��fos��sil?n.A fossil large enough to be examined without a microscope. record from Lake Suigetsu (Kitagawa & van der Plicht 2000). In thistime range calibration for archaeological purposes is possible. However,such calibration is more provisional and there could be some minorchanges as new data accumulate, particularly from terrestrial records,which might be significant in certain contexts (see Figure 3). Further back than 26 000 cal BP, the situation is radicallydifferent. Here the records are neither based on purely terrestrialmaterial, nor do they agree with one another (see Figure 4). As statedabove, some of these discrepancies are being addressed actively andwithin a couple of years the situation is likely to be much better.However, the possible major discrepancies between the marine andatmospheric data need to be viewed with particular caution as they implythat even with consistent marine records we may still not understand howto interpret the records in the context of terrestrial archaeologicalsamples. Given this, it is clear why the radiocarbon community does notthink that calibration as such is possible in this time range, since itis not clear which, if any, of the present records provide a goodindication of the atmospheric radiocarbon concentration. Thus far thereis only one record that represents true atmospheric [sup.14]Cmeasurements (Lake Suigetsu); however, this record stands alone in thesense that it is not confirmed by others. We know for the period inwhich we do have an atmospheric record that there are many short-termfluctuations, which are missing from the marine record. This is likelyto be even more significant in periods where the climate is much lessstable, there may be major magnetic excursions, and the resolution ofthe marine measurements we do have is poorer. The highest resolution record in this time range, that from theCariaco basin (Hughen, Lehman et al. 2004), illustrates many of thesepoints clearly and also shows what can and cannot be done with thecurrent data. The samples for this record are marine, and they areabsolutely dated by matching changes in the characteristics of thesediments to changes in the climate as recorded by the Greenland icecores, in this case GISP2. This means that the timescale used is theGISP2 timescale, which is based on a model of ice accumulation beyond 41000 cal BE Recent work on the NGRIP core (presentation of J.P.Steffensen at the Oxford Radiocarbon Conference) suggests that the GISP2timescale has non-linear errors, which means that not only are theabsolute ages wrong, but that rates of change estimated from thistimescale may be significantly incorrect too. This in turn thensignificantly impacts archaeological assessments made using the 2004Cariaco record (as in Mellars 2006). As reported at the OxfordRadiocarbon Conference, this particular problem is likely to beaddressed by linking to other absolutely dated records, most likely thatat Hulu Cave (Wang et al. 2001). However, the uncertainty in thedifference between the atmospheric and marine radiocarbon concentrationwill not be so easily addressed. Even though these differences seem tobe fairly well behaved Adj. 1. well behaved - (usually of children) someone who behaves in a manner that the speaker believes is correct; "a well-behaved child"well-behaved in the late glacial we cannot assume that this isalways the case. That said, comparison of radiocarbon dates to thisrecord can undoubtedly be valuable, particularly if what is of interestis how the dates lie in relation to the changes in climate as recordedin the GISP2 [delta][sup.18]O record--but where this is done it shouldalways be made clear that the comparison is made against this record andis on the GISP2 or NGRIP timescale (as discussed in Gravina et al.2005). Other records are also valuable for archaeologists. Coral data,although also marine, link into a more absolute uranium-series-basedchronology. This is better from the point-of-view of absoluteages--though not as useful if you wish to compare them to the oxygenisotope records of the Greenland ice cores. Furthermore, the coral-basedrecords such as that of Fairbanks et al. (2005), are not continuousrecords, since they are based on chance finds of corals; nor is itlikely that they are an entirely random sample since formation factorslinking to climate and environmental changes are likely to bias therecovered sample set. Thus any curve generated from such datasets lookssmooth. But we must remember that absence of evidence is not evidence ofabsence and such a curve almost certainly fails to show even the scaleof fluctuations in the radiocarbon concentration of the oceans, letalone the levels of variability in the atmosphere. Available climateindicators suggest similar (e.g. Roig et al. 2001) or greater (e.g. Bond& Lotti 1995; Dansgaard et al. 1993) periods and cycles of changefor the later Pleistocene compared to the Holocene (e.g. Bond et al.2001). These would be reflected in an atmospheric [sup.14]C recordgiving at least as many, and very likely more, variations and cyclical cyclicalOf or relating to a variable, such as housing starts, car sales, or the price of a certain stock, that is subject to regular or irregular up-and-down movements. features than for the record available for the Holocene. At present weare largely lacking such information for the periods before terrestrialtree-based records, and measurement errors on very old radiocarbon ageswill anyway tend to mask some of the expected century-scale variation.The record for Lake Suigetsu is potentially very useful as it is purelyterrestrial, but it lacks a good absolute timescale. The speleothemrecords are partly terrestrial and so also provide useful information onthe possible scale of differences between the radiocarbon concentrationof the surface oceans and the terrestrial groundwater. No one record isright in all respects but all give information that is potentiallyuseful. Because their problems are all different it is also potentiallymisleading to compile them into a composite curve for calibration sincethis merely serves to mask the underlying complications. This is thereason for the ironically named NOTCal curve (van der Plicht et al.2004), and the criticisms levelled at aspects of the CalPal programreferred to by van Andel (2005). So what should the archaeological researcher working in this perioddo? Ignoring the problem, either by assuming that radiocarbon ages inthis period can be treated as some approximate proxy for age, or byusing some ad hoc For this purpose. Meaning "to this" in Latin, it refers to dealing with special situations as they occur rather than functions that are repeated on a regular basis. See ad hoc query and ad hoc mode. compilation of data into a 'comparison'curve as if it were a 'calibration' curve cannot be regardedas good scholarship. It is almost bound to result in conclusions andassertions which will have to be changed (and quite possiblysignificantly) within a very few years--indeed often before the researchis physically published. The uncertainties need to be fullyacknowledged. The correct approach will depend very much on theapplication. In many cases it may be appropriate to compare dates to anumber of different specific records--unless there are very particularreasons for one record being most appropriate. The timescale to whichthe comparison has been made (for example Uranium Series or NGRIP icecore) should be made explicit and the ages deduced would be betterreferred to as 'estimated' rather than 'calibrated'dates. Most crucially all should be aware that these estimates may wellchange significantly as our understanding of the Earth's systemduring the last glaciation improves. If absolute ages are the primaryinterest then there is not really much of a substitute for comparisonagainst all of the main records since this demonstrates the range ofpossible true ages depending on which of the records most closelyreflect the relevant reality. As the datasets improve, this exercisewill hopefully provide a narrower and narrower range of possibilities. Conclusions There has been considerable progress in recent years in the levelof information available for assessing the past radiocarbonconcentration of the atmosphere and oceans. This information is veryvaluable for archaeologists since it helps them to interpret theirradiocarbon dates in terms of absolute chronology. However, the cost ofthis progress is increasing complexity in the nature of the data, andthis means that archaeologists need to have a critical understanding ofwhat sort of analyses the data can and cannot support. Back to about 12 400 cal BP, the data are fairly robust and theIntCal04 calibration curve should provide accurate calibration for mostpurposes. Where very high precision is required, with large Bayesianmodels or wiggle-matching of tree ring sequences, it may also bevaluable to compare the results of such analyses against the IntCal98curve because it is compiled differently (even though it does have knowndeficiencies), or against individual datasets (such as Irish oak in thecase of British sites, for example). In the period between 12 400 and 26 000 cal BP any calibration ismore provisional since the data used for construction of the calibrationcurve are marine-derived. However, given the level of agreement betweenrecords in this region, such calibration is likely to be fairly accurateand for most purposes the IntCal04 calibration curve can be used as itis. In some critical applications, it may also be useful to compare suchcalibration to estimates from individual records. Beyond 26 000 cal BP, there is no accepted calibration curve simplybecause of the disparity in the records we have for this time period asof June 2006, and so comparison should be made to a range of individualrecords to estimate ages on the timescale relevant to the specificrecords. The records used for such comparisons will depend on thedetails of the application. If climatic correlations are important, thenrecords that link to climatic data will be most useful. On the otherhand, if absolute ages are the main issue, then the full range ofdatasets should be considered to see the range of possibilities. In order to prevent confusion, it makes a lot of sense to reservethe terms 'calibration' and 'calibrated dates' foranalyses based on the recognised calibration curves (IntCal04, SHCal04& Marine04). In the periods covered by these curves it may also beuseful to make a 'comparison' against other records. The term'estimated dates' for the results of such analyses seems mostappropriate. Where calibration is not yet possible,'comparison' against the different records now available maystill be useful but the provisional nature of such analyses should befully appreciated. As with the paper by Mellars (2006), speculationabout the implications of the data as they emerge are entirelyappropriate but the caveat at the end of that piece is important to keepfully in mind: 'A final, definitive calibration curve for this timerange will depend on the results of new calibration studies, at presentbeing pursued in several different laboratories. The full implicationsof these studies for the interpretation of the human archaeological andevolutionary record will need to be kept under active and vigilantreview'. If we always remember this, we should avoid the inevitabledisappointment when new facts emerge to overturn a beautiful and eleganthypothesis constructed on the basis of preliminary data. 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Manning(3), Paula Reimer (4) & Hans van der Plicht (5) (1) Research Laboratory for Archaeology and the History of Art The Research Laboratory for Archaeology and the History of Art is a laboratory at the University of Oxford which develops and applies scientific methods to the study of the past.As of 2005, the Laboratory is directed by Prof. Mark Pollard. ,University of Oxford, UK (2) Department of Probability and Statistics See the separate articles on probability or the article on statistics. Statistical analysis depends on the characteristics of particular probability distributions, and the two topics are normally studied together. , University ofSheffield The University of Sheffield is a research university, located in Sheffield in South Yorkshire, England. ReputationSheffield was the Sunday Times University of the Year in 2001 and has consistently appeared as their top 20 institutions. , UK (3) Department of Classics and The Malcolm and Carolyn WienerLaboratory for Aegean and Near Eastern Dendrochronology dendrochronology:see dating. dendrochronologyMethod of scientific dating based on the analysis of tree rings. Because the width of annular rings varies with climatic conditions, laboratory analysis of timber core samples allows scientists to , CornellUniversity Cornell University,mainly at Ithaca, N.Y.; with land-grant, state, and private support; coeducational; chartered 1865, opened 1868. It was named for Ezra Cornell, who donated $500,000 and a tract of land. With the help of state senator Andrew D. , USA; School of Human and Environmental Sciences, Universityof Reading, UK (4) 14CHRONO Centre for Climate, the Environment and Chronology,Queen's University Belfast, Belfast, Northern Ireland Northern Ireland:see Ireland, Northern. Northern IrelandPart of the United Kingdom of Great Britain and Northern Ireland occupying the northeastern portion of the island of Ireland. Area: 5,461 sq mi (14,144 sq km). Population (2001): 1,685,267. (5) Centre for Isotope Research, Rijksuniversiteit Groningen,Netherlands; Faculty of Archaeology, Leiden University The Faculty of Creative and Performing Arts is a cooperation between Leiden University and the Royal Conservatoire and Royal Academy of Art. The university has never had a faculty of economics, business or management, since all these decades one thought this would not fit into its , NetherlandsTable 1. Summary of different calibration records showing the sampletypes and the methods used to assess independently the (calendar)ages; the examples given are not intended to be an exhaustive list Sample material Plant fragments (terrestrial; Independent assumed young dating method Wood (terrestrial) on deposition)Tree rings Tree ring records (accurate to the (see main records year) in Reimer et al. 2004)Uranium series (quality depends on samples)Ice cores (subject to modelling or counting errors)Varved sediments Varved lake (susceptible to records (e.g. missing varves Kitagawa & van and counting der Plicht 1998) errors) Sample material Foraminifera (oceanic; depth Independent Corals (surface depends on dating method ocean) species)Tree rings (accurate to the year)Uranium series Coral records (e.g. (quality depends Bard et al. 1998; on samples) Chin et al 2006; Cutler et al. 2004; Fairbanks et al. 2005)Ice cores (subject Ocean sediment to modelling or records (e.g. counting errors) Bard et al. 2004; Hughen et al. 2004b)Varved sediments Varved ocean (susceptible to sediments missing varves (Hughen et al. and counting 2004b) errors) Sample material Speleothems tufas, etc. (mixed Independent terrestrial and dating method geological carbon)Tree rings (accurate to the year)Uranium series Speleothems and (quality depends Tufa records (e.g. on samples) Beck et al. 2001; Stein et al. 2004; Vogel & Kronfeld 1997)Ice cores (subject to modelling or counting errors)Varved sediments (susceptible to missing varves and counting errors)