Replication is often considered the gold standard of science, yet there are multiple potential reasons why a replicative test might fail, beyond the validity of a technique alone e. Another barely misses that standard by years but matches it at two standard deviations.
The shift from botryoidal to lamellate micromorphology is currently understood as the change from the LU1 to LU2 paleoclimatic periods terminology from [ 6 — 8 ]; see Figure 2 and is set at about 12, yrs cal BP. The microstratigraphic distinction proved to be more easily seen in thin section, and Dorn identified seven Pleistocene wet event dark layers that are rich in Mn and Ba, matched by six intervening less wet period light layers that are rich in Si and Al.
The sequence initially extended from 12, to about 60, cal yrs BP. The only visible microstratigraphic change whether seen with micrographs or thin sections in the archaeological time range then was the Pleistocene-Holocene boundary, thereby effectively precluding dating California rock use of the technique for most of the New World archaeological record. Though I discuss the validity of CR dating in more detail below, it is worth noting here simply that it has analytical weaknesses, probably the most pronounced of which is that it is based on geochemical processes and these potentially can reverse over time.
This question is addressed starting with a reconsideration of rock varnish chronometrics as applied to Great Basin, eastern California, petroglyphs. This reporting convention, however, makes cumbersome the comparison of VML to other chronometric ages in standard form using a comparison between calculated standard errors. The comparability of the using the two dating techniques has implications at two levels: 1 the validity of CR dating in general terms, important in light of early controversies about this technique; 2 the dating California rock of specific individual petroglyph ages.
The CR technique was developed by Dorn and first applied to petroglyphs in the early s [ 24 — 2729 — 3135 — 38 ]. Although the initial CR dates were experimental, their recent combination with VML dating below provides greater scientific confidence in the resulting ased petroglyph ages.
Sixty-seven petroglyphs and one natural control rock surface are included in the sample, representing independent chronometric assays 3 Table 1 —one of the largest chronometric rock art data sets in the world. When these dates were first reported, many North American archaeologists considered them an empirical impossibility, due to the then-prevailing consensus concerning the Clovis first peopling of the Americas 2.
In addition to multiple replications, the technique has also been successfully subjected to petroglyph dating blind tests [ 66 — 69 ]. Great Basin petroglyphs are well known due to influential early syntheses of this art [ 1112 ]. The rate of change can be calibrated regionally, using independently dated control surfaces, such as varnish-covered basalt flows. This demonstrates, conservatively, that the petroglyph tradition began before 11, YBP, probably before 12, YBP, and potentially in the 14, years range.
The lengths of the intervals between wet events vary from to years, with an average of —roughly years.
Dating california: stratigraphy and geochronology
This too was conducted as a blind test in the sense that the primary goal was to resample ly dated petroglyphs to confirm or reject their pre-existing calculated ages, as well as to sample additional engravings. On the archaeological side, the initial research focused on pd earliest motifs and unexpectedly obtained a handful of pre-Clovis aged [ 26274344 ], that is, dates older than 11, years old, then assumed to be the start of the Clovis period.
The result was extreme skepticism, if not outright rejection, of the newly introduced technique, for straightforward reasons: the chronometric ages were older than the pd first peopling of the Americas. These studies are summarized below with the intent of providing a hemisphere-wide overview of early symbolic behavior.
Rock art dating and the peopling of the americas
These data were obtained and analyzed in the following fashion. The resulting correlated VML ages are certainly not as precise as radiocarbon ages, but they are adequate for age asment to the broad time periods comprising the regional cultural historical sequence. Few New World archaeological problems have received more attention than and experienced as much debate as the initial peopling of the Americas.
His use of this convention is intended to emphasize the fact that these are minimum-limiting ages, not specific calendrical determinations. The result is evidence for the development of regional cultural diversity in the Americas by Paleoindian times.
A key attribute of archaeological modernity is the ability to conceptualize and employ symbols, including the capacity to make and use art [ 4 ]. Included in these is my own replication, fully independent of Dorn and his lab, in southern Africa [ 65 ]. The objective was to obtain confident temporal asments. Over a decade of research also resulted in a Holocene VML calibration [ 57 — 10 ]making this approach widely dating California rock in archaeology.
In order to reevaluate the Great Basin petroglyph chronology, a series of chronometric analyses were conducted, some of which involved blind tests, with the assistance of Dorn and Liu, each working independently. It is believed to be the largest rock art concentration in the Americas, and perhaps one of the biggest in the world. Liu reports his VML ages as single dates e. These processes leach out mobile trace elements notably potassium and calciumthrough capillary action, more rapidly than less mobile trace elements particularly titanium; see [ 293039 — 42 ].
Two points need emphasis in order to understand the .
What part did art play in the peopling of the Americas? In light of the varnish dating controversy and the uncertainties and misunderstandings that it generated, I discuss the current status of these two chronometric techniques before turning to the revised rock art chronology.
It resulted in improved temporal resolution of the Pleistocene sequence and the extension of the technique back to aboutcal yrs BP [ 75 ]. David S. The peopling of the Americas is both the oldest and most frequently researched question in American archaeology. As noted above, during the Holocene these periods average about years in length, but their spans range from to years.
Though initially employed in the Americas, it was subsequently used in South America, Africa, Asia, and Australia discussed below. The Coso petroglyphs vary from the remainder of the Great Basin art not due to ificant stylistic or known cultural differences but instead as a function of variations in the proportional motif emphases at individual sites and within smaller regions.
More confidently, there is a consensus that the initial immigrants where behaviorally modernin the archaeological sense of these terms e. Although we still do not know when humans arrived in the Americas, we can assume dating California rock they were fully capable of producing art, and potentially had the proclivity to do so. But what and where is the evidence for the earliest American art, and what does it tell us about the peopling of the hemisphere?
VML analyses involve the identification of the time period during which revarnishing began to develop. Liu was given no information on the name of the sites visited, the petroglyph ages, or their sample identifications in the literature.
Dating rocks and fossils using geologic methods
Nonetheless, the VML ages are adequate for the construction of a petroglyph chronology, and for a generalized independent evaluation of CR dating. Note however that none of the VML readings were rejected as clearly in error by either analyst; instead in some cases Liu or Dorn felt that additional sampling and thin sections are required for full confidence in specific inferred minimum-limiting ages. My point of departure in this discussion is a chronometric reanalysis of Great Basin petroglyphs rock engravingsdirected specifically at identifying the earliest art in far dating California rock North America.
Despite these uncertainties, the majority opinion currently seems to maintain that humans first arrived sometime prior to 13, years ago, though how much earlier is unknown e. Two decades later, archaeological opinion has reversed and now supports greater time depth for the initial colonization of the hemisphere. This calibration was developed for the Mojave Desert, thereby facilitating its application to the Coso Range and other eastern California petroglyph localities, and accommodating its use as an independent check on CR dating.
At other Great Basin sites, outside the Cosos, the same motif assemblage typically occurs, although geometric des commonly predominate. But simple and complex geometric des are typical roughly one-third of the total and are frequently intermingled with the ostensibly identifiable images.
There are two implications of this circumstance. Fourteen of the 32 have CR ages that statistically overlap the minimum-limiting VML ages confirmed by both analysts at one standard deviation. Although regional microlamination sequences appear to vary and require local calibration, the Saharan, South American, and other international studies [ 8586 ] indicate that VML has the potential for widespread application in arid and semiarid environments.
This change in circumstance does not prove that the pre-Clovis CR ages were correct, but it negates the original archaeological argument against and dating California rock towards the technique. Although AMS-WRO dating is in fact currently unusable [ 3334 ], the controversy strictly had no implications for the other techniques, and ificant geomorphological research on them has occurred in the interim.
Liu more recently has defined the Holocene VML sequence for the Mojave Desert [ 5 — 10 ], making the technique fully applicable for archaeological purposes in this region. The initial utility of VML dating in fact was hampered because it was restricted to a layering sequence for the Late-Terminal Pleistocene.
Using the published Holocene calibration dating California rock 5 — 10 ], Dorn chronologically interpreted and annotated images of each of these thin sections. This demonstrates that it is more common than generally recognized, includes both portable and landscape i. Even with decades of research, basic questions like the earliest entry date and colonizing route remain elusive. No accurate figure has been obtained for the of Coso motifs, but credible estimates vary from hundreds of thousands to millions of individual engravings.
Although I use these provisional VML ages below, they should be recognized as not yet independently verified by a second analyst. Although rarely considered, early art has the potential to provide insight into questions that may be obscured by other kinds of evidence, particularly stone tools. As is often the case with research on new topics, this resulted partly from simple confusions; first over techniques and analytical measurements used to assay the varnish sample chemistry e. Liu prepared the thin sections and interpreted the. Perry and Adams [ 70 ] first observed continuous orange microlayers in thin sections of rock varnish, interbedded with black bands, representing manganese-poor and manganese-rich layers, respectively.
Micrographs showed the Pleistocene-Holocene boundary as a change in micromorphology, with botryoidal layers developing during the earlier wetter period and lamellate varnish during the drier Holocene. These were submitted to Liu blindly: he received no information on the identification or ly determined age of these samples.
It also included a blind test matching VML against cosmogenic dating [ 7778 ]. An electron microprobe is typically used to measure the chemical constituents of varnish samples, which are pin-head sized flakes that are removed from the engraved-out portion of a revarnished petroglyph.
This necessarily requires a discussion of recent advances in petroglyph dating techniques before turning to the larger issues at hand. Comparison of these ages dating California rock evidence from other regions in the hemisphere demonstrates substantial artistic and stylistic variation in rock art by the Paleoindian period circa 10,—11, YBP. This suggests that, while art may have been part of the baggage of the first immigrants, regional cultural traditions had already been developed by the Terminal Pleistocene, if not earlier.
Subsequent research has shown that these microlaminations occur over wide regions and correlate with major paleoenvironmental changes—alternating dry and wet periods [ 29 — 313771 — 76 ]. Any discussion of potential early art requires a consideration of dating; in this case, rock art chronometrics and much of the early rock art chronometrics research was conducted in the Coso Range and Mojave Desert e. The first CR petroglyph dating were controversial in two ways. Importantly, VML dating has been independently replicated and applied to the archaeological record by different research teams working in the Sahara Desert [ 80 — 83 ], and Argentina [ 84 ].
It is based on cation exchange processes in the rock varnish that coats many petroglyphs in arid regions. An unfortunate controversy developed concerning this last technique in the late s 1 ; one result of which is the widespread but incorrect assumption among archaeologists that rock varnish dating as a whole is no longer viable [ 32 ]. That said, it is still apparent that the concordance between the two dating techniques is good, especially when both analysts agreed on the VML age asments. It is conceptually analogous to dendrochronology in that it involves the identification of regional microstratigraphic dating California rock similar to tree growth patterns caused by climate change, although it lacks the temporal resolution of tree-ring dating.
Although originally developed by Dorn, Tanzhuo Liu improved and extended the technique subsequently, working at the Lamont-Doherty labs at Columbia University with Wallace Broecker. As noted above, the comparison of the from these two techniques involves different mathematical conventions so that the standard criterion—whether the sets of ages overlap at one or two standard deviations—cannot be employed.