Articulating the ancient Maya and modern European calendars depends on a correlation constant that has been debated for over a century1,2,3,4,5. Correlation is required because the Maya Long Count system fell into disuse before the arrival of the Spanish in the 16th century, leaving only a few clues to their correct alignment in early colonial chronicles and native documents3,6. Many solutions to the problem have been proposed, employing a variety of historical and astronomical data7, with results separated in time by ~1000 years. The most widely accepted was first put forward by Joseph Goodman in 19051, which, after certain modifications, is known as the Goodman-Martínez-Thompson (GMT) correlation. In no small part the acceptance of the GMT correlation is based on a radiocarbon study that was carried out in the 1950s using gas counting of β particles from 14C decay in two wooden lintels from the ancient Maya city of Tikal (Guatemala) that carry carved dates that can be fixed in the Long Count system8. The analytical error of these measurements and other uncertainties associated with this early radiocarbon study do not fully resolve the problem and support multiple correlations at the 95% confidence interval (Supplementary Fig. 1, 2). Here we report a series of high-resolution AMS 14C dates from one of these wooden lintels (Manilkara zapota; commonly chico zapota or sapodilla) at Tikal (Lintel 3, Temple I; Fig. 1) with Maya calendar dates indicating that it was carved between AD 695 and 712 using the GMT correlation. These dates are wiggle-matched9,10 to a mixed 14C calibration curve (IntCal0911, SHCal0412) using a Bayesian statistical model that includes tree growth rates estimated from changing calcium (Ca/C) concentrations that are linked to differential uptake seasonally13. The combination of high-resolution AMS 14C dating and calibration using tree growth rates provides a more definitive test of the GMT correlation.

Figure 1 Temple I (a, Photo: D. Webster) and Lintel 3 (b, Photo: Courtesy Museum der Kulturen Basel and UPenn Museum) at Tikal. This is what remains of the carved panels from Lintel 3 (b) memorializing Jasaw Chan K'awiil and his victory over Yich'aak K'ahk' of Calakmul. The carved lintel beams in color are at the Museum der Kulturen Basel (Switzerland) and the black and white panels are at the British Museum. (c) Cross section of lintel beam e showing sequential 14C sampling locations through the trees growth. The number of years between AMS 14C samples was determined using seasonal Ca/C cycles measured via LA-ICP-MS (Fig. 2, Supplementary Fig. 6). Full size image

The Long Count calendar is one of the defining features of Classic Maya civilization (AD 300–900, GMT correlation). These were not the first such dates in Mesoamerica and the system was likely adopted from adjacent regions where dates appear on stone monuments hundreds of years earlier (36 BCE, Chiapa de Corzo)14. The Classic Maya franchised the system and it proliferated to more than 40 different centers across the lowlands between AD 600–90015,16,17,18,19. These dates were used to anchor major historical events in time and the result is a remarkable chronicle of royal successions, rituals, victories and defeats in war, hierarchical relationships and regal marriages17. These events are ordered in time by a count of individual days, the Long Count, but correlation is necessary to tie this rich record to the European calendar and to make comparisons with other sources of archaeological, environmental, or climatic data with chronologies based on 14C and uranium-series dating20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35.

The Long Count consists of a sequence of five time units: Bak'tun (144,000 days), K'atun (7,200 days), Tun (360 days), Winal (20 days) and K'in (1 day). The numbers 0–19 (represented by a bar [5], dot [1] system; with a zero symbol) were then used as multipliers for each unit so that the date 9 Bak'tuns, 13 K'atuns, 3 Tuns, 7 Winals, 18 Kins (noted as 9.13.3.7.18) is 1,390,838 days from a mythical starting point on August 11, 3114 BC using one variant of the GMT correlation. In this case a coefficient of 584283 days is added to the Long Count to obtain the equivalent day in the European calendar. This date was carved on Lintel 3, Temple 1 at Tikal (Fig. 2) and in the European Calendar is August 6, 695, the day that King Jasaw Chan K'awill I of Tikal defeated Yich'aak K'ahk' (‘Claw of Fire'), a long-standing rival king of the powerful center of Calakmul located 90 km to the NW. Alternative correlation constants span nearly a millennium and range from 450,000 and 775,000 days and are based upon historical and astronomical data7,36.

Figure 2 Tikal lintel Ca/C data (Green) shown relative to a stalagmite δ18O regional rainfall record from Yok Balum cave in southern Belize (Blue)16. Spectral analysis of the Ca/C record indicates annual growth rates between 0.94 and 1.43 mm per year (see Supplementary Fig. 6). Incremental δ18O measurements of wood cellulose (red) from the Tikal lintel between AD 615 and 631 are shown relative to the rainfall record from southern Belize. Radiocarbon date distributions (2σ ranges, gray) are shown along with the number of years between these dates estimated from spectral analysis of the Ca/C data (see supporting documentation for details). The upper panel shows a series of historical events recorded in the region prior to the dedication of Temple 1, Lintel 317 that occurred during the growth of the M. zapota tree (beam e). Full size image

The GMT correlation hinges on historical texts that describe specific events (e.g. a massacre at Otzmal in the Yucatan) and the European year that they occurred (AD 1536) along with a date in a derivative Maya calendar that counts a series of K'atuns that reoccur every ~260 years (in this example 13 Ahau). Other historical and astronomical data is used to bolster the result2,3 and in 1960 a University of Pennsylvania radiocarbon study of wooden lintel beams from Tikal bearing Maya dates was thought to provide independent verification8. Two lintels and multiple roof beams from Temple I and Temple IV were radiocarbon dated and compared to the expected European calendar dates using different correlation constants. The samples were large and taken from beam exteriors only. In addition, all of the 14C dates from Temple I and IV have large analytical errors so that they show some overlap with the GMT along with several other correlation constants at the 95% confidence level when calibrated using the IntCal09 calibration curve (Supplementary Fig. 1, 2 & Tab. 1, 2)11. These alternative correlations (X and C) cannot be ruled out definitively. This is significant because even the archaeologists/epigraphers that championed the GMT correlation (Thompson 1935) never considered it to be infallible because the uncertainties in the historical and astronomical records left the possibility for other solutions. With only a few dissenting voices4,37, the GMT correlation is widely accepted and used, but it must remain provisional without some form of independent corroboration.