SURMOUNTING THE PROBLEMS OF RADIOMETRIC DATING IN THE SUBMERGED CAVES OF THE YUCATAN: THE LA MINA AND HOYO NEGRO EXAMPLES (Invited Presentation)

The submerged caves of the Yucatan are a rich in fossils of late Pleistocene mammals and pre-ceramic humans, and actively mined by Paleoindians. The scientific potential of these finds is immense but won’t be fully realized if they re not accurately placed in time. Dating Pleistocene remains can often be difficult, but it is especially problematic when they have been submerged for millennia in waters rich in carbonate and organic material. Opportunities for 14C dating are limited: bone collagen is rarely preserved; charcoal and wood float, and may be moved about the caves long after they are originally deposited. Direct bone dating is thus difficult and superimposed charcoal is an unreliable medium for establishing even minimum ages. The food chain in dark caves can be based on ancient organic material, like guano, or include dissolved inorganic carbon (DIC) from limestone bedrock, requiring us to suspect any organic material of uncertain origin. Bioapatite can be contaminated by DIC, producing falsely ancient dates. Uranium-thorium dating can only be applied in restricted cases and should never be applied directly to tooth or bone material. Sea level history is only now becoming understood well enough to be a chronological guide.

We have been able to surmount, or at least limit these problems in studies of the deep natural trap of Hoyo Negro and the ochre mines of upper tunnels in coastal Quintana Roo using a variety of approaches. In the ochre mines, we established associated charcoal as artifacts and likely produced by human activity 12.0-10.2 ka by identifying burned tree species and distinguishing them from forest fire charcoal found in Hoyo Negro. At Hoyo Negro itself, with its rich fauna and complex depositional setting, we combined 14C dating of bioapatites as terminus post quem and overlying charcoal, guano, or calcite as terminus ante quem, findings backed up by the chronology of postglacial sea-level rise. Animal fossils date between 43 and 10 ka. Comparison of bone apatite ages with actual ages based on organic ¹⁴C or U-Th calcite ages in three unusual cases (bat bones from guano, bone and charcoal from an isolated overhung ledge, flowstone on a complete skeleton) demonstrated a consistent ~3.2 ka offset between the two. This has enhanced confidence in the ~12.9 ka enamel-age of “Naia” the young woman from this site, verifying her as early Paleoindian in age.