Port Eliza Cave: The sedimentology, stratigraphy and palaeontology of cave deposits and their implications for a human coastal migration route. [La grotte de Port Eliza : sédimentologie, stratigraphie et paléontologie des dépôts des grottes et leurs implications concernant la route côtière de migration].

Résumé

The timing of late Pleistocene glacial advance, retreat, relative sea level and environmental viability between 25 and 12.5 ka (14C yrs BP) remain a key issue in the feasibility of a coastal migration route for the first North Americans. Here we present stratigraphic, radiometric and faunal data for Port Eliza cave, a raised sea cave, on the west coast of Vancouver Island, British Columbia. A 2.5 m deep excavation near the back of the cave revealed three units: (1) > 50 cm of relatively massive, silty-sandy diamicton that contains bones, striated clasts and small fragments of dripstone (2) 2 m of laminated clay and (3) 20 to 30 cm of oxidized, weakly laminated to massive silts and clays containing dripstone fragments, rare bones and charcoal, capped by stalagmites. The genesis of unit 1 is problematic however, the unit likely represents the surface of the cave floor into which bones became incorporated over time by resedimentation and bioturbation. Unit 2 represents deposition by suspension settling in a subglacial lake, indicating ice cover during the LGM. Unit 3 is interpreted as early postglacial and Holocene accumulation of resedimented fines (from unit 2) in standing water in topographic lows. Unit 1 has yielded 4 radiocarbon dates from single bone fragments of known species ranging from 18.0-16.3 ka. These dates show that ice cover on the outer coast was brief, from ca. 15.5-14 ka. A diverse vertebrate fauna of marmot, vole, marten, cervid and various species of birds and fish indicates a partially treed landscape with the sea near its present level. The fact that relative sea level was close to the cave at a time when eustatic lowering would otherwise have placed the shoreline ~ 15 km away, implies significant isostatic depression. The terrestrial vertebrate fauna is consistent with a cool, open parkland environment with maximum summer temperatures cooler than present. Although most fossils represent small mammals, the presence of at least one cervid confirms viability for ungulates, a significant source of food for humans. The marine fauna also indicate the proximity of a rich marine environment and suggest that salmon runs, affording an abundant seasonal source of protein, likely existed prior to the LGM. The existence of this diverse vertebrate fauna as late as 16 ka demonstrates both marine and terrestrial resources available to support humans, confirming the viability of the coastal migration hypothesis for this portion of the route.