A NEWLY DISCOVERED MAMMOTH IN SAN JOSE, CALIFORNIA, POSES A STRATIGRAPHIC PUZZLE

A partial skeleton of a Columbian mammoth ( Mammuthus columbi ) found in San Jose, CA, in July, 2005, poses a stratigraphic dilemma: either the mammoth is significantly younger than others found in North America, or stratigraphic details and correlations recently developed in the same area are not representative.

A well preserved ventral skull with molar teeth and articulated tusks, a partial ilium, a partial femur, distal toe bones, and rib fragments occurred together in deposits previously mapped as Holocene in a modern stream bank at an elevation ~ 4.5 m above mean sea level and ~ 3.5 m below the modern floodplain. The silty sand fossil matrix is mottled yellowish gray and reddish brown and has clay-lined cracks. C-14 ages of two charcoal samples from sediment correlated to 65 cm below the mammoth bed are 12,290 and 12,730 radiocarbon years (corrected to 14,400 and 14,900 ybp), and the age of a very low carbon, bulk soil sample from the matrix near the mammoth femur is 3,530 radiocarbon years (corrected to 3,550 ybp). The youngest mammoth on record from North America is 7,700 yrs (Agenbroad, 2004), making a 3,500-yr-old mammoth in San Jose highly unlikely.

Post-glacial alluvial sediment is about 18 m thick in a nearby well, consistent with our model of accumulation soon after the glacial maximum at 18 ka in a basin subsiding at 0.4 m/ka (Wentworth and Tinsley, 2005). This makes a stratigraphic age of 14-15 ka at 3.5 m depth much too old, given any reasonable rate of sediment accumulation. A C-14 age of 14,000 ybp on plant fragments at a depth of 6 m in the well is also anomalously shallow, but the fragments are detrital. Similar reworking of older mammoth remains into post-glacial deposits seems unlikely, however, given the closely associated preservation of the skull and postcranial elements representing a single animal about 1.8 m tall at the shoulder.

A latest Pleistocene age for the mammoth matrix, suggested by the apparently considerable degree of soil development, would require that the unconformable top of the previous alluvial cycle (about 140-20 ka) be anomalously shallow, due either to preservation of a locally unusual thickness of that cycle or to local uplift on an unknown fault. But the 14-15 ka dates are too young for such an interpretation, as they define a maximum age for the stratum just beneath the fossil.

The dilemma is unresolved.