Sunday, December 04, 2005

40,000 Year Old Footprints: Revisited

Something is bothering me about this story. Consider this quote (from the press release:

Renne determined the new date using the argon/argon dating technique, which reliably dates rock as young as 2,000 years or as old as 4 billion years. The British-led researchers, however, relied mainly on carbon-14 dates of overlying sediments. Carbon-14 cannot reliably date materials older than about 50,000 years.

This is incorrect. This is a list of the dating methods used by Dr. Gonzalez's team:

1)Accelerator Mass Spectrometer radiocarbon dating (AMS) was carried out on mollusc shells and organic balls at the Oxford Radiocarbon Accelerator Unit (ORAU) by Drs Tom Higham and Chris Bronk Ramsey of Oxford University.
2)Electron Spin Resonance (ESR) on a mammoth molar found in the Barranca Caulapan (Valsequillo Basin), was carried out by Professor Rainer Grün, Australian National University.
3)Optically Stimulated Luminescence Dating (OSL) on sediments and the Xalnene Ash was carried out by Dr Jean-Luc Schwenninger from Oxford University
4)Argon-Argon dating on the Xalnene Ash and overlying lava was carried out by Dr Simon Kelley, Open University.
5)Uranium series dating on animal bones from the Valsequillo Gravels was carried out by Dr Alistair Pike, Bristol University.

above is a schematic of the stratigraphy with locations were they took the samples and what type of method was used. For those who don't like drawings:

The results derived from different dating methods indicate that the Valsequillo gravels from the Barranca Caulapan area range in age from the Late Pleistocene, around 40,000 years ago, to the early Holocene, around 9,000 years ago. It is important to note that the Barranca Caulapan sands and gravels analysed were taken from strata geologically younger than the Xalnene ash layer in which the footprints were preserved.

The mammoth molar was dated at approximately 27,000 years old using ESR; while an organic ball in the gravels was dated at 25,000 years old using C14. These materials were transported and incorporated into the fluvial sequence of the Barranca Caulapan.

However, the mollusc shells are in situ in the deposits and were dated between 27,000 and 39,000 years old using C14.

Attempts were made to date the Xalnene ash using Argon-Argon dating but the amount of potassium present in the ash was too low for a reliable age determination.

More successful was the Optically Stimulated Luminescence (OSL) method to date quartz crystals found in baked lake sediments incorporated into the ash (as xenoliths) and secured an age of around 40,000 years. The sample was taken from the working quarry, 200m south from where the footprints were found.

I'm in the process of tracking down the Nature article to see what it says, but certainly this press release mischaracterizes the work of Dr. Gonzalez and his team. I'm wondering if a correction needs to be issued on this, but I'm not sure if one can actually issue a correction on a press release. I'm debating emailing the contact on the press release, anybody have any advice on the subject?