The Analytical Chemistry article was based on studies done of an 800-year-old site near Lyon, France. The site appears to have been a garbage dump where many old leather shoe soles were found in an excellent state of preservation. The news article notes:
They used laboratory technology called nuclear magnetic resonance to compare composition of the ancient leather to modern leather. It turned out that tannin, which helps to preserve leather, had been washed out of the old soles and replaced by iron oxides. The iron oxides, which leached into the leather from surrounding soil, helped preserve the soles in the absence of tannins.
In case anyone wishes to track down the Analytical Chemistry article, the news blurb dates back to March 2009. Analytical Chemistry is a semi-monthly journal, and the news blurb said that the article about the soles study was in the "current issue". The news blurb itself can be found here.