No. 100: Jul-Aug 1995
In the 1980s, skeptics had a lot of fun debunking the Shroud of Turin, the supposed burial cloth wrapped around Christ. Their glee was unbounded when radiocarbon dating "proved" that the shroud could not be older than 700 years. The skeptics may have been too quick to celebrate, because the samples that were sent to the radiocarbon lab may not have been wholly cloth.
The reality of our biosphere is that virtually everything is permeated with microbes and their products. S.J. Mattingly and L.A. Garza-Valdes, of the University of Texas at San Antonio, have been studying "biogenic varnishes" for years. These plastic-like coatings are produced by bacteria and fungi. Sure enough, microscopic examination of a few linen fibers from the Shroud of Turin show that they, too, are coated with such varnishes. These biogenic varnishes may introduce carbon that has been recently fixed from the atmosphere and thus make the sample's age appear younger than it really is.
(Travis, John; "Microbes Muddle Shroud of Turin's Age," Science News, 147:346, 1995.)
Comment. More than the Shroud is at stake here. Bacteria contaminate just about everything, including wood and bone from archeological sites. Bacteria may, therefore, "rejuvenate" samples sent in for radiocarbon dating. The importance of this phenomenon is still unclear.
Cross reference. Radiocarbon-dated samples may also appear erroneously "aged" by the uptake of primordial carbon (C13) present in the earth's crust. See SF#99.