Progress in studying the active biosphere in the deep seafloor is currently
limited by the unknown amount of drilling-induced microbial contamination.
Contamination tests were conducted during Leg 185 of the Ocean Drilling Pr
ogram on the drillship JOIDES Resolution to assess the suitability of this
platform for deep biosphere research. Tests using both a chemical tracer [p
erfluoro(methylcyclohexane)] and a particulate tracer (0.5-mu m-diameter fl
uorescent microspheres) were conducted during coring with the advanced hydr
aulic piston core in unconsolidated sediments and with the rotary core barr
el and diamond core barrel in igneous rock. Detection of both types of trac
ers on the exterior of recovered cores confirmed successful delivery. The p
articulate tracer was never detected in the interior of unconsolidated sedi
ment (n = 24). The average concentration of the chemical tracer (n = 12) wa
s equivalent to 0.35 mu L of drilling fluid per gram of sediment. The parti
culate tracer was not found in the interiors of igneous rock samples that w
ere crushed (n = 4) but was found in the interiors of 64% of the thin secti
ons examined (n = 12), indicating that the samples were contaminated during
sectioning rather than drilling. Perfluorocarbon data indicate that drilli
ng fluid in the igneous rock samples averaged 0.01 mu L g(1) rock. Based on
the abundance of bacteria in the surface seawater (4.2 x 10(8) L-1), which
was used as the drilling fluid, the potential contamination of both sample
types is on the order of a few bacteria per gram of cored material. This e
stimate is conservative, given the high pervasiveness of the perfluorocarbo
n tracer relative to microbes.