FIGURE 3.

Potential rates of carbon fixation by seafloor-exposed basalt-hosted microbial communities (expressed as nmol C incorporated g^-1_rock d^-1) as a function of incubation time, based on incubation experiments of basalt samples with ¹³C-labeled bicarbonate. Error bars represent the range of the coefficient of variance from the measurement of organic carbon weight percent, since the mean value for each sample set was applied to all samples in that set (see Materials and Methods); in some cases, data symbol is larger than the error bar range. Samples with isotopic composition differences of less than 2‰ from the time zero sample (Table 3) were assumed to be below the limit of detection of the method (see Materials and Methods) and are not reported here.