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Recent and older (early Holocene to Pleistocene) hot-spring travertine carbonates from central Italy have two distinctive macrofabrics, crystalline crusts and shrubs. Crystalline crusts are laminated slope deposits that formed abiotically following CO ₂ degassing from spring water. The formation of shrub travertine--irregular, dendritic precipitates from pool environments--is controversial and has been attributed to both abiotic and microbial processes. Oxygen isotope variation in our travertines can be explained by abiotic processes, mainly CO ₂ degassing. In contrast, our carbon isotope data cannot be wholly explained by abiotic CO ₂ degassing invoked in earlier studies. Because photosynthesis is known to preferentially remove ¹² C, leaving ambient waters enriched in ¹³ C, this fractionation should be recorded in delta ¹³ C values of microbially influenced travertine. Our shrub carbon isotope values are …