Depositional facies and aqueous-solid geochemistry of travertine-depositioning hot springs (Angel Terrace Mammoth Hot Springs, Yellowstone National Park, U.S.A.) - Reply

The thoughtful discussion by Andrews and Riding (2001) highlights several fundamental issues regarding the use of terrestrial hot springs as natural laboratories in which to identify biotic versus abiotic products of carbonate mineral precipitation. An urgent need exists to develop criteria that will consistently discriminate between carbonate precipitation mediated by microorganisms (biologically controlled and biologically induced) and precipitation resulting solely from inorganic processes (Lowenstam 1981; Lowenstam and Weiner 1982; Mann et al. 1982; McIntosh and Groat 1997). Results will permit accurate interpretation of the absence or presence and role of microbial life during the precipitation of ancient carbonate minerals within the context of aqueous paleoenvironmental conditions. Foulke et al. (2000) illustrate that both abiotic and biotic processes influence modern travertine precipitation at Angel Terrace in the Mammoth Hot Springs complex of Yellowstone National Park. On the basis of systematic changes in aqueous chemistry and travertine crystalline texture and composition along a hot-spring drainage flow path, a five-component depositional facies framework has been established (Foulke et al. 2000). The facies context creates a process-oriented model (e.g., Wilson 1975; Walker and James 1992) for paleoenvironmental reconstruction of ancient travertine from crystal fabric, chemical composition, and spatial distribution. Modern facies also provide a comparative baseline with which to evaluate postdepositional (diagenetic) alteration of ancient travertine. The Angel Terrace study indicates that abiotic factors such as degassing and temperature change dominate over biological mechanisms in controlling the δ ¹³C and δ ¹⁸O composition of travertine precipitating in high-temperature ( ∼ 45 to 73°C) facies (fig. 3 in Fouke et al. 2000). Conversely, microbial aerobic respiration is detectable above the background degassing and temperature fractionation effects in travertine δ ¹³C and δ ¹⁸O precipitated in low-temperature (25 to ∼ 45°C) facies (Fig. 3 in Fouke et al. 2000). Angel Terrace was compared with several other previously studied hot springs by Fouke et al. (2000), including Terme San Giovanni near Rapolano in Tuscany, Italy (Guo et al. 1996). Andrews and Riding (2001) have clarified the environmental context of travertine precipitation at Terme San Giovanni, which was incorrectly interpreted in Fouke et al. (2000). The environmental relationships of Guo et al. (1996) travertine data is therefore amended accordingly in this reply and compared to the Angel Terrace travertine. Andrews and Riding (2001) also suggest that the Angel Terrace study does not adequately demonstrate that CO ₂ degassing is the dominant agent for δ ¹³C fractionation at Angel Terrace. They further imply that Fouke et al. (2000) do not acknowledge the possible influence of microbial activity and associated water flow on the Angel Terrace travertine isotopic compositions. This reply will illustrate that neither assertion by Andrews and Riding (2001) is accurate via comparison of the hydrogeological systems, sampling strategies, and travertine fractionation processes at Terme San Giovanni (Guo et al 1996) and Angel Terrace (Fouke et al. 2000).