Figure 3. Typical profiles of scalar irradiance (E_s), O₂, pH and H₂S in the Socompa stromatolites, as measured with microsensors in the field at 3500 masl.

(A) E_s was normalized to the value at the stromatolite surface, and is shown for selected wavelengths (see legend). E_s in the UV region was estimated based on the profiles measured at 676 nm and 750 nm. (B–C) Steady state profiles of O₂ and pH were measured at 25°C. Incident intensity of the ambient PAR was 130 W m⁻² (corresponding to 600 µmol photons m⁻² s⁻¹). (D) Transient profiles of H₂S were measured in a shaded place (PAR intensity <50 µmol photons m⁻² s⁻¹) during gradually increasing ambient temperature (see legend). For all measurements salinity of the overlying water was 48 g L⁻¹, as estimated from the measured conductivity (70 mS cm⁻¹) assuming seawater salt composition. Due to the lack of calibration standards, H₂S is given only as a raw signal linearly proportional to H₂S concentrations. Depth 0 corresponds to the stromatolite surface. Note the 10-fold difference in the depth scale in panels A–B and C–D.