*2.5. Analysis of Stable Isotopes*

For the carbon (δ13C) and oxygen (δ18O) stable isotope analysis, water samples (*n* = 37) were collected in 100 mL glass vials and treated with HgCl2 to prohibit biological activity. The samples were stored in a cool, dark place until measurements were carried out at the isotope laboratory at the Friedrich-Alexander University Erlangen-Nürnberg. Samples were analyzed through a standard procedure with an isotope-ratio-mass spectrometer (Gasbench 2; Thermo Fisher Scientific, Bremen, Germany), corrected to instrumental drift, and normalized to the VPDB (Vienna Pee Dee Belemnite) or to the VSMOW/SLAP (Vienna Standard Mean Ocean Water/Standard Light Antarctic Precipitation) scale, respectively [75]. An acid treatment on a Gasbench was used for carbonate stable isotope analyses. The precision of the control sample was better than 0.1‰ (1 sigma) for δ13C and better than 0.05‰ (±1 sigma) for <sup>δ</sup>18O. Oxygen samples were not corrected for the isotope salt effect as this effect has been reported to be neglectable for seawater consisting mainly of NaCl [76,77].
