Calcium (Ca) is a key element for the understanding of the chemical evolution of the ocean and for the global climate on long geological time scales. This is because Ca is interacting with the carbon cycle and is a major constituent of continental weathering. Beside continental runoff, mid-ocean ridges are of quantitative importance for the marine Ca elemental and isotope budget. Variations of hydrothermal circulation of seawater through oceanic crust have been recognized to play a significant role for the oceanic Ca mass and isotope balance. Hydrothermal activity leads to a chemical alteration of the circulating seawater at low- and high temperatures during water-rock interaction, the formation of Ca-bearing minerals, and during phase separation. Within the framework of the subproject 'CARLA' in the 'Special Priority Program SPP 1144' Ca isotope ratios (d44/40Ca) in hydrothermal fluids sampled from the Logatchev hydrothermal field (15°N/45°W) and the Ascension area (4 11°S) have been investigated in detail in order to further constrain the global Ca cycling
