Coralline sponge skeletons are excellent tools for reconstructing the carbon isotope history of dissolved inorganic carbon (DIC) in tropical surface waters. Carbon isotope records from coralline sponges clearly reflect the industrial 12C increase in atmospheric CO2 with a precision that permits quantitative interpretations. We find from a set of δ13C records of four Caribbean sponge specimens that the isotopic response of surface water DIC to the changing isotopic composition of atmospheric CO2 varied dynamically during the last century, depending on the rate of atmospheric change. Three of our sponges provide 600 year long δ13C records. For the first time, we can reconstruct surface water δ13CDIC for the full history of the industrial CO2 release as well as the preceding preindustrial period back to the beginning of the Little Ice Age. This provides a well-founded estimate of the anthropogenically uninfluenced, preindustrial background level of surface water 13C/12C ratios. Our records show small but systematic variations that appear to be linked to the climate fluctuations of the Little Ice Age.