Evidence for preindustrial variations in the marine surface water carbonate system from coralline sponges

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 12 C increase in atmospheric CO 2 with a precision that permits quantitative interpretations. We find from a set of δ 13 C records of four Caribbean sponge specimens that the isotopic response of surface water DIC to the changing isotopic composition of atmospheric CO 2 varied dynamically during the last century, depending on the rate of atmospheric change. Three of our sponges provide 600 year long δ 13 C records. For the first time, we can reconstruct surface water δ 13 C DIC for the full history of the industrial CO 2 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 13 C/ 12 C ratios. Our records show small but systematic variations that appear to be linked to the climate fluctuations of the Little Ice Age.