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 ofdissolved inorganic carbon (DIC) in tropical surface waters. Carbon isotope records from corallinesponges clearly reflect the industrial12C increase in atmospheric CO2with a precision that permitsquantitative interpretations. We find from a set of d13C records of four Caribbean sponge specimensthat the isotopic response of surface water DIC to the changing isotopic composition of atmosphericCO2varied dynamically during the last century, depending on the rate of atmospheric change. Three ofour sponges provide 600 year long d13C records. For the first time, we can reconstruct surface waterd13CDICfor the full history of the industrial CO2release as well as the preceding preindustrial periodback to the beginning of the Little Ice Age. This provides a well-founded estimate of theanthropogenically uninfluenced, preindustrial background level of surface water13C/12C ratios. Ourrecords show small but systematic variations that appear to be linked to the climate fluctuations of theLittle Ice Age