Isotope signals as climate proxies:: the role of transfer functions in the study of terrestrial archives

The use of stable isotopes as climate proxies is widely accepted for climate reconstruction. Re-establishing climate signals, e.g. temperature, from isotope values of biological materials requires some knowledge of the system's response behaviour. The related problems are discussed in conjunction with stable isotopes of two different terrestrial archives: carbon isotope data of tree rings from southern Germany and oxygen isotope data of diatoms from Lake Holzmaar, Germany. Carbon isotope temperature coefficients (Delta delta(13)C/Delta T) derived from tree rings were chosen as an example for non-linear transformation of environmental signals through biological systems explaining negative and positive temperature coefficients. Thin radial tree ring sections taken from tree rings of different species (poplar, beech and oak) show carbon isotope variations of up to 3 parts per thousand with a characteristic, annually recurring isotope pattern. This behaviour is discussed in view of time resolution, isotope signature conservation and the question of storage and remobilisation of photosynthates with time. For Lake Holzmaar it is shown that isotope proxy signals derived from diatoms may not be unambiguously translated into abiotic environmental forcing factors, such as for example temperature. Corresponding measurements reveal that the isotopic input signal varies considerably and nonlinearly with temperature. (C) 1999 Elsevier Science Ltd. All rights reserved.