Description: 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 (Δδ13C/ΔT) 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‰ 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.

Description: Stable carbon isotope investigations have been carried out on Miocene brown coal from the Garzweiler Seam of the German Lower Rhine Embayment. Material studied included fossil wood from seven different taxa, and brown coal matrix. Isotope results from macrofossil analysis show variations of more than 6‰ within individual samples and reveal a general isotopic difference between angiosperm and gymnosperm wood specimens. According to mean carbon isotope values found for gymnosperms, angiosperms and brown coal matrix (−23.3‰, −26.0‰ and −25.8‰), the peat-forming vegetation of the Garzweiler Seam was dominated by angiosperm taxa. Results from brown coal matrix establish a continuous high-resolution depth profile of carbon isotope variations during the late Middle Miocene. They show a significant and characteristic isotope pattern with distinct medium- and short-term cycles (high-frequency variations) in the two main units of Garzweiler Seam (locally split into three units). The medium-term δ13C variations are most likely caused by varying proportions of gymnosperms within the peat-forming vegetation while high-frequency oscillations seem to be a direct signal of environmental changes. A long-term decline of carbon isotope values observed within the complete Garzweiler Seam from base to top is presumably due to a cooling trend in the Miocene.