Stable carbon- and hydrogen-isotope ratios of subfossil oaks in southern Germany: methodology and application to a composite record for the Holocene

The &D and δ¹³C values of tree rings from dendrochronologically dated subfossil Holocene oaks have been studied for palaeoenvironmental and palaeoclimate reconstructions. The wood material mainly originates from fluvial deposits of the river Main and to a lesser extent from the Danube and Rhine regions in southern Germany. The chronology consists of the isotope records of 82 trees that cover a large part of the Holocene. The periods 8230 BC to AD 905 are represented by 6'¹³C values; AD values exist for the time interval 6470 BC to AD 905. Not all records have the same time resolution and not all of them were prepared in the same way for isotope analysis. However, investigations on the influence of the preparation technique on the isotope values and on the isotope composition of different wood compartments (latewood and earlywood, respectively) allowed us to homogenize the records. The comparison of δD values of wood with different cambial ages showed that hydrogen-isotoperatios were affected by growth trends. The uniformity of the trends implies physiological rather than microclimatic causes of these δD growth trends. The growth trends were corrected by subtracting a standardized trend curve, which in most cases resulted in a higher conformity in the overlap of tree-ring sequences. The δ¹³C records showed no uniform trends. However, the differences between δ¹³C values of overlapping trees in several cases were markedly offset, making it difficult to interpret the δ¹³C long-term trends in some sections of the chronology. Despite that, there are some characteristics common to both isotope chronologies. Most noticeable is the occurrence of comparatively high values in the interval 4500 to 2000 BC indicating higher temperatures (δD) and possibly lower water availability (δ¹³C), and a decrease in both isotope chronologies since about 2000 BC.