Alpine Holocene tree ring isotope records

The environment of the Alps reacts sensitively to changes in temperature and precipitation as visible, e.g., in the current retreat of glaciers as a result of actual warming. This environmental reaction proves the role of alpine glaciers as an important climate archive. Glaciers have been used to understand Holocene climate evolution for many decades. However, recent finds of wood remains of long-lived trees changed the concept of Holocene Alpine glacier variability and our understanding of Holocene climate dynamics. Moreover, these finds prove that the natural variability of postglacial climate is still not sufficiently known. Such knowledge, however, is essential for climate model input and, finally, to disentangle natural from anthropogenic influences on the Earth`s climate. Knowledge on Holocene climate evolution was mainly based on low-frequency archives like Alpine glaciers and treeline fluctuations over long time periods. Holocene proxy records from the Alpine region with up to yearly resolution from archives like speleothems and tree-rings became available only recently. However, by comparing low- and high-frequency Alpine proxy records a difference can be recognized: Alpine glaciers and treeline proves a Holocene evolution from a generally warm early and mid to a relatively cool late Holocene, whereas the highly resolved records often don`t indicate such a general long-term trend. The causes behind are not understood yet. This project will improve our knowledge on Holocene environmental and natural climatic variability by the establishment of new highly resolved, multi-millennial long stable isotope records and by comparing those with other proxy records of similar resolution. For the construction of the new isotope records we will utilize calendar- dated wood material from Swiss glacier forelands (Tschierva, Unteraar, Morteratsch & Ried and Mont Miné) and other high-elevated Alpine sites. The already analysed wood material covers the last c. 9100 years continuously. Collected and available material will be supplemented with samples gathered during carefully planned fieldwork. The new isotope records will have a 5-year resolution with calendar dating accuracy over the last c. 9000 years. Within this project we will i) establish for the first time multi-millennial long Alpine stable isotope (C, O and H) records based on calendar-dated 5-year tree-ring blocks as well as latewood density (blue intensity) measurement on the same material, ii) document natural Holocene climate sensitivity and variability by analysing tree-ring material from climate optima as well as minima periods based on a multi-proxy approach (stable isotopes, tree-ring width, late-wood density(blue intensity) - a priority target is the well documented 8.2 ka BP event; iii) test the temporal stability of sensitivities by comparing our new isotope records with other high resolution Alpine stable isotope as well as with Greenland ice core records; and iv) use these sensitivities for improving the currently limited understanding of water isotope incorporation into plant materials. This is required since newly established millennial years long tree ring isotope records from an Alpine area show unexpected but interesting composition differences between oxygen and hydrogen isotopes on extracted cellulose. This project is based on an established, successfully and smoothly running co-operation between the Physics Institute and the Institute of Geological Sciences, University of Bern, and the Institute of Geography, University of Innsbruck.

During the last years and decades dozens of tree remains could be collected on forefields of several glaciers in the Alps within an Austrian-Swiss research cooperation. These tree remains partly sampled near current glacier termini date mainly into the early and middle Holocene and prove long-term retreat phases of Alpine glaciers and with that also relatively warm conditions during these phases. Proceeding from this low-resolution climate archive the Swiss-Austrian D-A-CH project "Alpine Holocene tree ring isotope records", supported by the Swiss and Austrian science funds SNF and FWF, aimed at the establishment of highly resolved climate records by utilizing the wood samples from the glacier sites completed with other samples from comparable sites. The research consortium consisted of two Swiss partners (PI Prof. Dr. Markus Leuenberger, Prof. Dr. Christian Schlüchter, both University of Bern) and an Austrian partner (PI ao. Univ.-Prof. Dr. Kurt Nicolussi, University of Innsbruck). The overall goal of the joint project was to improve our knowledge on Holocene environmental and natural climatic variability by the establishment of new and highly resolved, calendar-dated and multi- millennial long stable isotope records. To achieve this goal the field surveys for collecting wood samples at glacier sites were continued. The Austrian sub-project was responsible for the field surveys on glacier forefields (in co-operation with the Swiss partner Prof. C. Schlüchter), for the tree-ring width measurements as well as dating of the new samples, preparation of 5-year tree-ring blocks of calendar-dated tree remains for the isotope analyses and wood density, i.e. blue intensity measurements on these wood samples. The triple- isotope measurements (18O, 13C, D) and isotope-climate relationship analyses were carried out at the University of Bern (Prof. M. Leuenberger, M.M. Ziehmer, PhD)Multi-millennial long isotope records covering the time period ca. 9000 to 3500 years before today were successfully established. I.e., the 18O clearly display the 8.2 ka cooling event and the highest values around 7 ka prove the Holocene Thermal Maximum in the middle Holocene. The surveys on several glacier forefields allowed the sampling of more than 220 additional tree remains, which were also partly used for the preparation of the isotope samples. The blue intensity measurements on wood sections also used for the preparation of the isotope 5-year tree-ring blocks allowed the establishment of a yet low-replicated wood- density record that covers the last 9000 years with few gaps. An improvement, i.e. an increase in sample depth, of this record is planned in a follow-up project.