Alpine Holocene Triple Tree Ring Isotope Records

Ziehmer, Malin Michelle (2017). Alpine Holocene Triple Tree Ring Isotope Records (Unpublished). (Dissertation, Universität Bern, Philosophisch–naturwissenschaftliche Fakultät, Physikalisches Institut, Abteilung für Klima– und Umweltphysik)

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The tree-ring archive and its diverse high-resolution proxy records contribute significantly in reconstructing and understanding past climate variability on various temporal and spatial scales. The annually-resolved and calendar-dated tree-ring records are particularly useful in disentangling natural climate variability from the anthropogenic imprint on the global climate and its impact on the climate system. This further allows an improvement of models in view of projecting future climate change. While the majority of tree-ring records is based on samples from living trees, recent finds of Holocene wood remains from glacier forefields, peat bogs and small lakes even allow an annual reconstruction of Holocene climate variability on a multi-millennial scale. Such unique records enable us to improve our understanding of Holocene climate dynamics, as the natural variability of postglacial climate is still not sufficiently known.
In the present thesis, multi-millennial tree-ring records are established in a multi-proxy approach to examine climate variability in the Holocene. For the construction of these novel records, calendar-dated wood material from Swiss glacier forefields (Mont Miné, Unteraar, Tschierva) and subfossil wood from peat bog sites located in Austria and Italy, which have been sampled within the past two decades, are utilized to establish triple isotope records. In addition, modern samples from living trees were collected close to the glacier sites to connect the Holocene records to present-day climate and examine climate-proxy relationships.
In a first step, Holocene wood remains and modern wood samples are separated into 5-year tree-ring blocks to reduce both time and cost during the sample preparation process. Subsequently, α-cellulose is extracted from the individual samples in a modified and standardized Jayme-Wise procedure. The quality of the extraction is tested by calculating the cellulose content in a tree-ring sample, which should be around 40-44 % in theory. However, the determination of cellulose content in thousands of wood samples revealed generally lower values and led to a closer investigation of cellulose content time series. The time series illustrate a species-specific low-frequency trend, and show potential for reconstructing growing season temperature, which further led to the suggestion of cellulose content as a novel supplementary proxy in dendroclimatology.
In a further step, the simultaneous determination of carbon (δ13C), oxygen (δ18O) and hydrogen (δ2H or δD) in 5-year tree-ring cellulose samples was performed on a coupled GC-IRMS system, which is connected to an equilibration chamber and a high-temperature elemental analyzer. The novel measurement setup allows for a high sample throughput and thereby for the establishment of multi-millennial triple isotope tree-ring records.
Modern triple isotope as well as the cellulose content (CC [%]) records are utilized to investigate the site and species-specific sensitivity of the two high-Alpine tree species Larix decidua Mill. (European larch) and Pinus cembra L. (Swiss stone pine), which are present in both modern and Holocene tree samples. Thereby, a species-specific signature is found in CC [%] and δD records, whereas a site sensitivity is found for the δ18O series. Climate-proxy relationships established for the modern proxy data and climate variables extracted from the HISTALP database revealed both species-specific and site-specific correlations with climate variables.
A novel high-resolution insight into Holocene climate variability is illustrated by the established triple isotope records based on Holocene wood remains, where interesting low-frequency variability is found in the three isotope records as well as the cellulose content series. Particularly the δ18O series show potential for a future temperature reconstruction, and is in agreement in the comparison with other proxy records and reconstructions established for the Holocene, especially during the well-known 8.2 ka BP cold event.
For the first time, Early and Mid-Holocene climate is investigated by high-resolution tree-ring records in a combined multi-proxy approach. The individual proxies show high potential for reconstructing past environments in the Holocene and will contribute to a better understanding of Holocene climate variability in the future.
This study was realized in the framework of the project Alpine Holocene Tree Ring Isotope Records in collaboration with (i) Prof. Dr. Markus Leuenberger from the division of Climate and Environmental Physics, Physics Institute, University of Bern, (ii) Prof. Dr. Kurt Nicolussi from the Institute of Geography, University of Innsbruck, and (iii) Prof. Dr. Christian Schlüchter from the Geological Institute, University of Bern, financed by the Swiss National Science Foundation within international collaborations with Germany, Austria and Switzerland (DACH-projects) and supported by the Oeschger Center for Climate Change Research, University of Bern, Bern Switzerland (OCCR).

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