Millennial stable isotope chronologies from tree rings and peat

Hangartner, Sarah (2010). Millennial stable isotope chronologies from tree rings and peat (Unpublished). (Dissertation, Universität Bern, Philosophisch–naturwissenschaftliche Fakultät, Physikalisches Institut, Abteilung für Klima– und Umweltphysik)

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Perennial plants in climatically extreme and mountain regions are exposed to a range of climate variables, namely temperature, precipitation, sunshine hours or relative humidity. Since the beginning of the industrialization around 1850 AD these climate variables began to alter. Plants which are adapted to extreme and sensitive biospheres are mainly affected by increasing temperature due to the anthropogenic global warming, changes in the hydrological cycle and the direct effect of the increasing CO2. Insights into past climate conditions as well as into the consequences of the anthropogenic impact on future climate are main purposes for investigating plant proxies.
Due to temperature controlled fractionation processes as evaporation and condensation, the ratio of stable water isotopes as 2H/1H (δD) and 18O/16O (δ18O) in meteoric water give information about ambient temperature during the formation. In colder climates, meteoric water is increasingly depleted in the heavier 2H and 18O 1iotopes. As plants favor the lighter 12C during the uptake of CO2 for photosynthesis, the ratios of stable carbon isotopes (13C/12C) in the atmosphere reflects the activity of the biosphere which is mainly influenced by temperature. If the biosphere stores more carbon, the 13C/12C (δ13C) ratio in the atmosphere increases. Organic plant material such as cellulose is formed from atmospheric CO2 and soil water which is eventually derived from meteoric water. Climate induced signals such as the isotopic composition of cellulose is stored in annual increments of centenarian plants such as trees or peat moss.
In the present study we investigated stable isotopes on one millennial and three 400-year old tree and one millennial peat isotope chronologies from climatically extreme sites in Switzerland and Spain. This dataset was built up within the framework of Millennium, a EU funded research project. δ13C and δ18O were investigated on a Sphagnum and Polytrichum, peat core dated to the period 970-2004 AD from a peat bog in Engadin valley, Switzerland. Furthermore, δ13C, δ18O and δD were measured on cellulose in a 1200-year larch (Larix decidua) tree-ring chronology from the Valais, Switzerland. In order to explore climate signals in stable hydrogen ratios of tree rings, we analyzed 400 years of δD in pine (Pinus nigra, Pinus uncincata and Pinus sylvestris) chronologies from three different mountain sites in Spain.
Oxygen bound hydrogen in tree ring cellulose continuously exchanges with atmospheric hydrogen and therefore complicates the analysis of δD. Within the scope of this study, the experimental setup for deuterium analysis was automatized. This resulted in an increased number of processed samples which is highly competitive with δ13C and δ18O measurements.
Uncertainties in the interpretation of δD on tree ring cellulose were revealed in the 400-year deuterium chronologies from Spain. Common correlations of the δD signals between sites are weak, as well as the correlation between δD and δ18O series. In addition, the climatic imprint on the deuterium signals is barely detectable. These unexpected findings point to a gap in the current understanding of deuterium fractionations in plants.
The 1200-year isotope chronology from Valais consists out of several 300-500 years old tree cohorts. This required the development of methods to merge these overlapping tree ring isotope series to assess the common signal within the different cohorts (each consisting of 4-5 trees). However, the unknown growth conditions of historic wood can lead to incoherent tree cohorts. The climatic signal strength is closely related to the signal that is measureable in the different isotopes: δ13C shows strong correlations with temperature and precipitation, indicating a drought signal in the carbon chronology. Temperature and precipitation signals in δ18O are significantly lower, with a strong influence of sunshine duration. Temperature and precipitation signals are hardly detectable in δD. Note that isotopic signals in tree-ring cellulose are not controlled by one dominating factor but are a combination of several climate variables such as temperature, relative humidity or precipitation.

Results from peat moss analyses from Engadin, Switzerland, point to strong vapor pressure signals in δ18O of Sphagnum and Polytrichum moss. Relative humidity from monthly station data as well as monthly gridded vapor pressure correlate significantly with the stable oxygen chronologies, with maximal explained variances of 35% for Sphagnum branch and 30% for Polytrichum, for water vapor pressure in summer months (JJA). This correlation is consistent over the investigated period from 1900-2004 AD. Climate signals in Sphagnum stem are significantly weaker.
The fact that several climatic factors influence δ18O and δ13C signals in tree ring cellulose or peat moss can instrumental in humidity reconstructions, as the water content of the atmosphere is closely related to temperature. To date, e.g. precipitation reconstrucitions are rare and often derived from documentary archives. The δ18O peat chronology and the δ13C tree chronology that were compiled within the scope of this thesis bear the potential to contribute worthwhile information to the understanding of ancient humidity variations on a regional scale.

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