Project

Discipline

Climate Change; Paleoclimate; Sedimentology; Holocene; Switzerland; limnology

Lead
Lay summary
Seasonal to annual quantitative reconstructions of spatially-explicit climate state variables for the last 2000 years across the world are recognized as one of the primary targets for current climate research. Varved (annually laminated) lake sediments play a key role in this context because they provide seasonally resolved paleoclimate archives with a very good chronology. The minerogenic varves in Lake Oeschinen, a proglacial lake in the northern Swiss Alps will be used to establish the first quantitative summer precipitation record back to ca. 1000 AD for Switzerland. In a pilot study for the varves AD 1920-1986, we have established that varve thickness and mineralogical composition is a good predictor for MJJA precipitation (r=0.64, p<0.05). Phyllosilicate influx is related to rain stroms whereas calcite influx is related to glacial meltwater. Novel scanning techniques (reflectance spectroscopy 380-730 nm) will be explored to analyze sediment compositions at very high (mm-scale) resolution.

Direct link to Lay Summary

Last update: 21.02.2013

Active participation

Number	Title	Start	Funding scheme

Seasonal to annual quantitative reconstructions of spatially-explicit climate state variables for the last 2000 years across the world are recognized as one of the primary targets for current climate research (IGBP-PAGES/WCRP-CLIVAR. This builds on the rationale that knowledge about the natural, forced and stochastic variability at different scales, rates of change and probabilities of extremes is critical (i) to discriminate the human fingerprint on the global climate in the 20th century (detection and attribution) and (ii) to reduce uncertainty for ecological, societal and economic risks when natural variability and anthropogenic forcings are superposed in a future warmer climate.Varved (annually laminated) lake sediments play a key role in this context. They provide very highly resolved (seasonal to annual) paleoclimate archives with a very good chronology spanning over long periods (several millennia). They may provide information in areas where other annually resolved paleoclimate archives are short in length or not available. It is also recognized that lake sediments may preserve signals of low-frequency climate variability, which is one of the moist critical challenges for climate reconstructions.However, well-calibrated, quantified annually resolved millennial-long records are very difficult to obtain from lake sediments: the analytical work is enormous, precise and accurate chronologies are difficult to establish, and calibration has remained a challenge. In consequence, research priorities include two targets: (i) development of rapid, scanning and non-destructive techniques and (ii) development of numerical tools for calibrations.This has been the target of research in the Paleolimnology Group U Bern over the past years. We have implemented and developed novel scanning Reflectance Spectroscopy (380 - 730 nm, VIS RS) with direct measurements of fresh split sediment cores, and have converted VIS RS data into quantitative measures of climate variables. This has produced remarkable data sets for climate reconstructions and generated growing interest and international recognition: In first case studies from Chile, Tasmania and the Alps we show that VIS RS data indicative of minerogenic compounds (mainly illite and chlorite) and sedimentary pigments provide excellent quantitative summer temperature and summer precipitation reconstructions for the past 1000 years. However, interpreting VIS RS data is still in its infancy and poorly understood. Calibration of VIS RS data with concentration and composition of specific sediment compounds is the recognized methodological gap.This proposal is the logic continuation of NF 200021-116005 (‘Late Holocene varved sediments from Lake Silvaplana’) and seeks funding to calibrate VIS RS data of minerogenic and biochemical varved lake sediments with specific compositions of minerals and sedimentary pigments, respectively as determined with established analytical methods (XRD, XRF elemental mapping, microscopy of thin sections, extraction and HPLC, biogenic Si and TOC/N). In consequence, the proposed research develops along two major thrusts:Thrust 1: Calibrate VIS RS data from minerogenic varves in Lake Oeschinen with phyllosilicate/calcite ratios and reconstruct MJJA precipitation back to ca. 1000 AD. In a pilot study for the varves 1920-1986, we have established that varve thickness is a good predictor for MJJA precipitation (r=0.64, p<0.05), that phyllosilicate influx is related to rain stroms whereas calcite influx is related to glacial meltwater. Using this information we will (i) Refine the “sediment proxy - climate” calibration, (ii) Calibrate/validate VIS RS data with ‘synthetic sediments’ and sediment short cores, and (iii) Reconstruct summer precipitation back to ca. 1000 AD.Thrust 2: Calibrate VIS RS data from biochemical varves in Lake Lazduny, Poland with sedimentary pigments. Many of the VIS RS-bases climate reconstructions target reflectance features indicative of sedimentary pigments. Currently, our understanding of these features is unspecific and interpretations speculative (or missing). We have carefully selected biochemical varves from lake Lazduny to (i) Calibrate VIS RS data with concentrations and composition of sedimentary pigments (HPLC), (ii) Calibrate autigenic calcite with VIS RS data, and (iii) asses the skill of this data set fro climate reconstructions. The project has a methodological component (relevant to many other research groups) and a data/result component (directly relevant to international climate research programmes IGBP-PAGES and WCRP-CLIVAR).