title: The varved sediments of Lake Żabińskie, Poland as a high-resolution archive of environmental change
Creators: name: Zander, Paul David
Publisher: name: Universität Bern
Year: 2021
Subjects: subject: 550 Earth sciences &amp; geology

description: Human impacts to the global environment have accelerated in the past several decades resulting in global warming, pollution, modification of biogeochemical cycles and other major impacts to terrestrial and aquatic ecosystems. The diverse and growing field of paleoenvironmental reconstruction offers a long-term perspective of environmental change that is necessary to understand natural processes and what conditions were like prior to human impacts. Lake sediments are a particularly important archive of paleoenvironmental information because of their wide distribution on the continents, the potential for well-constrained chronologies, and the vast array of information that can be obtained from lake sediments. In this thesis, a 10,800-year record of environmental change is presented from Lake Żabińskie, Poland. This site is relatively unique because annual laminations (varves) are preserved over the majority of the past 10,800 years and a high sedimentation rate enables analyses at high temporal resolution. The site has been investigated extensively to understand modern processes as well as the past 2,000 years of the sedimentary record, but the full Holocene record had not been investigated until this thesis.
The first case study of the thesis (Chapter 3) investigates the application of cutting-edge techniques in radiocarbon measurements for dating lake sediments. Samples of plant macrofossils containing as little as 11 μg of carbon were measured for radiocarbon using a gas-source input to the mass spectrometer. Multiple samples were taken from the same stratigraphic levels in order to assess the accuracy and precision of these miniature radiocarbon samples. Additionally, the impact of miniature samples on age-depth models was assessed using simulations of different sampling scenarios. We found no evidence of age bias from miniature radiocarbon samples, and the precision of ages was not strongly affected by the technique used (gas-source or graphitized input). Simulated sampling scenarios indicate that increasing the number of ages can compensate for increased uncertainty produced by samples with small masses.
The second study (Chapter 4) focuses on reconstructing primary production and water column anoxia over the past 10,800 years. This was accomplished primarily by the use of ultra-high-resolution pigment measurements from hyperspectral imaging (HSI) core scanning, with complimentary evidence provided by elemental data from X-ray fluorescence scanning and pigment data from high-performance liquid chromatography. Reconstructions of primary productivity and anoxia were compared with local and regional climate and vegetation data to understand how these factors influenced conditions in the lake. The presence of bacteriopheopigments, a specific biomarker for water column anoxia, indicates that an extensive anoxic zone was present in the lake nearly continuously prior to approximately 2,800 cal yr BP. Several shifts in lake mixing regime occurred after that time and lead to variable water column oxygen concentrations. These shifts were linked to human modifications of forest cover, leading to the conclusion that forest cover was a primary control on lake mixing over the Holocene. Since the 17th century, intensive agricultural development and deforestation surrounding the lake caused major increases in erosional and nutrient inputs leading to major increases in primary production and biogeochemical cycling.
The third study (Chapter 5) investigates relationships between meteorological conditions and the structure and composition of varves over the period 1966-2019. This period was selected because a varve chronology could be established with no uncertainty over this interval. Ultra-high resolution spatially resolved biogeochemical data was obtained using micro X-ray fluorescence and hyperspectral imaging. Sub-varve scale patterns of geochemical data were used to classify varves into four varve types. The years associated with these varve types were shown to experience differing seasonal weather conditions, providing evidence that weather affects the composition of varves at sub-varve scale. Statistical analysis showed that total C and Ti could be used to reconstruct spring and summer temperature, and mass accumulation rate and Si could be used to reconstruct the frequency of windy days. This study shows the potential of bulk geochemical data from biochemical varves for paleoclimate reconstructions.
In the fourth study (Chapter 6), the relationship between spring temperatures sedimentary chloropigments was investigated, building upon previously published work that indicated chloropigments could be used as a proxy for spring temperature at Lake Żabińskie. Although a significant relationship between these two variables was found for the period 1779-2016 CE, there is no significant correlation over the period 1779-1906 CE. This result suggests that chloropigments are not a reliable temperature indicator at this site, and highlights the potential problems inherent to quantitative climate reconstructions based on the assumption that climate - proxy relationships remained stable through time.
Together, these studies demonstrate the variety of insights that can be gained from high quality lake sediments records. High-resolution scanning techniques, such as micro X-ray fluorescence (μXRF) and hyperspectral imaging (HSI), applied to varved sediments represent the cutting-edge of paleolimnological research in terms of measurement resolution. The feasibility of using hyperspectral imaging on resin-embedded sediment slabs was assessed; this would enable alignment of μXRF and HSI data with the greatest possible precision. However, for Lake Żabińskie sediments, resin-embedded sediments absorbed too much light, and it was not possible to apply HSI to resin-embedded slabs. Nonetheless, these high-resolution measurements enable investigation of sub-annual processes, and make it possible to detect rapid changes that cannot be observed with conventional proxy techniques. Additionally, the speed and low-cost of these non-destructive techniques make it possible to study long records in a relatively short amount of time. Therefore, these methods should be increasingly applied to cover sites across a wide variety of environments.