Call number:
AWI G5-23-95172
Description / Table of Contents:
Throughout the last ~3 million years, the Earth's climate system was characterised by cycles of glacial and interglacial periods. The current warm period, the Holocene, is comparably stable and stands out from this long-term cyclicality. However, since the industrial revolution, the climate has been increasingly affected by a human-induced increase in greenhouse gas concentrations. While instrumental observations are used to describe changes over the past ~200 years, indirect observations via proxy data are the main source of information beyond this instrumental era. These data are indicators of past climatic conditions, stored in palaeoclimate archives around the Earth. The proxy signal is affected by processes independent of the prevailing climatic conditions. In particular, for sedimentary archives such as marine sediments and polar ice sheets, material may be redistributed during or after the initial deposition and subsequent formation of the archive. This leads to noise in the records challenging reliable reconstructions on local or short time scales. This dissertation characterises the initial deposition of the climatic signal and quantifies the resulting archive-internal heterogeneity and its influence on the observed proxy signal to improve the representativity and interpretation of climate reconstructions from marine sediments and ice cores.
To this end, the horizontal and vertical variation in radiocarbon content of a box-core from the South China Sea is investigated. The three-dimensional resolution is used to quantify the true uncertainty in radiocarbon age estimates from planktonic foraminifera with an extensive sampling scheme, including different sample volumes and replicated measurements of batches of small and large numbers of specimen. An assessment on the variability stemming from sediment mixing by benthic organisms reveals strong internal heterogeneity. Hence, sediment mixing leads to substantial time uncertainty of proxy-based reconstructions with error terms two to five times larger than previously assumed.
A second three-dimensional analysis of the upper snowpack provides insights into the heterogeneous signal deposition and imprint in snow and firn. A new study design which combines a structure-from-motion photogrammetry approach with two-dimensional isotopic data is performed at a study site in the accumulation zone of the Greenland Ice Sheet. The photogrammetry method reveals an intermittent character of snowfall, a layer-wise snow deposition with substantial contributions by wind-driven erosion and redistribution to the final spatially variable accumulation and illustrated the evolution of stratigraphic noise at the surface. The isotopic data show the preservation of stratigraphic noise within the upper firn column, leading to a spatially variable climate signal imprint and heterogeneous layer thicknesses. Additional post-depositional modifications due to snow-air exchange are also investigated, but without a conclusive quantification of the contribution to the final isotopic signature.
Finally, this characterisation and quantification of the complex signal formation in marine sediments and polar ice contributes to a better understanding of the signal content in proxy data which is needed to assess the natural climate variability during the Holocene.
Type of Medium:
Dissertations
Pages:
xx, 167 Seiten : Illustrationen, Diagramme
URL:
https://doi.org/10.25932/publishup-58286
Language:
English
Note:
Dissertation, Universität Potsdam, 2023 (publikationsbasierte Dissertation)
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CONTENTS
1 Introduction
1.1 Introduction to climate reconstructions
1.1.1 Radiocarbon as a tracer of time
1.1.2 Environmental information stored in snow
1.2 Challenges of climate reconstructions
1.2.1 The particle flux
1.2.2 Modifications after the initial deposition
1.2.3 Sampling and measurement uncertainty
1.3 Objectives and overview of the thesis
1.4 Author contributions to the Manuscripts
2 Age-heterogeneity in marine sediments revealed by three-dimensional high-resolution radio-carbon measurements
2.1 Introduction
2.2 Methods
2.2.1 Study approach
2.2.2 Core setup and sampling
2.2.3 Estimation of the sediment accumulation rate
2.2.4 Estimation of the sediment mixing strength
2.2.5 Estimation of the net sediment displacement
2.2.6 Visual assessment of mixing
2.3 Results
2.3.1 Radiocarbon measurements
2.3.2 Sediment accumulation rate
2.3.3 Sediment mixing estimates
2.3.4 Spatial structure of sediment mixing
2.3.5 Components of age uncertainty
2.4 Discussion
2.4.1 Spatial scale of sediment heterogeneity
2.4.2 Potential implications for palaeo-reconstructions
2.4.3 Suggested 14C measurement strategy
2.5 Conclusions
2.6 Supplementary Material
2.6.1 Supplementary figures and tables
2.6.2 Supplementary table
3 Local-scale deposition of surface snow on the Greenland ice sheet
3.1 Introduction
3.2 Data and methods
3.2.1 Study site
3.2.2 SfM photogrammetry
3.2.3 Additional snow height and snowfall data
3.2.4 Estimation of surface roughness
3.3 Results
3.3.1 Relative snow heights from DEMs
3.3.2 Temporal snow height evolution
3.3.3 Day-to-day variations of snowfall
3.3.4 Changes in surface roughness
3.3.5 Implied internal structure of the snowpack
3.4 Discussion
3.4.1 Changes of surface structures
3.4.2 Implications for proxy data
3.4.3 Implications for snow accumulation
3.4.4 SfM as an efficient monitoring tool
3.5 Conclusions
3.6 Appendix
3.6.1 Additional information
3.6.2 Accuracy estimates and validation
3.6.3 Validation
3.6.4 Overall snow height evolution
3.6.5 Surface roughness
4 A snapshot on the buildup of the stable water isotopic signal in the upper snowpack at east-grip, Geenland ice sheet
4.1 Introduction
4.2 Methods and data
4.2.1 Study site
4.2.2 DEM generation
4.2.3 Isotope measurements
4.2.4 Simulation of the snowpack layering
4.2.5 Expected uncertainty
4.3 Results
4.3.1 Snow height evolution
4.3.2 Mean isotopic records
4.3.3 Combining isotopic data with snow height information
4.3.4 Observed vs. simulated composition
4.3.5 Changes in the isotope signal over time
4.4 Discussion
4.4.1 Evolution of the snow surface
4.4.2 Two-dimensional view of isotopes in snow
4.4.3 Buildup of the snowpack isotopic signal
4.5 Conclusion
5 General discussion and conclusions
5.1 Heterogeneity in sedimentary archives
5.1.1 Quantifying archive-internal heterogeneity
5.1.2 Relation between signal and heterogeneity
5.2 Methods to improve climate reconstructions
5.3 Implications for climate reconstructions
5.4 Concluding remarks
Bibliography
A the role of sublimation as a driver of climate signals in the water isotope content of surface snow: laboratory and field experimental results
A.1 Introduction
A.2 Methods
A.2.1 Laboratory experimental methods
A.2.2 Field experimental methods
A.3 Results
A.3.1 Laboratory experiments
A.3.2 Field experiments
A.4 Discussion
A.5 Conclusions
B Atmosphere-snow exchange explains surface snow isotope variability
Acknowledgments
Eidesstattliche Erklärung
Location:
AWI Reading room
Branch Library:
AWI Library
