Assessing the response of plant and algal biomarkers to high CO\(_{2}\) levels

Warren, Bridget Alison ORCID: https://orcid.org/0000-0002-3931-7596 (2023). Assessing the response of plant and algal biomarkers to high CO\(_{2}\) levels. University of Birmingham. Ph.D.

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Abstract

Biomarker-based reconstruction of climatic and environmental variables can and have provided great insight into the processes of the earth system. However, biomarkers are by definition produced by living organisms via complex processes of biosynthesis, and interpretation of information encoded in biomarkers relies heavily on knowledge of these processes observed in the modern. A robust understanding of the modern mechanisms underpinning biomarker behaviour is essential to accurate interpretations of their trends geologically. In particular, understanding how biomarkers and their source organisms responded to increased levels of CO\(_{2}\) in the geologic record is of extreme importance for understanding how they will respond to anthropogenic climate change, and how climate mechanisms and feedbacks function in high CO\(_{2}\) worlds. Since gradients of CO\(_{2}\) concentration are rare in nature, observing biomarker response to CO\(_{2}\) requires manipulation of environmental conditions in settings which are limited in size and complexity compared to the real world. This thesis examines the response of leaf wax n-alkane biomarkers to elevated CO\(_{2}\) as part of a Free Air CO\(_{2}\) Enrichment (FACE) experiment in two species over four years, with the aim of determining how, and why, plant biomarkers respond to CO\(_{2}\), and what this could mean for their use in reconstructing past environments. In addition, this thesis will reconstruct atmospheric pCO\(_{2}\) using the alkenone δ\(^{13}\)C proxy in the early Eocene: a time interval noted for its high CO\(_{2}\) conditions. By examining the performance of the proxy in extreme CO\(_{2}\) concentrations on geologic timescales, the underlying mechanics of the proxy can be evaluated, and the circumstances at which our understanding of alkenone biomarker response to pCO\(_{2}\) breaks down can be examined.

Changes to the distribution and carbon isotopic composition of leaf wax n-alkane homologues under elevated CO\(_{2}\) allow for examination of a plant biomarker’s functional response to experimental CO\(_{2}\) changes. This data shows that production of n-alkanes changes under elevated CO\(_{2}\) in one of the two species studied: the observed changes likely acted to optimise plant response to its environment. This shows that n-alkane production can be changeable within a single generation of plants on extremely short timescales geologically, but that response is highly species-specific and cannot necessarily be extrapolated to the scale of entire catchments. It also provides qualitative evidence that some plants may adjust their production of molecules known to reduce water loss under elevated CO\(_{2}\) in response to water savings made elsewhere in the plant, suggesting a hitherto undescribed feedback mechanism in plant-CO\(_{2}\) relations, that may have implications for understanding of forest water use under climate change.

This data also shows that biosynthetic processes act to alter the δ\(^{13}\)C of n-alkane biomarkers under elevated CO\(_{2}\) in both studied species. Alongside previously published studies, this data suggests that plants often adjust biosynthesis of n-alkanes on short timescales in response to changes in atmospheric CO\(_{2}\) concentration in ways which influence their δ\(^{13}\)C, but do so via multiple mechanisms. This suggests that post-photosynthetic fractionations of δ\(^{13}\)C should be considered when interpreting n-alkane δ\(^{13}\)C geologically, as these provide an additional mechanism by which n-alkane δ\(^{13}\)C can respond to environment geologically alongside the currently accepted model of plant δ\(^{13}\)C response.

Finally, this thesis examines the response of alkenone biomarkers in a sediment core dating from the hot, high-CO\(_{2}\) conditions of the early Eocene. The early Eocene environment represents the highest CO\(_{2}\) and warmest sustained conditions of the last 66 Ma, and existing pCO\(_{2}\) reconstructions give widely varying results depending on proxy used. The alkenone data presented here represent some of the earliest alkenone δ\(^{13}\)C data ever measured, and thus if the proxy can be successfully applied, the alkenone δ\(^{13}\)C-derived pCO\(_{2}\) record could be extended backward through the early Eocene and could further constrain atmospheric pCO\(_{2}\) reconstructions for the interval. The data presented here suggests that in these extreme conditions the commonly used assumptions made with algal biology in order to produce quantitative pCO\(_{2}\) reconstructions may be inaccurate, and alkenone-producing algal biology should be regarded as more changeable on long timescales than is reflected in current data used for calibration. However, although quantitative pCO\(_{2}\) reconstruction using the alkenone proxy is challenging due to extremely high uncertainty, this data provides a valuable lower constraint on pCO\(_{2}\) through the interval: even with the combination of input parameters to the model that would produce the lowest pCO\(_{2}\) estimations, pCO\(_{2}\) levels remain at or above 1000 ppmv over the interval. This is compatible with the pCO\(_{2}\) estimations for the interval made using Boron- and phytane-based proxies, but is much higher than the <700 ppmv predictions made using measurements taken from higher plant fossils. Thus, although the alkenone data presented here cannot quantify pCO\(_{2}\) during this interval, it suggests that Eocene pCO\(_{2}\) is best reconstructed from non-higher plant sources.

Type of Work:	Thesis (Doctorates > Ph.D.)
Award Type:	Doctorates > Ph.D.
Supervisor(s):	Supervisor(s) Email ORCID Bendle, James UNSPECIFIED orcid.org/0000-0002-6826-8658 Hilton, Jason UNSPECIFIED orcid.org/0000-0003-0286-8236 Greene, Sarah UNSPECIFIED orcid.org/0000-0002-3025-9043 Dunkley Jones, Tom UNSPECIFIED orcid.org/0000-0002-9518-8143
Licence:	All rights reserved
College/Faculty:	Colleges (2008 onwards) > College of Life & Environmental Sciences
School or Department:	School of Geography, Earth and Environmental Sciences, Department of Earth and Environmental Sciences
Funders:	Leverhulme Trust
Subjects:	G Geography. Anthropology. Recreation > GE Environmental Sciences Q Science > QE Geology Q Science > QH Natural history > QH301 Biology
URI:	http://etheses.bham.ac.uk/id/eprint/13917

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