Development and calibration of 3-hydroxy fatty acid-based palaeoclimatic proxies in pedogenic and lacustrine environments

Hardman, Alice (2024). Development and calibration of 3-hydroxy fatty acid-based palaeoclimatic proxies in pedogenic and lacustrine environments. University of Birmingham. Ph.D.

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Abstract

Terrestrial environments are where the majority of climate change impacts will affect human societies, and the Intergovernmental Panel on Climate Change (IPCC) have called for high-resolution palaeoclimatic data to improve future temperature predictions in (often arid) regions with uncertainty in hydroclimatic regime, such as the Southwestern U.S. Environmental proxies derived from organic biomarkers are important tools for reconstruction of past climatic conditions, however their development and application between terrestrial environments is complex due to environmental heterogeneity. C\(_{10}\)-C\(_{18}\) 3-OH FAs are bacterial lipids investigated as a novel palaeoclimate biomarker group in recent years in soils, lake and marine sediments. However proxy-MAAT relationships are highly variable between regional soil transects, and studies of lacustrine 3-OH FA distributions are limited. This thesis aims to investigate environmental drivers of 3-OH FA variability in pedogenic and lacustrine environments at a regional scale, to assess the performance of RAN\(_{17}\) as a temperature proxy in soils that span moisture and pH gradients, and compare 3-OH FA temperature proxy development and performance between and within soils and lakes, to improve regional calibration of terrestrial 3-OH FA-based environmental proxies for robust applicability.

In Chapter 3, relationships between 3-OH FA temperature and pH proxies (RAN\(_{15}\), RAN\(_{17}\), and RIAN) and contemporary environmental conditions are assessed in soils from the East Coast and Southwestern U.S., and the 3-OH FA distributions of these soils investigated for novel environmental proxy development. 3-OH FA proxy performance was compared with brGDGT-based MBT and CBT for the soils. pH and MAP are first-order drivers of 3-OH FA variability in U.S. soils, best described by the novel pH proxy -log(RIN\(_{USA}\)). We find RAN\(_{17}\) is the primary 3-OH FA temperature response in U.S. soils with pH<7.2, which may be associated with pH and moisture stress invoked on 3-OH FA bacterial soils in arid environments. In Chapter 4, relationships between 3-OH FA temperature proxies (RIN\(_{17}\) and RAN\(_{13}\) are investigated in 26 lake sediments from the SW U.S. and compared with existing data from Chinese lake sediments and the SW U.S. soils. 3-OH FA proxy performance was compared with brGDGT-based MBT’\(_{5ME}\). [(\(i\)-C\(_{12}\) + a-C\(_{13}\) + a-C\(_{17}\)) / n-C\(_{18}\)] is the 3-OH FA response to MAAT in U.S. lake sediments and is most responsive to TMAM, differing from the MAAT responses of RIN\(_{17}\) in Chinese lakes and RANS in SW soils. Different 3-OH FA temperature proxies are viable within and between lacustrine and pedogenic environments, suggesting that lacustrine temperature calibrations from 3-OH FAs have a regional calibre at present. In Chapter 5, the potential to streamline 3-OH FA extraction from environmental samples is investigated by testing a low temperature long duration (70 °C, 14 hrs; 'Low n' Slow') acid hydrolysis method on 3-OH FA recovery and proxy data from re-extracted SW U.S. lake sediments and soils. The Low n' Slow method is successful at streamlining the laboratory procedure for 3-OH FA extraction and extracting 3-OH FAs, but raises implications for palaeoclimatic interpretations from 3-OH FAs extracted by different hydrolysis temperatures. In Chapter 6, the Low n' Slow method is applied to a 105 yr lacustrine sequence from Esthwaite Water (UK). 3-OH FA proxies are able to track environmental conditions at a sub-decadal resolution, however most proxies have a unique and contradicting response to temperature, which is potentially related to a history of nutrient enrichment at Eshwaite Water.

This thesis reveals important insights into the environmental drivers of 3-OH FA production in lacustrine and pedogenic environments, and produces regional lake and soil 3-OH FA-based proxy calibrations applicable to the U.S. The proxies likely represent both homeoviscous adaptation of bacterial membranes to ambient environmental conditions and bacterial community changes. There is much more to learn about 3-OH FA variability in response to environmental conditions, particularly in lacustrine environments, and we recommend a global-scale investigation of 3-OH FAs in lakes across different environmental gradients in future studies.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):