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The terrestrial carbon cycle in transition: tracking changes using novel tracers on multiple timescales

Soils store more carbon than the atmosphere and terrestrial vegetation combined, therefore, changes in soil carbon storage can affect the global carbon budget. One of the forms in which carbon is exported from soil is as dissolved organic carbon (DOC). Upon release from soil, components of DOC can be oxidised to CO₂, thus potentially contributing to the global climate crisis. In freshwater systems, DOC is considered a pollutant because of its detrimental impact on freshwater ecology and biodiversity (by blocking light and reducing photosynthesis and altering thermal-mixing regimes in lakes), increasing freshwater acidity, transporting nutrients (particularly N and P) and trace metal pollutants. Further, during the water treatment process (for human consumption), DOC can form toxic by-products. This project aimed to shed new light on terrestrial carbon dynamics by testing the hypothesis that climate can drive increases in soil DOC export. This research focused on past soil DOC exports in New Zealand, an archipelago located in the mid-latitudes of the Southern Hemisphere. New Zealand’s palaeo-environmental archives (e.g. speleothems and lake sediments) provide a unique opportunity to reconstruct the concentrations and characteristics of DOC under changing climate conditions over centennial/millennial timescales. New Zealand was uninhabited by humans until the 13th century. Palaeo-environmental records that extend beyond the 13th century, therefore, exclude interference from anthropogenic factors, thus enabling the climate hypothesis to be assessed. Using lake sediment archives to reconstruct DOC concentrations and characteristics through the past 14,000 years This study explored the use of 3D EEM (excitation-emission matrix spectroscopy) fluorescence of water extractable dissolved organic matter (WEDOM) from lake sediments in reconstructing both past soil export of DOM and past trophic status. DOM and water quality are linked because DOM can contain nutrients (N, and P), provide an energy source to heterotrophs and algae, can reduce light penetration, and lead to reduced dissolved oxygen concentrations. Mean trophic level index (TLI) monitoring data of the lake water columns were compared against protein-like fluorescence from contemporary sedimentary WEDOM from ten lakes, producing strong positive correlations with total phosphorous (R²= 0.81), and TLI scores (R²= 0.74 ), and weak positive correlations with total nitrogen (R²= 0.5) and chlorophyll a (R²= 0.44). The equation produced from the correlation between TLI scores and protein-like fluorescence of WEDOM was used to reconstruct TLI scores through the past 13,700 years at Adelaide Tarn (a climatically sensitive, sub-alpine lake located in the north west of the South Island), indicating predominantly oligotrophic conditions throughout the record. Humic-like DOM fluxes in Adelaide Tarn responded to known climate shifts over the course of the Holocene, indicating a climatic control on soil DOM production and/or export. The results indicate that WEDOM fluorescence can be used as a reliable indicator of past soil export of DOM as well as past trophic status. To examine total organic carbon concentration; a partial-least square regression (PLSR) model was produced using Fourier transform infrared spectroscopy (FTIRS) data vs conventionally measured TOC of 141 sediment samples from 13 lakes, resulting in strong positive correlation (R² =0.88). The equation from this correlation was then used to reconstruct past TOC concentrations from sediments in Adelaide Tarn. The FTIRS-TOC and humic-like DOM records showed a clear relationship with known temperature changes over the past ~14,000 years BP. Testing the reliability of calcite to record DOC concentrations from its parent solution Natural carbonates (such as speleothems) are known to contain DOM. Speleothems accumulate for hundreds of thousands of years, thus potentially containing long-term records of DOM export from their overlying soil. However, the ability of carbonates to reliably record DOM concentrations from their parent solutions is unknown. A calcite precipitation experiment was undertaken in growth solutions (diluted peat water) of differing [DOC]aq (0, 5, 10, 15 ppm) with the aim of testing the reliability of calcite to record [DOC]aq, and the potential of 3D excitation emission (EEM) analysis to assess the DOC concentration in dissolved calcite samples. A case study of flowstones and dripwaters from three New Zealand caves was also conducted. In the case study, 3D EEM fluorescence-inferred DOC concentrations were measured in the dripwaters and speleothems, thus enabling calculation of DOC partition coefficients between dripwaters and their associated flowstones. Overall, the study demonstrated that calcite [DOC] is controlled by aqueous [DOC] from the parent solution, in natural and experimental environments. Further, [DOC]aq within the experimental range did not alter the calcium carbonate polymorph (calcite), yet heavily influenced calcite crystal structure; smooth-faced, rhombohedral crystals formed in growth solutions with low [DOC]aq (0–5 ppm), whereas prismatic, ‘impure’ crystals were produced at high [DOC]aq (10 and 15 ppm). Using speleothem archives to test the impact of climate on aquatic DOC concentrations in a pre-human continent 3D EEM fluorescence was used to measure humic-like DOC concentrations in modern dripwaters and within three flowstones (secondary carbonate deposits) from three caves distributed along a 7° latitudinal gradient. Calculated DOC concentrations from recently deposited flowstone calcite and dripwater monitoring were used to calculate empirical distribution coefficient (Kd) values, enabling an estimate of dripwater DOC concentrations through the past ~14,000 years BP. Correspondence between heightened aquatic DOC concentrations during periods of known temperature increases (especially the Holocene climatic optimum) was observed at Hodges Creek (north-west South Island) and Dave’s Cave (south-west South Island), whilst climatic cooling (during the mid-Holocene) yielded lower DOC values. This occurrence was not clear at Waipuna Cave (western North Island), perhaps owing to its relatively low altitude and lack of climatic sensitivity compared to the other sites. Ratios of Mg to Ca in the flowstone showed a strong correspondence between periods of drier climate and increases in DOC dripwater concentration at each site. The findings of this study show that DOC concentrations have been higher in the past at the high-altitude sites, and that climate (in the absence of anthropogenic impacts) plays a pivotal role in soil DOC export in sub-alpine and alpine environments. Summary The thesis produced several outcomes. Chapter 3 demonstrated that protein-like fluorescence of sedimentary WEDOM is associated with trophic level index score and can be used as an indicator of past trophic level (i.e. productivity). Further, the Adelaide Tarn palaeo-environmental reconstruction demonstrated that humic-like fluorescence responded to climate shifts through the Holocene, indicating a temperature control on soil C production and/or export. At large, the DOC concentration over the ~10 kyr period approximately halved with a decrease in temperature of 1.2 °C. The project also demonstrated (in Chapter 4) that speleothems can reliably record DOC from their parent solutions, showing a linear response between dissolved and solid phase organic concentration. This finding formed the foundation of Chapter 5, which demonstrated that high humic-like DOC concentrations in flowstones were associated with drier conditions at Waipuna Cave (lower-altitude North Island site), however there was no clear correspondence between temperature and [DOC] in the Waipuna record. At the high altitude, South Island sites (Hodges Creek cave, Adelaide Tarn and Dave’s Cave), DOC concentrations were higher during periods with relatively low precipitation and high temperature. Hodges Creek Cave (940 m a.s.l.) is situated at a similar altitude to Adelaide Tarn (1,250 m a.s.l.) the two sites being only 32 km apart. Adelaide Tarn’s WEDOM and FTIR-TOC records show strikingly similar patterns of humic-like fluorescence and TOC concentrations to the Hodge’s Creek fluorescence record, providing support for a regionally coherent signal. The relationship between climate and soil DOC export was most clear during the Holocene climatic optimum. At this time, temperatures were approximately 1.5-2.5 °C warmer than present, whilst humic-like DOC values (relative to modern) were 57 % higher at Hodge’s Creek, 36 % higher at Adelaide Tarn and 40 % higher at Dave’s Cave. The HCO was also drier than most of the Holocene at these sites. Following the HCO, temperature declined, and wetness increased, corresponding with a notable decline in DOC concentrations at these sites. These findings imply that in the absence of human interference, higher temperatures and aridity were important drivers of elevated soil DOC export through the past 14,000 years BP was controlled by temperature and wetness at the high-altitude sites, whilst only wetness was important at low altitude.
Type of thesis
Pearson, A. (2020). The terrestrial carbon cycle in transition: tracking changes using novel tracers on multiple timescales (Thesis, Doctor of Philosophy (PhD)). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/13559
The University of Waikato
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