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Title: Palaeo-perspectives on surface water brownification
Author: Russell, Fiona
ISNI:       0000 0005 0288 1929
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2021
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'Brownification', or increases in surface water Dissolved Organic Carbon (DOC) concentrations across the northern Hemisphere, are often regarded as upland waters responding to declining atmospheric acid deposition. Discriminating between this and alternative hypotheses requires data sets longer in duration than monitored records. Palaeoenvironmental approaches provide a solution and this thesis iterates through method development, applications to UK sediments, and explores the implications for understanding of changes in surface water DOC. A new approach to interrogating Near Infra-red Spectra (NIRS) was developed that avoided the empirical chemometric statistical procedures and used the direct fitting of spectra for known end member materials to spectra of samples using multiple regression (EMS-MR). This approach avoids the need for building training sets and produces simultaneous estimation of multiple sediment components. Throughout, data generated by NIRS EMS-MR were compared with parallel independent sediment burial flux data from loss-on-Ignition, geochemistry and sediment chronology measurements to corroborate the reconstructions. The stages were: I) The EMS-MR method was developed and trialled for end members including total organic matter, biogenic silica, and mineral matter. For Loch Grannoch (SW Scotland), analysis of the Holocene sediments showed and corroborated changes in biogenic silica, organic and mineral content over 11,500 years. II) Motivated by the methodological successes, the NIRS EMS-MR approach was applied to a stratigraphy dominated by fluctuations in humic acid (DOC) using an ombrotrophic peatland, Holcroft Moss (NW England). This allowed direct quantification of the organic matter fractions associated with DOC, using International Humic Substances Society (IHSS) humic and fulvic acid and other standards. There, the approach struggled to differentiate between plant taxa due to the similarity of different end member spectra and a strong sensitivity of NIRS to plant decomposition. The approach showed more success in the rapid non-destructive characterisation of humic acid concentrations and peat humification. III) Records of DOC concentrations were inferred from the sediments at four lakes in the UK Upland Waters Monitoring Network (UK-UWMN). Using a simple process model, the aqueous DOC concentration was calculated from the DOC sediment burial flux. Those DOC concentrations agreed well with the monitoring record (1988-2016) for DOC in the water (r2 = 0.78). DOC increases recorded in the sediment predate lake acidification and it appears enhanced terrestrial productivity may have driven brownification. The timing is broadly consistent with fertilisation of upland soils by enhanced atmospheric N deposition, though other land use factors and recent climate warming are likely contributing factors. IV) Lake water DOC throughout the Holocene was reconstructed for dystrophic Loch Grannoch and oligotrophic Llyn Cwm Mynach to explore the controls over DOC at longer timescales. The results show significant variation in the DOC flux, in the early Holocene driven predominantly by climate and catchment vegetation cover, but from 5000 BP human activity drives the flux of terrestrial materials showing importance of land management in limiting carbon loss from soils. DOC flux declines with cooler climate of the past 2000 BP, but without falls in total organic matter suggesting changes in either supply or in the processing of organic matter. In summary, climate and vegetation cover have been strong drivers of terrestrial organic matter supply to surface waters through the Holocene, and although it is widely accepted that recovery from acid deposition may have enhanced DOC flux during the late 20th century, my findings conclude that this is not responsible for these recent rises, which are driven instead by increasing terrestrial productivity related to warming. Future increases in DOC flux are likely given expected future climate warming, but careful management of catchment land cover change could mitigate surface water "brownification".
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral