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Title: Investigating oxygen and hydrogen isotopes in the routing of freshwater to highlight the considerations when using Lithothamnion glaciale as an in-situ palaeo-runoff indicator
Author: Smiley, Crystal Renee
Awarding Body: University of Glasgow
Current Institution: University of Glasgow
Date of Award: 2016
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Abstract:
Many parts of the Holocene climate system (e.g. impacts of freshwater input on polar oceans) are not fully understood due to a paucity of data. These data gaps limit our ability to examine the drivers of Holocene climate variations, restricting the accuracy of climate projection models. Freshwater input to polar oceans may have significant impacts on the behaviour of the thermohaline circulation (THC). In particular, freshwater input to the North Atlantic sector of the THC may cause reduced salinity slowing water mass sinking which drives the THC. Currently, our understanding of Holocene salinity change is temporally restricted due to limited instrumental runoff and salinity records. One approach to extend the records is to use runoff proxies; environmental recorders of historic runoff. The high temporal resolution, marine, red coralline algae (Lithothamnion glaciale), has been suggested as a potential solution to this problem. Coralline algae are long-lived with slow growth rates that display well developed annual growth bands at seasonal time-scales. In Greenland, they have been shown to record changes in water salinity and temperature at annual—5 year resolution. However, before L. glaciale, is used more widely to reconstruct freshwater runoff, we need to understand; 1) the water source that drives changes to salinity and 2) if the different water sources can be detected in the marine environment isotopically (δ18OVSMOW and δDVSMOW). To achieve that, this thesis aims to understand the oxygen (δ18OVSMOW) and hydrogen (δDVSMOW) isotope compositions in the routing of freshwater, from its source into the marine environment. This will enable determination of the environmental conditions which L. glaciale is exposed to and thus recording. Isotopic composition of source and marine waters were determined at locations that have a terrestrial-fjord/loch system and locations that are influenced by snow and ice melt (Glencoe, Scotland and Kangerlussuaq, Greenland respectively). At both locations, snowmelt, ice melt, riverine, lacustrine and marine waters were sampled for δ18OVSMOW and δDVSMOW analysis to determine; 1) the starting source isotope compositions in areas of freshwater influences into the marine environment, 2) any processes influencing the oxygen and hydrogen isotope composition change from the source to ocean, 3) the extent to which the isotopic composition changes throughout the runoff season and 4) the consequences of 1)-3) for using L. glaciale as an in-situ palaeo-runoff indicator. Similar results were found at both collection sites; 1) isotope compositions enriched from the main freshwater source, through a river system and into the marine environment, 2) δ18OVSMOW and δDVSMOW compositions could pinpoint the exact areas of freshwater input and mixing along the fjord/loch, 3) the fjord/loch have fast hydrological responses to changing runoff conditions, 4) additional freshwater inputs and changes in the source are the primary processes altered the isotopic composition of the freshwater surface layer, 5) established individual sources can be separated from the bulk water mass and can be detected 118 km (Søndre Strømfjord) and 20.4 km (Loch Etive) away from the main freshwater terminus and 6) a suitable collection site can be determined from the isotopic signal ensuring that the algae record the full runoff signal. Knowing the exact areas of freshwater input along with spatial mixing and tidal action within the surface layer of the fjord/loch has given insights into the environmental forcing acting on the algae at the time of growth. To relate key findings of this thesis with the algal signals detected in previous studies; salinity-temperature-δ18OPDB relationships (within Søndre Strømfjord) were used to determine any considerations for the use of L. glaciale as a climate recorder if salinity-driven δ18O patterns are not accounted for. However, it is important to note that as algal samples were from other studies, the temporal miss-match between water (this study) and algal collection may drive some of the observations made. It was found that; 1) Isotopic compositions extracted from L. glaciale growth bands in previous studies are enriched by 8.7-10.72 ‰ compared to calculated δ18OVSMOW at the algae collection sites, 2) the δ18OVSMOW environmental signal is thus not directly incorporated into the algae’s skeletal structure providing a consistent offset or enrichment of the freshwater signal, 3) calibrating algal δ18O with predicted sea surface temperatures (SST) at the algae collection site generates an algal-derived summer SST of ~4 °C which is comparable to instrumental records, 4) variations in the marine δ18OVSMOW composition due to a freshwater input can cause a δ18OPDB change within the algae and may influence the reconstructed temperature in areas of high freshwater input, 5) greater GrIS runoff corresponds to depleted δ18OPDB and cooler reconstructed temperatures and thus indicates that freshwater provides a temperature error in reconstructed temperatures. However, the runoff itself cools the water and this effect could be less than initially expected. This investigation has suggested the optimal collection locations for algae to increase the likelihood of them having recorded freshwater discharge signals. This had made a major advance as in the past algal collection locations were largely chosen using chart assessments. A dedicated study, which uses Lithothamnion glaciale collected from the suggested optimal recording locations, is now needed. Overall, knowledge generated in this study will enable advances in reconstructions of runoff from ice sheets, glaciers and calibrations using marine proxies, enabling detailed reconstruction of Holocene climate variability.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.716872  DOI: Not available
Keywords: Q Science (General)
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