Late Quaternary palaeoenvironmental reconstruction from Lake Ohrid

Lake Ohrid is a large, deep, ancient lake located on the Balkan Peninsula and has an extraordinary degree of endemic biodiversity. A deep drilling campaign was carried out in 2013 as part of the Scientific Collaboration on Past Speciation Conditions in Lake Ohrid (SCOPSCO) project to better understand the influence of climate and environmental change on evolutionary patterns and endemism. Initial results from a 569 m sediment succession drilled in the centre of the basin indicate continuous lacustrine conditions over the past ca. 1.2 Ma. This thesis presents new stable isotope data from carbonate within the upper 248 m of the composite profile, covering the last ca. 640 ka at a millennial-scale resolution (≈ 0.5 ka). Isotope data are used to provide a long-term palaeoenvironmental reconstruction, which is supported by a quantitative model of modern lake water isotope composition and a detailed multi-proxy investigation of climate evolution since the last glacial period. Water balance calculations confirm Ohrid to be an evaporative system with a complex hydrology. Variations in the isotope composition of modern lake water are suggested to represent long-term fluctuations in water balance. A trend from wetter to drier conditions through the Holocene is consistent with regional and hemispheric processes related to changes in insolation. Over the last ca. 640 ka, endogenic calcite is precipitated in abundance during warm stages, however carbonate is negligible during glacial periods with the exception of discrete horizons comprising early diagenetic authigenic siderite. The oxygen isotope composition of lake water, calculated using calcite and siderite, indicates more evaporated conditions during warm stages and fresher lake water in glacial phases. Low frequency variability shown by calcite isotope data suggests relatively stable conditions before ca. 450 ka, a transition to a wetter climate between ca. 400-250 ka, and a trend to drier climate conditions after ca. 250 ka. Higher frequency millennial-scale oscillations and abrupt climate events observed during warm stages are likely associated with regional climate change as a function of orbital forcing. This study emphasises the potential of Lake Ohrid as a valuable archive of climate change in the central Mediterranean region and demonstrates the efficacy of isotope data for palaeoenvironmental reconstruction at Lake Ohrid.