LATE-HOLOCENE LAKE DIATOM-INFERRED PALAEOCLIMATE FROM CENTRAL TURKEY
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In the semi-arid environment of the Eastern Mediterranean, water can be a limiting resource and its availability is influenced by different climate factors. Knowledge of late Holocene water balance is limited for this region. Lake systems and organisms respond to environmental variability and can be used as a proxy for palaeoclimate. The aims of this research project were to reconstruct late Holocene palaeoclimate using diatom frustules preserved within crater lake sediments in central Turkey. Two lakes (Nar Gölü and Kratergöl), located in the same climate region, were selected for this purpose. Modem lake samples and sediment cores collected between 1999-2006 were subsampled at high resolution for diatom analysis, Nar G610 provided an uninterrupted annually-laminated late Holocene sequence covering the last 1720 years. The varied lake sedimentation rate of Kratergöl was evident in sediment core coarse sandy sections and the sequence was thought to represent the mid-late 20`h century. A diatom-salinity transfer function was employed using existing training sets from the European Diatom Database to infer past water balance. The reconstruction was calibrated with instrumental meteorological data. Reconstructed salinity was limited by poor analogue matching between the palaeo-diatom assemblage and the modem training set. This was partly associated with the presence of a previously undescribed diatom genus (newly named Clipeoparvus anatolicus), which was highly abundant in the Nar modern environment and sediment record. Additional methods to extract palaeoenvironmental information from the diatom record were explored. This included calibrating diatom DCA axes with instrumental temperature in order to reconstruct palaeo-temperature, identifying mono-specific diatom bloom events in situ on core thin section slides, calculating diatom biovolume, concentration, diversity and grouping species according to their habitat preferences. Comparison of the Nar and Kratergöl records highlighted the advantages of annually laminated lake sediments for palaeoenvironmental research and the limitations of sediment sequences from lakes with a varied sedimentation rate and poor chronological control. The primary meteorological control on the Nar diatom population was identified as summer temperature, via the link with lake water salinity. The Nar diatom sequence was compared with an oxygen isotope (palaeo-evaporation) and pollen record (human land use) from the same sediment cores and palaeoclimate reconstructions from other sites and regions. Nar diatoms and oxygen isotopes revealed that Cappadocia experienced high aridity prior to AD 540 and mono-specific diatom bloom events have become increasingly common during the most recent -400 years. A diatom assemblage shift at AD 2001 also indicated a recent change in the system. Human land use evident in the pollen sequence may have influenced the diatom relationship with climatic variability in the later part of the record. The Kratergöl diatom record indicated environmental variability throughout the mid-late 20`h century; however, interpretations were limited due to chronological discrepancies. The annually laminated Nar diatom record has provided a detailed account of palaeoenvironmental variability in central Anatolia throughout the late Holocene and contributes towards our understanding of Eastern Mediterranean palaeoclimate.
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