Primmer, Nick
(2019)
Reconstructing high resolution environmental change from carbonate-rich lakes in Turkey and Mexico using varve microfacies analysis.
PhD thesis, University of Nottingham.
Abstract
Analysis of lacustrine sediment records provide insight into terrestrial environmental change throughout the Holocene; however, poor chronological control often limits a record’s ability to identify the precise dynamics and drivers of these changes. Exploring these important aspects of environmental change can develop our understanding of the mechanisms driving long-term climatology and its influence on regional hydrological systems. To fully investigate these past environmental systems, high resolution, climatically-sensitive proxies are required.
Here, microfacies analysis of varved sediment records from Nar Gölü, Turkey and Yaal Chac, Mexico provide long annually-resolved records of environmental, limnological and climatic change, from two regions lacking in continuous, high resolution archives. From Nar Gölü, varves were analysed from discrete sequences deposited during the early-mid and late Holocene (9,064 – 7,630 cal yrs BP and 2,540 – -51 cal yrs BP respectively), and from Yaal Chac, the early-mid Holocene (8,812 – 7,245 cal yrs BP and 5,346 – 4,295 cal yrs BP). These long, continuous depositional archives have permitted the reconstruction of both low- and high-frequency limnological change, where the sequences’ incremental chronologies have further enabled the pace and timing of change to be identified and robustly dated.
In both lakes, organic-calcareous varve deposition is driven by the regions’ wet-dry seasonality. At Nar Gölü, sedimentological calibration using meteorological data identified the March-May evaporation/precipitation ratio as the primary control on the deposition of endogenic carbonates. Comparing specific carbonate sublayers’ mineralogy and thickness with their δ18Ocarbonate value further indicates that regional water balance over the last 2,600 years primarily drives carbonate deposition. Considering the sites’ similar seasonality and sedimentary microfacies, changes to the endogenic carbonate deposition and mineralogy are used to indicate high resolution changes to lake hydrology and the timing/intensity of dry-season E/P conditions in both lakes.
Spectral analysis of these varved records, which provide incremental timeseries, is used to identify the multi-decadal drivers to depositional variability. The Atlantic Multi-decadal Oscillation (AMO) is likely detected within both lake records at various stages through the Holocene, where changes to sea surface temperatures are known to influence moisture supply and rainfall during each region’s respective wet-seasons. From Nar Gölü, changes to cyclicity are also proposed to reflect shifts in the relative influence of Atlantic-and tropical-derived climates, as the multi-decadal variability of Indian monsoon intensity is also known to drive dry-season aridity.
Sedimentological-inferred changes to past conditions, specifically changes to the regional evaporation/precipitation (E/P) ratio, are finally used to provide the environmental context to societal change. Each region’s wet-dry seasonality increases societal vulnerability to drought, where periods of elevated evaporation/precipitation conditions may have influenced food and/or water security, and hence socio-economic stability. Temporal correlation between microfacies and societal changes provide circumstantial evidence that climatic change may have contributed to major societal shifts through the Holocene in both Turkey and Mexico.
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