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Paleoenvironmental Variability during the Middle Miocene Climatic Optimum Reconstructed using a Lake Sediment Record from the Otago Region, New Zealand

Climate archives indicate that the varied interaction of several cycles across a range of timescales exerted a prevailing control on climate throughout the Holocene and Pleistocene. However, due to the scarcity of highly resolved lacustrine records, it has so far been difficult to recognize sub-Milankovitch scale cycles in earlier epochs. Climate variability during the Middle Miocene was particularly extensive, and included the Miocene Climatic Optimum, representing the maximum temperatures experienced during the Cenozoic. A high resolution account of Southern Hemisphere paleoenvironmental variability during the Miocene Climatic Optimum has never previously been produced and is thus of significant importance. The Hindon Maar diatomite is a laminated lacustrine deposit dominated by Aulacoseira Spp. diatoms. This lake formed due to a phreatomagmatic eruption as part of the Waipiata volcanic field, and accumulated during the Langhian Age (~15.2 Ma; Middle Miocene) in the Otago region of New Zealand. A high-resolution multi-proxy analysis was conducted on a 10.8-m-long core of this annually resolved lake sediment, which varve thicknesses suggest represents 7600 years of deposition. Gamma density, magnetic susceptibility, CIELAB L*a*b* colour, particle size, compositional data and petrographic images were collected. Compositional data included biogenic silica, total organic carbon and schist, determined using Fourier transform infrared spectroscopy. Principal components analysis of these variables enabled the identification of underlying processes which I interpret to represent precipitation, wind speed and temperature. Aulacoseira Spp. diatoms are highly sensitive to variations in wind speed, and thus most changes in lake productivity reflect wind strength changes relating to underlying cyclicity. In addition, electrofacies analysis is used to identify six reoccurring and persisting paleoenvironmental states. Based on electrofacies analysis of principal component factor scores, a high resolution reconstruction is proposed and related to the underlying cyclical factors. It is suggested that wind strength on this time scale during the Middle Miocene may partially respond to the interaction of two solar cycles and one orbital cycle. These are the DeVries cycle (200 years), the Hallstatt cycle (2300 years) and precession (~26000 years). These cycles and others were identified using a combination of sum of sinusoidal signal analysis, Tukey-Hamming spectral analysis and wavelet transformations. Peaks in these cycles thus likely generated warm conditions, causing prevailing westerly winds to move northwards and intensify, whilst troughs correspond to a decline in oceanic temperatures and wind speeds. This investigation therefore demonstrates the influence of sub-Milankovitch scale cycles and solar forcing on conditions during the Miocene Climatic Optimum for the first time in the Southern Hemisphere.
Type of thesis
Garrity, F. D. A. (2017). Paleoenvironmental Variability during the Middle Miocene Climatic Optimum Reconstructed using a Lake Sediment Record from the Otago Region, New Zealand (Thesis, Master of Science (MSc)). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/11790
The University of Waikato
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